5 - Early Human Development

Question 1

What are the three periods of human development, and when do they occur?

Answer 1

Pre-embryonic - 0-2weeks
Embryonic - 3-8 weeks
Fetal - 9-40 weeks

Question 2

What are cleavage divisions?

Answer 2

Quick no growth mitotic divisions of the zygote into the morula.

Question 3

Define morula

Answer 3

 solid ball of 16-32 cells (cells called blastomeres)

 SAME size as ovum/zygote

Question 4

Define a blastocyst and it’s parts before implantation

Answer 4

Structure after morula
features:
a) fluid filled cavity develops = blastocoel/blastocyst cavity
b) inner cell mass
 cells will divide and differentiate (undergo changes to become more specialized) to form the embryo
c) trophoblast (made of trophoblast cells)
 cells will divide and differentiate to form chorion - provides nutrients to developing embryo

 blastocyst moves from uterine tube and attaches to endometrium of uterine wall = implantation
 ~ 5-7 days after fertilization

Question 5

What are the parts of a blastocyst after implantation

Answer 5

 inner cell mass forms a 2-layered (bilayered) embryonic disc composed of:
a) Epiblast (epi means top – think epidermis)
 will undergo mitosis to form:
i. amnion – with a fluid filled amniotic cavity
ii. 3 germ layers of the embryo
b) Hypoblast (Hypo means below)
 will undergo mitosis to form the yolk sac

Question 6

What is the embryo?

Answer 6

The three germ layers created from the epiblast and surrounded by the amnion.

Question 7

What are the three germ layers, and what will they become?

Answer 7

1) Ectoderm
 will form the nervous system and the epidermis of the skin
2) Mesoderm
 will form muscles, cardiovascular system, dermis, and most bones
3) Endoderm
 will form epithelial linings of digestive, respiratory, urinary and reproductive systems and their associated glands

Question 8

What functions begin during the embryonic period?

Answer 8

 major organ systems almost completely formed
 heart begins to beat
 limb buds differentiate
 embryonic membranes form

Question 9

What are the embryonic membranes? What do they do?

Answer 9

a) chorion – 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
b) amnion – derived from epiblast
 forms amniotic cavity – surrounds embryo / fetus with amniotic fluid
 function of fluid - shock absorption, temperature regulation
c) 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 + primary spermatocytes)
d) allantois – out-pocket of yolk sac
 forms the umbilical cord and the urinary bladder

Question 10

What occurs during the fetal period?

Answer 10

 growth and maturation or organs occurs