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Biology 233
Human Anatomy and Physiology
Chapter 29 Lecture Outline
DEVELOPMENT AND INHERITANCE
PRENATAL DEVELOPMENT
fertilization till birth
9 months (38 weeks)
3 trimesters – 3 month periods
first trimester – most vulnerable
differentiation of zygote to establish all organ systems
teratogen – environmental agent that causes defect in embryo
second trimester – continued differentiation and growth
third trimester – mainly growth, most organs become fully functional
Ovum
ovulation – secondary oocyte arrested in metaphase II
zona pellucida – inner glycoprotein shell
corona radiata – outer shell of granulosa cells
large cell containing organelles and nutrients to support developing zygote
until implantation (about 1 week)
viable for 24 hours after ovulation
Spermatozoan
ejaculation of semen into vagina
sperm and secretions that protect and nourish it
capacitation – activation of sperm by male and female secretions
motility – flagellum begins propellor-like movement
secretions thin cell membrane of acrosome
sperm propelled by flagella through cervix, uterus, to uterine tube
secretions and nerve impulses stimulate vaginal and uterine contractions
aid in moving sperm into uterus
viable for 48 hours
FERTILIZATION – usually occurs in uterine tube
fertilization window – 2 days before till 1 day after ovulation
sperm penetrate corona radiata by releasing acrosomal enzymes
requires multiple sperm to clear a path
receptors in zona pellucida bind heads of sperm with intact acrosomes
triggers release of acrosomal enzymes
enzymes dissolve through zona pellucida
oocyte activation – first sperm to contact oocyte cell membrane fuses with it
depolarization of oocyte membrane prevents fusion with other sperm
inactivates sperm receptors and hardens zona pellucida
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triggers completion of meiosis II in secondary oocyte
ovum and second polar body formed
nuclei of ovum and sperm differentiate
male pronucleus – haploid
female pronucleus – haploid
pronuclei fuse – diploid zygote formed
EMBRYONIC DEVELOPMENT – first 2 months
zygote divides and differentiates to establish all organ systems
CLEAVAGE – mitotic divisions of zygote form a blastocyst
blastomeres – cells formed by cleavage of zygote
morula – ball of cells formed by day 4
monozygotic (identical) twins – cells separate into 2 separate embryos
[dizygotic (fraternal) twins – develop from 2 different zygotes]
blastocyst – fluid-filled ball of cells formed by day 5
morula enters uterus
nutrient-rich uterine secretions collect between blastomeres
blastocoel – fluid-filled cavity in center
trophoblast – blastomeres forming wall of blastocyst
develops into fetal placenta
inner cell mass – cluster of blastomeres in one wall
develops into embryo
zona pellucida is digested by trophoblast and shed
IMPLANTATION – blastocyst attaches to uterine endometrium in fundus or body
inner cell mass orients toward endometrium
ectopic pregnancy – abnormal site of implantation
uterine tube – may rupture and cause death
Differentiation of Trophoblast into 2 Layers
1) syncytial trophoblast – outer layer of rapidly growing, fusing cells
secretes enzymes – digest endometrium
embryo burrows into endometrium (day 8-9)
secretes human chorionic gonadotropin (hCG)
maintains corpus luteum
2) cellular trophoblast – distinct inner layer
lacunae – spaces develop between syncytial trophoblast cells
uterine secretions and blood fill lacunae
diffusion of nutrients and wastes occurs between lacunae and
developing embryo
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DEVELOPMENT OF EMBRYO INTO BILAMINAR EMBRYONIC DISC
inner cell mass differentiates into 2 layers
primitive ectoderm – dorsal layer of embryo
primitive endoderm – ventral layer of embryo
DEVELOPMENT OF EXTRAEMBRYONIC MEMBRANES
AMNION – primitive ectoderm splits
outer layer of ectoderm becomes amnion
amnionic cavity – fluid-filled space between amnion and embryo
amniotic fluid – amniocentesis
protects fetus – trauma, temperature, dessication, adhesion
