General Features 1 Morphogenesis

The brain and spinal cord originate from the embryonic ectoderm through the action of inductive signals from underlying mesoderm and within the ectoderm. During this process, a region of dorsal ectoderm called the neural plate is delineated, folds together at the midline to form the neural tube, and invaginates into the dorsal aspect of the embryo. Even before closure and invagination of the neural tube are complete, three initial subdivisions of the brain can be discerned, first by the appearance of indentations within the neural plate and subsequently by the expansion of the intervening regions into vesicles of the neural tube. These are the primary brain vesicles, the prosencephalon, mesencephalon, and rhombencephalon (Fig. 3). Within a short time (by about 4 weeks of development), further subdivisions of the neural tube, called neuromeres, can be seen. The neuromeres of the rhombencephalon are called rhombomeres (Fig. 3A).

During flexure of the developing neural tube, the rhombencephalon assumes a shape approximating a rhombus, hence the name (Fig. 4). This occurs at least in part because the mesencephalic and cervical flexures create constrictions in the neural tube, whereas the pontine flexure creates a lateral expansion. Initially, the wall of the rhombencephalic neural tube has approximately the same thickness around its entire circumference. As the flexures appear, however, mor-phometric changes occur that presage the mature structure (Fig. 2). The ventral portion starts proliferating extensively and eventually gives rise to the bulk of the pons and medulla oblongata. A specialized structure at the ventral midline known as the floor plate eventually gives rise to the raphe. The dorsal portion, also called the roof plate, becomes relatively much thinner and eventually gives rise to the velum of the fourth ventricle with its associated choroid plexus. No neurons differentiate within the roof plate and its derivatives. At pontine levels, the dorsal portion of the neural tube, on either side of the roof plate, develops into progressively thicker flaps of neural tissue that establish the major portion of the cerebellar primor-dium. A longitudinal indentation, called the sulcus limitans, appears in the floor of the fourth ventricle about midway between the floor plate and roof plate. This sulcus divides the wall of the neural tube into two longitudinal plates or columns, the basal plate and the alar plate.

Shortly after the hindbrain neural tube forms, and during the period when rhombomere and flexure formation is shaping the hindbrain, a population of progenitor cells at the dorsal aspect of the tube emigrates into the periphery. This population, a part of the cranial neural crest, contributes much of the mesenchyme of the cranium and branchial arches and

Figure 3 Important morphogenetic changes in the neural tube that shape the hindbrain [reproduced with permission from Larsen (1997), Human Embryology. Churchill Livingstone, New York].

of certain regions of the thorax including the develop- including cartilage and other connective tissue struc-ing thymus and heart. The neural crest cells proliferate tures, smooth muscle, melanocytes, peripheral neu-and differentiate into a large number of cell types, rons, and Schwann cells.

Figure 4 Dorsal views of the human embryo at approximately 4 weeks (A) and 5 weeks (B), showing the development of structural features in the floor of the fourth ventricle and the dorsolateral aspect of the hindbrain. The dotted line indicates the level of section shown in Fig. 2 [modified from Larsen (1997), Human Embryology. Churchill Livingstone, New York].

Figure 4 Dorsal views of the human embryo at approximately 4 weeks (A) and 5 weeks (B), showing the development of structural features in the floor of the fourth ventricle and the dorsolateral aspect of the hindbrain. The dotted line indicates the level of section shown in Fig. 2 [modified from Larsen (1997), Human Embryology. Churchill Livingstone, New York].

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Understanding And Treating Autism

Understanding And Treating Autism

Whenever a doctor informs the parents that their child is suffering with Autism, the first & foremost question that is thrown over him is - How did it happen? How did my child get this disease? Well, there is no definite answer to what are the exact causes of Autism.

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