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Development of Bruch's membrane in the chick: an electron microscopic study.

The development of fibrous connective tissue in Bruch's membrane within the choroid of chronologically staged chick embryos was observed by means of transmission electron microscopy. The appearance of these connective tissue elements follows an orderly developmental sequence. The first component of Bruch's membrane to appear is a continuous basal lamina at the basal surface of the presumptive retinal pigment epithelium (RPE) at 21/2 days. Microfibrils and associated amorphous material are present in the adjacent connective tissue space. A discontinuous inner collagenous layer is observed at the basal aspect of the RPE during the fourth day. A definitive elastic layer is present during the ninth day and becomes more apparent following subsequent stages of development. An outer collagenous layer begins development during the tenth and twelfth days. Collagenous fibrils average 37 nm in diameter and display axial periodicity measuring 46 nm between major periods. These measurements increase with age as do the number of collagenous fibrils. Only isolated patches of basal lamina are observed in association with the choriocapillary endothelium by the twentieth day.