Human Anatomy IV: Main Exam, 2 August 2018

Mount Kenya University · Department of Dental Sciences · Diploma in Dental Technology · DDS2112

Human Anatomy IV (DDS2112) past exam from Mount Kenya University 2017-2018 with model answers.

Instructions: Answer all questions

SECTION A

1. Define the following terms; (12 marks)

  1. a) Embryoblast
    Model answer
    • Embryoblast: The inner cell mass of the blastocyst that gives rise to the embryo proper.
  2. b) Meiosis
    Model answer
    • Meiosis: A specialized cell division that reduces chromosome number by half, producing four genetically diverse haploid gametes.
  3. c) Trophoblast
    Model answer
    • Trophoblast: The outer cell layer of the blastocyst that forms the placenta and mediates implantation.
  4. d) Zona pellucida
    Model answer
    • Zona pellucida: A glycoprotein-rich extracellular matrix surrounding the oocyte and early embryo, essential for sperm binding and protection.
  5. e) Cleavage
    Model answer
    • Cleavage: The rapid series of mitotic divisions of the fertilised egg that partition the cytoplasm into smaller cells without increasing overall size.
  6. f) Morula
    Model answer
    • Morula: A solid ball of ~16‑32 cells formed after several cleavage divisions, preceding blastocyst formation.

2. Briefly explain how blastocyst is formed (4 marks)

Model answer
  • Fertilisation produces a zygote (single totipotent cell).
  • The zygote undergoes rapid mitotic divisions (cleavage) forming a solid ball of cells called the morula.
  • Fluid begins to accumulate between cells of the morula, creating a cavity.
  • The cavity expands and the structure becomes a blastocyst with an outer trophoblast and an inner cell mass.

3. Explain the stages of cleavage (4 marks)

Model answer
  • Day 1‑2: Cleavage divisions produce 2‑cell, then 4‑cell, then 8‑cell embryos; cells remain totipotent.
  • Day 3: Morula forms as a compacted ball of ~16‑32 cells.
  • Day 4: Compaction intensifies; cells differentiate into outer trophoblast and inner cell mass.
  • Day 5‑6: Blastocoel cavity forms, giving the blastocyst.

4. Briefly explain how hypoblast and epiblast is formed (6 marks)

Model answer
  • The inner cell mass (ICM) of the blastocyst differentiates into two layers.
  • The epiblast forms on the dorsal side of the ICM and remains pluripotent.
  • The hypoblast (primitive endoderm) spreads ventrally beneath the epiblast.
  • The hypoblast lines the blastocyst cavity and contributes to the yolk sac.
  • The epiblast will give rise to the embryo proper and all three germ layers.
  • Interaction between epiblast and hypoblast establishes the embryonic‑extraembryonic axis.

5. Explain the first stage of embryonic development in cell division. (6 marks)

Model answer
  • Zygote (1 cell): Fertilisation completes the diploid genome.
  • 2‑cell stage: First mitotic division (cleavage) occurs ~24 h post‑fertilisation.
  • 4‑cell stage: Second cleavage produces four totipotent blastomeres.
  • 8‑cell stage: Third cleavage yields eight cells; compaction begins.
  • Morula (16‑32 cells): Further divisions create a solid ball of cells.
  • Blastocyst formation: Fluid cavity (blastocoel) appears, marking the transition to the first embryonic stage.

6. Explain the process of gastrulation (5 marks)

Model answer
  • Initiation: Epiblast cells migrate toward the primitive streak on the dorsal surface.
  • Ingression: Cells delaminate and move between the epiblast and hypoblast, forming the three germ layers.
  • Formation of endoderm: Ingressed cells replace the hypoblast to become the definitive endoderm.
  • Formation of mesoderm: Cells that remain between epiblast and endoderm differentiate into mesoderm.
  • Ectoderm retention: Epiblast cells that do not ingress become the ectoderm.
  • Result: A trilaminar embryo with distinct ectoderm, mesoderm and endoderm layers.

7. Briefly explain the mitotic(M) phase in cell cycle. (3 marks)

Model answer
  • Prophase: Chromosomes condense, spindle forms, nuclear envelope breaks down.
  • Metaphase: Chromosomes align at the cell equator.
  • Anaphase: Sister chromatids separate and move to opposite poles.
  • Telophase: Nuclear membranes re‑form around each set of chromosomes, chromosomes decondense.
  • Cytokinesis: Cytoplasm divides, producing two daughter cells.