YOLK SAC – endoderm cells migrate to line inner blastcoel
contains nutrients secreted by uterus
helps nourish developing embryo until placenta forms
site of hemopoiesis (3-6 weeks)
site of primordial germ cell formation
migrate to gonads
differentiate into spermatogonia and oogonia
ALLANTOIS – outpouching of yolk sac that forms later
develops into lining of urinary bladder
extraembryonic mesoderm – yolk sac & allantois produce mesoderm cells
connective tissue that surrounds yolk sac and amnion
extraembryonic mesoderm migrates to line surface of cellular trophoblast
CHORION – extraembryonic mesoderm + trophoblast
surrounds extraembryonic coelom (cavity around amnion & yolk sac)
connecting stalk – embryo connected to chorion by a band of mesoderm
cells; develops into umbilical cord
Functions of Chorion:
protects embryo from maternal immune system
produces hCG – maintains corpus luteum
develops into fetal placenta – site of exchange with maternal blood
GASTRULATION (15 days)
migration and differentiation of primitive ectoderm of embryonic disc
results in formation of a trilaminar (3 layer) embryonic disc
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Development of 3 Primary Germ Layers:
ectoderm – dorsal epithelial layer
mesoderm – middle loose connective tissue layer
endoderm – ventral epithelial layer
primitive streak – groove in dorsal ectoderm
begins at caudal embryo and extends towards head
invagination – primitive ectoderm cells multiply and migrate through primitive
streak to form a layer between ectoderm and endoderm (mesoderm)
FATES OF PRIMARY GERM LAYERS
MESODERM
notocord – central column of mesoderm cephalad to primitive streak
induction – one tissue stimulates differentiation of adjacent tissue
notocord induces adjacent mesoderm to form vertebra
(notocord also induces formation of CNS)
notocord remnants form nucleus pulposis of intervertebral discs
somites – paired segments of mesoderm lateral to notocord
divisions of somites:
myotomes – develop into skeletal muscles
sclerotomes – develop into vertebrae
intermediate mesoderm – longitudinal cylinders of mesoderm lateral to somites
develops into kidneys, adrenal cortex, and reproductive organs
lateral plate mesoderm – flat sheets of mesoderm lateral to intermediate
mesoderm which split into 2 layers
visceral layer
forms visceral serous membranes
forms walls of cardivascular, respiratory, and digestive systems
parietal layer
forms parietal serous membranes
forms connective tissues of limbs and body wall
space between visceral and parietal layers becomes ventral body cavity
ECTODERM
notocord induces overlying ectoderm to differentiate into neural plate
neural plate grows, folds and fuses
neural folds
neural groove
neural tube – develops into CNS structures
neural crest – develops into PNS structures
superficial ectoderm develops into epidermis of skin
ENDODERM
primitive gut – tube of endoderm pinched off within ventral body cavity
develops into inner lining of digestive and respiratory organs
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DEVELOPMENT OF PLACENTA
required to supply nutrients and oxygen to growing embryo
Chorionic Villi (Fetal Component) – projections of chorion
Angiogenesis – formation of blood vessels and blood from mesoderm
first occurs in yolk sac, connecting stalk, and chorion
angioblasts – differentiated mesoderm cells
blood islands – clusters of angioblasts
spaces appear in islands – form lumen of vessel
peripheral cells become walls of blood vessels
central cells become blood cells
heart develops from visceral layer of lateral plate mesoderm
begins beating at end of third week – first functional organ
system; circulates nutrients to growing embryo
Umbilical Cord
angiogenesis in connecting stalk forms;
umbilical arteries – carry deoxygenated blood from embryo
umbilical vein – carries oxygenated blood to embryo
link vessels in chorion to vessels forming in embryo
Wharton’s jelly – mucous connective tissue fills cord
Maternal Component of Placenta – from functional zone of endometrium
