Chapter 20: Integrating Cells into Tissues

Integrating Cells into Tissues from Molecular Cell Biology (Ninth Edition) presents a comprehensive analysis of how individual cells assemble into complex, functional tissues through precise molecular interactions. It details the structural and functional mechanisms of cell-cell and cell-matrix adhesion, mediated by distinct families of cell-adhesion molecules (CAMs) and adhesion receptors, including the calcium-dependent cadherins, the immunoglobulin (Ig) superfamily, selectins, and the heterodimeric integrins. The text explores the critical role of cis (lateral) and trans (intercellular) interactions in stabilizing these contacts. A major focus is placed on the organization of epithelial tissues, differentiating between the apical, lateral, and basal surfaces, and examining the specific architecture of cell junctions: anchoring junctions (adherens junctions utilizing actin and desmosomes utilizing intermediate filaments), tight junctions (composed of occludin, claudins, and JAMs) that form permeability barriers and maintain polarity, and gap junctions (formed by connexins) that enable direct cytoplasmic communication. The chapter further distinguishes between the specialized extracellular matrix (ECM) of the basal lamina—characterized by sheet-forming Type IV collagen, laminin, perlecan, and nidogen—and the fibrillar ECM of connective tissue, which is dominated by Type I, II, and III collagens, elastin, and proteoglycans containing glycosaminoglycans (GAGs) like hyaluronan and chondroitin sulfate. Key physiological processes are explained, including mechanotransduction, where mechanical forces unfold proteins like talin to recruit vinculin and reinforce adhesion, and the dynamic regulation of integrin affinity (switching between bent-closed and extended-open conformations) which is essential for cell motility and leukocyte extravasation. The discussion concludes by contrasting animal tissues with plant tissues, detailing the composition of the plant cell wall (cellulose, hemicellulose, and pectin) and the unique structure of plasmodesmata, which facilitate intercellular transport via desmotubules connecting the endoplasmic reticulum of adjacent cells.