SECTION B: Answer Two Questions Only

8. Discuss the development of fetal membranes and associated factors. (15 marks)

Model answer
  • Formation of the yolk sac: Early extra‑embryonic mesoderm splits to form the primary yolk sac, which later contributes to the chorionic villi.
  • Development of the amnion: The amniotic cavity appears by cavitation of the epiblast; the amnion encloses the embryo, providing a fluid‑filled protective environment.
  • Formation of the chorion: The chorionic plate forms from the extra‑embryonic mesoderm and trophoblast; it later fuses with the allantois to become the chorioallantoic membrane.
  • Allantois and its role: Outpouching of the hindgut forms the allantois, which contributes to the umbilical vessels and the fetal portion of the placenta.
  • Chorioallantoic membrane: Fusion of chorion and allantois creates a highly vascularised membrane for gas exchange and nutrient transfer.
  • Development of the yolk sac membrane: The secondary yolk sac persists, supplying early nutrition before placental circulation is established.
  • Factors influencing membrane development:
  • Genetic regulation: Homeobox genes (e.g., Hox, BMP) guide extra‑embryonic tissue differentiation.
  • Maternal hormones: hCG and progesterone support trophoblast proliferation and vascularisation.
  • Oxygen tension: Low early oxygen levels stimulate angiogenesis in the chorion.
  • Nutrient availability: Glucose and amino acids affect trophoblast growth and membrane integrity.
  • Clinical relevance: Abnormal membrane development can lead to conditions such as placenta previa, vasa previa, or premature rupture of membranes.
  • Timing: By the end of the 4th week the amnion, chorion, and yolk sac are distinct; the chorioallantoic membrane is functional by weeks 5‑6.

9. Discuss fertilization (15 marks)

Model answer
  • Location of fertilization: Typically occurs in the ampulla of the fallopian tube.
  • Capacitation of sperm: Biochemical changes in the female tract render sperm capable of penetrating the zona pellucida.
  • Acrosome reaction: Enzyme release (hyaluronidase, acrosin) enables sperm to bind to and digest the zona pellucida.
  • Sperm‑egg binding: Interaction between sperm surface proteins (e.g., IZUMO) and egg zona proteins (ZP2, ZP3).
  • Cortical reaction: Release of cortical granules from the oocyte prevents polyspermy by modifying the zona pellucida.
  • Fusion of pronuclei: Male and female pronuclei migrate and fuse to form the diploid zygote nucleus.
  • Activation of the oocyte: Calcium oscillations trigger completion of meiosis II and initiate embryonic development.
  • Timing: Fertilization occurs ~12‑24 h after ovulation; the zygote begins cleavage as it travels toward the uterus.
  • Factors affecting fertilization:
  • pH and ionic environment of the fallopian tube.
  • Presence of cervical mucus plug and its selective permeability.
  • Sperm count and motility; abnormalities reduce likelihood of successful fertilization.
  • Clinical relevance: Failures at any step can cause infertility; assisted reproductive technologies (IVF, ICSI) bypass specific barriers.

10. Discuss the development of skin and the appendages. (15 marks)

Model answer
  • Ectodermal origin: Surface ectoderm gives rise to epidermis, hair follicles, nails, and glands.
  • Formation of the epidermis: Single‑layered ectoderm thickens, proliferates, and differentiates into basal keratinocytes.
  • Stratification: Basal cells divide and move outward forming the stratum spinosum, granulosum, lucidum (palms/soles), and corneum.
  • Keratinisation: Production of keratin and loss of nuclei in the stratum corneum provide a waterproof barrier.
  • Melanocyte migration: Neural‑crest derived melanocytes migrate into the basal layer, imparting pigment.
  • Hair follicle development: Interaction between epidermal placode and underlying dermal papilla induces downgrowth forming the hair peg, which elongates into a follicle.
  • Sebaceous gland formation: Sebaceous glands arise from ectodermal cells at the base of hair follicles.
  • Sweat gland development:
  • Eccrine glands: Form as buds from the epidermis throughout the body.
  • Apocrine glands: Develop in axillary and genital regions after puberty.
  • Nail formation: Nail plate derived from the dorsal epidermis of the digit tip; nail matrix proliferates to produce keratinised nail plate.
  • Regulatory factors:
  • Growth factors: EGF, FGF, and BMP signaling guide epidermal proliferation and appendage patterning.
  • Transcription factors: p63, SOX9, and LEF1 are essential for hair follicle initiation.
  • Hormonal influence: Androgens stimulate sebaceous gland activity and apocrine gland development.
  • Clinical relevance: Disruptions lead to conditions such as ectodermal dysplasia, alopecia, or ichthyosis.

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