Download Embryo final study tips

STUDY TIPS FOR THE EMBRYO FINAL
SPRING 1999
1. Where do somites come from? What structures do they form?
Paraxial mesoderm  somites  skin, bone and muscle
Tongue myoblasts derived from occipital somites
2. How do the fates of the mesonephric and paramesonephric ducts differ in the male and female?
In males the
Paramesonephric ducts regress
Mesonephric  Wolfian ducts  epididymus, ductus deferens, ejaculatory duct
In females the
Mesonephric ducts regress
Paramesonephric  Müllerian ducts  oviducts, uterus, upper vagina
3. Where does fertilization occur?
Fertilization occurs in the ampulla mostly, other locations may result in inappropriate implantation.
4. When does the lung complete development?
Alveolar development is from late fetal development to about age 8 years. During this time the alveoli mature
completely, their walls become very thin to allow for extensive capillary contact.
5. How does the gut rotate and how much? The stomach?
During the 6th week the duodenum rotates 270 degrees counterclockwise around the axis of the superior
mesenteric artery accompanied by further elongation and coiling.
The stomach rotates around the longitudinal axis first, by 90 degrees clockwise, leaving the anterior side inferiorly.
Next it rotates around the anterior posterior axis by 90 degrees clockwise, so the left side is inferior and forms the
greater curvature.
6. What cells arise from neuroepithelium of the neural tube? Neural crest?
Neural tube gives rise to cells of the CNS
Retina
Pineal body
Neurohypophysis
Neural crest gives rise to cells that form most of the PNS and ANS
cranial, spinal, and autonomic ganglia
sensory ganglia of spinal nerves (dorsal root g. )
postganglionic neurons of the ANS
Schwann cells
Odontoblasts
meninges
Medulla of the adrenal gland
Melanocytes
Pharyngeal arch mesenchyme (First arch syndrome – failure of neural crest migration )
Head mesenchyme and CT ( cranial facial bones )
Bulbar and conal ridges of the heart (Abnormal valve leaflets – neural crest defect )
7. Where is the primitive streak? What does it do? When does it form?
The first thing to appear during gastrulation is the primitive streak. It forms at the caudal end of epiblast ( ~day 16 –
beginning of the 3rd week) and is a thickened linear band of epiblast. It results from the proliferation and migration
of cells of the epiblast to the median plane of the embryonic disc. As it elongates, it forms the primitive node. The
primitive streak helps differentiate cranial /caudal end, dorsal /ventral surfaces, and right/ left sides. Shortly after
the P.S. appears, cells leave its deep surface and form a loose network of embryonic CT called mesenchyme 
supporting tissue of the embryo, most of the body CT and stromal components of the glands and mesenchyme
intraembryonic mesoderm. The primitive streak actively forms mesoderm until the early part of the 4th week, then
it diminishes in size to become an insignificant structure in the sacrococcygeal region of the embryo. Normally it
disappears by the end of the fourth week.
8. Syncytiotrophoblast: what does it do? Where does it come from?
Develops about 6 days after fertilization. The outer mass of the trophoblast, consisting of a multinucleated
protoplasmic mass in which no cell boundaries can be seen. The syncytiotrophoblast’s fingerlike projections extend
through the endometrial epithelium and invade the connective tissue, thereby allowing the blastocyst to superficially
implant. The syncytiotrophoblast expands quickly adjacent to the inner cell mass. It is probably responsible for
hormone production. The Cytotrophoblast is highly mitotic and contributes cells to syncytiotrophoblast .
9. Know the phases of the ovarian and uterine cycles and the hormones responsible.
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At the beginning (day 0) of the menstrual cycle all hormone levels are low. Low levels of GnRH are released, and it
stimulates FSH and LH. FSH will stimulate follicular development over the next 14 days, and the LH stimulates
estrogen production by the follicle. Estrogen levels subsequently rise and this causes neg. feedback on GnRH.
In turn FSH and LH also rise. When levels of estrogen become “high”, it exerts positive feedback on GnRH and in
turn on LH (great effect) and FSH (some effect). The LH surge causes rupture of the fully mature secondary oocyte
from the Graffian follicle. After this happens, LH luteinizes the corpus hemorrhagicum and it becomes the corpus
luteum. The C.L. under influence of LH, secretes Progesterone ( levels now rise ) and some estrogen – not as much
as the developing follicle (levels are falling). The life span of the corpus luteum is short and it begins to degenerate
into the corpus albicans less than 2 weeks after ovulation. Progesterone and estrogen levels drop off and GnRH is
once again stimulated and the cycle starts again.
