Question

How is the notochord formed?

Answer 1

Hint: Notochord is a rod-like structure that is present in the embryonic stage of the organisms. It serves as the basis for the axial skeleton which is derived from the mesoderm. It grows in all types of organisms but is persistent only in chordates in future.

Complete answer:
The notochord is a flexible rod-like structure formed of mesodermal cells and is a principal longitudinal structure of the chordates that are present in the early embryo of the vertebrates where it plays an organizational role in the nervous system development. Later in the vertebrate, it becomes part of the vertebral column. It derives during the process of gastrulation mainly from the cells that migrate anteriorly in the midline that is present in between the hypoblast and the epiblast.

It plays an important role in signalling and in the coordination development in animals. It is found ventral to the neural tube. Notogenesis is the process of development of the notochord by the epiblast present in the amnion cavity. The notochord formed during the gastrulation soon induces the formation of the neural plate which later helps in the formation of the neural tube. On the ventral aspect, the endoderm becomes thickened which appears as a furrow and the margins of which anastomose (come into contact). It is then converted into a solid rod of polygonal-shaped cells which is later separated from the endoderm.

Invertebrates extends in the entire body of the vertebral column and then reaches as far as the anterior end of the midbrain where it ends up like a hook in the region of dorsum sellae of the sphenoid bone. As it originates from the primitive node and is placed within the mesodermal space, it is considered to be derived from the mesoderm.

Note: In humans, by the age of 4 all the notochord residue is replaced by chondrocytes like cells. The persistence of notochordal cells in vertebrates can cause pathologic conditions in humans. If the notochord does not get separated from the nasopharynx during the process of embryonic development, a depression or Tornwaldt cyst may get formed. It can cause a rare type of cancer in humans.