Cell lineage tracing during Xenopus tail regeneration.

The tail of the Xenopus tadpole will regenerate following amputation, and all three of the main axial structures - the spinal cord, the notochord and the segmented myotomes - are found in the regenerated tail. We have investigated the cellular origin of each of these three tissue types during regeneration. We produced Xenopus laevis embryos transgenic for the CMV (Simian Cytomegalovirus) promoter driving GFP (Green Fluorescent Protein) ubiquitously throughout the embryo. Single tissues were then specifically labelled by making grafts at the neurula stage from transgenic donors to unlabelled hosts. When the hosts have developed to tadpoles, they carry a region of the appropriate tissue labelled with GFP. These tails were amputated through the labelled region and the distribution of labelled cells in the regenerate was followed. We also labelled myofibres using the Cre-lox method. The results show that the spinal cord and the notochord regenerate from the same tissue type in the stump, with no labelling of other tissues. In the case of the muscle, we show that the myofibres of the regenerate arise from satellite cells and not from the pre-existing myofibres. This shows that metaplasia between differentiated cell types does not occur, and that the process of Xenopus tail regeneration is more akin to tissue renewal in mammals than to urodele tail regeneration.

Ectopic pharynxes arise by regional reorganization after anterior/posterior chimera in planarians.

To elucidate the mechanisms underlying pharynx regeneration in planarians, we transplanted pieces excised from various regions of the body into the prepharyngeal or postpharyngeal region, since it has been shown that such transplantation experiments can induce ectopic pharynx formation. We confirmed the ectopic formation of pharynxes by expression of the myosin heavy chain gene specific to pharynx muscles (DjMHC-A). To investigate the cellular events after grafting, we also stained such transplanted worms by in situ hybridization using neuronal cell- and mucous producing cell-type-specific marker genes which can detect formation of brain and prepharyngeal region, respectively. When the head piece was transplanted into the tail region, ectopic formation of the head, prepharyngeal and pharynx region was observed in the postpharyngeal region anterior to the graft, while these organs were formed in the reversed polarity along the anterior-posterior (A-P) axis. Furthermore, in the tail region posterior to the graft, ectopic formation of the prepharyngeal and pharynx region was observed. In the reverse combination, when a tail piece was transplanted into the prepharyngeal region, ectopic formation of prepharyngeal and pharynx region was observed in the region between the head and the graft, and an additional ectopic pharynx was also formed in reverse polarity in the region between the graft and host pharynx. These results clearly indicated that ectopic pharynxes were formed as a consequence of the regional reorganization induced by interaction between the host and graft. Furthermore, chimeric analyses demonstrated that the cells participating in ectopic pharynx formation were not exclusively derived from the host or donor cells in the worm, suggesting that the stem cells of the host and donor may change their differentiation pattern due to altered regionality. To further investigate if regional reorganization is induced after grafting, expression of a Hox gene was analyzed in the transplanted worms by whole-mount in situ hybridization. The expression of the Hox gene along the A-P axis was apparently rearranged after grafting of the head piece into the tail region. These results suggest that grafting of the head piece may rearrange the regionality of the host tail, and that stem cells in the region newly defined as pharynx-forming may start to regenerate a pharynx.

Epithelial-mesenchymal interactions in development of the mouse fetal genital tubercle.

Epithelial-mesenchymal interactions play a central role in the development of urogenital organs. We hypothesized that normal development of the external genitalia depends upon proper mesenchymal-epithelial signaling. The mesenchyme of the adult mouse penis consists of a corpus cavernosum and proximal and distal bones. The differentiation of penile mesenchyme into bone and cartilage begins after birth and can be accelerated by androgens. After determining the sex, genital tubercles of fetal mice at gestational day 15 were removed. The genital tubercles were trypsinized and microdissected to remove the epidermis and urethra from the mesenchyme. Recombinant specimens were created by combining genital tubercle mesenchyme with genital tubercle epithelium, bladder epithelium or tail epidermis. Tissues were grafted under the renal capsule of male athymic mice. After 3 weeks of growth, grafts were removed from the kidney, weighed and stained with hematoxylin and eosin, Alcian blue and peanut agglutinin. Male and female grafts showed no difference in growth or differentiation. Intact grafts and recombinant grafts, irrespective of the epithelial source, grew significantly more than grafts of the mesenchyme only. Recombinant grafts demonstrated a significantly higher prevalence of cartilage formation and mesenchymal differentiation compared to grafts of the mesenchyme without epithelium. Since heterologous epithelium is able to induce equivalent growth and differentiation of phallic mesenchyme, epithelium carries a permissive, but critical, role in genital mesenchyme development.

