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Biomedical subjects

C R Braekevelt

Publications and source records attributed to C R Braekevelt.

At least 73 records · Page 4Linked to original sources
PubMed1989

Photoreceptor fine structure in the bobtail goanna (Tiliqua rugosa).

The fine structure of the retinal photoreceptors has been studied by light and electron microscopy in the bobtail goanna (Tiliqua rugosa) an Australian diurnal lizard. The photoreceptors in this species are readily divisible into rods or cones based on morphological criteria. Single cones are the dominate cell type with a cone:rod ratio of about 80:1. No multiple photoreceptors were present nor was a photoreceptor mosiac observed. Cones are large cells with a short, tapering outer segment. The inner segment contains a large apical oil droplet, an ellipsoid of mitochondria and a prominent paraboloid of glycogen. Rods are slender cells with a longer non-tapering outer segment. Rod inner segments display an ellipsoid but no oil droplet or paraboloid. The nuclei of cones are large and vesicular while rod nuclei are smaller and displayed more heterochromatin. The synaptic pedicle of cones is larger than the spherule of rods. Both rods and cones displayed both invaginated (ribbon) and conventional synapses with the large cones having more synaptic sites. As only light-adapted specimens were examined it is uncertain if either rods or cones are capable of retinomotor responses.

Animals↗
PubMed1989

Fine structure of the conus papillaris in the bobtail goanna (Tiliqua rugosa).

The structure of the conus papillaris in an Australian lizard, the bobtail goanna (Tiliqua rugosa) was investigated by light and electron microscopy. In this strongly diurnal species, the conus papillaris consists of a heavily vascularized and pigmented, finger-like structure about 1 mm in diameter and 3-4 mm in length. It is situated over the optic nerve head and projects into the vitreous chamber. Within the conus are numerous capillaries and larger blood vessels, melanocytes and occasional mast cells. Many of the capillaries display prominent luminal and abluminal microfolds. Other capillaries show no microfolds while still others display an intermediate number of microfolds. The larger blood vessels are usually indistinguishable as to being either arterioles or venules. The endothelial cells of all blood vessels show a population of cytoplasmic granules. The melanocytes are large pleomorphic cells usually rich in microfilaments. Unmyelinated nerve processes are plentiful within the conus and the Schwann cells enclosing these nerve fibres are occasionally seen to be pigmented. The morphology of the conus papillaris indicates a heavy involvement in the transport of materials. It is considered to be homologous to the pecten oculi of the avian eye; to the falciform process of the teleost eye; to the supraretinal vessels of amphibians and to the intraretinal vessels of the mammalian eye.

Actin Cytoskeleton↗
PubMed1989

Retinal pigment epithelial fine structure in the bobtail goanna (Tiliqua rugosa).

The retinal pigment epithelium (RPE), the choriocapillaris and Bruch's membrane (complexus basalis) have been studied by light and electron microscopy in the bobtail goanna (Tiliqua rugosa) an Australian diurnal lizard. The RPE consists of a single layer of cuboidal cells which display very deep and tortuous basal (choroidal) infoldings as well as numerous apical (vitreal) processes which interdigitate with the photoreceptor cells. The lateral cell borders are relatively smooth and joined by basally located tight junctions. Internally smooth endoplasmic reticulum is abundant while rough endoplasmic reticulum is not. The RPE cell nucleus is large and vesicular and basally located in the light-adapted state. Polysomes, mitochondria and myeloid bodies are present and widely distributed. Melanosomes are plentiful in the apical region of the epithelial cells in light-adaptation. Bruch's membrane is pentalaminate with the basal lamina of the choriocapillaris being exceptionally thick. The choriocapillaris is a single layer of large-caliber capillaries with thin but only moderately fenestrated endothelium. Numerous dense granules are always present within these endothelial cells.

Animals↗
PubMed1988

Fine structure of the pecten of the pigeon (Columba livia).

