Cellular contribution to symmetrical forelimbs from triploid-marked "polarizing region" in the embryo of axolotl, Ambystoma mexicanum

Grafts of posterior tissue placed anterior to the limb bud in the salamander embryo exert a polarizing influence. To explain this result, the idea that the anteroposterior axis of the developing forelimb is polarized by a diffusible morphogen has been proposed. An alternative hypothesis, and the working hypothesis of the present study, is that the polarization of the developing salamander forelimb is accomplished by short-range cellular interactions resulting in intercalation rather than by the more global influence of a diffusible morphogen. One prediction of this intercalation hypothesis is that cells will be contributed to the limb from the "polarizing tissue." To test this idea, grafts of triploid marked polarizing tissue were implanted anterior to the limb bud in 82 diploid axolotl embryos at stages 32-34 of development. A total of 27 (33%) of the limbs that resulted were symmetrical and ranged in complexity from one to seven digits. Histological analysis of a subgroup of the original symmetrical limbs revealed that mesodermally derived tissues in the anterior side of these limbs (the side which formed as a duplication in response to the influence of the graft) contained high percentages of trinucleolate cells (muscle, 12.1%; connective tissue tissue, 12.5%; and cartilage, 13.4%) when compared to similar tissues in the posterior side of the same symmetrical limbs (muscle, 1.8%; connective tissue , 0.7%; and cartilage, 0.6%). When symmetrical limbs were amputated, 73% regenerated symmetrical limbs. When these regenerated limbs were again amputated, 63% formed symmetrical secondary regenerates. Histological analysis of the first generation of regenerated limbs revealed that the pattern of distribution of trinucleolate cells in each regenerate was similar to the pattern seen in the original symmetrical limb. These results indicate that there is considerable cellular contribution to the anterior side of the symmetrical forelimb from the mesoderm of grafted "polarizing tissue." This result supports the idea that short-range cellular interaction are sufficient for formation of symmetrical forelimbs in salamander embryos.     

The failure of double-half forelimbs to undergo distal transformation following amputation in the axolotl, Ambystoma mexicanum

Although capable of initiating early regenerative responses, axolotl forelimb stumps which are composed of double-half limb tissues fail to undergo the events that normally lead to the replacement of missing parts. In the present study, the posterior halves of right forelimbs were exchanged with the anterior halves of left forelimbs, or the dorsal halves of right forelimbs were exchanged with the ventral halves of left forelimbs. Forelimbs were amputated through the graft region 30 days after grafting. Limb stumps bearing double-dorsal, double-ventral or double-posterior tissues either produced hypomorphic regenerates or failed to form any externally visible outgrowth. When the limb stump bore double-anterior tissues, no externally visible structures were formed. Normal and multiple regenerates were never formed by double-half limbs. These results are discussed in terms of the polar coordinate model and suggest that the regeneration blastema requires a complete circumference of positional values in order to complete distal transformation.     

The role of grafted skin in the regeneration of X-irradiated axolotl limbs

The primary aim of these experiments was to follow the cells descended from limb skin through the process of limb regeneration to determine what range of differentiations these cells may assume. Triploid hindlimb or forelimb skin was grafted to the denuded thighs of diploid host axolotls that had previously received 3000 R of X irradiation across both hindlimbs and the intervening pelvic area. The host limbs were then amputated through their grafts and permitted to regenerate. Cartilage, perichondrium, joint connective tissue, general connective tissue, dermis, and epidermis were present in all the regenerated limbs, but only 10% of the regenerates contained muscle. Tabulation of nucleolar numbers showed that the majority of cells in each regenerated tissue originated from the grafted skin. A strong correlation was demonstrated between the forelimb or hindlimb origin of the skin grafts and the number of digits regenerated.     

