Electron microscopy of the development of the cell junctions in the embryonic and fetal human corneal endothelium.

The eyes of 12 human embryos and. fetuses between the seventh week and the eighth month of pregnancy are examined. Immature cell junctions, connecting the loosely arranged mesenchymal cells which later form the comeal endothelium, are observed between the seventh and eighth week of gestation. Occluding junctions in the form of maculae occludentes appear in the eighth week when a solid cell layer has formed. In the middle of the fourth month beltlike zonulae occludentes exist between the endothelial cells at their apices. This coincides with the commencement of aqueous humor formation and suppoHs the theory that the corneal endothelium performs a barrier and transport function. From the fifth month on, the junctional attachments look like the cell junctions in the mature corneal endothelium and there are no essential changes until the end of pregnancy.