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Biomedical subjects

C R Braekevelt

Publications and source records attributed to C R Braekevelt.

At least 91 records · Page 5Linked to original sources
PubMed1984

Retinal pigment epithelial fine structure in the nighthawk (Chordeiles minor).

The retinal pigment epithelial (RPE) layer of a nocturnally active bird, the nighthawk (Chordeiles minor), has been investigated by electron microscopy. This layer is composed of a single layer of low cuboidal cells joined laterally by tight junctions. Basally (sclerally), the epithelial cells display deep infoldings while apically (vitreally) numerous processes enclose photoreceptor outer segments. Internally, the nucleus is large, vesicular and basally located. Smooth endoplasmic reticulum and mitochondria are abundant. Rough endoplasmic reticulum (RER) is scarce but numerous polysomes are present. Myeloid bodies are widespread and often appear to be continuous with the RER membranes. Phagosomes of outer segment material are present but not numerous in the light-adapted condition. Large spindle-shaped melanosomes as well as numerous rounded tapetal spheres are present in the apical region and apical processes of the epithelial cells. The composition of the reflective tapetal spheres is unknown. Bruch's membrane (complexus basalis) in most locations displays a pentalaminate appearance while in some regions it is trilaminate. The endothelium of the choriocapillaris facing Bruch's membrane is extremely thin but only sparsely fenestrated.

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PubMed1984

Electron microscopic observations on the pecten of the nighthawk (Chordeiles minor).

The pecten of a nocturnally active bird, the nighthawk (Chordeiles minor) has been examined by light and electron microscopy. In this species the pecten consists of a pleated, highly vascular pigmented structure. It is situated over the optic nerve head and projects into the vitreous. Several accordion folds (4-5) are joined apically by a bridge of tissue which holds the pecten in a fan-like shape widest at the base. The entire pecten is enclosed in a fine basal lamina. Within each fold are numerous capillaries, larger blood vessels which could not be differentiated as to arterioles or venules and melanocytes. The capillaries are surrounded by extremely thick basal laminae composed of several fibrillar layers. These capillaries display extremely numerous microfolds on both the luminal and abluminal borders. The cell body is extremely thin with most organelles present in a paranuclear location. Pericytes are often associated with the capillaries. The melanocytes form an incomplete sheath around the capillaries and other blood vessels. The morphology of the pecten is indicative of a heavy involvement in the transport of materials.

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PubMed1984

Retinal fine structure in the european eel Anguilla anguilla. II. Photoreceptors of the glass eel stage.

The eye of the European eel (Anguilla anguilla) undergoes marked morphological changes during its long and complex life cycle. This report describes the fine structure of the photoreceptors of the glass eel stage and forms part of a comparative study of the retina of the eel during the major stages of its life cycle. The photoreceptors of the glass eels are readily divisible into rods and cones based on morphological criteria. Rods are more numerous and much longer than cone cells, reaching to the pigment epithelial layer. In rods the inner and outer segments are of the same diameter whereas in cones, inner segments are much wider than the outer segments which taper distally. While the nuclei of rods are located at all levels within the outer nuclear layer, in the light-adapted condition at least, cone nuclei are predominantly located scleral to the external limiting membrane. The synaptic terminal of rods displays 2-3 invaginated (ribbon) synaptic sites while cones have 8-10 such sites. Both rods and cones also possess superficial synaptic sites. Apparent cellular degeneration is quite widespread amongst rods and is also occasionally noted for cone photoreceptors.

Anguilla↗
PubMed1984

Retinal fine structure in the European eel Anguilla anguilla. III. Pigment epithelium of the yellow eel stage.

The catadromous migrations of the European eel (Anguilla anguilla) are accompanied by a number of morphological and biochemical alterations within the visual system. This report describes the morphology of the retinal pigment epithelium, choriocapillaris and Bruch's membrane of the yellow eel and forms the third in a series describing the morphological changes in the retina of the eel during its life cycle. The retinal epithelium of the yellow eel stage consists of a single layer of cuboidal cells joined laterally by tight junctions. Apically, numerous processes enclose photoreceptor cells while basally and laterally the cell borders are relatively smooth. Internally the epithelial cells display abundant smooth endoplasmic reticulum, mitochondria, melanosomes, phagosomes and myeloid bodies. Rough endoplasmic reticulum, polysomes and Golgi zones are present but not plentiful. The nucleus is pleiomorphic and basally located. Bruch's membrane (complexus basalis) is trilaminate. The choriocapillaris is typically fenestrated facing the retina. A stratum argenteum is present in the outer choroid and wandering phagocytes are noted at the retinal epithelium-photoreceptor interface.

