6T: Trilaminar Germ Disc

Question 1

Formation at Day 15

Answer 1

Primitive Streak Formation

• primitive node acts as “organizer” of gastrulation and as a signaling center to pattern germ layers
• extension caudally and then regression

Question 2

Formation at Day 15-16

Answer 2

Gastrulation/Trilaminar Germ Disc

gastrulation: formation of endoderm, mesoderm, and ectoderm
- occurs because of invagination of epiblast cells at primitive streak
- cell movement is influenced by FGF signaling

Question 3

notochord

Answer 3

• cord that forms cranial to caudal beginning at day 17 from the notochordal plate.
• lies just caudal to the prechordal mesoderm
• prenotochordal cells ingress through the primitive streak and migrate cranially to form the notochordal plate

Question 4

Establishment of the L/R axis

Answer 4

• left side is established first by release of Nodal and FGF8. SHH (sonic hedgehog) is expressed in the notochord and repressed lefty genes to establish the right side. The node also contains ciliated cells that set up fluid flow patters. Cilia defects lead to laterality defects in mice.

Question 5

Establishment of the D/V axis

Answer 5

BMP4 (Bone Morphogenic Protein 4) ventralizes the mesoderm (IM & LPM). Chordin (activated by Goosecoid), noggin, and folistatin (secreted by node and prechordal mesoderm) antagonize BMP4 and dorsalize cranial mesoderm to form the notochord and paraxial mesoderm. Too much Goosecoid (dorsalization) could possibly lead to 2 heads like in conjoined twins

Question 6

Establishment of the A/P axis

Answer 6

AVE (anterior visceral endoderm) acts with the primitive node as a second signaling region in the head.
- expresses genes essential for head formation and genes that inhibit primitive streak formation in the head
The cranial end is established before gastrulation

Question 7

How do axis determination effect:

a. conjoined twins
b. caudal dysgenesis

Answer 7

a. conjoined twins: Goosecoid (dorsalizes head mesoderm) helps regulate head development. Over or under expression results in head malformations. Over expression could possibly lead to two heads, like in conjoined twins
b. Caudal Dysgenesis: If Brachyury (dorsalizing gene) is absent, then there is decreased formation of the mesoderm and shorteninig of embryonic axis. This may result in fusion of limb buds (aka 1 central lower limb)

Question 8

Primary, Secondary, and Tertiary villi

Answer 8

Beginning of 3rd week
| Primary Villi: 2 trophoblast cell layers
| Secondary Villi: extraembryonic mesoderm core
| plus the two trophoblast cell layers
| Tertiary Villi: Capillaries present. Depletion of
V cytotrophoblast layer.
End of 3rd week

• villous system ready for heartbeat to start in the 4th week of development

Question 9

Teratogenesis and gastrulation

Answer 9

Beginning 3rd week (when gastrulation begins) the embryo is highly sensitive to teratogenic insult
Holoprosencephaly: increast alcohol consumption (before mom knows she’s pregnant) can lead to craniofacial defects
Caudal Dysgenesis (Sirenomelia): insufficient mesoderm formation = abnormal lower limbs, UG system, and lumbosacral vertebrae
Sinus Inversus: transposition of viscera in thorax and abdomen (possibly because of cilia defects?)
Sacrococcygeal Teratoma: not usually malignant. Cause by the primitive streak not going away. Contains all 3 germ layers