[The embryonic development of iridial muscles]

Development of iridial muscles was observed in 116 fetal eyes (72 human embryos 7 weeks to full gestation). Bundles of microfilaments with local high density spots appeared in the cytoplasm of the anterior pigment epithelium early in 10 week embryos. The high density spots later grew to be the dense-bodies in the myofilaments, the original structure of the pupillary sphincter. In embryos of 4 1/2 months, blood capillaries grew into the sphincter, which was fully developed in the 6th month, and myofilaments of the dilator muscles began to appear in the cytoplasm of peripheral anterior pigment epithelium, with villous protrusions toward the stroma. Many myofilaments with scattered dense-bodies were seen in the protrusions. It was thus confirmed that both the sphincter and dilator muscles originated from the iridial anterior pigment epithelium of neuro-ectoderm; however, the former developed fully to form independent smooth muscle bundles, while the latter was less developed and remained part of the pigment epithelium (myo-epithelium). The appearance of both muscles was earlier than previously reported.