Inhibition and axial deviation of limb regeneration in the newt by means of a digit implanted into the amputated limb

This research was designed to follow up the observation of Thornton and Kraemer ('51) that regressed, denervated limbs of Ambystoma larvae will not regenerate upon reinnervation if all digits on the limbs were not completely resorbed. The object of this experiment was to determine whether the presence of an apical structure, protruding past the amputation surface, would affect the regenerative process. Both forearms of adult newts were amputated midway between the elbow and the wrist. One limb served as a normal regeneration control, and in the other limb the third digit from the removed hand was implanted in place of the removed radius, so that the three distal phalangeal segments protruded past the plane of amputation. Blastema formation in the experimental limbs was delayed by several weeks as compared with control limbs. Approximately one third of the experimental limbs did not regenerate. The regenerates that did form were strongly deviated (45–90°) radially from the longitudinal axis of the limb. Experimental analysis showed that the delay in regeneration is due largely to the projecting part of the digit. The radial deviation of the regenerates is not due to the digital implant, but rather to the removal of the radius. Trauma alone does not account for this phenomenon.     

The effect of healing time on the proximodistal organization of double-half forelimb regenerates in the axolotl,Ambystoma mexicanum

The effect of healing on the proximodistal organization of regenerates from double-half forelimbs was studied. Double-anterior and double-posterior upper forelimbs were prepared surgically and amputated at 5, 10, 15, 20, 30, and 60 days after grafting. All experimental groups regenerated hypomorphic skeletal patterns. Double-half forelimbs amputated at Days 5 and 10 regenerated more distally complete skeletal patterns than did limbs amputated at Days 30 and 60. The mean numbers of skeletal elements regenerated were seen to decrease as a function of time after grafting, with the maximal suppression of skeletal patterns observed to occur when limbs were amputated 30 days following grafting. There was no appreciable difference between limbs amputated at Days 30 and 60. These results suggest that healing time has a profound effect on the proximodistal organization of limbs regenerated from double-half forelimb stumps.     

Retinoic acid modifies positional memory in the anteroposterior axis of regenerating axolotl limbs

The effects of retinoic acid (RA) on anteroposterior (AP) positional memory of regenerating axolotl limbs were tested after removing the anterior or posterior half from the zeugopodium (lower arm or leg). RA (150 micrograms/g body wt) was injected into groups of animals bearing the following types of limbs: (1) anterior and posterior half zeugopodia grafted to the eyesocket and amputated distally 7 days later; (2) unamputated anterior and posterior half zeugopodia in situ; (3) double anterior and double posterior half zeugopodia amputated distally 7 days after their construction; (4) sham-operated zeugopodia amputated distally 7 days after operation. Controls consisted of these four groups injected with the retinoid solvent, dimethyl sulfoxide, or not injected. Control half zeugopodia grafted to the eyesocket regenerated no more than one or two digits. Control unamputated half zeugopodia in situ underwent partial or complete regeneration of the missing half from the proximal and midline wound surfaces exposed during construction of the half zeugopodia. Control double anterior and posterior zeugopodia both regenerated symmetrical, hypomorphic regenerates with 1-3 digits in the double anteriors and 1-6 digits in the double posteriors. Sham-operated controls regenerated normally. Regenerating anterior and posterior halves responded differently to RA. RA-treated anterior half zeugopodia in the eyesocket, and anterior half stumps adjacent to the unamputated posterior half zeugopodia in situ both produced regenerates that duplicated stump structures in the proximodistal axis and formed a complete and normal AP pattern. RA-treated double anterior zeugopodia regenerated proximodistal-duplicated pairs of mirror-imaged limbs, each with a complete and normal AP pattern. In contrast, half posterior zeugopodia in the eyesocket, the posterior half stumps of unamputated half anterior zeugopodia in situ, and double posterior zeugopodia all failed to regenerate. These results suggest that RA modifies positional memory in only one direction in the AP axis, posterior.     

