early human development Flashcards
three periods
pre-embryonic period
embryonic period
fetal period
pre-embryonic
first 2 weeks
stages : zygote, morula, blastocyst
zygote
a cell 2n
-undergoes rapid mitotic divisions called cleavage divisions
morula
72 hours
- a sold ball of 16-32 cells 9cells called blastomeres)
- same size as ovum/zygote
blastocyst
5days
- features:
1. fluid filled cavity develops=blastoceol/blastocyst cavity
2. Inner cell mass
3. trophoblast - blastocyst moves from uterine tube and attached to endometrium of uterine wall= implantation (5-7days after fertilization)
inner cell mass
cells will divide and differentiate (undergo changes to become more specialized) to form the embryo
trophoblast
made of trophoblast cells
-cells will divide and differentiate to form chorion
chorion
provides nutrients to developing embryo
-derived from trophoblast cells
surrounds all embryonic membranes
chorionic villi= projections of the chorion containing the blood vessels that will form fetal part of placenta
after implantation
inner cell mass forms a 2 layered (bilayered) embryonic disc composed of
a) epiblast
b) hypoblast
epiblast
will undergo mitosis to form:
- amnion-with a fluid filled amniotic cavity
- 3 germ layers of the embryo
hypoblast
will undergo mitosis to form the yolk sac
embryonic period
3rd to end of 8th week
- 3 germ layers are now present forming the embryo (each layer was formed from mitosis and differentiation of epiblast cells:
1. ectoderm
2. mesoderm
3. endoderm
ectoderm
will form the nervous system and the epidermis of the skin
mesoderm
will form muscles, cardiovascular system, dermis and most bones
endoderm
will form epithelial linings of digestive, respiratory, urinary and reproductive systems and their associated glands
during embryonic period
-major organ systems almost completely formed
-heart begins to beat
-limb buds differentiate
-embryonic membranes form:
chorion, amnion, yolk sac, allantois
amnion
derived from epiblast
- forms amniotic cavity- surround s embryo/fetus with amniotic fluid
- function of fluid-shock absorption, temperature regulation
yolk sac
derived from hypoblast
- forms part of gut (gastrointestinal tract)
- produces earliest blood cells and vessels
- source of primordial germ cells (reproductive cells= primary oocytes and primary spermatocytes)
allantois
out-pocket of yolk sac
-forms the umbilical cord and the urinary bladder
fetal period
9th week to end of 40th week
-growth and maturation of organs occurs
