Biomedical subjects

C R Braekevelt

Publications and source records attributed to C R Braekevelt.

At least 55 records · Page 3Linked to original sources

PubMed1992

Retinal photoreceptor fine structure in the red-backed salamander (Plethodon cinereus).

The retinal photoreceptors of the red-backed salamander (Plethodon cinerus) have been studied by light and electron microscopy. Rods and single cones are present in this duplex retina in a ratio of about 25:1. The photoreceptors in this amphibian species are much larger than is reported for most vertebrates. In the light-adapted state, rods reach deep into the retinal epithelial (RPE) layer. The rod outer segment is composed of discs of uniform diameter displaying several very deep incisors. The rod inner segment displays a distal elliposid of mitochondria and a short stout myoid region. Rod nuclei are electron dense and often protrude through the external limiting membrane. Rod synaptic spherules are large and display several invaginated synaptic sites as well as superficial synapses. It is felt that the rods do not undergo retinomotor movements. The cone photoreceptors are much smaller than the rods and display a tapering outer segment, an unusual modified ellipsoid and a large parabolid of glycogen in the inner segment. Cone nuclei are less electron dense than rods and are located at all levels within the outer nuclear layer. The synaptic pedicle of the cones is larger, more electron lucent and display more synaptic sites (both invaginated and superficial) than that of rods. It is felt that cone photomechanical responses are minimal.

Animals↗

PubMed1992

Retinal pigment epithelial fine structure in the red-backed salamander (Plethodon cinereus).

The retinal pigment epithelium (RPE) of the red-backed salamander (Plethodon cinerus) consists of a single layer of large squamous shaped cells. The RPE cells are but minimally infolded basally (sclerally) but show many large apical (vitreal) processes interdigitating with the rod outer segments. These epithelial cells are joined laterally by prominent tight junctions located in the mid region of the cells. Internally smooth endoplasmic reticulum is very plentiful while rough endoplasmic reticulum is not. Polysomes, small dense mitochondria and small round to oval melanosomes are plentiful. Golgi zones and lysosome-like bodies are also present as are phagosomes of outer segment material and myeloid bodies. The RPE cell nucleus is large and vesicular. It is felt that the melanosomes undergo retinomotor movements but as only light-adapted specimens were examined it is not known how extensive are these movements. Bruch's membrane or complexus basalis shows the typical pentalaminate structure noted for most vertebrates. The choriocapillaris is a single layer of large anastomosing capillaries which are minimally fenestrated facing Bruch's membrane.

Animals↗

PubMed1992

Retinal epithelial fine structure in the southern fiddler ray (Trygonorhina fasciata).

The morphology of the retinal epithelium (RPE), choriocapillaris and Bruch's membrane (complexus basalis) has been investigated by light and electron microscopy in an elasmobranch, the southern fiddler ray or guitarfish (Trygonorhina fasciata). The RPE consists of a single layer of cuboidal cells which display basal (scleral) infoldings as well as numerous apical (vitreal) finger-like processes which interdigitate with the photoreceptor outer segments. The lateral cell borders are relatively smooth and are joined in the mid-region by a series of tight junctions. Internally the RPE nucleus is large, vesicular and centrally located. Smooth endoplasmic reticulum (SER) is abundant while rough endoplasmic reticulum (RER) is scarce. Polysomes are however widespread and mitochondria are plentiful. Two unusual organelles are also noted. One consists of a membrane bound array of tubules while the other is a membrane bound structure consisting of a granular matrix with again an internal tubular array. This species possesses a choroidally located tapetum lucidum in the superior fundus and over this tapetal area, melanosomes are absent from the RPE cells. In non-tapetal locations a few melanosomes are present that do not appear to undergo photomechanical movements. Bruch's membrane is a pentalaminate structure with an almost continuous central elastic layer (lamina densa). The choriocapillaris forms a single layer of capillaries with a thin but only minimally fenestrated endothelium facing Bruch's membrane.

Animals↗

PubMed1992

Photoreceptor fine structure in the southern fiddler ray (Trygonorhina fasciata).

