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The outer cornea of larval Xenopus laevis can reprogram cell differentiation when cultured in medium conditioned by X. laevis neural retina (XRCM) or by Rana esculenta neural retina (RRCM). Under these experimental conditions corneal cells showed the same series of cytological changes of fibre cell differentiation observed during ontogenesis and in vivo lens regeneration: enlargement of nuclei and nucleoli, increase of ribosomal population (cytoplasm-basophilia), cell elongation gradual loss of basophilic properties and acquisition of acidophilic properties for crystallin synthesis and accumulation. These events were completely dependent on XRCM or RRCM, suggesting that the neural retina secretes a factor(s) which initiates and sustains lens fibre transdifferentiation of the corneal epithelial cells. This culture system appears to be a suitable one for investigating the control of lens fibre transdifferentiation in vitro.

Implants of lensectomized eye placed between the outer and inner cornea, denatured by ethanol treatment and implants of total protein pellets from homogenates of lensectomized eyes, induce lens-forming transformations of the outer cornea of larval Xenopus laevis.

The present research aims at showing the role played by the lens in inhibiting lens-forming transformations of the outer cornea of Xenopus laevis tadpoles. Two types of experiment were carried out: 1) lentectomy and insertion of a Millipore filter disk at the side of the lens, and 2) lentectomy and insertion of a Millipore disk with a central hole. Results indicate that when the lens is replaced by a mechanical obstacle capable of preventing direct communication between vitreous and anterior chambers, definite inhibition of the lens-forming transformations of the outer cornea occurs. These data strongly suggest that the inhibition exerted by the lens on lens-forming transformations of the outer cornea in Xenopus laevis eyes is mechanical rather than chemical in nature.

Outer cornea of lensectomized Xenopus laevis tadpoles at state 50 (according to Nieuwkoop, P.D. and Faber, J., ('56) Normal Table of Xenopus laevis, Daudin, North-Holland, Amsterdam, pp. 1-243) was removed 3, 7 and 10 days after lensectomy and implanted between the outer and the inner cornea of larvae of the same species at stage 51-52. In these conditions, the implanted outer cornea remained isolated from the retinal factor of the vitreous chamber, although it received the nutritional factors normally reaching the outer cornea. Results show that lens-forming transformation process of the outer cornea is arrested, and lens-forming structures undergo regression at speed which increases with increasing precocity of the stage of lens-forming transformation undergone by the implanted cornea. These data suggest that the process of lens-forming transformation is not a single-step process, but a sequence of interactions extending over a long period of time requiring the continuous presence of the retinal factor in the vitreous chamber until complete differentiation of the lens is achieved.

Three different types of experiments were carried out to investigate the role of the lens in lens-forming transformations of the outer cornea of Xenopus laevis tadpoles (at stage 51, as defined by Nieuwkoop and Faber, '56): 1) simple lentectomy; 2) incision of outer and inner cornea in the region opposite the dorsal iris without lens removal; and 3) incision of outer and inner cornea and perforation of dorsal iris without lens removal. The results indicate that the outer cornea of Xenopus laevis larvae has lens-forming competence, even in the presence of the old lens, only when direct communication has been set up between the outer cornea and the environment of the vitreous chamber. These data indicate that the inhibition exerted by the lens on lens-forming transformations of the outer cornea is mechanical in nature.

Larval Xenopus laevis at stage 56 (Nieuwkoop and Faber, '56) were subjected to various types of lentectomy: (1) simple lentectomy, from the pupillary space after incision of outer and inner cornea; (2) lentectomy from the dorsal region of the eye; (3) lentectomy from the dorsal region of the eye and simultaneous incision of the outer cornea; (4) lentectomy from the dorsal region of the eye and simultaneous incision of the outer and inner cornea. The results obtained show that the outer cornea underwent lens-forming transformations only when the inner cornea had been incised, thus permitting outer cornea (Experiments I-IV). No lens regeneration occurred when the inner cornea was left intact (Experiments II, III). It was concluded that the factor(s) allowing the lens-forming transformations of the outer cornea is not an aspecific nutritional factor(s) but a more specific factor(s) that cannot reach the outer cornea when the inner cornea is intact. Therefore, the absence of the lens and sufficient nutrient available to the outer cornea are not enough to allow lens regeneration from the outer cornea. When lens removal was carried out through the dorsal part of the eye (Experiments III-IV) the lens regenerated from the pericorneal epidermis of this region in a large number of cases.

The present research was carried out using Xenopus laevis tadpoles at stage 50-51 to ascertain whether eye territories other than the outer cornea and the pericorneal epidermis have lens-forming competence, particularly the iris and/or retina. Five experiments were performed: simple lentectomy; removal of lens, outer cornea, and pericorneal integument; removal of lens, outer cornea, and pericorneal integument, and simultaneous incision of retina; removal of lens, outer cornea and pericorneal integument, and simultaneous incision of dorsal iris; removal of lens and simultaneous lesion of iris and retina. Results show that when lentectomy is performed without damaging any territories other than the outer and inner cornea, lens regeneration is always from the outer cornea, and the iris and retina do not exhibit any lens-forming competence. The iris and retina show no clear-cut lens-forming competence even after they have been stimulated by comparatively extensive lesions.