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Anatomy - General Embryology: Board Exam Answers (BDS 1st Year, Kathmandu University)

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Q1. What is Oogenesis? Explain its mechanism, adding notes on "Ovulation" [1+2+2=5 marks]

Definition (1 mark)

Mechanism (2 marks)

• During early fetal life, oogonia proliferate by mitosis
• Oogonia enlarge to form primary oocytes before birth; flattened follicular cells surround each to form a primordial follicle
• Primary oocytes begin meiosis I before birth but arrest in prophase I (dictyotene stage) - maintained by oocyte maturation inhibitor secreted by follicular cells
• At birth: ~2 million primary oocytes; by puberty only ~40,000 remain

• Each month, FSH stimulates follicular maturation; the zona pellucida (acellular glycoprotein layer) forms around the oocyte
• Just before ovulation, the primary oocyte completes meiosis I → produces a secondary oocyte (receives most cytoplasm) + 1st polar body (degenerates)
• Meiosis II begins but arrests at metaphase II
• The secondary oocyte is ovulated at this stage
• Meiosis II is only completed if fertilization occurs, producing the mature oocyte + 2nd polar body

Ovulation (2 marks)

• Triggered by a LH surge (midcycle, day ~14)
• The Graafian follicle ruptures and releases the secondary oocyte surrounded by the corona radiata and zona pellucida
• The oocyte is swept into the fallopian tube (uterine tube) by fimbriae
• If not fertilized within ~24 hours, the secondary oocyte degenerates
• The ruptured follicle becomes the corpus luteum, which secretes progesterone to maintain the endometrium

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Q2. Short Notes [1+2+2=5 each; written as individual 5-mark notes]

(a) Neurulation

1. At ~day 18, the notochord induces the overlying ectoderm to thicken, forming the neural plate
2. The lateral edges of the neural plate elevate as neural folds; the groove between them is the neural groove
3. Neural folds approach the midline and fuse (starting in the cervical region, proceeding cranially and caudally) to form the neural tube
4. Fusion is complete by day 26 (cranial neuropore closes day 25; caudal neuropore day 27)
5. Neural crest cells migrate from the margins of the fusing folds

• Cranial: Anencephaly (failure of anterior neuropore to close)
• Caudal: Spina bifida (failure of posterior neuropore to close)

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(b) Spermatogenesis

1. Spermatogonium (2n) - stem cells in seminiferous tubules; proliferate by mitosis
   • Type A: self-renewing stem cells
   • Type B: differentiate into primary spermatocytes
2. Primary spermatocyte (2n) - undergoes meiosis I → two secondary spermatocytes (n)
3. Secondary spermatocyte (n) - undergoes meiosis II → four spermatids (n)
4. Spermiogenesis - spermatids undergo morphological transformation (no cell division):
   • Acrosome formation (from Golgi)
   • Condensation of nucleus
   • Formation of flagellum (tail)
   • Loss of excess cytoplasm as residual bodies (phagocytosed by Sertoli cells)
   • Result: mature spermatozoon

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(c) Gastrulation

1. The primitive streak appears in the midline of the epiblast at the caudal end of the embryo (day 15)
2. At its cranial end is the primitive node (Hensen's node)
3. Epiblast cells migrate toward the streak, invaginate, and displace the hypoblast → form endoderm
4. Cells invaginating between epiblast and new endoderm form the intraembryonic mesoderm
5. Remaining surface epiblast cells become ectoderm

• Establishes the three primary germ layers
• Establishes the embryonic body axes (cranio-caudal, dorso-ventral, left-right)
• All future organs derive from these three layers

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(d) Palate Formation (Palate Teratology)

• Shelves initially project vertically on either side of the tongue
• Elevate to horizontal position (due to intrinsic shelf-elevating force + tongue dropping)
• Shelves fuse with each other in the midline, with the primary palate anteriorly, and the nasal septum superiorly
• Fusion complete by week 12

• Cleft lip: failure of fusion of maxillary and medial nasal prominences
• Cleft palate: failure of palatine shelves to fuse
• Can be unilateral or bilateral
• Associated with maternal folic acid deficiency, corticosteroid use, phenytoin, alcohol
• Incidence: ~1/1000 live births

