Human retinal development: Ultrastructure of the outer retina

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Abstract
The morphogenesis of the outer human retina from 6.5 to 18 weeks of gestational age (16–156 mm) was studied by light and electron microscopy. During this period the retinal pigment epithelium changes from a pseudostratified columnar epithelium, with mitoses at its inner edge, to a single layer of cuboidal cells. Initially, mature pigment granules, premelanosomes, polysomes, rough endoplasmic reticulum and a Golgi complex occupy the epithelial cytoplasm. Lateral epithelial surfaces show infoldings and neighboring epithelial cells are bound at their inner borders by a junctional complex consisting of a zonula occludens and zonula adherens. Later, pigment epithelial development is marked by an increase in cytoplasmic organelle content and the appearance of slender processes at the inner epithelial border. Initially, the outer neural retina is formed by a uniform population of elongated cells containing oval nuclei with multiple nucleoli and, in the cytoplasm, mitochondria, polysomes and the Golgi complex. Cell division is common in the proliferative layer at the outer edge of the posterior neural retina until 120 mm crown‐rump length, and cell membrane junctions between outer neuroblasts remain intact during mitosis. Cilia project outwards from outer neuroblasts and invaginate the immediately adjacent pigment epithelium. At 120 mm crown‐rump length, differentiating rods, cones and Müller cell processes are distinguishable and developing ribbon synapses and surface contacts are present at the base of the cone prior to outer segment formation.

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