Week 2 of Human Development

Question 1

Further Development of Embryoblast

Answer 1

• embryoblast ⇒ dorsal epiblast layer and ventral hypoblast layer
  A. epiblast + hypoblast ⇒ bilaminar embryonic disc
  B. cleft within epiblast and coalesce ⇒ amniotic cavity
  C. hypoblast cells migrate to inner surface of cytotophoblast ⇒ exocoelomic membrane
• additional cells migrate along exocoelomic membrane ⇒ secondary yolk sac ⇒ portions of exocoelomic cavity pinched off ⇒ exocoelomic cysts
  D. at site of future mouth ⇒ hypoblast cells become columnar and fuse with epiblast ⇒ prochordal plate

Question 2

Syncytiotrophoblast

Answer 2

• outer multinucleated zone of trophoblast ⇒ no mitosis
• arises from cytotrophoblast
• invasion of endometrial stroma⇒ rods endometrial blood vessels and glands
  1. Lacunae form mostly at embryonic pole ⇒ maternal blood and glandular secretion
• form lacunar network
• uteroplacental circulation
  2. embryo blast receives nutrition by diffusion
  3. progressibe involvement of maternal blood vessels

Question 3

Cytotrophoblast

Answer 3

• mitotically active
  1. new cells migrate into syncytiotrophoblast ⇒ growth
  2. new cells produce primary chorionic villi

Question 4

Development of Extraembryonic mesoderm

Answer 4

A. New layer of loosely arranged cells, derived from epiblast
B. layer fills space between exocoelomic membrane and cytotrophoblast
C. large spaces in extraembryonic mesoderm and coalesce ⇒ extraembryonic coelom
D. extraembryonic coelom divides mesoderm ⇒ extraembryonic somatic mesoderm and visceral mesoderm
E. somatic mesoderm + cytotrophoblast + syncytiotrophoblast ⇒ chorion
F. extraembryonic coelom ⇒ chorionic cavity
G. embryoblast suspended by connecting stalk