Morphologic characteristics of the human ciliary muscle

The present review focusses on the morphological characteristics of the normal human ciliary muscle. It develops in the second and third trimenon during pregnancy but continues to change after birth with developing accommodation. The mature muscle forms three distinguishable portions which are connected but show individual characteristics. The outer portion origins and inserts with tendon-like structures, while the inner portion forms a sphincter-like appearance. The distinct different innervation is not yet completely described, nor exist portion-dependent summaries of the cytological characteristics. The specific intracellular, membraneous, and extracellular components described in the literature are listed and supplemented with own observations. Dedication This review is dedicated to my scientific instructors Prof. Johannes W. Rohen and Prof. Elke Lütjen-Drecoll Introduction Accommodation is one of the crucial conditions for foveal vision. The active change of the lens curvature is induced by the ciliary muscle, its counterpart are passive forces like the elasticity of the lens itself and of Bruch’s membrane. In the present review the specifics of this muscle in the human are highlighted to better understand its unique role in the concert of uveal smooth muscle cells. Since there are known differences in the morphology and function of the ciliary muscle in different species, only data from investigations using human tissue or human cell cultures were included. The step-wise discovery of the human ciliary muscle in the early 19th century was only described recently [1] and is not part of this review. Three-dimensional architecture Development The early development of the ciliary muscle was first described by Herzog [2] and Seefelder & Wolfrum [3] using light microscopy of fetal human eyes; extended views were described in ophthalmic text books [4,5] and a first electron-microscopic analysis was performed by Sellheyer and Spitznas [6]. According to this literature, differentiation of the human ciliary muscle starts around week 12 forming at first the outer portion (meridional portion or Brücke’s muscle). The inner portion (circular portion or Müller’s muscle) shows first characteristic arrangements at week 14, the intermediate portion (radial/ reticular portion or Iwanoff’s muscle) develops successively. The muscle cells and the interwoven fibroblasts derive from a common mesenchymal cell. Population present from week 10 on. Muscle cell differentiation continues up to week 16, followed by continuing maturation. First axons demonstrating an innervation of the smooth muscle cells were described in week 15. A detailed description of the development of human ciliary muscle innervation remains to be determined. At birth, the human ciliary muscle appears still somewhat immature [7-9] and develops in the first year of life to its mature appearance. Although the different muscle portions act as one unit, Correspondence to: Christian Albrecht May, Anatomisches Institut, Fetscherstr. 74, 01307 Dresden, Germany, Tel: +49 351 4586105; Fax: +49 351 4586303; E-mail: [email protected]