Positional information in the forelimb of the axolotl: experiments with double-half tissues

It has been demonstrated recently that upper forelimbs of axolotls comprised of symmetrically arranged soft tissues do not regenerate (P. W. Tank, 1978,J. Exp. Zool.204, 325–336). These double-half forelimb stumps contained skin, muscle, and loose connective tissues in symmetrical arrangement. The present study explores the roles of muscle, skin, and epidermis in the regeneration of double-half forelimbs by grafting them separately to create forelimb stumps bearing symmetrical arrangements of these individual tissues. Forelimb stumps bearing symmetrically arranged flexor and extensor muscles and normally arranged skin underwent complete regeneration (96%). Forelimbs comprised of double-half skin overlying normally arranged muscles and deep tissues formed hypomorphic structures and nonregenerates (56%) with some single and multiple regenerates. Limbs with double-half deep tissues and complete epidermis either regenerated distally incomplete patterns (47%), single patterns (33%), or multiple patterns (20%). Those forelimbs comprised of double-half skin and no muscle regenerated incomplete patterns in the majority of cases (56%) but single and multiple limbs also were formed. Based on these results it can be concluded that no single type of tissue is solely responsible for the regenerative failure experienced by double-half forelimbs in the earlier study. The complete failure of forelimb regeneration occurs only when all types of soft tissues tested (skin, muscle, and deep connective tissues) are present in symmetrical arrangement.     

Regeneration of digits and forelimbs in the Kenyan reed frog Hyperolius viridiflavus ferniquei.

Newly metamorphosed individuals of the Kenyan reed frog, Hyperolius viridiflavus ferniquei, are able completely to regenerate amputated digits, including the morphologically complex digital pad. The sequence of morphological events is very similar to that seen in the typical epimorphic regeneration of amphibian extremities. Amputated forearms typically produce hypomorphic spike regenerates. Despite the lack of underlying hand and digital structures, an apical "digital" pad commonly differentiates.     

Evidence that regenerative ability is an intrinsic property of limb cells in Xenopus

Xenopus laevis exhibits an ontogenetic decline in the ability to regenerate its limbs: Young tadpoles can completely regenerate an amputated limb, whereas post metamorphic froglets regenerate at most a cartilagenous "spike." We have tested the regenerative competence of normally regenerating limb buds of stage 52-53 Xenopus tadpoles grafted onto limb stumps of postmetamorphic froglets. The limb buds become vascularized and innervated by the host and, when amputated, regenerate limbs with normal or slightly less than normal numbers of tadpole hindlimb digits. Reciprocal grafts of froglet forelimb blastemas onto tadpole hindlimb stumps resulted in either autonomous development of tadpole hindlimb structures and/or formation of a cartilaginous spike typical of froglet forelimb regeneration. Our results suggest that the Xenopus froglet host environment is completely permissive for regeneration and that the ability to regenerate a complete limb pattern is an intrinsic property of young tadpole limb cells, a property that is lost during ontogenesis.     

Forelimb regeneration in the postmetamorphic bullfrog: stimulation by dimethyl sulfoxide and retinoic acid

To examine the effects of retinoic acid and dimethyl sulfoxide on regenerative ability of anuran amphibians, the left forelimbs of 60 postmetamorphic froglets of Rana catesbeiana (bullfrogs) were amputated through the distal zeugopodium. Fifteen of the froglets had their left forelimb stumps immersed in dimethyl sulfoxide (DMSO) for 3 minutes, once immediately after amputation and once on each of 5 subsequent days. Another 15 frogs had their left forelimb stumps immersed in a 0.01 M solution of retinoic acid dissolved in DMSO for the same period of time. The remaining 30 control froglets did not regenerate structures distal to the amputation surface, while all limbs in both treated groups produced regenerates by 120 days postamputation. Regenerates of limbs treated with both DMSO alone and DMSO combined with retinoic acid, although hypomorphic, were composed of multiple cartilage elements, which in many cases (46.7%) were organized as patterns partially resembling the skeletal arrangement of a normal forelimb. All of these regenerates exhibited bundles of striated muscle. In addition, nearly half (46.7%) of the regenerates in the DMSO + retinoic acid group possessed two separate regenerate outgrowths. The results demonstrate that young bullfrogs (Rana catesbeiana) possess a latent epimorphic regenerative capability, which can be stimulated by topical application to the wound surface of DMSO alone or DMSO combined with retinoic acid.     