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Q3. What are the stages of Spermatogenesis and how does it differ from Oogenesis? [4+3=7 marks]

Stages of Spermatogenesis (4 marks)

• Golgi phase: pro-acrosomal granules coalesce to form acrosome vesicle
• Cap phase: acrosome cap spreads over nuclear surface
• Acrosomal phase: nucleus elongates, flagellum develops, mitochondria arrange in midpiece
• Maturation phase: excess cytoplasm shed, spermatozoon formed

Differences: Spermatogenesis vs. Oogenesis (3 marks)

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Q4. Explain the process of formation of the Notochord with its significance and fate [2+2+2=6 marks]

Formation (2 marks)

1. Epiblast cells migrate through the primitive node cranially in the midline as a cord of cells
2. They initially form a tube called the notochordal process (hollow), which extends toward the prechordal plate (oropharyngeal membrane)
3. The notochordal process fuses with the endoderm → notochordal plate (temporarily creates a communication - the neurenteric canal)
4. The notochordal plate detaches from the endoderm and rolls into a solid rod - the definitive notochord

Significance (2 marks)

1. Induces neurulation: notochord secretes signaling molecules (Sonic Hedgehog - SHH) that induce overlying ectoderm to form the neural plate
2. Defines the body axis: forms the central axis around which the vertebral column develops
3. Organizer function: produces floor plate of the neural tube and dorsal-ventral patterning signals
4. Somite specification: induces adjacent paraxial mesoderm to form somites
5. Separation of embryo layers: ensures endoderm and ectoderm separate properly

Fate (2 marks)

• The vertebral bodies form around the notochord during ossification
• The notochord at the level of each intervertebral disc persists as the nucleus pulposus (the central gelatinous core of each intervertebral disc)
• The nucleus pulposus acts as a shock absorber
• Clinical relevance: Nucleus pulposus herniation (prolapsed intervertebral disc / "slipped disc") causes pressure on spinal nerves → radiculopathy; the notochord can also give rise to chordoma, a rare malignant tumor

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Q5. Draw a Labeled Histological Diagram of the Fallopian Tube [5 marks]

LUMEN
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[1] MUCOSA (innermost):
     - Epithelium: simple columnar with two cell types:
         a) Ciliated cells - cilia beat toward uterus
         b) Secretory (peg) cells - nourish ovum
     - Lamina propria: loose CT with vessels
     - Highly folded (tall longitudinal folds) - especially prominent in ampulla
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[2] MUSCULARIS:
     - Inner circular layer
     - Outer longitudinal layer
     - (No distinct submucosa)
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[3] SEROSA (PERIMETRIUM):
     - Visceral peritoneum (mesothelium + CT)

1. Ciliated columnar epithelial cells (long cilia)
2. Secretory (peg) cells (intercalated between ciliated cells)
3. Mucosal folds (plicae) - tallest in ampulla, absent in isthmus
4. Lamina propria (loose connective tissue)
5. Inner circular smooth muscle layer
6. Outer longitudinal smooth muscle layer
7. Serosa (mesothelium)

• Infundibulum: widest; finger-like fimbriae; tall folds
• Ampulla: site of fertilization; most elaborate mucosal folds
• Isthmus: narrow; few, low folds; thick muscularis
• Intramural part: within uterine wall; minimal folds

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Q6. Describe the Development of Placenta with its Anomalies [3+2=5 marks]

Development of Placenta (3 marks)

• Syncytiotrophoblast invades decidua; lacunae form and fill with maternal blood → uteroplacental circulation begins

• Primary villi: solid cytotrophoblast cores covered by syncytiotrophoblast
• Secondary villi: mesoderm grows into primary villi core
• Tertiary villi: fetal blood vessels form within mesoderm core → functional gas exchange begins

• Villi on the embryonic pole proliferate (chorion frondosum = future fetal side)
• Villi on the abembryonic pole regress (chorion laeve = smooth chorion)
• Decidua basalis = maternal component (deep to chorion frondosum)
• Together: chorion frondosum + decidua basalis = placenta

