Differentiation of the ciliary muscle in the human embryo and fetus.

In human embryos and fetuses with a gestation age of 10.3 to 22 weeks, differentiation of the ciliary muscle was examined both by light and transmission electron microscopy. The smooth muscle cells develop from mesenchymal elements or early fibroblasts located in the region between the anterior scleral condensation and the ciliary pigment epithelium. The first musclelike cells exhibiting myofilaments and dense bodies could be distinguished during week 12. Smooth muscle cells with an adultlike appearance become apparent during week 15/16. Up to week 22, a cellular maturation process can be observed. Fibroblasts separating the muscle cell layers from each other also derive from the same precursor cells, as do the smooth muscle cells, thereby pointing to the close relationship between the two cell types.