The role of growth factors in the embryogenesis and differentiation of the eye

Transgenic models for eye malformations

Several lines of transgenic mice developing eye malformations have been described in the literature and appear to be of increasing interest for the study of eye teratology in humans, since gene expression and regulation can be studied in the developing animal. Transgenic applications are briefly described here and an overview of existing transgenic mouse models carrying different eye abnormalities is given according to the major diagnosis (e.g., cataract, microphthalmia, anterior segment dysgenesis, retinal dysplasia). Interestingly, many transgenic models exhibit pathological findings similar to those observed in human pediatric ophthalmology. Unfortunately, detailed embryological studies in transgenic mice bearing congenital eye malformations are not available for all lines. Thus, the importance of creating further transgenic models to study the function of morphogenes and growth factors in eye development is also discussed.

Ontogeny of the head of the Pacific hagfish (Eptatretus stouti, Myxinoidea): development of the lateral line system

The head of adult hagfishes (jawless craniates, Myxinoidea) of the family Eptatretidae displays a number of skin grooves of uncertain origin. These grooves have been homologized to the neuromast lines of other craniates, and they are innervated by two ganglionated cranial nerves that have been interpreted as lateral line nerves. The grooves do not, however, contain the compound receptors that are typical of a lateral line (i.e. neuromasts or electroreceptors), and both their development and function have remained enigmatic. To elucidate the embryonic origin of the grooves (which should develop from placodes if they are homologues of the lateral line system), embryos of Pacific hagfish were examined by means of three-dimensional reconstructions from serial sections. Because of the scarcity of specimens of embryonic hagfishes, only two embryos were reconstructed, but these reconstructions clearly show that a number of placodes and placodal derivatives (i.e. sensory ridges, receptor primordia, and cranial ganglia) occur in the head of embryonic eptatretid hagfishes. Some of these placodes correspond to the lens and epibranchial placodes of other craniates, but there are also three other placodes which represent possible homologues of lateral line placodes. The topology of the placodes in this latter group corresponds to the topology of the grooves of adult hagfishes, and we therefore reach three conclusions: (i) that an embryonic lateral line system is present in hagfishes; (ii) that the grooves of adult hagfishes in all probability derive from lateral line placodes; and (iii) that the presence of lateral line placodes is a primitive character of craniates.

TGF alpha can act as a chemoattractant to perioptic mesenchymal cells in developing mouse eyes

Growth factors are believed to play an important role in regulating cell fate and cell behavior during embryonic development. Transforming growth factor alpha (TGF alpha), a member of the epidermal growth factor (EGF) superfamily, is a small polypeptide growth factor. Upon binding to its receptor, the EGF receptor (EGFR), TGF alpha can exert diverse biological activities, such as induction of cell proliferation or differentiation. To explore the possibility that TGF alpha might regulate cell fate during murine eye development, we generated transgenic mice that express human TGF alpha in the lens under the control of the mouse alpha A-crystallin promoter. The transgenic mice displayed multiple eye defects, including corneal opacities, cataracts and microphthalmia. At early embryonic stages TGF alpha induced the perioptic mesenchymal cells to migrate abnormally into the eye and accumulate around the lens. In situ hybridization revealed that the EGFR mRNA is highly expressed in the perioptic mesenchyme, suggesting that the migratory response is mediated by receptor activation. In order to test this model, the TGF alpha transgenic mice were bred to EGFR mutant waved-2 (wa-2) mice. We found that the eye defects of the TGF alpha transgenic mice are significantly abated in the wa-2 homozygote background. Because the EGFR mutation in the wa-2 mice is located in the receptor kinase domain, this result indicates that the receptor tyrosine kinase activity is critical for signaling the migratory response. Taken together, our studies demonstrate that TGF alpha is capable of altering the migratory decisions and behavior of perioptic mesenchyme during eye development.

Conserved expression of the opioid growth factor, [Met5]enkephalin, and the zeta (zeta) opioid receptor in vertebrate cornea

In addition to neuromodulation, endogenous opioids serve as growth factors. The naturally occurring opioid peptide, [Met5]enkephalin, termed opioid growth factor (OGF), has been found to be a potent and tonic inhibitor of processes related to growth and renewal, particularly cell proliferation. OGF mediates its actions through the zeta (zeta) opioid receptor. In order to determine if OGF and/or the zeta receptor are present in human corneal epithelium, immunocytochemistry was utilized. Immunoreactivity with regard to OGF and to the zeta receptor could be detected in the cortical cytoplasm of both basal and suprabasal epithelial cells, but was not associated with the cell nucleus. Investigation of the ubiquity of OGF and zeta receptor in the vertebrate cornea showed that both elements are present in a wide variety of classes of the phylum Chordata, including mammalia, aves, reptilia, amphibia, and osteichthyes. These results suggest that an endogenous opioid system related to growth may have originated as early as 300 million years ago, and that the function of this system in cellular renewal and homeostasis is a requirement of the vertebrate corneal epithelium.