Anguilla↗
PubMed1983

Fine structure of the retinal rods and cones in the domestic pig.

The structure of the retinal photoreceptors of the domestic pig has been studied by both light and electron microscopy. In this species the photoreceptors can be traditionally and adequately differentiated into rods and cones, with rods being the more numerous. Rods are long, slender cells, while cones are shorter and stouter. Both rods and cones consist of an outer segment, a connecting cilium, an inner segment, a nuclear region and a synaptic process leading to a synaptic ending. Differences are noted between rods and cones for the various regions of these cells. What are presumed to be the displaced cell bodies of both rods and horizontal cells are often noted within the outer plexiform layer of this species.

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PubMed1983

Fine structure of the choriocapillaris, Bruch's membrane and retinal epithelium in the sheep.

The fine structure of the choriocapillaris, Bruch's membrane and retinal epithelium was investigated by light and electron microscopy in both the tapetal and non-tapetal fundus of the domestic sheep. The choriocapillaris are heavily fenestrated on the side facing the retina. Overlying the tapetum, the choriocapillaris also displays fenestrae on the choroidal border. Bruch's membrane is pentalaminate throughout the retina and slightly thicker over the tapetal region. In all locations the retinal epithelium consists of a single layer of cells, which vary in height depending upon their location. The epithelial cells are joined laterally by apically located tight junctions and throughout the retina display numerous basal infoldings and apical processes which enclose rod outer segments. The epithelial cell nucleus is large and vesicular. All retinal epithelial cells are rich in smooth endoplasmic reticulum and phagosomes. Although not as abundant, mitochondria, Golgi zones, profiles of rough endoplasmic reticulum and polysomes are also noted in all locations. In non-tapetal areas, melanosomes are numerous whereas over the central tapetum fibrosum they are absent. While lysosomes are present throughout the epithelial layer, over the tapetal area they appear to be more numerous. The absence of melanosomes over a functional tapetum fibrosum is to be expected. The apparent increase in lysosomal numbers in this location may indicate an enhanced shedding of outer segment material over the tapetal region. Although some retinal epithelial cells are modified to accommodate a tapetum lucidum their morphology is basically similar throughout the retina and probably indicates that all regions of the retinal epithelium are capable of the normal functions of this indispensible retinal layer.

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PubMed1983

Retinal photoreceptor fine structure in the domestic sheep.

The structure of the photoreceptors of the domestic sheep has been investigated by light and electron microscopy. In the sheep retina the photoreceptors are readily differentiated and adequately described by the classical terminology of rods and cones. Rods are more numerous then cones. They appear as tall slender cells measuring 45-50 microns in length and 2-3 microns in diameter with inner and outer segments of the same diameter. Rod nuclei are located at all levels of the outer nuclear layer. The rod synaptic ending is small and rounded with two to three invaginated (ribbon) synaptic sites. Cone cells are shorter than rods, measuring only 30-35 micron in total length. The cone inner segment is wider than the outer segment which tapers distally. Cone nuclei are located adjacent to the external limiting membrane and are more vesicular than rod nuclei. Cone synaptic endings are larger spherules with ten to twelve (ribbon) synaptic sites. Both rods and cones also display superficial synaptic regions. An unusual form of apparent degeneration is noted in cone photoreceptors. This involves a disorganization and loss of the outer segment and a progressive distal movement of the nucleus through the inner segment towards the retinal epithelial layer. The structure and ratio of rods and cones remains similar in both the tapetal and nontapetal regions of the fundus.

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PubMed1983

Fine structure of the canine tapetum lucidum.

The structure of the canine tapetum lucidum appears to be basically similar to that described in some other carnivores, but specific distinctions are present. The reflecting rodlets of the dog are similar in their dimensions to those of the cat, but they are more closely packed, resulting in a unique value for the wavelength of maximal reflectance. In addition, the rodlets are less precisely orientated, as compared to those of the cat, a condition which appears to result in a less efficient reflectance from the dog tapetum. It has also been shown that the dog does not possess a retinal tapetum lucidum, as had previously been suggested.

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PubMed1983

Photoreceptor fine structure in the domestic ferret.

