neurulation

Neurulation

refers to notochord-induced transformation of ectoderm into nervous tissue. The process begins during the third week in the region of the future brain and then progresses caudally into the region of the future spinal cord.

Neuroectoderm

ectodermal cells overlaying the notochord become tall columnar (--); they form a thickened area designated the neural plate. The other ectodermal epithelium is flattened.

neural plate

Neuroectoderm form a thickened area designated the .

Flattened

The other ectodermal epithelium is

neural groove

is formed as edges of the neural plate become raised on each side of a midline depression. (Apical ends of individual neuroectodermal cells constrict.)

neural tube

is then formed as the neural groove undergoes midline merger of its dorsal edges. The tube separates from non-neural ectoderm which unites dorsal to it (Tube formation begins in the cranial cervical region of the central nervous system and progresses cranially and caudally until anterior and posterior neuropores, the last openings, finally close.)

neural crest

bilaterally, where the neural groove is joined to non-neural ectoderm, cells detach as the neural groove closes; the cells proliferate and assume a position dorsolateral to the neural tube—forming

Neural tube

becomes the central nervous system, i.e., the brain and spinal cord.

Neural crest cells

are remarkable for the range of structures they form. Some cells migrate dorsally and become pigment cells in skin. Other cells migrate ventrally and become neurons and glial cells of the peripheral nervous system, or adrenal medulla cells.

mesenchyme (ectomesenchyme)

In the head, neural crest forms --- which becomes meninges, bone, fascia, and teeth.

• Mesoderm blocks

located just lateral to the notochord, which induced somite development.

• A pair of somites

develops for every vertebra, plus a half dozen somite pairs in the head.

• Number of somites

in an embryo is indicative of age, individual somites develop chronologically, in craniocaudal order.

paraxial mesoderm

accumulates on each side of the notochord

rostral to caudal over time

transverse fissures divide the paraxial mesoderm into blocks

Somite

each block becomes a (epithelioid cells within a somite block re-orient 90°, from transverse to the notochord to longitudinal)

head (occipital) somites

develop from proliferation of local mesenchyme lateral to the cranial end of the notochord

Somitomeres

rostral to the notochord, mesenchyme forms less-developed somites, called --; these migrate into pharyngeal arches and form muscles of the jaw, face, pharynx, & larynx.

Sclerotome

ventromedial region) gives rise to vertebrae, ribs, and endochondral bones at the base of the skull.

Dermatome

(lateral region) gives rise to the dermis of skin

Myotome

(intermediate region) gives rise to skeletal muscles of the body