Download LECTURE Ectoderm, neurulation

Developmental Biology
Neurulation
Reading: editions. 6,7,8: chap. 12 | edition 9:chap. 9 | edition 10: chap. 10.
+ selected pages relevant to neural crest topics we cover.
**Gastrulation made a three layered embryo- endoderm, ectoderm, mesoderm
A. In vertebrates, the next major event is an inductive one - called Neurulation
1. A mesodermal tissue, the notochord, directs the adjacent ectoderm to form the neural
tube- this is true for fish, amphibians, reptiles, birds and mammals, and chordates
a. The notochord directly underlies the most dorsal part of the embryo
b. The ectoderm overlying the notochord forms the neural tube which will become
the brain and central nervous system
c. The remaining ectoderm, from which the neural tube pinched off, becomes skin,…
d. Cells at the point of fusion of neural folds become the migrating neural crest cells
2. the regions of neural plate, neural folds, neural groove, and neural tube describe the
structures seen in vertebrates
a. Neurulation proceeds from anterior to posterior of the embryo (especially evident
in birds and mammals (just like in gastrulation)
b. somites form along side the notochord from the adjacent mesoderm
c. rostal (posterior) portions of the neural tube form by a secondary mechanism by
which ectodermal cells "sink" as a cord, then undergo "hollowing"
d. the ends of the neural tube close last. In mammals, the failure of these closures
cause specific anomalities, well known as anomalities in humans
I. failure of posterior neuropore closure causes spina bifida in humans
(quite common, of varying severity of consequences)
II. failure of anterior neuropore closure causes anacephaly, which is fatal
& causes severe forebrain degeneration - 1/1000 human pregnancies
3. The neural tube induction is called primary embryonic induction (historically)
a. The dorsal vegetal cells (in the blastula) induce the dorsal marginal cells to
become the future notochord and somites (mesoderm)
b. the underlying endoderm enables the notochord to have inducing activity
c. the notochord (in the gastrula) then induces the overlying ectoderm to become the
neural tube ("primary induction" proper)
d. this series of inductions can be shown by transplantation experiments of the
amphibian & bird organizer (expt’s of Spemann &Mangold, Waddington)
4. The mechanism of formation of the neural tube is by change in cell shapes as mediated by
actin microfilaments and tubulin microtubules
a. The microtubules cause the cells to elongate and become columnar
b. The actin filaments at the apical surfaces of the cells then contract, and cause the
curvature of the neural tube by a "purse string" like tightening movement
B. The neural tube becomes the central nervous system, with different portions becoming different
parts of the brain and spinal cord
1. The optic vesicle contacts the epidermis and causes a secondary induction of ectoderm
to become the lens and eye
C. The neural crest cells dissociate and distribute throughout the body and become pigment cells
and a large variety of nerve cells
1. They migrate along ECM tracts (largely directed by fibronectin)
2. they are pluripotent nerve precursors, and their identity is determined by location in
the embryo, and by the ‘history’ of their sequential interactions en-route