Download Lecture 23: Pharmacogenetics Pharmacogenomics: study of genetic

Lecture 23: Pharmacogenetics
1. Pharmacogenomics: study of genetic variations that influence drug responses
a. Asses the effect of many genes simultaneously
b. A drug’s toxicity is related to single-nucleotide polymorphisms (genetic changes)
c. Multiple variants that may have smaller effects
2. Pharmacogenetics: study of individual genetic variants that will affect an individual’s
response to drugs
a. Single variant that has a large effect
3. Genetic factors are important in determining drug efficacy and toxicity
4. Genes control intake, absorption, distribution, drug-cell interactions, breakdown, and
excretion
5. Genetics of drug responses
a. Continuous variation: multifactorial control of drug metabolism
i. unimodal distribution = drug metabolism is under control of many genes
b. discontinuous bimodal variation
i. individuals who are RR or Rr are indistinguishable
c. discontinuous trimodal variation
i. normal drug metabolism is under control of dominant gene
1. homozygous recessive people can’t metabolize the drug
2. there are three types of responders: RR, Rr, rr
3. trimodal is seen when you can differentiate RR from Rr
6. drug biotransformation
a. after drugs are absorbed they can:
i. go through cytochrome p450 system
ii. become a drug metabolite with modified activity
iii. become an inactive drug metabolite
b. some biotransformation reactions may have toxic side effects
c. phase I reactions
i. introduce a functional group to the original drug
ii. this will typically cause the drug to lose its activity
iii. occur via cyt p450 – typically add OH groups to make the drug more
hydrophilic
7. CYP2D6
a. a person’s ability to metabolize many drug types depends on this individual
genotype
b. phentoypes include:
i. ultrarapid metabolizers
1. require higher doses to achieve the needed plasma concentration
ii. extensive metabolizers
iii. intermediate metabolizers
iv. poor metabolizers
1. require smaller doses of nortiptyline (anti-depressant drug)
8. testing for disease-specific or disease-enriched drug targets
a. imantinib drug is used to block production of fusion proteins from bcr-abl in CML
cells
i. normal cells do not have these fusion proteins
b. trastuzumab drug binds to the Her2/nue receptor (breast cancer cell) to prevent
receptor activation
9. estimating drug efficacy and dosing
a. pharmacokinetics
i. warfarin blocks metabolism of vitamin K (affects blood clotting)
b. pharmacodynamics
i. clopidogrel blocks platelet activation
c. predicting and avoiding side effects
i. simvastatin must be absorbed in the intestines
ii. it reduces cholesterol synthesis by reducing HMG-CoA reductase activity
10. disease genetics
a. examines disease prognostic/diagnostics
i. rare mendelian diseases
ii. common complex diseases
b. new disease insights leads to future medicines
11. pharmacogenetics
a. examines medicine response profiles
i. genes for drug metabolism
ii. SNPs/haplotypes for drug metabolism
b. Helps to determine optimal medicine response
12. Effects of genetic polymorphisms on drug metabolism
a. CYPs can mediate and activate drug dtox
b. codeine: most of its effects are seen after it’s converted to morphine
i. morphine has a 10-fold higher potency
ii. this conversion occurs via CYP2D6
c. people who are poor metabolizers have a loss-of-function allele in their CYP2D6
i. codeine isn’t converted to morphine so it has little benefit to them
d. ultrafast metabolizers can be intoxicated at low codeine doses
13. pharmacogenetics polymorphism effects in cancer seen in:
a. proteins that affect drug availability to the tumor
b. proteins that affect tumor drug sensitivity
c. proteins that affect toxicity
14. cancer pharmacogenetics
a. 6-mercaptopurine (6-MP)
i. the pro-drug (azothioprine) is inactive
ii. the metabolines are active
iii. azathiprine can be etabolized into 6-MP which can then be broken down
into 3 metabolites via their respective enzymes
1. 6-thiouric acid via xanthine oxidase
2. 6-thioguanine nucleotide (6-TGN) via HGPRRT – toxic metabolite
3. 6-methylmercaptopurine (6-MMP) via thiopurine
methyltransferase (TPMT) – inactivates 6-MP
iv. TPMT enzyme inactivates 6-MP and prevents it from becoming TGN
(toxic)
1. RBC availability of this enzyme is a codominant autosomal trait
2. Low TPMT = high toxicity due to high TGN
a. can cause myelosuppression and fever
3. high TPMT = reduced therapeutic activity of 6-MP
4. the most common dysfunctional TPMT alleles are 2, 3A, and 3C
v. thio guanine nucleotides kill quickly growing cells by inhibiting DNA/RNA
synthesis
vi. used to treat autoimmune disorders (rheumatoid arthritis) and to
prevent organ rejections
vii. can also treat ALL – but can cause toxicity
viii. most people metabolize this drug quickly – doses need to be high enough
for cancer patients so the drug can have its effects
ix. those who metabolize this drug slowly need lower doses to prevent
toxicity
x. those who are TPMT deficient (very slow metabolism of the drug) require
very low dose
b. irinotecan are used to reduce toxic events
c. 5-FU
i. drug that inhibits thymidylate synthase (TS is overexpressed in tumors)
ii. TS expression depends on the number of tandem repeats (TSER—tandom
repeat polymorphic copies of 28 BP) in the 5’ promoter region of the TS
gene
1. 3 repeats of TSER = 2.6 times higher TS activity than those with 2
repeats = poor antitumor response
2. 2 tandom repeats = lower TS activity = good antitumor response
3. 3-10 tandom repeats = higher TS activity
iii. drug’s inactivated in liver by DPD (first enzyme needed for uracil and
thymine catabolism)
1. deficiency in DPD causes severe 5-FU toxicity
2. most common DPD mutation is G --> A
3. mutation causes truncated mRNA with no enzyme activity
4. this allele is DPYD*2A and causes severe 5-FU toxicity
15. warfarin metabolism
a. inhibits vitamin K epoxide reductase (VKOR)
b. drug acts as an anticoagulant by clock ability of VKORC1 to regenerate reduced
vitamin K (which is needed as a cofactor for the clotting factors)
c. maturation of vitamin K is inhibited = reduced coagulation
d. warfarin is a mixture of both R and S enantiomers
i. S is 5-fold more potent
1. Metabolized by CYP2C9
ii. R is metabolized by CYP1A1, CYP1A2, CYP3A4, CYP2C19
iii. Genetic variation in the CYP’s can lead to individual dosage variation
16. Glucose-6-P DH deficiency causes increased sensitivity to anti-malarial drug primaquine
(this can lead to hemolytic anemia)
17. succinylcholine is used in surgeries to cause short-term muscle paralysis
a. mutations in the butrylcholinersterase gene will decrease the enzymes activity,
causing the inability to metabolize succinylcholine
b. this causes malignant hyperthermia in patients
18. Personalized medicine: use of individual’s unique genome and environmental profile to
predict risk factors of a disease as well as the person’s response to treatment
a. predicts disease outcome by gene expression profile
b. lung cancer is tested by looking for genes that are abnormally regulated
19. on an EKG, a long QT interval is caused by mutations in 10 genes
a. an individual genetic profile is used to determine most effective drug
Lecture 24: Regenerative Medicine
1. gene therapy: delivers therapeutic genes to correct disease conditions
a. involves only somatic cells NOT germline
b. ex vivo gene therapy
c. in vivo gene therapy
2. most genetic disorders are untreatable
3. genetic disorders with drug treatment:
a. familial hypercholesterolemia
i. statin drugs inhibit HMG-coA reductase
ii. causes an up-regulation of LDL receptor
iii. increase in LDL clearance
b. glucose-6-P DH
i. avoid sulfonamides
c. angioneurotic edema: aminocaprioc acid
d. porphyria: avoid barbiturates
e. CF: pancreatic enzymes
f. Wilson disease: penicillamine
4. Genetic disorders with dietary restrictions
a. PKU: remove F from diet
i. Mutation in phenylalanine hydroxylase
ii. Some people are missing tetrahydrobipterin (BH4)
b. maple syrup urine disease: remove BCAA from diet