Analytical study of Xenopus hindlimb regenerate with special reference to muscle regeneration

Amputated hindlimbs of Xenopus laevis, develop various types of regenerates in relation with amputation level as well as stage development. The present experiments is an attempt to study the histological characteristics of Xenopus regenerations, i.e., rational changes of tissue components along the length of the regenerated part with special emphasis on the degree of muscle regeneration. Four types of regenerates were studied viz; a 4th toe obtained from a completely restored regenerated limb at 126 days after amputation of limb at base level in stage 51. An amputated limb with no external sign of regeneration of limb at thigh level in stage 60. A spike-shaped regenerate at 96 days after amputation of limb at shank level in stage 63. A spike-shaped regenerate at about 2 years after amputation of limb at shank level in stage 60. Cross sectional areas of muscle, skin gland, epidermis and cartilage in each of the four types of regenerates were measured with Image Analyzing Apparatus (VIP 121 CH, Olympus Co.). The relative area of each tissue was expressed as a percentage of the cross sectional area of the limb. The obtained values were plotted along the length of the regenerate. Digitiform regenerates were found to be more or less similar to the control limbs, i.e., provided joints and muscle, while the heteromorphic spike or rod shaped regenerates were simply provided with cartilaginous axial core without joint formation. Muscle area were reduced rapidly near the amputation area of these heteromorphic regenerates with no more continuation in the regenerated tissue. It is interesting to mention that percentage cartilage area of about 2 years old spike regenerate was higher than that of similar 96 days regenerate. In addition muscle regeneration was completely absent even in such an aged regenerate. The area showed fairly similar ratio irrespective of the external appearance of the regenerate. In 32 regenerates of which limbs were amputated at various developmental stages ranging between stage 51 and adult stage, the histological condition of muscle at the amputation site, were well observed. In all digitated types of regenerates even in those with reduced number of toes, muscles were found grown well in the regenerates. In heteromorphic regenerates without toe formation muscle did not usually regenerate. In few cases, however, a small mass of myoblastic like cells or small aggregation of differentiated muscle cells without any structural continuation with the stump muscles, were seen to develop in the midst of the regenerate.(ABSTRACT TRUNCATED AT 400 WORDS)     

The effect of retinoic acid on positional memory in the dorsoventral axis of regenerating axolotl limbs

We investigated the effect of retinoic acid (RA) on pattern regulation in the dorsoventral (DV) axis of regenerating axolotl limbs. Half and double half dorsal and ventral zeugopodia (lower arms or legs) were amputated through their distal ends, and 4 days later the animals were injected intraperitoneally with 50 (large animals) or 100 (small animals) micrograms RA/g body wt. Half and double half dorsal and ventral zeugopodia of uninjected axolotls, and sham-operated zeugopodia of untreated and RA-treated limbs served as controls. Skeletal patterns and the DV muscle patterns of control and experimental regenerates were then analyzed. Sham-operated zeugopodia of uninjected animals regenerated normally. Sham-operated, RA-treated zeugopodia regenerated normally with proximodistal duplications. Sixty percent of uninjected control dorsal half zeugopodia, 80% of control ventral half zeugopodia, and 100% of control double dorsal and double ventral zeugopodia regenerated distally, but the regenerates did not reconstitute the muscle pattern of the missing half. Thirty-eight percent of RA-treated ventral half zeugopodia and 78% of RA-treated double ventral zeugopodia failed to regenerate distally. Of those cases that did regenerate distally, none regenerated the muscle pattern of the missing half. By contrast, 100% of RA-treated dorsal half zeugopodia regenerated distally and all completed the normal DV muscle pattern. Forty-one percent of RA-treated double dorsal zeugopodia failed to regenerate, but of the remainder that did regenerate, 50% completed the normal DV muscle pattern. These represented eight cases, six of which regenerated single limbs, and two of which regenerated twin limbs, each with a normal DV muscle pattern. We interpret these data to mean that RA ventralizes the positional memory of blastema cells in the DV axis.     

