Early Embryonic Development 3

Question 0

Describe the formation of the coelom

Answer 0

With differentiation of the mesoderm, intraembryonic coelom is formed. When the embryo undergoes lateral folding the lateral branches of mesoderm connect to form the coelom within

Question 1

Describe the differentiation of mesoderm into the notochord, somites, intermediate mesoderm, and lateral plate

Answer 1

Somites are segments of the paraxial mesoderm
There are 31 pairs in total (31 pairs of spinal nerves)
Organisation into somites gives rise to repeating structures e.g. vertebrae, ribs, intercostal muscles) and guides innervation

Question 2

List the derivatives of each region of the mesoderm

Answer 2

Paraxial - axial skeleton, dermis, muscles of axial body wall, some limb muscles
Somatic - connective tissue of limbs, contributes to axial body wall
Splanchnic - smooth musculature, connective tissue and vasculature of gut
Immediate - urogenital system e.g. kidneys, ureters, gonads

Question 3

Describe the formation and fate of the neural plate

Answer 3

Ectoderm thickens (slipper shaped neural plate) to create the neural tube

Question 4

Describe the formation of the head fold, tail fold and lateral body folds

Answer 4

Saggital cross section = cephalocaudal folding - heart lies at cranial end but moves inward until it lies in future chest, yolk sac extends out to form a connection between yolk sac and gut
Transverse section = lateral folding - folding draws together the margins of the disc

Question 5

What does folding achieve?

Answer 5

3D structure, creates ventral body wall, pulls amniotic membrane around disc so embryo becomes suspended in amniotic sac, pulls connective stalk ventrally, creation of a new cavity within embryo (coelom)

Question 6

Describe the differentiation of somites into dermatome, myotome, and sclerotome and the fate of each of these in later development

Answer 6

Somites appear as regular blocks of mesoderm cells arranged around a small cavity
This is followed by organised degeneration - the ventral wall of the somite breaks down, leading to the formation of the sclerotome (hard tissue - bone)
Further organisation of the dorsal portion forms the combined dermomyotome
Myotome (muscle) proliferates and migrates, dermatome (skin) disperses

Question 7

Explain the use of the terms ‘dermatome’ and ‘myotome’ in describing the innervation of skin and muscles in the adult

Answer 7

Developmentally:
Dermatome - part of somite that gives rise to the dermis
Myotome - part of somite that gives rise to muscle
Clinically:
Dermatome - strip of skin supplied by a single spinal nerve
Myotome - a group of muscles supplied by a single spinal nerve
Neural tube produces the nerves that innovate the dermomyotome