Notochord definition: Definition, Types, Structure, Coelomic Cavity, Function

Definition of Notochord

In vertebrate development, the notochord plays an important role in providing structural support and secreting signalling molecules that are necessary for the differentiation and patterning of adjacent structures, such as the neural tube and somites. These signals guide the development of the central nervous system and axial skeleton, and thus, the notochord plays a major role in the embryogenesis of all vertebrates.

Structure of the Notochord

The notochord is long and cylindrical and is a rod-like structure that is made up of tightly packed cells and surrounded by a sheath. It is present between the digestive tube and the nerve cord in the embryo. Understanding the structure of the notochord helps explain its role in vertebrate development and evolution.

Location in the Embryo and Adult Vertebrate

• In the embryo, it extends along the dorsal midline from head to tail.

• It lies just beneath the neural tube and above the gut.

• During embryonic development, it provides the central axis around which the vertebral column forms.

• Most of the notochord is replaced by the vertebral column in most adult vertebrates, although remnants are found in humans and other mammals.

Cellular Composition

• The notochord core contains large, vacuolated cells sheathed by a network of collagenous fibres and, more superficially, epithelial-like cells.

• Such vacuolated cells are rich in glycosaminoglycans, which are responsible for turgor pressure, which helps in the structural integrity of the notochord.

• These cells are highly specialised and arise from mesodermal precursors to provide mechanical support at an early stage of development.

Comparative Anatomy across Vertebrates

• The structure and persistence of the notochord vary greatly in vertebrates.

• In primitive chordates, the notochord is persistent during life and forms the primary axial support.

• In cartilaginous fish, such as sharks, the notochord persists in the adult as a continuous rod running along with the vertebrae.

• This is mostly replaced by the vertebral column during development in bony fish, amphibians, reptiles, birds, and mammals.

Types of Notochord

The types of notochord vary across the phylum Chordata depending on whether it persists, is replaced, or modified during development. Understanding these types helps in classification and evolutionary study, as given in the table below:

Notochord and Coelomic Cavity

The notochord and coelomic cavity are closely linked in embryonic development. The notochord acts as the central axis, guiding animal tissue organisation and signalling pathways that shape the vertebrate body plan.

Developmental Significance

• The notochord helps guide the formation of the central nervous system, particularly the neural tube.

• It plays an important role in signalling and patterning during early embryonic development.

• The notochord is central to vertebrate embryonic development: it guides the formation of many structures and lays out the vertebrate body plan.

Formation during Embryogenesis

• The notochord is a very early embryonic structure that develops from the mesodermal layer.

• In particular, it arises from the primitive streak. This happens in human embryos at about the third week. The cells migrate to form something known as the notochordal process.

• The mature notochord is differentiated from this, and then it guides the organisation of tissues around it.

Role in the Development of the Vertebral Column

• The notochord is important in the induction of the mesoderm to form vertebral bodies around it in the development of the vertebral column.

• In most vertebrates, this notochord gets replaced by the vertebrae and intervertebral discs, with remnants in the nucleus pulposus of these discs in mammals.

Molecular Composition

• It comprises vital proteins, genes, extracellular matrix components, and signal pathways related to its developmental function in its molecular makeup.

• Expression of the Brachyury gene, a transcription factor, is a key event in notochord formation known to occur in all vertebrates.

• The gene modulates other genes necessary for cell differentiation and tissue patterning.

Function of the Notochord

The function of the notochord is central to vertebrate development. It provides structural support, guides embryonic patterning, and influences neighbouring tissues during early growth.

In vertebrates, it then contributes to the formation of the vertebral column. The notochord also forms the backbone of important functions in vertebrate development, including structural integrity and embryonic patterning.

1. Mechanical Support and Flexibility

The notochord is the vital force of support during early embryonic development. Its flexible rod shape assists in the prolongation and flexing movements of the embryo while forming a body axis.

2. Role in Embryonic Patterning

Besides its mechanical role, the notochord generates some diffusible signalling molecules with important roles in embryonic patterning. For example, Sonic Hedgehog, a signal molecule given off by the notochord, is very important for the dorsoventral patterning of the neural tube and the birth of motor neurons.

3. Influence on Neighbouring Tissues (Neural Tube, Somites)

The notochord is a key tissue with strong interaction with surrounding tissues, such as the neural tube and somites. It is the tissue that directs the induction of the neural tube of the overlying ectoderm, which later gives rise to the brain and spinal cord. The notochord also signals to the paraxial mesoderm to form somites that would later give rise to skeletal muscle, vertebrae, and dermis.

Notochord vs Vertebral Column

The notochord and vertebral column are related structures but differ in origin, persistence, and function. The notochord is the primitive axial support in embryos, while the vertebral column is the advanced skeletal muscle framework in vertebrates.

Evolutionary Perspective of the Notochord

The notochord is an ancient structure, over 500 million years old, first appearing throughout early chordates and persisting throughout vertebrate evolution. It had majorly provided the axial support in primitive chordates such as lancelets and tunicates, and later evolved adaptations in vertebrate groups. In jawless fish, cartilaginous fish, and bony fish, this notochord serves transiently during embryonic development to provide for the initial formation of the vertebral column. Despite these evolutionary changes, remnants of the notochord often persist into adulthood in some vertebrates. This emphasises how this structure has been evolutionarily conserved and also plays an important role in the development of vertebrate body plans.

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