The pecten oculi of the diurnally active pigeon (Columba livia) has been examined by light and electron microscopy. In this species the pecten is of the pleated type. It is situated over the optic nerve head and extends out into the vitreous. Fifteen to 17 accordion folds are joined apically by a heavily pigmented ridge of tissue which holds the pecten in a fan-like shape, widest at the base. The entire pecten is enclosed by a fine basal lamina. Within each fold are numerous capillaries, larger blood vessels and melanocytes. The capillaries are surrounded by thick fibrillar basal laminae which often contain pericytes. These capillaries display numerous microfolds on both the luminal and abluminal borders. The endothelial cell body is extremely thin with most organelles present in a paranuclear location. The melanocytes which are more plentiful in the bridge region form an incomplete sheath around the capillaries and other blood vessels. The morphology of the pecten in the pigeon is indicative of a heavy involvement in the transport of materials.

Animals↗
PubMed1988

Retinal epithelial fine structure in the vervet monkey (Cercopithecus aethiops).

The morphology of the retinal pigment epithelium (RPE) and closely associated Bruch's membrane (complexus basalis) and choriocapillaris have been investigated by electron microscopy in the vervet monkey (Cercopithecus aethiops). The RPE is composed of a single layer of cuboidal cells joined laterally by apically-located junctional complexes. Basally (sclerally) these cells display numerous infoldings while apically (vitreally) abundant processes enclose and interdigitate with rod outer segments. Internally the large vesicular nucleus is centrally located and smooth endoplasmic reticulum, mitochondria and lysosome-like bodies, are plentiful. Rough endoplasmic reticulum, polysomes and melanosomes while present are not abundant. Phagosomes of outer segment discs are noted in various stages of uptake and degradation. The choriocapillaris is highly fenestrated over large areas. Bruch's membrane shows the typical pentalaminate structure noted in other mammalian species without a tapetum lucidum.

Animals↗
PubMed1988

Retinal fine structure in the European eel Anguilla anguilla. V. Pigment epithelium of the sexually immature silver eel stage.

The fine structure of the retinal pigment epithelium (RPE), Bruch's membrane and choriocapillaris has been investigated in the sexually immature silver eel and compared with previous observations made at the glass and yellow eel stages of the lengthy life cycle of the European eel. As in the previous stages the retinal pigment epithelium of the sexually immature silver eel consists of a single layer of cuboidal to squamous cells joined laterally by apically located cell junctions. The basal (scleral) and lateral borders of these cells are relatively smooth while apical (vitreal) processes enclosing photoreceptor outer segments are plentiful. Internally the epithelial cells display a vesicular pleomorphic nucleus, abundant mitochondria and melanosomes as well as myeloid bodies, phagosomes and polysomes. Smooth endoplasmic reticulum (SER) is abundant while rough endoplasmic reticulum (RER) is uncommon. Lipid droplets are also scarce. Wandering phagocytes are a regular feature amongst the retinal epithelial cells. Bruch's membrane is trilaminate and remains thin as in the previous stages investigated. The choriocapillaris is a single discontinuous layer of capillaries showing fenestrations on the side facing Bruch's membrane. A stratum argenteum remains in the choroid but separated from the retinal epithelium by pigment cells of the choroid.

Anguilla↗
PubMed1988

Retinal fine structure in the European eel Anguilla anguilla. VI. Photoreceptors of the sexually immature silver eel stage.