Transfer of the rat tail: an experimental model for free osteocutaneous transfer.

A new experimental model for free transfer of the rat tail has been developed. The vascular and nerve anatomy of the tail was studied in ten Wistar rats. The caudal artery (average diameter: 0.8 mm) provides the principal arterial supply to the tail. The main venous drainage is from bilateral dorsolateral subcutaneous veins (average diameter: 1.2 mm). The inferior caudal trunk establishes the cutaneous nerve branches to the skin of the tail. Free transfer of the tail to the carotid artery and external jugular vein, with the attachment of the tail to the dorsum, allows for clinical monitoring of the transfer, without risk of autophagia. This transfer is easy to perform, simple to monitor, and provides consistent yet challenging microsurgery for the beginning microvascular surgeon. In addition to its utility as a training model, it may also have an application as an easy research model for composite tissue transfer.

Continuing organizer function during chick tail development.

Development of the posterior body (lumbosacral region and tail) in vertebrates is delayed relative to gastrulation. In amniotes, it proceeds with the replacement of the regressed node and primitive streak by a caudal blastema-like mass of mesenchyme known as the tail bud. Despite apparent morphological dissimilarities, recent results suggest that tail development in amniotes is in essence a continuation of gastrulation, as is the case in Xenopus. However, this has been inferred primarily from the outcome of fate mapping studies demonstrating discrete, regionalized cell populations in the tail bud, like those present at gastrulation. Our analysis of the tail bud distribution of several molecular markers that are expressed in specific spatial domains during chick gastrulation confirms these results. Furthermore, we present evidence that gastrulation-like ingression movements from the surface continue in the early chick tail bud and that the established tail bud retains organizer activity. This 'tail organizer' has the expected properties of being able to recruit uncommitted host cells into a new embryonic axis and induce host neural tissue with posteriorly regionalized gene expression when grafted to competent host cells that are otherwise destined to form only extra-embryonic tissue. Together, these results indicate that chick tail development is mechanistically continuous with gastrulation and that the developing tail in chick may serve as a useful experimental adjunct to investigate the molecular basis of inductive interactions operating during gastrulation, considering that residual tail organizing activity is still present at a surprisingly late stage.

The somitogenetic potential of cells in the primitive streak and the tail bud of the organogenesis-stage mouse embryo.

The developmental potency of cells isolated from the primitive streak and the tail bud of 8.5- to 13.5-day-old mouse embryos was examined by analyzing the pattern of tissue colonization after transplanting these cells to the primitive streak of 8.5-day embryos. Cells derived from these progenitor tissues contributed predominantly to tissues of the paraxial and lateral mesoderm. Cells isolated from older embryos could alter their segmental fate and participated in the formation of anterior somites after transplantation to the primitive streak of 8.5-day host embryo. There was, however, a developmental lag in the recruitment of the transplanted cells to the paraxial mesoderm and this lag increased with the extent of mismatch of developmental ages between donor and host embryos. It is postulated that certain forms of cell-cell or cell-matrix interaction are involved in the specification of segmental units and that there may be age-related variations in the interactive capability of the somitic progenitor cells during development. Tail bud mesenchyme isolated from 13.5-day embryos, in which somite formation will shortly cease, was still capable of somite formation after transplantation to 8.5-day embryos. The cessation of somite formation is therefore likely to result from a change in the tissue environment in the tail bud rather than a loss of cellular somitogenetic potency.

Neurites show pathway specificity but lack directional specificity or predetermined lengths in Xenopus embryos.

