Development of microglia in the albino rabbit retina.

In this study the development of ameboid microglia and resting microglia in the retina of the albino rabbit has been examined by means of a lectin derived from Griffonia simplicifolia. Ameboid microglia are present in the retina as early as E12, when the optic fissure is in the process of closure, and appear to be concentrated initially at the vitreal surface. At E14 many ameboid microglia can be seen to extend processes to the ventricular surface of the cytoblast layer, but in subsequent ages these cells are rare and ameboid microglia are largely confined to the ganglion cell layer, inner plexiform layer, and occasionally the developing inner nuclear layer. By adult life, mature (or resting) microglia are confined to the inner plexiform and ganglion cell layers. Numbers of microglia increase steadily throughout fetal life from a mean of 400 at E14, the earliest age quantified, to a peak of 28,600 at E30. There is a small postnatal drop in numbers to 17,150 at P9. Microglia could only be labelled faintly in animals older than P11, but analysis of two adult (P130) retinas with adequate labelling suggested that numbers rise to a value of about 23,800 at this age. Ameboid microglia thus appear in the retina 11 days prior to the onset of axon loss in the optic nerve (about E23) and 14 days prior to the beginning of the period of reduction of retinal ganglion cell numbers (about E26). The present findings indicate that while some microglial precursors may enter the retina in response to debris generated during the natural retinal ganglion cell death period, most enter the retina well before this period. Also, microglia present a uniform density distribution with apparently regular spacing as early as E16, so the uniform regular distribution cannot simply be the consequence of regularly distributed pyknotic figures as previously suggested.