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GENERAL EMBRYOLOGY - Complete Answers

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SAQ ANSWERS

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1. Somites (***)

• Cells from the primitive node form paraxial mesoderm alongside the notochord
• Toward the end of the 3rd week, paraxial mesoderm differentiates, condenses, and divides into paired cuboidal bodies - the somites
• They form a craniocaudal sequence beginning in the future occipital region
• The first pair forms a short distance caudal to where the otic placode forms
• Between days 26-32, approximately 38-39 pairs form (the somite period)
• By end of 5th week: 42-44 pairs present
• Molecular regulation: WNT, FGF, and NOTCH pathway genes; Tbx6 (T-box gene); FoxC1 and FoxC2 transcription factors; Delta-Notch signaling governs the craniocaudal pattern; a molecular "clock" sequences their formation

1. Sclerotome - ventromedial cells migrate to surround notochord and neural tube; forms vertebrae, ribs, and part of skull
2. Myotome - forms skeletal muscles of the back (epimere) and ventrolateral body wall/limbs (hypomere)
3. Dermatome - forms dermis of the skin of back and neck

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2. Spermatogenesis (*****)

• Loss of cytoplasm (residual body shed)
• Development of tail (flagellum)
• Formation of acrosome (from Golgi region) - contains hyaluronidase and acrosin
• Nuclear condensation

• Sertoli cells - nurture developing germ cells, form the blood-testis barrier, regulate spermatogenesis
• Leydig cells - produce testosterone (essential for spermatogenesis)

• Head (nucleus + acrosome covering anterior 2/3)
• Neck (junction of head and tail)
• Middle piece, principal piece, end piece (collectively the tail/flagellum)

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3. Oogenesis (*****)

• Neonate: ~2 million primary oocytes
• Puberty: ~40,000 remain
• Only 400-500 are ovulated during reproductive life
• Remainder degenerate by atresia

• Oocyte maturation inhibitor (from follicular cells) maintains meiotic arrest
• The long arrest in prophase (up to 45 years) explains the high frequency of meiotic errors (e.g., non-disjunction) with advancing maternal age
• Primary oocytes arrested in dictyotene are vulnerable to radiation

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4. Implantation (***)

• Zona pellucida must degenerate (hatching of blastocyst)
• Endometrium must be in the secretory phase (luteal phase)
• "Window of implantation" - days 20-24 of cycle
• Blastocyst must be "activated"

1. Apposition - blastocyst loosely contacts endometrial epithelium (polar trophoblast over ICM attaches first)
2. Adhesion - firm attachment mediated by HB-EGF (heparin-binding EGF-like growth factor) binding to ErbB1/ErbB4 receptors on trophoblast; also LIF, integrins, selectins
3. Invasion - syncytiotrophoblast invades decidua (weeks 2-3); trophoblast erodes uterine stroma; blastocyst sinks beneath epithelium
4. By day 10-12, blastocyst is completely embedded; defect in endometrium is sealed by a fibrin coagulum (closing plug)

• 95-98% occur in uterine tubes (most common: ampulla > isthmus)
• Also: ovarian, abdominal, cervical
• Risk: tubal rupture, hemorrhage, death

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5. Formation of Germ Layers & their Derivatives (*****)

• Primitive streak appears in the epiblast (day 15)
• Epiblast cells migrate toward the primitive streak, ingress, and spread laterally and cranially
• Cells that displace hypoblast form endoderm
• Cells that remain between ectoderm and endoderm form mesoderm
• Epiblast cells that do not ingress become ectoderm

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• CNS (brain + spinal cord)
• Peripheral nervous system
• Sensory epithelium (eye, ear, nose)
• Epidermis, hair, nails, skin glands
• Mammary and pituitary glands
• Enamel of teeth
• Neural crest cells (give rise to additional structures)

• Connective tissue, cartilage, bone
• Striated & smooth muscle
• Heart, blood vessels, blood (cardiovascular system)
• Kidneys, ureters (excretory system)
• Gonads, genital ducts (reproductive system)
• Spleen
• Adrenal cortex
• Serous membranes (pleura, peritoneum, pericardium)
• Dermis of skin

• Epithelial lining of GI tract, respiratory tract
• Urinary bladder, urethra (epithelium)
• Thyroid, parathyroid glands
• Liver and biliary apparatus
• Pancreas
• Thymus, tonsils
• Tympanic cavity and Eustachian tube lining

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6. Formation of Notochord (*****)

1. Cells from the primitive node migrate cranially to form the notochordal process (a cellular tube)
2. The primitive pit extends into the process, forming a notochordal canal
3. The floor of the notochordal process fuses with the underlying endoderm
4. These fused layers degenerate, forming openings connecting the notochordal canal with the umbilical vesicle
5. As openings become confluent, the floor disappears and a notochordal plate is formed
6. The notochordal plate cells proliferate and undergo infolding, forming the solid notochord

