Human cornea: Superior and central oxygen demands

Corneal oxygen demands, expressed as ratios of uptake rate relative to baseline rates derived from unstressed corneas, were determined with a micropolarographic system for central (closed eye) and superior (open eye) locations of one cornea of each of seven human subjects. The closed-eye central corneal measurements were repeated during two additional experimental sessions. Intrasubject variability of the three mean closed-eye central corneal rate ratios ranged as high as 23%, possibly representing effects of homeostatic mechanisms on the palpebral conjunctival capillaries of some subjects. For the open-eye superior cornea, which was covered by the upper eyelid of every subject prior to measurement, oxygen demand was found to have a greater intersubject range, but was diminished in magnitude relative to the demand associated with the closed-eye central cornea. Superior corneal oxygen demand was not found to be predictable from closed-eye central corneal oxygen demand or extent of eyelid overlap onto the cornea and thus indicated localized open-eye superior corneal environments that were significantly different from those of the corresponding closed-eye central corneas. Such localized environments may be critically important when gauging the susceptibility of particular eyes to superior corneal pathology during contact lens wear.     

Human cornea: Individual responses to hypoxic environments

Using a micropolarographic system, the responses of seven human corneas to oxygen atmospheres ranging from 20.9% to 0% were measured. Under normal atmospheric conditions, ratios of oxygen demand were observed between corneas as great as 1.23:1. Following 0% oxygen conditions, however, these ratios rose to a maximum of 1.93:1, i.e., representing demand differences between particular corneas of nearly 2:1. Such differences may prove increasingly important in the gauging of patient suitability and safety in relation to hypoxic stress, e.g., the extended wear of contact lenses.     

Human immunodeficiency virus p24 antigen testing in cornea donors

PURPOSE: Testing for the p24 antigen of the human immunodeficiency virus (HIV) may detect early HIV infection in the seronegative window; however, falsely reactive results may occur in cadaver specimens. Although neither the Food and Drug Administration (FDA) nor the Eye Bank Association of America requires p24 testing of cornea donors, many tissue banks using other organs from cornea donors do perform this assay, and the FDA requires that eye banks reject corneal tissue if a reactive p24 assay is reported. We investigated the impact of p24 testing on eye banking and corneal transplantation. METHODS: Two clinical cases and records from the Lions Eye Bank of Delaware Valley (LEBDV) were reviewed retrospectively. RESULTS: Two corneas from the LEBDV were transplanted before the reporting of p24 reactivity by other tissue banks. In one case, because of the young age of the recipient, the surgeon elected to replace the cornea with new tissue hours after the original transplant, and later polymerase chain reaction (PCR) testing was negative. In the other case, there was not enough specimen to perform Western blot or PCR confirmatory testing. The patient was followed with periodic serologic testing for HIV and has remained seronegative. To avoid such problems in the future, the LEBDV initiated testing of all donors with p24 and other nonrequired screening tests. Over a 2-month period, 22 corneas (from 11 donors) were discarded because of these tests: 4 donors had reactive p24 tests, 6 were reactive for antibody to hepatitis B core antigen, and 1 had a reactive syphilis test. CONCLUSIONS: Results from p24 assays by other tissue banks may cause difficult clinical situations when the results are received after transplantation of the tissue, but the use of the p24 assay in the screening of cornea donors may result in excessive waste of donor tissue. Further guidance is needed regarding the management of positive results from this and other nonrequired screening tests.     

Human cornea: Oxygen uptake immediately following graded deprivation

Amicropolarographic system was used to measure the flux ofoxygen crossing the tear epithelial interface of thehuman cornea just following stabilized exposure to eight oxygen atmospheres ranging from 0% to 20.9%. In all, 336 measurements were made on the corneas of seven young healthy subjects. The rate coefficients of their immediate postexposure oxygen uptakes were found to increase nearly linearly over the atmospheric range of 20.9% (air) down to about 1.5%. Below that level, however, a marked increase in the immediaterecovery rate coefficient occurred, suggesting that at least two oxygen demand phases can be distinguished for the aerobically dependent pathways of the cornea.     

Immunoprotein deposition in the cornea

A 63-year old woman had bilateral, multi-level corneal deposits distributed as fine, discrete crystals and in dense, deep geographic patches. She had a long history of sero-positive rheumatoid arthritis. Autopsy revealed an unsuspected lymphoproliferative disorder and immune-complex disease. Histologic examination of the eyes revealed eosinophilic, PAS-positive, noncollagenous deposits in the cornea at all levels and also in the ciliary processes, pars plana, and choroid. Stains for gold, amyloid, and acid mucopolysaccharides were negative. Immunoperoxidase stains were positive for IgG most strongly, and also for IgA, kappa and lambda light chains. Transmission electron microscopy showed needle-like electron-dense extracellular particles which we presume are immunoglobulins.     

