Question

What is notochord? What is its function?

Answer 1

Hint: The notochord is a flexible rod-like structure of mesodermal cells that is the primary longitudinal structural element of chordates and the early embryo of vertebrates, in which it plays an organisational role in nervous system development.

Complete answer:
The notochord is a flexible rod that runs through Chordate organisms and provides rigid support. The notochord is replaced by the vertebral column in the more derived Chordates, the Vertebrates, and becomes the cartilaginous substance between vertebrae.

The notochord is composed of a variety of structural molecules, including glycoproteins, and is similar to cartilage in many ways. When you cut the notochord and look at the cross section under a microscope, it looks like a series of concentric rings. These different sized rings that surround each other are notochord layers, and they are made of various structural molecules to give the notochord strength and elasticity.

The glycoproteins and other structural molecules protrude from the notochord's widely spaced cells. Each of these cells contains a large vacuole that can be pressurised. When the cells are under pressure, they push against one another and the surrounding structural materials. This results in a more rigid notochord, which is required for fast swimming.

Notochord Function:
The notochord's strength makes it an excellent structure for attaching muscles to. Muscles require attachment points in order to flex properly. Small invertebrates can swim using muscles all over their bodies by attaching muscles down the length of the notochord. Even in larger fishes that rely on a notochord, it provides enough support for the majority of their body's muscles to attach to.

The turgor pressure generated by the notochord's cells makes it extremely rigid. While this provided adequate support for many organisms, vertebrates took this body plan a step further by incorporating the spine. Because it is made of bone, the spine increases the rigidity of the notochord and protects the spinal cord by completely encasing it.

It has also been discovered that the notochord plays critical signalling roles during normal vertebrate embryogenesis. A notochord secretes proteins that promote the development of organ systems. This process, known as organogenesis, begins when the embryo is still a hollow ball of cells known as the gastrula. A notochord is derived from the middle layer, or mesoderm, of this tiny ball.

Once formed, the notochord begins to secrete a variety of chemical signals that aid in the process of organogenesis. Eventually, bones form and the spine begins to form. The notochord is sandwiched between these vertebrae and protects them from rubbing or smashing together.

Note: The bony vertebrae that grow around the spinal cord in vertebrates protect it on all sides. Because the spinal cord sits between the notochord and the skin, animals with only a notochord do not have this protection. To protect their spinal cord and notochord, animals such as the sturgeon and coelacanth have developed armoured plates and thick skin. Vertebrates convert the notochord into the cushioning intervertebral discs that keep the vertebrae from collapsing. However, by the time a human is four years old, the original notochord has been completely replaced in the spine with other materials.