The fine structure of the retinal photoreceptors has been studied by light and electron microscopy in the southern fiddler ray or guitarfish (Trygonorhina fasciata). The duplex retina of this species contains only rods and single cones in a ratio of about 40:1. No multiple receptors (double cones), no repeating pattern or mosaic of photoreceptors and no retinomotor movements of these photoreceptors were noted. The rods are cylindrical cells with inner and outer segments of the same diameter. Cones are shorter, stouter cells with a conical outer segment and a wider inner segment. Rod outer segment discs display several irregular incisures to give a scalloped outline to the discs while cone outer segment discs have only a single incisure. In all photoreceptors a non-motile cilium joins the inner and outer segments. The inner segment is the synthetic centre of photoreceptors and in this compartment is located an accumulation of mitochondria (the ellipsoid), profiles of both rough and smooth endoplasmic reticulum, prominent Golgi zones and frequent autophagic vacuoles. The nuclei of rods and cones have much the same chromatin pattern but cone nuclei are invariably located against or particularly through the external limiting membrane (ELM). Numerous Landolt's clubs which are ciliated dendrites of bipolar cells as well as Müller cell processes project through the ELM, which is composed of a series of zonulae adherentes between these cells and the photoreceptors. The synaptic region of both rods (spherules) and cones (pedicles) display both invaginated (ribbon) synapses and superficial (conventional) synapses with cones showing more sites than the rods.

Animals↗

PubMed1991

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.

Animals↗

PubMed1991

Electron microscopic study of the occlusible tapetum lucidum of the southern fiddler ray (Trygonorhina fasciata).

The choroidally located tapetum lucidum of the southern fiddler ray (Trygonorhina fasciata) has been examined by light and electron microscopy in both light- and dark-adaptation. 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 are situated immediately external to the choriocapillaris. These tapetal cells alternate with and are separated from one another by melanocytes which have an inner extension that curves and intervenes between the tapetal cells and the choriocapillaris. The tapetal cells and the melanocytes are flattened cells with their widest dimension facing the retina. Internally the tapetal cells display a peripherally-located, vesicular nucleus with most of the cell 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 small melanosomes of the melanocytes are widely dispersed and fill the portion of the cell intervening between the tapetal cells and the incoming light. This effectively occludes the tapetum as light is unable to reach the reflective material. In dark-adaptation the melanosomes withdraw from this location, exposing the tapetum to light and allowing it to act as a reflective layer. The retinal epithelium overlying the tapetal area is totally unpigmented so as not to interfere with the passage of light.

Animals↗

PubMed1991

Electron microscopic observations on the pecten of the great blue heron (Ardea herodias).

The pecten oculi of the great blue heron (Ardea herodias) has been examined by both light and electron microscopy. In this species the pecten is large and of the pleated type. It consists of 14-15 acordion folds that are joined apically by a more heavily pigmented bridge of tissue which holds the pecten in a fan-like shape widest at its base. As in other species it is situated over the optic nerve head and projects out into the vitreous. Within each fold are numerous capillaries, larger supply and drainage vessels and many melanocytes. The capillaries are extremely specialized vessels which display extensive microfolds on both their luminal and abluminal borders. The endothelial cell bodies are extremely thin with most organelles present in a paranuclear location. The capillaries are surrounded by thick fibrillar basal laminae which are felt to be structurally useful. Pericytes are a common feature of these capillaries. The numerous pleomorphic melanocytes which form an incomplete sheath around the capillaries and other blood vessels are also felt to be important in structural support of the pecten. The morphology of the pecten of the great blue heron is indicative of a heavy involvement in the transport of materials.

Anatomy, Comparative↗

PubMed1991

Retinal epithelial fine structure in the brush-tailed possum (Trichosurus vulpecula).

As part of a comparative morphological study, the fine structure of the retinal pigment epithelium (RPE), the choriocapillaris and Bruch's membrane (complexus basalis) has been studied by electron microscopy in the brush-tailed possum (Trichosurus vulpecula), an Australian marsupial. In this species, the RPE consists of a single layer of squamous cells which show numerous shallow basal (scleral) infoldings as well as extensive apical (vitreal) processes enclosing rod outer segments. The epithelial cells are joined laterally by prominent, apically-located tight junctions. Internally smooth endoplasmic reticulum is the most abundant cell organelle with only small amounts of rough endoplasmic reticulum present. Basally-located mitochondria and apically-located melanosomes are abundant while polysomes and the occasional Golgi zone are also noted. The epithelial cell nucleus is large and vesicular. Bruch's membrane shows the typical pentalaminate structure noted for mammalian species. The choriocapillaris is a single layer of anastomosing capillaries which are heavily fenestrated facing the retinal epithelium.

Animals↗

PubMed1990

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.

Animals↗

PubMed1990

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.

Animals↗

PubMed1990

Fine structure of the feline tapetum lucidum.

The morphology of the tapetum lucidum of the domestic cat (Felis catus) was studied by electron microscopy. The tapetum is a tapetum cellulosum, localized in the choroid of the dorsal fundus. The cells comprising this structure are described.

