The reparative process begins at approximately the same time as the injury and is interwoven with inflammation. Healing pro-ceeds after the inflammatory debris has been removed. Healing may occur by regeneration, in which gradual repair of the defect occurs by proliferation of cells of the same type as those de-stroyed, or by replacement, in which cells of another type, usu-ally connective tissue, fill in the tissue defect and result in scar formation.
The ability of cells to regenerate depends on whether they are la-bile, permanent, or stable. Labile cells multiply constantly to re-place cells worn out by normal physiologic processes; these include epithelial cells of the skin and those lining the gastroin-testinal tract. Permanent cells include neurons—the nerve cell bodies, not their axons. Destruction of a neuron is a permanent loss, but axons may regenerate. If normal activity is to return, tis-sue regeneration must occur in a functional pattern, especially in the growth of several axons. Stable cells have a latent ability to re-generate. Under normal physiologic processes, they are not shed and do not need replacement, but if they are damaged or de-stroyed, they are able to regenerate. These include functional cells of the kidney, liver, and pancreas.
Depending on the extent of damage, tissue healing may occur by primary intention or by secondary intention. In primary inten-tion healing, the wound is clean and dry and the edges are ap-proximated, as in a surgical wound. Little scar formation occurs, and the wound is usually healed in a week. In secondary inten-tion healing, the wound or defect is larger and gaping and has necrotic or dead material. The wound fills from the bottom up-ward with granulation tissue. The process of repair takes longer and results in more scar formation, with loss of specialized func-tion. People who have recovered from myocardial infarction, for example, have abnormal electrocardiographic (ECG) tracings be-cause the electrical signal cannot be conducted through the con-nective tissue that has replaced the infarcted area.
The condition of the host, the environment, and the nature and severity of the injury affect the processes of inflammation and repair. Any of the injuries previously discussed can lead to death of the cell. Essentially, the cell membrane becomes impaired, re-sulting in a nonrestricted flow of ions. Sodium and calcium enter the cell, followed by water, which leads to edema, and energy transformation ceases. Nerve impulses are no longer transmitted; muscles no longer contract. As the cells rupture, lysosomal en-zymes that destroy tissues escape, and cell death and necrosis occur.
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