Human Anatomy IV: Main Exam, November 2017

Mount Kenya University · Department of Dental Sciences · Diploma in Community Oral Health · DDS1305

Human Anatomy IV past paper from Mount Kenya University 2017-2018 with model answers for students.

ANSWER ALL QUESTIONS

SECTION A (30 MARKS) (30 marks)

1. Define the following terms;

  1. a) Mitosis
    Model answer
    • Mitosis: a type of cell division that results in two genetically identical daughter cells, each with the same number of chromosomes as the parent cell.
  2. b) Gastrulation
    Model answer
    • Gastrulation: the embryonic process during which the single‑layered blastula reorganises into a multilayered structure (gastrula) forming the three primary germ layers (ectoderm, mesoderm, endoderm).
  3. c) Zygote
    Model answer
    • Zygote: the single cell formed by the fusion of a sperm and an ovum, containing a complete diploid set of chromosomes.
  4. d) Cleavage
    Model answer
    • Cleavage: the series of rapid, mitotic cell divisions of the zygote that partition its cytoplasm into smaller cells (blastomeres) without overall growth.
  5. e) Notochord
    Model answer
    • Notochord: a flexible, rod‑like midline structure derived from mesoderm that provides axial support in the embryo and signals the development of the neural tube.

2. Draw a well labeled diagram of a blastocyst.

Model answer
  • Diagram description: a spherical structure showing the outer trophoblast layer surrounding a fluid‑filled cavity (blastocoel) with an inner cell mass attached to the trophoblast.
  • Labels to include:
  • Trophoblast (outer layer)
  • Blastocoel (central cavity)
  • Inner cell mass (embryoblast)
  • Zona pellucida (remnant outer membrane, if shown)
  • Polar trophoblast (region opposite the inner cell mass)
  • Mural trophoblast (region adjacent to the zona pellucida)
  • Amniotic cavity (future cavity forming from inner cell mass)
  • Yolk sac (early extra‑embryonic membrane, if depicted)

3. Development of circulatory system occurs in the mesoderm which differentiates into blood islands.

  1. a) Define blood islands
    Model answer
    • Blood islands: clusters of mesodermal cells in the yolk sac that differentiate into primitive blood cells (angioblasts) and endothelial cells forming the first blood vessels
  2. b) Explain the derivatives of blood islands
    Model answer
    • Derivatives of blood islands
    • Hematopoietic cells → primitive erythrocytes, myeloid cells, and megakaryocytes
    • Endothelial cells → lining of early capillaries and vasculature of the yolk sac
    • Formation of the primary capillary plexus (vasculogenesis)
    • Contribute to the development of the dorsal aorta and vitelline vessels

4. Briefly explain two parts of cell cycle.

Model answer
  • G1 phase: cell grows, synthesises proteins and organelles, prepares for DNA replication
  • S phase: DNA replication occurs, chromosomes duplicated
  • G2 phase: further growth, checks DNA integrity, prepares for mitosis
  • M phase: mitosis (prophase, metaphase, anaphase, telophase) and cytokinesis

5. Explain four organs that are present in fetal membrane.

Model answer
  • Amnion: encloses the amniotic cavity, provides a fluid‑filled protective environment for the embryo
  • Chorion: outer fetal membrane, contributes to formation of the placenta and exchange of gases
  • Yolk sac: early source of nutrients and site of primitive blood formation (blood islands)
  • Allantois: forms the umbilical cord and contributes to the fetal portion of the placenta

6. Briefly differentiate between cell division and cell cycle.

Model answer
  • Cell division: the actual physical process of splitting one cell into two daughter cells (mitosis or meiosis)
  • Cell cycle: the entire series of events that a cell undergoes from one division to the next, including interphase (G1, S, G2) and the mitotic phase