Progesterone
Menses 0-4
Proliferative 4-14
destruction of functionalis
repair/regeneration
Ovulation
Luteal Phase 15-28
Graffian follicle
UTERINE
seretory phase
Follicular Phase 1-14
follicle develops
Secretory phase 15-28
Ovulation
OVARIAN
corpus luteum  matures  corpus albicans
10. How is the notochord formed? Where does it persist in the adult?
The notochord is a cellular rod that develops by transformation of the notochordal process. It defines the primordial
axis of the embryo and gives it some rigidity. It also serves as the basis for development of the axial skeleton and it
is an intricate structure around which the vertebral column forms. The notochord degenerates as the bodies of the
vertebrae form but it persists as the nucleus pulposus of each IVD.
The developing notochord inducs the overlying embryonic ectoderm to thicken and form the neural plate, the
primordium of the the CNS. (page 70)
Notochord---mesenchymal(mesoderm) cells in midline from primitive pit to prechordal plate
Notochordal process (hollow tube)  notochordal plate  notochord
11. Know the adult derivatives of the embryonic brain
at about 5 weeks
3 primary vescicles
5 secondary vescicles
Prosencephalon 
Telencephalon 

Diencephalon 
Mesencephalon 
Mesencephalon 
Rhombencephalon 
metencephalon 

Myelencephalon 
adult derivatives of walls …
cerebral hemispheres
thalamic structures
Midbrain
Pons
Cerebellum
Medulla
and cavities
lateral ventricles
3rd ventricle
aqueduct
upper part of 4th ventricle
lower part of 4th ventricle
12. How are the limbs formed?
Limbs come from somatic mesoderm from the lateral plate. Development begins at the 4 th week. The upper limbs
appear first, but both upper and lower finish their morphogenesis around the 8th week. The apical ectodermal ridge
(AER) is induced by mesoderm and itself induces mesoderm to form specific structures. It is essential for limb
outgrowth and develoment and acts as a clock for the mesoderm: it tells when and what to form. The AER keeps
underlying mesoderm in a highly mitotic state and the tissues that stay associate with it longer form the more distal
structures. Around week 6 a flattened hand and foot plate appear. The AER is confined to the tips of the digital rays.
Digits are formed by apoptosis. The zone of polarizing activity ( ZPA) at the caudal tip of the plate is made of
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ectoderm and determines the order of the digits. It is thought that retinoic acid is responsible for this process. Cell
fate determination occurs in steps based on temporal and spatial location and structures develop in proximal to
distal sequence. The bones are formed by endochondral development starting at week 6 when cartilage appears,
ossification begins during week 7. The upper limbs rotate laterally by 90 degrees, and the lower limbs rotate
medially by 90 degrees.
13. What muscles develop from epimeres? hypomeres?
Epimere ( dorsal )  extensor muscles of the neck and back
Hypomere ( ventral )
thorax – external, internal, and innermost intercostals and transversus thoracic
abdomen – external and internal obliques, transverse ebdominal
ventral tip – rectus abdominis and infrahyoids
diaphragm – myoblast migration into pleuroperitoneal membranes and leteral body wall, innervation by
phrenic nerve carried in with myoblasts from cervical somites.
Myotome regions of somites  most skeletal mm
Pharyngeal arch mesoderm  some head and neck mm
Somites  myogenic precursor cells  limb mm
Splanchnic mesoderm  cardiac and smooth mm
14. Know the divisions of the gut and their blood supply.
Foregut  supplied by celiac artery
 Esophagus
Esophageal artresia, TE fistula ( both polyhydramnios )
Stomach
Pyloric stenosis ( hypertrophy of mm)
Duodenum
Liver
Artresia, double gall bladders
Gall bladder
Mesenteries
dorsal greater o., ventral lesser o.
Pancreas
Annular pancreas ( incomplete rotation or 2 p.buds), accessory p. duct ( often in Meckel’s d. )
Midgut  supplied by the superior mesenteric artery
 small intestine ( including most of the duodenum)
cecum
vermiform appendix
ascending colon
proximal 2/3 of the transverse colon
Hindgut  supplied by inferior mesenteric artery
 distal 1/3 of the transverse colon
descending colon
sigmoid colon
rectum
superior part of the anal canal
bladder epithelium
most of the urethra
15. What are the flexures of the developing brain and where are they?
Flexures appear between the 4th and 8th week. The mesencephalic or cranial flexure is in the area of the
midbrain. The cervical flexure is at the brain stem and the spinal cord, approximately at the level of the foramen
magnum. Both the mesencephalic and the cervical flexures are ventral flexures. Later, unequal growth of the brain
between these 2 flexures produces the pontine flexure which is located in the future pontine region and goes in the
opposite direction . It divides the brain into the met- and myelencephalon.
