2011 (SBTE) 25th Annual Meeting Proceedings - International ...

R.C. Cheb
hebel.
el. 2011. Use of Applied Reproductive Technologies (FTAI, FTET) to Improve the Reproductive Efficiency in
Dairy Cattle. Acta Scientiae Veterinariae. 39(Suppl 1): s203 - s221.
The neural folds continue to elevate, appose
in the midline, and, eventually, fuse to create the neural
tube which becomes covered by the surface
ectoderm. Primary neurulation creates the brain and
most of the spinal cord, whereas in the tail bud, the
posterior neural tube is formed by secondary neurulation,
where the spinal cord initially forms as a
solid mass of epithelial cells, and a central lumen
develops secondarily by cavitation.
The primary neurulation is accompanied by
a bending of the neural plate, which occurs at three
principal sites: the median hinge point (MHP), overlying
the notochord, and the paired dorsolateral hinge
points (DHLP) at the points of attachment of the surface
ectoderm. The MHP is induced by signals from
the notochord.
Gradually, the neural folds approach each
other in the midline, where they eventually fuse.
Cellular protrusions extend from apical cells of the
neural folds as they approach one another in the
dorsal midline and interdigitate as the folds come
into contact. This allows a first cell-cell recognition
and provides an initial adhesion pending later
establishment of permanent cell contacts.
The subsequent fusion of the neural folds
begins in the cervical region and proceeds in a zipperlike
fashion anteriorly and posteriorly from there. As
a result of these processes, the neural tube is formed
and separated from the overlying surface ectoderm.
Until fusion is complete, the anterior and posterior
ends of the neural tube communicate with the
amniotic cavity via two openings, the anterior and
posterior neuropores. Closure of the neuropores
occurs at approximately Days 24 to 26 and Days 15
to 16 in cattle and pig, respectively; the anterior
neuropore one to 2 days prior to the posterior.
Neurulation is then complete. The central nervous
system is represented at this time by a closed tubular
structure with a narrow posterior portion, the anlage
of the spinal cord, and a much broader cephalic
portion, the primordium of the encephalon. During
neurulation, the neuroepithelium is entirely proliferative;
cells do not begin to exit the cell cycle and
start neuronal differentiation until after the neural tube
closure is complete.
During neurulation, cell proliferation is accompanied
by some degree of apoptosis in the neuroepithelium.
The rate of apoptosis appears to be
finely tuned and it seems to be equally detrimental if
the intensity of apoptosis is increased or decreased.
Apoptosis at the tips of the neural folds may serve a
special function. After opposing neural folds have
made contact and adhered to each other, midline
epithelial remodelling by apoptosis breaks the
continuity between the neuroepithelium and surface
ectoderm.
4.2 Neural crest
Along with the elevation and fusion of the
neural folds, certain cells at the lateral border or crest
of the neural folds become detached. This cell
population, known as the neural crest cells, will not
participate in formation of the neural tube; instead
they migrate widely and participate in the formation
of many other tissues, such as the integument
(melanocytes), other parts of the nervous system
(including neurons for the central, sympathetic and
enteric nervous system as well as glial and Swann
cells), and large parts of the craniofacial mesenchymal
derivatives [16].
The mechanism whereby the neural crest cells
detach from the neural folds is comparable with that
occurring during ingression of epiblast cells in the
primitive streak and node - a second example of
epithelio-mesenchymal transition. The term mesenchyme
refers to loosely organized embryonic tissue
regardless of germ layer origin. Thus, both neuroectoderm
(through the neural crest cells) and
mesoderm (at gastrulation) may give rise to mesenchyme.
The induction of neural crest cells is possibly
mediated by a gradient of BMP4, BMP7, and WNT
secreted by the surface ectoderm. In the chick and
pig, the neural crest cells express the transcription
factor PAX7 [2].
V. DEVELOPMENT OF THE PRIMORDIAL GERM CELLS
(PGCS)
The development of the germ line involves
specification of the cell linage and subsequent
migration of the individual cells through various embryonic
tissues to the final destination in the genital
ridges. After reaching the genital ridges, the cells of
the germ line integrate and initiate the final steps of
differentiation towards mature germ cells; a process
not completed until adulthood.
At the stage where the embryo presents a clear
primitive streak the OCT4 expression gradually
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