Choroidal Capillaries Research Articles

Bovine retinal pigment epithelium promotes proliferation of choroidal endothelium in vitro.

The invasion of Bruch's membrane and subsequently the subpigment epithelium and subretinal spaces by proliferating choroidal capillaries is a prominent feature of age-related macular degeneration. The ability to isolate and propagate pure cultures of bovine choroidal microvascular endothelium, choroidal pericytes, turbinate fibroblasts, and bovine retinal pigment epithelium allowed us to identify a growth-promoting activity produced by the retinal pigment epithelium that stimulated the proliferation of these cells in vitro. This mitogenic activity is present in both retinal pigment epithelium-conditioned medium and retinal pigment epithelium extract and enhances the mitogenic activity of retinal-derived extract. The activity is resistant to extremes of temperature and pH but partially sensitive to trypsin inactivation.

A Model of Subretinal Neovascularization in the Pigmented Rat

We produced krypton laser photocoagulation lesions of mild to moderate whiteness in the posterior retinas of one eye of 23 pigmented rats, and identical appearing argon laser burns in the fellow eyes. We observed foci of subretinal neovascularization, histopathologically markedly similar to that which occurs in several human retinal diseases, in the krypton laser treated eyes of 6 of the 14 rats that were followed for one to three months after photocoagulation. No such lesions were observed in the argon laser treated fellow eyes, nor in krypton or argon laser treated eyes examined earlier than one month after photocoagulation. The photocoagulation damaged only the choriocapillaris, the retinal pigment epithelium (RPE), and the photoreceptor layer. In the acute lesions, we did not observe ruptures in Bruch's membrane. The neovascularization was surrounded by multiple layers of RPE cells, a histopathologic finding that has also been reported in some human eyes with subretinal neovascularization in age-related macular degeneration. These observations suggest that the RPE cells may be modifying the proliferative behavior of adjacent choroidal capillaries. This model differs from previous models of subretinal neovascularization in primates, and may be useful for additional studies of this important pathological process.

Ultrastructure of Bruch's Membrane After Krypton Laser Photocoagulation

Previous observations on rats suggested that subretinal neovascularization does not require a prior break in Bruch's membrane (BM). We verified this observation by using scanning electron microscopy to examine large expanses of BM that became exposed following ophthalmoscopically white krypton laser burns. Bruch's membrane appeared intact in the acute phase after injury. Subsequently, slitlike defects in BM were associated with penetrating choroidal capillaries. These observations were consistent with transmission electron microscopic findings of cellular protrusions arising from the choriocapillary endothelial cells and from regenerating retinal pigment epithelial (RPE) cells invading BM. These cell-formed defects in BM differed from thermal defects in appearance, size, and onset of occurrence. Endothelial cells penetrated all layers of BM, including the RPE basement membrane. We conclude that endothelial cells can erode their own basement membrane and the RPE basement membrane, and krypton laser burns with or without immediate rupture of BM induce cellular activity that may result in defects in BM.

Retinal and Choroidal Blood Flows in Hypoxic and Hypercarbic Newborn Lambs

We studied the effect of changes in the arterial tensions of oxygen and carbon dioxide on blood flow to the retinal (RBF) and choroidal ( ChBF ) capillary beds in 20 lambs. One to three days after placement of catheters in the left ventricle, abdominal aorta, and brachiocephalic artery, different gas mixtures were delivered to a bag enclosing the lamb's head. One group of lambs was studied at normal and low oxygen tensions while normocarbic , and another group was studied at normal and high carbon dioxide tensions while normoxic. RBF and ChBF were measured using the radioactive microsphere technique. RBF increased as PaO2 and, thus, arterial oxygen content [( O2]a) fell; in contrast, ChBF was not related to [O2]a. Oxygen delivery to the capillary bed of the retinal artery (i.e., RBF X [O2]a) was independent of arterial oxygen content because the change in [O2]a was balanced by a reciprocal change in RBF. Oxygen delivery to the choroidal capillary bed, however, rose with [O2]a because there was no reciprocal decrease in ChBF . Both RBF and ChBF increase as arterial carbon dioxide tension rose. Although an increase in arterial carbon dioxide tension produced an increase in RBF with no change in [O2]a, oxygen delivery to the retinal capillaries rose. Similarly, oxygen delivery to the capillaries of the choroid rose with carbon dioxide tension. Oxygenation of retinal tissue, which depends on oxygen diffusion from both the retinal and choroidal capillary beds, may change with variations in arterial oxygen content (increase in oxygen delivered to the choroidal capillary bed) or arterial carbon dioxide tension (increase in oxygen delivered to both the retinal and choroidal capillary beds).

