Qualitative changes in fetal trabecular meshwork fibers at the human iridocorneal angle.

We examined a series of changes that occur in the trabecular meshwork fibers of human eyes during fetal development at 12-30 weeks of gestation. At 12 and 15 weeks, the uveal meshwork was stained black with silver impregnation (indicating the predominance of collagen types III and IV) in the endomysium of the ciliary muscle. At 20 weeks, in combination with Schlemm's canal, a dense fibrous tissue mass corresponding to the trabecular meshwork anlage appeared and was colored black. The anlage was continuous with the corneal endothelium rather than with the ciliary muscle. Until 25 weeks, the trabecular meshwork was identifiable as fragmented fiber bundles that stained red-black, suggesting a mixture of collagen types I, III, and IV. At 30 weeks, half of the ciliary muscle fibers were inserted into the scleral spur and not into the meshwork. Therefore, any contribution of ciliary muscle contraction to the differentiation of the trabecular meshwork would appear to be limited. We hypothesize that an uneven distribution of mechanical stresses in the area of the cornea-sclera junction causes a tear thereby creating Schlemm's canal and is accompanied by a change in the collagen fiber types comprising the meshwork.