Studies on the Eyes of Anchovies Anchoa mitchilli and A. hepsetus (Engraulidae) with Particular Reference to the Pigment Epithelium

A study has been made of the eyes of the anchovies Anchoa mitchilli and A. hepsetus (Engraulidae) with particular reference to the pigment epithelium. Eyes of dark (d.a.) and light (l.a.) adapted fish were investigated by optical and electron microscopy. In light adapted eyes the pigment epithelium is radially extended with the vitread region of the cells inserted amongst the cones and lying in vertical rows above them. They are separated from adjacent rows of pigment epithelial cells by masses of rods arranged in similar vertical sheets. The vitread end of the cell, where it fits between the cones, is cuneate in shape. In l.a. eyes lateral lobes develop from the cuneate portion of the cell, extend into the spaces between the cone rows, and serve to shield the lateral margins of the cones. On dark adaptation of the eye the lateral lobes are withdrawn as the cell shortens radially. The cytoplasm of the pigment epithelium contains numerous myeloid bodies composed of stacks of smooth endoplasmic reticulum distributed throughout the cell. In l.a. eyes masses of less well ordered e.r. cisternae and tubules are also common, especially in the vitread region among the tapetal material and melanosomes. Mitochondria, some large vacuoles, lysosomes containing ingested portions of rod outer segments, and also occasionally melanosomes and tapetal material occur in the basal region of the cell in the vicinity of the nucleus. Structurally differentiated ground substance and ribosomes are poorly represented throughout all regions. Spherical and cylindrical melanosomes and tapetal material composed of guanine are plentiful within the cell. In the l.a. state melanosomes occur in the vitread region, at the level of the cones, but in the d.a. state they become concentrated in the basal region forming a dark scleral zone throughout the tissue. The tapetal material consists of approximately isodiametric crystallites, truncate needles and thin almost square platelets. All forms are strongly birefringent. The crystallites are distributed throughout the l.a. cell but in the d.a. condition they become concentrated mainly in the middle region forming a prominent whitish zone throughout the tissue vitread to the melanosome layer. In l.a. eyes the needles are arranged axially and are distributed mainly around the periphery of the cell in the vitread region. On dark adaptation of the eye some of the needles are dispersed amongst the crystallites in the middle region. The platelets are grouped into two stacks which remain essentially unaltered in organization and location during photomechanical movements of the eye. The stacks are grouped into a V in the vitread cuneate region with the platelets abutting the cone units and lying in the same vertical plane as the cone lamellae. Each stack is composed of several regularly spaced rows with each row made up of two files of platelets. The average thickness of platelets measured from vertical sections through stacks is about 82 nm and the distance between platelets of adjacent rows is about 112 nm. A cisterna of endoplasmic reticulum runs between each row and is connected to the membrane sacs of adjacent platelets by tubular connections. Non-membrane cross-bridging material also links the cisterna and platelet sacs. The consequence of the organization of the platelet stacks as a system for the reflexion of light into the cones by constructive interference is discussed.