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Carnegie Stage 8 - First Trimester
Primitive Pit,
Notochordal Canal
and Neurenteric Canals


SIZE: 1.0 - 1.5 mm

TIME PERIOD: 17-19 days post-ovulation

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.

Sonic Hedgehog - shh
The Appearance of Sonic Hedgehog (shh)

The sonic hedgehog (Shh) gene got its name from the popular Sega Genesis video game. But the hh gene had already been named after a mutation in the gene which produced an embryo covered with pointy denticles making it resemble an actual hedgehog.

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).

Important Roles of Shh

• Purkinje neurons secrete shh to sustain the division of granule neuron precursor cells in the external granule layer diring cerebral development.

• Shh plays a role in a left-right axis patterns.

• Shh is involved in the separation of the single eye field into two bilateral fields. If the shh gene is mutated, the result is cyclopia, a single eye in the center of the face (Gilbert, 2000).

• Shh has been implicated in the growth of cutaneous appendages such as hair, feathers or scales. It is said to be the activator because it does not diffuse rapidly, while BMP4 and BMP2 are possible inhibitors.

• Shh is thought to effect the specification of the mesoderm in gut formation.

• Shh plays an important role in limb development, specifically effecting the zone of polarizing activity (ZPA).

• Shh specify motor neurons in early development by inducting certain transcription factors at different concentrations (see above explanation Shh functions in Early Development).

• By repressing Shh expression in the endoderm, the notochord initiates pancreatic development (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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