i. mutation in alpha-ketoacid DH
c. galactosemia: remove galactose from diet
i. deficiency in enzymes that metabolize galactose, galactokinase,
galactose-1-P uridyl transferase, or UDP galactose epimerase
5. genetic disorders that require replacement
a. congenital adrenal hyperplasia: administer cortisone
i. mutation in CYP21A2 in the glucocorticoid pathway
b. congenital hypothydoidism: supply thyroxine as treatment
i. caused by iodine deficiency, TSH resistance, idiotyrosine deiodinase
mutation
c. homocystinuria: supply B6 in diet if that’s what deficient
i. defect in mtheionine metabolism
d. diabetes: replacement with insulin
6. Enzyme replacement theory (ERT)
a. Originally used for gaucher disease (lysosomal storage disease)
i. Type I patients get enzyme doses isolated from human placentas
b. Also used in other lysosomal storage diseases (Fabry’s and MPS)
7. Some ERT had negative results
a. hGH isolated from cadavers transmitted vCJD to patients
b. hemophilia A factor replacement transmitted HIV to patients
8. human genes can be cloned to produce synthetic human proteins
a. restriction enzymes cut the plasmids
b. Gene of interest can be inserted into the plasmid
c. Recominant DNA is then inserted into bacteria cell and clones are produced
d. Bacteria + antibiotic are plated – only the bacteria with the recombined DNA will
grow
e. DNA can then be purified
9. DNA construct for gene therapy
a. Regulatory elements determine: amount of expression, tissue specificity, and
timing of expression
b. The protein coding sequence of the plasmid gives the functional human gene
c. The gene delivery element has a retrovirus (RNA  DNA)
d. The functional gene packaged in viral vector or the plasmid containing the gene
of interest are then cultured
e. The protein of interest is isolated and the infused back into patient (i.e. ex vivo
therapy)
10. Gene delivery methods
a. Ex vivo: cells treated outside the body infused back into the patient’s body
i. Cell transfection: liposomes and other transfection methods
ii. Viral transfection: retrovirus, lentivirus, adenovirus
iii. You can check transduced cells before injecting back into donor
iv. Treatement cell source: hemopoietic stem cells or stem cells from other
organs
v. Steps:
1. Cells with gene defect are isolated from patient and grown in
culture
2. Target cells are infected with retrovirus (which has recombinant
RNA of normal gene form)
3. DNA is produced from the viral recombinant RNA by reverse
transcriptase
4. Translation of the corrected protein
5. Reinfusion of the engineered cells into patient
b. In vivo: cells treated directly in body
i. Direct injection of virus/carrier into infected organ
ii. Stem cell transplant
iii. Receptor mediated endocytosis of DNA in liposomes
1. DNA released into cytoplasm then goes to nucleus for mRNA then
protein production
2. Efficiency is low but less potential complication than viruses
iv. Potential organ targets
1. Liver – OTCD, Gaucher
2. Muscle –DMD
3. Pancreas – Diabetes I and II
4. Bone marrow – sickle cell
5. Cancer treatment with suicide vectors
c. The objective is to reach threshold level of therapeutic product to effect cure
11. Gene therapy can be used via adenovirus cell vector
a. Retrovirus structure
i. Long terminal repeats at 5’ and 3’ ends
ii. Open reading frames for gag, pol, and env proteins
iii. Splice acceptor
iv. Protease
v. Reverse transcriptase
vi. Integrase
vii. Surface, transmembrane proteins
b. Virus has a gene delivery (GOI) vector
c. 3 plasmids can deliver functional viral proteins but only GOI piece has the ψ
packaging signal to make virus particles
d. retrovirus lifecycle
i. retroviral particle with its envelope binds to target cell surface receptors
ii. virus core enters cell cytoplasm
iii. reverse transciption (RNA DNA)
iv. cDNA is integrated into host chromatin
v. integrated DNA is transcribed/translated into viral proteins
vi. proteins assemble in host cell then bud from membrane
e. retrovirus gene delivery vector
i. newest version of retrovirus vectors are SIN vectors (self-inactivating)
ii. SIN vector advantages:
1. Decrease risk of creating viruses that can replicate
2. Decrease use of internal promoter deletion of U3’LTR enhancer
3. Internal promoter can be cell-specific
4. Can regulate GOI/trasngene expression at a post-transciptional
level
iii. SIN disadvantages
1. Insertional mutagenesis into a gene creating a loss of function
a. Vector can insert in reverse orientation upstream of gene
– this would activate enhancer and up-regulate gene
expression
b. Integration into a transcription unit can mediate
premature termination
c. Integration between promoter and 1st coding exon
uncouples cell transcription unit from its promoter –
causes expression of viral promoter
f. Steps
i. Purification and cultivation of HSC
ii. Ex-vivo retroviral gene transfer
iii. Condition of patient to accept ex-vivo genetically modifies cells
iv. Transplant modified cells
g. Used in treatment of:
i. SCID
1. Some patients with SCID-X1 who underwent retroviral treatments
saw that any clinic benefits were modulated by leukemia
2. Proto-oncogene activation was seen occasionally due to the
retroviral integration into host genome
3. Development of this disease could be mediated through growth
control genes
ii. Wiskott-Alrich Syndrome (mutations in WASP proteins)
1. Ex vivo: used a SIN lentivirus vector to introduce functional WASP
2. Improvement was seen; no proto-oncogene activation
iii. Chronic granulomatous disease
iv. ALD
12. Transfection efficiency
a. Co-transfect a marker that will show how many cells might express the virus
b. GFP is used as an indicator
13. Jesse Gelsinger case
a. In vivo gene therapy (adenoviral vector) trial for ornithine transcarbamylase
(OTC) deficiency (urea cycle deficiency)
b. Jesse was on medication to capture the excessive NH4+ in blood
c. Infused a high dose of vector containing the OTC gene – he died from multiple
system failure
14. Metachromatic leukodystrophy (MLD) gene therapy trial
a. Lysosomal storage disorder due to deficiency in arylsulfatase A
b. Lentivirus transduction of ARSA gene – saw no progession of disease
15. Reprogramming peripheral blood mononuclear cells (PBMCs)
16. Induced pleuripotent stem cells can differentiate into embryoid bodies  neuron
differentiation
17. Undifferentiated stem cells  directed differentiation  application of differentiated
stem cells
18. Regenerative medicine could replace diseased cells of spinal cord injury, pancreas (beta
cells), or heart
Lecture 25: Genetic Counseling
1. Genetic counseling: communication process which deals with human problems
associated with risk or genetic disorder in a family
a. As a doctor, you have to explain to a patient their genetic diseases in the best
way that allows them to make an informed decision
b. You must not put forth your own feelings
2. Family history depends on aspects that are actionable (interventions are available)
a. Family planning
i. You can only tell a patient why you think something is happening
b. Personalized medicine
i. Disease prevention
ii. Delay disease expression
iii. Earlier diagnosis
c. Genetic therapy
3. Family history red flags
a. F: family history
i. Look for diseases that seem to have more prevalence in family than in
general population
b. G: groups of congenital anomalies
i. Anyone born with more than 1 birth defect
c. E: extreme presentation of common disorders
i. EX: early breast cancer
d. N: neurodevelopmental delay or degeneration
e. E: extreme pathology
f. S: surprising laboratory values
i. Blood work
4. Pedigree interpretation
a. Variable expressivity
b. Reduced penetrance
c. Limited family structure
d. Genetic/allelic heterogeneity
e. Delayed age of onset
i. Ex: hereditary cancer, Huntingdon’s chorea
f. Sex-limited expression
g. Mosaiscism/gonadal mosaicism
i. Sporadic errors in chromosomes that are not supposed to be passed on
but they seem to be present multiple times in a single family
ii. Ex: down syndrome
h. De novo mutations
5.
6.
7.
