
Lens Regenerationin the Newt, Triturus viridescens:A Teacher-Student Research Project * Richard J. Brady, East Leyden High School, Franklin Park, Illinois A teacher research project is described here which has been used as a vehicle to encourage student research efforts. The embryology of the eye is the basis of the project. The purpose of this article is two-fold. of Reyer (1954). The more recent review First, acquainting the reader with the process by Stone (1959), which is a summary of of lens regeneration. The ability of animals his own work for the most part, deals with to regenerate missing parts such as limbs and lens regeneration in adult animals and does tails has received considerable attention, and not give much consideration to larval forms Downloaded from http://online.ucpress.edu/abt/article-pdf/27/3/185/21190/4440885.pdf by guest on 02 October 2021 I am sure most biology teachers have read as does the review by Reyer. articles dealing with regenerating planaria and The process of lens regeneration from the salamander limbs and tails. The process of pupillary margin of the dorsal iris is called lens regeneration, on the other hand, is usually "Wolffian" lens regeneration after Gustav mentioned only in passing as an example of Wolff who was among the first investigators another tissue which can regenerate. Although of this process in 1894. Thirteen stages of the capacity to regenerate a missing lens in an lens regeneration have been designated by adult animal is a unique attribute of only a Sato (1940). These, with slight modification, single family of Urodele amphibians, the Sala- can be applied to the process as it occurs in mandridae, the experimental procedures which both larvae and adults. The stages can be have been employed to elucidate the factors divided into four groups and are summarized involved in this process are so straightforward as follows: that the subject could very effectively be in- I. Latent Period: Stages 1 and 2 cluded in a school biology course as il- high Thickening of the iris along pupillary lustrative material in a number of places. If margin with a formation of a cleft be- the teacher is looking for another example of tween the inner and outer laminae the "scientific method" at work, for instance, (Fig. 1, A and B). the experiments described herein will provide material which even the most well-read among II. Initial Period: Stages 3 to 6 the students will find new and interesting. Appearance of depigmented cells The second purpose of this article is to along pupillary margin followed by describe the development of this teacher-stu- formation of a vesicle. The cells of dent project at East Leyden High School. the inner wall of this vesicle become elongate to form the primary lens Part I. Lens Regeneration in the Newt, fibers (Fig. 1 C, D, E, and F). Triturus V. Viridescens III. Period of Lens Fiber Differentiation: Stages 7 to 11 The capacity of an animal to regenerate an Completion of the primary lens fiber lens has been known for many extirpated nucleus which fills the lumen of the and been the of intensive years has subject vesicle and the symmetrical enclosure There are two study by many investigators. of this nucleus by secondary lens especially ac- men, however, who have beeii fibers (Fig. 1, G). tive in this area: Dr. Leon S. Stone, Yale 12 13 Medical School, and Dr. Randall W. Reyer, IV. Period of Growth: Stages and University of West Virginia. For a more Disappearance of nuclei in primary complete development of the ideas presented lens fibers and secondary lens fibers in this paper, the reader is referred to several except in the outermost portions (Fig. excellent reviews by these investigators (Re- 1, H). yer, 1954; Stone, 1953, 1959). The most In adult Triturus v. viridescens the entire comprehensive review of the topic is that process takes approximately five weeks at or- 185 186 THE AMERICAN BIOLOGY TEACHER that there is a stimulus of a chemical nature involved which he calls the "neural retina factor." In a series of experiments, simple to com- prehend but difficult to carry out, Stone (1953) has clearly shown the relationship of the dorsal iris to the process of lens regenera- tion (Figs. 2, 3 and 4). From these experi- mental results and others which have been re- ported by Dr. Stone, the following conclusions concerning the dorsal iris can be drawn: 1. Injury to the dorsal iris neither stimu- Downloaded from http://online.ucpress.edu/abt/article-pdf/27/3/185/21190/4440885.pdf by guest on 02 October 2021 lates nor inhibits lens regeneration. 2. Maximum capacity for lens regenera- tion resides in the dorsal iris at the 12- o'clock position and decreases com- pletely as the 9 and 3-o'clock positions are reached. 