Carbonic anhydrase distribution in the human and monkey eye by light and electron microscopy.

The distribution of carbonic anhydrase (CA) in the eye of man and three species of monkeys was studied by light and electron microscopy using the histochemical method of Hansson. Carbonic anhydrase staining was found in the corneal endothelium. In monkeys the endothelial cells covering the inner surface of the operculum were also stained, whereas in the human the staining stopped at Schwalbe's line. In the trabecular meshwork no cells exhibited CA staining. The iris dilator muscle showed some staining, while no clear staining was found in the pigment epithelium (PE). In the ciliary body both the PE and the non-pigmented epithelium (NPE) displayed CA staining, most prominently in the basal and lateral membranes but also discernible in the cytoplasm. The staining of the NPE (but not the PE) showed clear-cut regional differences, and the presence of CA coincided with morphological indicators of secretory activity. Heavy staining was found in the capillaries under the ciliary epithelium. In the pars plana many capillaries showed staining of only that part of the circumference of the vessel wall which faced the epithelium. Unstained segments were also found in many choroidal capillaries under the pigmented epithelium of the retina. In the retina itself CA stain was found in the pigmented epithelium, in the Müller cells and in capillaries supplying the inner retina (only monkey retina was studied).(ABSTRACT TRUNCATED AT 250 WORDS)

Talc retinopathy in primates: a model of ischemic retinopathy. II. A histopathologic study.

Experiment talc retinopathy was produced in four adult rhesus monkeys by biweekly intravenous injections of talc for 31/2 to ten months and was studied by retinal vascular flat preparations and by light microscopy. Talc particles were lodged in the walls of the precapillary arterioles and capillaries, producing focal occlusion of retinal and choroidal capillaries. The pericyte-endothelial cell ratio was 1:0.77 in the posterior pole and 1:0.53 in the retinal periphery. The horseradish peroxidase study showed leakage of tracer from the retinal vasculature into the extracellular interstitial space, but the barrier of the retinal pigment epithelium was intact. Microinfarcts produced small cystoid spaces in the outer plexiform layer, inner nuclear layer, and ganglion cell layer of the macula. Cytoid bodies and macrophages were scattered in the retina. No retinal or vitreal neovascularization was observed.

Talc retinopathy in primates: a model of ischemic retinopathy. III. An electron microscopic study.

Electron microscopic study of experimental talc retinopathy provides an opportunity to study the basic cytologic response of retinal and choroidal vessels to small, relatively inert emboli. Occlusion of the retinal and choroidal capillaries resulted primarily from the cellular reaction to, rather than directly from, the emboli. Mitotic figures in pericytes were observed. Even though we failed to produce retinal neovascularization (as is sometimes seen in human talc retinopathy) in this experimental model, our observations suggest that proliferation of pericytes may be an initial step in the process of retinal neovascularization.

Fine structure of ependymal cysts in and around the area postrema of the rat.

Peculiar cells forming cysts were observed in the area postrema and sometimes also in the choroid plexus and the tela chorioidea near the area postrema, and were studied in detail by electron microscopy. The cytological features of the cyst cell and its junctional relationship to neighboring cells imply that cyst cells are derived from ependymal and choroid epithelial cells. The cyst cells usually contact directly the perivascular spaces of postremal, choroidal or pial capillaries, where the cytoplasm is often considerably attenuated. The cystic lumen is commonly filled with a flocculent material. The limiting membrane of the cystic lumen, which frequently bears cilia and microvilli, has the same thickness as the surface cell membrane. In many cases the cyst is surrounded by the cytoplasm of a single cell. In some cases, however, two cells participating in the formation of the cyst, although one is only a slender process and joined by a zonula occludens with the main cyst cell. Horseradish peroxidase (HRP) injected into the cerebrospinal fluid (CSF) space failed to enter the cystic lumen. A possible significance of the cyst in relation to the CSF and blood circulation was considered.

