Biomedical subjects

C R Braekevelt

Publications and source records attributed to C R Braekevelt.

At least 37 records · Page 2Linked to original sources

PubMed1994

Retinal pigment epithelial fine structure in the great blue heron (Ardea herodias).

The fine structure of the retinal epithelium (RPE), choriocapillaris and Bruch's membrane (complexus basalis) has been studied by light and electron microscopy in the great blue heron (Ardea herodias). In this species the RPE consists of a single layer of cuboidal cells which display numerous basal (scleral) infoldings and plentiful apical (vitreal) processes which surround photoreceptor outer segments. These epithelial cells are joined laterally by a series of tight junctions located in the mid to basal region. Within the epithelial cells, smooth endoplasmic reticulum is very abundant while rough ER is not. Mitochondria (some of which are ring-shaped) and polysomes are abundant. In light-adaptation the RPE nuclei are large vesicular and basally located while the melanosomes of these cells are almost exclusively located within the apical processes. Myeloid bodies are large and numerous and often show ribosomes on their outer surface. Bruch's membrane (complexus basalis) shows the typical pentalaminate structure noted in the majority of vertebrates except teleosts. The choriocapillary endothelium is very thin facing Bruch's membrane but is only moderately fenestrated. The majority of these fenestrations show a single-layered diaphragm but double-layered diaphragms are also noted.

Animals↗

PubMed1994

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.

Animals↗

PubMed1994

Retinal pigment epithelial fine structure in the American crow (Corvus brachyrhynchos).

The fine structure of the retinal epithelial (RPE) region has been investigated by light and electron microscopy in the American crow (Corvus brachyrhynchos). In this species the RPE consists of a single layer of cuboidal cells which display numerous deep basal (scleral) infoldings and plentiful apical (vitreal) microvillar processes which surround photoreceptor inner and outer segments. The RPE cells are joined laterally by a series of tight junctions (Verhoeff's membrane) located in the mid to basal region. Within the epithelial cells, smooth endoplasmic reticulum is very abundant while rough endoplasmic reticulum (RER) is scarce. Mitochondria of various shapes are abundant basally while polysomes are plentiful and widespread. In the light-adapted state RPE nuclei are large and vesicular and basally located while the melanosomes of these cells are predominantly located with the apical processes indicating photomechanical movements. Myeloid bodies are large and numerous and often have ribosomes on their outer surface. Bruch's membrane (complexus basalis) is typical of avian species in that it is pentalaminate and the lamina densa is displaced near the choriocapillaris. The endothelium of the choriocapillaris is thin facing Bruch's membrane but is only moderately fenestrated. Some of these fenestrations display a double-layered diaphragm while the majority show the more typical single-layered diaphragm noted in most species.

Animals↗

PubMed1994

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.

Animals↗

PubMed1994

Fine structure of the tapetum lucidum in the short-tailed stingray (Dasyatis brevicaudata).

The tapetum lucidum of the short-tailed stingray (Dasyatis brevicaudata) is located in the choroid of the superior fundus immediately external to the choriocapillaris. In this species the tapetum consists of a single layer of overlapping cells oriented at an angle of about 30 degrees to the incoming light. These tapetal cells alternate with and are separated from one another by melanocytes which extend beyond the tapetal cells to intervene between the tapetal cells and the choriocapillaris. The tapetal cells and the melanocytes are flattened plate-like cells with their widest dimension facing the retina. Internally the tapetal cells display a peripherally located vesicular nucleus with most organelles in a paranuclear location. The bulk of the cell is packed with regularly spaced crystals reported to be guanine. The size and spacing of these reflective crystals is commensurate with constructive interference. In light-adaptation, the melanosomes of the intervening melanocytes are widely dispersed and for the most part block the passage of light to the tapetal cells. Although dark-adapted specimens were not examined, it seems reasonable to assume that in dark-adaptation, the melanosomes will retreat to unmask the tapetum and allow it to function as a reflective layer.

Adaptation, Physiological↗

PubMed1994

Retinal pigment epithelial fine structure in the short-tailed stingray (Dasyatis brevicaudata).

