Embryology week 2 Moon: cell types, gastrulation, and neurulation

zona pellucida purpose

prevents early implantation and polyspermia

gastrulation

formation of the 3 primary germ layers

neurulation

formation of the neural tube

neural tube later becomes…

brain and spinal cord

epithelial cells

polarized, apical and basal surface, sit atop a basement membrane

mesenchymal cells

not polarized in ECM, surrounded by ECM, migrate through ECM

epithelial-mesenchymal cell transformation in adults

cancer metastases. Needs to be turned off at some point in development to avoid this.

ectoderm

outer layer that forms skin and nervous system

mesoderm

middle layer that forms muscles, the heart, and the skeleton

endoderm

inner layer that forms lining of the GI tract

how does gastrulation help form adult structures

it allows for complexity

when does gastrulation take place

weeks 3-4

what structure enables gastrulation

the primitive streak, which is formed by epiblast thickening in the midline embryo

how is the primitive streak formed?

from thickened epiblasts in the midline

what forms the mesoderm?

ingression of epiblasts in the midline

what forms definitive endoderm?

epiblasts that ingress through the primitive groove and replace hypoblasts

future caudal end of embryo

allantois and connecting stalk

future cranial end of embryo

prochordal plate

ingression of epiblast cells

loss adhesion molecules dip into ECM, epithelial to mesenchymal transformation to form mesoderm and endoderm, then transform back to epithelial.

migratory mesodermal cells

are a mesenchymal cell type

from what cell type do all three germ layers arise?

epiblasts

what do surface epiblasts become?

ectoderm

what do middle layer epiblasts become?

mesoderm

what do epiblasts that replace the hypoblasts become?

endoderm

where does the primitive streak begin and what must occur to fill the embryo with mesoderm

begins at the caudal end. Mesoderm has to migrate cranially along the midline (forming notocord) and laterally out from there.

why are there still hypoblasts at oropharyngeal and cloacal membranes and no mesoderm?

because the membrane has to die in order to form body cavities

what forms from the regression of the primitive streak?

notochord

where does the primitive streak terminate?

sacrococcygeal region

sacrococcygeal teratoma

tumor from lack of regression of primitive streak. It keeps forming germ layers

neurulation

process by which the neural tube is formed

when does the neural tube close

end of 4th week

neural crest cells

form as neural tube closes and become peripheral ganglia and schwann cells of the nervous system.