Formation of prechondrogenic mesenchymal cell aggregates in the regenerating newt limb

Perspective: a suggested role for basement membrane structures during newt limb regeneration

BACKGROUND

Interactions between epithelium and mesenchyme, which occur across a basement membrane (BM) zone, are essential to generate a growth bud, or blastema, from which a new limb regenerates. An intact BM at that interface is believed to inhibit regeneration, but that mechanism of inhibition is not understood.

METHODS

Interference contrast microscopy and antibodies to laminin have been used to describe reformation of the BM and the basal lamina (BL) and their relationships to wound epithelium and mesenchyme in successive stages of blastema formation.

RESULTS

The BL is initially absent from the amputation surface and is reestablished to continuity by the late bud stage of regeneration. It forms generally from base to apex, precedes reticular lamina (RL) formation, and is absent beneath most of the wound epithelium. Our inability to correlate mesenchymal cell accumulation exclusively with the area lacking BL apically and postaxially prompted rethinking of the significance of the BL.

CONCLUSIONS

Consistent with these and other observations, we suggest that the BL, when it forms during blastema formation, appears to function as in other developing systems to stabilize the phenotype of adjacent cells. Thus, epithelium becomes epidermis and adjacent mesenchyme synthesizes RL and becomes dermis. Accordingly, the feature that distinguishes regenerating from nonregenerating appendages is the ability of regenerating appendages to delay BL closure until after a critical mass of mesenchymal cells has accumulated.

Stabilizing role of the basement membrane and dermal fibers during newt limb regeneration

BACKGROUND

Following amputation of a newt limb, tissues at the amputation site undergo histolysis to give rise to a growth bud, or blastema, but they also provide a base on which the regenerate is constructed. Studies suggest that dermal tissues may differentially resist histolysis.

METHODS AND RESULTS

To examine stability of tissues at the amputation site, more than 80 preblastemal staged regenerating limbs were examined histologically. Initially, all soft tissues not attached to bone retracted and were covered by migrating epithelium. The dermis was seen to be stable during the first week postamputation. Muscle dedifferentiated and was heavily stained with anti-tenascin antibodies, but the intact overlying dermis was unstained. Fiber bundles, revealed by staining with phosphotungstic acid hematoxylin, isolated the dermis from dedifferentiating deeper tissues during the first week postamputation, but partially broke down during the second week. However, the basement membrane (BM) remained as the distalmost intact structure at the amputation site in all limbs examined. The BM was the foundation for new BM synthesis which preceded dermis synthesis in the base of the blastema during the second week, even while undifferentiated cells were accumulating centrally.

CONCLUSIONS

We suggest that the dermis resists histolysis long enough for new BM to form in continuity with that of the stump. Dermis formation (dermogenesis) distal to the amputation plane begins early as in mammalian healing but is not completed until after blastema formation. Thus, factors that inhibit dermal closure appear to distinguish regenerating from non-regenerating appendages.

Epidermis, basement membrane, and connective-tissue healing after amputation of mouse digits: implications for mammalian appendage regeneration

Soft tissues from amputation sites of mice were examined at both light and electron microscope levels to determine whether features of growth buds (blastemas), which are necessary for amphibian limb regeneration, exist in nonregenerating mice. Several such features were found. A small area of the wound bed was covered by wound epithelium which, as in regenerating newt limbs, initially lacked an underlying basement membrane. Serially sectioned digits revealed blastemalike growth in the subdermal layer surrounding periosteal chondrogenic cells. Mesenchymelike cells were seen among the fibroblasts and leucocytes within the proliferating tissues. However, no evidence of dedifferentiation was seen in the dermis, which persisted as an apparent intact obstruction to growth bud formation. Existence of the essential ingredients of growth buds and soft-tissue proliferation adjacent to chondrogenic cells proximally suggest that the tissues of mammalian healing may differ quantitatively rather than qualitatively from tissues of appendage regeneration. This premise is encouraging for efforts at growth enhancement in mammals.

Expression and function of neural cell adhesion molecule during limb regeneration

The neural cell adhesion molecule (NCAM) has been detected in regenerating limb bud of adult newts in addition to brain and peripheral nerves. In the regenerating tissue, NCAM was found primarily on mesenchymal cells and also in wound epidermis. Infusion of Fab fragments of antibodies to NCAM into limb buds at the early blastema stage delayed the regenerative process. Previous studies have indicated that NCAM serves as a homophilic ligand for adhesion among cells that express this molecule and, in doing so, can influence the interaction of nerves with their environment. The expression of NCAM in regenerating limb and the effects of antibody infusion are therefore consistent with the observation that limb regeneration requires interactions among axons and mesenchymal cells.

