Fine structure of the retinal epithelial regions of the red kangaroo (Macropus rufus)

The fine structure of the retinal epithelium, Bruch's membrane, and choriocapillaris has been studied by electron microscopy in the red kangaroo (Macropus rufus). The retinal epithelium consists of a single layer of pigmented, squamous cells. The epithelial cells are joined laterally by tight junctions and throughout the retina, display numerous basal infoldings and apical processes that enclose photoreceptor outer segments. The retinal epithelial cells are rich in smooth endoplasmic reticulum and mitochondria. Polysomes are a constant feature and the cells contain few profiles of rough endoplasmic reticulum. Membrane bound phagosomes of outer segment discs are frequently seen. All cells examined contain spherical to oval shaped pigment granules. We find no evidence of a tapetum either in the retinal epithelium or the choroid. Wandering phagocytes are occasionally observed at the photoreceptor-retinal epithelial junction. The presence of these cells has been reported in lower vertebrates but less frequently in mammals. Bruch's membrane displays the typical pentalaminate structure described for most vertebrates. The endothelium of the choriocapillaris is very thin and is highly fenestrated facing Bruch's membrane with a few fenestrations also present on the side bordering the choroid.