Outer segment

Schematic representation of rods and cones in the retina. The outer segments are designated with OS.

The outer segment is a specialized cell compartment of the photoreceptors in the retina of the eye . In the outer segments (also known as the “outer segment”, OS), a substantial part of the phototransduction takes place by means of the retinal- coupled seven transmembrane protein rhodopsin (“visual purple”).

The structure of the outer segment differs between the two types of photoreceptors: The outer segments of the rods are long, narrow and border on the retinal pigment epithelium (RPE), which phagocytes constricted, old stacks of membranes . The outer segments of the cones , on the other hand, like the entire cone photoreceptor cell, are shorter and wider than the rods and they are tapered. An outer segment is connected to the rest of the cell via a modified cilium in a decentralized position, the connecting cilium ("Connecting cilium", CC). Nine microtubule doublets in a nonagonal arrangement form the inner structure of this immobile cilium.

Furthermore, the outer segments have either many membranous "disks" (rods) or membrane folds (cones) in which the rhodopsin is stored, as well as other required proteins such as transducin , arrestin and a cGMP phosphodiesterase. The rhodopsin makes up over 90% of the protein mass in the outer segment and is the starting point for the conversion of an external light stimulus into a physiological signal.

Individual evidence

1. ↑ Walther Graumann, Dieter Sasse: Compact textbook anatomy: in 4 volumes . Schattauer Verlag, 2005. ISBN 9783794520640 . Pp. 28, 43 - 49.
2. ↑ Joachim Wachtlin, Ulrich Kellner: Retina: Diagnosis and therapy of diseases of the posterior segment. , Georg Thieme Verlag 2008. ISBN 9783131569813 . Pp. 6-11.