During embryonic development of the neural tube, mesenchymal somites differentiate into different tissues such as the muscles, skin and cartilage (Gilbert, 2000). The somites are the triangular-shaped bodies that are positioned at both sides of the developing neural tube. The changes that occur in somites are triggered by different factors such as proteins which are generated by tissues that are situated nearby the somites. In the normal pathway, somites are expected to further develop into three regions.
The sclerotome is the differentiated mesenchymal somitic tissue that gives rise to the skeleton of the developing organism. This structure provides the axis of the animal. Another mesenchymal somitic tissue that is generated during differentiation is the myotome, which is known to produce the muscular tissues of the organism. The third mesenchymal somatic tissue that is produced after differentiation is the dermatome, which is also known as the epidermal tissue or skin of the developing organism.
When an additional notochord is grafted into an embryo on the lateral side of the neural tube, the neighboring somite that is situated between the grafted notochord and the original axis develops into only sclerotome. This result is generally due to the patterning signals that are released by the notochord which influences what type of tissue located in an adjacent region will result into after differentiation (Dessaud et al. , 2008). On another situation, when a segment of the normal notochord in an embryo is removed, the somites in that region develop only dermatome and myotome.
The same result is due to the influence of the proteins that are produced by the notochord, whereby it releases a specific protein, specifically sonic hedgehog that is responsible for the differentiation of somitic tissue (Marigo and Tabin, 1996). When isolated somitic tissue is incubated in culture with sonic hedgehog (shh), sclerotome-specific differentiation markers appear in the somitic tissue as a function of time and concentration. The specific protein that is responsible for the differentiation of somites is sonic hedgehog which is produced by the notochord and the cells of the neural tube of the developing embryo.
The presence of this protein induces the somite to differentiate into sclerotome cells. There are also other proteins that play specific roles in neural tube development. Another protein is Pax1 which is a transcription factor that induces somitic tissue to transform into cartilage, which in turn, eventually changes into the vertebral column. Another reason why the somitic tissue follows an axial orientiation of dorsal and medial positioning is due to the effect of another protein produced by the neural tube, called neurotrophin 3 (NT-3).
The production of NT-3 by the neural tube cells influences the identification of the epaxial and hypaxial regions of the dermatome. Consequently, the epaxial region of the dermatome is influenced by the proteins Wnt1 and Wnt3, as well as sonic hedgehog into further differentiation as well as migration to their ultimate location in the neural tube of the developing embryo. The epaxial region eventually forms the muscles of the back side of the developing embryo, while the hypaxial region consequently differentiates into the muscles that are found between the rib bones.
Neural tube development in an embryo thus involves the differentiation of specific regions of the embryo and this is mainly influenced by specific genes that are producing proteins that convey signals that are associated with what type of tissues will be formed in each specific region of the embryo. In addition, neural tube development also involves the migration of specific mesenchymal tissues in order to position themselves at their ultimate destination when development of the embryo has been completed.
Among the most important proteins involved in embryonic development is sonic hedgehog because it signals the somitic tissue to differentiate in relation to its position and distance to the notochord.
References
Dessaud E, McMahon AP, Briscoe J (2008): Pattern formation in the vertebrate neural tube: a sonic hedgehog morphogen-regulated transcriptional network. Development 135:2489-503. Gilbert S (2000): Developmental Biology, 6th ed. New York: Sunderland. Marigo V and Tabin CJ (1996): Regulation of Patched by Sonic hedgehog in the developing neural tube. Proc. Natl. Acad. Sci. USA 93:9346-9351.