Types of Morphogenetic Phenomena in Vertebrate Regenerating Systems

Within the regenerating amphibian limb the regeneration blastemarepresents the focus of the dominant epimorphic regenerativeprocess. More proximally, tissue regenerative processes areprominent within certain tissues of the stump. It is not yetknown whether all tissues present within the limb regenerateare laid down as part of a single allinclusive plan or whetherthe morphogenesis of some tissues is dependent upon secondarycues. Initial axial deviation of regenerates arising from obliqueamputation surfaces (Barfurth's rule) is compared with the permanentaxial deviation of regenerates from zeugopodial segments oflimbs from which one skeletal element has been removed. Homeoticregenerates are commonly produced in arthropods, but they haveyet to be demonstrated in regenerating vertebrate extremities.Individual tissue components of limbs regenerate well, and theirmorphogenesis appears to be controlled largely by the functionalenvironment in which the regenerative process is taking place.Under appropriate conditions, terminally amputated mammalian,including human, digits produce well organized outgrowths completewith nails or claws. Whether or not this form of outgrowth isdirectly comparable with an epimorphically regenerating amphibianlimb remains to be seen.     

Retinoic acid-induced pattern duplication in regenerating urodele limbs

The effects of varying doses of retinoic acid on forelimb regeneration in larval Ambystoma mexicanum amputated through the wrist joint and in adult Notophthalmus viridescens amputated through the basal carpals were compared. In both species, the major effect of retinoic acid was to cause the proximodistal duplication, in the regenerate, of stump segments proximal to the amputation plane. Transverse axial duplications (anteroposterior and dorsoventral) occurred in a smaller percentage of cases; these consisted of cartilage spurs in axolotls, and extra digits in newts. The frequency and magnitude of the proximodistal and (in the newt) transverse duplications were dose dependent, and the regenerating limbs were maximally sensitive to the retinoid during the period of dedifferentiation and accumulation of blastema cells. The effect of retinoic acid is exerted on cells local to the amputation surface, as shown by the fact that retinoic acid caused the proximodistal duplication of stump segments in regenerates derived from amputated distal lower arm segments grafted to the eyesocket.     

Development and regeneration of the neonatal digit tip in mice

The digit tips of children and rodents are known to regenerate following amputation. The skeletal structure that regenerates is the distal region of the terminal phalangeal bone that is associated with the nail organ. The terminal phalanx forms late in gestation by endochondral ossification and continues to elongate until sexual maturity (8 weeks of age). Postnatal elongation at its distal end occurs by appositional ossification, i.e. direct ossification on the surface of the terminal phalanx, whereas proximal elongation results from an endochondral growth plate. Amputation through the middle of the terminal phalanx regenerates whereas regenerative failure is observed following amputation to remove the distal 2/3 of the bone. Regeneration is characterized by the formation of a blastema of proliferating cells that appear undifferentiated and express Bmp4. Using chondrogenic and osteogenic markers we show that redifferentiation does not occur by endochondral ossification but by the direct ossification of blastema cells that form the rudiment of the digit tip. Once formed the rudiment elongates by appositional ossification in parallel with unamputated control digits. Regenerated digits are consistently shorter than unamputated control digits. Finally, we present a case study of a child who suffered an amputation injury at a proximal level of the terminal phalanx, but failed to regenerate despite conservative treatment and the presence of the nail organ. These clinical and experimental findings expand on previously published observations and initiate a molecular assessment of a mammalian regeneration model.     

