Musculoskeletal Development | Main Anatomy Index | Eye Development

This page was contributed by David Boshell

Development of the subdivisions of the brain

• On day 19, indentations appear on the neural plate, demarcating the future forebrain (prosencephalon), midbrain (mesencephalon) and hindbrain (rhombencephalon)
• In the 4^th week:

• The cerebral aqueduct drains cerebrospinal fluid from the lateral and third ventricles, and if it becomes blocked, they will swell, causing hydrocephaly.

Flexures of the brain

• The brain folds into 3 flexures:

1. The mesencephalic (cephalic) flexure, between the fore and midbrain, that folds the forebrain ventrally
2. The pontine flexure: a dorsal flexure between the metencephalon and myelencephalon
3. The cervical flexure, between the myelencephalon and spinal cord, that folds the brain ventrally relative to the spinal cord

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Development of the brain stem

• The brainstem, consisting of the mesencephalon, metencephalon and myelencephalon is very similar to the spinal cord, initially being organised into paired basal (ventral) and alar (dorsal) gray columns.
• Generally:
• The basal columns give rise to motor neurons
• The alar columns give rise to sensory neurons as well as special, brain-specific neurons.

Development of the cranial nerves

• The cranial nerve nuclei appear in the 4^th and 5^th weeks, with motor nuclei developing in the basal columns and sensory nuclei developing from the alar columns.
• The cranial nerves may carry fibres from multiple cranial nerve nuclei, and thus may be motor, sensory or mixed.

Development of the cranial nerve nuclei

• The brainstem cranial nerve nuclei are generally organised by their function into 7 longitudinal columns, with 3 motor columns coming from the basal columns, and 4 sensory columns coming from the alar columns.
• They traverse around the rhombencephalic wall as follows (although some migrate after forming), from ventromedial (motor – efferent) to dorsolateral (sensory – afferent):

• Parasympathetic and sensory ganglia appear late in the 4^th week:
• The enteric ganglia are innervated by the vagus (X) nerve
• The cranial parasympathetic ganglia are innervated by cranial nerves III, VII and IX close to their target organs in the head.

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Development of the forebrain

• The prosencephalon gives rise to the telencephalon and the diencephalon.

Development of the diencephalon

• At the end of the 5^th week, the thalamus and hypothalamus appear as swellings in the wall of the diencephalic neural canal, separated by the hypothalamic sulcus.
• At the end of the 6^th week, the epithalamus appears as a swelling in the dorsal wall of the diencephalon, separated from the thalamus by the sulcus dorsalis, and will give rise to the pineal gland, the habenular nuclei and the posterior commisure.
• The pituitary gland is formed in the 4^th week from:

Development of the Telencephalon

• On day 32, the cerebral hemispheres arise as bubble-like, lateral diverticula of the telencephalon, and proceed to grow cranially, laterally and caudally, covering the diencephalon.
• Folding of the cerebral cortex begins in the 4^th month, and continues throughout foetal life, and after birth.
• The central sulcus has appeared by 6 months, and the lateral ventricles become narrowed by thickening of the cerebral cortex.
• Ventromedial strips of the cerebral hemispheres form choroid fissures along the walls of the lateral ventricles, along which some choroid plexus can develop.
• In the 4^th and 5^th weeks, cells of the nasal placodes differentiate into primary neurosensory cells of the olfactory epithelium, whose axons grow into the adjacent telencephalon to synapse in the 6^th week with the secondary neurosensory cells of the olfactory pathway.

Cell differentiation in the spinal cord and brainstem

• A ventricular layer of cells surrounding the neural canal proliferates to produce most of the cell types of the future central nervous system.
• The 1st wave of proliferation from this layer produces neuroblasts (future neurons) that migrate peripherally to form an outer mantle zone (future grey matter of spinal cord and brain stem)
• The nerve fibres from the neuroblasts form a marginal zone, superficial to the mantle zone, which contains fibre tracts (future white matter of the spinal cord and brainstem).
• The 2^nd wave of proliferation produces glioblasts that migrate peripherally and become astrocytes and oligodendrocytes
• Finally, ventricular layer differentiates into the ependyma lining the brain ventricles and central canal of the spinal cord.
• At the end of the 4^th week, the mantle layer becomes organised into the paired basal and alar columns of the ventral and dorsal quadrants of the spinal cord, respectively.

Cell differentiation in the cerebral cortex

• Cell differentiation is more complex in the cerebral cortex than the rest of the nervous system, but follows an essentially similar pattern.
• The first neuroblasts from the ventricular zone form a marginal zone, before migrating peripherally to form an intermediate zone between these zones.
• Some neuroblasts of the ventricular and intermediate zones will then migrate to form a cortical plate between the intermediate and marginal zones.
• Neuroblast production then ceases in the ventricular zone, and begins in a new subventricular zone just peripheral to the ventricular zone, from which cells will migrate to form a subplate, just under the cortical plate.
• The intermediate layer gives rise to white matter.

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Neural crest

• Neural crest cells grow on the lateral lips of each neural fold, before detaching during neurulation, creating a layer of neural crest between the neural tube and the surface ectoderm.
• The neural crest of the cephalic neural tube will give rise to many diverse structures of the head and neck, whilst the occipital and spinal neural crest will produce major components of the peripheral nervous system, including:
  • Autonomic postganglionic neurons
  • The chromaffin cells of the adrenal medulla
  • The spinal ganglia

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