The sphincter and dilator muscles of the iris are derived from neural ectoderm, ie, from the anterior epithelial layer of the primitive optic cup. These two muscles therefore form an exception to the general rule that muscle fibers, both striated and unstriated, are developed from mesenchyme. The myo-epithelial cells of various glands form another exception in man but these contractile cells are derived from surface ectoderm. In spite of its origin the sphincter muscle is usually regarded as a true smooth muscle both morphologically and functionally.3,14,19,20 On the contrary, the less distinct dilator muscle is sometimes thought to be composed of myo-epithelial cells, and it is thus not a typical smooth muscle.3 Except the study of specific cholinesterase in the sphincter muscle of the cat by Koelle et al,12 no information is available about the histochemical characteristics of those two muscles. The purpose of this communication is
NIEMI M, TARKKANEN A. Cholinesterases, Monoamine Oxidase, and Phosphorylase in the Iris Muscles. Arch Ophthalmol. 1964;72(4):548–553. doi:10.1001/archopht.1964.00970020548021
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