Neurogenesis - Anatomy & Physiology

BACK TO DEVELOPMENTAL BIOLOGY

Introduction

The nervous system develops from ectoderm. Some of the ectoderm will also develop into the epidermis. Signalling factors from elsewhere in the embryo determine which parts of the ectoderm become neural tissue or epidermis.

In the anterior part of the embryo the ectoderm forms the neural plate, a flat layer of ectodermal cells lying directly above the node. The morphogenic changes from this neural plate to the nervous system is split into two parts; primary and secondary neuralation.

Neural Plate Induction

• Cells of the ectoderm are directed to develop into either neural ectoderm or epidermis.
• How is this choice made?
• Formation of the neural ectoderm is the default pathway.
  • Therefore, in order to make epidermis, a signal from the environment must be diverting cells from going down this default pathway.
  • This signal comes from the lateral plate mesoderm (LPM) which directs ectoderm to become epidermis.
    • The notochord (lying along the midline) secretes an antagonist to the factor of the LPM so that neural ectoderm is formed above the notochord.
• Therefore, in lateral regions ectoderm becomes epidermis and along the midline ectoderm becomes neural ectoderm.

Neuralation

Primary Neuralation

1. Cells of the neural plate proliferate causing the neural plate to thicken. Cells then converge at the midline and become wedge shaped, which drives the neural plate to become long and narrow.
2. Cells along the midline descend ventrally and contact the notochord, forming a "hinge". This forms a depression which is called the neural groove.
3. Either side of the neural groove, ectoderm converges towards the midline. This causes elevations either side of the neural groove, called neural folds.
4. Hinges form at the foot of each neural fold, allowing the folds to be brought together. This forms a tube.
5. The tube sinks as non - neural ectoderm fuses above the neural tube, "zipping" the neural tube up. The non - neural ectoderm will form epidermis.
   • Failure of the neural tube to close causes spina bifida (spinal cord protrusion) and exencephaly (brain located out of the skull).

Secondary Neuralation

• Occurs posteriorly from the lumbar level onwards.

1. The neural plate descends ventrally and medially.
2. Cells of the neural plate condense and form a solid rod called the medullary cord.
3. It is then made hollow by cavitations which join up to make a lumen.

• Tubes formed by primary and secondary neuralation fuse together to make one continuous tube called the neural tube.

Neural Tube Development

• The neural tube is one cell thick.
• The lumen of the neural tube enlarges as cells at the periphery of the lumen under mitosis.
• Cell division is governed by oscillations of the nucleus.
  • When the nucleus reaches the luminal side of cell, the cell undergoes mitosis.

Neural Tube Regionalisation

• The neural tube undergoes regionalisation to define the anterior as the brain and posterior as the spinal cord.
• In the anterior, the neural tube swells into the three ventricles of the brain; forebrain; midbrain; and hindbrain.
  • This occurs as fluid is pumped into the lumen of the neual tube.
  • Pressure is regionalised to the anterior by occlusion of the neural tube at the base of the hindbrain. The occlusion reopens after enlargement of the ventricles.