Retinal Pigment Epithelial Fine Structure in the Red-Tailed Hawk (Buto jamaicensis)

As part of a comparative morphological study, the fine structure of the retinal epithelium (RPE), choriocapillaris and Bruch's membrane (complexus basalis) has been studied by electron microscopy in the red-tailed hawk (Buteo jamaicensis). In this species the RPE consists of a single layer of low cuboidal cells which display numerous basal (scleral) infoldings and extensive apical (vitreal) processes which interdigitate with photoreceptor outer segments. These epithelial cells are joined laterally by a series of basally located tight junctions. Internally SER is the most abundant cell organelle while only small amounts of RER are present. Polysomes are however abundant as are mitochondria. The RPE cell nucleus is large and vesicular. Melanosomes are mainly located in the apical processes of the RPE cells in light-adaptation. Myeloid bodies are large and numerous in light-adaptation and often show ribosomes on their outer border. Bruch's membrane (complexus basalis) shows the typical pentalaminate structure noted in most vertebrates but with only a poorly defined central elastic layer. The endothelium of the choriocapillaris is very thin facing the RPE but is only moderately fenestrated. The choriocapillaris in this species is unusual however in that many of the fenestrae show a double-layered diaphragm.     

Fine Structure of the Pecten Oculi in the American Crow (Corvus brachyrhynchos)

The pecten oculi of the American crow (Corvus brachyrhynchos) has been examined by both light and electron microscopy. In this species the pecten is very large and of the pleated type. It consists of 22 - 25 accordion folds that are joined apically by a bridge of tissue which holds the pecten in a fan-like shape widest at its base. Within each fold are numerous capillaries, larger supply and drainage vessels and many melanocytes. The capillaries are extremely specialized for transport functions and display extensive microfolds on both their luminal and abluminal borders. Except for the nuclear region which contains most of the organelles, the endothelial cell bodies are extremely thin. These capillaries are surrounded by thick fibrillar basal laminae which are conjectured to be structurally important. Pericytes are a common feature of these capillaries. The numerous pleomorphic melanocytes interspersed between the capillaries are also felt to be important in structural support of the pecten. The pecten is considered to be comparable to the falciform process of some teleosts, the conus papillaris of reptiles, the supraretinal vessels of amphibians and teleosts and the intraretinal vessels of mammals which are all felt to be alternative methods of bringing nutrients to the inner retina.     

Retinal Photoreceptor Fine Structure in the Great Blue Heron (Ardea herodias)

The morphology of the retinal photoreceptors of the great blue heron (Ardea herodias) has been investigated by light and electron microscopy. They consist of rods, single cones and double (unequal) cones present in a ratio of about 2:1:1 respectively. The rods are slender elongated cells with outer segments that reach to the retinal epithelial (RPE) cells and are surrounded by pigment-rich apical processes of the RPE cells in the light-adapted state. The rod inner segment displays an ellipsoid of mitochondria, an hyperboloid of glycogen, much rough ER, numerous polysomes, Golgi zones and autophagic vacuoles. The rod nucleus is located deep in the outer nuclear layer and the rod synaptic pedicle displays both invaginated and superficial synaptic sites. Single cones display a slightly tapered outer segment, a large electron lucent oil droplet and an ellipsoid of mitochondria in the apex of the inner segment. Double cones consist of a long thin chief member which shows an electron dense oil droplet and a shorter, stouter accessory cone with no oil droplet but a paraboloid of glycogen below the ellipsoid. As in the single cone, polysomes, RER and Golgi zones are present in the myoid region of both members of the double cone. All photoreceptor types have a connecting cilium joining inner and outer segments. Near the external limiting membrane, the chief and accessory cones show intercellular junctions. All cone photoreceptors are relatively small in diameter and hence tightly packed. While rods are felt to undergo retinomotor movements, cones are felt to move minimally or not at all. Both single and double cones display several invaginated (ribbon) synapses as well as numerous superficial (conventional) synaptic sites.     

