Müller Cells in the Healthy Retina 2.1 Morphology and Cellular Properties of Müller Cells as Constituents of Retinal Tissue 2.1.1 Basic Morphology of Müller Cells

Müller cells (Figs. 1.17c, d, 2.1, 2.5, 2.9, 2.13, and 2.31) are radial glial cells that reside in a part of the adult CNS. As such, they share the basic bipolar morphology of radial glial cells (Figs. 1.1 and 1.4) and possess the complete set of principal glial cell processes/contact types (Fig. 1.3) (Reichenbach, 1989b). Originating from soma which generally is located in the inner nuclear layer (where the somata of all Müller cells may even constitute a sublayer) two stem processes extend into opposite directions. The outer stem process reaches to the subretinal space (i.e., the relict of the optic ventricle) into which it sends numerous microvilli. The inner stem process approaches the vitread surface of the neuroretina where it forms a so-called endfoot abutting the basal lamina between the vitreous body and the neuroretina (the “inner limiting membrane”, ILM). Both processes and the soma extend side branches which contact and/or ensheath virtually all neuronal elements of the retina (as well as the blood vessels in vascularized retinas; see Section 2.1.4). Basically, the functional structure and ultrastructure of these three types of Müller cells processes are very similar throughout the various vertebrate species (Figs. 2.1 and 2.9) (Uga and Smelser, 1973). This is probably due to the fact that these processes are locally adapted to the microenvironment which they contact (Reichenbach, 1989b; Reichenbach et al., 1989a). However, the Müller cell morphology undergoes considerable quantitative (and even qualitative) modifications throughout the variety of vertebrate species. This variability is largely dependent on the photopic vs. scotopic specialization of a retina (or retinal area) which, in turn, mainly determines the absolute and relative thickness of the retinal layers (cf. Figs. 1.13 and 2.16); another factor is the presence or absence of intraretinal blood vessels. The outer process is rather short and stout in diurnal animals with photopically specialized retinas, as it has to span a thin ONL consisting of only one (pure cone retinas) to about three (mixed but cone-dominant retinas) rows of photoreceptor cell nuclei. This is the typical situation for most reptilians and