A correlated light and electron microscope study on the pecten oculi of the domestic fowl (Gallus domesticus)

Cytological features and certain histological relationships in the avian pecten oculi in the domestic chicken are described in detail at both the light and electron microscopic levels. Comparisons and contrasts are made to relate the authors' electron microscopic observations on this specialized vascular organ with those of Tanaka (1960). The cleared, doubly injected in toto preparations of the adult pecten oculi shows that the basic plan of organization of the blood vessels in each pectineal fold consists of a supplying artery and a draining vein, and between these two main vessels are numerous anastomosing capillaries of varying calibre. The main artery (longitudinal artery), together with its ventrally associated longitudinal vein, runs along the base of the pecten and gives off a supply and draining branch to each of the pectineal folds (16–18 in number). Histologically, the avian pecten oculi consists of two tissue components: (1) a plexus of modified blood vessels and, interdigitated between the blood vessels, (2) numerous pigmented intervascular cells. No glial elements have been observed. The pectineal capillaries which form the main portion of the pecten are lined by a continuous layer of endothelial cells which typically possess some interesting ultrafine structural modifications and specializations. There are two membrane systems in a pectineal endothelial cell: (1) apically, the plasma membrane is highly plicated and (2) basally, there is also a folded membrane system which appears at intervals confluent with the bounding thick, laminar and fibrillar basement membrane. Groups of small vesicles are associated with the infoldings of the membrane systems. It is suggested that the vesicular profiles, as well as the infoldings of the membrane systems, may represent and function as an agranular endoplasmic reticulum in the pectineal endothelial cells. Preliminary studies have shown that intravenous injection of Diamox, which reduces intraocular pressure within 60 min after administration, in the avian eye causes marked changes in the ultrafine morphology of the membrane system of the pectineal endothelial cell. Both membrane systems are labile and are transformed into more complex highly interdigitated membrane systems within a relatively short time. Exactly how this striking morphological transformation of the membrane is accomplished is unknown, and is being investigated at the present time. The larger calibre pectineal blood vessels contain well defined intramural cells (pericytes, pericapillary cells, Rouget cells) which are completely surrounded by a laminar and fibrillar basement membrane. The intravascular pigmented tissue contains numerous dense osmiophilic pigment granules. Associated with each pigment granule are two or three mitochondria. No intercellular spaces are present between the pigment cells and the neighbouring pectineal capillaries in this material. The ultrafine morphology of elaborate membrane systems of the endothelial cells of the pectineal capillaries suggests (a) that it is a device for increasing cell surface, and (b) that this epithelium is probably concerned in active fluid transport. Experimental studies are in progress to elucidate, if possible, the dynamics of fluid transport in this specialized vascular organ.