The microglia comprise tiny cells throughout the CNS (Fig. 15, M). Resembling oligodendrocytes, they are even smaller and darker, with dense oval, elongate, or triangular nuclei, meagre somata, and short tortuous processes with minute pointed thorns (also found on the soma).

The embryologic origin of the microglia is disputed. Most believe they are of mesodermal origin, invading the young CNS with the blood vessels. In context, the mesoderm in question derives from the neural crest: cells at the crests of the neural folds that break free to enter the mesodermal compartment at the time of neural tube closure. These cells undergo ectomesench-ymal transformation. Accordingly, many embryolo-gists characterize them as ectomesoderm. Almost taken up in the CNS, at the final moment they depart, differentiate, and rapidly migrate away to form a host of derivatives, including dorsal root ganglion cells, autonomic ganglion cells, and Schwann cells, i.e., peripheral neurons and glia. Thus, it may be that the microglia are not invading the young CNS, but coming home.

Others favor hematopoietic origin of the microglia, citing blood-borne mononuclear cells derived from bone marrow. Most neuropathologists recognize this added source of macrophages after brain insult. If vessels are damaged (they usually are), both the resident microglia and monocytes from the blood participate in repairs. Microglial cells proliferate, enlarge, and become puffy and phagocytic, clearing away cellular debris and ingesting products of degenerating myelin. Blood-borne macrophages do the same. The term microglia has been suggested for the minimally active resident population and phagocytes for cells responding to insults. The latter is appropriately noncommittal. Phagocytes may derive from the microglia, circulating monocytes, capillary peri-cytes, and outlying connective tissue. A third origin that has been postulated is that microglial cells arise from the same neuroepithelial stem cells that give rise to the macroglia: the astrocytes and oligodendrocytes.

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