Notogenesis: Development of axial structures

Notogenesis is the embryological process by which the notochord forms and establishes the embryonic axis during early development. The notochord is an axial midline structure located between the ectoderm and the endoderm, and it plays a central role in tissue patterning and inductive interactions, especially during gastrulation (days 15-17).

Embryological Sequence of Notogenesis

1. Primitive Streak and Node Formation

Notogenesis begins during gastrulation, when the primitive streak forms on the dorsal surface of the epiblast. At its cranial end is primitive node (Hensen's node), which coordinates midline cell migration and axial organization.

Notochord formation

2. Notochordal Process

Epiblast cells migrate through the primitive node and form a hollow tube called the notochordal process. This structure extends cranially towards the prechordal plate and represents the earliest form of the axial mesoderm.

3. Notochordal Plate and Definitive Notochord

The floor of the notochordal process temporarily fuses with the endoderm, flattening into a notochordal plate. Later, this plate folds and detaches from the endoderm to form the definitive, solid notochord running along the midline.

4. Position and Extent

In its final form, the notochord stretches from the prechordal plate cranially toward the primitive pit caudally and lies ventral to the developing neural tube.

Function of the Notochord

Tissue Induction

The notochord secretes signaling molecules such as Sonic Hedgehog (SHH), which are essential for:

  1. Neural tube induction & patterning
  2. Specifying ventral neural fates including motor neurons
  3. Initiating sclerotome formation from paraxial mesoderm and somitic tissue

Axis Establishment

By developing along the embryo's midline, the notochord helps define the cranio-caudal and left-right body axes and guides somite segmentation

Future of the Notochord

Vertebral Development

Most of the notochord disappears as vertebral bodies form around it. Residual notochordal tissue persists as the nucleus pulposus inside the intervertebral discs.

Clinical Correlate

Rare tumors called chordomas arise from persistent notochordal remnants, usually at the skull base or sacrum.

Molecular Signals

Notochord development and function involve several signaling pathways, especially:

  1. SHH (Sonic Hedgehog gene) - guides ventral neural tube and somite differentiation
  2. BMP Antagonists - secreted by notochord and allows neural induction by inhibiting BMP signaling

Resources

  1. Sadler, T. W. Langman’s Medical Embryology, 14th edition
  2. Junqueira, L. C., Carneiro, J. Basic Histology: Text and Atlas, 15th edition
  3. Moore, K. L., Persaud, T. V. N., & Torchia, M. G. The Developing Human: Clinically Oriented Embryology.
  4. Moore, K. L. The Developing Human.