Chapter 6: Connective Tissue

The ECM is structurally complex, consisting of protein fibers—collagen, reticular, and elastic—and an amorphous ground substance made of highly hydrated molecules like glycosaminoglycans (GAGs), proteoglycans, and multiadhesive glycoproteins. Connective tissues are broadly classified into embryonic CT, CT proper, and specialized CT (e.g., bone and cartilage). Embryonic CT includes mesenchyme, the precursor tissue, and mucous connective tissue, or Wharton jelly, found in the umbilical cord, distinguished by its gelatin-like, hyaluronan-rich ground substance. Connective tissue proper is divided into loose connective tissue, characterized by abundant ground substance, numerous cells, and sparse fibers, functioning as the primary site for immune responses beneath epithelia; and dense connective tissue, which prioritizes fiber density over cellularity and ground substance. Dense irregular CT has randomly organized collagen bundles to resist multi-directional forces, while dense regular CT, found in structures like tendons and ligaments, features parallel, densely packed fibers for maximal tensile strength. Structurally, collagen fibers, the most prevalent type, exhibit a distinctive 68-nm banding pattern arising from the staggered assembly of triple-helix collagen molecules. Collagen biosynthesis is a tightly regulated process beginning intracellularly with procollagen formation (requiring Vitamin C for hydroxylation) and concluding extracellularly with fibrillogenesis and covalent cross-linking. Reticular fibers, primarily composed of Type III collagen, form delicate, branching networks that create the supportive stroma in lymphatic and hemopoietic organs. Elastic fibers, made of elastin and fibrillin microfibrils, grant flexibility and recoil to tissues. CT cells are categorized as resident—stable inhabitants like fibroblasts (ECM synthesizing cells), myofibroblasts (contractile cells vital for wound healing), macrophages (phagocytic and antigen-presenting cells), mast cells (mediators of inflammation), and adult stem cells—or wandering cells, such as lymphocytes and plasma cells (antibody-producing B cell derivatives). Pathology related to defective fiber production, known as collagenopathies (e.g., osteogenesis imperfecta) or elastin abnormalities (e.g., Marfan syndrome), underscores the structural and functional importance of the ECM.