19. The placodes, the neural crest and their derivatives. Development of the peripheral nervous system

Learning objectives

  • What is a placode?
  • Name all placodes and their derivatives
  • What is the neural crest?
  • What structures and cells do neural crest cells give rise to?
  • Describe the development of the peripheral nervous system

The placodes

A placode is a thickening of ectoderm which develops into a sensory organ or ganglion. There are multiple placodes, all of which are paired.

Lens placode

The lens placodes, also called the optic placodes, appear in the cephalic region of the embryo by the time neurulation is finished. These placodes give rise to the lens of the eye. Development of the eye is described further in topic 20.

Otic placode

The otic placodes appear simultaneously as the lens placodes and will give rise to the structures of the internal ear. They invaginate and form the otic vesicles, sometimes called the otocysts, which will give rise to the membranous labyrinth. They also give rise to the acoustic and vestibular ganglia of the vestibulocochlear nerve.

Development of the ear is described further in topic 21.

Nasal placode

The nasal placodes, also called the olfactory placodes, give rise to the olfactory epithelium of the nose.

Trigeminal placode

The trigeminal placodes arise close to the first pharyngeal groove. It gives rise to the trigeminal ganglion.

Epibranchial placodes

The epibranchial (or epipharyngeal) placodes, of which there are four pairs, arise close to the pharyngeal arches. They give rise to the following sensory cranial nerve ganglia:

  • Inferior (geniculate) ganglion of facial nerve
  • Inferior (petrosal) ganglion of glossopharyngeal nerve
  • Inferior (nodose) ganglion of vagus nerve

Adenohypophyseal placode

The adenohypophyseal placode is the only unpaired placode – it lies in the midline. This placode invaginates to form the Rathke’s pouch, which will give rise to the anterior lobe of the pituitary gland, also called the adenohypophysis.

Development of the pituitary is described further in topic 18.

The neural crest

Formation of the neural crest

During fusion of the neural folds many cells dissociate from the neural folds. These cells deposit between the neural tube and the surface ectoderm, forming the neural crest.

Derivatives of neural crest

Neural crest cells give rise to a great variety of structures, including:

Connective tissue and bones of the face and skull

Cranial nerve ganglia C-cells of the thyroid gland Aorticopulmonary septum in the heart
Odontoblasts Dermis of the face and neck Dorsal root ganglia

Sympathetic chain

Preaortic ganglia

Adrenal medulla Schwann cells Glial cells
Meninges of the forebrain Melanocytes Smooth muscle cells of the vessels of the face and forebrain

Parasympathetic ganglia of the GI tract

Development of the peripheral nervous system

The peripheral nervous system is divided into the somatic nervous system and the autonomic nervous system. Both are primarily derived from neural crest cells.

Somatic nervous system

The somatic nervous system contains afferent (sensory) and efferent (motor) nerves. The peripheral afferent nerves are peripheral branches of the axons of the pseudounipolar cells of the dorsal root ganglia, which originate from neural crest.

The peripheral efferent nerves are axons of the motor neurons in the ventral horn of the spinal cord, which originate from basal plates of the neural tube.

Autonomic nervous system

The sympathetic and parasympathetic ganglia arise from the neural crest and neural tube. The enteric nervous system also arises from neural crest cells which migrate to the GI tract, which is a testament to these cells’ ability to migrate long distances.

Summary

  • What is a placode?
    • A placode is a thickening of ectoderm which develops into a sensory organ or ganglion
  • Name all placodes and their derivatives
    • Lens placodes -> lenses of the eyes
    • Otic placodes -> membranous labyrinths and vestibulocochlear ganglia
    • Nasal placodes -> olfactory epithelium
    • Trigeminal placodes -> trigeminal ganglion
    • Epibranchial placodes -> Sensory ganglia of VII, IX, X
    • Adenohypophyseal placode -> Rathke’s pouch -> adenohypophysis
  • What is the neural crest?
    • Cells which dissociate from the neural folds and form a population of cells called the neural crest
  • What structures and cells do neural crest cells give rise to?
    • See table above
  • Describe the development of the peripheral nervous system
    • Somatic sensory nerves are peripheral branches of axons of the pseudounipolar neurons of the dorsal root ganglia
    • Somatic motor nerves are axons of motor neurons of the ventral horn of the spinal cord
    • Sympathetic and parasympathetic ganglia arise from neural crest and neural tube
    • The enteric nervous system arises from neural crest cells which migrate to the GI tract

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18. Development of the diencephalon and the telencephalon. Development of the commissural pathways and the fornix

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20. Development of the visual system

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