This report deals with the fine structure of the photoreceptors of the sexually immature silver eel and compares these findings with previous observations on the glass and yellow eel stages of the life cycle of the European eel. The photoreceptors of the sexually immature glass eel are readily categorized as either rods or cones. No multiple cones are observed. The rod:cone ratio is about 100:1, which is greater than that of either the glass or yellow eel stages. Rods and perhaps cones also are capable of retinomotor responses. In the light-adapted condition, rod inner segments are the same width as the outer segments but narrow abruptly below the ellipsoid to form the myoid. Rod outer segments in the immature glass eel are longer but much the same diameter as that noted for the glass and yellow eel. Cone inner segments are much wider than the outer segments which taper distally. Both rods and cones display organelles in the inner segment region indicative of protein production. Most of the cones observed at this stage showed some degenerative signs mostly as empty vacuoles within their cytoplasm. Cone nuclei are large and vesicular and in the light-adapted state are invariably located sclerad to the external limiting membrane while rod nuclei are small and dense and located vitread to this membrane. Both rods and cones display both invaginated and superficial synaptic sites. The most obvious changes noted in the immature silver eel photoreceptor population are a marked rise in the rod:cone ratio and a lengthening of rod outer segments.

Anguilla↗
PubMed1988

Retinal fine structure in the European eel Anguilla anguilla. VII. Pigment epithelium of the sexually mature silver eel stage.

This report describes the morphology of the retinal pigment epithelium (RPE), Bruch's membrane and choriocapillaris in the sexually mature silver eel and compares these observations with previous descriptions of this region in the glass, yellow and sexually immature silver eel stages of the lengthy life cycle of the European eel. The retinal epithelium in the sexually mature silver eel is a single layer of cuboidal to squamous cells joined laterally by apically located cell junctions. Both the basal (scleral) and lateral borders of the epithelial cells are relatively smooth while apically (vitreally) numerous processes enclose a photoreceptor inner and outer segments. The epithelial cells contain abundant profiles of smooth endoplasmic reticulum (SER), numerous small mitochondria and melanosomes and a single pleomorphic, vesicular nucleus. Lipid droplets are large and abundant at this stage. Myeloid bodies, phagosomes and lysosomes are also present. Rough endoplasmic reticulum (RER) is not plentiful. Wandering phagocytes are a constant feature of this region. Bruch's membrane is still trilaminate but is now much thicker than at any previous stage. The choriocapillaris remains as a single layer of fenestrated capillaries adjacent to Bruch's membrane. The stratum argenteum is still present in the choroid and separated from the RPE by pigmented cells. Single cells presumed to be from the stratum argenteum are, however, often located close to Bruch's membrane.

Anguilla↗
PubMed1988

Retinal fine structure in the European eel Anguilla anguilla. VIII. Photoreceptors of the sexually mature silver eel stage.

The morphology of the photoreceptors of the sexually mature silver eel has been investigated by electron microscopy and these findings compared with observations made on the photoreceptors of the glass, yellow and sexually immature silver eel stages of the lengthy life cycle of the European eel (Anguilla anguilla). As in previous stages the photoreceptors of the sexually mature silver eel are adequately described as either rods or single cones. The rod: cone ratio at this stage is about 200: 1 which is the highest ratio of any of the stages investigated. Rods and perhaps also cones are capable of photomechanical movements. In the light-adapted stage studied, rod inner segments are the same width in the ellipsoid region as the outer segments but the rod myoid region is much thinner. Rod outer segments in the mature silver eel are slightly longer than that observed in the immature silver eel but remain much the same diameter as in previous stages. Cone outer segments taper distally and are not as wide as the inner segments. Both rods and cones display organelles in the inner segment region indicative of active protein production as it is well established that the inner segment is the synthetic center of these cells. In most cones degenerative changes were noted in the mitochondria of the ellipsoid and in the presence empty vacuoles within their cytoplasm. Cone nuclei are large and vesicular and in the light-adapted state are always located sclerad to the external limiting membrane. Rod nuclei are smaller and more electron dense and located within the outer nuclear layer. Both rods and cones still show both invaginated (ribbon) and superficial synaptic sites. In the sexually mature silver eel, therefore, the rod: cone ratio becomes higher still; the rod outer segments become somewhat longer in length and cone cell degenerative changes are even more widespread.

Anguilla↗
PubMed1987

Photoreceptor fine structure in the vervet monkey (Cercopithecus aethiops).

