Collagen and mucopolysaccharide biosynthesis in mass cultures and clones of chick corneal fibroblasts in vitro

Wounding the cornea to learn how it heals

Corneal wound healing studies have a long history and rich literature that describes the data obtained over the past 70 years using many different species of animals and methods of injury. These studies have lead to reduced suffering and provided clues to treatments that are now helping patients live more productive lives. In spite of the progress made, further research is required since blindness and reduced quality of life due to corneal scarring still happens. The purpose of this review is to summarize what is known about different types of wound and animal models used to study corneal wound healing. The subject of corneal wound healing is broad and includes chemical and mechanical wound models. This review focuses on mechanical injury models involving debridement and keratectomy wounds to reflect the authors' expertise.

Human fetal keratocytes have multipotent characteristics in the developing avian embryo

The human cornea contains stem cells that can be induced to express markers consistent with multipotency in cell culture; however, there have been no studies demonstrating that human corneal keratocytes are multipotent. The objective of this study is to examine the potential of human fetal keratocytes (HFKs) to differentiate into neural crest-derived tissues when challenged in an embryonic environment. HFKs were injected bilaterally into the cranial mesenchyme adjacent to the neural tube and the periocular mesenchyme in chick embryos at embryonic days 1.5 and 3, respectively. The injected keratocytes were detected by immunofluorescence using the human cell-specific marker, HuNu. HuNu-positive keratocytes injected along the neural crest pathway were localized adjacent to HNK-1-positive migratory host neural crest cells and in the cardiac cushion mesenchyme. The HuNu-positive cells transformed into neural crest derivatives such as smooth muscle in cranial blood vessels, stromal keratocytes, and corneal endothelium. However, they failed to form neurons despite their presence in the condensing trigeminal ganglion. These results show that HFKs retain the ability to differentiate into some neural crest-derived tissues. Their ability to respond to embryonic cues and generate corneal endothelium and stromal keratocytes provides a basis for understanding the feasibility of creating specialized cells for possible use in regenerative medicine.

Corneal keratocytes retain neural crest progenitor cell properties

Corneal keratocytes have a remarkable ability to heal the cornea throughout life. Given their developmental origin from the cranial neural crest, we asked whether this regenerative ability was related to the stem cell-like properties of their neural crest precursors. To this end, we challenged corneal stromal keratocytes by injecting them into a new environment along cranial neural crest migratory pathways. The results show that injected stromal keratocytes change their phenotype, proliferate and migrate ventrally adjacent to host neural crest cells. They then contribute to the corneal endothelial and stromal layers, the musculature of the eye, mandibular process, blood vessels and cardiac cushion tissue of the host. However, they fail to form neurons in cranial ganglia or branchial arch cartilage, illustrating that they are at least partially restricted progenitors rather than stem cells. The data show that, even at late embryonic stages, corneal keratocytes are not terminally differentiated, but maintain plasticity and multipotentiality, contributing to non-neuronal cranial neural crest derivatives.

Lysophosphatidic acid, serum, and hyposmolarity activate Cl- currents in corneal keratocytes

The influence of serum, lysophosphatidic acid (LPA), and hyposmotic stress on the ion channel activity of normal and cryo-injured rabbit corneal keratocytes was investigated. Whole cell currents were examined using the amphotericin perforated-patch technique. In cells from wounded corneas, fetal bovine serum activated large, holding voltage-insensitive, fast-activating, 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS)-, flufenamic acid-, and 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB)-blockable outward currents showing inactivation at depolarized voltages. LPA activated identical currents, also only in cells from wounded corneas. Blocker and reversal potential experiments characterized the current as a Cl- currents (Icl). Lysophosphatidylcholine (10 microM) failed to activate the current. An identical current was activated by hyposmotic stimulation in cells from control and wounded corneas. Hyposmotic stimulation also activated Icl in cells from wounded corneas that were unresponsive to LPA. We conclude that serum, LPA, and hypotonic stress activate Icl in keratocytes from wounded corneas. We also conclude that LPA is a serum factor that can activate Icl and that hyposmotic activation may work through a signaling pathway separate from that of LPA.