The morphology of the retinal photoreceptors has been studied in the domestic ferret by electron microscopy. 2 types of photoreceptor are present and are differentiated morphologically into the classical categories of rods and cones. Rods are longer, more slender and more numerous than the cones. The rod outer segment is uniform in diameter along its entire length while the cone outer segment tapers distally. Cone outer segments are short and large apical processes from the pigment epithelium extend some distance to wrap around them. Both rods and cones possess a connecting cilium and a large cross-striated fibril which begins near the ciliary basal body and extends through the outer segment to end near the external limiting membrane. Cone nuclei are more vesicular than rod nuclei. They form a single layer immediately beneath the external limiting membrane while rod nuclei form several layers beneath the cone nuclei. The synaptic region of the cone cells is larger and more elaborate than that of the rods. Synapses in both rods and cones appear to be formed by both superficial and invaginated contacts.

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PubMed1983

Smooth muscle in human intracerebral arterioles.

Aterioles from the human cerebral cortex have been examined by electron microscopy in several patients aged 43 to 58 years. The wall of all arterioles (ranging in size from 10-70 microns in diameter) is composed of a single layer of endothelium and from one to 5 layers of smooth muscle cells. A basal lamina supports the endothelium and encloses the smooth muscle cells. Smooth muscle cells were identified by morphological criteria that included the presence of dense bodies, abundant microfilaments and numerous pinocytotic vesicles. Some of the intracerebral arterioles were observed in a constricted state indicating functional smooth muscle cells. The presence of functional smooth muscle cells in human intracerebral arterioles indicates that these arterioles are capable of vasoconstriction which may be a possible factor in some clinical conditions.

Adult↗
PubMed1982

Photoreceptor fine structure in the goldeye (Hiodon alosoides) (teleost).

The retinal photoreceptors of the goldeye (Hiodon alosoides) are arranged in large bundles of 40-50 cells optically isolated from other bundles by the retinal epithelial cells. Within each bundle are found both rods and cones in roughly equal numbers. Rod photoreceptors show marked retinomotor responses to project beyond the photoreceptor bundle in light-adaptation and to lie entirely within the bundle in dark-adaptation. In all stages of the light cycle cone outer segments remain at the apex of the photoreceptor bundle. In light-adaptation, rod inner segments display an apical ellipsoid separated from a basal ellipsoid by the greatly elongated myoid. In dark-adaptation the rod inner segment is much the same diameter throughout its length. In both rods and cones, profiles of rough and smooth endoplasmic reticulum and Golgi zones are present in a supranuclear location. The nuclei of rods display little heterochromatin and are located vitreal to the external limiting membrane in light-adaptation, whereas in dark-adaptation more heterochromatin is noted and the nuclei lie scleral to the external limiting membrane. Cone nuclei display the same changes in chromatin pattern as rods but they show changes in nuclear location opposite that of the rods. Throughout its length, the rod photoreceptor cytoplasm is more electron dense than that of the cone. The synaptic spherule of rods displays 2-3 invaginated synaptic sites while the cone pedicle is larger and presents 8-10 invaginated synaptic sites. Both rods and cones also appear to have superficial synaptic sites. Membrane specializations are found along the length of the inner segments where rods and cones are contiguous. These may act as sites of intercellular communication and the whole photoreceptor bundle may therefore be considered as a macroreceptor.

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PubMed1982

Fine structure of the retinal epithelium and retinal tapetum lucidum of the goldeye Hiodon alosoides.

The fine structure of the retinal epithelium of the goldeye (Hiodon alosoides) has been investigated with electron microscopy in both the light- and dark-adapted condition. The retinal epithelial cells are extremely tall columnar cells. They form a single layer abutting on Bruch's membrane basally (sclerally) and interdigitating with bundles of photoreceptor cells apically (vitreally). The retinal epithelial cells and the photoreceptor bundles are arranged in a hexagonal fashion so that each epithelial cell normally contacts three photoreceptor bundles and three other epithelial cells. In the light-adapted state, melanosomes are sparsely but evenly distributed throughout the cell body except for the apices, which contain more melanosomes. In dark-adaptation, melanosomes become very concentrated in the cell apex between photoreceptor bundles, with some accumulation of melanosomes also noted basally. The predominant feature of these epithelial cells is the presence of numerous reflective bodies which act as a retinal tapetum lucidum. During light-adaptation these reflective bodies are scattered throughout the cell but in dark-adaptation they accumulate centrally. In the body of the epithelial cell these reflective particles are diffusely arranged, whereas they are regularly arranged where epithelial cells ensheath photoreceptor bundles. Circadian changes are also noted in the morphology of the myeloid bodies and mitochondria and in the location of the epithelial cell nucleus. Wandering phagocytes are present at the retinal epithelial-photoreceptor interface. Bruch's membrane is thick and trilaminate and the choriocapillaris endothelium is thin and sparsely fenestrated.