The structure of the photoreceptors of the vervet monkey (Cercopithecus aethiops) 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 while cones are shorter and stouter. Both rods and cones are highly differentiated cells and consist of an outer segment, a connecting cilium, and inner segment, a nuclear region and a synaptic process leading to a synaptic ending. Morphological differences are noted between rods and cones for the various regions of these cells.

Animals↗
PubMed1986

Numbers of axons in the optic nerve and of retinal ganglion cells during development in the marsupial Setonix brachyurus.

Numbers of axons during postnatal development in the optic nerve of the marsupial Setonix brachyurus were estimated from electron micrographs. Between 14 and 45 days axon totals rose 10-fold from 95,000 to 961,000. Thereafter, counts decreased gradually by almost 5-fold stabilising at the adult level of 180,000-224,000 between 130-150 days. Myelinated axons were first seen at 85 days and constituted 76% of all axons by 150 days, compared to almost 100% in the adult. At comparable stages of development, estimates of cell number in the ganglion cell layer were made from sectioned and whole-mounted retinae. Ganglion cells, labelled with horseradish peroxidase (HRP) injected into the optic tract and visual brain centres, reached a maximum of 304,000 by 50 days. This value was only one-third higher than the adult ganglion cell estimate of 202,000, to which counts fell by 70 days. Such a fall of ca. 100,000 ganglion cells was matched by a reduction from 443,000 to 361,000 in the total number of cells in the retinal ganglion cell layer, that is, cells labelled by HRP plus those which remained unlabelled. Thus, during development axon counts exceeded adult values 4-fold whereas ganglion cell totals were only one-third higher than the mature value. The shortfall of ganglion cells to optic axons indicates that during development the transiently high axon number cannot be explained by a correspondingly raised count of ganglion cells. It is therefore necessary to hypothesise a substantial fibre population which temporarily projects into the nerve from either the brain or the opposite eye. Alternatively, optic axons may branch extensively during development with subsequent loss of collaterals.

Age Factors↗
PubMed1986

Fine structure of the tapetum cellulosum of the grey seal (Halichoerus grypus).

The morphology of the tapetum lucidum of the grey seal (Halichoerus grypus) has been studied by light and electron microscopy. The reflective layer in this species is a tapetum cellulosum situated in the choroid and covering the entire effective fundus. Posteriorly the tapetum is composed of 30-35 layers of flattened polygonal cells. This number gradually declines to 15-20 layers in the extreme periphery. Near the retinal epithelial layer the tapetal cells are larger and more regular (brick-like) in arrangement whereas further from the retina the tapetal cells become more irregular in outline and more widely separated by collagen fibrils and connective tissue cells. In this outer region the tapetal cells are gradually replaced by melanocytes of the choroid. Within the tapetal cells a few mitochondria and profiles of smooth endoplasmic reticulum are scattered peripherally while the majority of the cell organelles are clustered near the centrally located vesicular nucleus. The dominant feature of the tapetal cells is, however, an accumulation of numerous electron-dense rodlets of presumed zinc cysteine. These rodlets are the reflective material of the tapetum and are arranged with their long axes perpendicular to the incoming light. The orientation of these rodlets is usually uniform within each tapetal cell but varies between adjacent cells. The diameter (0.10 micron) and spacing (0.15 micron) of these rodlets is consistent with the principles of constructive interference. Blood vessels penetrate the tapetum at right angles to supply the choriocapillaris which is indented into the amelanotic retinal epithelium to give a flat reflective surface to the tapetum.

Animals↗
PubMed1986

Retinal epithelial fine structure in the grey seal (Halichoerus grypus).

