Topic 5: Early Human Development - Embryonic Period and Fetal Period Flashcards
What weeks are the embryonic period?
- 3rd week to the end of the 8th week
but note that by the zygote stage, it is still an embryo in the first 2 week!
What is the purpose of the embryonic period
- 3 germ layers are now present, forming the embryo
What are the 3 germ layers?
1) ectoderm
2) Mesoderm
3) Endoderm
How were the germ layers formed?
- each layer was formed from mitosis and differentiation of epiblast cells
What does the ectoderm form?
- the nervous system and epidermis of the skin
What does the mesoderm form?
- muscles, cardiovascular system, dermis, and most bones
What does the endoderm form?
- epithelial lining of the digestive, respiratory, urinary, and reproductive system and their associated glands
What parts of the embryo have formed in this period?
- major organ systems almost completely formed
- heart begins to beat
- limb buds differentiate
- embryonic membranes form
As previously stated, embryonic membranes form. Name these membranes. (4)
- chorion
- amnion
- yolk sac
- allantois
What does the chorion form? (3)
- derived from trophoblast cells
- surrounds all embryonic membranes
- chorionic villi
What is chorionic villi?
projections of the chorion containing the blood vessels that will form the fetal part of the placenta
What does the amnion form? (4)
- derived from epiblast
- forms the amniotic cavity
- amniotic cavity surrounds embryo/fetus with amniotic fluid
- the fluid’s function is shock absorption, temperature regulation
What does the yolk sac form? (4)
- derived from hypoblast
- forms part of the GI (gastrointestinal tract)
- produced earliest blood cells and vessels
- source of primordial germ cells
What are primordial germ cells?
- reporductive cells composed of primary oocytes and primary spermatocytes
How many weeks does the fetal period span?
- 9th to end of the 40th week
What is the Allantois?
Out of pocket yolk sac
What does the allantois form? (2)
Umbilical cord and urinary bladder