The retinal pigment epithelium (RPE), choriocapillaris and Bruch's membrane (complexus basalis) have been studied by light and electron microscopy in the short-tailed stingray (Dasyatis brevicaudata). The RPE consists of a single layer of cuboidal cells which display numerous basal (choroidal) infoldings as well as many apical (vitreal) processes which interdigitate with photoreceptor outer segments. The lateral cell borders are relatively smooth and joined by a series of tight junctions. Within these epithelial cells, smooth endoplasmic reticulum is the dominant organelle with only scattered profiles of rough endoplasmic reticulum. Polysosomes, mitochondria and phagosomes are abundant. Melanosomes are totally absent over the tapetum and are scarce in non-tapetal locations. The RPE nucleus is large, vesicular and centrally located. Bruch's membrane is a pentalaminate structure. The choriocapillaris is a single layer of large capillaries. The endothelium of these capillaries is typically thin but only minimally fenestrated.

Adaptation, Physiological↗

PubMed1994

Retinal photoreceptor fine structure in the short-tailed stingray (Dasyatis brevicaudata).

The fine structure of the retinal photoreceptors has been studied by light and electron microscopy in the short-tailed stingray (Dasyatis brevicaudata). The duplex retina of this elasmobranch contains rods and cones in a ratio of about 10:1. No multiple receptors were noted nor was a repeating or mosaic arrangement of the cones obvious. Only light-adapted specimens were studied but retinomotor movements of the photoreceptors were felt to be minimal or absent. The rods are large cylindrical cells with inner and outer segments of much the same diameter. Cones are short stout cells with a conical outer segment and a wider inner segment. Rod outer segment discs show peripheral incisures while cones do not. The inner segment of rods and cones are rich in organelles indicating much synthetic activity. The nuclei of rods and cones appear quite similar but cone nuclei are invariably at least partially protruded through the external limiting membrane which is formed by a series of zonulae adherentes between photoreceptor cells and Müller cells. The synaptic region of both rods and cones display both invaginated (ribbon) synapses and superficial (conventional) synapses.

Adaptation, Physiological↗

PubMed1993

Fine structure of the retinal epithelial regions of the red kangaroo (Macropus rufus).

The fine structure of the retinal epithelium, Bruch's membrane, and choriocapillaris has been studied by electron microscopy in the red kangaroo (Macropus rufus). The retinal epithelium consists of a single layer of pigmented, squamous cells. The epithelial cells are joined laterally by tight junctions and throughout the retina, display numerous basal infoldings and apical processes that enclose photoreceptor outer segments. The retinal epithelial cells are rich in smooth endoplasmic reticulum and mitochondria. Polysomes are a constant feature and the cells contain few profiles of rough endoplasmic reticulum. Membrane bound phagosomes of outer segment discs are frequently seen. All cells examined contain spherical to oval shaped pigment granules. We find no evidence of a tapetum either in the retinal epithelium or the choroid. Wandering phagocytes are occasionally observed at the photoreceptor-retinal epithelial junction. The presence of these cells has been reported in lower vertebrates but less frequently in mammals. Bruch's membrane displays the typical pentalaminate structure described for most vertebrates. The endothelium of the choriocapillaris is very thin and is highly fenestrated facing Bruch's membrane with a few fenestrations also present on the side bordering the choroid.

Animals↗

PubMed1993

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 2:1:5. 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.

Animals↗

PubMed1993

Fine structure of the tapetum lucidum of the paca Cuniculus paca.

The tapetum lucidum of the paca or spotted cavy (Cuniculus paca) has been studied by light and electron microscopy. The reflective layer in this species is a tapetum cellulosum situated in the choroid of the superior fundus. Posteriorly, the tapetum is composed of several layers of ovoid to flattened cells while peripherally the tapetum gradually thins down and disappears. Within the tapetal cells most cell organelles are located near the nucleus. The dominant feature of these cells is, however, a large accumulation of electron-dense rodlets which show a variety of sizes, shapes and orientations. These rodlets are the reflective material of the tapetum, and energy dispersive studies indicate that they are rich in sulphur. The diameter and spacing of these irregular rodlets is too varied to be consistent with the principles of constructive interference, and the tapetum in the paca is probably capable of only diffuse reflectance. The retinal epithelium over the tapetum is nonpigmented while in non-tapetal locations it is normally pigmented.

Animals↗

PubMed1993

Fine structure of the retinal pigment epithelium of the great horned owl (Bubo virginianus).