8.
i. Some trait that should be inherited but only one patient of the family has
it
i. Inbreeding
j. Incest
k. Adoption
l. Non-parentage
i. Normally non-paternity
ii. You may find a disease that is not possible with the current parents
(normally it’s the father who is not the actual father)
m. Egg/sperm donor
Risk assessment—the risk for developing multifactorial disease depends on risk profile
of that disease
a. Is it hereditary
b. What’s the chance the patient/offspring will get the disease
i. Risks need to be placed in context -- try to explain the risk without using
numbers
1. Relative to what you see in your practice, is it necessary to take
aggressive action
ii. Qualification of risk
1. Diagnostic criteria met or not met
2. Increased/decreased compared to the general population
iii. Quantification of risk
1. Bayesian analysis
2. Empiric data
a. If a disease is more frequent in M, then M have a higher
liability
b. If there are severe/mild forms of the disease, the
recurrence risks are higher in family members of a
proband with the disease
Single nucleotide polymorphisms (SNPs): a form of variant (mutation) that can be
pathogenic (disease-causing), benign
a. There are about 1000 SNPs associated with over 100 common diseases
b. You can test for SNPs – most SNPs are not very predictive (they will not increase
your risk enough to make a medical decision)
c. If something is a pathogenic variant with a strong predictive likelihood of causing
disease, then something should be done
The lower the threshold for the disease expression, the higher the genetic liability
a. If you pass the threshold, then disease will be expressed
b. If you carry a certain SNP for a genetic disease, it could lower your threshold
before there is any disease expression
Risk stratification
a. Genes + environment = complex disease
b. A person with no red flags for heart attack may just be advised to maintain a
normal BMI and to exercise
c. A person who is high risk for heart attack may be prescribed a statin
d. A person with high risk to breast cancer may want to undergo a double
mastectomy
9. Barriers to ordering genetic tests
a. Constant introduction of new tests, guidelines, laboratories
b. Cost/insurance coverage
c. Patient/physician doesn’t understand purpose of the test
d. Test is not interpreted appropriately
e. You do not have access to the best candidate in the family
f. Fear of genetic discrimination
10. Pre-test counseling
a. Informed consent
i. Does the patient understand risks/benefits of the test
ii. Will testing address the patient’s concerns
iii. What’s the plan if the test is positive/negative/uninformative
11. Screening tests provide you with
a. Risk stratification
b. Economical way to ID most common carriers
c. May require follow-up testing
12. Diagnosis tests provide a discrete answer and usually doesn’t require follow- up (gives
you a yes/no answer)
13. Post-test counseling
a. Pathogenic variant = positive
i. Duty to warn relatives at risk
b. No variant = negative
c. Variant of unknown significance
i. You still need to have a management plan for the disease in front of you
ii. Majority are benign – and are not clinically actionable
14. Psychosocial component of genetic counseling
a. Emotional response to answer
b. Normalize positive/negative results
15. Ethical, legal, and social implications of genetic medicine
a. Autonomy
b. Beneficence – do good to the patient
c. Non-maleficence – don’t harm the patient
d. Justice – no discrimination
e. Informed consent, duty to warm, unexpected findings, eugenics, abortion,
wrongful birth, genetic discrimination, medical futility (end of life decisions)
16. Genetic info non-discrimination act (GINA)
a. Provides protection against discrimination by employers and health insurers
i. But doesn’t cover life, disability, or long-term insurances
b. Those affected by discrimination may be eligible for pre-existing condition
insurance plan
17. Types of genetic disorders
a. Chromosomal (Down syndrome)
b. Single gene (sickle cell)
c. Trinucleotide repeat (fragile X)
d. Imprinting (Prader-Willi)
e. Mitochondrial (MELAS)
f. Multifactorial/complex (cancer)
18. Chromosomal disorders – sec chromosome abnormalities are most common
a. Down syndrome
b. Klinefelter syndrome (XXX)
c. Turner syndrome (XO)
d. Velocardiofacial syndrome (22q microdeletion)
19. Autosomal dominant disorders
a. Neurofibromatosis
b. Marfan syndrome
c. AD polycystic kidney disease
d. Hereditary hemorrhagic telangiectasia
e. Hereditary thrombophilia – most common genetics test ordered
f. Familial hypercholesterolemia
g. Osteogenesis imperfecta type I
h. Hereditary cancer
20. Autosomal recessive disorders
a. Hemoglobinopathies (beta-thalassemia and sickle cell)
b. CF
c. Alpha-1 antitrypsin deficiency
d. Hemachromatosis
21. X-linked disorders
a. Hemophilia A and B
b. DMD
c. Fragile X
Lecture 26: Reproductive Genetics
1. Reasons for genetic referrals
a. Maternal age
b. Ethnicity-related risks
c. Multiple miscarriages
d. Family history of birth defects
e. Teratogen exposures
f. Assistive reproductive technology
g. Ultrasound/serum screening results
2. Advanced maternal age
a. Oogenesis is stalled in prophase I of meiosis in 3rd/4th month of embryonic
development
3.
4.
5.
6.
7.
i. If they don’t divide properly in meiosis I you get two trisomy eggs and
two monosomy eggs
ii. If they don’t properly divide in meiosis II, half the eggs can be normal,
one will be monosomy and the last will be trisomy
b. Oogenesis is not completed until ovulation or fertilization
c. The older the egg, the more likely anaphase lag will occur – due to the failure of
spindle to attach to chromosome
Down syndrome – trisomy 21
a. Most common aneuploidy syndrome that can survive birth
b. Most common cause of mental retardation
c. Most cases are due to errors in maternal meiosis I resulting in trisomy
d. Most down syndrome pregnancies are lost through miscarriages
e. Characteristic facial features, heart defects, increased risk for diabetes,
leukemia, obesity, Alzheimer’s
f. On ultrasound: AV canal, increased nuchal thickness, echogenic foci of heart and
bowel, short long bonds
Trisomy 18
a. Second most common trisomy at birth
b. Normally due to nondisjunction in maternal meiosis II
c. Most cases result in miscarriage/stillbirth
d. The survivors typically die in first year
e. 3X more common in F than in M
f. Ultrasound findings: VSD (ventricular septal defects), rocker bottom feet,
clenched fists, intrauterine grorwth restriction
Turner syndrome-- triosomy X
a. Only monosomy that can survive to birth
b. Normally due to the paternal X missing
c. Short stature, webbed neck with low hairline, broad chest, heart defects, failure
or puberty to occur, infertility, intelligence with normal limits
d. Most cases are miscarried due to hydrops/fetal edema
e. Ultrasound findings: increased nuchal thickness, coarctation of the aorta
Advanced paternal age
a. Increased risk for de novo mutations
b. Fathers over the age of 40
c. Conditions with paternal effects: Pfieffer syndrome, crouzon syndrome, apert
syndrome, achondroplasia, thanatophoric dysplasia, MEN2A, MEN2B,
schizophrenia, autism, spectrum disorders
Ethnicity-related risks
a. Carrier frequencies are high in certain populations due to:
i. Over/under-representation in research
ii. Inbreeding
iii. Founder effect/genetic drift
iv. Heterozygote advantage
1. Sickle cell carriers protects against malaria – malaria will not
attack sickle RBC but there are not enough sickle cells to produce
severe anemia
2. CF protects against cholera – these patients have defective CFTR;
these channels in CF patients won’t lose lots of Cl- due to cholera
so they are less likely to die
3. Tay Sachs protect against TB
b. Ashkenazi Jewish disorders
i. Tay-sachs
ii. Canayan disease
iii. Gaucher disease
iv. CF
v. Neimann Pick disease
vi. Familial dysautonomia
vii. Mucolipidosis type IV
viii. Bloom syndrome
ix. Fanconi anemia
8. Multiple miscarriages
a. Half of miscarriages are due to chromosomal abnormalities (trisomy 16)
i. If mother is over 35, aneuploidy accounts for majoritiy of miscarriages
b. Some are due to (unbalanced) chromosomal translocations
i. T(11;22) is the most common
ii. Balanced chromosome translocations are usually benign
c. Stillbirth can have identifiable cause at autopsy or have genetic etiology
(maternal thyroid disease, diabetes, lupus, clotting disorder, hormone disorders,
etc.)
