Early development and Somites (Anatomy)

Question 1

When does gastrulation happen and explain it…

Answer 1

• 3^rd week of development coincides with the week following first missed menstrual period. At approx. 5 weeks, a normal pregnancy can be detected on USS
• Gastrulation is the formation of a trilaminar embryo from a bilaminar embryo

Question 2

Explain the bilaminar embryo

Answer 2

• Formed at the end of the 2^nd week (week of 2s)
• Embryoblast differentiates into bilaminar embryonic disc
• Epiblast – adjacent to amniotic cavity
• Hypoblast – adjacent to blastocystic cavity
• Prechordal plate – future site of mouth

Question 3

Where do the 3 layers of the trilaminar embryo come from?

Answer 3

• Formation of the primitive streak in epiblast
• Primitive streak grows cranially – caudally
• At cranial end is the primitive node,** the **primitive groove stretches from the node
• Epiblastic mesenchymal cells pass through primitive streak to enter into deeper regions of developing embryo (ingression, invagination)

Question 4

What is the fate of the germ layers (what do they become)?

Answer 4

Ectoderm

• Epidermis
• CNS & PNS
• Eyes and internal ears
• Connective tissues structures of head

Endoderm

• Epithelial linings
• Glandular cells

Mesoderm

• Everything else!!!!
• Including bones and muscles

Question 5

What is the notochodal progress and notochord?

Answer 5

• Some mesenchymal cells migrating through the primitive node extend cranially to form notochordal process
• Notochordal process** extends to **prechordal plate
• Notochordal process acquires a lumen
• Floor of notochordal plate obliterates – notochordal canal is not continuous with umbilical vesicle
• What remains of the notochordal plate proliferate and undergo infolding to form notochord

Question 6

What is the fate of the notochord?

Answer 6

• Important
  
  • Defines the longitudinal axis
  • Signals development of axial MSC structures and CNS
• Remnants – nucleus pulposus of IV disc
• Vestigial remnants can lead to development of chordomas

Question 7

Explain somites

Answer 7

• Develop from paraxial mesoderm
• Paired cuboidal blocks that form lateral to the developing neurtube
• 38 pairs between days 20-30, 42-44 by end of 5^th week
• First somites appear in future occipital region. Develop craniocaudally
• Give rise to most of axial skeleton and musculature and adjacent dermis of skin