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Capillary loops grow into the coagulum, and migrated leucocytes from the adjacent blood vessels destroy the red corpuscles, and are in turn disposed of by the developing fibroblasts, which by their growth and proliferation fill up the gap with young connective tissue. It will be evident that this process only differs from healing by primary union in the amount of blood-clot that is present.
The transformation of this temporary granulation tissue into scar tissue is effected by the fibroblasts, which become elongated and spindle-shaped, and produce in and around them a fine fibrillated material which gradually increases in quantity till it replaces the cell protoplasm.
Among the fibroblasts, larger multi-nucleated cells giant cells are sometimes found, particularly when resistant substances, such as silk ligatures or fragments of bone, are embedded in the tissues, and their function seems to be to soften such substances preliminary to their being removed by the phagocytes.
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.
The raw surface is first covered by a layer of coagulated blood and fibrin. An extensive new formation of capillary loops and fibroblasts takes place towards the free surface, and goes on until the gap is filled by a fine velvet-like mass of granulation tissue.
These fibroblasts are large irregular nucleated cells derived mainly from the proliferation of the fixed connective-tissue cells of the part, and to a less extent from the lymphocytes and other mononuclear cells which have migrated from the vessels.
The spaces between these loops are filled by cells of various kinds, the most important being the fibroblasts, which are destined to form cicatricial fibrous tissue.
As a result of explosions, particles of carbon, in the form of coal-dust or gunpowder, or portions of shale, may lodge in a wound. The embedded foreign body at first acts as an irritant, and induces a reaction in the tissues in which it lodges, in the form of hyperæmia, local leucocytosis, proliferation of fibroblasts, and the formation of granulation tissue.
Numerous polymorpho-nuclear leucocytes, which have wandered from the vessels, are also present in the spaces. These act as phagocytes, their function being to remove the red corpuscles and fibrin of the original clot, and this performed, they either pass back into the circulation in virtue of their amœboid movement, or are themselves eaten up by the growing fibroblasts.
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