• Disc-shaped, 15-20 cm diameter, ~500g
• Fetal surface: covered by smooth amnion; umbilical cord attaches
• Maternal surface: 15-20 cotyledons separated by decidual septa
• Umbilical cord: contains 2 umbilical arteries (deoxygenated blood) + 1 umbilical vein (oxygenated blood), embedded in Wharton's jelly

Anomalies (2 marks)

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Q7. Write Short Notes on Intraembryonic Mesoderm and its Subdivisions [3+2=5 marks]

Intraembryonic Mesoderm - Formation (3 marks)

• Epiblast cells migrate through the primitive streak (sides of streak), invaginate, and spread laterally between the ectoderm and endoderm
• They fill the space on either side of the notochord and neural plate as the intraembryonic mesoderm
• Prechordal plate (cranial) and cloacal membrane (caudal) remain as bilaminar plates (no mesoderm penetrates)

Subdivisions and Derivatives (2 marks)

• Forms paired blocks - somites (~42-44 pairs; formed days 20-30)
• Each somite differentiates into:
  • Sclerotome → vertebrae and ribs
  • Dermomyotome → Myotome (skeletal muscles of back, body wall, limbs) + Dermatome (dermis of skin)

• Connects paraxial and lateral plate mesoderm
• Forms the urogenital system: kidneys (pronephros, mesonephros, metanephros), gonads, genital ducts (Wolffian/Müllerian ducts)

• Splits into two layers with the intraembryonic coelom (body cavity) between them:
  • Somatic (parietal) layer: lines body wall; forms serous membranes parietal layer, bones/CT of limbs
  • Splanchnic (visceral) layer: surrounds gut tube; forms heart, smooth muscle of gut, serous membranes visceral layer, spleen
• The intraembryonic coelom becomes the pericardial, pleural, and peritoneal cavities

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Q8. Describe Paraxial Mesoderm and its Derivatives. What is the Source of its Origin? [marks]

Source of Origin

Formation of Somites

• Paraxial mesoderm condenses into somitomeres (cranialy around brain) and somites (in the trunk)
• Somites appear from day 20 onward at a rate of ~3 pairs/day (craniocaudal direction)
• By end of week 5: ~42-44 pairs (8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 8-10 coccygeal)
• Used clinically to estimate embryonic age

Differentiation of Somites

SOMITE
  ├── SCLEROTOME (ventromedial cells)
  │     → Vertebrae + ribs + intervertebral discs (surrounds notochord/neural tube)
  │
  └── DERMOMYOTOME (dorsolateral cells)
        ├── MYOTOME
        │     → Skeletal muscles:
        │         - Epimere (dorsal) → intrinsic back muscles
        │         - Hypomere (ventral) → muscles of body wall and limbs
        │
        └── DERMATOME
              → Dermis and subcutaneous tissue of trunk and limbs

Derivatives Summary

Paraxial Mesoderm vs. Other Mesodermal Subdivisions

• Cranial paraxial mesoderm (somitomeres): contributes to muscles of mastication, facial expression, extraocular muscles, and connective tissue of the head
• Controlled by Wnt, FGF, Notch-Delta signaling pathways that regulate segmentation clock (somitogenesis)

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Implantation (brief note)

• Occurs at day 6-7 post-fertilization in the posterior wall of the uterus (most commonly)
• Blastocyst attaches via trophoblast cells; syncytiotrophoblast invades decidua
• Human chorionic gonadotropin (hCG) is secreted from day 8-9, maintains corpus luteum
• Abnormal implantation: ectopic pregnancy (95% in fallopian tube ampulla) - can cause rupture and hemorrhage

Fertilization (brief note)

• Occurs in the ampulla of the fallopian tube
• Capacitation of sperm occurs in female genital tract (~7 hours)
• Acrosome reaction: sperm penetrates zona pellucida using acrosin (proteolytic enzyme)
• Cortical reaction: prevents polyspermy by zona reaction
• Result: zygote (2n); cleavage begins immediately

Monozygotic vs. Dizygotic Twins

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