fThe CNS originates in the ectoderm and appears as the neural plate at the middle of the third week (Fig. 19.1). After the edges of the plate fold, the neural folds approach each other in the midline to fuse into the neural tube (Figs. 19.2 and 19.3). The cranial end closes approximately at day 25, and the caudal end closes at day 27. The CNS then forms a tubular structure with a broad cephalic portion, the brain, and a long caudal portion, the spinal cord. Failure of the neural tube to close results in defects such as spina bifida (Figs. 19.15 and 19.16) and anencephaly (Fig. 19.38), defects that can be prevented by folic acid.

The spinal cord, which forms the caudal end of the CNS, is characterized by the basal plate containing the motor neurons, the alar plate for the sensory neurons, and a floor plate and a roof plate as connecting plates between the two sides (Fig. 19.8). SHH ventralizes the neural tube in the spinal cord region and induces the floor and basal plates. Bone morphogenetic proteins 4 and 7, expressed in nonneural ectoderm, maintain and up-regulate expression of PAX3 and PAX7 in the alar and roof plates.

The brain, which forms the cranial part of the CNS, consists originally of three vesicles: the rhombencephalon (hindbrain), mesencephalon (midbrain), and prosencephalon (forebrain).

The rhombencephalon is divided into (a) the myelencephalon, which forms the medulla oblongata (this region has a basal plate for somatic and visceral efferent neurons and an alar plate for somatic and visceral afferent neurons) (Fig. 19.18), and (b) the metencephalon, with its typical basal (efferent) and alar (afferent) plates (Fig. 19.19). This brain vesicle is also characterized by formation of the cerebellum (Fig. 19.20), a coordination center for posture and movement, and the pons, the pathway for nerve fibers between the spinal cord and the cerebral and the cerebellar cortices (Fig. 19.19).

The mesencephalon, or midbrain, resembles the spinal cord with its basal efferent and alar afferent plates. The mesencephalon's alar plates form the anterior and posterior colliculi as relay stations for visual and auditory reflex centers, respectively (Fig. 19.23).

The diencephalon, the posterior portion of the forebrain, consists of a thin roof plate and a thick alar plate in which the thalamus and hypothalamus develop (Figs. 19.24 and 19.25). It participates in formation of the pituitary gland, which also develops from Rathke's pouch (Fig. 19.26). Rathke's pouch forms the adenohypophysis, the intermediate lobe, and pars tuberalis, and the di-encephalon forms the posterior lobe, the neurohypophysis, which contains neuroglia and receives nerve fibers from the hypothalamus.

The telencephalon, the most rostral of the brain vesicles, consists of two lateral outpocketings, the cerebral hemispheres, and a median portion, the lamina terminalis (Fig. 19.27). The lamina terminalis is used by the commissures as a connection pathway for fiber bundles between the right and left hemispheres (Fig. 19.30). The cerebral hemispheres, originally two small outpocketings (Figs. 19.24 and 19.25), expand and cover the lateral aspect of the diencephalon, mesencephalon, and metencephalon (Figs. 19.26-19.28). Eventually, nuclear regions of the telencephalon come in close contact with those of the diencephalon (Fig. 19.27).

The ventricular system, containing cerebrospinal fluid, extends from the lumen in the spinal cord to the fourth ventricle in the rhombencephalon, through the narrow duct in the mesencephalon, and to the third ventricle in the dien-cephalon. By way of the foramina of Monro, the ventricular system extends from the third ventricle into the lateral ventricles of the cerebral hemispheres. Cerebrospinal fluid is produced in the choroid plexus of the third, fourth, and lateral ventricles. Blockage of cerebrospinal fluid in the ventricular system or subarachnoid space may lead to hydrocephalus.

The brain is patterned along the anteroposterior (craniocaudal) and dorsoventral (mediolateral) axes. HOX genes pattern the anteroposterior axis in the hindbrain and specify rhombomere identity. Other transcription factors containing a homeodomain pattern the anteroposterior axis in the forebrain and midbrain regions, including LIM1 and OTX2. Two other organizing centers, the anterior neural ridge and the rhombencephalic isthmus, secrete FGF-8, which serves as the inducing signal for these areas. In response to this growth factor, the cranial end of the forebrain expresses BF1, which regulates development of the telencephalon, and the isthmus expresses engrailed genes that regulate differentiation of the cerebellum and the roof of the midbrain. As it does throughout the central nervous system, SHH, secreted by the prechordal plate and notochord, ventralizes the forebrain and midbrain areas. Bone mor-phogenetic proteins 4 and 7, secreted by nonneural ectoderm, induce and maintain expression of dorsalizing genes.

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