Embryology Flashcards
Name the three stages and their timings in embryology
Week 0-3 = conceptus/embryo
Week 3-8 = embryonic period
Week 9-40 = foetal period
Name the phases of embryogenesis
1 - gametogenesis
2 - fertilisation
3 - cleavage
4 - gastrulation
5 - formation of body plan
6 - organogenesis
What happens at fertilisation?
Fusion of sperm and oocyte forming diploid zygote
What happens in cleavage stage?
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
What happens in gastrulation?
Epiblast cells move through primitive streak to form three germ layers (ecto, meso and endoderm). Body axes established. Epiblast becomes ectoderm.
What happens in organogenesis?
Formation of organs and organ systems - basis of these all in place by end of embryonic period. Development continues through foetal period
What happens in foetal period?
Growth and weight gain, tissue matures and becomes functional, overt sexual differentiation, bone laid down, CNS connections made
What are the primary processes of development?
Cell division, differentiation, cell attachment, apoptosis, induction (ability of one cell type to cause another to differentiate), cell migration
What are secondary processes of development?
Axis formation/polarity, folding/rotation
How is development regulated?
Genetic (gene regulation) and epigenetic (preferential expression of maternal or paternal copy of gene) control
Stages of fertilisation
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
Where does fertilisation occur?
Ampulla
Layers of blastocyst
Trophoblast round outside (single cell layer) - becomes foetal part of placenta
Inner cell mass - will become embryo
What must happen before implantation can occur?
Blastocyst must hatch from zona pellucida
When and where does implantation occur?
Typically by day 6. Occurs on posterior or anterior uterine wall.
Describe what happens on day 7.5
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)
Describe what happens on day 9
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
Describe what happens on day 12
Uteroplacental circulation is established - maternal blood flows into lacunae and diffuses to embryoblast. Extraembryonic mesoderm develops and degenerates, forming chorionic cavity
Describe what happens on day 13
Further development of chorionic cavity and presence of connecting stalk. Second wave of hypoblast cells migrate to form definitive yolk sac
Describe formation of primitive streak
Occurs in week 3. Appears in midline at caudal end of epiblast. Cranial end consists of primitive pit and node. Sets up body axis
What does ectoderm form?
Epidermis, hair, nails and associated glands. Nervous system - brain, spinal cord, peripheral nerves
What does mesoderm become?
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
What does endoderm become?
Lining of gut tube, respiratory tract, bladder and urethra