The earliest outgrowth of nerve fibers from identified spinal neurons labeled with horseradish peroxidase (HRP) was traced along surgically rearranged pathways in the central nervous system (CNS) of Xenopus embryos. Parts of the CNS were misaligned or inverted rostrocaudally by grafting a segment of labeled spinal cord in place of the same or different spinal cord segment of an unlabeled embryo or by joining two rostral half embryos (head-to-head) or two caudal half embryos (tail-to-tail), one half of which was derived from a labeled embryo in each combination. Donor embryos were labeled by injection of HRP into a selected blastomere at the 16- or 32-cell stage. Host embryos were unlabeled. Grafts from labeled donors to unlabeled host embryos were made at early neural tube stages before outgrowth of any nerve fibers had started (Jacobson and Huang, 1985). Routes taken by labeled nerve fibers growing into unlabeled CNS were observed at later stages, and the rates of nerve fiber elongation were calculated. Labeled nerve fibers were normal in appearance, and elongated without branching, at normal rates (22-71 micron/h). In head-to-head and tail-to-tail embryos and in embryos with inverted spinal cord grafts, nerve fibers continued elongating without branching in the direction opposite to normal in the CNS. Many fibers reached lengths that were far greater than normal. No reorientation of such maldirected nerve fibers was seen. These results indicate that nerve fiber elongation is not guided by axially polarized pathway cues or markers and that nerve fibers do not grow to predetermined lengths. However, neurites preferred to grow along stereotyped nerve fiber pathways even when forced to grow in the wrong direction or when confronted with nonneural tissue.

Remodeling of bone and bones: effects of altered mechanical stress on the regeneration of transplanted bones.

We divided 116 rats weighing 50 gm into four groups with tails either left in situ or transplanted as follows: straight in situ: untreated controls; bent in situ: five caudal vertebrae (CV) in the loop; straight transplants: three CV skinned and transplanted autologously; and bent transplants: five CV skinned, bent to form a loop, and transplanted autologously. Tails were radiographed weekly up to 6 weeks and at 12 weeks, and microradiographic and histological studies were undertaken on selected specimens. At 12 weeks the bones in the apex of the loop of tails left in situ appeared bent with a straight-to-convex shaft on the outer side and a thicker, more concave one on the inner side. In the transplanted bent segments the bone shaft died and initially the reverse occurred: the outer shaft thickened and the inner resorbed completely. A new concave inner diaphysis then formed so that the bones in both instances were essentially similar in final shape. In the bent transplants the surviving osteogenic tissues regenerated and, adapting to the altered forces, formed a new bone shaft. This involved a change in the direction, amount, and nature of endochondral, periosteal, and regenerative growth and subsequent remodeling of bone. The results support previous observations that, within limits, the strain in the osteogenic envelope is an important factor in adaptation of bones to changing stress and that, where the envelope is deficient, the surviving tissues have the capacity to regenerate and repair defects in the bone so that it best resists the changing stresses applied to it.

Tolerance maintenance depends on persistence of the tolerizing antigen: evidence from transplantation studies on Xenopus laevis.

In order to assess the role of antigen persistence in the tolerant state, tolerance was induced in Xenopus laevis by the embryonic transplantation of whole eyes or tail tissue. Both types of transplants were seen to heal in and persist, with no signs of immunological incompatibility. At metamorphosis, tail resorption occurred and grafted tail tissue was lost. Eye transplants were maintained through metamorphosis in most eye grafted animals. Eye graft recipients which had maintained the transplant were observed to accept challenge skin allografts from donors of the same genotype as the eye donor in all but one case, while recipients which had lost the eye transplant at metamorphosis or had the eye transplant experimentally removed sometimes did not accept the challenge skin graft. Animals tail grafted as embryos did not accept post metamorphic skin grafts from donors of the same genotype as the tail tissue donor, but rejection was not accelerated. It is proposed that tolerance induction is dependent on the presence of appropriately presented antigen at a time when precursor thymocyte cells are migrating to the thymus, prior to their processing into alloreactive cells, and that tolerance maintenance is dependent upon the persistence of the tolerizing antigen.

