Topic 5: Early Human Development

Question 1

Pre-embryonic period

Answer 1

first 2 weeks

1) Zygote
2) Morula
3) Blastocyst

Question 2

gestation period

Answer 2

from last menstrual period to birth (280 days)

Question 3

Zygote

Answer 3

(1 cell, diploid)

undergoes rapid mitotic divisions called cleavage divisions (cutting into bunch of smaller cells)

Question 4

Morula

Answer 4

72 hrs

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

— SAME size as ovum/zygote

Question 5

Blastocyst

Answer 5

5 days)

— features:
a) fluid filled cavity develops = blastocoel/blastocyst cavity
coel= hollow blast= generation of something new

b) embryoblast (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

Question 6

chorion

Answer 6

provides nutrients to

developing embryo

Question 7

implantation

Answer 7

blastocyst moves from uterine tube and attaches to endometrium of uterine wall

~ 5-7 days after fertilization

Question 8

Describe what happens after implantation

Answer 8

— embryoblast (inner cell mass) forms a 2-layered (bilayered) embryonic disc composed of:

a) Epiblast
b) Hypoblast

Question 9

Epiblast

Answer 9

will undergo mitosis to form:

i. amnion – with a fluid filled amniotic cavity
ii. 3 germ layers of the embryo

Question 10

Hypoblast

Answer 10

will undergo mitosis to form the yolk sac

Question 11

Embryonic period

Answer 11

— 3rd week to end of the 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

Question 12

Ectoderm

Answer 12

will form the nervous system and the epidermis of the skin

Question 13

Mesoderm

Answer 13

will form muscles, cardiovascular system, dermis, and most bones

Question 14

Endoderm

Answer 14

will form epithelial linings of digestive, respiratory, urinary and reproductive
systems and their associated glands

Question 15

During the embryonic period

Answer 15

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

Question 16

chorion

Answer 16

derived from trophoblast cells
— surrounds all embryonic membranes
- develops into placenta

Question 17

chorionic villi

Answer 17

projections of the chorion containing the blood vessels

that will form fetal part of placenta

Question 18

amnion

Answer 18

erived from epiblast

— forms amniotic cavity

Question 19

amniotic cavity

Answer 19

surrounds embryo / fetus with amniotic fluid

¦ function of fluid - shock absorption, temperature regulation

Question 20

yolk sac

Answer 20

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)

Question 21

allantois

Answer 21

out-pocket of yolk sac

— forms the umbilical cord and the urinary bladder

Question 22

Fetal period

Answer 22

— 9th week to end of 40th week

— growth and maturation or organs occurs