Retinal Photoreceptor Fine Structure in the Red-Tailed Hawk (Buteo jamaicensis)

The retinal photoreceptors of the red-tailed hawk (Buteo jamaicensis) consist of rods, single cones and double (unequal) cones present in a ratio of about 2: 11 5. In the light-adapted state, the rods are slender elongated cells with outer segments that reach to the retinal epithelial (RPE) cells. The inner segment displays an ellipsoid of mitochondria, plentiful polysomes, some rough ER and Golgi zones. The rod nucleus is located deep within the outer nuclear layer and the synaptic spherule displays both invaginated (ribbon) and superficial (conventional) synaptic sites. Single cones show a thin tapering outer segment, a large electron lucent oil droplet at the apex of the inner segment and an ellipsoid of mitochondria. Double cones consist of a larger chief member which displays a thin tapering outer segment and an electron dense oil droplet as well as a smaller accessory cone which shows no oil droplet, an ellipsoid and a paraboloid of glycogen. As in the single cone, polysomes, RER and Golgi zones are also noted in the inner segments of both members of the double cone. Near the external limiting membrane the chief and accessory cones show membrane specializations indicative of junctions on their contiguous surfaces. All cone photoreceptors are of a smaller diameter than is normally reported for avian species. Both single and double cones display several invaginated synapses as well as numerous superficial synaptic sites.     

Fine Structure of the Retinal Photoreceptors of the Domestic Cat (Felis catus)

The morphology of the retinal photoreceptors of the domestic cat (Felis catus) has been investigated by light and electron microscopy. In this species the photoreceptors can be readily differentiated and adequately described by the classical terminology of rods and cones, with rods being the more numerous. Rods are long slender cells with inner and outer segments of much the same diameter. Cones are shorter cells with a conical outer segment that tapers distally and an inner segment which is wider than the outer segment. Both rods and cones are highly polarized and compartmentalized cells and consist of an outer segment, a connecting cilium, an inner segment, a nuclear region and a synaptic process leading to an expanded synaptic ending. Morphological similarities and differences are noted between the rods and cones for the various regions of these cells.     

Fine Structure of the Pecten Oculi of the Red-Tailed Hawk (Buteo jamaicensis)

The pecten oculi of the red-tailed hawk (Buteo jamaicensis) has been examined by light and electron microscopy. In this species the pecten is very large and of the pleated type. It consists of 17-18 accordion folds which are joined apically by a heavily pigmented bridge of tissue which holds the pecten in a fan-like shape, widest at its base. It is situated over the optic nerve head and extends into the vitreous. Within each fold are numerous capillaries, larger supply and drainage vessels and melanocytes. The capillaries are specialized and display extensive microfolds on both the luminal and abluminal borders. The endothelial cell bodies are thin with most organelles present in a paranuclear location. The capillaries are surrounded by thick fibrillar basal laminae which are probably structurally important and which often enclose pericytes. The melanocytes which are most plentiful in the bridge region and peripherally in the pecten, form an incomplete sheath around the capillaries and other blood vessels. These melanocytes are also felt to be fulfilling a structural role within the pecten. The morphology of the pecten of the red-tailed hawk is indicative of a heavy involvement in the transport of materials to the avascular avian retina.     

Retinal Photoreceptor Fine Structure in the American Crow (Corvus brachyrhynchos)

The morphology of the light-adapted retinal photoreceptors of the American crow (Corvus brachyrhynchos) has been investigated by light and electron microscopy. They consist of rods, single cones and double (unequal) cones present in a ratio of about 4:3:3 respectively. The rods are stout cells with a long inner segment and an outer segment that reaches to the RPE cell body. In the light-adapted state, the pigment-laden apical processes of the RPE cells surround cell photoreceptor types for most of their length. The rod inner segment displays an ellipsoid of mitochondria, a large hyperboloid of glycogen, much RER, numerous polysomes, Golgi zones and autophagic vacuoles. Single cones show a slightly tapered outer segment, a large and usually heterogeneous oil droplet and an ellipsoid of mitochondria at the apex of the inner segment. Double cones consist of a longer, stouter chief member which displays a more homogeneous oil droplet and a prominent paraboloid of glycogen and a slightly shorter and thinner accessory member with no oil droplet or paraboloid. Both members of the double cone as well as the single cones show a prominent ellipsoid and plentiful polysomes, RER and Golgi zones in the inner segment. Along the length of the contiguous membranes of the two members of the double cone are presumed interreceptor junctions. All cone photoreceptors are relatively small in diameter and hence are tightly packed. Judging by their morphology in the light-adapted state neither rods nor cones are felt to undergo photomechanical movements in this species. Rods and cones (both types) display both invaginated (ribbon) synapses as well as numerous flat (conventional) synaptic sites.     

