Development of the Nervous System

. THE NEURAL TUBE (Figure 4-1) gives rise to the central nervous system (CNS) [i.e., brain and spinal cord].

A. The brain stem and spinal cord have:

1. An alar plate that gives rise to the sensory neurons

2. A basal plate that gives rise to the motor neurons (Figure 4-2)

B. The neural tube gives rise to three primary vesicles, which develop into five secondary vesicles (Figure 4-3).

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.

Neural plate

Neural groove

Neural plate

Neural groove

Surface Ectoderm

Surface ectoderm

Notochord

Surface ectoderm

Notochord

Neural folds

Neural crest

Neural tube

Neural folds

Neural crest

Neural tube

Sulcus Limitans The Neural Canal

Dorsal root ganglion

Alar plate (sensory) Sulcus limitans Basal plate (motor)

Central canal

Figure 4-1. Development of the neural tube and crest. The alar plate gives rise to sensory neurons. The basal plate gives rise to motor neurons. The neural crest gives rise to the peripheral nervous system.

Dorsal root ganglion

Alar plate (sensory) Sulcus limitans Basal plate (motor)

Central canal

Figure 4-1. Development of the neural tube and crest. The alar plate gives rise to sensory neurons. The basal plate gives rise to motor neurons. The neural crest gives rise to the peripheral nervous system.

Alar Plate

Choroid w^ plexus ^

Sulcus limitans

Development of the Nervous System 25 Tela choroidea

Semicircular ducts

Pia mater

Roof plate (ependymal layer) Pial blood vessels Fourth ventricle

SSA nuclei GSA column SVA column GVA column GVE column SVE column

GSE column Floor plate

Somatic striated muscle (tongue)

Ampullae Cochlea

Skin

Taste bud cell of tongue

Visceral epithelium Smooth muscle

Branchial striated muscle (larynx)

Figure 4-2. The brain stem showing the cell columns derived from the alar and basal plates. The seven cranial nerve modalities arc shown. GSA = general somatic afferent; USE = general somatic efferent; GVA = general visceral afferent; GVE = general visceral efferent; SSA = special somatic afferent; SVA = special visceral afferent; SVE = special visceral efferent. (Adapted with permission from Patten BM: Humcm Embryology, 3rd cd. New York, McGraw-Hill, 1969, p. 298.)

II. THE NEURAL CREST (see Figure 4-1) gives rise to:

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

B. The following cells:

1. Pseudounipolar ganglion cells of the spinal and cranial nerve ganglia

2. Schwann cells (which elaborate the myelin sheath)

3. Multipolar ganglion cells of autonomic ganglia

4. Leptomeninges (the pia-arachnoid) 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. Aorticopulmonary septum of the heart

Three primary Five secondary Adult derivatives of:

vesicles vesicles Walls Cavities

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

9. Parafollicular cells (calcitonin-producing C-cells) 10. Skeletal and connective tissue components of the pharyngeal arches

III. THE ANTERIOR NEUROPORE. The closure of the anterior neuropore gives rise to the lamina terminalis. Failure to close results in anencephaly (i.e., failure of the brain to develop).

IV. THE POSTERIOR NEUROPORE. Failure to close results in spina bifida (Figure 4-4).

V. MICROGLIA arise from the monocytes.

A. Spina bifida occulta

B. Meningocele

C. Meningomyelocele

Neural tissue

Neural tissue

Posterior Rachischisis
D. Rachischisis

Figure 4-4. The various types of spina bifida. (Reprinted with permission from Sadler TW: Langman's Medical Embryology, 6th ed. Baltimore, Williams & Wilkins, 1990, p. 363.)

Anterior Cranial Meningocele

C. Meningomyelocele

Dura

Hairs

Spina Bifida Occulta Pictures

A. Spina bifida occulta

Spinal cord Arachnoid

Subarachnoid Space Zebrafish

Subarachnoid space

B. Meningocele

Dura

Hairs

Spinal cord Arachnoid

Subarachnoid space

Transverse process

VI. MYELINATION begins in the fourth month of gestation. Myelination of the corticospinal tracts is not completed until the end of the second postnatal year, when the tracts become functional. Myelination in the cerebral association cortex continues into the third decade.

A. Myelination of the CNS is accomplished by oligodendrocytes, which are not found in the retina.

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

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