Effect of low density lipoprotein on glycosaminoglycan secretion by cultured human smooth muscle cells and fibroblasts. Influence of serum concentration and cell proliferation rate

Glycosaminoglycan (GAG) secretion was studied in cultures of human fibroblasts and arterial smooth muscle cells. Supplementation of culture medium with whole human serum increased the secretion of GAG but this effect disappeared as cell density increased. Lipoprotein-free serum (LFS) supported cell growth but led to a decrease in GAG secretion and in cell cholesterol. Addition of human low density lipoprotein (LDL) to the medium containing 10% LFS produced increases in GAG secretion (200%) and cell cholesterol (300%) and a decrease (60%) in cell population. The effects of LDL were considerably smaller in medium containing 5% LFS; this was related to the lower rate of proliferation in this medium, since there was a close relationship between rate of proliferation and stimulation of GAG secretion by LDL independent of serum concentration. In addition, fetal smooth muscle cells showed a qualitatively different response to LDL in 5% LFS, with a biphasic dose-response of GAG secretion and cell number. It is concluded that: (1) whole human serum stimulates GAG secretion by sparse cell cultures, (2) LFS can support cell growth but not GAG secretion, (3) LDL stimulates GAG secretion but has a cytotoxic effect, (4) the degree of GAG stimulation by LDL is dependent on the proliferative state of cells, (5) at low serum concentrations fibroblasts and fetal smooth muscle cells show differences in response to LDL which are not evident at higher serum concentration.

DNA synthesis during the development of the chick cornea

The frequency and pattern of DNA synthesis were analysed autoradiographically in the developing chick cornea. Each cellular population was studied in time-sequence fashion from the time of its appearance until hatching. There is a sharp drop in the synthetic index (number of labeled cells/total number of cells) in the anterior corneal epithelium soon after its formation, corresponding in time to the secretion of extracellular matrix material by this tissue. A similar decrease does not occur in adjacent tissues. Continous labeling experiments show that about 20% of the corneal cells are not in the proliferative pool at this time while 100% of the cells in the underlying lens epithelium and the surrounding head epidermis and head mesenchyme are labeled. Cell cycle measurements indicate that the proliferative kinetics of both the corneal epithelium and the head epidermis are similar at this time even though the percentage of labeled cells in each region differs. The formation of the corneal endothelium and the movement of fibroblasts into the stromal region are events which involve extensive cellular migration. Labeled cells are observed at all stages of both endothelial and stromal fibroblast migration, indicating that DNA synthesis occurs during the course of cellular migration in the developing cornea.

Synthesis of sulfated glycosaminoglycans by the three cell types of the rabbit cornea in culture

Rabbit corneal cells were cultivated for 21 days and then exposed to Na235SO4, a precursor of sulfated glycosaminoglycans (GAG). All 3 cell types of the cornea, the fibroblasts, the epithelial as well as the endothelial cells, synthesize GAG. The fractionation-patterns of the epithelial and endotherlial GAG are almost identical and differ clearly from the one of fibroblastic GAG.

Biosynthesis of glycosaminoglycans by mammalian corneal epithelium and fibroblasts in vitro. II. Approach to specify the GAG from the two cell types

35S- as well as 3H-labeled glycosaminoglycans (GAG) produced by cultivated epithelium and fibroblasts of the rabbit cornea were treated with testicular hyaluronidase, leech hyaluronidase and chondroitinase-ABC or -AC. The fractionation-patterns of enzyme-treated GAG were compared with blanks not exposed to enzymes. The epithelial GAG revealed to be generally more resistant to the enzymatic degradation than the GAG synthesized by the fibroblasts, but--depending on the enzyme--in the GAG of both cell types the same fractions were attacked. The decline of the radioactivity in the fractions of enzyme-treated GAG allows the conclusions that both cell types produce relatively small amount of keratan sulfate but mainly chondroitin sulfates with a different degree of sulfation. In addition GAG, not present in the normal cornea, are synthesized: hyaluronic acid chiefly by fibroblasts and probably dermatan sulfate. The possible role of the fibroblastic and epithelial GAG in corneal wound repair is discussed.

