and Neurenteric Canals
SIZE: 1.0 - 1.5 mm
The embryonic area is now shaped like a pear, and the head region is broader than the tail end.
The ectoderm has thickened to form the neural plate. The edges of this plate rise and form a concave area known as the neural groove. This groove is the precursor of the embryo's nervous system and it is one of the first organs to develop.
By stage 8, the blood cells are already developed and begin to form channels along side of the epithelial cells forming at the same time.
As the primary germ layers develop, cells lose their adhesive properties they detach from sheets of connected epithelial cells. Called the 'epithelial-mesenchymal transition', this process gives rise to the mesodermal layer of the embryo, a process that continues to occur many times in development as in cellular proliferation and tissue repair.
The reverse of this process is the 'mesenchymal-epithelial transition', or when loose cells of mesenchyme develop adhesive properties and arrange themselves into organized sheets of cells. Also common during development, and seen in kidney formation.
Drosophila and other invertebrates, have only a single hh gene. But vertebrates have a family of genes that are homologous to the hh gene: Sonic hedgehog (Shh), Indian hedgehog (Ihh), and Desert hedgehog (Dhh). All hedgehog genes encode for molecules involved in patterning in the creation of the embryo. Shh has been implicated as the key inductive signal in patterning the ventral neural tube, the anterior-posterior limb axis, and the ventral somites.
Shh is secreted from the notochord and different amounts of Shh cause different types of cells to be formed in the embryo (Gilbert, 2000).
|SHH sonic hedgehog
[ Homo sapiens (human)]
Gene ID: 6469, updated 5-Oct-2014, National Center for Biotechnology Information, U.S. National Library of Medicine 8600 Rockville Pike, Bethesda MD, 20894 USA
Gilbert, S.F. (2000).
Developmental Biology (Sunderland, Massachusetts: Sinauer Associates, Inc., Publishers).
Herzog, W. et al. (2003).
Adenohypophysis formation in the
zebrafish and its dependence
on sonic hedgehog.
Developmental Biology. 254. 1.
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