The fine structure of the retinal epithelium (RPE), choriocapillaries and Bruch's membrane (complexus basalis) has been studied by light and electron microscopy in the great horned owl (Bubo virginianus). The RPE consists of a single layer of cuboidal cells joined laterally in the mid to basal region by a series of tight junctions forming part of the blood-ocular barrier. Basally (sclerally) the epithelial cells show numerous deep infoldings while apically (vitreally) a wealth of microvillar processes interdigitate with the photoreceptor cells. Internally the RPE cells display a large vesicular nucleus, plentiful smooth endoplasmic reticulum (SER) and polysomes with only small scattered profiles of rough endoplasmic reticulum (RER). Numerous pleomorphic mitochondria are basally located. In the light-adapted state the melanosomes are located almost exclusively within the apical processes indicating retinomotor movements. Myeloid bodies are numerous and often show ribosomes on their outer surface. Bruch's membrane is typical of avian species in that it is pentalaminate and the lamina densa is displaced towards the choriocapillaris. The choriocapillaris itself is but minimally fenestrated facing Bruch's membrane. Most fenestrations present show a single layered diaphragm while others display a double-layered diaphragm.

Animals↗

PubMed1993

Fine structure of the retinal photoreceptors of the great horned owl (Bubo virginianus).

The retinal photoreceptors of the great horned owl (Bubo virginianus) consist of rods, single cones and unequal double cones present in a ratio of about 30:1.2. In the light-adapted state the rods are stout cells which are not felt to undergo retinomotor movements. The rod outer segment consists of a stack of scalloped membranous discs enclosed by the cell membrane. The rod inner segment shows an ellipsoid of mitochondria and a wealth of rough endoplasmic reticulum (RER) and polysomes, Golgi zones and autophagic vacuoles but not hyperboloid of glycogen. Single cones show a slightly tapered outer segment, a heterogenous oil droplet and an ellipsoid of mitochondria at the apex of the inner segment. Double cones consist of a larger chief member which also displays an oil droplet and a slightly smaller accessory member which does not. Both members of the double cone as well as the single cone show a prominent ellipsoid, plentiful polysomes and RER and Golgi zones in the inner segment. Neither single nor double cones possess a condensed paraboloid of glycogen but instead show plentiful scattered glycogen particles. Along the contiguous membranes between accessory and chief cones a few presumed junctional complexes are seen near the external limiting membrane. Judging by their morphology in light-adaptation the cones of this species do not undergo photomechanical movements. Rods and cones (both types) have both invaginated (ribbon) and numerous superficial (conventional) synaptic sites. Rods are more numerous in this nocturnally active bird than is usually noted in avian species.

Animals↗

PubMed1993

Fine structure of the pecten oculi in the great horned owl (Bubo virginianus).

The pecten oculi of the great horned owl (Bubo virginianus) has been examined by light and electron microscopy. The pecten in this species is of the pleated type and is small in comparison to the size of the eyeball. It consists of 7-8 accordion folds which are joined apically by a pigmented bridge of tissue. Within each fold are numerous capillaries, larger supply and drainage vessels and plentiful pleomorphic melanocytes. The capillaries are extremely specialized vessels, most of which display plentiful microfolds on both their luminal and abluminal surfaces although some capillaries show but a few microfolds. The endothelial cell bodies are extremely thin with most organelles located near the nucleus. All capillaries are surrounded by a thick fibrillar basal lamina which is felt to be structurally important. Pericytes are a common feature within these thickened basal laminae. The numerous melanocytes form an incomplete sheath around the capillaries and are also presumed to be fulfilling a structural role. While the morphology of the pecten in the great horned owl is certainly indicative of a heavy involvement in transport, when compared to the pecten in species that are more visually oriented it is smaller, displays fewer folds and a reduced number of microfolds within the capillaries.

Animals↗

PubMed1993

Fine structure of the retinal epithelium of the tiger salamander (Ambystoma tigrinum).

The retinal pigment epithelium (RPE) of the tiger salamander (Ambystoma tigrinum) consists of a single layer of large squamous-shaped cells. These RPE cells are but minimally infolded basally (sclerally) but show numerous apical (vitreal) processes that interdigitate with the photoreceptor cells. A series of apically-located tight junctions joins the RPE cells to form part of the blood-ocular barrier. Internally the RPE nucleus is large, vesicular and flattened. Smooth endoplasmic reticulum predominates in these cells with only occasional small profiles of rough endoplasmic reticulum present. Polysomes and basally-located mitochondria are plentiful. Golgi zones, lipid droplets, lysosome-like bodies and phagosomes of outer segment material are also present. Myeloid bodies which may show ribosomes on their surface are also numerous. The round to oval melanosomes undergo retinomotor movements and in the light-adapted state reach almost to the external limiting membrane. Bruch's membrane or complexus basalis shows a pentalaminate structure with the central elastic lamina poorly represented. The choriocapillaris is a single layer of large anastomosing capillaries which are but minimally fenestrated.

