C H A P T E R 2

Development of the

Nervous System

Objectives

1.Describe the development of the neural tube, including the stages of development and the adult derivatives of each brain vesicle.

2.Trace the lineage of the cells of the neural tube wall, including the alar and basal plates.

3.Identify the derivatives of the neural crest.

4.Describe the development of the brainstem as well as the general arrangement of motor versus sensory components and somatic versus visceral components.

5.Describe the development of the pituitary (hypophysis).

6.List and characterize major congenital malformations of the central nervous system.

IThe Neural Tube (Figure 2-1) gives rise to the central nervous system (CNS)

(i.e., brain and spinal cord).

A.The brainstem and spinal cord are composed of plates separated by the sulcus limitans:

1.An alar plate—gives rise to sensory neurons.

2.A basal plate—gives rise to motor neurons (Figure 2-2).

3.Interneurons are derived from both plates.

B.The neural tube gives rise to three primary vesicles (forebrain, midbrain, and hindbrain), which develop into five secondary vesicles (telencephalon, diencephalon, mesencephalon, metencephalon, and myelencephalon) (Figure 2-3).

C.Alpha-fetoprotein (AFP) is found in the amniotic fluid and maternal serum. It is an indicator of neural tube defects (e.g., spina bifida, anencephaly). AFP levels are reduced in mothers of fetuses with Down syndrome.

IIThe Neural Crest (see Figure 2-1) gives rise to:

A.The peripheral nervous system (PNS) (i.e., peripheral nerves and sensory and autonomic ganglia).

B.The following cells:

1.Pseudounipolar cells of the spinal and cranial nerve ganglia

2.Schwann cells (which elaborate the myelin sheath)

3.Multipolar cells of autonomic ganglia

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4.Cells of the leptomeninges (the pia-arachnoid), which envelop the brain and spinal cord

5.Chromaffin cells of the suprarenal medulla (which elaborate epinephrine)

6.Pigment cells (melanocytes)

7.Odontoblasts (which elaborate predentin)

8.Cells of the aorticopulmonary septum of the heart

9.Parafollicular cells (calcitonin-producing C-cells)

10.Skeletal and connective tissue components of the pharyngeal arches

Figure 2-2 The brainstem showing the cell columns derived from the alar and basal plates. The seven cranial nerve modalities are shown. GSA, general somatic afferent; GSE, general somatic efferent; GVA, general visceral afferent; GVE, general visceral efferent; SSA, special somatic afferent; SVA, special visceral afferent; SVE, special visceral efferent. (Adapted from Patten BM. Human Embryology. 3rd ed. New York: McGraw-Hill; 1969:298, with permission.)

Spinal cord

Figure 2-3 The brain vesicles indicating the adult derivatives of their walls and cavities. (Reprinted from Moore KL. The Developing Human: Clinically Orienting Embryology. 4th ed. Philadelphia, PA: WB Saunders; 1988:380, with permission.)

VMicroglia arise from blood-born monocytes.

A.Myelination of the CNS—accomplished by oligodendrocytes.

B.Myelination of the PNS—accomplished by Schwann cells.

VII The Optic Nerve and Chiasma—derived from the diencephalon. The optic nerve fibers occupy the choroid fissure. Failure of this fissure to close results in coloboma iridis.

VIII The Hypophysis (pituitary gland)—derived from two embryologic

substrata (Figures 2-5 and 2-6).

E.Dandy–Walker malformation has a frequency of 1:25,000. It may result from riboflavin inhibitors, posterior fossa trauma, or viral infection (Figure 2-8).

F.Hydrocephalus—most commonly caused by stenosis of the cerebral aqueduct during development. Excessive CSF accumulates in the ventricles and subarachnoid space. This condition may result from maternal infection (cytomegalovirus and toxoplasmosis). The frequency is 1:1,000.

G.Fetal Alcohol Syndrome—the most common cause of mental retardation. It manifests with microcephaly and congenital heart disease; holoprosencephaly is the most severe manifestation.

Figure 2-8 Dandy–Walker malformation. Midsagittal section. An enormous dilation of the fourth ventricle results from failure of the lateral foramina (of Luschka) and median foramen (of Magendie) to open. This condition is associated with occipital meningocele, elevation of the confluence of the sinuses (torcular Herophili), agenesis of the cerebellar vermis, and splenium of the corpus callosum. (Reprinted from Dudek RW, Fix JD. BRS Embryology. Baltimore, MD: Williams & Wilkins; 1997:97, with permission.)