Skin and Connective Tissue.—The mode of regeneration of these tissues under aseptic conditions has already been described as the type of ideal repair. In highly vascular parts, such as the face, the reparative process goes on with great rapidity, and even extensive wounds may be firmly united in from three to five days. Where the anastomosis is less free the process is more prolonged. The more highly organised elements of the skin, such as the hair follicles, the sweat and sebaceous glands, are imperfectly reproduced; hence the scar remains smooth, dry, and hairless.
Epithelium.—Epithelium is only reproduced from pre-existing epithelium, and, as a rule, from one of a similar type, although metaplastic transformation of cells of one kind of epithelium into another kind can take place. Thus a granulating surface may be covered entirely by the ingrowing of the cutaneous epithelium from the margins; or islets, originating in surviving cells of sebaceous glands or sweat glands, or of hair follicles, may spring up in the centre of the raw area. Such islets may also be due to the accidental transference of loose epithelial cells from the edges. Even the fluid from a blister, in virtue of the isolated cells of the rete Malpighii which it contains, is capable of starting epithelial growth on a granulating surface. Hairs and nails may be completely regenerated if a sufficient amount of the hair follicles or of the nail matrix has escaped destruction. The epithelium of a mucous membrane is regenerated in the same way as that on a cutaneous surface.
Epithelial cells have the power of living for some time after being separated from their normal surroundings, and of growing again when once more placed in favourable circumstances. On this fact the practice of skin grafting is based (p. 11).
Cartilage.—When an articular cartilage is divided by incision or by being implicated in a fracture involving the articular end of a bone, it is repaired by ordinary cicatricial fibrous tissue derived from the proliferating cells of the perichondrium. Cartilage being a non-vascular tissue, the reparative process goes on slowly, and it may be many weeks before it is complete.
It is possible for a metaplastic transformation of connective-tissue cells into cartilage cells to take place, the characteristic hyaline matrix being secreted by the new cells. This is sometimes observed as an intermediary stage in the healing of fractures, especially in young bones. It may also take place in the regeneration of lost portions of cartilage, provided the new tissue is so situated as to constitute part of a joint and to be subjected to pressure by an opposing cartilaginous surface. This is illustrated by what takes place after excision of joints where it is desired to restore the function of the articulation. By carrying out movements between the constituent parts, the fibrous tissue covering the ends of the bones becomes moulded into shape, its cells take on the characters of cartilage cells, and, forming a matrix, so develop a new cartilage.
Conversely, it is observed that when articular cartilage is no longer subjected to pressure by an opposing cartilage, it tends to be transformed into fibrous tissue, as may be seen in deformities attended with displacement of articular surfaces, such as hallux valgus and club-foot.
After fractures of costal cartilage or of the cartilages of the larynx the cicatricial tissue may be ultimately replaced by bone.
Tendons.—When a tendon is divided, for example by subcutaneous tenotomy, the end nearer the muscle fibres is drawn away from the other, leaving a gap which is speedily filled by blood-clot. In the course of a few days this clot becomes permeated by granulation tissue, the fibroblasts of which are derived from the sheath of the tendon, the surrounding connective tissue, and probably also from the divided ends of the tendon itself. These fibroblasts ultimately develop into typical tendon cells, and the fibres which they form constitute the new tendon fibres. Under aseptic conditions repair is complete in from two to three weeks. In the course of the reparative process the tendon and its sheath may become adherent, which leads to impaired movement and stiffness. If the ends of an accidentally divided tendon are at once brought into accurate apposition and secured by sutures, they unite directly with a minimum amount of scar tissue, and function is perfectly restored.
Muscle.—Unstriped muscle does not seem to be capable of being regenerated to any but a moderate degree. If the ends of a divided striped muscle are at once brought into apposition by stitches, primary union takes place with a minimum of intervening fibrous tissue. The nuclei of the muscle fibres in close proximity to this young cicatricial tissue proliferate, and a few new muscle fibres may be developed, but any gross loss of muscular tissue is replaced by a fibrous cicatrix. It would appear that portions of muscle transplanted from animals to fill up gaps in human muscle are similarly replaced by fibrous tissue. When a muscle is paralysed from loss of its nerve supply and undergoes complete degeneration, it is not capable of being regenerated, even should the integrity of the nerve be restored, and so its function is permanently lost.
Secretory Glands.—The regeneration of secretory glands is usually incomplete, cicatricial tissue taking the place of the glandular substance which has been destroyed. In wounds of the liver, for example, the gap is filled by fibrous tissue, but towards the periphery of the wound the liver cells proliferate and a certain amount of regeneration takes place. In the kidney also, repair mainly takes place by cicatricial tissue, and although a few collecting tubules may be reformed, no regeneration of secreting tissue takes place. After the operation of decapsulation of the kidney a new capsule is formed, and during the process young blood vessels permeate the superficial parts of the kidney and temporarily increase its blood supply, but in the consolidation of the new fibrous tissue these vessels are ultimately obliterated. This does not prove that the operation is useless, as the temporary improvement of the circulation in the kidney may serve to tide the patient over a critical period of renal insufficiency.
Stomach and Intestine.—Provided the peritoneal surfaces are accurately apposed, wounds of the stomach and intestine heal with great rapidity. Within a few hours the peritoneal surfaces are glued together by a thin layer of fibrin and leucocytes, which is speedily organised and replaced by fibrous tissue. Fibrous tissue takes the place of the muscular elements, which are not regenerated. The mucous lining is restored by ingrowth from the margins, and there is evidence that some of the secreting glands may be reproduced.
Hollow viscera, the œsophagus and urinary bladder, in so far as they are not covered by peritoneum, heal less rapidly.
Nerve Tissues.—There is no trustworthy evidence that regeneration of the tissues of the brain or spinal cord in man ever takes place. Any loss of substance is replaced by cicatricial tissue.
The repair of Bone, Blood Vessels, and Peripheral Nerves is more conveniently considered in the chapters dealing with these structures.
Rate of Healing.—While the rate at which wounds heal is remarkably constant there are certain factors that influence it in one direction or the other. Healing is more rapid when the edges are in contact, when there is a minimum amount of blood-clot between them, when the patient is in normal health and the vitality of the tissues has not been impaired. Wounds heal slightly more quickly in the young than in the old, although the difference is so small that it can only be demonstrated by the most careful observations.
Certain tissues take longer to heal than others: for example, a fracture of one of the larger long bones takes about six weeks to unite, and divided nerve trunks take much longer—about a year.
Wounds of certain parts of the body heal more quickly than others: those of the scalp, face, and neck, for example, heal more quickly than those over the buttock or sacrum, probably because of their greater vascularity.
The extent of the wound influences the rate of healing; it is only natural that a long and deep wound should take longer to heal than a short and superficial one, because there is so much more work to be done in the conversion of blood-clot into granulation tissue, and this again into scar tissue that will be strong enough to stand the strain on the edges of the wound.