Ambystoma↗

PubMed1993

Fine structure of the retinal photoreceptors of the tiger salamander (Ambystoma tigrinum).

The retinal photoreceptors of the tiger salamander (Ambystoma tigrinum) have been studied by light and electron microscopy in both light- and dark-adaptation. Rods and cones are present in this duplex retina in a ratio of about 20:1. As in other urodele species these photoreceptors are very large cells. The rod outer segment is composed of bi-membranous discs of uniform diameter displaying several very deep incisures. The rod inner segment displays an ellipsoid of mitochondria and a myoid region which changes in diameter during the lighting cycle indicating that rods undergo photomechanical movements. Rod nuclei are located at all levels of the outer nuclear layer and rod spherules are large and display several invaginated and superficial synaptic sites. Cone photoreceptors while large cells are smaller than the rods. They show a smaller tapering outer segment, a large distal ellipsoid of mitochondria and a prominent paraboloid of glycogen but no oil droplet within the inner segment. Judging by the width of the myoid region which remains similar throughout the lighting cycle, cones in this species show no retino-motor responses. Cone nuclei are less electron dense than rods and are also located at all levels of the outer nuclear layer. The cone synaptic pedicle is larger than that of the rods and also shows several invaginated and superficial synaptic sites.

Adaptation, Ocular↗

PubMed1992

Retinal photoreceptor fine structure in the velvet cichlid (Astronotus ocellatus).

The structure and arrangement of the retinal photoreceptors of the velvet cichlid fish (Astronotus ocellatus) have been studied by light and electron microscopy. Rods, single cones and double (twin) cones are present. In the light-adapted state, rods are very tall cells that reach deep into the retinal epithelial (RPE) layer. The long outer segment is composed of discs of uniform diameter displaying one or two incisures. The rod inner segment shows a distal ellipsoid of mitochondria, and then narrows dramatically in the myoid region. Rod nuclei are electron dense and located deep in the outer nuclear layer. Rod synaptic spherules are small and show two to three invaginated synaptic sites as well as superficial synapses. Single cones are similar to the individual members of a double cone and all display a short tapering outer segment, a large ellipsoid of mitochondria and a myoid rich in rough endoplasmic reticulum, polysomes, Golgi zones and autophagic vacuoles. Double cones have extensive subsurface cisternae along their entire contiguous surfaces. Cone nuclei are large and vesicular and located close to or through the external limiting membrane. The synaptic pedicles of cones are larger, more electron lucent, and display more invaginated (ribbon) synapses as well as conventional (superficial) synaptic sites than do the rod spherules. Rod photoreceptors certainly undergo retinomotor movements and it is probable that cones do as well. In the light-adapted state the cone photoreceptors are arranged in a repeating square mosaic pattern with one single cone surrounded by four double (twin) cones.

Animals↗

PubMed1992

Retinal pigment epithelial fine structure in the velvet cichlid (Astronotus ocellatus).

The morphology of the retinal pigment epithelium (RPE), choriocapillaris and Bruch's membrane (complexus basalis) have been studied by light and electron microscopy in the velvet cichlid (Astronotus ocellatus). The RPE is composed of a single layer of large columnar cells. The basal (scleral) border of these cells is minimally infolded, whereas the apical (vitreal) surface displays numerous pigment-laden processes which in light-adaptation surround both rod and cone outer segments. Laterally the RPE cells are joined by a series of basally located tight junctions. Wandering phagocytes are a constant feature within this epithelial membrane. The RPE cells display a large, vesicular nucleus, numerous mitochondria, much smooth endoplasmic reticulum, polysomes, myeloid bodies, phagosomes and melanosomes. Rough endoplasmic reticulum is relatively scarce within these cells. Although only light-adapted specimens were examined, it is thought that the melanosomes are capable of extensive retinomotor movement. The endothelium of the choriocapillaris facing Bruch's membrane is typically very thin but shows few fenestrations. Bruch's membrane is typical of other teleost species in that it is composed of only three layers.

Animals↗

PubMed1992

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.

Animals↗