Stromal-epithelial interactions in the cornea

Stromal-epithelial interactions are key determinants of corneal function. Bi-directional communications occur in a highly coordinated manner between these corneal tissues during normal development, homeostasis, and wound healing. The best characterized stromal to epithelial interactions in the cornea are mediated by the classical paracrine mediators hepatocyte growth factor (HGF) and keratinocyte growth factor (KGF). HGF and KGF are produced by the keratocytes to regulate proliferation, motility, differentiation, and possibly other functions, of epithelial cells. Other cytokines produced by keratocytes may also contribute to these interactions. Epithelial to stromal interactions are mediated by cytokines, such as interleukin-1 (IL-1) and soluble Fas ligand, that are released by corneal epithelial cells in response to injury. Other, yet to be identified, cytokine systems may be released from the unwounded corneal epithelium to regulate keratocyte viability and function. IL-1 appears to be a master regulator of corneal wound healing that modulates functions such as matrix metalloproteinase production, HGF and KGF production, and apoptosis of keratocyte cells following injury. The Fas/Fas ligand system has been shown to contribute to the immune privileged status of the cornea. However, this cytokine-receptor system probably also modulates corneal cell apoptosis following infection by viruses such as herpes simplex and wounding. Pharmacologic control of stromal-epithelial interactions appears to offer the potential to regulate corneal wound healing and, possibly, treat corneal diseases in which these interactions have a central role.     

Gene therapy in the cornea

Technological advances in the field of gene therapy has prompted more than three hundred phase I and phase II gene-based clinical trials for the treatment of cancer, AIDS, macular degeneration, cardiovascular, and other monogenic diseases. Besides treating diseases, gene transfer technology has been utilized for the development of preventive and therapeutic vaccines for malaria, tuberculosis, hepatitis A, B and C viruses, AIDS, and influenza. The potential therapeutic applications of gene transfer technology are enormous. The cornea is an excellent candidate for gene therapy because of its accessibility and immune-privileged nature. In the last two decades, various viral vectors, such as adeno, adeno-associated, retro, lenti, and herpes simplex, as well as non-viral methods, were examined for introducing DNA into corneal cells in vitro, in vivo and ex vivo. Most of these studies used fluorescent or non-fluorescent marker genes to track the level and duration of transgene expression in corneal cells. However, limited studies were directed to evaluate prospects of gene-based interventions for corneal diseases or disorders such as allograft rejection, laser-induced post-operative haze, herpes simplex keratitis, and wound healing in animal models. We will review the successes and obstacles impeding gene therapy approaches used for delivering genes into the cornea.     

角膜细胞凋亡

细胞凋亡是一种程序性细胞死亡 ,在维持正常组织器官的稳定平衡及病理状态组织器官的损伤、修复中起着重要作用。许多因素对细胞凋亡既有诱导作用 ,又有抑制作用 ,如细胞因子、基因、病毒、类固醇、辐射、营养不良、T淋巴细胞等。在正常和损伤角膜中也有凋亡的证据。上皮、内皮及基质中的IL l通过角膜细胞凋亡来调节角膜细胞和损伤愈合 ,上皮损伤 (如紫外线照射、准分子激光PRK手术、病毒感染等 )导致前基质下大量角膜细胞凋亡和IL l释放。许多角膜疾病 (如病毒性角膜炎、圆锥角膜、大泡性角膜病变等 )IL l、上皮基质系统可起到保护作用 ,或与其发病机理有关。对正常和疾病角膜细胞凋亡的进一步研究可能会发现稳定正常角膜组织 ,促进损伤愈合过程的更好方法。     

Endothelial repair in the rat cornea

Destruction of the central endothelium of the rat cornea was produced by mechanical injury, total debridement, or transcorneal freezing. Endothelial repair was then studied using specular microscopy, histological staining, pachymetry, and autoradiographic analysis of the incorporation of tritiated thymidine into nuclear DNA. Following an initial process of cell slide to cover the endothelial defect, extensive cellular division occurred at the margins of the wound, with approximately 45% of cells in the wound area showing incorporation of tritiated thymidine. An intact monolayer of irregularly shaped cells was reestablished by 2-14 days, depending on the wound. These results suggest that the corneal endothelial repair processes in the rat are more analogous to those of the rabbit than to those of the cat or primate.