9. Structural rearrangements of chromosomes are less common than aneuploidy
a. T(11;22) is the most common
b. Balanced translocations are benign normally
c. Unbalanced translocations can cause pregnancy loss, mental retardation, birth
defects
d. Normally due to female translocation bc unbalanced sperm normally won’t
reach the egg
e. Robertsonian translocation: entire chromosome is stuck to another one
10. Family history
a. Congenital anomalies occur in 3-6% of live births -- most are sporadic
i. Folic acid helps to keep this risk low
b. Vertical transmission (parent to child to grandchild) is dominant
c. Horizontal transmission (only siblings are affected) is recessive
d. Fragile X is most common inherited form of mental retardation (down syndrome
is most common genetic form)
e. Come congeneticl anomalies don’t have known genetic etiology
i. VACTERL (vertebral defects, anal atresia, cardiac defects, tracheoesophageal fistula, renal anomalies, limb abnormalaties)
ii. CHARGE (coloboma, heart defect, atresia choanae, retarted growth and
development, genital abnormality, ear abnormality)
11. Teratogens
a. Timing and dose-dependent
b. Antiepileptic drugs can cause heart defects, neural tube defects
c. SSRIs can cause heart defects, neonatal adverse effects (pulmonary HTN, low
birth weight, withdrawal)
d. Maternal disease/infections
i. CMV, toxoplasmosis, parovirus
ii. Maternal diabetes, seizures, lupus
e. Organ systems develop between 3 and 10 weeks
12. Prenatal screening
a. Used for risk stratification
i. High, mediate, or low risk
ii. High, low, or no intervention
b. Parental choice -- speeds up diagnosis; allows option for termination
c. Early intervention – fetal therapy; timely postnatal care
i. Amniocentesis and percutaneous umbilical cord sampling (PUBS) can be
used for treatment
ii. Fetal surgery can be done to treat spina bifida, twin-twin transfusion,
tumors, abdominal wall defects, UT obstruction
d. First trimester
i. Biochemical screening
ii. Ultrasound
1. Nuchal translucency (10-13 weeks)
2. Detects 70% of fetuses with Down syndrome
iii. Blood work
1. Cell-free fetal DNA
2. Population/ethnic carrier screening
3. Universal carrier screening
iv. Chorionic villus sampling
e. Second trimester (18-20 weeks)
i. Biochemical screening
ii. Ultrasound
1. Fetal anatomy scan
a. Looks for congenital anomalies and soft markers present in
half of pregnancies affected with Down syndrome and
those with trisomy 13/18
2. Fetal EKG
a. Performed at 24 weeks to evaluate heart defects
iii. Blood work
1. MSAFP
iv. Amniocentesis
v. PUBS – blood is taken from the fetus through the umbilical cord
f. Ultrasound anomalies
i. Cystic hygroma
ii. Hydrops
iii. Holoprosencephaly
iv. Cardiac defects – AV canal
v. Omphalocele
vi. Isolated neural tube defects
vii. Gastroschisis
g. Ultrasound soft markers
i. Choroid plexus cyst
ii. Echogenic foci of heart/bowel
iii. Borderline ventriculomgealy
iv. Borderline renal pelvis dilation
v. Single umbilical artery
h. Biochemical screening
i. First trimester: combines maternal serum free beta-hCG and PAPP-A and
nuchal translucency measurement
1. 85-90% detection rate and 5% false positive for down syndrome
2. 95% detection rate and 0.3% false positive for trisomy 18
ii. Second trimester: maternal serum AFP, hCG, uE3, dIA
1. 81% detection rate and 5% false positive for down
2. 80% detection and <1% false positive for trisomy 18
3. elevated AFP = open spina bifida
i. Cell-free fetal DNA: non-invasive prenatal testing (NIPT)
i. Performed after 10 weeks
ii. 99% detection of trisomy 21, 18, 13
iii. Can detect sex chromosome aneuploidy
13. Prentatal diagnosis
a. Invasive procedures have a risk for pregnancy loss
b. More tissue obtained = increased detection and accuracy (but not as accurate as
postnatal evaluation)
c. A confirmation of ultrasound/lab findings may require patient to help mother
with
i. Psychosocial wellbeing
ii. Termination vs. preparing for birth
iii. Risk for recurrence
14. Chorionic villus sampling
a. Performed between 10.5 and 12 weeks
b. Chromosome analysis of trophoblast cells
c. Cannot detect neural tube defects
d. 99% sensitivity
15. Amniocentesis
a. Performed after 15 weeks
b. Chromosome analysis of amniocytes (fetal bladder epithelium)
c. Can measure AFP (plasma protein made by fetus) in amniotic fluid for 95%
detection of open neural tube defects
d. 99.4% sensitivity
16. Cordocentesis
a. Very rare
b. Samples PUBS – percutaneous umbilical cord blood
c. Performed after 17 weeks
d. Needed if amniocentesis can’t be performed due to fetal malformations, fetal
infection, fetal platelt count in mother, fetal anemia, iboimmunisation
17. Karytope: picture of all chromosomes
a. Takes 2 weeks
b. Requires cultured cells
18. Chromosomal microarray: detects general banding patterns of chromosomes; more
comprehensive genetic info about fetuses
a. Can’t detect single gene disorders caused by point mutation or small
deletion/duplication
19. FISH
a. Quick test for aneuploidy (2-3 days)
b. Detects microdeletions/duplications
20. Assistive reproductive technology
a. ICSI (intracytoplasmic sperm injection)/IVF (in vitro fertilization) associated with
increased risk for birth defects/chromosomal abnormalities
b. Preimplantation genetic screening/diagnosis
i. Polar body biopsy – can be preformed on unfertilized oocytes
ii. Day 3 blastomere biopsy (8-cell embryo)
iii. Blastocyst biopsy
Lecture 27: Development Genetics
1. About 150,000 babies born/year will have congenital abnormality
2. 4% will have major congenital abnormality
3. Population risk  factors that influence/modify/increase the risk
a. Family history
b. Pre-existing medical conditions
c. Environmental factors
d. Ethnicity
e. Genetic factors
4. No factors are known to lower the risk of birth defects
5. A child with one significant congenital anomaly has a 50% chance of a second one
6. Many gene products interact in a time-based pattern to produce a human
a. Gene expression changes over time as the cells go from pleuripotent stem cells
to differentiated cells
b. The first studies of this were done in Drosophila
7. Many syndromes are due to mis-expression of genes
8. The Hox gene (aka homeobox genes)
a. Large gene family that direct early embryonic development
b. They are transcription factors that contain a homeodomain (60 AA sequence that
recognizes sequences in DNA and control expression of other genes)
c. Genes in homeobox family have many functions
i. Formation of limbs along A-P axis
ii. Regulate cell maturity to differentiate into functional clusters
iii. Some hox genes act as tumor suppressors
d. Paralogs: genes that have been duplicated within the Hox clusters
i. The paralogs evolved new functions over time
ii. They are involved in limb development
e. The hox promoter can be linked to a reporter gene (lacZ from the lac operon) in
order to view developmental processes (they will stain blue)
f. Mutation in homeobox genes can produce early developmental disorders or
could be involved in cancer later in life
i. HOX genes  associated with limb disorders
ii. PAX genes  eye disorders
iii. MSX genes  abnormal head, face, and tooth development
g. Drosophila has 1 set of homoebox genes which are expressed 3’to 5’
h. The genes at the 3’ end are expressed before those at the 5’ end
i. Humans have Hoxa, b, c, d clusters which function as transcription
factors/regulators – they will turn on key genes at certain times in development
j. The arrangement of 5’ to 3’ tells the cells how to differentiate as the embryo
grows
k. Hox genes are highly conserved across species
i. They regulate organ pattern
ii. Pathological tissue development is linked to mutations/dysregulation of
hox gene
iii. They are morphoregulatory genes bc they set up chemical gradients
iv. In humans, there are two miRNA genes (can be regulatory genes) in HoxB
and HoxC
9. Cleidocranial dysplasia (CCD)
a. Results from a mis-expression of CBFA gene (a homolog of the drosophila gene
runt)
b. Characterized by the absence of clavicles
10. T-Box transcription factors
a. Family of about 20 genes that generate lineage diversity and help to form
developing embryo
i. T-box transcription factors function in bone morphogenetic protein
(BMP) and fibroblast growth factor (FGF) pathways
ii. If you lost the T-box TF, you get an oversized neural tube where the
