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Biomedical subjects

H I Covington

Publications and source records attributed to H I Covington.

18 recordsLinked to original sources
PubMed1990

Proteoglycan distribution in developing rabbit cornea.

We used a staining procedure specific for sulfated glycosaminoglycans, cuprolinic blue dye (CBD), and immunohistochemical techniques to determine the histological distribution and ultrastructural organization of proteoglycans in developing rabbit cornea. We found several types of CBD-stained structures located throughout the corneal stroma, indicative of the distribution and perhaps the chemical heterogeneity of proteoglycans in this tissue. Keratan sulfate-specific immunohistochemical evidence supports our cytochemical findings. Our results suggest that low-sulfated keratan sulfate proteoglycans are found throughout most of the developing stroma, with the exception of the posterior margin of this tissue. Highly sulfated keratan sulfate proteoglycans in young fetal corneas, initially restricted to the subepithelial stroma, progressively extend to deeper portions of the stroma with development. Dermatan sulfate proteoglycans are located throughout the stroma, including the posterior margin. Invoking a recently published "oxygen-lack hypothesis" and correlating the tissue location of proteoglycans with the source of oxygen, we hypothesize that the distribution of proteoglycans in the developing rabbit cornea is related to the selective synthesis of keratan sulfate glycosaminoglycans under hypoxic conditions.

Animals↗
PubMed1990

Morphologic analyses of proteoglycans in rabbit corneal scars.

Ultrastructural localization of proteoglycans (PGs) in 1-week- to 2-year-old scar was determined by staining with cuprolinic blue dye (CBD) after specific enzymatic digestion of keratan sulfate (KS) glycosaminoglycans (GAGs) or chondroitin sulfate glycosaminoglycans (CSs). High critical electrolyte conditions were maintained for CBD-staining, specific for high-sulfated GAGs. Although KS was detected in the 1-week-old wound, no CBD-stained KS was seen in the anterior stroma adjacent to the wound. The CS was present throughout the 1-week-old wound and adjacent stroma, and PGs were biosynthetically 35SO4-labeled in normal stroma. Subsequently, radioactivity from labeled PGs in normal stroma adjacent to the wound moved into scar tissue during healing. Marked sensitivity of PGs to Chondroitinase ABC indicated an abundance of CS in 2-week-old scars. Punctate CBD-staining and immunohistochemical evidence suggested chemically altered KS is present in the 2-week-old anterior scar. The pattern of CBD-staining in 1- and 2-week scars, after chondroitinase treatment, suggested KS in the younger scar is similar to adult high-sulfated GAG, whereas KS in the 2-week scar contains primarily newly synthesized low-sulfated KS. The latter is consistent with previous immunochemical and biochemical analyses. Cytochemical and immunohistochemical evidence indicated that KS is not present in the 2-week-old posterior scar. By the week 8 of healing, CBD-stained KS was present throughout most of the scar, except along the posterior margin, consistent with earlier stages of healing. The CBD-stained structures in the first 8 weeks of healing were reminiscent of stained GAGs in normal developing cornea. This fetal-like CBD-staining pattern seen in scar, however, changed to that of the normal adult by the 2nd year of healing. The significance of these observations relate to our contention that healing adult cornea recapitulates some ontogenetic events of the normal cornea, and that the nonuniform distribution and chemical properties of GAGs in scar tissue are a function of the movement of existing proteoglycans and de novo synthesis of altered macromolecules.

Animals↗
PubMed1990

Immunolocalization of type VI collagen in developing and healing rabbit cornea.

We have localized type VI collagen in normal developing and corneal scar tissue. Indirect immunofluorescence showed that type VI collagen was distributed throughout the normal stroma and most of the scar. No fluorescence was detected along the posterior margin of the scar and in a retrocorneal membrane continuous with the scar. Since the corneal endothelium in rabbits contributes to the formation of scar tissue and retrocorneal membrane, our observations suggest that the endothelium does not synthesize type VI collagen. Indirect immunoelectron microscopy showed that type VI collagen was located abundantly between collagen fibrils as fine filamentous structures containing beads with a periodicity of 100 nm, consistent with published observations of other tissues. Because these filaments are more prominent when stained with ruthenium red, and predigestion of tissue with Chondroitinase ABC enhances binding of monoclonal antibody to type VI collagen, proteoglycans probably are associated with this collagen in the cornea. Ultrastructural observations supported by previous biochemical analyses show that the proportion of type VI collagen to fibrillar collagen is smaller in scar tissue compared with fetal cornea. The abundance of type VI collagen and its distribution and association with proteoglycans in rabbit corneal tissues suggest that this macromolecule plays a role in the tensile strength and transparency of the stroma.