The morphology of the retinal epithelium and the closely associated choriocapillaris and Bruch's membrane (complexus basalis) has been investigated in the eye of the grey seal (Halichoerus grypus) by electron microscopy. The retinal epithelium is composed of a single layer of cuboidal cells joined laterally by apically-located junctional complexes. Basally (sclerally) these cells display numerous infoldings while apically (vitreally) abundant processes enclose rod outer segments. Internally the large vesicular nucleus is centrally located. Smooth endoplasmic reticulum, mitochondria and lysosome-like bodies are abundant. Rough endoplasmic reticulum and polysomes while present are not plentiful. Phagosomes of outer-segment discs are only occasionally noted in the light-adapted state. As the entire fundus is overlain with a choroidally located tapetum cellulosum, only at the extreme periphery is an occasional melanosome present in these epithelial cells. The choriocapillaris endothelium is highly fenestrated and the profiles of these capillaries are deeply indented into the epithelial layer. Bruch's membrane (complexus basalis) is reduced to a trilaminate structure rather than the typical pentalaminate membrane seen in most mammalian species and when associated with capillary profiles is further reduced to a single thick basal lamina.

Animals↗
PubMed1985

Fine structure of the retinal pigment epithelial region of the archerfish (Toxotes jaculatrix).

The structure of the retinal pigment epithelium, choriocapillaris and Bruch's membrane has been studied by electron microscopy in the archerfish Toxotes jaculatrix. The pigment epithelium is composed of a single layer of large columnar cells. Each epithelial cell displays numerous small mitochondria, smooth endoplasmic reticulum, myeloid bodies, phagosomes and melanosomes. Rough endoplasmic reticulum and ribosomes while present are not abundant. The scleral or basal border of the epithelial cells is but minimally infolded whereas the vitreal or apical surface displays many elongated processes which surround the inner and outer segments of the photoreceptors. Laterally the epithelial cells are joined by a series of basally located cell junctions. Unattached phagocytic cells are a constant feature both between the retinal epithelial cells and within Bruch's membrane. The endothelium of the choriocapillaris bordering Bruch's membrane is typically very thin with a few fenestrations. Bruch's membrane lacks a central elastic layer and is composed of only three layers. Although only light-adapted specimens were examined, it is felt that the melanosomes are capable of retinomotor movements.

Adaptation, Physiological↗
PubMed1985

Retinal fine structure in the European eel Anguilla anguilla. IV. Photoreceptors of the yellow eel stage.

The complex life cycle of the European eel (Anguilla anguilla) involves a long catadromous migration during which the eye undergoes a number of morphological alterations. This report deals with the fine structure of the photoreceptors of the yellow eel stage and is part of a comparative study of the retina of the eel during the major stages of its life cycle. The photoreceptors of the yellow eel are easily divisible into rods and single cones and the rods at least appear to be capable of retinomotor responses. Rods are more elongated and more numerous than cones. In the light-adapted state the inner segment of rods is of the same diameter as the outer segment but narrows abruptly in the myoid region. Cone inner segments on the other hand are much wider than the outer segments which taper distally. Both rods and cones display organelles in the inner segment indicative of high metabolic activity. Cone nuclei are located scleral to the external limiting membrane while rod nuclei are vitreal to this boundary. Both rods and cones display both invaginated and superficial synaptic sites. Except for an apparent increase in number and outer segment width of the rods, the photoreceptors of the yellow eel have changed only slightly from that described for the glass eel stage.

Age Factors↗
PubMed1985

Further observations on the presence of wandering phagocytes within the teleostean retina.

A previous report by this author has noted the presence of wandering phagocytes at the photo-receptor-retinal epithelial interface in five teleostean species. A further 5 species have now been investigated by electron microscopy and again all specimens examined consistently show the presence of this population of phagocytes. The new species studied are the goldeye (Hiodon alosoides), the yellow eel (Anguilla anguilla), the archer fish (Toxotes jaculatrix), the goldfish (Carassius auratus) and the burbot (Lota lota). A population of ameboid phagocytes appears to be a normal feature of the teleostean retina. In some species (i.e., the goldeye and the goldfish) the phagocytes were extremely numerous and could be located in groups of 20-25. In addition, preliminary experiments with the goldfish have shown that these wandering phagocytes undergo a cyclic pattern to show changes in both morphology and location dependent upon environmental lighting.

Anguilla↗