16. What is the choroid plexus and where does it come from?
The choroid plexus is a differentiation of the tela choroida, which consists of ependymal cells plus vascular
mesenchyme. There are a total of 4 plexi, one in each ventricle. Their function is to secrete CSF which circulates
around the brain and the spinal chord in the subarachnoid space which it enters from the fourth ventricle.
17. Know the anomalies associated with the CNS development.
Spina bifida cystica – failure of neural arch to form over SC, leaving neural tissues &/or meninges exposed deficits
loss of motor function potentially due to mechanical injury in utero or during delivery
Spina bifida occulta – failure of bony structure to form  no neural deficits but localized hypertrichosis
Meningiocele – herniation of the meninges
Meningioencephalocele – herniation of part of the brain
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Meningiohydroencephalocele – herniation of brain and maybe ventricles
Anencephaly – due to failure of anterior neural tube closure  degeneration of neural tissues
Hydrocephalus –abnormal accumulation of CSF due to obstruction of drainage into subarachnoid space, most
commonly foramen of Luschka or cerebral aqueduct.
Arnold-Chiari malformation – herniation of cerebellar tissue through the foramen magnum, sometimes associated
with spina bifida cystica
18. Know the derivatives of the pharyngeal arches, pouches and clefts.
ARCH
BONES
MUSCLES
Arch I
Maxillary process: maxilla, lat palatine
Mm of mastication,
process, zygomatic, lat. part of upper
myelohyoid, ant. belly of
lip, part of temporal.
digastric, tensor tympani,
Mandibular process: Meckel’s cart.,
tensor palatine
mandible, incus, malleus
Arch II Stapes, styloid process of temporal,
Stapedius, stylohyoid, post.
stylohyoid ligament, Reichert’s cart.
belly of digastric, mm of facial
lesser horn/upper body of hyoid,
expression, zygomaticus
Arch III Greater horn/ lower body of hyoid
Stylopharyngeus
Arch IV
Cartilage of the larynx:
thyroid, cricoid,
arytenoid, corniculate,
cuneiform
Arch VI
( poorly developed)
Cricothyroid
Constrictors of pharynx
( swallow)
Intrinsic mm of the pharynx
(voice production)
BLOOD SUPPLY
( mostly regresses )
maxillary arteries,
external carotid artery
NERVES
CN V2 –
motor
V 1, 2, 3 –
sensory
(mostly regresses)
hyoid arteries, stapedial
aa of middle ear
Proximal: common &
external carotid
Distal: proximal internal
carotids
Lpart of aorta btwn L
com carotid & L
subclavian
Rproximal portion of
R subclavian
Pulmonary arch  L 
proximal L pulmonary artery;
distal ductus arteriosus;
Pulmonary arch  R 
proximal R pulmonary artery;
distal  degenerates
CN VII
CN IX
CN X,
Superior
laryngeal
nerve
CN X,
recurrent
laryngeal
PHARYNGEAL POUCHES
DERIVATIVES
POUCH I
Tubotympanic recess – contacts cleft I – between arch I & II
Distal portion  middle ear ( primitive tympanic cavity )
Proximal portion  Eustacian tube
POUCH II
Invaginates into mesenchyme  stroma of palatine tonsil
POUCH III
2 wings
dorsal wing inferior parathyroid gland
ventral wing  thymus
POUCH IV & V
Never distinctly separated, always found together
IV  superior parathyroid gland
V  ultimobranchial body  incorporated into thyroid  parafollicular cells  Calcitonin
PHARYNGEAL CLEFTS – ( external ) – between Arch I & II
First cleft  external accoustic meatus – lined with ectoderm
Second – fourth cleft  cut off from surface by rapid growth of Arch II  temporary cervical sinus
19. Remember the anomalies of the heart
ASDs
PDA – if accompanied by pulmonary stenosis  cyanosis
as ostium secundum defect: due to faulty septum primum or septum secundum
Common atrium – failure of septum primum and secundum
Endocardial cushions and AV septal defects – need surgical correction since these contribute to Bi- and
Tricuspid valves
VSDs
Membranous VSD – most common – bulbar ridges start too high
Muscular VSD – usually multiple small openings
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Absence of interventricular septum – failure of muscular septum to form
Persistent trunkus arteriosus – no formation of trunkal ridges or aorticopulmonary septum – always w/ VSD
Transposition of the great vessels – always w/ VSD, maybe a PDA, failure of aorticopulmonary septum to spiral
Coarctation of the aorta
Postductal – no problems post-natally
Pre-ductal – BIG problems post-natally
Double aortic arch – failure of distal right aorta to regress  constriction of trachea
Tetralogy of Fallot – cono-trunkal region due to unequal division of trunkus
1. Pulmonary stenosis
2. VSD
3. Overriding aorta
4. R ventricular hypertrophy
20. Remember how and where the foramen ovale is formed.
The septum primum grows down from the roof of the primordial atrium towards the fusing endocardial cushions. As
this septum grows, the foramen primum forms between the free edge and the endocardial cushions. As it gets
smaller and smaller the septum primum grows and fuses with the endocardial cushions. Before the foramen primum
completely closes, a number of small holes appear that eventually make the foramen secundum within the septum
primum. The purpose of the foramen secundum is to ensure the continued blood flow between the left and right
atria. The septum secundum grows on the right side of the septum primum. It grows inferiorily, overlaps the foramen
secundum in the septum primum. The septum secundum forms an incomplete partition between the atria which
results in the foramen ovale.
