Postnatal development of tyrosine hydroxylase immunoreactive amacrine cells in the rabbit retina: I. Morphological characterization

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Abstract
The present and accompanying (Casini, G., and N.C. Brecha, J. Comp. Neurol. 326:302–313, 1992) papers investigate the postnatal development of tyrosine hydroxylase (TH)‐immunoreactive (IR) amacrine cells in the rabbit retina. This study is focused on a detailed analysis of the patterns of cellular growth and differentiation of TH‐IR amacrine cells, which serve as a model to gain insights into the mechanisms underlying developmental changes associated with the maturation of amacrine cells. Faintly staining TH‐IR neurons are present in the proximal inner nuclear layer of newborn retinas. They are characterized by a large nucleus and usually a single primary process lacking varicosities. At postnatal day (PND) 6, TH‐IR processes display more complex morphological characteristics, including a few varicosities, and second‐ and third‐order ramifications. Growth cones are often seen. At PNDs 10 and 12 (eye opening), TH‐IR cells have general morphological characteristics similar to adult TH‐IR amacrines. They display 2–5 primary processes, which start forming a complex network of fibers in lamina 1 of the inner plexiform layer (IPL). TH‐IR processes are also present in lamina 3 and rarely in lamina 5 of the IPL. Many fibers ending in growth cones are observed. In addition, very rare, thin TH‐IR fibers are present in the outer plexiform layer. At PND 19, TH‐IR fibers form a complex, dense network in lamina 1 of the IPL, and loose networks in laminae 3 and 5. Growth cones are not observed at this age. At PND 26, a few “ring‐like” structures formed by TH‐IR fibers in lamina 1 of the IPL are observed for the first time. In adult retinas, the “ring‐like” structures are more numerous than at PND 26. A second, rare type of TH‐IR cell (“type B”) is encountered in all retinal regions beginning at PND 10. These cells are characterized by weak immunostaining and a small soma size. The present findings show that a significant differentiation of TH‐IR neurons occurs during the first 10–12 PNDs. Eye opening is an important period for the maturation of TH‐IR amacrines and, more generally, for the maturation of the IPL.