Embryology 1

Question 1

Define congential disease

Answer 1

• Congenital disease, birth defect
• Structural or functional (e.g. metabolic disorder)
  defects
• Condition present at or before birth (regardless of
  cause)
• Genetic, infectious, nutritional and/or
  environmental in origin
• Occurs in approximately 3% of live birth

Question 2

Describe the stages of human embryology

Answer 2

> Conceptus: Fertilisation to end of 2nd week
Embryo: beginning of 3rd week to end of 8th week
Fetus: 3rd month to birth

Step 1: Ovulation
o A secondary oocyte is released from the ovary and
swept into the oviduct

Step 2: Fertilisation:
o A single sperm penetrates the secondary oocyte
o Eventually, the sperm and egg nuclei will fuse,
resulting in a fertilised egg or zygote

Step 3: Cleavage:
o The zygote undergoes rapid mitotic cell divisions as
it moves along the oviduct toward the uterus,
becoming a pre-embryo that consists of two cells,
then four cells, then eight cells and so on

Step 4: Morula
o By day 4, successive divisions produce a morula, a
solid ball of cells that enters the uterus

Step 5: Blastocyst
o By day 6, the pre-embryo becomes a blastocyst, a
hollow ball of cells with a fluid-filled cavity. The
blastocyst has freed itself from the zona pellucida
and can increase in size
–> Has two types of cells:
- Outer cell mass (epithelial layer) (called
trophoblast)
- Inner cell mass
- Trophoblast forms extraembryonic structures
(parts of the placenta)
- Between 5 to 10 days, blastocyst implants into the
uterine wall

Step 6: implantation
o The blastocyst attaches to the uterine lining
(endometrium) and begins to digest its way inward.
The cells of the inner cell mass begin to form
primary germ layers.

Question 3

Describe the bilaminar stage (Two Germ Layer)

Answer 3

• Inner cell mass splits into hypoblast and epiblast
• Cavities form (amniotic cavity and yolk sac)
• Forming bilaminar embryonic disc epiblast above
  and hypoblast below
• Outer cell mass splits into outer trophoblast and
  inner trophoblast

Question 4

Describe the trilaminar stage (gastrulation)

Answer 4

Gastrulation:
- Formation of primitive streak that defines all major
body axes
- Formation of the three germ layers: ectoderm,
mesoderm, endoderm
- Give rise to distinct tissues in adult

> Formation of primitive streak:
 On the upper surface of bilaminar disc (on epiblast)
a line of thickened cells appear (primitive streak)
 Primitive streak invaginates to form primitive
groove

• Cells of epiblast migrate medially and into primitive
  groove
• First cells move into hypoblast to form embryonic
  endoderm
• Later cells move into space between epiblast and
  endoderm become embryonic mesoderm
• Cells left in epiblast become embryonic ectoderm

Question 5

Briefly list what organs each germ layer will produce

Answer 5

Ectoderm: (outer layer)
o Outer surface: epidermal cells of skin
o CNS: neuron of brain
o Neural crest: pigment cell (melan-octye)

Mesoderm: (middle layer)
o Dorsal: Noto-chord
o Paraxial: Bone tissue
o Intermediate: Tubule cell of the kidney
o Lateral: red blood cells
o Head: Facial muscle

Endoderm: (internal layer)
o Digestive tube (stomach cell)
o Pharynx (thyroid cell)
o Respiratory tube (lung/alveolar cell)

Germ cells:
o Male (sperm)
o Female (egg)

Question 6

Explain how the internal asymmetry of organs is determined.

Answer 6

Primitive streak contains a primitive node which is fluid-filled, cells beneath this pit have cilia that are motile (rotate), they all rotate in the same direction)

Question 7

What is situs invertus condition?

Answer 7

• organs are mirrored from normal position
• Only some organs or partially mirrored (situs
  ambiguous or heterotaxis)
• Often associated with other problems (especially heart defects)

Question 8

Summarise the early development by a flow chart perspective, indicating which are the extra-embryonic and embryonic tissues.

Answer 8

Blastocyst
- Trophoblast
> Cytotrophoblast
> Syncytiotrophoblast
- Inner cell mass
1. Hypoblast (Primitive endoderm) –> extraembryonic
endoderm –> Yolk sac
2. Epiblast
- amniotic ectoderm
- (Embryonic tissues:)
> Embryonic epiblast (forms embryonic ectoderm)
>
> primitive streak
> embryonic mesoderm
> embryonic endoderm

Question 9

Describe the formation of the notochord and the condition teratomas

Answer 9

• Important for induction of neural tube
• Endoderm and mesoderm are formed by cells
  migrating through the primitive streak/groove
• Some cells also migrate through the primitive
  node/pit – these cells don’t form a “sheet” but a
  cartilage-like/rod-like structure that goes from the
  node to the cranial end and is within the
  mesoderm (because the cells still migrate through
  the primitive streak
• After the remaining cells form the endoderm and
  mesoderm, they start migrating in and joining the
  cells that form the notochord, the notochord then
  also starts to grow towards the caudal end –
  achieving a rod-like structure in the midline that
  goes from the cranial to caudal end
• Primitive streak regresses (declines)

Teratomas from primitive streak:
 The primitive streak should disappear
 Sometimes cells of primitive streak retained
 Most common tumour in newborns
 Benign, corrected by surgery

Question 10

Describe formation of neural plate and neural tube and what neural crest cells form

Answer 10

• Ectodermal cells above notochord thicken and
  differentiate
• Extension folding of the neural plate
• Convergence of neural folds
• Neural tube closure

1. Neural plate:
   - Induced by notochord
   - Cranial to primitive node
   - Ectodermal cells differentiate into thick plates of
   pseudostratified, columnar neuroepithelial cells =
   neuroectoderm
2. Neural tube:
   - Formation = neurulation
   - Invagination of neural plate = neural groove
   - Closure
3. Neural crest cells:
   - Epithelial cells: form sheets/tubes, characterised by
   cell-cell junctions
   - Mesenchymal cells: irregular, found in connective
   tissue (cartilage, bone, blood)
   - Derived from ectoderm, undergo epithelial-
   mesenchymal transition (EMT)
   > Give rise to:
   o Dorsal root ganglia
   o Enteric ganglia
   o Schwann cells
   o Melanocytes – pigment cells in skin
   o Sympathetic and parasympathetic ganglia
   o Dentine
   - Segmentation of neural tube:
   o Neural tube initially one cell layer thick and hollow
   o Cranial end starts to swell forming vesicles
   o Vesicles give rise to the brain
   o Remainder to spinal cord

Question 11

Briefly describe the development of skin

Answer 11

• Two layers: epidermis & dermis
• Epidermis: from embryonic ectoderm, colonised by
  melanocytes (neural crest) and Langerhans cells
  (immune cells from bone marrow)
• Dermis: Mesodermal (except face: neural crest)