Embryonic Period (week 3-8)

Question 1

Embryonic period

Answer 1

• all major organ systems begin to develop
• development of the cardiovascular system essential
• folding of embryo
  ⇒ craniocaudal bc CNS development
  ⇒ lateral folding bc somites etc
  ⇒⇒ 3D cylinder

Question 2

Gastrulation

Answer 2

• establishment of 3 germ layers ⇒ trilaminar embryonic disc
• first indicated by formation of primitive streak
  1. primitive streak: primitive groove + primitve node + primitive pit
• epiblast cells migrate to primitive streak
• some of migrating epiblast cells displace hypoblast ⇒ endoderm
• other epiblast cells laterally, cranial and along midline ⇒ mesoderm
• epiblast ⇒ ectoderm
  2. cloacal membrane: future anus, epiblast + hypoblast fuse
  3. intraembryonic mesoderm ⇒ various tissues + organs
  extraembryonic mesoderm ⇒ placental formation

Question 3

Paraxial mesoderm

Answer 3

• becomes organized into somitomeres in craniocaudal sequence
• Somitomere 1-7: contribute mesoderm to pharyngeal arches
• Rest of somitomeres: form 42-44 somites
• somites closest to caudal end disappear ⇒ 35 pairs of somites
• Somites ⇒ sclerotome, myotome, dermatome

Question 4

Intermediate mesoderm

Answer 4

• connects paraxial + lateral mesoderm

- forms Urogenital ridge ⇒ future kidneys and gonads

Question 5

Lateral mesoderm

Answer 5

• thin plate in which intraembryonic coelom forms and divides mesoderm into ⇒ somatic and visceral

Question 6

Notocord

Answer 6

• solid cylinder of mesoderm from primitive node to prochordal plate
• induces overlying ectoderm to differentiate into neuroectoderm ⇒ neural plate
• induces formation of vertebral bodies
• forms nucleus pulposus

Question 7

Cardiogenic region

Answer 7

• horseshoe shaped region of mesoderm
• in cranial end of trilaminar embryonic disc
• formation of future heart

Question 8

Ectoderm

Answer 8

• formation of future nervous system + epidermis of skin

Question 9

Endoderm

Answer 9

• formation of future digestive system, respiratory system, urogenital system and pharyngeal pouches

Question 10

Further development of Trophoblast

Answer 10

• Syncytiotrophoblast ⇒ lacunar network

- Cytotrophoblast ⇒ penetrates syncytiotrophoblast ⇒ outer cytotrophoblast shell⇒ attaches chorion to endometrial stroma

Question 11

Further development of Extraembryonic Mesoderm

Answer 11

A. Secondary chorionic villi
B. Angiogenesis
- occurs initially in visceral mesoderm around yolk sac , but after also in somatic mesoderm around connecting stalk ⇒ umbilical vessels + tertiary chorionic villi
- extraembryonic mesoderm ⇒ angioblasts⇒ aniogenic cell clusters
- angioblast at periphery of cluster ⇒ endothelial cell
C. Hematopoesis
- occurs in extraembryonic mesoderm of yolk sac in week 3
- angioblast in center of cluster ⇒ primitive blood cells
- week 5, hematopoiesis taken over by organs