Observations of the ultrastructure of the developing primate choroid coat

Abstract

A descriptive analysis of some of the fine structural features of the choroidal components is made in a chronological sequence of Macaca mulatta embryos and fetuses. The observations indicate the emergence of primitive choroidal capillaries from locally differentiated cells of the retrobulbar mesenchymal tissue. Early capillaries are of two types: one represented by the choriocapillary with low endothelial cells, having extremely thin, fenestrated regions covered by basal lamina, the other by small channels in the retrobulbar mesenchymal network, with cuboidal endothelium bulging into slit-like lumina and bearing pronounced marginal folds. Discontinuous endothelium, large intercellular clefts and interrupted basal lamina permitting the passage of blood elements into the extracellular space may be associated with both types of capillaries. Primitive perieytes and muscle cells appear to be modifications from the existing mesenchymal cells adjacent to the developing vascular endothelium. The adult choroidal layering is well defined by mid-term. The fibrillar and vesicular content of the endothelial cytoplasm increases early during the second half of gestation. Cell to cell adhesions become more pronounced in the choriocapillary endothelium during this period. In the arterioles the intercellular spaces remain incompletely scaled off till birth. Appearance of the structural components indicate the involvement of some of these cells in transport mechanisms shortly after mid-gestation.