Pax6

Figure 19.14 Molecular regulation of spinal cord development. A. Sonic hedgehog (SHH), secreted by the notochord, ventralizes the neural tube and induces a floor plate region (F) that also expresses this gene. Bone morphogenetic proteins 4 and 7 are secreted by the nonneural ectoderm and contribute to differentiation of the roof and alar plates. B. Initially, PAX3 and 7 and MSX I and 2 are expressed uniformly throughout the neural plate. SHH represses expression of these genes in the ventral half of the neural tube that will become the floor and basal plates. Simultaneously, BMPs upregulate and maintain expression of PAX 3 and 7 in the dorsal half of the neural tube that will form the roof and alar plates. PAX 6 begins expression throughout the neural ectoderm as the neural folds elevate and close. The exact roles of the PAX and MSX genes in differentiation of these regions have not been determined.

neural tube defects (NTDs), may involve the meninges, vertebrae, muscles, and skin. Severe NTDs involving neural and non-neural structures occur in approximately 1 in 1000 births, but the incidence varies among different populations and may be as high as 1 in 100 births in some areas, such as Northern China.

Spina bifida is a general term for NTDs affecting the spinal region. It consists of a splitting of the vertebral arches and may or may not involve underlying neural tissue. Two different types of spina bifida occur:

1) Spina bifida occulta is a defect in the vertebral arches that is covered by skin and usually does not involve underlying neural tissue (Fig. 19.15A). It occurs in the lumbosacral region (L4 to S1) and is usually marked by a patch of hair overlying the affected region. The defect, which is due to a lack of fusion of the vertebral arches, affects about 10% of otherwise normal people.

2) Spina bifida cystica is a severe NTD in which neural tissue and/or meninges protrude through a defect in the vertebral arches and skin to form a cystlike sac (Fig. 19.15). Most lie in the lumbosacral region and result in neurological deficits, but they are usually not associated with mental retardation. In some cases only fluid-filled meninges protrude through the defect (spina bifida with meningocele) (Fig. 19.15B); in others neural tissue is included

Hairs Dura Arachnoid

Skin \ / t Arachnoid pinal cord

Hairs Dura Arachnoid

Skin \ / t Arachnoid

Spina Bifida Occulta

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