Growth factors: importance in wound healing and maintenance of transparency of the cornea
Introduction
The cornea is a transparent and elastic tissue without blood vessels and about 0.5 mm thick in the central part. It consists of three cell layers, namely epithelial cells, interstitial cells (stromal) and endothelial cells. Furthermore, there are two important membranes, Bowman’s membrane between the epithelial cell layer and the interstitial cell layer and Descemet’s membrane between the interstitial cell layer and the endothelial cell layer.
The corneal epithelium, a 5–6 layered, stratified tissue, is derived ontologically from the surface ectodermal tissue, but is phenotypically different from conjunctival epithelial cells. Therefore, it is thought that the mechanism of repair of corneal epithelial cells is different from that of conjunctival epithelial cells. The interstitial cell layer includes keratocytes, type I, V and VI collagens, mucopolysaccharides, etc. The endothelial cell layer consists of monolayered endothelial cells. The epithelium and interstitial layers of normal cornea are innervated with trigeminal nerve and sympathic nerves, which are considered to play important roles not only in sensory transmission, but also in maintenance for the viability of corneal epithelial cells. In fact, disturbance of the corneal epithelium occurs frequently from the paralysis of the first branch of the trigeminal nerve (paralytic keratitis). In normal corneal epithelium, cells divide in the basal layer, migrate gradually to the upper layer and fall out of the epithelial layer 1–2 weeks later. Terminally differentiated epithelial cells in the most superficial layer form a solid tight junction, which functions as a barrier.
It has been thought since the 1980 s that authentic corneal stem cells may exist, because the biological features of the corneal epithelial cells are very different from those of conjunctival epithelial cells. Early in the 1980 s, it was clarified that epithelial cells in the limbus regenerated in a manner similar to the corneal epithelial cells (Kinoshita et al., 1982)). It was shown in 1986 that basal cells in the limbal epithelial cells, which were positive for keratin 3, were the stem cells of the corneal epithelial cells (Shermer et al., 1986). A study conducted in 1989 showed a slow-cycling of cells with high growth potential in the limbus. Although there is only indirect evidence because a direct marker for the corneal epithelial stem cells is not available, the concept that corneal stem cells exist in the limbus has been accepted.
Since the normal cornea is in close contact with tear film and aqueous fluid, the various kinds of growth factors and cytokines in the tear film and aqueous fluid could play an important role in the turnover of corneal cells and wound healing of the cornea. It is important to understand the mechanism of corneal wound healing in the maintenance of good visual function. Until recently, the mechanism of corneal wound healing had mainly been studied from the perspective of extracellular matrices, etc. From a number of studies, it is now clear that many kinds of growth factors and cytokines are also involved in corneal wound healing. Signals of injury to the cornea, which are not yet well known, are transmitted to the corneal stem cells, which is localized in the limbal region and then the stem cells begin to proliferate and differentiate. Thereafter, sufficient corneal cells are provided to the damaged area of the cornea for repair to be successfully accomplished.
In the present article, we review the roles of growth factors, in particular, epidermal growth factor (EGF), keratinocyte growth factor (KGF), transforming growth factor (TGF)-β and platelet-derived growth factor (PDGF), and other cytokines in corneal wound healing and the maintenance of corneal transparency.
Section snippets
Epidermal growth factor (EGF) and the distribution of its receptor
Epidermal growth factor (EGF) is a protein of 6 kDa, which is secreted from a number of tissues such as the submaxillary gland, lacrimal gland, kidney, thyroid gland, and pancreas. It stimulates various kinds of epithelial cells. EGF, which exists as a component of human tears (Ohashi et al., 1989, Van Setten et al., 1989), accelerates eyelid opening and incisor teeth eruption in newborn mice (Cohen, 1962) and, in addition, stimulates the proliferation of corneal epithelial (Frati et al., 1972,
Keratinocyte growth factor (KGF)
KGF has been identified from a human embryonic lung fibroblast cell line as a member of the fibroblast growth factor (FGF) family (Finch et al., 1989, Rubin et al., 1989). Although the mitogenic activity of KGF is restricted to epithelial cells (Rubin et al., 1989), the KGF transcript is present in human skin fibroblasts and in the mouse skin mesenchymal layer but not in human epithelial cells nor in the mouse skin epithelial layer (Finch et al., 1989). KGF is thus considered to be a paracrine
Transforming growth factor (TGF)-β
The transforming growth factor (TGF) family is divided into two subfamilies: TGF-α and -β. TGF-α is a substance similar to EGF and has a common receptor. On the other hand, TGF-β is very different from EGF. TGF-β is a protein of 25 kDa (expressed type), which is secreted from almost all nucleated cells, especially cancer cells. TGF-β is further subdivided into TGF-β1, TGF-β2 and TGF-β3, and their genes have been localized on chromosome 19q13, 1q41 and 14q24 in human, respectively.
Platelet-derived growth factor (PDGF)
PDGF is a dimer composed of two chains, which have been classified as chain A and B. Therefore, three types of isomers (PDGF-AA, PDGF-AB and PDGF-BB) exist from the combination of these chains. It is thought that these different isoforms may have different biological characteristics.
Cytokines and corneal cell migration
It is well known that various kinds of cytokines are involved in corneal wound healing and the maintenance of corneal transparency. In other words, many kinds of inflammatory cytokines besides growth factors contribute to corneal wound healing. For example, we examined the effects of cytokines on the fibronectin-induced migration of corneal epithelial cells by using a modified Boyden’s chamber (Wang et al., 1994). Namely, cells plated in the inner chamber were incubated with FN, cytokines or
Conclusions
Clearly, various kinds of growth factors are involved in the maintenance of corneal structure and function, especially corneal transparency through wound healing (Fig. 4). In fact, many growth factors, such as EGF, FGF, TGF, KGF, HGF, PDGF and IGF, and their receptors are expressed in the corneal cells and enhance the proliferation of corneal cells. For example, although EGF is secreted mainly from the lacrimal gland (Wilson et al., 1991; Kasayama et al., 1990; Watanabe et al., 1993), it is
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