implantation stimulates increased vascularity and glandular development
3 regions of decidua formed (will be shed with fetus at birth)
decidua basalis – between embryo and basal zone
forms maternal placenta – secretes nutrients
decidua capsularis – between embryo and uterine cavity; around amnion
decidua parietalis – lines remaining uterus
blood sinusoids – endometrial capillaries expand and are eroded by
embryonic enzymes
maternal blood and secretions fill lacunae around chorionic villi
maternal and fetal blood doesn’t mix – diffusion occurs across the chorion
diffusion of oxygen and nutrients to fetal blood
diffusion of wastes and carbon dioxide to maternal blood
Placental Hormones
hCG – maintains corpus luteum
estrogen and progesterone – support endometrium
corpus luteum no longer essential after 3-4 months
relaxin – relaxes pubic symphysis and cervix; inhibits oxytocin (prevents labor)
placental lactogen & placental prolactin – help prepare mammary glands
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EMBRYOGENESIS
embryonic folding – embryo goes from trilaminar disc to tubular shape
3 tubes:
neural tube – ectoderm (also forms epidermis of skin)
coelom (body cavity) – mesoderm
primitive gut – endoderm
rapid growth of neural tube
embryo forms C-shaped longitudinal curve
amnion grows over entire embryo and umbilical cord
head and tail folds
ectoderm and endoderm meet
oropharyngeal membrane – ruptures to form mouth
cloacal membrane – ruptures to form anal and urogenital openings
Pharyngeal Development
pharyngeal arches – 4 paired outgrowths of ectoderm and mesoderm
pharyngeal clefts – grooves between arches
pharyngeal pouches – endoderm pockets within arches
develop into structures of head and neck
mandible, hyoid, larynx, thyroid, parathyroid, middle & external ear
Ear
otic placode – thickened ectoderm
forms membranous labyrinth of inner ear
Eye
lens placode – thickened ectoderm
forms lens and cornea
optic vesicle – outpocket of neural tube (ectoderm)
forms retina and vascular tunic of eye
Limb Buds – outgrowths of mesoderm covered by ectoderm
develop into upper and lower limbs
somite myotomes form muscles
lateral plate mesoderm forms connective tissues, including bones
Respiratory System
Lung Buds – outpockets of pharyngeal endoderm
form tracheal, bronchial and lung inner epithelium
surrounding mesoderm forms muscle, blood vessels, and CT
Digestive System
primitive gut (endoderm) forms inner epithelium of all digestive organs
surrounding mesoderm forms muscle, blood vessels, and CT
Urinary System
kidneys and ureters form from intermediate mesoderm
allantois – outpocket of yolk sac within umbilical cord
develops into epithelium of urinary bladder and urethra
surrounding mesoderm forms muscle and CT
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Reproductive System
gonads and tubular structures develop from intermediate mesoderm
primordial germ cells develop in yolk sac and migrate to gonads
FETAL DEVELOPMENT
9 weeks till birth
growth (1 gram – 7 pounds)
differentiation – most organ systems become fully functional
PREGNANCY
EFFECTS OF PREGNANCY ON MOTHER
cardiovascular
increased blood volume (50%)
lying on back may compress descending aorta
fetus may compress inferior vena cava – edema in feet and legs
respiratory
increased tidal volume
increased respiratory rate
dypnea in late pregnancy
vital capacity decreases – compression of thoracic cavity
digestive
increased appetite – 10-30% increase in nutrient demands
nausea, gastroesophageal reflux
urinary
increased GFR and compression of bladder
increased frequency of urination
stress incontinence
mammary glands
lactiferous glands grow and produce secretions
weight gain – uterus, fetus, blood, breasts
LABOR & PARTURITION – Hormones of Pregnancy
endometrium is supported by progesterone (hormone of pregnancy)
early pregnancy – corpus luteum produces
supported by hCG from chorion of embryo
level of hCG decreases as pregnancy progresses
later in pregnancy – placenta produces progesterone and estrogen
both help prepare mammary glands for lactation
