Chapter 13: Embryology and Congenital Disorders

INTRODUCTION

This chapter provides you with the highest yield information on the development of the nervous system as well as disorders that arise during this process. It will be important to be familiar with the various congenital diseases, their genetic basis (if there is one), and the treatment and management of each.

EMBRYOLOGY AND DEVELOPMENT OF THE NERVOUS SYSTEM

Development of the nervous system fits into the larger process of creating the whole person from a fertilized egg. Disruptions of development create predictable neurologic syndromes.

• Week 3-4: Neurulation—After gastrulation, the mesoderm develops the notochord which induces the ectoderm to differentiate into nervous system tissue (neural plate) (Fig. 13-1).

  • Neuronal stem cells accumulate on the neural plate over the notochord which begins to fold as the weight of the cells increases, becoming a groove, then a fold, and then a tube.

  • By the 28th day after fertilization, there is a neural tube (derived from the ectoderm) surrounded by somites (mesodermal structures that will become the bony spinal cord and paraspinus muscles).

  • The neural fold zips itself up (becomes a true tube) from the middle toward the top and bottom simultaneously.

  • Folate is necessary for this process to occur, which is why all prenatal vitamins contain folate.

Neurologic Development

• Week 4-5:

  • Brain development—Once the cranial neuropore is closed, the developing nervous system divides into three primary and five secondary vesicles (Table 13-1).

  • Spinal cord development—Cells of the developing spinal cord produce two separate plates.

    • - Dorsally, the Alar plate will have all the sensory neurons.

    • - Ventrally, the Basal plate will have all the motor neurons.

    • - Some neurons migrate to the intermediolateral cell column to form the autonomic nervous system, and some move to the edges to become the peripheral ganglia (ie, the dorsal root ganglia).

Almost all brain tumors arise from neuronal stem cells (such as Primitive Neuronal Ectodermal Tumors—PNETs), glial cells (eg, astrocytomas, oligodendroglimos, and glioblastoma multiforme), ependymal cells (eg, ependymomas), meninges (eg, meningiomas), and neuronal ...