Early Embryonic Development 2

Question 0

Define the terms ‘mesoderm’, ‘primitive streak’, ‘gastrulation’, ‘trilaminar embryonic disc’

Answer 0

Mesoderm - middle germ layer giving rise to muscle, cartilage, vascular system
Primitive streak - a narrow groove that develops on the dorsal surface of the epiblast, has an important role in orientation, determining the front and back
Gastrulation - reorganising of the germ layers into ectoderm, mesoderm, endoderm and establishing all tissues of the body, starts with appearance of primitive streak
Trilaminar embryonic disc - three germ layers (ectoderm, mesoderm, endoderm)

Question 1

Define the terms ‘bilaminar disc’, ‘germ layer’, ‘ectoderm’, ‘endoderm’, ‘dorsal’ and ‘ventral surfaces’

Answer 1

Bilaminar disc - epiblast and hypoblast
Germ layer - layer from which other developed tissues originated from
Ectoderm - outermost layer of cells giving rise to epidermis, nervous system
Endoderm - innermost layer of cells giving rise to respiratory and GI tracts
Dorsal surface - outermost surface during early development
Ventral surface - innermost surface during early development

Question 2

List the most important ectodermal, mesodermal and endoderm all derivatives

Answer 2

Ectodermal - epidermis, skin derivatives (hair, nails, lens and cornea of eye, vagina and anal lining), nervous system, peripheral nerves, retina and iris of eye
Mesodermal - muscle, cartilage, bone, connective tissue, vascular system, urinary system
Endodermal - respiratory, GI tract, parenchyma of glands, lungs

Question 3

Describe the migration of mesoderm through the primitive streak

Answer 3

Cells on epiblast migrate towards primitive streak then invaginate and displace epiblast, creating a third layer (mesoderm)
As more cells migrate through they spread laterally and towards the head
Mesoderm spreads out, leaving two gaps (future mouth and future anus)

Question 4

Explain the role of the notochord

Answer 4

Has an important signalling role, drives neurulation, basis for axial skeleton, defines midline, regresses to nucleus pulposus of intervertebral disc

Question 5

How are the axes of the body defined?

Answer 5

Primitive streak defines front and back (anterior/posterior), primitive node emanates signals to ensure correct left/right and dorsal/ventral development