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SIZE: 0.4 mm
TIME PERIOD: 16 days post-ovulation
Neurulation
In Stage 6, gastrulation began with the appearance of the primitive streak. In Stage 7, gastrulation continues with the formation of a new cell layer - the ectoderm - changing the two-layered disc into a three-layered disc.
Neural crest cells originate at the top of the neural tube and migrate extensively, differentiating into many cell types such as neurons, glial cells, pigmented cells of the epidermis, epinephrine producing cells of the adrenal gland, and various skeletal and connective tissues of the head. It appears that the fate of a migrating cell is largely determined by its final destination.
RhoB and Slug protein are the proteins now present in Stage 7 which promote migration. The loss of N-cadherin, a protein required for left-right asymmetry, also helps to initiate the migration of neural crests cells.
Gastrulation
Gastrulation in full force. Cells are migrating across the surface of the 2 layered embryonic disc and into the newly formed Primitive Streak. After migrating through the Primitive Streak, these new cells collect into a new 3rd layer - the endoderm.
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