Notochord

The notochord is a midline structure that develops in the midline between the primitive streak and prochordal plate. It develops from the primitive node of the primitive streak.

The notochord is the primitive beginning of the backbone and it is a flexible rod-like structure. The notochord is only found in chordates. Chordata is a phylum that includes humans. In human postnatal life, a part of notochord remains in the discs present in our back as a gelatinous material.

To understand the development of notochord is essential for the students because it forms the central axis of the embryonic disc and induces the formation of the neural tube.

Formation of notochord

The notochord develops from the primitive node of the primitive streak. The various stages through which the notochord develops are

1. The cells of the primitive knot of the primitive streak proliferate and form pre notochordal cells that move inward and produce a central depression called blastopore. From the bottom of the blastopore, the pre notochordal cells of the primitive knot migrate forward in the midline between the ectoderm and endoderm to form a solid cord of cells called the notochordal process.

2.  The notochord extends caudally that increases their length as the primitive streak recedes casually and regress. A canal is formed in the notochordal process called the notochordal canal that is continuous with the blastopore od the primitive streak.

3. At this stage, the floor of the notochordal canal fuses with the endoderm that is a germ layer of the embryo.

4.  At this stage, the fused part of the notochordal canal (wall of the notochordal canal and part of the endoderm) breakdown. Due to the breakdown of the fused part the notochordal canal now communicates with the yolk sac at one end and with the amniotic cavity at the other end. At this stage, the amniotic cavity and yolk sac are in communication with each other.

5.  Gradually the walls of the notochordal canal become flattened and form a flattened plate called notochordal plate.

6. The notochordal plate becomes curved and forms a tube.

7.  The cells of the tube proliferate and convert it into a solid cord of cells that form a definitive notochord.

8.  At this stage, the endoderm is restored and now the notochord is completely separated from the endoderm.

Function of notochord

The functions of the notochord are

1. it forms the central axis of the developing embryo mean embryonic disc.
2.  It induces the formation of the neural tube from the overlying ectoderm.
3. It provides a central column around which vertebral bodies and intervertebral discs develop.

The fate of the notochord

It is present in all the animals that belong to the phylum Chordata. Humans also belong to the phylum Chordata. In human beings, the notochord appears only in the embryo in prenatal life. In postnatal life, it disappears but its remnants are seen in the form of nucleus pulposus of the intervertebral discs and apical ligament of dens of the second cervical vertebra.

Chordoma

This is a tumor that arises from the remnants of the notochord. It is a rare and slow-growing nucleoplasm. It is formed either in the cranial region or in the sacral region. In the cranial region, it is seen at the base of the cranium and has the tendency to spread into the nasopharynx. It commonly occurs in men over the age of 50. About 30% of these tumors are malignant.

Difference between the notochord and nerve cord

Notochord and nerve cord are the two basic features of the animals that belong to the phylum Chordata. Both the notochord and nerve cord are present from neck to tail in the dorsal part of the chordate body. The notochord is a rod-like structure while the nerve cord is a solid strand of nervous tissue.

The main difference between these two structures is that the notochord belongs to the chordate skeleton while the nerve cord belongs to the central nervous system.

The notochord occurs between the CNS and nerve cord while the nerve cord occurs below the notochord.

What is neurulation?

Neurulation is the process in which the folding of the embryo occurs. In neurulation, the neural plate is transformed into the neural tube after which the embryo is called neurula.

The process of neurulation starts when the notochord induces the formation of CNS by stimulating the ectoderm to form a neural plate. This plate then folds and forms the neural tube. The neural tube then differentiates into the spinal cord and brain forming the CNS.

Neural plate

It is a structure that serves as the basis for the nervous system. Opposite to the primitive streak of the embryonic disc the ectodermal tissues thicken and flattened to become the neural plate. The region that lies anterior to the primitive knot of the primitive streak can be generally referred to as the neural plate.

The neural plate ends called neural folds push the ends of the neural plate up and together to form a neural tube that is a very important structure critical to the spinal cord and brain development in the chordates.

Neural fold

It is a structure that arises during the process of neurulation in embryonic development and is associated with primary neurulation. The anterior end of the neural tube is formed by the neural folds in chordates. The neural folds are derived from another structure called the neural plate which itself form from the ectoderm of the embryo. The functions of the neural folds are

1. Neural folds give rise to another type of cells called neural crest cells.
2.  Neural folds also bring the formation of the neural tube.

The formation of neural folds arises from the area where the ectoderm and neural plate converge and this region is formed after the process of gastrulation. Gastrulation is the process in which two layered embryones is converted into a tri-laminar disc. The three layers that formed during gastrulation are ectoderm, endoderm, and mesoderm.

Neural tube

It is the precursor of the CNS. The neural folds present at the neural plate become elevated and meets in the middle and form a tube called the neural tube.

The neural tube is divided into four regions each of which forms the distinct regions of the central nervous system. The opening of the neural tube is called neuropores that close during the fourth week of embryonic development.