Developmental regulation of insulin-like growth factor binding protein-2 in chick embryo serum and vitreous humor

The chick embryo is a useful vertebrate model for studying developmental embryogenesis. Insulin-like growth factor I (IGF-I), a potent mitogen, is thought to contribute to the general growth of the embryo as an endocrine factor, and as a paracrine factor to the development of the early embryo and of specific organs such as the eye. Recent data suggest that a family of at least six IGF binding proteins (IGFBPs) complex IGF-I and modulate its biological actions. In the present study, we examine the expression of IGFBPs in chicken serum and vitreous humor at different stages of embryonic development, and compare it with that of IGF-I. As determined by ligand blotting, the predominant IGFBP in chick serum and vitreous humor between embryonic days 4 and 22 (E4-E22) is a 30 kDa IGFBP. This IGFBP was specifically immunoprecipitated by a polyclonal antiserum raised against rat IGFBP-2, the predominant IGFBP in fetal human and rat serum. Although IGFBP-2 is present in both chick fluids at all times examined, serum IGFBP-2 increased progressively between E10-E22, whereas vitreous IGFBP-2 was highest during eye organogenesis (E4-E8). This suggests that vitreous IGFBP-2 is synthesized locally. Like serum IGFBP-2, levels of immunoreactive IGF-I in serum are higher in the second week of embryogenesis than the first. Despite this correlation, changes in IGFBP-2 do not appear to be regulated by IGF-I: (a) serum IGF-I decreases after day 15, whereas IGFBP-2 levels remain stable until hatching; (b) vitreous IGF-I, like serum IGF-I, is higher in the second week of embryogenesis, whereas vitreous IGFBP-2 is highest in the first week; (c) embryos cultured ex ovo express IGFBP-2 at E15-E19, although they lack the normal mid-embryogenesis surge in IGF-I. We conclude that vitreous IGFBP-2 is synthesized locally in the eye, and that the expression of IGFBP-2 in chick embryos is not directly regulated by IGF-I.

Ocular findings in cerebro-ocular-myopathy syndrome (COMS). A possible role of growth factors?

The clinical and histopathological findings are described in a case of cerebral and ocular abnormalities associated with (congenital) muscular dystrophy. Histopathological examination of the eyes revealed a fetal configuration of the anterior chamber angle, elongated ciliary processes, cataract, persistence of primary hyperplastic vitreous and total retinal detachment with retinal dysplasia. The similarity of ocular findings in Walker-Warburg syndrome, muscle-eye-brain disease and Fukuyama's congenital muscular dystrophy, and the role of growth factors as a possible unifying (foetal) cause are discussed. This case has been presented at the meeting of the EOPS Verhoeff Society, Nürnberg 1991.

Detection, quantification, and significance of basic fibroblast growth factor in the aqueous humor of man, cat, dog and pig

By using a highly specific and sensitive heterologous radioimmunoassay, we determined the content of basic fibroblast growth factor (bFGF) in fresh samples of aqueous humor obtained from human (n = 18), feline (n = 8), canine (n = 12), and porcine (n = 10) eyes by paracentesis. The content of bFGF in human aqueous humor ranged from 0.480 to 1.44 ng ml-1 (mean +/- S.D. = 1.074 +/- 0.297 ng ml-1); in feline samples, from 0.672 to 1.152 ng ml-1 (1.016 +/- 0.158 ng ml-1); in canine samples, from 0.640 to 1.232 ng ml-1 (1.026 +/- 0.171 ng ml-1); and in porcine samples, from 0.627 to 0.957 ng ml-1 (0.728 +/- 0.115 ng ml-1). These values were confirmed by means of a dot/slot-blot technique. For all species, the aqueous samples had normal protein levels that ranged from 5 to 19 mg dl-1. There was no correlation of the content of bFGF with the level of protein or with age of the human subjects. The similarity in the concentrations of bFGF in the aqueous humor as well as the stability of the blood-aqueous barriers of all four species indicate that cats, dogs, and pigs can serve as suitable animal models for the study of the role of bFGF in health and disease. We suggest the possible involvement of bFGF in the pathogenesis of anterior-segment disorders, such as neovascular glaucoma, and in the wound-healing response of limbal tissues after glaucoma filtration surgery.