Embryology

Question 1

Name the three stages and their timings in embryology

Answer 1

Week 0-3 = conceptus/embryo
Week 3-8 = embryonic period
Week 9-40 = foetal period

Question 2

Name the phases of embryogenesis

Answer 2

1 - gametogenesis
2 - fertilisation
3 - cleavage
4 - gastrulation
5 - formation of body plan
6 - organogenesis

Question 3

What happens at fertilisation?

Answer 3

Fusion of sperm and oocyte forming diploid zygote

Question 4

What happens in cleavage stage?

Answer 4

Rapid mitotic cell division with no increase in size. Blastomere forms then morula (16+ cells) - cells confined within zona pellucida. Blastocyst develops around day 4-5 when fluid moves into cell mass and forms cavity

Question 5

What happens in gastrulation?

Answer 5

Epiblast cells move through primitive streak to form three germ layers (ecto, meso and endoderm). Body axes established. Epiblast becomes ectoderm.

Question 6

What happens in organogenesis?

Answer 6

Formation of organs and organ systems - basis of these all in place by end of embryonic period. Development continues through foetal period

Question 7

What happens in foetal period?

Answer 7

Growth and weight gain, tissue matures and becomes functional, overt sexual differentiation, bone laid down, CNS connections made

Question 8

What are the primary processes of development?

Answer 8

Cell division, differentiation, cell attachment, apoptosis, induction (ability of one cell type to cause another to differentiate), cell migration

Question 9

What are secondary processes of development?

Answer 9

Axis formation/polarity, folding/rotation

Question 10

How is development regulated?

Answer 10

Genetic (gene regulation) and epigenetic (preferential expression of maternal or paternal copy of gene) control

Question 11

Stages of fertilisation

Answer 11

1 - sperm binds with zona pellucida of oocyte
2 - acrosomal enzymes released from sperm - digests its way into egg
3 - egg and sperm plasma membranes fuse and sperm contents enters egg
4 - sperm triggers completion of meiosis 2 and release of cortical granules by oocyte to prevent polyspermy

Question 12

Where does fertilisation occur?

Answer 12

Ampulla

Question 13

Layers of blastocyst

Answer 13

Trophoblast round outside (single cell layer) - becomes foetal part of placenta
Inner cell mass - will become embryo

Question 14

What must happen before implantation can occur?

Answer 14

Blastocyst must hatch from zona pellucida

Question 15

When and where does implantation occur?

Answer 15

Typically by day 6. Occurs on posterior or anterior uterine wall.

Question 16

Describe what happens on day 7.5

Answer 16

Embryo organises into two cell layers - epiblast (dorsal surface) and hypoblast (ventral surface). Amniotic cavity begins to form. Trophoblast divides into cytotrophoblast and syncytiotrophoblast (produces hCG)

Question 17

Describe what happens on day 9

Answer 17

Amniotic cavity forms within epiblast and primitive yolk sac forms within hypoblast. Implantation is complete and endometrium is closed by fibrin coagulum. Vacuoles appear in syncytiotrophoblast and unite to form lacunae

Question 18

Describe what happens on day 12

Answer 18

Uteroplacental circulation is established - maternal blood flows into lacunae and diffuses to embryoblast. Extraembryonic mesoderm develops and degenerates, forming chorionic cavity

Question 19

Describe what happens on day 13

Answer 19

Further development of chorionic cavity and presence of connecting stalk. Second wave of hypoblast cells migrate to form definitive yolk sac

Question 20

Describe formation of primitive streak

Answer 20

Occurs in week 3. Appears in midline at caudal end of epiblast. Cranial end consists of primitive pit and node. Sets up body axis

Question 21

What does ectoderm form?

Answer 21

Epidermis, hair, nails and associated glands. Nervous system - brain, spinal cord, peripheral nerves

Question 22

What does mesoderm become?

Answer 22

Divides into paraxial, intermediate and lateral plate mesoderm. Lateral plate includes somatic and visceral layers.
Paraxial = axial skeleton, voluntary muscle, parts of dermis (back)
Intermediate = urogenital systems
Somatic lateral plate = most of dermis, lining of body wall, parts of limbs
Visceral lateral plate = CV system, mesothelial coverings of organs, smooth muscle

Question 23

What does endoderm become?

Answer 23

Lining of gut tube, respiratory tract, bladder and urethra