1. Defines the longitudinal axis of the embryo (acts as the primary structural scaffold)
2. Primary inductor (signaling center) - induces overlying ectoderm to thicken and form the neural plate (primordium of CNS)
3. Provides positional signals for somite differentiation

• Degenerates as vertebral bodies form
• Persists as the nucleus pulposus of each intervertebral disc

• Remnants can form chordomas (benign or malignant tumors) - 1/3 occur at the base of the cranium and extend to the nasopharynx

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7. Formation of Neural Tube (*****)

1. Notochord induces the overlying ectoderm to thicken and form the neural plate (day 18)
2. Neural plate elongates and its lateral edges elevate to form neural folds
3. A groove appears in the center - the neural groove
4. Neural folds meet in the midline and fuse, forming the neural tube (day 22-23)
5. Fusion begins in the cervical region and proceeds cranially and caudally
6. The openings at each end are the anterior neuropore (closes day 25) and posterior neuropore (closes day 27)

• Brain (prosencephalon, mesencephalon, rhombencephalon)
• Spinal cord
• Motor neurons
• Ependymal cells, oligodendrocytes, astrocytes

• Failure of anterior neuropore to close → Anencephaly (incompatible with life)
• Failure of posterior neuropore to close → Spina bifida
  • Spina bifida occulta (most common, benign)
  • Meningocele (meninges herniate)
  • Meningomyelocele (meninges + cord herniate)
• Folic acid (400 mcg/day preconception) significantly reduces NTD risk

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8. Chorionic Villi & their Functions (*****)

1. Primary villi (end of 2nd week) - solid cores of cytotrophoblast covered by syncytiotrophoblast
2. Secondary villi (early 3rd week) - extraembryonic mesoderm grows into primary villi core
3. Tertiary villi (end of 3rd week) - blood vessels develop within the mesoderm core; forms the definitive placental villus

• Stem (anchoring) villi - anchor placenta to decidua basalis
• Free (floating) villi - project into intervillous space; primary site of exchange

• Outer syncytiotrophoblast (direct contact with maternal blood)
• Inner cytotrophoblast (less prominent at term)
• Villous mesoderm (stroma)
• Fetal capillaries inside

1. Nutrition - glucose, amino acids, fatty acids cross to fetus
2. Respiration - O₂ transfer to fetus; CO₂ removal
3. Excretion - fetal waste (urea, creatinine) transferred to maternal blood
4. Endocrine - secretes hCG (maintains corpus luteum), hPL, progesterone, estrogen
5. Immunological - IgG crosses (passive immunity to fetus); acts as an immunological barrier
6. Barrier - prevents some microorganisms; but TORCH agents can cross
7. Drug transfer - many drugs cross (thalidomide, alcohol, cocaine)

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9. Neural Crest Cells & their Derivatives (***)

• Bones and cartilages of the face and skull (viscerocranium)
• Dental papilla (dentine)
• Melanocytes of the head
• Cranial nerve ganglia (V, VII, IX, X - sensory parts)
• Connective tissue of thyroid, parathyroid, thymus

• Dorsal root ganglia (sensory)
• Autonomic ganglia (sympathetic chain, parasympathetic)
• Adrenal medulla (chromaffin cells)
• Schwann cells (peripheral myelin)
• Melanocytes of the skin

• Aorticopulmonary septum (partitions aorta and pulmonary trunk)
• Cardiac ganglia

• Treacher Collins syndrome (failure of cranial NC migration)
• Hirschsprung disease (failure of NC cells to colonize distal colon)
• Neuroblastoma (malignant NC derivative - adrenal medulla)
• Melanoma (from neural crest-derived melanocytes)
• Waardenburg syndrome (NC migration defect - deafness + pigmentation)
• DiGeorge syndrome (cardiac/pharyngeal NC defect)

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10. Primitive Streak (***)

• Appears in the caudal midline of the epiblast
• Has a primitive groove running along its length
• Cranial end has a thickened area: the primitive node (Hensen's node)
• Primitive node has a central depression: the primitive pit

1. Establishes the cranial-caudal axis and bilateral symmetry of the embryo
2. Establishes left-right axis - cilia at primitive node rotate and create a leftward flow of fluid; this asymmetry is essential for normal organ siding (situs solitus)
3. Through gastrulation, produces mesoderm and endoderm from epiblast
4. Defines the dorsal (posterior) midline

• Epiblast cells migrate toward the primitive streak
• Cells ingress through the streak into the subembryonic space
• They spread cranially and laterally between ectoderm and endoderm = intraembryonic mesoderm
• Some cells displace hypoblast = definitive endoderm
• Epiblast remaining on surface becomes ectoderm

• Sacrococcygeal teratoma - most common tumor of newborns; arises from primitive streak remnants; contains tissues derived from all three germ layers