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PubMed1982

Fine structure of the retinal epithelium, Bruch's membrane (Complexus basalis) and choriocapillaris in the domestic ferret.

The fine structure of the retinal epithelium, choriocapillaris, and Bruch's membrane has been studied by electron microscopy in the domestic ferret. The ferret possesses a tapetum cellulosum in the superior fundus, and the morphology of the retinal epithelium and its associated structures varies depending upon its relation to the tapetum. Over the tapetum the retinal epithelium is a single layer of low cuboidal cells with basal infoldings and two types of apical process. Internally the cells are non-pigmented, display much smooth endoplasmic reticulum, little rough endoplasmic reticulum, lysosomes, phagosomes, and a basally located nucleus. The choriocapillaris is composed of large capillaries indented into the retinal epithelium. Bruch's membrane is thin and composed of three layers. In non-tapetal regions the retinal epithelial cells are pigmented but display most of the other features noted over the tapetum. The choriocapillaris is not indented in non-tapetal areas. Bruch's membrane appears identical to that noted over the tapetum. Wandering phagocytes are occasionally noted at the retinal epithelium-photoreceptor junction.

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PubMed1982

Retinal fine structure in the European eel Anguilla anguilla. I. Pigment epithelium of the glass eel stage.

The European eel (Anguilla anguilla) has a complex life cycle during which it changes habitat and to some extent morphology. During this time the eye undergoes modifications in both size and photopigments. This report forms the first of a comparative morphological study of the retina of the eel during the major stages of its life cycle. The retinal pigment epithelium of the glass eel stage consists of a single layer of cuboidal cells joined laterally by cell junctions. Both the basal (scleral) and lateral borders of these cells are relatively smooth while apically (vitreally) numerous processes enclose photoreceptor outer segments. Within the epithelial cells are abundant mitochondria, melanin granules, lipid droplets, myeloid bodies and a peculiar "multitubular body". Phagosomes are present but not numerous. Bruch's membrane is relatively thin and homogeneous. The choriocapillaris although present is not widespread and displays typically fenestrated endothelium.

Anguilla↗
PubMed1980

Fine structure of the retinal epithelium and tapetum lucidum in the giant danio (danio malabaricus) (teleost).

The normal morphology of the retinal epithelium and tapetum lucidum has been studied by transmission electron microscopy in the eye of a teleost; the giant danio (Danio malabaricus). The retinal epithelium forms a single layer of cells joined laterally by cell junctions. Centrally in the retina these cells are tall columnar, while more peripherally they become more cuboidal in shape. Apical processes of these cells enclose photoreceptor inner and outer segments. Few basal (scleral) infoldings are seen in the epithelial cells. Throughout the epithelial layer the cells display smooth endoplasmic reticulum, numerous mitochondria and phagosomes. Within the epithelial cells centrally located in the retina are found numerous lipid spheres which act as a tapetum lucidum. Only at the extreme periphery are these reflective spheres scarce or absent from the epithelial cells. Melanosomes, although found throughout the epithelial layer, are scarce centrally and more numerous peripherally. A population of wandering phagocytes appears to be a normal feature within the retinal epithelial layer of this species. Bruch's membrane shows a trilaminate composition rather than the usual pentalaminate structure seen in most vertebrates. The choriocapillaris endothelium facing Bruch's membrane is very thin but only minimally fenestrated.

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PubMed1980

The fine structure of the retinal epithelium in the scissortail (Rasbora trilineata) (Teleost).

The normal morphology of the retinal epithelium has been studied by transmission electron microscopy in the eye of a teleost; the scissortail (Rasbora trilineata). The retinal epithelium forms a single layer of cells joined laterally by cell junctions. Centrally in the retina these cells are columnar while moving peripherally they are cuboidal and eventually squamous in shape. Apical processes of these cells enclose photoreceptor inner and outer segments and reach almost to the external limiting membrane. Few basal infoldings are seen. In addition to the usual cell organelles seen in retinal epithelial cells, i. e., abundant smooth endoplasmic reticulum, numerous mitochondria and phagosomes, the central epithelial cells contain numerous lipid tapetal spheres and a few melanosomes. Peripherally the cells contain few or no tapetal spheres and more melanosomes. A population of wandering phagocytes within the epithelial layer is described. Bruch's membrane shows a three-layered composition rather than the usual pentalaminate structure seen in most vertebrates. The choriocapillaris endothelium facing Bruch's membrane is very thin but displays only a few fenestrae.

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