Enzyme-histochemical demonstration of microglial cells in the adult and postnatal rabbit retina.

Enzyme-histochemical methods for thiamine pyrophosphatase (TPPase) and nucleoside diphosphatase (NDPase) were applied to wholemounted rabbit retinae to demonstrate the shape and distribution of microglial cells in early postnatal and adult animals. At birth, microglial cells were already present in the entire retina. They acquired their adult "resting shape" during the first 3 postnatal weeks. Early postnatally labeled microglial cells were scattered throughout the nerve fiber layer, the inner plexiform layer, and the outer plexiform layer (OPL); at adulthood, they were not detected in the OPL. Nissl-stained retinae revealed that the number of microglial cells continuously increased during postnatal development. The same Nissl-stained preparations were used to evaluate the topography of degenerating cells in the developing postnatal retina of the rabbit. Large numbers of degenerating pyknotic cells were observed throughout the entire retinal ganglion cell layer during the first postnatal week. Later their number decreased, and from the third postnatal week onward degenerating cells were rare. Also discussed is that the emergence of microglial cells during development may be related to cell death, whereas at adulthood the function(s) of microglial cells remains obscure. Evidence for the blood-derived origin of microglia was not obtained in this study. It is argued here that if this mode of development, which has been demonstrated for other species, is also applied to the rabbit retina, then microglia would have to migrate over considerable distances, since, postnatally, the rabbit retina is avascular for more than 1 week.