Morphogenetic interactions occurring between blastemas and stumps after exchanging blastemas between normal and double-half forelimbs in the axolotl, Ambystoma mexicanum.

Regeneration blastemas were exchanged between surgically constructed forelimbs comprised of symmetrical tissues (double-anterior and double-posterior) and normal, unoperated forelimbs. Normal blastemas grafted at the stage of medium bud (MB) onto double-half forelimb stumps regenerated normal skeletal patterns in nearly all cases. Double-half blastemas transplanted at the stage of MB onto normal forelimb stumps did not regenerate complete limb patterns. These results indicate that a double-half blastema cannot be “rescued” by transplantation to a normal stump and that a double-half limb stump does not interfere with the ability of a normal blastema to distally transform. The regeneration blastema possesses sufficient positional information at the stage of MB to permit it to develop autonomously. Supernumerary forelimbs resulted from several types of graft-stump combinations. The location and handedness of these supernumerary limbs are predicted by the rules of a recently presented model for pattern regulation in epimorphic fields [French, V., Bryant, P. J., and Bryant, S. V. (1976). Science193, 969–981].     

Regenerative capability in the hindlimb of Xenopus laevis during ontogenetic development

The regenerative capacity of hindlimb of Xenopus laevis was investigated by amputating the limbs at four levels in various developmental stages including younger postmetamorphosed froglets. Amputations of limbs were performed at the base of limb in stages 50, 51, 52, 53, 54, 55, 58, and 60 (Nieuwkoop and Faber's table), at the middle of limb bud in stages 50, 51, 52 and 54, and at mid-thigh and mid-shank in stages 58 and 60, and the froglets in 2 and 3 cm in snout-vent length. In the present experiments the regenerative capacity of limbs was expressed by the rate of regeneration and morphogenesis. Tadpoles in the stages after 55 failed to regenerate when the limbs were amputated at base level, but individuals in all the other experimental series exhibited regeneration in various rates irrespective of the level of amputation and the stage. The regenerative capacity increased distally along the proximo-distal axis of the limb when amputated at the same stage, while regeneration was better in younger stages than that in older stages when amputations were made at the same levels. The regenerates obtained by amputation of limbs in stages between 50 and 54, were mainly digitated in that they had 5 toes with 3 claws which is the same pattern with the normal limb, 4 toes with 2 claws, 3 toes with 2 claws or one, and 2 toes with one claw etc. Tadpoles at stage 50 could regenerate toes and claws without defect, but in the later the regenerative capacity gradually declined by reducing the number of toes and claws and accompanied by malformation of skeleton as the stage proceeded. The tadpoles in stages after 58, and the froglets of 2 and 3 cm, produced various types of heteromorphic regenerates of shapes such as cone, spike or rod of which the centra were occupied with cartilage rods. However these regenerates showed no morphological differences according to the developmental stages. These heteromorphic regenerates continued their growth even after one year without any sign of development of digitated feet.     

The interaction between the blastema and stump in the establishment of the anterior-posterior and proximal-distal organization of the limb regenerate

