Development of the choroid and related structures.

The development of the choriocapillaris and the choroid is described using light and electron microscopy. Up to the seventh week after conception, the endothelium of the choriocapillaris is thick and contains many cytoplasmic vesicles. By the ninth week the endothelium flattens and becomes vesiculated. Fenestrations are found as early as the seventh week, whereas the continuous basement membrane is only observed at the ninth week. The first choroidal arterioles and venules can be seen during the fifteenth week and the arteries and veins become distinguishable at the twenty-second week. Haller's and Sattler's layers are both venous and arterial at the time of their first appearance.