[Light and electron microscopy studies of the development of the human ciliary body]

In human embryos and fetuses with a gestation age of 9.5 to 24 weeks, differentiation of the ciliary muscle was examined by both light and transmission electron microscopy, and the surface morphology of the ciliary body was analyzed by scanning electron microscopy. Smooth muscle cells develop from mesenchymal elements or early fibroblasts located at the anterior rim of the optic cup. Musclelike cells exhibiting dense bodies and myofilaments were first distinguished during week 12 and smooth muscle cells with an adultlike appearance become apparent during week 15/16. A cellular maturation process can be observed up to week 22. Fibroblasts separating the muscle cell layers from each other also derive from the same precursor cells, as do smooth muscle cells, thereby pointing out the close relationship between the two cell types. The first radial ciliary folds could be observed at week 10.3 by scanning electron microscopy. However, true ciliary processes have only been described at week 24. The ciliary processes observed during week 24 only differed from those of the adult eye by the dimensions and lack of surface infoldings. A primitive pars plana was first identified during week 24. The morphological basis for aqueous humor production is discussed.