Interactions between the limb stump and the developing regenerate were studied in the limbs of adult newts, Notophthalmus viridescens. Forelimb blastemas at various stages were transplanted to the contralateral forelimb such that the anterior-posterior axes of stump and blastema were opposed. The blastemas were transplanted either from a proximal to distal, distal to proximal, proximal to proximal, or distal to distal level limb stump. The results indicate that at the earliest stage studied the anterior-posterior axis of the blastema is established but is not stable. An interection between the stump and blastema at this early stage results in the production of a variety of limbs intermediate in polarity between the graft and the stump. At all later stages, the original anterior-posterior axis of the blastema can be retained, although under certain grafting conditions the stump can still exert considerable influence over the anterior-posterior organization of the final regenerate. In those circumstances in which the blastema retains its original handedness, the interaction between stump and blastema results in the production of separate anterior and posterior supernumerary regenerates.The results of transplanting proximal blastemas to a distal limb level indicate that the proximal boundary of the blastema has been established by the earliest stage studied, leading to the production of limbs with serially duplicated segments. However, irrespective of the stage of a blastema transplanted from a distal to proximal level, there are no deleted structures in the proximal-distal axis of the resulting limb. From both histological examination of transplanted regenerates and the arrangement of skeletal elements of the resulting limbs, it is postulated that the stump plays an important role in the production of the intercalary regenerate.     

Background to work on retinoids and amphibian limb regeneration: Studies on anuran tadpoles—a retrospect

Studies on the effects of exogenous vitamin A palminate on limb development and regeneration in anuran tadpoles carried out since late 1960s at the author’s laboratory are reviewed and discussed. Most significant was the initial discovery that vitamin A causes regeneration of complete or nearly complete limbs instead of only the missing distal part, thus altering the P-D pattern of regeneration—a phenomenon now called proximalization. Often more than one such regenerates develop per stump. Vitamin A produces proximalizing effect on regeneration cells during their dedifferentiation and blastema formation but inhibits regeneration if given once redifferentiation begins. Shank-level blastemas from treated tadpoles grafted into orbits of previously treated/untreated host tadpoles formed complete limbs. Proximalizing effect is proportionate to vitamin A concentration, duration of treatment, amputational level and stage of tadpoles. Vitamin A produces this effect also if given only prior to amputation. Its influence persists after cessation of treatment, declining with time. Proximalizing effect is correlated with natural ability in limbs to regenerate. Vitamin A improves regenerative ability and can induce it to some extent in non-regenerating limbs. Vitamin A excess retards limb development and produces stage dependent teratogenic defects. Further development of only that limb region is prevented in which differentiation is beginning when vitamin A is given. Short treatment of tadpoles beginning with limbs at spatula/paddle stage inhibited foot development in the unoperated limbs hut promoted regeneration of complete limbs from the contra-lateral amputated limbs. These dual effects were due to cells of the former differentiating and of the latter dedifferentiating when exposed to vitamin A palmitate.     

Cellular behavior in the anteroposterior axis of the regenerating forelimb of the axolotl, Ambystoma mexicanum

Cellular behavior along the anteroposterior axis of the regenerating axolotl forelimb was studied by use of triploid (3N) tissue grafted into diploid (2N) hosts and three-dimensional computer reconstructions. Asymmetrical upper forelimbs were surgically constructed with one half (anterior or posterior) 3N and the other half 2N. Limbs were amputated immediately after grafting or were permitted to heal for 5 or 30 days prior to amputation. When regenerates had attained the stage of digital outgrowth, the limbs were harvested and sectioned in the transverse axis for histological analysis. When all limbs bearing anterior grafts were considered as a group, 77% of the 3N mesodermal cells were observed in the anterior side of the regenerates and 23% were located in the posterior side of the regenerates. When all limbs bearing posterior grafts were considered as a group, 76% of the 3N mesodermal cells were found in the posterior side of the regenerate and 24% had crossed into the anterior side. Healing times of 0, 5, or 30 days prior to amputation had no effect on the experimental outcome. Three-dimensional computer reconstructions revealed that most 3N cells of mesodermal origin underwent short-distance migration from anterior to posterior or from posterior to anterior and intermixed with diploid mesodermal cells near the midpoint of the regenerated anteroposterior axis. Some 3N cells were observed at greater distances from the graft-host interface. By contrast, labeled epidermal cells from both anterior and posterior grafts exhibited long-distance migration across all surfaces of regenerated limbs. Details of a computer-assisted reconstructive method for studying the three-dimensional distribution of labeled cells in tissues are presented.