Biosynthesis of glycosaminoglycans by mammalian corneal epithelium and fibroblasts in vitro. I. Isolation and fractionation-differences of GAG from the two cell types

Labeled glycosaminoglycans (GAG) were isolated and fractionated from cultured rabbit corneal epithelium, rabbit or bovine corneal fibroblasts and rabbit bone fibroblasts that had been previously incubated with either Na235SO4 or D-glucosamine-6-3H precursors. Thus it was demonstrated that the epithelium produces GAG. The fractionations indicated that the epithelial cells predominantly synthesize GAG with a lower degree of sulfation than those produced by corneal fibroblasts. The GAG-fractionation-patterns of the corneal epithelium and bone fibroblasts were similar.

Location of procollagen in chick corneal and tendon fibroblasts with ferritin-conjugated antibodies

Three distinct antiprocollagen preparations were characterized and used in immunocytochemical staining of chick embryo corneal and tendon cells. The several ferritin-conjugated antibody preparations permitted similar location of procollagen in the cisternae of the rough endoplasmic reticulum and in Golgi elements in both cell types. The ability to demonstrate and interpret specific ferritin staining was dependent on the extent of membrane breakage in each of those organelles, coupled with adequate retention of cell morphology. Corneal fibroblasts appeared to suffer more extensive intracellular membrane damage under controlled conditions of homogenization than tendon fibroblasts, facilitating the identification of procollagen in Golgi vacuoles of these cells. None of the labeled material appeared to by cytoplasmic in origin since ferritin was observed in the cytoplasm only in the vicinity of Golgi elements that were extensively broken. This study extends previous immunological evidence for the presence of procollagen in the Golgi complex and calls attention to the problems to be encountered in locating the antigen in small Golgi vesicles and lamellae.

Stimulation of extracellular matrix synthesis in the developing cornea by glycosaminoglycans

Previously, it was demonstrated that the embryonic corneal epithelium produces the chondroitin sulfate and heparan-sulfate-like compounds and the collagen of the primary corneal stroma. Synthesis of all of these extracellular materials is greatly enhanced in vitro when isolated epithelium is grown on collagenous substrata instead of Millipore filters. We report here that chondroitin sulfate, heparin, and heparan sulfate added to the culture medium at a concentration of 200 mug/ml enhance the synthesis by the epithelium of chondroitin sulfate and heparan-sulfate-like compounds 2-fold, whether or not collagenous substrata are employed. Collagen synthesis is unaffected by adding glycosaminoglycan to the medium. Chondroitin sulfate proteoglycan (chondromucoprotein) has the same stimulatory effect as chondroitin sulfate, but dermatan sulfate and hyaluronate have no measurable effect on glycosaminoglycan production by epithelial cells. Keratan sulfate however, seems to depress glycosaminoglycan synthesis. Thus, in this system, only sulfated polyanions like those produced by the corneal epithelium have a stimulatory effect on glycosaminoglycan synthesis. The results are discussed in terms of how the tissues of the cornea (epithelium, endothelium, keratocytes) may interact by changing the composition of the stromal extracellular matrix.

Secretion of collagen by corneal epithelium. I. Morphology of the collagenous products produced by isolated epithelia grown on frozen-killed lens

Corneal epithelium from 5-7-day old chick embryos was isolated with EDTA and grown in culture on frozen-killed lens as a substratum. Autoradiographs showed that in the presence of [(3)H]proline, the corneal epithelium synthesized and secreted onto the lens substratum, radioactive materials resistant to extraction by sodium hydroxide. The radioactive label was associated with newly formed striated collagen fibrils, large "sheets" of collagen, and basal lamina. The repeat period and interband pattern of the abundant new collagen sheets and fibrils was typical of "native" or so-called "mesenchymal" collagen. Collagen-like materials were observed in secretory (Golgi) vacuoles within the corneal cells and collagen fibrils within the intercellular canals (lateral interfaces) of the epithelium, as well as at the base of the cells. Both the granular endoplasmic reticulum and Golgi complexes were highly developed in the corneal epithelium. In the discussion, the role of cytoplasmic organelles in collagen secretion, the origin and structure of the basal lamina, and variations in collagen polymerization patterns in vitro are reviewed and evaluated. The morphogenetic significance of the synthesis and secretion of collagen by embryonic epithelium is appraised and the production of true native-striated collagen by epithelium is stressed.