Animals↗
PubMed1988

Morphogenesis of rabbit corneal endothelium.

We studied ultrastructurally the development of rabbit corneal endothelium from the 13th day of gestation to 3 days after birth. Precursor corneal endothelial cells, stromal cells, and a vascular network migrate in close association with each other between the developing corneal and lens epithelia. During development, newly deposited extracellular fibrous matrices separate the prospective endothelium from the capillaries and corneal stroma. The extracellular matrix between the apical endothelial surface and the vascular network loses its fibrous appearance early in development. Simultaneously, randomly organized fibrils are deposited on the basal endothelial surface facing the stroma. These fibrils, gradually obscured by the deposition of a nonfibrous component, eventually become part of Descemet's membrane. Early in development, prospective endothelial cells cannot be distinguished morphologically from the overlying corneal stromal cells. Morphologic differentiation of the endothelial cell is characterized by the formation of sinuous lateral borders that interdigitate with those of adjacent cells to form a continuous single-cell layer of tissue. The basal endothelial membrane forms a pitted surface, distinguishing it from the apical cell membrane. Intercellular junctions between lateral membranes, a cilium projecting into the anterior chamber, and deposition of Descemet's membrane on the basal endothelial surface contribute to the polarization of the endothelium. Throughout most of corneal development the vascular pupillary membrane maintains a close association with the apical surface of the differentiating endothelium. We conclude that fetal corneal endothelium develops within a complex extracellular matrix environment and in proximity to the underlying vascular network. These structures play an important role in the morphogenesis of corneal endothelium.

Animals↗
PubMed1988

Organ culture of rabbit cornea: morphological analyses.

Tissues maintained in vitro often undergo changes in the pattern of protein synthesis that result in the deposition of macromolecules quantitatively or qualitatively unrelated to those normally synthesized. In these preliminary studies, we modified published techniques to maintain adult and neonate rabbit corneas in vitro for 24 to 48 h. Measurements of corneal wet weight, and histologic and ultrastructural analyses were made to determine the success of maintaining rabbit corneas in culture. The results show that rabbit corneas can maintain normal corneal hydration and tissue structure for at least 48 h when incubated in Coon's modification of Ham's F12, 5% fetal or newborn calf serum, 2 mM glutamine, and 2% chondroitin sulfate or 2% 50 kDa dextran sulfate at 37 degrees C in a 5% CO2/air atmosphere. In addition, we confirmed previous observations that corneal explants must have a 1 to 2 mm rim of limbal sclera, and the organ placed in the culture dish with the epithelial side down to guard against damage and insure endothelial functioning. Normal ultrastructure of neonate rabbit cornea is also maintained when organ-cultured with these procedures. Moreover, neonate corneas continue to synthesize collagen in culture.

Animals↗
PubMed1988

Immunoanalysis of keratan sulfate proteoglycan from corneal scars.

Corneal keratan sulfate proteoglycan (KSPG) from scar tissue of experimental penetrating corneal wounds in rabbits was analyzed 2-8 weeks after injury using three previously characterized antibodies. Keratan sulfate (KS) was identified in 2 week scars and normal corneal tissue by indirect immunofluorescence using a monoclonal antibody against sulfated KS epitopes. KSPG was measured in unfractionated extracts of scar and of normal corneal tissue using a "sandwich" enzyme-linked immunosorbent assay (ELISA). In extracts of 2 week scars, KSPG molecules reacting with two different anti-KS monoclonal antibodies were 55% and 82% as abundant as in normal tissue extracts. Ion exchange high performance liquid chromatography (HPLC) of tissue extracts found qualitatively similar elution profiles of KSPG antigens from both scar and normal tissues. Direct ELISA of the HPLC-purified KSPG showed identical quantitative binding of antibodies against core protein and KS from normal and scar tissue. KS in the HPLC-purified extracts was sensitive to digestion with endo-beta-galactosidase, whereas core protein antigens were not affected by this enzyme, as expected. Alteration of the antigenic characteristics of the KSPG of scars was detected with a competitive immunoassay using immobilized monoclonal antibodies against KS. KS in extracts from 2, 6, and 8 week scars competed only 5-11% as effectively as KS from normal cornea, although core protein antigens in the scar extracts competed 61-80% as well as those of normal cornea.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1988

Heterogeneity of collagens in rabbit cornea: type III collagen.

