Mononuclear phagocytes in the retina of developing rats

Mononuclear phagocytes were labeled with colloidal carbon injected into the circulation or stained with cytochemical techniques for the detection of marker enzymes in whole-mounted retinae of rats from birth to 10 days after birth. Positive cells were found apposed to or scattered among the blood vessels of the immature vascular network located just vitread to the developing retina. A few cells only had carbon distributed in the cytoplasm, but all retinae tested had positive cells. The enzymes located cytochemically in the phagocytes were non-specific esterase, acid phosphatase and endogenous peroxidase. When stained with aniline dyes, the phagocytes had a morphology similar to blood monocytes. Such cells were not found in the retina of adult rats. It is concluded that mononuclear phagocytes reside just vitread to the ganglion cell layer during the period of natural cell death in that layer. The phagocytes are probably associated with the removal of cell debris during the late period of retinal histogenesis.