Animals↗

PubMed1990

Circadian changes in the retinal pigment epithelium of the butterfly fish (Pantodon buchholzi).

The fine structure of the retinal pigment epithelium (RPE) of the butterfly fish (Pantodon buchholzi) has been investigated in both light- and dark-adaptation. In this species which possesses a retinal tapetum lucidum, the RPE melanosomes move basally (sclerally) in dark-adaptation and apically (vitreally) during light-adaptation. In addition a number of other morphological changes are noted which also appear to be influenced by the light cycle. In dark-adaptation when the melanosomes move basally the lipid spheres of the tapetum lucidum move apically to form an efficient reflective layer with the tapetal spheres often coalescing at this time. In light-adaptation when the melanosomes move more apically, the tapetal material moves more basally and is probably masked to some extent by the melanosomes. Other morphological features which appear to change at least to some degree are; the shape and location of the RPE nucleus (more basally located and more pleomorphic in light-adaptation); the localisation and electron density of the mitochondria (more scattered and more electron dense in dark-adaptation) and the number and size of the myeloid bodies (small and relatively numerous in dark-adaptation and larger but more widely scattered in the light-adapted state). Wandering phagocytes between the RPE cells were only noted in light-adaptation when they were situated adjacent to Bruch's membrane. These observations would indicate that a number of morphological changes take place within the RPE cells of this species in addition to be obvious movement of melanosomes and that these changes should perhaps be included in descriptions of photomechanical or retinomotor movements.

Animals↗

PubMed1990

Photoreceptor fine structure in light- and dark-adaptation in the butterfly fish (Pantodon buchholzi).

The morphology of the retinal photoreceptors of the butterfly fish Pantodon buchholzi has been studied by electron microscopy in both light- and dark-adaptation. The photoreceptors in this species are readily divisible into rods and cones based on morphological criteria. No double or twin cones are present. The rod photoreceptors show marked retinomotor movements. In light-adaptation they are extremely elongate cells while in the dark-adapted state they are much shorter. Cones seem to respond but minimally to the circadian cycle. Rod outer segments are composed of membranous discs of uniform diameter displaying several incisures. The inner segment has a small distal ellipsoid and a thin myoid region which is lost in dark-adaptation. The nuclei of rods are condensed and always located vitread to the external limiting membrane. The rod synaptic spherule displays 2 or 3 invaginated sites. The single cones display a tapering outer segment. The wider inner segment contains a large electron-dense ellipsoid with small glycogen deposits located peripherally. The cone nuclei are large and vesicular and usually located sclerad to the external limiting membrane. The synaptic pedicle of cones is larger and more electron-lucent and contains more synaptic sites than do the rods. No mosaic pattern of arrangement of the photoreceptors is apparent. Except for the obvious lengthening or shortening of the rods, the morphology of the photoreceptors changes but little during the circadian cycle.

Adaptation, Physiological↗

PubMed1990

Retinal photoreceptor fine structure in the mallard duck (Anas platyrhynchos).

The retinal photoreceptors of the mallard duck (Anas platyrhynchos) consist of rods, single cones and double (unequal) cones present in a ratio of about 1:2:1. The rods have relatively large cylindrical outer segments which in the light-adapted state reach to the retinal epithelial (RPE) cell bodies and are surrounded by the pigment-laden apical processes of these RPE cells. The inner segment displays an apically located ellipsoid of mitochondria and plentiful polysomes, some rough ER and Golgi zones. The rod nucleus is located within the outer nuclear layer and the synaptic pedicle displays both invaginated (ribbon) and superficial (conventional) synaptic sites. Single cones display a thin tapering outer segment, a large often heterogeneous oil droplet in the apical region of the inner segment, an ellipsoid of mitochondria and a prominent paraboloid of glycogen. Double cones consist of a larger chief member which shows a thin tapering outer segment, a large heterogeneous oil droplet and a prominent paraboloid. The small accessory cone shows a thin outer segment, a small granular oil droplet and a paraboloid. As in the single cone, polysomes, RER and Golgi zones are also present within the inner segment. Near the outer limiting membrane the two members of the double cone show a membrane specialization on their contiguous surfaces. Both single and double cones display several invaginated (ribbon) synapses as well as numerous superficial synaptic sites.

Animals↗

PubMed1990

Fine structure of the retinal pigment epithelium of the mallard duck (Anas platyrhynchos).

