Immunocytochemical study on the triple origin of the sphincter iris in thechick embryo

The ontogenic development of the sphincter iris has been studied by immunoc ytochemistry and standard staining on chick embryos from stage 25 HH to the time of hatching. We have used the monoclonal antibody 13F4, a highly spec ific marker of muscular cells. We have observed three different regions in the iris. Tn the pupillary region, immunoreactive cells are in continuous c ontact with the inner epithelium of the pupillary margin. In the intermedia te region, the outer epithelium forms buds of pigmented cells that emigrate toward the stroma. In this epithelium cells that are totally or partially unpigmented exist, and they are 13F4 positive. In the sphincter we have obs erved 13F4 positive cells with melanin granules. In the ciliary region, the immunoreactivity appears in dispersed mesenchymal cells. The present findi ngs are consistent with a triple origin of the sphincter iris in the chick embryo. This muscle is derived from the inner epithelium of the pupillary m argin, the intermediate region of the outer epithelium, and from the mesenc hymal cells. The cells of the inner epithelium of the pupillary margin are differentiated into smooth muscle cells, and the remaining cells form stria ted muscle cells.