Regeneration

by Thomas Hunt Morgan

The new tail that regenerates does not contain a new series of vertebræ, as does the new tail of the salamander, but, instead, a cartilaginous tube that is attached to the half of the broken seventh caudal vertebra.

The regeneration of the new tissues of the tail of the lizard takes place as follows: A scab forms over the cut-surface, composed in part of clotted blood, in part of broken-down tissues from the injured cells. In the course of a week the necrotic tissue falls off, and a smooth surface of ectoderm is found covering the end of the tail. The new ectoderm appears to come from the old, but its method of development has not been studied. The deeper layer of the skin of the lizard is composed of mesodermal connective tissue, and in the new part this layer arises from the connective tissue of the old part. The tissue that forms the cartilaginous tube of the new tail develops from the skeletal tissue of the broken vertebra. The remnants of the old notochord that are present in the vertebra, have nothing to do with the new structure, nor does the new tube represent in any way a notochord, but it appears to be a structure sui generis. In later stages, osseous plates may be formed in the cartilage, but these are too irregular to be compared to vertebræ. A tube grows out from the cut-end of the nerve-cord, which in some forms, as Fraisse shows, is only an extension of the lining epithelium of the nerve-cord. In other forms it is possible that other cells of the old cord may also grow backward, divide, and produce new cells. The fine thread that is formed in this way does not send out any nerve fibres into the surrounding parts. In Anguis fragilis, however, a few ganglion cells are present in the new cord. It is probable, Fraisse states, that while the new tube is morphologically a nerve-cord, yet physiologically it is not functional in any of the reptiles.

The new muscles come from the old ones. Fraisse thinks that the new muscle fibres come from the so-called “spindle fibres” that split off from the primitive muscle bundles. These fibres, Fraisse believes, originate normally during the process of physiological regeneration of the muscles, and also after injury to the muscles. From these spindle cells the new muscle fibres develop in the same way as the muscle cells of the embryo.

Fraisse sums up the results of his studies of regeneration as follows: (1) Both in amphibians and reptiles, injured tissues can only produce new tissues like themselves. The leucocytes assume only the function of nutrition and of devouring the broken-down parts of tissues. They never become fixed tissues—neither connective tissue nor any other sort. (2) All tissues are capable of regenerating themselves, either directly out of their differentiated elements, or out of a matrix. As a matrix for the epidermis, there is the Malpighian layer of the skin; for the central nervous system, the epithelium of the central canal of the nerve-cord; and for the musculature, the spindle fibres.

Fraisse also formulates the following general statements: (a) Regeneration is neither a pure recapitulation of the ontogeny nor of the phylogeny. The process is rather a hereditary one, with which complicated adaptations of the tissues are often involved that follow the laws of correlated development. (b) We cannot explain the phenomenon of regeneration, as the result of wounding the tissues, or as the outcome of an increase in the food supply, or as due to the removal of a resistance to growth. Far more important are the principles covered by the former paragraph, (a).

Barfurth has studied in detail the regeneration of the tail in some amphibia; and his results, while not covering as much ground as do those of Fraisse, yet give a more detailed account of the origin of the new tissues. Barfurth’s results on triton and siredon are not essentially different from those of Fraisse. In the tadpole of the frog, Barfurth finds that the notochord regenerates from the sheath of the old notochord. In the larval urodele, he finds that the new notochord arises as in the tadpole, and not from the skeletal sheath, as Fraisse maintains. In very young larvæ of siredon the chordal cells themselves seem to give rise to the cells of the new notochord. In older larvæ, in which the skeletal tissue is developed around the notochord, regeneration takes place both from this tissue and also from the sheath of the notochord. He concludes that in the regeneration of the new notochord, and also of the skeleton, the origin of the cells depends upon the developmental stage of the supporting tissues.