As part of a comparative morphological study, the fine structure of the retinal pigment epithelium (RPE), the choriocapillaris and Bruch's membrane (complexus basalis) has been investigated by light and electron microscopy in the mallard (Anas platyrhynchos). In this species the RPE consists of a single layer of cuboidal cells which display numerous very deep basal (scleral) infoldings and extensive apical (vitreal) processes which enclose photoreceptor outer segments. The RPE cells are joined laterally by prominent basally-located tight junctions. Internally smooth endoplasmic reticulum is the most abundant cell organelle with only small amounts of rough endoplasmic reticulum present. Polysomes are abundant as are basally-located mitochondria which often displayed a ring-shaped profile. The cell nucleus is large and vesicular. Melanosomes are plentiful only within the apical processes of the RPE cells in the light-adapted state. Myeloid bodies are large and numerous and very often have ribosomes on their outer surface. Bruch's membrane (complexus basalis) shows a pentalaminate structure but with only a poorly represented central elastic lamina. Profiles of the choriocapillaris are relatively small and the endothelium of these capillaries while extremely thin facing the retinal epithelium is but minimally fenestrated.

Animals↗

PubMed1990

Fine structure of the retinal photoreceptors of the ranch mink Mustela vison.

The structure of the retinal photoreceptors of the ranch mink (Mustela vison) has been investigated by light and electron microscopy. In this mammalian species, the photoreceptors can be readily differentiated and adequately described by the classical terminology of rods and cones, with the rods being the more numerous. Rods are long slender cells while cones are shorter and stouter in appearance. Both rods and cones are highly differentiated and extremely polarized cells consisting of an outer segment, a non-motile connecting cilium, an inner segment, a nuclear region and a synaptic process extending to an expanded synaptic ending. Morphological differences are noted between rods and cones for most of the various regions of these cells. While rods reach to the cell body of the retinal pigment epithelial (RPE) cells, larger apical processes from the RPE extend to the shorter cone cells, so that both photoreceptor types are in intimate contact with the retinal epithelial cells.

Animals↗

PubMed1989

Fine structure of the retinal epithelium and tapetum lucidum of the ranch mink Mustela vison.

The morphology of the retinal pigment epithelium (RPE), Bruch's membrane (complexus basalis), choriocapillaris and tapetum lucidum has been studied in the eye of the ranch mink (Mustela vison) by light and electron microscopy. The RPE is composed of a single layer of cells joined laterally by apically located junctional complexes. Basally (sclerally) these cells display numerous infoldings whereas apically (vitreally) two types of processes are associated with rod and cone outer segments. Smooth endoplasmic reticulum and mitochondria are abundant in these cells whereas rough endoplasmic reticulum and polysomes, although present, are not plentiful. An occasional wandering phagocyte is noted at the RPE-photoreceptor interface. In the posterosuperior part of the fundus, a degenerative tapetum lucidum is present. The presence of only a few layers of tapetal cells containing but little reflective material and the haphazard arrangement of this material makes it very unlikely that this area functions as an effective tapetum lucidum. The RPE over the aberrant tapetum, however, shows the morphology that is seen when a functioning tapetum cellulosum is present, namely the absence of melanosomes and an indented choriocapillaris. Bruch's membrane in non-tapetal areas is pentalaminate but, over the tapetum and where it is associated with capillary profiles, it is reduced to a single, thickened basal lamina. The choriocapillary endothelium is highly fenestrated and in nontapetal areas these capillaries are not indented into the epithelial layer.

Animals↗

PubMed1989

Development of the retinal tapetum lucidum of the walleye (Stizostedion vitreum vitreum).

The development of the retinal tapetum lucidum within the cells of the retinal pigment epithelium (RPE) has been investigated by both light and electron microscopy in the walleye (Stizostedion vitreum vitreum) in specimens ranging in total length from 25-140 mm. In addition changes in the arrangement of the photoreceptors (both rods and cones) in both light and dark-adaptation have also been studied. At 25 mm no evidence of a tapetum is present. At about 30 mm it makes its initial appearance as granular bodies formed within the apical smooth endoplasmic reticulum (SER) cisternae of the RPE cells in the superior temporal fundus. The developing tapetum then spreads peripherally and continues to thicken in existing areas. By 90 mm it is well established throughout the fundus but always appears better developed in the superior fundus. By 125-140 mm it is essentially adult in appearance. At 60-70 mm the rods and cones begin to form bundles producing macroreceptors of 20-30 photoreceptors. In dark-adaptation the rod bundles are retracted and have one or more cone cells centrally located in each bundle, with the bundles separated from one another by melanosomes. Initially when no tapetal material is present, post-larval walleye are positively phototactic and feed on zooplankton. In the adult condition when a tapetum lucidum and large macroreceptors are present, the walleye is negatively phototactic and feeds almost exclusively on larger organisms such as other fish.

Adaptation, Ocular↗