Joint changes in transplanted caudal vertebrae.

Both straight and bent segments of tails from 4-d-old and weaning Sprague-Dawley rats were used to study the changes which occur in symphyseal joints on transplantation to non-functional sites. In the joints from the younger donors ankylosis occurred almost invariably in the proximal end of the tail, while distally it was rarely seen unless the tail was curved, when ankylosis was visible on the inner side of the bend. The joints from the older donors showed a more varied response on transplantation. Some appeared unaltered, in others where growth continued, calcific changes were seen. In bent segments, unlike in younger ones, ankylosis occurred preferentially on the outer side of the bend. Histological examination revealed that ankylosis of the joint occurred through a process of chondroid metaplasia of the intervertebral connective tissue with subsequent replacement by bone. The metaplastic joint changes were primarily the results of pressure producing compression of the annulus fibrosus in tissues with a reduced vitality due to transplantation and lack of function.

The fate of transplanted tails in frog larvae.

Frog larvae recieved transplants of autografts, allografts, and xeonografts using Rana brevipoda and Rana japonica. Tail tip autografts heal rapidly, continue to grow but undergo degeneration at metamorphosis. Allografts and xenografts also heal but show signs of rejection by the host's immune system. Onset of rejection depends upon the stage when grafts were performed. If transplants were made during early stages, onset was late; if performed later, onset was early. Because frog larvae will eventually undergo metamorphosis as a result of thyroxine effects, they are excellent experimental models for correlating the immune and endocrine systems with differentiation, growth, and aging.

Growth of transplanted tails of infant rats in adolescent allogeneic recipients.

The tails of newborn rats, consisting of slender continuous cones of avascular cartilage, were transplanted in subcutaneous spaces of allogeneic adolescents; colloidal carbon was injected in a vein before the transplants were harvested on days 7-28. Between 97 and 100% of the transplants were accepted, underwent differentiation into bone with bone marrow, and grew at a brisk rate. Acceptance was recognized by (i) a zebra-stripe effect, visible in the gross, resulting from accumulation of carbon in reticuloendothelial cells; (ii) increase of alkaline phosphatase [or-thophosphoric-monoester phosphohydrolase (alkaline optimum), EC 3.1.3.1] and of incorporated 45Ca in the transplant; and (iii) the presence of dense cortical bone with lacunae populated with osteocytes demonstrable by histology. Differentiation of the cartilaginous transplant into bone with subsequent growth of the transplant occurred in recipients fed sucrose as their sole ration.

Regeneration from different levels along the tail of the newt, Notophthalmus viridescens.

Some aspects of the influence of position on regeneration have been examined by comparing regeneration from different levels along the newt tail. Tails amputated such that either three-fourths one-half or one-fourth of the tail was removed pass through the same morphological and histological stages at the same times after amputation. In tails amputated at these three different levels, the rate of elongation of regenerates from more proximal levels is greater than that of regenerates from more distal levels. The total lengths of regenerates from different levels are proportional to the lengths of tail removed by amputation. Furthermore, the number of vertebrae formed in a tail regenerate is directly proportional to the number of vertebrae removed by amputation. When a tail blastema is transplanted to a more proximal level tail stump, intercalary regeneration between the stump and transplant occurs and the resulting regenerate has a complement of vertabrae appropriate to its new level along the tail. The results indicate that position along the appendage does not influence the developmental sequence of events of regeneration, but that it does influence the rate of growth and the structures to be replaced.

A comparison of the survival of H-Y incompatible ear, tail, and body skin grafts.

In the H-Y incompatible C57BL/6 male to female system, both tail and ear skin grafts were found to persist longer than body (thorax) skin grafts of the same size and shape. Ear skin grafts of long standing did not affect the survival of subsequent body skin grafts but, in one case, a long-term tail skin graft prolonged the survival of a subsequent body skin gratf, both transplants persisting throughout the life of the host. Whereas ear skin grafts from which cartilage was removed were invariably rejected, several ear skin grafts with cartilage intact survived permanently.