high progesterone inhibits uterine contractions and lactation
level of estrogen increases throughout pregnancy
late pregnancy – high estrogen
inhibits progesterone – uterine muscle tone increases
increases sensitivity of myometrium to oxytocin
stimulates oxytocin secretion by fetus – increases uterine tone
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weight of fetus and increased uterine tone stretches cervix
stimulates oxytocin secretion – increases uterine tone
placental hormones (stimulated by estrogen and oxytocin)
prostaglandins – increase uterine tone
relaxin – relaxes cervix
fetus descends into cervix = stretches cervix = more oxytocin secretion
positive feedback
more oxytocin = more contraction = more stretch = more oxytocin
contraction waves in uterus (like peristalsis) expel fetus
STAGES OF LABOR
1) dilation stage (takes 8+ hours)
regular contractions of uterus, increasing in strength and frequency
dilate cervix (10cm)
usually ruptures amnion – “water broke”
2) expulsion stage (takes < 2 hours)
powerful contractions expel fetus (delivery or parturition)
compression of umbilical cord may cause hypoxia
stress to infant causes sympathetic response
prepares infant for extrauterine life
3) placental stage (takes < 1 hour)
powerful contractions expel placenta (afterbirth)
maternal decidua + fetal chorion
involution – uterus returns to original size (takes 6 weeks)
lochia – discharge during involution
dystocia – difficult labor
breech – fetus presents legs or buttocks into cervix
INHERITANCE – passage of hereditary traits to offspring
genetics – science of inheritance
GENOTYPE – genetic makeup of an individual
gene – part of chromosome that codes for specific structural, functional or
regulatory protein
trait – a genetically regulated characteristic (eg. hair color)
locus – site on a chromosome where a particular gene is located
homologous chromosomes – similar chromosomes from mother and father
(form tetrads during meiosis I)
have genes controlling the same traits at the same loci
individual has 2 of each gene (one from mother, one from father)
alleles – alternative forms of a given gene
(eg. coarse hair or fine hair)
may be 2 or more different alleles of a gene
homozygous – individual has 2 identical alleles of a gene
heterozygous – individual has 2 different alleles of a gene
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PHENOTYPE – anatomical or physiological expression of a genotype
Inheritance Patterns:
dominant / recessive inheritance
dominant allele – dominates when present; masks presence of other allele
expressed as a capital letter
recessive allele – masked by a dominant allele; only seen in phenotype if
individual is homozygous recessive
expressed as a lower-case letter
incomplete dominance – heterozygous genotype is expressed as a unique
phenotype (something new)
codominance – heterozygous genotype expresses both dominant and recessive
alleles in phenotype
Punnett Square
used to determine probability of genotypes and phenotypes occurring in offspring
EXAMPLE OF DOMINANT / RECESSIVE INHERITANCE
PKU (phenylketonuria) – disease resulting from inability to metabolize
phenylalanine due to lack of the enzyme phenylalanine hydroxylase (PH)
allele P – dominant allele; gene for normal PH enzyme
allele p – recessive allele; gene for nonfunctional PH
possible genotypes
PP (homozygous dominant)
Pp (heterozygous)
pp (homozygous recessive)
possible phenotypes
normal (has PP or Pp genotype) – can metabolize phenylalanine
PKU (has pp phenotype) – cannot metabolize phenylalanine
carrier – heterozygous individual
doesn’t express recessive gene in phenotype, but can pass it to offspring
2 carrier parents can be healthy but produce children with PKU
EXAMPLE OF INCOMPLETE DOMINANCE
sickle-cell disease – disease resulting from production of abnormal hemoglobin
which forms sickle-shaped RBCs when deoxyhemoglobin is formed
allele HbA – normal hemoglobin gene
allele HbS – sickle-cell hemoglobin gene
HbAHbA genotype = normal phenotype
HbSHbS genotype = sickle-cell disease phenotype