Whole neonate rabbit corneas and adult corneas containing 2-week-old scars were incubated in the presence of [14C] glycine. Radiolabeled collagen extracted from the corneas and scar tissue were analyzed by sodium dodecylsulfate/polyacrylamide gel electrophoresis and fluorography to determine the types and relative quantity of collagen polypeptides present and synthesized by these tissues. In addition to other collagen types, type III was found in both neonate cornea and scar tissue from adult cornea, albeit in relatively small quantities. Type III collagen in normal cornea was associated with the residue after pepsin digestion and formic acid extraction of the tissue, and the same type of collagen was extracted from scar tissue after similar treatment. Type III collagen-specific monoclonal antibody bound to developing normal corneas and healing adult tissue sections, as determined by immunofluorescence. Antibody binding was localized to the endothelium and growing Descemet's membrane in fetal and neonate corneas, and restricted to the most posterior region of the corneal scar tissue. Although monoclonal antibody to keratan sulfate, used as a marker for stromal fibroblasts, bound to most of the scar tissue, the antibody failed to bind to the posterior scar tissue positive for type III collagen. We conclude that endothelial cells from fetal and neonate rabbit cornea and endothelium-derived fibroblasts from healing wounds of adult cornea synthesize and deposit type III collagen. Moreover, this collagen appears to be incorporated into the growing Descemet's membrane of normal corneas and narrow posterior portion of the scar tissue.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1988

Developmental changes in proteoglycans of rabbit corneal stroma.

Proteoglycans have been extracted from rabbit corneal stromas at developmental stages from fetal to adult. Ion exchange fractionation and gel chromatography show that proteodermatan sulfates decrease in sulfation and relative amount and proteokeratan sulfates increase in sulfation and relative amount during development. There are small increases in size of all of the proteoglycans up to 2 weeks after birth, and the final adult composition is achieved by 8 weeks.

Animals↗
PubMed1982

Human ocular mucus. Scanning electron microscopic study.

Twenty biopsy specimens of the human conjunctival epithelial surface with its associated mucus from normal subjects and contact lens wearers with biomicroscopic evidence of excess mucus were studied by scanning electron microscopy. Mucus existed in strands, sheets, and granules; each morphologic type of mucus was observed in all samples. No qualitative difference in morphologic features of mucus was found between normal subjects and those with excess mucus. Mucous forms observed by scanning electron microscopy closely resembled structures previously shown by biomicroscopic and light microscopic techniques, providing evidence that careful scanning electron microscopic preparation of biopsy specimens containing mucus may not greatly alter in situ morphologic characteristics of mucus.

Conjunctiva↗
PubMed1982

Cell surface variations of the human upper tarsal conjunctiva.

Fifteen specimens of the upper tarsal conjunctiva were obtained from subjects with biomicroscopically normal conjunctivae. Ten specimens were examined by scanning electron microscopy and five by transmission electron microscopy. This examination was performed to examine cells with variant surface morphologic appearance that may be signs of the initial changes typical of those seen in the diseased conjunctiva. Although most specimens had cells with a characteristic uniform distribution of microvilli, seven specimens had cells with nonuniform microvillar patterns. In these seven specimens, less than 5% of the cells showed these changes. Cell surface variations marked by reduced cell diameters and alterations in the microvillar distribution were categorized into two groups: cells with microvilli that were grouped together to form tufts of varying height and number, and cells with a single, centralized group of microvilli. These variations were seen with increased frequency in disease, and it is postulated that the microvillar changes described in this study represent a continuum of adaptive responses of cells to insult and disease.

Conjunctiva↗
PubMed1981

Goblet cells of the human conjunctiva.

We correlated histologic findings concerning conjunctival goblet cells with findings concerning the mucous membrane surfaces of the conjunctiva. Ten biopsy specimens were obtained from the upper tarsal conjunctiva and ten from the perilimbal bulbar conjunctiva. Changes in goblet cells just before and during secretion included changes in surface cell diameter, evagination of the apical surface of the cell membrane, decrease in the number of microvilli, and alteration in the arrangement and morphologic characteristics of microvilli. Changes seen by light and transmission electron microscopy correlated with those observed by scanning electron microscopy. Goblet cells were often associated with crypt openings; some crypts served as conduits for the secretion of mucus from underlying goblet cells. This study supports the hypothesis that crypts in the conjunctiva seen by scanning electron microscopy are often associated with goblet cells and their mucus secretion.