Immune-Complex Deposition in the Eye in Systemic Lupus Erythematosus

A patient with systemic lupus erythematosus (SLE) and lupus retinopathy showed resolution of subretinal edema documented with fluorescein angiography. Subsequently at autopsy, immunofluorescence studies disclosed ocular deposition of immunoglobulins in the vascular layer of choroid capillaries and basement membranes of ciliary processes and bulbar conjunctivas. To our knowledge, these findings represent the first reported documentation of probable immune-complex ocular vasculitis in lupus retinopathy using immunofluorescent techniques, and they support the hypothesis that lupus retinopathy is caused by immune complex deposition as are other manifestations of SLE.

Observations of the ultrastructure of the developing primate choroid coat

A descriptive analysis of some of the fine structural features of the choroidal components is made in a chronological sequence of Macaca mulatta embryos and fetuses. The observations indicate the emergence of primitive choroidal capillaries from locally differentiated cells of the retrobulbar mesenchymal tissue. Early capillaries are of two types: one represented by the choriocapillary with low endothelial cells, having extremely thin, fenestrated regions covered by basal lamina, the other by small channels in the retrobulbar mesenchymal network, with cuboidal endothelium bulging into slit-like lumina and bearing pronounced marginal folds. Discontinuous endothelium, large intercellular clefts and interrupted basal lamina permitting the passage of blood elements into the extracellular space may be associated with both types of capillaries. Primitive perieytes and muscle cells appear to be modifications from the existing mesenchymal cells adjacent to the developing vascular endothelium. The adult choroidal layering is well defined by mid-term. The fibrillar and vesicular content of the endothelial cytoplasm increases early during the second half of gestation. Cell to cell adhesions become more pronounced in the choriocapillary endothelium during this period. In the arterioles the intercellular spaces remain incompletely scaled off till birth. Appearance of the structural components indicate the involvement of some of these cells in transport mechanisms shortly after mid-gestation.

PERMEABILITY PROPERTIES OF THE TISSUES IN THE OPTIC NERVE HEAD REGION IN THE RABBIT AND THE MONKEY

The distribution of the protein tracer, horseradish peroxidase, has been studied in the optic nerve head region in rabbits and monkeys by light and electron microscopy. Following intravenous injection of the tracer, the eyes were enucleated after varying time intervals. Leaking out of the choroidal capillaries in the peripapillary choroid, peroxidase rapidly spread to the adjacent sclera and to the intraneural connective tissue, that is lamina cribrosa and the connective tissue surrounding the intraneural vessels. The tracer then diffused from both the perineural and intraneural connective tissue into the adjacent optic nerve tissue. This diffusion could take place because the intercellular spaces in the astrocytic cell layer interposed between the connective and neural tissues were freely permeable to the tracer. The present investigation confirms the presence of a defect in the blood optic nerve barrier in the optic nerve head. The clinical importance of this defect in the permeability barrier is not known. It may represent a predilection point for pathological events, e.g. in retrobulbar neuritis.

Histologic Studies of the Vasculature of the Anterior Optic Nerve

Thirty-one normal human anterior optic nerves were studied in thin serial sections and their vessels stained by standard histologic stains and a new modification of the silver reticulin stain. The retrolaminar cribrosa was profusely supplied by centripetal and longitudinal vessels of pial origin; small branches of the central retinal artery were also seen. Longitudinal continuity of small vessels extended from the retrolaminar nerve to the retinal surface. This system freely anastomosed with three transverse systems; a significant anteriolar input from short posterior ciliary arteries in the sclera to the lamina cribrosa; smaller short posterior ciliary branches and occasional choroidal capillaries to the prelaminar portion; and branches derived from the central retinal artery in and around the disk. Vessels entering the prelaminar region at the level of the choroid were mainly derived from scleral branches of short posterior ciliary vessels entering through the border tissue of Elschnig, rather than from choroidal branches of ciliary arteries. Venous drainage for the anterior optic nerve was primarily through the central retinal vein, although alternate smaller paths to surrounding tissues were also identified.

Retinal detachment and pathology following experimental embolization of choroidal and retinal circulation.

By means of a micro-surgical technique, the central retinal artery of the owl monkey (Aotus trivirgatus) was embolized with spheric plastic beads (7-25 mu in diameter). Beads (4-15 mu) were also injected into choroidal capillaries through a supero-temporal vortex vein. Choroidal ischemia induced proliferation or degerneration of the pigment epithelium, frequently accompanied by photoreceptor degeneration and circumscribed retinal detachment. Occlusion of retinal vessels resulted in posterior vitreous detachment and micro-cystoid degeneration in inner retinal layers, followed by retinal schisis. In combined retinal and choroidal ischemia long-lasting retinal detachments regularly developed.