Retinal Epithelial Fine Structure in the Domestic Cat (Felis catus)

The morphology of the retinal epithelium (RPE) and closely associated choriocapillaris and Bruch's membrane (complexus basalis) has been investigated in the eye of the domestic cat (felis catus) by light and electron microscopy. The RPE consists of a single layer of cuboidal cells joined laterally by apically-located junctional complexes. Basally (sclerally) these cells display numerous infoldings while apically (vitreally) two distinct types of processes enclose photoreceptor outer segments. Internally the large vesicular nucleus is centrally located. Smooth endoplasmic reticulum, mitochondria, polysomes and lysosome-like bodies are abundant. Rough endoplasmic reticulum while present is not plentiful. Over the tapetum lucidum, melanosomes are absent from the RPE cells while in non-tapetal locations melanosomes are large and plentiful. The endothelium of the choriocapillaris is highly fenestrated and these capillaries are deeply indented into the epithelial layer over the tapetum. Bruch's membrane (complexus basalis) is non-tapetal regions is the typical pentalaminate structure noted for most mammals. Over the tapetum however it is reduced to a trilaminate structure and when associated with the indented capillary profiles is further reduced to a single thickened basal lamina.     

Fine Structure of the Bovine Tapetum Fibrosum

The fine structure of the tapetum lucidum of the domestic cow was studied by electron microscopy. In this ungulate the reflective layer is a tapetum fibrosum situated in the choroid and mainly restricted to the superior fundus. The tapetum is composed of a large array of fine extracellular collagen fibrils separated into lamellae by fibrocytes. The collagen fibrils display the normal cross-striations of native collagen, are 0.2 μm in diameter and are arranged in a hexagonal pattern with a center-to-center spacing of about 0.2 μm. The diameter and spacing of these fibrils is consistent with the constructive interference of reflected light. A vascular lamella adjacent to the retinal epithelial layer contains the choriocapillaris and some irregularly arranged collagen fibrils. The tapetum is pierced by blood vessels which supply the choriocapillaris. Over the tapetum the retinal epithelium is non-pigmented while in nontapetal regions this layer is pigmented. The choriocapillaris is not indented into the retinal epithelium as is the case in species with a tapetum cellulosum.     

Fine Structure of the Choriocapillaris, Bruch''s Membrane and Retinal Epithelium of the Cow

The fine structure of the choriocapillaris, Bruch's membrane and retinal epithelium was investigated in both the tapetal and non-tapetal fundus of the bovine eye. In ail locations the retinal epithelium consists of a single layer of cuboidal cells. The epithelial cells are joined laterally by apically located tight junctions and throughout the retina display numerous basal infoldings and fine apical processes which enclose rod outer segments. All retinal epithelial cells are rich in smooth endoplasmic reticulum and mitochondria and contain phagosomes. Although not as abundant, profiles of rough endoplasmic reticulum and polysomes are also noted in all locations. In non-tapetal areas, melano-somes are numerous whereas over the central tapetum fibrosum they are absent. The absence of melanosomes over a functional tapetum is to be expected. While lysosomes are present throughout the epithelial layer, over the tapetal region they appear to be more numerous. The apparent increase in lysosomal numbers in this location may indicate an enhanced shedding of outer segment material over the tapetum. Although some retinal epithelial cells are modified to accomodate a tapetum lucidum their morphology is basically similar throughout the retina and probably indicates that ail regions of the retinal epithelium are capable of the normal functions of this indispensible retinal layer. The choriocapillaris is heavily fenestrated on the border facing the retina and overlying the tapetum also displays fenestrae on its choroidal edge. Bruch's membrane ( complexus basalis ) is pentalaminate throughout the retina and is slightly thicker in the posterior fundus.