21. Know the unique characteristics of the fetal circulation and how it changes at birth.
Nutrient and O2 rich blood from umbilical vein bypasses liver by way of ductus venosus to inferior vena cava to R
atrium. Here most blood goes through foramen ovale into L atrium to L ventricle and from there into the ascending
aorta to supply the head, neck, heart, and upper limbs. The blood that stays in the R atrium goes into the R ventricle
then up the pulmonary trunk but bypasses the lungs by way of ductus arteriosus to go into the descending aorta to
supply the lower body.
At birth several changes take place:
1. loss of placenta lowers BP in the inferior vena cava and in the R atrium
2. aeration of lungs  lowers pulmonary resistance
 increases pulmonary blood flow
 increases BP in L atrium, lowers BP in R atrium
3. closure of foramen ovale
4. closure of ductus arteriosus due to decreased pulmonary and increased systemic resistance
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22. What is the adult fate of embryonic structures of the heart and vessels; or what is the embryonic origin of adult
vessels and chambers of the heart.
EMBRYONIC ADULT
Umbilical veins  ligamentum teres
Ductus venosus  ligamentum venosus – w/i the liver between the portal vein and inferior vena cava
Foramen ovale  fossa ovalis
Ducuts arteriosus  ligamentum arteriosum
Umbilical artery  medial umbilical ligament
EMBRYONIC
ARCH I

ARCH II
ARCH III
ARCH IV
ARCH VI
Dorsal aortae
Vitelline arteries
Aortic sac
Cardinal veins
R subcardinal btwn liver & kidney
Hepatic vein btwn liver & heart 
Sub-/supracardinal anastemosis 
R supracardinal vein 
ADULT
( arch mostly regresses )
maxillary arteries, external carotid artery
( arch mostly regresses )
hyoid arteries,
stapedial aa of middle ear
Proximal: common & external carotid
Distal: proximal internal carotids
Lpart of aorta btwn L com carotid & L subclavian
Rproximal portion of R subclavian
Pulmonary arch  L 
proximal L pulmonary artery;
distal ductus arteriosus;
Pulmonary arch  R 
proximal R pulmonary artery;
distal  degenerates
Median sacral artery
Distal portion of internal carotids
Intersegmental arteries vertebral arteries in neck
 intercostal arteries in thorax
 5th lumbar intersegmentals  common iliac
aa
 7th intersegmental  L  L subclavian
 R  distal R subclavian
branches  umbilical arteries fuse with common iliac artery
Celiac trunk
Superior mesenteric
Inferior mesenteric
Ascending aorta  2 horns
1. L horn  arch of the aorta
2. R horn  brachiocephalic trunk
Anterior  L brachiocephalic vein
R anterior cardinal & common cardinal veins superior vena cava
Posterior cardinals  azygos vein & common iliac veins  replaced by:
Subcardinals &
Supracardinals
Inferior vena cava: 4 segments
Pre-renal segment
Hepatic segment
Renal segment
Post-renal vein
Finally, in your spare time think about all of the tissues and organs you studied in histology and give the embryonic
germ layer of origin for each.
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Bilaminar embryonic disc  Hypoblast  endoderm of yolk sac extraembryonic mesoderm
 Epiblast  ectoderm of amnion
 embryonic ectoderm
 primitive streak
 extraembryonic mesoderm
 embryonic mesoderm
 notochordal process
 embryonic endoderm
3 germ layers
Ectoderm
 epidermis
 epidermal derivatives
 CNS and PNS ( neural plate fold  neural tube , neural crest )
 pituitary gland
 sensory epithelia of the eye, ear, and nose ( otic and lens placodes )
Endoderm
 epithelial lining of gut – 3 regions – foregut, midgut, hindgut
 epithelial lining of respiratory tract
 parts of urinary system
 liver
 pancreas
 thyroid
 parathyroid glands
Mesoderm
 paraxial mesoderm  somites  skin, bone, muscle
 intermediate mesoderm  kidneys and gonads
 lateral plate mesoderm  somatic / parietal mesoderm lines the body wall
 splanchnic / visceral mesoderm covers viscera
 CT
 muscle
 circulatory system
 kidneys
 gonads
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