Neurulation and Pattern Formation

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Neurulation

During neurulation a broad central region of ectoderm thickens, rolls up into a tube and pinched off to form the neural tube. The neural tube will form the brain and the spinal cord.

Induction of neurulation

Neurulation of the epithelial cells is induced by the underlying notochord (made of mesoderm derived from the organizer).

If you transplant dorsal mesoderm to the ventral side, the ventral epithelium will form a second neural tube.

The neural crest

Along the midline of the nerual tube, ectodermal cells break loose to form the neural crest. The neural creast cells migrate to form the: peripheral nervous system, pigment cells in the skin, cartilage, bone and connective tissue in the head

Somotogenesis

On either side of the newly formed neural tube is a slap of mesoderm. The slab soon breaks into blocks called SOMITES. Pairs of somites form one after another from head to tail. Each soite roughly corresponds to one segment of the body.

If you transplant somites they demonstrate positive identity (they acquire their fate before developing). E.g transplanting thoracic forming region into cervical region, the cells will still develop as thoracic.

Mechanism to Assign Positional Identity are conserved

The mechanisms for ASSIGNING positional identity along the anterior-posterior axis can be completely independent from the interpretation of the position identity. The cellular mechanisms for conferring positional identity are conserved among different species even though the interpretations of the positional identities are very different(eg limbs vs wings)

Hox genes

The boundaries of Hox gene Expression in the somites determine the types of vertebrae. An insight into the function of the homeobox factors came from work on drosophila transcription. Mutation cause homeotic mutations that transform parts of the bod into structures appropriate fore other positions.

Antennapedia: legs sprout from head in place of antennae

Ultrabithrox: an extra pair of winds appears in the place where halteres should appear.

Bithorax:

Normal flies have wings that arise from the second thoracic fragment.

Loss of function mutations in Ubx cause a second pair of winds to form from the third thoracic segement( third thoracic segment has taken the position identity of the second) .

Gain of function in Ubx cause a loss of the normal winds in the seocnd thoracic segment (second thoracic segemnt has taken positional identity of the third).

The order of Hox genes is conserved

In mice, Hox

A10 is normally expressed in the lumbar region. If you overexpress

HoxA10, throughout the developing vertebral tissue, the cervical and thoracic regions will become lumbar in character and ribs will not form.

In the mouse, deleting Hox A10, Hox

C10, and Hox

D10 results in vertebrae that should be lumbar becoming thoracic,resulting in additional ribs.