Development of the notochord and its significance for development of other structures

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The Notochord is a transient, rod-like, flexible structure that is the defining anatomical characteristic of all organisms in the phylum Chordata, including vertebrates. It is a critical, early embryonic midline structure that serves two main functions in human development: providing axial skeletal support and acting as a central signaling center to pattern surrounding tissues.

Development and Formation

The notochord develops from the axial mesoderm during the process of gastrulation and the third week of human embryonic development.

Stages of Notochord Formation

1. Primitive Node Migration: During gastrulation, cells from the epiblast layer ingress through the primitive streak and migrate cranially (toward the head end) from the primitive node.
2. Notochordal Process: These migrating cells form a median cord called the notochordal process (or axial process). This process extends along the midline between the ectoderm and endoderm until it reaches the prechordal plate.
3. Notochordal Plate: The floor of the notochordal process temporarily fuses with the underlying endoderm. Both layers degenerate in places, forming transient openings (the neurenteric canal) that connect the amniotic cavity and the yolk sac. The fused layers flatten out, forming the notochordal plate.
4. Definitive Notochord: Starting around day 25, the cells of the notochordal plate detach from the endoderm and fuse again to form a solid, midline cord known as the definitive notochord. This structure now lies centrally in the mesoderm layer.

Significance for Development of Other Structures

The notochord's significance lies in its role as a master organizer, directing the fate and differentiation of the surrounding germ layers—specifically the ectoderm above it and the mesoderm and endoderm beside and below it—through the secretion of signaling molecules.

1. Neural Induction (Neurulation)

The notochord is the primary inducer of the central nervous system (CNS).

• Induction of the Neural Plate: The notochord secretes inductive factors that signal the overlying ectoderm to thicken and differentiate into the neural plate, a process known as primary neurulation.
• Establishment of Dorso-Ventral Polarity: The notochord is crucial for patterning the developing neural tube (which forms from the neural plate). It secretes the diffusible protein Sonic Hedgehog (Shh), which establishes the ventral identity of the neural tube.
  • High concentrations of Shh near the notochord induce the formation of the floor plate in the ventral neural tube.
  • The floor plate and notochord then further signal the differentiation of ventral structures, notably motor neurons and inhibitory interneurons.

2. Skeletal and Axial Support

The notochord serves as the primary longitudinal skeletal axis of the early embryo.

• Vertebral Column Formation: While the notochord provides the axis, it does not directly form the vertebral bodies. It plays a key role by inducing the surrounding paraxial mesoderm (specifically the sclerotome component of the somites) to migrate and surround the notochord and the neural tube, forming the vertebrae and ribs.
• Adult Derivative (Nucleus Pulposus): In humans, the notochord largely degenerates and disappears where the vertebral bodies form. However, remnants persist and expand to form the gelatinous center of the intervertebral discs: the nucleus pulposus.

3. Mesoderm and Organ Patterning

The signaling activity of the notochord extends beyond the neural tube and sclerotomes.

• Somite Differentiation: It patterns the somites (blocks of paraxial mesoderm), influencing their differentiation into:
  • Sclerotome (cartilage and bone).
  • Myotome (muscle).
  • Dermatome (dermis of the skin).
• Foregut and Vascular Development: Notochord signals, primarily through Shh, are involved in:
  • Inducing differentiation within the endoderm, such as inhibiting Shh in the dorsal endoderm to allow for pancreas development.
  • Controlling the development and identity of the major axial blood vessels (aorta and vena cava).

Clinical Significance

Abnormalities in the development or regression of the notochord can lead to congenital defects and tumors:

• Chordoma: A rare, slow-growing malignant tumor that arises from remnants of the notochordal tissue, typically located at the ends of the vertebral column (skull base or sacrum).
• Neurenteric Cysts: Anomalies resulting from the incomplete closure of the neurenteric canal, leading to communication between the notochord and endoderm/gut derivatives.
• Skeletal Defects: Defects in notochordal signaling can indirectly result in abnormalities of the vertebral column and the organization of the central nervous system.