7. Explain briefly on;

  1. a) Early cleavage
    Model answer
    • Early cleavage: rapid, synchronous mitotic divisions of the fertilised ovum without overall growth, producing smaller cells (blastomeres) that form the morula
  2. b) Late cleavage
    Model answer
    • Late cleavage
    • Occurs after the embryo reaches the blastula stage, typically at the morula to early blastocyst transition.
    • Cell divisions become asynchronous, with some cells dividing faster than others.
    • Blastomeres become smaller and begin to differentiate, forming inner cell mass and trophoblast layers.
    • Cell polarity increases, leading to the establishment of the embryonic axis.
    • Gap junctions and adhesion molecules increase, facilitating communication between cells.
    • More answers:
    • Cytoplasmic determinants are redistributed, influencing cell fate decisions.
    • The rate of cleavage slows compared to early rapid divisions.

8. Explain the function of vitelline membrane.

Model answer
  • The vitelline membrane surrounds the ovum and separates the plasma membrane from the yolk.
  • Provides mechanical protection against physical damage.
  • Acts as a barrier to polyspermy by participating in the cortical reaction after fertilisation.
  • Maintains the shape of the egg and supports the embryo during early cleavage.
  • Facilitates gas exchange and nutrient diffusion between the external environment and the oocyte.
  • More answers:
  • Serves as a site for sperm binding in some species.
  • Contributes to the formation of the chorion during later embryonic development.

SECTION B: (30 MARKS) ANSWER ANY TWO QUESTIONS (30 marks)

1. Discuss on cell cycle.

Model answer
  • Cell cycle phases
  • G1 phase: cell growth, synthesis of proteins and organelles.
  • S phase: DNA replication producing identical sister chromatids.
  • G2 phase: further growth, preparation for mitosis, synthesis of mitotic proteins.
  • M phase (mitosis): division of nucleus (prophase, metaphase, anaphase, telophase) followed by cytokinesis.
  • Regulatory checkpoints
  • G1 checkpoint: assesses DNA integrity before replication.
  • G2 checkpoint: ensures DNA replication is complete and undamaged.
  • Metaphase (spindle) checkpoint: verifies proper chromosome attachment to spindle fibers.
  • Key regulators
  • Cyclins (D, E, A, B) bind to cyclin‑dependent kinases (CDKs) to drive phase transitions.
  • CDK inhibitors (p21, p27) can halt cycle progression.
  • Tumour suppressor proteins (p53) trigger cell‑cycle arrest or apoptosis in response to DNA damage.
  • More answers:
  • Role of retinoblastoma protein (Rb) in G1‑S transition.
  • Importance of growth factors and extracellular signals in stimulating cell‑cycle entry.

2. Discuss formation of tubes.

Model answer
  • Primary tube formation (neurulation)
  • Neural plate folds to create the neural groove; edges elevate to form neural folds.
  • Neural folds converge and fuse dorsally, forming the neural tube.
  • Secondary tube formation (gut tube)
  • Endodermal cells roll inward at the primitive streak, creating the archenteron (primitive gut).
  • The archenteron elongates and differentiates into foregut, midgut and hindgut.
  • Other tubular structures
  • Somites segment and give rise to the notochordal tube, providing axial support.
  • Branching morphogenesis of lung and kidney tubules from epithelial buds.
  • More answers:
  • Formation of the otic and optic vesicles as outpouchings of the neural tube.
  • Development of the ureteric bud forming the collecting duct system.

3. Discuss stages of embryogenesis.

Model answer
  • Fertilisation – union of sperm and oocyte forming a zygote.
  • Cleavage – rapid mitotic divisions producing a morula and then a blastocyst.
  • Implantation – blastocyst attaches to uterine wall and embeds within endometrium.
  • Gastrulation – formation of three germ layers (ectoderm, mesoderm, endoderm) via the primitive streak.
  • Organogenesis – differentiation of germ layers into specific organs and tissues.
  • Growth and maturation – fetal growth, organ refinement, and preparation for birth.
  • More answers:
  • Formation of the extra‑embryonic membranes (amnion, chorion, yolk sac).
  • Development of the placenta and establishment of maternal‑fetal circulation.

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