Association of mesenchyme with attenuated basement membranes during morphogenetic stages of newt limb regeneration

The interface between epithelium and mesenchyme may be involved in inductive interactions which occur during development. This interface within the growth bud, or blastema, of a regenerating limb has been examined to determine whether changes in basement-membrane structures are visible in regions of putative epithelial-mesenchymal inductive interaction. Regenerating forelimbs of adult newts were fixed by perfusion with osmotically balanced aldehydes. Late-bulb to early-digit stage regenerates were collected and processed either for light and transmission electron microscopy or for scanning electron microscopy. Light microscopy confirmed that regions characterized by increased numbers of subepithelial mesenchymal cells were covered by a diffusely stained basement membrane. Transmission electron microscopy of these regions revealed two structural components of the basement membrane. The thin basal lamina was continuous in all regions of all stages examined, but it was attenuated apically in areas of mesenchymal cell accumulation. The thicker underlying reticular lamina was markedly attenuated in these regions near the blastemal apex. Scanning electron microscopy of de-epithelialized blastemas revealed that, apically, the reticular lamina formed only a delicate lacelike network. On the base of the blastema, it formed a dense fibrillar meshwork which was further organized into a geometric pattern in the adjacent stump skin. Cumulatively, these observations suggest that physical contact between epithelial and mesenchymal cells is not essential at these stages, but that regions of putative epithelial-mesenchymal interaction are characterized by a distinctly diminished reticular lamina. Structural changes in basement-membrane components may be related to termination of local inductive events.

Changes in the extracellular matrix and glycosaminoglycan synthesis during the initiation of regeneration in adult newt forelimbs

The extracellular matrix (ECM) of the distal tissues in a newt limb stump is completely reorganized in the 2-3-week period following amputation. In view of numerous in vitro studies showing that extracellular material influences cellular migration and proliferation, it is likely that the changes in the limb's ECM are important activities in the process leading to regeneration of such limbs. Using biochemical, autoradiographic, and histochemical techniques we studied temporal and spatial differences in the synthesis of glycosaminoglycans (GAGs) during the early, nerve-dependent phase of limb regeneration. Hyaluronic acid synthesis began with the onset of tissue dedifferentiation, became maximal within 1 weeks, and continued throughout the period of active cell proliferation. Chondroitin sulfate synthesis began somewhat later, increased steadily, and reached very high levels during chondrogenesis. During the first 10 days after amputation, distributions of sulfated and nonsulfated GAGs were both uniform throughout dedifferentiating tissues, except for a heavier localization near the bone. Since nerves are necessary to promote the regenerative process, we examined the neural influence on synthesis and accumulation of extracellular GAGs. Denervation decreased GAG production in all parts of the limb stump by approximately 50%. Newt dorsal root ganglia and brain-derived fibroblast growth factor each produced twofold stimulation of GAG synthesis in cultured 7-day regenerates. The latter effect was primarily on synthesis of hyaluronic acid. The results indicate that the trophic action of nerves on amphibian limb regeneration includes a positive influence on synthesis and extracellular accumulation of GAGs. Since the ECM exerts a major influence on cellular proliferation and migration, the effect of nerves on GAG metabolism may have considerable importance for growth and development of the early regenerate.

Bone healing after amputation of mouse digits and newt limbs: implications for induced regeneration in mammals

Postamputational healing was compared in nonregenerating and regenerating animals to determine whether bone healing might interfere with a regenerative response in mice. More than 150 mouse toes and 100 newt limbs were examined at the light microscope level. Stages of normal bone healing with approximate times of occurrence were established. Major differences in healing of these two species were seen. The periosteum produced hyaline cartilage, woven bone, and chondroid bone in mice, but only hyaline cartilage in newts. The endosteum produced woven bone in mice but no new growth in newts. Dead bone persisted in mice but was removed in newts. The marrow cavity became sealed in mice but remained open in newts. Despite these differences both animals produced skeletal tissue distal to the amputation plane. Woven bone formed distal to the amputation plane of mice. Cartilage formed distal to the amputation plane of newts, but cartilage was never seen distal to the plane of mice. Results of previous studies reveal that cartilage can be formed distal to the amputation plane of experimentally treated mice. Thus, although it does not regenerate, mouse bone is capable of producing, distal to the amputation plane, the type of skeletal tissue which appears at that location during an epimorphic regenerative response. This observation, in combination with other experimental results, indicates that both skeletal and soft tissues at the amputation site of treated mammals can resemble comparable tissues of newt limbs at an early stage of regeneration.

Abnormal limb regeneration in adult newts exposed to the fungicide Maneb 80. A histological study

The effects of the fungicide Maneb 80 (manganese ethylenebisdithiocarbamate, 80% active ingredient) on the regenerating limb of the adult crested newt, Triturus cristatus carnifex, was studied. Female newts were exposed percutaneously to 5 ppm Maneb 80. One group of control newts was exposed to the inert ingredients of Maneb 80 (sodium lignin sulfonate and n-butylnaphthalene sulfonate), and another control group was kept in tap water. The limbs were examined histologically at weekly intervals throughout the regeneration period and at the end of the experiment (10-12 wk postamputation). The regenerating limbs of all the animals exposed to Maneb 80 showed growth retardation and skeletal abnormalities. Histological examination provided evidence that vascular disturbances are important for the genesis of the developmental abnormalities induced by Maneb 80. The inert ingredients had a promoting effect on limb growth and had no teratogenic effects under our experimental conditions. There were no histological differences between the two control groups.