In regard to the regeneration of the muscles, Barfurth comes to the following conclusions: In very young larvæ of siredon, the degenerative changes in the muscle cells are often very slight. Regeneration takes place by growth from and the displacement of the old muscles. During this time bud-like terminal and lateral formations occur in the muscle fibres. These outgrowths contain nuclei and form sarcoblasts; and these pass into the new part, where they make the new muscle fibres in the same way as do the cells of the embryo. In older larvæ of the frog, and in mature animals in general, the changes are more complicated. Two processes can be distinguished: (a) degenerative and (b) regenerative. (a) Broken-down muscle fibres that have been cut, and torn-off pieces of muscle fibres, are found present. There follows an accumulation of leucocytes and of giant cells. The nuclei in the degenerating muscle fibres atrophy, and the substance of the fibres breaks down. (b) The muscle fibres split lengthwise to form spindle fibres, and there is an increase in the number of nuclei at the same time. Sarcoblast-like outgrowths of the old muscle fibres are formed, which produce the sarcoblasts that become new muscle fibres.

Barfurth agrees with Fraisse in two main points, viz. that all the tissues of the tail have the power of regeneration, and that each tissue produces only tissue like itself. The law which Kölliker attempted to establish, viz. that the elements of the formed tissues have lost the power of producing other kinds of tissue,—the law of the specification of the tissue,—is supported by these results of Fraisse and of Barfurth, but is contradicted, as has been shown above, by the results on the earthworm, and also as we shall see even in the amphibia, as for instance in the regeneration of the lens of the eye.

Spallanzani[95] was the first to study the regeneration of the limb in salamanders, and found that the skeleton in the new part is like that in the normal limb. Bonnet, Philipeaux,[96] as well as other naturalists,[97] also examined the regeneration of the limbs of salamanders. Götte (’79) has studied the embryonic development and the regeneration of the limb of triton, especially in regard to the origin of the new bones. He found that the skeleton develops in much the same way in the embryonic limb and in the regenerated limb, and the process in the latter may be said to repeat that in the former. This is especially true for the regeneration of the limb of a very young larva, but the older the larva the more it departs from the embryonic type of development. If the limb is cut off through the upper arm, or through the thigh, new tissue develops over the cut-end. If the larva is quite young, so that formation of the cartilages in the leg has not gone very far, the new tissue differs very little from the old; but if the leg of an older larva is amputated, the difference between the old and the new parts is more striking. If the bones of the leg have become ossified, the transition from the old to the new part is at first very sharp. The new tissue, that will make the new cartilages of the new limb, develops as a cap over the cut-end of the old bone. Götte does not give an explicit statement in regard to the origin of the new cartilage, but his account leads one to suppose that it develops from the old cartilage or from some part of the bone. This is, in fact, the case, as I have observed in preparations of the regenerating leg of Plethodon cinereus, in which the new cartilaginous tissue comes from the periosteum of the old bone. Götte shows that two long rods of tissue are formed, that are separate for the greater part of their length. They give rise to the two bones of the lower leg, or forearm, as the case may be. The broken end of the femur or humerus also completes itself by a short cartilaginous cap, which is at first continuous with the two rods just described. The ends of these two rods break up into a series of pieces that form the tarsalia, or the carpalia, and the digits. Two digits are first formed, and the others are added as outgrowths from the side of one of the two rods. It is important to note that the new cartilages are formed, in large part, out of a continuous substratum (or rather of two) which separates into proportionate parts to produce the elements of the new limb.

The regeneration of the muscles of the limb of an adult animal, plethodon, has been recently worked out by Towle. The leg was cut off in the middle of the forearm. Extensive changes take place in all the muscles that extend across the level of the cut. The old fibres in the lower end of the muscle, i.e. those near the cut-end, disintegrate, and the number of nuclei greatly increases. The division of the nuclei seems to be direct, each retaining some of the old muscle substance about itself. From some of these cells the new muscle tissue is formed in the new part. Higher up in the forearm the muscle fibres break down to a smaller extent, and still higher up some of the old fibres may remain intact. New muscle fibres are also formed in the old muscle, especially in the region near the cut-end.