somite don’t close around it
b. T is a novel sequence-specific DNA binding protein that binds to the T-box DNA
binding consensus sequence
c. T-box can be activators or repressors of TF
d. Have a large role in developing cardiovascular organs
e. First T-box gene was described by a Russian scientist
i. Mice with short tails (T) showed that genes were linked to gene activity
and cell behavior during embryogenesis
f. T-box has a certain consensus sequence in the DNA that can be bound by a
protein that affects development
g. T-box genes act as both regulators and repressors of transcription
h. Mutations to T-box genes cause dysmorphic syndromes
11. Mutation in T-box gene can result in DiGeorge Syndrome
a. Most common deletion syndrome
b. Tbx1 deletion  22q11 microdeletion
i. This gene deletion causes the loss of TBX1
c. Causes cardiac outflow tract and aortic arch anomalies, pharyngeal
malformations, behavioral problems
12. Key factors in normal morphogenesis
a. Cell growth rate – physical features can be explained by different cell growth
rates
i. Gives unique fingerprint – due to growth rate of finger tip cells
1. Whorl patterns = increased growth rate
2. Arch patterns = lower growth rate
b. Cell migration – cells migrate form their origin to their definitive body position
c. Cell-cell interactions – induction, adhesion, apoptosis
i. Interference with any of these can cause dysplasia
ii. Reactivation of any of these processes can cause cancer
d. Selected cell death – certain cells are pre-programmed for apoptosis
i. Failure of normal apoptosis in cells between fingers can cause syndactyly
(fusion of digits)
e. Growth factors/hormones – directly affect cell growth and migration
i. Paracrine: chemical affects nearby cells
1. Ex: FGF, WNT, hedgehog
ii. Autocrine: chemical affects the cell that secreted it
iii. Juxtacrine: chemicals act on receptors of adjacent cells
1. Can be cell-cell, cell-ECM, cell-cell via gap junction
13. Types of congenital anomalies
a. Malformation: tissue is malformed from the start
i. Primarily occurs in 1st trimester
ii. Can be caused by:
1. Agenesis: lack of development
2. Hypogenesis: underdevelopment
3. Heterotopia/ectopia: abnormal migration of cells/organs or
incomplete tissue closure/separation
b. Deformations: mechanical forces (from mother or fetus) applied to developing
structures
i. Maternal sources: small uterus, uterine malformation
ii. Fetal sources: multiple gestations, large fetus
c. Disruption: loss of cells or tissues
i. Ex: vascular accident, radiation, infections
d. Dysplasia: aberrant formations at levels of cells that are organizing into tissues
i. Occur later in development
ii. Can be a predisposition to cancer later in life
14. Dysmorphology: study of human malformations of prenatal origin
a. Associated with genetic signaling pathways (tissue differentiation,
organogenesis, morphogenesis)
b. Can occur singly or in combinations
15. Syndrome: a set of symptoms that consistently occur together
16. Single defect: malformation, deformation, disruption, or dysplasia
a. Etiology: chromosomal, single gene, or multifactorial
17. Multiple anomalies: syndrome, association, or sequence
a. Etiology: complex, epigenesis, or teratogens
18. Teratogens: substances encountered during pregnancy that affect fetus and can cause
birth defects
a. Commonly act in 3-8 weeks of embryonic life
b. Ex: alcohol (can cause fetal alcohol syndrome), drugs, hormones, cigarettes,
German measles, lead/mercury, radiation
c. Fetal susceptibility depends on gestational age
d. Factors that influence taratogenicity
i. Timing of exposure
1. Early 1st trimester – miscarriage
2. 1st trimester – malformations
3. 2nd/3rd trimester – brain malformations, fetal growth and
maturation
ii. Teratogen’s ability to cross placenta
1. Molecular size/weight – smaller molecules cross more easily
2. Highly charged molecules can’t cross as easily
3. Lipohilic molecules cross more easily
iii. Characteristics of exposure
1. Dosage
2. Duration and pattern of exposure
iv. Maternal/fetal genetic susceptibility
1. Alters DNA
2. Changes rate of cell growth
3. Changes apoptosis
e. Not susceptible to teratogens during first 2 weeks
f. Thalidomide: first use as a tranquillizer than as a cancer drug
i. Given to pregnant women to relieve them from morning sickness
ii. Caused newborns with short limbs, in utero death, blind/death
Lecture 28: Blastogenesis
1. Mitosis: in all cells except germ cells
a. Condensation, replication of DNA, chromatid separation, daughter cell formation
2. Meiosis
a. Synapsis: sister chromatids pair; crossing over occurs
b. 1st division: haploid, paired chromosomes
c. 2nd division: sister chromatids divide into gametes
d. F: 1 ovum
e. M: 4 spermatids
3. Oogenesis
a. Begins before birth
b. Primordial germ cells enter ovary
c. Follicular cells surround primordial germ cell at end of 3rd month
d. Oogonia divide by MITOSIS to increase in number – some enter meiosis and stop
in prophase I (primary oocytes)
e. 5th month: max number of germ cells
f. 7th: most germ cells have died and all remaining are primary oocytes
i. primordial follicles form
g. At birth: 600,000 to 800,000 oocytes
h. At puberty: 40,000
i. Less than 500 are ovulated
4. Female puberty
a. 15-20 primordial follicles finish meiosis I and begin meiosis II (due to LH
presence) but stop in metaphase unless fertilization occurs
b. Some will enter pre-antral stage (bc they have developed an antrum)
c. Antral follicles will swell to form Graffian follicle
d. Theca externa and interna form from follicular epithelial cells
e. Zona pellucida forms on surface of occyte – interior to theca interna
i. Some of the zona cells will form the cumulus oophorus (directly
surrounding the oocyte)
5. Ovulation
a. Hypothalamus release GnRH  anterior pituitary secretes FSH  pre-antral
follicles will survive and follicular cells around oocyte will mature
i. FSH causes follicle maturation
b. Theca interna produce androgens that are converted to estrogens by granulose
cells
i. Estrogen causes:
1. Proliferation of uterine endometrium
2. Thinning of cervical mucus
3. Production of LH by pituitary
c. LH surges in mid-cycle
i. Causes oocyte to complete meiosis I and enter meiosis II
ii. Stimulates production of progesterone by follicular cells
iii. Causes follicular rupture – ovulation
1. Ovary buldges at site of maturing follicle
2. Avascular stigma appears on ovary surface
3. Collagenase break down CT
4. LH cause increase in PG which will cause smooth muscle
contraction
5. Oocyte + cumulus oophorus are ovulated and will enter uterine
tube due to the sweeping motion of fimbriae
d. Remaining thecal and granulose cells will form corpus luteum
i. Secretes progesterone and estrogen – causes secretory stage
ii. Will degenerate (luteolysis) 9 days after ovulation if there’s no
fertilization and will leave a scar tissue called corpus albicans
1. Progesterone stops
2. Menstrual bleeding
iii. If there’s fertilization, then hCG will support corpus luteum
1. CL will continue to secrete progesterone until 4th month when
placenta takes over
2. In pregnancy, the CL is termied the corpus luteum graviditatis and
can take up 1/3 to ½ of ovary volume
6. Spermatogenesis
a. Primary germ cells are embedded in sex cords of testes
i. After puberty, sex cords form a lumen and will become seminiferous
tubule
b. 2 types of sex cords
i. Primordial germ cells – will form spermatogonial stem cells
ii. Sustentacular cells which will become sertoli cells
c. Type A spermatogonia undergo mitosis to form many clones
d. Type B spermatogonia form from last division of type A and will go on to form
primary spermatocyte
e. Primary spermatocyte undergo meiosis I to form secondary spermatocyte
i. Regulated by LH
ii. LH binds to Leydig cells  secrete testosterone  stimulate sertoli cells
(along with FSH) to form androgen receptors and testicular fluid
f. Spermatids mature to spermatozoa
i. Acromosome formation -- has enzyme needed to bind to ova
ii. Shedding of cytoplasm
iii. Neck, tail, midpiece formation
g. Type B spermatogonium  mature spermatozoa takes 74 days
h. 300 million sperm made daily
i. Sperm get their full motility in epididymis
7. Fertilization
a. Occurs in ampulla of uterine tube
b. Spermatozoa are viable in F for 6-7 days
c. Only 1% of sperm enter cervix (takes 30 mins to 6 days)
