The spinal cord is a crucially important information highway. It connects the brain with most of the body. It is the target of a number of disease processes, some of which (eg, spinal cord compression) are treatable but rapidly progressive if not treated. Failure to diagnose some disorders of the spinal cord, such as spinal cord compression, can be catastrophic and may relegate the patient to a lifetime of paralysis. A knowledge of the architecture of the spinal cord and its coverings, and of the fiber tracts and cell groups that comprise it, is essential.
During the third week of prenatal development, the ectoderm of the embryonic disk forms the neural plate, which folds at the edges into the neural tube (neuraxis). A group of cells migrates to form the neural crest, which gives rise to dorsal and autonomic ganglia, the adrenal medulla, and other structures (Fig 5–1). The middle portion of the neural tube closes first; the openings at each end close later.
Schematic cross sections (A–F) showing the development of the spinal cord.
During development the cells in the wall of the neural tube divide and differentiate, forming an ependymal layer that encircles the central canal and is surrounded by intermediate (mantle) and marginal zones of primitive neurons and glial cells (Figs 5–1 and 5–2). The mantle zone differentiates into an alar plate, which contains mostly sensory neurons, and a basal plate, which is primarily composed of motor neurons. These two regions are demarcated by the sulcus limitans, a groove on the wall of the central canal (see Fig 5–1D). The alar plate differentiates into a dorsal gray column; the basal plate becomes a ventral gray column. The processes of the mantle zone and other cells are contained in the marginal zone, which becomes the white matter of the spinal cord (see Fig 5–2A).
Cross section showing two phases in the development of the spinal cord (each half shows one phase). A: Early phase. B: Later phase with central cavity.
An investing layer of ectodermal cells around the primitive cord forms the two inner meninges: the arachnoid and pia mater (pia) (see Fig 5–2B). The thicker outer investment, the dura mater (dura), is formed from mesenchyma.
EXTERNAL ANATOMY OF THE SPINAL CORD
The spinal cord does not run the full length of the adult spinal column. It occupies the upper two-thirds of the adult spinal canal within the vertebral column (Fig 5–3). The cord is normally 42 to 45 cm long in adults ...