Structure and Function of the Choroid Plexus and Other Sites of Cerebrospinal Fluid Formation

Publisher Summary This chapter discusses the structure and function of the choroid plexus and other sites of cerebrospinal fluid (CSF) formation. A considerable volume of the CSF is formed continuously within the cerebral ventricles. The choroid plexuses contribute to this formation, but a significant fraction of the CSF is formed extrachoroidally. The exact contributions of fluid from choroidal and extrachoroidal sites remain to be determined. The elaboration of choroidal fluid probably involves the following steps: filtration of the .blood plasma across fenestrated choroidal capillaries, formation of a protein-rich interstitial fluid within the choroidal stroma, and movement of constituents of the interstitial fluid across the choroidal epithelium by the combined processes of ultrafiltration and active transport. Contributing to the choroid plexus blood-CSF barrier are at least three specialized features: a system of circumferential tight functions joining adjacent epithelial cells; a heterolytic system of pinocytotic vesicles and lysosomes within epithelial cells; and a system of epithelial cell enzymes concerned with the active bidirectional transport of substances between the plasma and CSF. It is likely that the choroid plexus epithelium elaborates a carefully regulated fluid and, at the same time, participates in the homeostasis of the CSF. Evidence exists that the extracellular fluid of the brain is continuously formed across the cerebral endothelium, that it drains in bulk to the adjacent CSF cavities, and that it serves as a vehicle for removing intracerebral metabolites. Overall, there is probably a steady, net addition of new fluid at all points along the pathways of CSF circulation until the major sites of absorption (arachnoid villi) are reached.

A combined physiological and morphological study of the secretory process in the rabbit choroid plexus.

ABSTRACT The ultrastructure of the secreting and non-secreting rabbit choroid plexus has been examined with the electron microscope. Secretion was inhibited by perfusion with an artificial cerebrospinal fluid containing one of the following drugs that have been shown to inhibit CSF formation and also the rate of penetration of sodium ions into the fluid: Ouabain, Diamox, Spirolactone and Amiloride. Perfusion with Kamovsky’s fixative enabled good preservation of the ultrastructure to be obtained in situ. In the inhibited state, large intercellular spaces, some of them reaching 5 μm diameter, were observed along the length of the intercellular cleft, whilst other regions of the cleft retained normal dimensions, apparently limited by desmosomes. In the normally secreting plexus subject to identical conditions of fixation, no such dilations were seen. This ‘reverse’ system contrasts with the ‘forward’ transporting system found in the gall bladder. These findings are in accordance with the Diamond concept of water transfer for ‘reverse’ systems since, if fluid is moving out of the cleft under the influence of active sodium transport, it could be predicted that no dilation would occur jn the transporting condition. A possible explanation of the mechanism is offered in terms of the high hydrostatic pressure which may exist in the choroidal capillaries which, unlike the rest of the cerebral vasculature, are of the fenestrated type and are thus most likely to be highly permeable.

Fine structural studies of capillaries in [formula omitted] mice

The fine structure of capillaries in young (12 weeks) and aging (40–52 weeks) NZB NZW F 1 mice, using Strong A mice of the same sex and similar ages as controls, was studied. Electron dense deposits and irregularly thickened basement membranes were noted in glomerular capillaries of the aging NZB NZW mice. Irregular thickening of the basement membranes was present in the choroidal capillaries of aging NZB NZW mice, but electron dense deposits were not seen. Myocardial capillaries in aging mice of both strains contained an increased number of pinocytotic vesicles.

A Possible Cytological Basis for the ‘R’ Membrane in the Vertebrate Eye

THE ‘R’ membrane was the name coined by Brindley1 for a hypothetical retinal barrier whose penetration by a microelectrode advancing from the vitreous chamber results in a marked drop in electrical resistance and decrease in capacitance. Later, Tomita, Murakami and Hashimoto2 found that this barrier could be demonstrated in eyes from which the retina was removed, thus placing the barrier behind the retina either at the level of the pigment epithelium or at Bruch's membrane. This was afterwards confirmed by Brindley3. Since the membrane of Bruch appears to be a substantial structure in light microscopy, it was a likely candidate for the ‘R’ membrane. Electrode marking methods employed by Brown and Tasaki4 gave results compatible with this hypothesis and ruling out the retina although in fact not clearly marking Bruch's membrane as distinct from the pigment epithelium. Cohen5, using electron microscopy, cast doubt as to the role of Bruch's membrane in this phenomenon by pointing out that Bruch's membrane consisted of the basement membranes of the pigment epithelium and choroid capillaries together with a layer of loosely interwoven connective tissue. Since there is a basement membrane at the retinal-vitreous junction where no electrical change of a comparable type is noted, it seemed unlikely that Bruch's membrane had the requisite structure for significant electrical resistance.