3. This polarization of the iris occurs dur- ing the formation of the optic cup. Experiments by Stone and Steinitz (1957) Fig. 1. Regeneration of the Lens from the Dorsal Iris. Darkly pigmented normal iris (A); depigmentation and involving hundreds of adult newts (Trititrus thickening of iris cells (B); cell proliferation resulting v. viridescens), demonstrated the relationship in the formation of a vesicle (C, D, and E); primary the pigment cells of the retina, the lens fiber formation at the inner pole of the vesicle between (F); secondary lens fiber formation (G); detachment of neural retina, and the capacity of the dorsal lens regenerate from dorsal iris and continuation of iris to regenerate an extirpated lens. They secondary lens fiber development (H). (Redrawn from Stone, 1953) furnish conclusive evidence that the pigment cells of the retina are capable of differentiat- ing into not only neural retina cells, but also dinary room temperature, whereas in other into iris and (subsequently) into lens cells as species it may take twice that long. well. For example, in a series of experiments The exact nature of the mechanism which in which the Entire eyes of adult newts were enables these phenomena to occur has been transplanted from one animal to another, the subject of much investigation. Stone there followed a complete breakdown of the (1953), in relating the embryological de- neural retina along with the destruction of the velopment of the lens to the process of lens lens by cataract formation and subsequent de- regeneration stated that: generation. Shortly following this destruction "At first the mechanism for lens formation period, the retinal pigment cells differentiated expresses itself in the nature of some stimulus, into a new neural retina layer and a lens re- perhaps of a chemical nature, that passes from generated from the dorsal iris. the embryonic eye to the responding surface Of all the experiments involving the rela- ectoderm with which it makes contact. As the tionship of the neural retina and lens re- eye develops, the mechanism apparently be- generation, there is one kind of experiment in comes embodied within the cells of the dorsal particular which will give an insight into this iris when the latter forms. The lens-forming interesting relationship (See Fig. 5, A, B, potentiality of the cells is then restrained from and C). expressing itself by the presence of living lens Other experiments have shown that the in- fibers or by some substance produced by troduction of wax spheres or small pieces of them." tissue (other than lens tissue) does not inhibit Since making this statement eleven years the formation of a new lens in a lentectomized ago, Dr. Stone has performed a great number eye (Stone, 1945), but the replacement of an of ingenious experiments to elucidate the intact lens from the same species of sala- nature of this mechanism. It seems probably mander or from another species of salaman- LENSREGENERATION IN THENEWT 187 r~~~--9-- Fig. 2. Dorsal iris from an intact newt eye transplanated under the ventral iris of a lensectomized eye results in the Downloaded from http://online.ucpress.edu/abt/article-pdf/27/3/185/21190/4440885.pdf by guest on 02 October 2021 development of lenses from both the normal and transplanted dorsal iris. (Redrawn from Stone, 1953) Fig. 3. Ventral iris from an intact newt eye transplanted under the ventral iris of a lensectomized eye results in the development of a lens from only the dorsal iris. (Redrawn from Stone, 1953) Fig. 4. Ventral iris transplanted to the 12-o'clock position in a lensectomized eye results in the development of lenses on each side of the transplant. (Redrawn from Stone, 1953) der which may Not even possess the capacity It seems logical to conclude that there is to regenerate an extirpated lens will inhibit present a factor in the lens which inhibits the lens regeneration if the transplanted lens does development of additional lens tissue. We may not degenerate. Removal of the loosely at- also conclude that there is present in the tached neural retina will also inhibit regenera- normal eye a "neural retina factor" which tion as will the daily injection of aqueous stimulates the formation of a new lens in the humor from intact eyes into lentectomized absence of the inhibiting lens factor. Experi- eyes, for many weeks in some experiments. ments by Dr. Stone (1958), using circular Following cessation of the injections, lens re- discs of plastic membranes of various poros- generation commences without delay (Stone, ities have shown that the neural retina factor 1953). will not pass through membranes which are 188 THEAMERICAN BIOLOGY TEACHER ER PLIOFILMDISCS CHOROID PIGMEN TED RETINA LEN3 A NEURP&LRETINA Downloaded from http://online.ucpress.edu/abt/article-pdf/27/3/185/21190/4440885.pdf by guest on 02 October 2021 NO LENS RF6CVA'A7S SMALLt ts RCCANCATCS Fig.
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