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11. Derivatives & Components of Mesoderm (***)

• Somatic (parietal) layer - with overlying ectoderm forms the body wall (somatopleure); gives rise to the parietal layer of serous membranes, limb bones, connective tissue of limbs
• Splanchnic (visceral) layer - with underlying endoderm forms the gut wall (splanchnopleure); gives rise to heart, blood vessels, blood cells, smooth muscle of viscera, visceral layer of serous membranes

1. All connective tissue proper (loose, dense, adipose)
2. Cartilage and bone
3. Striated muscle (from somites) and smooth muscle
4. Cardiovascular system (heart, blood vessels, blood cells)
5. Lymphatic system
6. Kidneys and ureters (intermediate mesoderm)
7. Gonads and genital ducts
8. Adrenal cortex
9. Spleen
10. Serous membranes (pleura, peritoneum, pericardium)
11. Dermis of skin (from dermatome)

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VSAQ ANSWERS

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1. Capacitation (***)

• A glycoprotein coat and seminal plasma proteins are removed from the surface of the sperm acrosome
• Plasma membrane components are extensively altered (cholesterol efflux from sperm membrane)
• Intracellular Ca²⁺ increases
• Regulated by tyrosine kinase (src kinase) and progesterone
• Capacitated sperms show no morphological change but become more active (hyperactivated motility)

• Acrosome binds ZP3 on zona pellucida
• Acrosomal membrane fuses with sperm plasma membrane
• Enzymes released: hyaluronidase (disperses corona radiata) and acrosin (penetrates zona pellucida)

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2. Divisions of Decidua (***)

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3. Functions of Placenta (***)

1. Nutrition (metabolic):
   • Transports glucose (facilitated diffusion), amino acids (active transport), fatty acids, vitamins, minerals, water
   • Also produces glycogen, fatty acids (histotrophic nutrition early on)
2. Respiration:
   • O₂ diffuses from maternal blood (high pO₂) across placental membrane to fetal blood
   • CO₂ diffuses in opposite direction
   • Fetal hemoglobin (HbF) has higher O₂ affinity than HbA, aiding transfer
3. Excretion:
   • Urea, uric acid, creatinine diffuse from fetal to maternal blood for elimination
4. Endocrine (hormone production):
   • hCG (human chorionic gonadotropin) - maintains corpus luteum (progesterone production) in first trimester; basis of pregnancy test
   • hPL (human placental lactogen) - promotes fetal growth; causes insulin resistance in mother
   • Progesterone - maintains pregnancy, prevents uterine contractions
   • Estrogens (mainly estriol) - uterine growth, breast development
   • hCT (human chorionic thyrotropin), relaxin
5. Immune:
   • Transfers maternal IgG antibodies to fetus (passive immunity for ~6 months after birth)
   • Also acts as immunological barrier (prevents rejection of fetus)
6. Barrier function:
   • Prevents most bacteria from crossing
   • Does NOT protect against viruses (HIV, rubella, CMV, HSV) or TORCH organisms
   • Many drugs, alcohol, and nicotine cross freely

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4. Trophoblast (*)

1. Cytotrophoblast (Langhans layer):
   • Inner, mononuclear, mitotically active
   • Proliferates and supplies cells to syncytiotrophoblast
   • Prominent in early pregnancy; decreases at term
2. Syncytiotrophoblast:
   • Outer, multinucleated, non-mitotic syncytium
   • Invasive - erodes decidua during implantation
   • Performs most placental functions (hormone synthesis, gas/nutrient exchange)
   • Forms lacunae that fill with maternal blood (begins intervillous circulation)

• Extravillous trophoblast - invades maternal spiral arteries; converts them from narrow, muscular vessels to wide, low-resistance channels (essential for adequate uteroplacental blood flow)
• Failure of this remodeling → preeclampsia

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5. Umbilical Cord (*)

• 2 umbilical arteries (carry deoxygenated blood from fetus to placenta) - branches of internal iliac arteries
• 1 umbilical vein (carries oxygenated blood from placenta to fetus)
• Wharton's jelly - mucoid connective tissue (proteoglycans) surrounding the vessels, providing cushioning and preventing compression
• Covered by amnion (amniotic epithelium)

• Single umbilical artery (SUA): Occurs in ~1% of births; associated with congenital anomalies (renal, cardiac)
• Nuchal cord: Cord wrapped around fetal neck; common (~25-30% deliveries); significant if tight
• Velamentous insertion: Cord inserts into membranes rather than placenta; risk of vasa previa
• Umbilical hernia: Persistence of herniation at umbilicus
• Allantoic stalk remnant: Urachus → if patent, forms urachal fistula (urine drains from umbilicus)
• After birth, umbilical arteries become medial umbilical ligaments; umbilical vein becomes ligamentum teres (round ligament of liver)

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