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Embryology and Brain Development
The human brain undergoes four phases of development: (1) dorsal induction (primary and secondary neurulation), (2) ventral induction (patterning of the forebrain), (3) neuronal proliferation and migration, and (4) myelination. During the third week of embryogenesis, initiation of the central nervous system evolves with the development of the notochordal process. This derivative of ectoderm grows rapidly in length so that by 20 days it is converted from a hollow tube to a solid rod—the notochord ( Fig. 24.1 ; e-Table 24.1 ). The notochord works with the axial mesoderm to induce the neural plate. The neuroepithelium of the neural plate begins the formation of the brain and spinal cord. It appears initially at the cranial end of the embryo and differentiates craniocaudally. At the beginning of the fourth week, the neural plate is composed of a broad cranial portion and a narrow caudal portion—the fetal brain and spinal cord.
Development of the notochord by transformation of the notochordal process.
(A) Dorsal view of the embryonic disc (about 18 days), exposed by removing the amnion. (B) Three-dimensional median section of the embryo. (C and E) Similar sections of slightly older embryos. (D, F, and G) Transverse sections of the trilaminar embryonic disc shown in (C) and (E).
(From Moore KL, Persaud TVN. Before We Are Born . 5th ed. Philadelphia: WB Saunders; 1998.)
e-TABLE 24.1
Schematic Chronology of the Major Events During Human Neocortical Development
From Gressens P, Helppi PS. Normal and abnormal brain development. In Martin R, Fanaroff A, Walsh M, eds. Neonatal-Perinatal Medicine . 8th ed. Philadelphia: Elsevier Mosby; 2005.
| Event | Time Event Occurs |
|---|---|
| Neuroectoderm induction | Third GW |
| Neurulation | Third to end of fourth GW |
| Proencephalic and hemispheric formation | Fifth to tenth GW |
| Neuronal proliferation | Tenth to twentieth GW |
| Neuronal migration | Twelfth to twenty-fourth GW |
| Programmed neuronal cell death | Twenty-eighth to forty-first GW |
| Neurogenesis | Fifteenth to twentieth GW to well into postnatal months or years |
| Synaptogenesis | Twentieth GW to puberty |
| Gliogenesis | Twenty to twenty-fourth GW to well into postnatal years |
| Myelination | Twenty-sixth to twenty-eighth GW to 2 to 3 postnatal years |
| Angiogenesis | Fifth to tenth GW to well into postnatal years |
GW, Gestational week.
Dorsal Induction
The process of neurulation or formation of the neural tube occurs when the lateral edges of the neural plate elongate to become neural folds and join together in the midline to form the neural tube ( Fig. 24.2 ). The process starts in the craniocervical region and proceeds superiorly and inferiorly. The space within the neural tube, the neural canal, initially is open at the cranial (or rostral) and caudal ends (cranial and caudal neuropores), which communicates with the amniotic cavity. The neuropores gradually decrease in size and close between the 24th and 26th day. The lowest portion of the spinal cord, the inferior sacral and coccygeal levels, are formed by a different process called secondary neurulation (see Chapter 40 ). Pluripotent tissue within the caudal eminence forms a solid neural cord; this neural cord forms a lumen that fuses with the caudal end of the neural tube to form a portion of the conus medullaris and the filum terminale ( e-Fig. 24.3 ). This secondary process is not completed until the eighth week after fertilization of the ovum.
Some neural crest cells (the population of neural cells arising at the lateral edge of the neural plate during neural tube formation) detach during neurulation and form many different tissues (e.g., melanocytes and chromaffin cells of the adrenal medulla), including the major components of the peripheral and autonomic nervous system ( e-Fig. 24.4 ).
Ventral Induction
The prechordal plate cephalic to the neural tube and notochord induces this stage of development. The three major divisions of the brain—the prosencephalon or forebrain, mesencephalon or midbrain, and rhombencephalon or hind brain—are more clearly differentiated during the rostral expansion of the neural tube (the formation of the primary brain vesicles). During the fourth and fifth weeks, a series of brain flexures occur in the midbrain, cervical, and later pontine regions so that by the end of the fifth week, five secondary brain vesicles are present ( Fig. 24.5 ). With further development, the prosencephalon becomes subdivided into the telencephalon and the diencephalon. The rhombencephalon subdivides into the metencephalon and myelencephalon. The mesencephalon does not divide. Within each of these five vesicles, the neural canal expands into a primary ventricle. The central canal of the spinal cord is continuous with the brain ventricles. The fate of these structures is shown in Fig. 24.5 . The cranial nerve nuclei appear in the brainstem during the fifth week (see the next section).
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