HbAHbS genotype = usually normal, but forms sickled cells if PO2 is very low
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EXAMPLE OF CODOMINANCE
ABO blood group – 3 alleles for blood antigen trait
allele IA – A antigen gene, codominant allele
allele IB – B antigen gene, codominant allele
allele i – gene produces no A or B antigen, recessive allele
possible phenotypes
genotype
shorthand
Type A
Type B
Type AB
Type O
IAIA or IAi
IBIB or IBi
IAIB
ii
AA or AO
BB or BO
AB
OO
COMPLEX INHERITANCE
polygenic inheritance – most traits are controlled by more than 1 gene
(eg. hair color, eye color, skin color, height)
environment affects gene expression – nutrition, disease, teratogens
usually expressed as a broad spectrum of phenotypes
ERRORS IN INHERITANCE
nondisjunction – abnormal division of homologous chromosomes or sister chromatids
during meiosis or early cleavage stages
aneuploid – cell with abnormal number of chromosomes
monosomic – missing a chromosome (2n – 1)
trisomic – extra chromosome (2n + 1)
Down’s syndrome – trisomy of chromosome 21
translocation – occurs during crossing-over in meiosis
may lose or gain genes
mutation – errors in replication of DNA result in altered genes
may form new alleles = new possible traits
change may be beneficial, harmful, indifferent, or lethal
SEX DETERMINATION
22 pairs of autosomes – homologous chromosomes that look the same and
carry similar genes
1 pair of sex chromosomes – homologs look different and carry different genes
X chromosome – large; carries genes for somatic (nonsexual) traits
Y chromosome – small; carries genes to turn on male development
XX = female
XY = male
ovum always contributes an X chromosome
sperm may contribute either X or Y chromosome
father determines sex of child
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SEX-LINKED INHERITANCE
X chromosomes carry genes for nonsexual traits
many genes present on X chromosome have no homolog on Y chromosome
single allele in males means the one inherited on the X chromosome will
determine his phenotype
sex-linked disorders occur more frequently in males
female has 2 alleles, so recessive disorders are not expressed in her
phenotype unless she is homozygous recessive
Example of Sex-Linked Inheritance
red-green color blindness – disorder resulting from production of nonfunctional
red or green light sensitive cones in retina
gene is found on X chromosome
allele C – normal color vision gene (normal cones)
allele c – color blind gene (abnormal cones)
possible genotypes
XCXC or XCXc
XcXc
XCY
XcY
phenotype
normal female
color blind female
normal male
color blind male
carrier mother and normal father (both have normal phenotype)
50% of male offspring and no female offspring are color blind
females will only be color blind if the father is color blind and mother is a carrier
or is color blind
GENETIC COUNSELING – advising on genetic problems or potential genetic
problems in offspring
TECHNIQUES TO EVALUATE GENOTYPE
pedigree – history of phenotypes in family tree
karyotype – microscopic examination of chromosomes from dividing cell
identifies aneuploids and some translocations
DNA testing
most severe genetic disorders are autosomal recessive
carriers are unknown
2 carrier parents may produce affected offspring
dominant disorders
most do not result in serious disease (eg polydactlyly)
serious dominant disorders usually prevent reproduction, so the disorder
is not passed on
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Biology 233
Lab Exercise 44
Lab Objectives Supplement
Chick Embryonic Development:
Study slides of chick development as introduced in lab. Review images are available
through the library on E-Reserves.
Be able to identify the following on chick embryo slides:
3 primary germ layers:
Ectoderm
Mesoderm
Endoderm
(Also know which of these layers form the following structures.)
Primitive streak
Notochord
Neural groove
Neural tube
Primitive gut
Somites
Blood islands / blood vessels
Developing heart
Otic placode
Optic placode
Limb buds
Allantois
Amnion
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