Myoepithelium of the Human Iris: A Stereoscopic Scanning Electron Microscopic Study

Abstract

Summary The dilator pupillæ of postmortem human eyes was examined by light microscopy and scanning electron microscopy. The results are reviewed in association with reports in the literature. The dilator pupilæ forms a dense layer with both muscular and epithelial functions (myoepithelium). During muscular activity the myoepithelial cells undergo marked intracellular rearrangement from long and fusiform in miosis, to columnar in wide mydriasis. During contraction of the dilator the myoepithelial cells change in a manner vastly different from mesodermal, visceral, smooth muscle cells.

Expression of smooth muscle-like effectors and core cardiomyocyte regulators in the contractile papillae of Ciona

“…Myoepithelial cells are also found in the eye, specifically the muscles of the iris. The iris dilator muscle is a smooth muscle-type myoepithelium, while the iris sphincter muscle (pupillary constrictor) loses its epithelial arrangement later in development [ 62 ]. Unlike the tunicate ACCs, mammalian iris muscles are thought to be derived from neurectoderm, while in chick they derive from neural crest [ 21 , 29 , 56 , 69 , 104 ].…”

“…Vertebrate smooth muscles are those muscles that lack repeated contractile actin–myosin units [ 44 , 45 ] and are primarily defined by smooth muscle-specific effectors of contractility whose regulation is independent of the myogenic regulatory factors (MRFs: in humans MYOD1, MYOG, MYF5, and MYF6) that specify striated muscles [ 6 , 96 ]. Myoepithelial cells are smooth muscle-like cells that are arranged as epithelia and are associated with vertebrate secretory glands and the iris dilator muscle [ 62 ]. Most myoepithelial cells are derived from surface ectoderm, non-migratory neurectoderm, and even endoderm [ 3 , 29 , 50 ], as opposed to the predominantly mesodermal or neural crest origin of conventional smooth muscles [ 28 , 78 ].…”

Altered Iris Aquaporin Expression and Aqueous Humor Osmolality in Glaucoma

“…The dilator muscle is a thin peripheral layer of myoepithelium with both muscular and epithelial functions. 45 It was originally believed that the iris myoepithelium is impervious to fluid flow and forms a barrier preventing mixing of aqueous humor from the posterior and anterior chambers. 45 Our data demonstrate the contrary and, in fact, reveal the iris myoepithelium as possibly actively participating in aqueous humor dynamics.…”

“… 45 It was originally believed that the iris myoepithelium is impervious to fluid flow and forms a barrier preventing mixing of aqueous humor from the posterior and anterior chambers. 45 Our data demonstrate the contrary and, in fact, reveal the iris myoepithelium as possibly actively participating in aqueous humor dynamics. Interestingly, AQP1 expression in myoepithelial cells have been reported in other tissues, including salivary and mammary glands.…”