d. Sperm movement occurs via muscular contractions of uterus – sperm are motile
again at ovulation
e. Sperm must be capacitated before they can fertilize (takes up to 7 hours)
f. Sperm pass through corona radiata and will penetrate zona pellucida
i. Acrosome reaction: enzymes allow penetration of zona
ii. Lysosomal enzymes are released to prevent other sperm from fusing
g. Oocyte and sperm fusion
i. ECM molecules will link the 2 cell membranes
ii. Plasma membrane of sperm won’t enter ovum
iii. Oocyte finishes meiosis II to form F pronucleus – it is now metabolically
active
iv. M pronucleus (determines sex) will fuse with F = diploid
h. Cleavage begins and zygote is seen at 2 cell stage (blastomeres)
i. Compaction at 8 cell stage
j. Morula (16 cells) seen 4 days after fertilization
i. Inner cell mass forms embryo
ii. Outer cell mass forms trophoblast/placenta
8. Morula will enter uterine cavity
a. Zona pellucida disappears to allow for implantation (107 cell stage)
Lecture 29: Implantation
1. Steps leading up to implantation
a. FSH stimulates follicular development
b. Estrogen stimulates endometrial growth
c. Spermatozoa continuously develop
d. Ovulation occurs – ovum drawn into ampulla
e. Sperm enter vagina and a few will enter cervix
f. Even less sperm will enter uterine tube
g. Capacitation occurs in female tract – final activation of the sperm
h. Sperm cross corona radiata and penetrate zona pellucida
i. One sperm fuses with oocyte – F and M pronuclei fuse
i. DNA duplication
ii. First cell division occurs
j. Cell divides to 16 cell stage –morula (4th day after fertilization)
k. 2 cell layers are formed
i. Embryoblast
ii. Trophoblast
l. Fertilized ovum travels down uterine tube and implants in uterine cavity on 6 th
day
m. Trophoblast will divide into cytotrophoblast and syncytiotrophoblast
i. Lacunae will form in syncytium
n. Embryoblast will form two layers
i. Hypoblast
2.
3.
4.
5.
6.
ii. Epiblast
o. Amniotic cavity will form
p. Exocoelomic cavity forms
i. Lined by hypoblast cells
ii. Forms primitive yolk sac
q. Maternal capillariers enlarge to form sinusoids
r. Syncytiotrophoblast continues to penetrate endometrium
s. Lacunae will fuse to form vessel network
t. Syncytium erods maternal vessels in order to fill the lucanae with blood
u. Extraembryonic mesoderm forms and will split into somatic and splanchnic
layers
v. Chorionic cavity enlarges and definitive yolk sac forms
w. Bilaminar embryo is suspended in chorionic cavity by the connecting stalk (which
will become the umbilical cord)
x. Secondary or definitive yolk sac forms as hypoblast cells line extraembryonic
splanchnic mesoderm
y. Lacunae exted around entire embryo
Retention of uterine lining depends on implantation
a. Without implantation, lining is shed even if there is fertilization
At beginning of menstrual cycle
a. FSH stimulates follicle maturaration in 15-20
b. Estrogen stimulates proliferation of uterine lining
Uttering lining phases – every 28 days
a. Proliferation phase
i. Basal cells divide to form spongy and compact layers
ii. Capillaries grow into uterine lining from the basal layer
iii. Basal layer is never shed
b. Secretory phase
i. Occurs 2-3 days after ovulation
ii. Due to pogesterone from corpus luteum
iii. Mucosa becomes secretory
iv. Large arteries form in uterine lining
v. Ready for implantation
c. Menstrual phase
i. Failure to implant fertilized ovum
ii. Breakdown of spongy and compact layers
iii. Uterine lining is shed bc there is not enough progesterone to stimulate its
maintenance
Implantation
a. Syncytiotrophoblast of zygote produces hCG which sustains corpus luteum
b. No menstruation occurs
Day 6
a. Trophoblast cells of outer cell mass will begin to implant into the uterine
epithelium
7. Day 8
a. Partially embedded in uterine wall – some bleeding may occur
b. Trophoblast forms two layers
i. Cytotrophoblast –inner layer
ii. Syncytiotrophoblast – outer layer
1. Forms one giant cell with many nuclei
2. Only has an out membrane
c. Inncer cell mass (embryoblast) divides into:
i. Hypoblast – forms extraembryonic membranes
ii. Epiblast
d. Amniotic cavity forms – surrounded by amnioblast cells
e. Uterine glands secrete mucus and glycogen
f.
8. Day 9
a. Blastocyste almost completely embedded
b. Fibrin plug closes defect
c. Trophoblast continues to develop
i. Vacuolizes
ii. Forms lacunae
d. Hypoblast cells inside ctyotrophoblast form exocoemic cavity (primitive yolk sac)
e. Maternal capillaries enlarge and start feeding embryo through placenta
9. Day 11/12
a. Blastocyst completely embedded in uterine endometrium
b. Lacunae in syncytium connect – vessel network
c. Syncytiotrophoblast cells penetrate deeper into endometrium
i. Erode endothelium of capillary sinusoids
ii. Blood fills lacunar network
d. Yolk sac cells between cytotrophoblast and exocoelomic cavity proliferate to
form loose network called extraembryonic mesoderm
i. Splanchnic mesoderm –viserca (inner parts)
ii. Somatic mesoderm – bodoy wall (external)
e. Vaculous in the extraembryonic mesoderm form chorionic cavity
f. The definitive yolk sac is formed by extraembryonic splanchnic mesoderm
g. Growth of bilaminar embryo is slower than trophoblast
h. Zygote is 0.2 mm
10. Day 13
a. Some bleeding may occur due to lacunae filling with blood
b. Cytotrophoblast proliferates and will form columnar structures that extend into
the syncytium (primary villi—allows for gas and nutrient exchange)
c. Hypoblast cells proliferate and will line the extracoelomic cavity to form
secondary or definitive yolk sac
d. Chorionic cavity expands
i. Embryo stays attached to endometrium by a connecting stalk (small
group of extraembryonic somatic mesoderm cells)
ii. Connecting stalk will become the umbilical cord when blood vessels grow
into it
e. The chorionic plate isolates the bilaminar embryo from surrounding tissue
f. Lacunae have formed and are filled with maternal blood – surround all of the
embryonic tissues
Lectures 30, 31 & 32: Gastrulation
1. Pre-embryo (0-2 weeks)
a. Increase in cell number
b. Fertilization and zygote moves down uterine tube
c. Zygote implants on 6th day
2. Embryo (3-8 weeks)
a. Morphological development
b. Most important point of development
c. In many cases, the female doesn’t know she’s pregnant during this time
3. Fetus (9 weeks-birth)
a. Functional development
4. Susceptibility to teratogens (anything that interferes with normal development)
a. Teratogen = any agent (virus, drug, radiation) that causes malformations
b. Teratogenesis = congenital malformation of an anatomical structure
i. Malformations occur prenatally and could be due to genetic or
environmental factors
c. During the first two weeks (pre-embryo), there are very few things that can
interfere with development bc it takes so long for implantation
d. If something does interfere with development during the first two weeks, it is
usually terminal
i. German measles in mother causes pre-embryo to abort
e. During embryo period, gastrulation is followed by organogenesis (organ
formation)
i. Each organ has a peak sensitivity
ii. Heart: one of the first organs formed around day 20/21
1. Very sensitive to teratogens
2. Its peak sensitivity is from 4th-6th week
3. After the 6th week, it is still sensitive to teratogens but not as
much
iii. Peak sensitivity for most organs is found in the embryo period
f. Thalamide was given for morning sickness
i. The administration of this drug was given during peak limb development
– so many babies were born with no limbs
5. At the end of the second week, there is a bilaminar disc (two layers of cells)
a. The upper cell layer (epiblast—next to amniotic cavity) secretes the protein
nodal (causes formation and maintenance of primitive streak
b. Hypoblast (next to blastocoel)
6. Primitive steak begins to form at the end of 2nd week
a. Nodal = protein secreted by cells in the area of the future primitive streak to
begin its formation
b. Cells of primitive node secrete nodal in order to maintain primitive streak
c. Nodal contributes to signaling pathways that promote genes needed for
proliferation and differentiation
d. Primitive pit: depression
e. Primitive node: raised area around the primitive pit
f. Over-expression of nodal causes duplication of structures – two headed people,
one head and two bodies, etc.