VERTEBRATE RETINAL CELLS AND THEIR ORGANIZATION

Summary1. Electron microscopy has added numerous details emphasizing the kinship of the retina with the central nervous system. The space between the receptors and pigment epithelium resembles neural tube lumen in that all cells facing this lumen possess terminal bars at their margins. The receptors, glial cells of Müller and cells of the pigment epithelium all appear to be derivatives of ependymal cell varieties. As is the neural tube and mature central nervous system, the entire optic cup derivative in the eye is jacketed in a basement membrane. Blood vessels run through glial tunnels lined by this membrane. The choroid capillaries and pigment epithelium resemble the choroid plexus.2. The choroid capillaries have endothelial cells which possess numerous focal attenuations or perhaps pores where they oppose the cells of the pigment epithelium. The latter show deep membranal infoldings and an extensive endoplasmic reticulum. Some species possess spindle‐shaped aggregations composed of stacked pairs of membranes which are continuous with the reticulum. The melanin pigment granules lie within microvilli which drape the receptors.3. The receptors are derived from ciliated ependymal cells. Their outer segments contain stacks of flattened membranous sacs which are apparently formed by ingrowth or infolding of the plasma membrane. The connexions of the resultant sacs to the membrane may be very narrow or perhaps absent in most rod sacs, but are generally persistent in cone sacs. Rod sacs possess one to many incisures, depending on the group of animals examined. Details of the molecular architecture of these membranes are highly speculative since they are based upon the appearance of the membranes after various techniques involving empirical fixation procedures and little fundamental research.4. Receptors are in intimate relation with processes of cells of the pigment epithelium and microvilli of the glial cell of Muller. The apex of the receptor inner segments may be reflected, calyx‐like, over the base of the outer segments.5. Receptor inner segments are joined to the outer by narrow stalks which contain nine pairs of filaments or microtubules like those of cilia, except for the absence of a central pair. The filaments end in relation to a centriole and a second centriole is also present. Rootlets run from the centrioles through the inner segments.6. The ellipsoid of the inner segment is a cluster of mitochondria. The paraboloid, when present, is seen to be an ellipsoidal configuration of expanded tubules of the endoplasmic reticulum with clusters of granules dispersed in the cytoplasm outside the tubules. In double cones, the inner segments and nuclear areas of two independent cells are closely applied to one another. Golgi regions overlie the receptor nuclei.7. The receptor synaptic area is highly complex. Processes, presumably from bipolar cells, end in pits in the receptor base. Above these pits specialized organelles are found in the receptor cytoplasm. Other processes terminate superficially. These may include processes from certain receptors ending on others, a situation possibly analogous to that existing between receptors of Limulus.8. The neural retina as a whole exhibits a paucity of extracellular space, with estimates of intercellular distances made precarious by our ignorance of the molecular composition of the light areas intervening between the osmiophilic lines partially representing the cell membranes but probably not the cell surfaces! However, more extracellular space must exist in areas where cell processes are small and numerous owing to geometric factors. Receptors and retinal neurons and their processes may have broad surfaces of contact, but glial processes, particularly of the cells of Muller, are rather pervasive, and fill in spaces not otherwise occupied.

Retinosis, Retinitis Externa and Choroiditis

Retinosis is a better word than retinitis to indicate conditions of the retina which are not inflammatory, but degenerative. The outer layers of the retina nourished from the choroidal capillaries are subject to inflammations properly called retinitis externa. Late stages of retinitis and of choroiditis may present similar pigmentary changes, causing confusion that would not arise were the lesions considered in connection with the anatomy of the parts and the history of their development. This paper to remove confusion and misunderstanding was read before the Colorado Congress of Ophthalmology and Oto-Laryngology, July, 1926.