This chapter describes the dynamic characteristic of tendon––the ability of a tendon to modulate its structural and material properties to meet mechanical requirements distinct from the usual need for strength in tension. Tendon performs the mechanical role of transmitting the tensional force generated by contraction of its muscle of origin to the bone upon which it inserts. The histology, biochemistry, and cell biology of regions of the flexor digitorum profundus tendon (deep digital flexor tendon) either subjected to tension or to compression are described in the chapter. The hypothesis that tendon fibroblasts and the extracellular matrix they produce are modulated according to their mechanical requirements is explored. The development of cartilaginous tissue in the region of tendon subjected to compressive forces is genetically programmed. The stimulus to produce large Proteoglycan (PG) is generated by one or more soluble factors that are released under defined conditions and carried systemically to a population of target cells. Many fibroblasts in adult tissue have the capacity to generate a cartilage like tissue.