Briefly describe the pathogenesis of granulation tissue formation in a wound, highlighting the important gross and microscopic features.
Answer: Granulation tissue formation is two major processes: fibroblast migration and proliferation and angiogenesis. Migration of the cells begins along the framework formed by plasma proteins and fibrin. This migration is triggered by growth factors. Angiogenesis begins with proteolysis of the basement membrane and ECM of parental vessel. This is followed by migration of immature endothelial cells into the wound. This proliferation of endothelial cells forms solid endothelial tubes. The lumen of the tubes mature into new capillaries. Then there is the formation of gap junctions and recruitment of pericytes and smooth muscle cells to support the new capillary. Grossly, granulation tissue appears as small nodules on the surface of the wound. They are red, hemorrhagic and bleeds easily. Microscopically, fibroblasts and connective tissue fibres grow parallel to wound surface. The proliferating capillaries are arranged perpendicularly to the fibroblasts and are often evenly spaced