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, like 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.