The role of thyroid hormone in the development of the chick corneal endothelium and epithelium.

Previous studies have established that thyroxine or thiouracil treatments affect the development of corneal transparency in the chick. The effects of these drugs on chick corneal epithelial and endothelial development were investigated because in the adult the integrity of these cell layers is necessary for the maintenance of corneal transparency. Chick embryos were treated with thiouracil or thyroxine at stage 36 or 38; the corneas were excised 2 to 12 days after treatment (between stages 38 and 45), and prepared for electron microscopy. The colloidal tracer ruthenium red was added during fixation to study epithelial and endothelial permeability and to stain the intercellular endothelial spaces. At all stages studied, the epithelium was impermeable to ruthenium red and this property was not affected by drug treatment. The epithelial barrier to this tracer is located in the outermost cell layer. The ease of penetration of ruthenium red through the endothelial intercellular spaces indicated the lack of zonular tight functions and the presence of gap functions in this cell layer. Thiouracil treatment delayed the development of the corneal epithelium so that at stage 45 it resembled epithelium from a normal embryo 3 stages younger. In normal animals, chick corneal endothelial development is characterized by an increasing degree of interdigitation of the lateral plasma membranes of adjacent endothelial cells with advancing embryonic age. Thiouracil treatment delayed this progressive development of adjacent cell membrane interdigitation. In contrast, thyroxine treatment accelerated the development of the lateral borders of the endothelial cells. It also appears that thyroid hormone can affect the development of the cell membranes of apposed cells in epithelium as well as the endothelium of the embryonic chick cornea.