7. By the end of the 3rd week, there is a trilaminar disc
8. Sacrococcygeal teratoma (“monster growth”)
a. Form from cellular remnants of primitive streak –lots of nodal secretion causes
uncontrolled division
b. Most common congenital tumor in newborns
c. Tumors are detected via ultrasound and require C-section
9. Totipotent cells can differentiate into any cell type based on their environment
10. Gastrulation: process where the epiblast cells (of bilaminar disc) migrate to and then
through the primitive streak to form 3 germ layers (ectoderm, mesoderm, endoderm)
a. These epiblast cells will migrate and push the hypoblast cells away
b. The epiblast cells at the top are now called ectoderm
c. The epiblast cells that migrated into the middle are called mesoderm
d. The epiblast cells that migrated and replaced the hypoblast cells are called
endoderm
e. This process will form the notochord
f. Mesenchymal cell: one that is not connected to other cells
i. It is free and can migrate
ii. Any migratory, undifferentiated cell
iii. If a cell loses its adhesion proteins, the cell can change shape, be
extruded and migrate (mesenchymal)
g. Initiated at beginning of week 3
h. Bilaminar disc seen at week 2
i. Trilaminar disc seen at week 3
11. Epiblast gives rise to all 3 germ layers
a. As cells migrate to the primitive streak, they have signals that tell them what to
become
b. They migrate in, push the hypoblast away and form the endoderm
12. Fate maps: technique used to determine embryonic origin (cell lineage) of adult tissues
a. A dye/tracer is injected into a region of the embryo
b. The labeled cells are followed to maturation
c. As the cells continue to divide, the dyes are diluted overtime
13. Oropharyngeal and cloacal membranes –no mesoderm is here
a. Form immediately after gastrulation
b. On either ends of the trilaminar layers, the ectoderm on the top will press up
against the endoderm on the bottom (mesoderm is excluded)
i. This will set the beginning and end of the GI tract
c. Small regions of fused ectoderm and endoderm
d. Oropharyngeal/buccopharyngeal membrane is positioned vertically at the
location of the future anterior 2/3 and posterior 1/3 tongue
i. Separates oral/nasal cavities from pharynx
ii. Typically degenerates during 4th week
iii. Sometimes it may persist—but it’s rate
e. Cloacal membrane is positioned over the cloaca (future anus, genital, and UT)
i. Degenerates in 7th week
14. Notochord formation
a. Notochord is derived from mesoderm cells that go into the primitive node, then
turn around and go anteriorly up the midline of primitive streak
i. Pre-notochordal cells pass through primitive node and migrate cranially
toward prechordal plate (cell population that signals migration of the
pre-notochordal cells to stop)
b. Forms along midline
c. Important in induction of CNS
d. It will eventually form the nucleus pulposus (central portion of intervertebral
discs)
e. Remnants of notochordal cells may develop into a chordoma (rare type of
malignant tumor)
15. Embryonic folding
a. Before folding, there are just 3 layers of cells stacked on top of each other, in an
elongated shape
b. Folding provides basic body form (cylinder)
i. Lateral folding
1. The sides of the trilaminar disc are bent down
2. The ectoderm forms lateral folds on either side that will
eventually meet
3. The ectoderm is now the outside
4. The bottom layer (endoderm) comes together to form internal
endodermal tube
5. Epidermis is from ectoderm
6. GI is from endoderm
7. Bones, blood, muscle are from mesoderm
8. Each germ layer meets its germ layer from the other side
ii. Cranial-caudal folding
1. The brain will grow so quickly that is causes the whole embryo to
bend/turn down
2. The tail will also bend
3. The two folds toward each other but do not meet
c. Folding occurs mainly in 4th week
16. Differentiated adult tissues (group of similar or identical cells)
a. Epithelium: lines body cavities (lumens) and body surface
i. Comprises glands and organs
ii. Derived from all 3 germ cells
iii. Ex: GI tract, pericardial cavity
b. Connective tissue: comprises skeletal system, blood, fat
i. Derived mostly from mesoderm
ii. Ex: ligaments, tendons, bone cartilage
c. Muscle: skeletal, smooth, cardiac
i. Derived mostly from mesoderm
d. Nervous: neurons and glia (neuronal support cell)
i. Derived mostly from ectoderm
e. Organs: comprised of 2 or more tissues (thus from cells derived from 2 or more
germ layers)
i. Organs develop through process called organogenesis
ii. Organogenesis occurs during weeks 4-8
iii. Any given organ can be composed of smooth muscle, loose connective
tissue, nervous, columnar epithelium
iv. Skin is considered an organ bc it’s composed to epithelium, dermis,
connective tissue
v. Although organs are made of more than 1 germ layer, there is one main
functional cell within each organ that is derived from 1 germ layer
1. Ex: stomach functional cells are the epithelium (line the lumen)
bc they secrete necessary enzymes
2. Ex: hepatocyes (functional cell in liver) are derived from
endoderm
f. Mesoderm is broken into different groups
i. Axial mesoderm: the notochord; found on the midline and runs lengthwise
ii. Paraxial mesoderm: on either side of the neural tube
iii. Intermediate mesoderm: just below paraxial mesoderm
iv. Lateral plate mesoderm
1. Splanchninc (organ/viscera) mesoderm: associated with GI tube
2. Somatic (body surface) mesoderm: next to the ectoderm
g. Initially the ectoderm is only on the surface, but overtime it will be moved
around
i. This allows brain and spinal cord to be made of ectoderm
17. Ectodermal derivatives
a. Surface ectoderm will form
i. Epidermis: very thin layer that sits on top of dermis
1. Many cells thick
2. The base contains living, dividing cells
3. The topmost layers are dead cells
ii. Epidermal derivatives
1. Hair
2. Nails
3. Sebaceous (oil) glands – maintain the dead cells
4. Sudoriferous (sweat) glands
5. Arrector pili muscles – cause hair to stand-up; under autonomic
control
6. Mammary glands – modified sweat glands
b. Neural ectoderm will form
i. Neural tube
1. Brain
2. Spinal cord
c. Neural crest ectoderm will form
i. Sensory neurons of PNS
ii. Pia mater and arachnoid mater (laptomeninges) – meninges
iii. Medulla of adrenal gland
iv. Membranous interventrivular septum of heart
v. Dermis and hypodermis of head
vi. Bones of head
vii. Odontoblasts
viii. Melanocytes
ix. Defect in neural crest cells may cause oddly associated issues (heart and
tooth issues as an example)
18. Epidermis  dermis  hypodermis  bone  cavity (depending on location)
19. Neuralation
a. Brain and spinal cord form via neurulation
b. Notochord secretes inductive proteins that diffuse up and stimulate overlying
ectoderm (neuroectoderm) to invaginate
c. The neuroectoderm is now called the neural plate (flat region) and the space
above neural plate is the neural groove (channel)
d. Invagination occurs until lateral margins (called neural folds) meet
e. Non-neural (surface ectoderm) fuses to give rise to epidermis of back
f. Neural fold cells detach just deep to the surface ectoderm and are called neural
crest cells
i. These neural crest cells are a type of mesenchymal cell that can migrate
throughout the body and form all of the neural crest derivatives
