Introduction

Abstract

Cells are continually lost and replaced by the tissues of multicellular organisms due to physiological turnover, disease, and injuries. Cell and tissue replacement that maintains or restores the original tissue structure and function is called regeneration. All organisms regenerate, though the degree of regenerative ability varies among species (Goss 1969). Within individual organisms, regeneration takes place on all levels of biological organization, from the molecular to tissues and organs. Tissues that do not regenerate are repaired by fibrosis, or scarring, which patches damaged areas with non-functional tissue. Fibrosis is due to an inflammatory response that results in a fibroblastic granulation tissue that organizes collagen fibrils into thick bundles instead ofthe normal reticular pattern (Linares 1992; Clark 1996). Examples of non-regenerating tissues are the dermis of the skin, pancreas, spinal cord and brain, neural retina and lens of the eye, cardiac muscle, lung, and kidney glomerulus. Scarring can also occur in tissues that have the ability to regenerate when damage exceeds their regenerative capacity.