Both the choroid and the sclera are of neural crest origin. The anterior sclera forms as a condensation of mesenchymal tissue that is continuous with the cornea (see Fig. 1-13). This conden sation progresses posteriorly toward the optic nerve and, by approximately 12 weeks, the mesenchymal condensation has enveloped the optic nerve. The lamina cribrosa consists of mesenchymal cells that have penetrated the optic nerve. The cornea and sclera are derived from the same mesenchymal tissue, except for the corneal epithelium, which is of surface ectoderm origin.
The choroid is a highly vascular pigmented tissue that develops from mesenchymal tissue (neural crest) surrounding endothelial blood spaces (mesoderm). The blood spaces organize and give rise to the embryonic choriocapillaris at approximately 2 months of gestation.16,45,80 At approximately 4 months, the choriocapillaris connects with the short posterior arteries and joins with the outer venous layer and the four vortex veins. Outside the choriocapillaris, multiple anastomoses between arterioles and venules occur, thus forming the fetal choroid plexus.
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