g. The neural plate fuses to form the neural tube, which is precursor to brain and
spinal cord
h. The neural folds initially meet in cervical region at day 22
i. Further fusion occurs bi-directionally (cranially and caudally simultaneously)
j. The anterior (cranial) and posterior (caudal) neuropores are the openings that
remain on either end of the neural tube
i. Anterior/cranial neuropore closes at day 24
ii. Posterior/caudal neuropore closes at day 26
iii. Neural tube defects occurs when neuropores fail to close
1. Anencephaly: failure of the anterior neuropore to close
a. “No brain”
b. Brainstem normally persists allowing the infant to carry
out basic functions for a limited time
c. Maternal intake of folate reduces likelihood
d. Incidence is 1/1000
2. Spina bifida: failure of posterior neuropore to close
a. Skin won’t be able to form over
b. Rachischisis: the worst kind of spina bifida
i. “Split spine”
ii. Vertebrae will still form anterior to the spinal cord
iii. Vertebrae below will have 2 spinous processes of
an incompletely formed vertebrae – bc they
weren’t able to come back around and fuse into
one
c. Frequently seen along with anecephaly
d. Infant is paralyzed completely form the point of deficit
caudally
20. Endodermal derivatives
a. Endodermal tube forms epithelial lining of the GI tract
i. Initially, this is just a single layer of endodermal cells that line the lumen
– these are the functional cells
ii. Eventually mesoderm cells will be added to the GI tube
iii. The GI tract is connected to the yolk sac
b. Endodermal bud (endodermal cells that bud off of the GI tract) will from
i. Thyroid gland -- buds completely off of the GI tract and moves caudally
ii. Parathyroid glands -- buds completely off of the GI tract
iii. Epithelial linings of respiratory system (lungs), urinary bladder, and
urethra
iv. Accessory digestive structures (must remain connected to GI tract bc
they make enzymes needed for digestion – remain attached via ducts)
1. Liver
2. Gall bladder
3. Pancreas
21. Mesodermal derivatives
a. Mesoderm is characterized based on its location within the embryo
i. Axial mesoderm (right on the membrane and is the notochord)
1. Located on midline
2. Form the notochord which secretes molecules to induce surface
ectoderm cells to undergo neurlulation (notochord drives
neurulation)
3. After neurulation occurs, sections of the notochord will die and
the remaining parts will differentiated into nucleus pulposus
(inner region of the intervertebral disc)
4. If someone has a herniated disc, it is the nucleus pulposus that is
coming out posteriorly which will pinch the spinal nerves
ii. Paraxial mesoderm
1. Condenses and forms bilateral, segmented populations of cells
called somites (they are called somitomeres in the head region)
2. Originally, 42-44 pairs develop craniocaudally – first somites form
closest to the head
a. Some will degenerate
b. Only 36 functional pairs remain
3. The somites will form a tail that is supposed to die off
4. Variable degrees of generation result in coccygeal vertebrae
number (3-5) – this range depends on how much of the somites
of the tail will degenerate
5. Somatic cells will differentiate into
a. Sclerotome (“hard”): gives rise to axial skeleton (base of
cranium, vertebrae, outer part of intervertebral discs, and
ribs)
i. Sclerotome is positioned medially – inner layer
b. Dermatome: gives rise to dermis on dorsal and lateral
body
i. Dermatome is the outer layer
c. Myotome: gives rise to skeletal muscle
i. Myotome is the middle layer between sclerotome
and dermatome
iii. Intermediate mesoderm
1. Forms urinary system (kidneys and ureters)
2. Failure of these cells to proliferate/differentiate may causes
kidney problems or caudal dysgenesis
3. Sirenomelia: mermaid syndrome
a. AKA caudal dysgenesis
b. Due to lack of proliferation of the intermediate mesoderm
iv. Lateral plate –somatic and splanchnic
1. Somatic (superficial to embryonic cavity)
a. Forms dermis of ventral body wall, hypodermis, and all
skeletal components (bones, ligaments, tendons)
2. Splanchnic (deep to embryonic cavity)
a. Forms smooth muscle and CT of endodermally-derived
organs
b. Forms most of the heart
3. Blood vessels and heart (pericardium), lungs (pleura), and
abdominal organs (peritoneum) have associated serous
membranes that are formed by both somatic (not in contact with
organ) and splanchnic (in contact with the organ) lateral plate
mesoderm
4. Large arteries that feed organs are made of splanchnic mesoderm
5. Arteries that feed limbs are from somatic lateral plate
22. Formation of body cavities (coeloms)
a. Extraembryonic coelom (chorionic cavity) are incorporated into the ventral
aspect of the embryo and now becomes the intraembryonic coelom due to
lateral embryonic folding
b. 3 central cavities will form
i. Pericardial -- in thorax
ii. Pleural – in thorad
iii. Peritoneal – in abdomen
c. Initial step in formation of 3 cavities is the movement of septurm transversum
(which will form diaphgram) will separate thoracic and abdominal cavities
23. Diaphragm formation
a. First step begins during cranial folding – results in relocation of the septum
transversum (ST) caudally
b. ST will contribute to division of the single intraembryonic cavity into two cavities
– thoracic and peritoneal
c. The ST doesn’t completely separate the 2 spaces – there are 2 channels dorsal
to the ST that allow the cavities to remain continuous for now =
pericardioperitoneal canals (PC)
d. Pleuroperitoneal membranes begin to grow over the PC
i. Myoblasts (embryonic muscle cells from somites) from cervical somites
3-5 (moved caudally with the ST) will grow over the pleuroperitoneal
membranes
ii. The ST becomes the central tendon and the myoblasts become the
muscular portion
e. Failure to close the PC results in diaphragmatic hernia
i. Abdominal organs (stomach, SI, spleen, liver, kidney) can move cranially
into thorax
ii. Lung tissue on affected side won’t fully develop
iii. Most affect the left side
iv. Seen in 1 of 2,200-5,000 births
24. External morphology of embryo
a. 4th week
i. Pharyngeal arches form craniocaudally
ii. Pharyngeal clefts form craniocaudally
iii. Eye, ear, and nasal primordial begin to form placodes
iv. Cardiac prominence
v. Limb buds emerge (upper begin to firm first)
th
b. 5 week
i. Hepatic prominence
ii. All pharyngeal arches and clefts formed
th
c. 6 week
i. “Paddle-shaped” hands have formed
ii. Facial tissue is beginning to converge
th
d. 7 week
i. Eyelids
ii. Philtrium—ridge formed where the lips meet
1. Not formed in fetal alcohol syndrome babies
iii. Digits begin to form
e. 8th week
i. External genitalia becomes recognizable
ii. Digits completely formed
1. If an image has all of its digits, then the pre-embryo Is now
considered an embryo
25. Limb rotation – begins around day 50 to day 56
a. Upper and lower limbe rotation occurs in opposite directions – opposition
position of elbows and knees
i. Elbow joints project posteriorly
ii. Knee joints project anteriorly
b. Direction/extent of limb rotation generates sensory patterns (dermatomes)
observed on body surface
i. Dermatome is a segment of the body surface that is supplied by a
specific spinal nerve
26. Digit issues
a. Syndactyly: incomplete cell death; fused digits
b. Polydactyly: completely development of extra digits