Early expression of recoverin in a unique population of neurons in the human retina.

The calcium-binding protein recoverin has been reported as present in photoreceptors, cone bipolar cells and sparse cells in the ganglion cell layer in the adult retinae of various vertebrate species. The present study was undertaken to clarify the developmental pattern of recoverin-immunoreactive cells in the human retina with particular attention to the cells in the inner retinal layers. In the adult human retina, small populations of recoverin-containing cells are present in the ganglion cell and nerve fiber layers. However, the precursors of these cells are quite numerous on the inner and outer borders of the nerve fiber layer in the fetal retina. By 13 weeks of gestation these cells express recoverin very intensely. By 24 weeks they are mature-looking with relatively large soma sizes (mean = 118 microns 2) and appear round, oval or multipolar in shape, with varying numbers of short processes. There follows a noticeable reduction of the mean soma size, but little change in morphology and process number during the remaining gestational stages up to and after birth. The mean numerical density of the recoverin-positive cells in the fetal inner retinal layers is gradually reduced from the high level at 13 weeks until birth, when there is a great drop to the adult level. The recoverin-immunoreactive cells in the ganglion cell layer demonstrate distinctively different developmental and morphological features from the principle neurons and glial cells in the retina. They are probably the neurons derived from the marginal zone of the retinal primordium that reside in the inner and outer borders of the nerve fiber layer due to the invasion of ganglion cell axons. The expression of recoverin in the neurons may be significant in maintaining an inside-out and centroperipheral gradient of calcium concentration in the premature retina, thereby playing a role in determining the polarity of the differentiating ganglion cells and the growth of their axons in a centrifugal spatiotemporal order.