Adult↗
PubMed1980

Branching of microvilli in the human conjunctival epithelium.

Biopsy specimens were obtained from the upper tarsal and limbal conjunctivae of ten normal persons and from the upper tarsal conjunctivae of five asymptomatic contact lens wearers and five patients with giant papillary conjunctivitis (GPC) associated with contact lens wear. The microvillar surface of the conjunctival epithelium was studied by transmission and scanning electron microscopy. Branches often stemmed from the side of a straight tubular microvillus and formed an acute angle with the main process. Branches sprouted at various distances from the origin of the microvillus. The most common branching pattern was the bifurcated (bifid) form. Occasionally, both primary and secondary bifurcations were primary and secondary bifurcations were observed on the same microvillus. Some microvilli were branched in the normal conjunctivae, but the conjunctivae of asymptomatic contact lens wearers and patients with GPC had more branched microvilli and greater polymorphism, which correlated with the degree of alteration. This report demonstrates the presence of branched microvilli and describes the types seen in normal conjunctivae and compares the frequency with which branched microvilli are seen in altered conjunctival surfaces.

Cell Membrane↗
PubMed1979

The human limbus. A scanning electron microscopic study.

Fourteen human limbal biopsy specimens were obtained from seven normal subjects. The epithelial surface was examined by scanning electron microscopy. Epithelial cell surfaces varied greatly in shape and size, and mosaics of three- to six-sided irregular polygons were formed. Microvillar borders of cells were distinct. Cell sizes ranged from 3 to 20 micron across. Light and dark cells were present and randomly distributed. A few cells were covered with microplicae; the remainder of the cells were covered with microvilli. Intercellular crypt openings measuring 1 to 10 micron in diameter were distributed randomly over all specimens. These openings were believed to be related to goblet cells. Many openings were plugged with what appeared to be mucus. Many surface-level intercellular structures were present; they corresponded in diameter to the surface openings. The human limbal epithelium varies from both the upper tarsal conjunctiva and the cornea.

Adult↗
PubMed1978

Conjunctiva in asymptomatic contact lens wearers.

Biopsy specimens of the upper tarsal conjunctiva from 11 asymptomatic, biomicroscopically normal contact lens wearers (five hard and six soft) were studied by scanning electron microscopy. The conjunctiva in lens wearers showed areas of normal epithelial cell surfaces interspersed among areas of altered cells. The altered cell surfaces were characterized by centralized clumped microvilli and baring of that portion of plasma membrane around the cell periphery. The degree of centralization of microvilli varied widely, from a mild tendency to centralize accompanied by slight reduction in cell diameter, to an exteme centralization and dramatic diameter reduction. In some cells, the centralized microvillar structure had a unique mucuslike covering. Numerous altered cells were present in all 11 specimens from asymptomatic contact lens wearers. The number of these cells found by conjunctival biopsy of the hard contact lens wearers and the number in the soft contact lens wearers apparently did not differ. The upper tarsal conjunctiva in these asymptomatic contact lens wearers has an altered epithelial surface, different from that observed in normal subjects without contact lens-wearing experience and from subjects with contact lens-associated giant papillary conjunctivitis.

Adult↗
PubMedDate not supplied

Elevated intraocular pressure secondary to rhegmatogenous retinal detachment.

Elevated intraocular pressure secondary to rhegmatogenous retinal detachment was described by Ariah Schwartz in 1972, an entity commonly known as Schwartz's syndrome. Photoreceptor outer segments identified in the aqueous of patients with this syndrome are thought to play a role in the elevation of the intraocular pressure. We present two patients with open angles and elevated intraocular pressure associated with retinal detachment. Retinal reattachment surgery resulted in normalization of the intraocular pressure. Electron microscopic examination of aqueous specimens from our patients demonstrated a predominance of photoreceptor outer segments in varying stages of degeneration. In these specimens, inflammatory cells, fibrin, and pigment granules were rarely observed or were absent. We review the literature regarding the epidemiology, clinical characteristics, and pathogenesis of Schwartz's syndrome.

Aged↗