Chapter Summary

Mesenchymal cells inhabit the spaces between the epithelial linings of the internal and external surfaces. These cells are embedded in an extracellular matrix that is rich in collagen, proteoglycan, and glycoproteins. Collagen is the most abundant protein in the extracellular matrix in animals. Collagen undergoes considerable posttranslational modification within the cell and is secreted as procollagen. Once outside the cell, procollagen undergoes final processing in the extracellular environment, resulting in elongated triple-stranded molecules. In some instances, collagen may also act as signaling molecule. Laminin and Fibronectin are proteins found in the extracellular matrix, mainly in the basal lamina between the epithelium and mesenchyme. In addition to laminin, collagen, and fibronectin, the extracellular matrix also contains another set of macromolecules called proteoglycans. Proteoglycans are basically assemblages of proteins and polysaccharides, which are called glycosaminoglycans. Proteoglycans are found in large amounts in most connective tissues of the body as well as the mesenchymal extracellular matrix of the developing embryo. In contrast to the animal cell wall, plant cell walls are composed of long fibrils of cellulose embedded in a matrix of hemicellulose and pectins. Plant cell walls are rigid enough to immobilize the plant cell encased within them. In addition to the aforementioned proteins, cell membranes also possess integral membrane proteins called integrins, which serve to link the extracellular matrix with internal structures and signaling apparatus within the cell.

Epithelial cells are closely adherent to one another through four distinct junctional complexes. The tight junctions form a circumferential seal between adjacent cells, thus functionally isolating the environment on the cells apical surface from the cells basal and lateral surfaces. The adherens junctions are located basal to the tight junctions and serve to link the microfilaments within the two cells. A specialized form of adherens junctions called desmosomes connects intermediate filaments in adjacent cells. Gap junctions are specialized juxtapositions of the apposing membranes that allow small molecules to pass from one cell to another.

Epithelial cells, as well as mesenchymal cells, possess receptor molecules as integral membrane proteins. The three known kinds of receptors are ion-channel-linked receptors, G-protein-linked receptors, and enzyme-linked receptors. These receptors are essential for cell-cell and cell-matrix interactions. Ligand-receptor interactions affect the conformation of the protein in the receptor, and this may have different downstream effects depending on the type of ligand and receptor [fig. 12.11]. For example, activation of an ion-channel linked receptor may result in the opening or closing of the ion channel. Signaling through G-protein-linked receptors may bring about a change in the levels of intracellular Ca+² and formation of molecules such as diacylglycerol, thereby resulting in a wide variety of effects including differential gene expression. Thus, these molecules form an intricate system of intercellular signaling that is crucial to normal development and cell fate specification

It has been shown that cells exhibit preferential adhesion properties. For example, when prospective epidermis and mesoderm are mixed, the two cell types sort out so that the epidermis is always on the surface of the reaggregated mass and the mesoderm is always internal to it. Adhesion between cells is due to the presence of a diverse set of molecules called cell adhesion molecules (CAMs), most of which are anchored to the cell membrane, usually by a transmembrane domain or by a specialized glycolipid anchor. Binding between CAM molecules is usually homophilic and is often dependent on the presence of calcium in the medium. Another large family of intercellular adhesion molecules is the cadherins, and these molecules always require calcium for their function. In junctional complexes such as the adherens junctions and desmosomes, cadherins interact with linker proteins called catenins on the cytoplasmic side of the junctions.

Normal embryonic development requires that different cells should come to lie in the correct location within the developing embryo. This is accomplished by one of the many morphogenetic movements: epiboly, intercalation, convergent extension, invagination, involution, migration, ingression, or proliferation [fig. 12.17]. Any kind of cell motility, in vitro or in vivo, requires an interaction between the cells and their substrate. Furthermore, changes in cellular shape are crucial in determining the final form. For instance, during epiboly, cells in a cuboidal epithelium stretch out so that the tissue thins, forming a squamous epithelium. Moreover, cellular shape changes may also be associated with changes in gene expression patterns as seen in the Drosophila blastoderm [fig. 12.18]. Final morphology of the tissue or organ is also affected by the rate of cell proliferation and cell death. This is further influenced by the presence of growth factors and signaling molecules Cell division planes also affect development and morphogenesis in those instances where cell fate specification is dependent on proper segregation of intracellular determinants to specific cells.

Chapter Summary

Further Reading