Ontogeny of somatostatin immunoreactivity in the cat retina

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Abstract
In the ganglion cell layer of the adult cat retina, subgroups of displaced amacrine cells and alpha ganglion cells are immunoreactive for somatostatin or a somatostatinlike substance. Both types of immunoreactive cells are found preferentially in inferior retina. We studied the development of somatostatin immunoreactivity in the prenatal and postnatal cat retina to determine how such unusual distributions of immunoreactive cells arise. Somatostatin‐immunoreactive profiles were first observed at embryonic day (E) 30, within the inner retina in a central region that included the optic disk and the area centralis. By E36, immunoreactivity had virtually disappeared from the central retina but was present throughout the periphery. The immunoreactive profiles could not be classified morphologically because of their immaturity but were most likely retinal ganglion cells, the earliest born cells of the inner retina. Of the two types of immunoreactive cell observed in the adult, the first to be recognized morphologically was the displaced amacrine cell, at E45. These cells were virtually adultlike in morphology and number by E51, two weeks before birth. In contrast, immunoreactive alpha ganglion cells were not apparent until five days after birth and did not achieve their mature numbers and immunoreactive staining characteristics until more than a month later. From the time they could initially be recognized, both immunoreactive displaced amacrine cells and alpha cells were distributed mainly in the inferior retina. A third type of somatostatin‐immunoreactive cell was transiently observed in the superior and inferior retina during prenatal and early postnatal development. These cells were characterized by granular staining in irregular shapes and few, if any, faintly stained processes. Injections of retrograde tracers into retinorecipient targets revealed that many of these cells were retinal ganglion cells. They disappeared by postnatal day 38. Our results indicate that somatostatin immunoreactivity initially follows a central‐to‐peripheral pattern of development, as is typical of other developmental events in the mammalian retina. They also indicate that the two types of somatostatin‐immunoreactive neurons present in the adult cat retina (displaced amacrine and alpha ganglion cells) attain their mature immunocytochemical properties with very different timecourses. Finally, the observation that somatostatin immunoreactivity appears transiently in the granular‐staining ganglion cells, distributed throughout the superior and inferior retina, suggests that the peptide may play a regulatory role in the development of the retina and/or retinofugal pathways.