9 - Chondrogenesis in Regenerating Systems

Abstract

This chapter discusses chondrogenesis in regenerating systems. Cartilage is unusual because it is rarely vascularized, never innervated, and devoid of fibroblasts. It is one of the few tissues in the body that are made up of only one kind of cell. Whether cartilage evolved before, during, or after bone is a matter for phylogenetic conjecture. In any case, it has persisted for two important reasons: (1) its tough resilience, which makes it useful for articular surfaces and in other sites and (2) its adaptation as a cartilaginous plate to regulate the elongation of bones. Cartilage plays a prominent role in many regenerating systems, both epimorphic and tissue, if only because most appendages possess endochondral skeletons. The chapter highlights a comparative distinction between the two types of regeneration, namely, epimorphic and tissue regeneration. Epimorphic regeneration of an amputated body part is achieved by direct outgrowth from the stump. The starfish can regenerate arms, the octopus can replace its tentacles, and arthropods can grow back legs and claws. Among vertebrates, there are numerous examples of appendages that can regenerate. For epimorphic regeneration to occur, there must be an epidermal wound, a mesodermal source of blastema cells, and in some cases, sufficient innervation. Tissue regeneration shares certain processes with epimorphic regeneration, but it comes in as many forms as there are tissues to regenerate. Among the more somatic tissues of the body, the healing of injuries in skin, muscle, tendons, bone, blood vessels, and nerves is especially relevant to epimorphic regeneration because these are the tissues normally present in appendages. Each of them is capable of repair following injury, a repair more appropriately classified as wound healing than regeneration proper. The chapter discusses the differences between epimorphic regeneration and tissue regeneration, which are much more than their similarities.