The evolution of multicellular organisms most likely began when cells remained associated in small colonies after division instead of separating into individual cells. A few prokaryotes and several unicellular eukaryotes exhibit such rudimentary social behavior. The full flowering of multicellularity, however, occurs in eukaryotic organisms whose cells become differentiated and organized into groups, or tissues, in which the tissue’s cells perform a specialized, common function.
The simplest multicellular organisms are single cells embedded in a jelly of protein and polysaccharide called the extracellular matrix. More complicated arrangements of cells into a chain, a ball, or a sheet require other means. The cells of higher plants, for instance, are connected by cytoplasmic bridges, called plasmodesmata, and are encased in a network of chambers formed by the interlocking cell walls surrounding the cells. Animal cells, in contrast, are “glued” together by cell-adhesion molecules (CAMs) on their surface. Some CAMs bind cells to one another; other types bind cells to the extracellular matrix, forming a cohesive unit. In animals, the matrix cushions and lubricates cells. A specialized matrix, the basal lamina, which is especially tough, forms a supporting layer underlying cell sheets and preventing the cells from ripping apart.
The specialized groups of differentiated cells form tissues, which are themselves the major components of organs. For example, the lumen of a blood vessel is lined with a sheetlike layer of endothelial cells, or endothelium, which prevents blood cells from leaking out (Figure 1-11). A layer of smooth muscle tissue encircles the lumen and contracts to limit the blood flow. During times of fright, constriction of smaller peripheral vessels forces more blood to the vital organs. The muscle layer of a blood vessel is wrapped in an outer layer of connective tissue, a network of fibers and cells that encase and protect the vessel walls from stretching and rupture. This hierarchy of tissues is copied in other blood vessels, which differ mainly in the thickness of the layers. The wall of a major artery must withstand much stress and is therefore thicker than a minor vessel. The strategy of grouping and layering of different tissues is used to build other complex organs. In each case the function of the organ is determined by the specific functions of its component tissues.
The human body consists of some 100 trillion cells, yet it develops from a single cell, the zygote, resulting from fusion of a sperm and an egg. The early stages in the development of an embryo are characterized by rapid cell division and the differentiation of cells into tissues. The embryonic body plan, the spatial pattern of cell types (tissues) and body parts, emerges from two influences: a program of genes that specify the pattern of the body and local cell interactions that induce different parts of the program. Remarkably, the basic body plan of all animals is very similar (Figure 1-12). This conservation of body plan reflects evolutionary pressure to preserve the commonalities in the molecular and cellular mechanisms controlling development in different organisms. The impressive strides made in understanding these mechanisms are detailed in several later chapters.
With only a few exceptions, most animals display axial symmetry; that is, their left and right sides mirror each other. This most basic of patterns is encoded in the genome. In fact, patterning genes specify the general organization of an organism, beginning with the major body axes - anterior-posterior, dorsal-ventral, and left-right - and ending with body segments such as the head, chest, abdomen, and tail. The conservation of axial symmetry from the simplest worms to mammals is explained by the presence of conserved patterning genes in the genomes. Some patterning genes encode proteins that control expression of other genes; other patterning genes encode proteins that are important in cell adhesion or in cell signaling. This broad repertoire of patterning genes permits the integration and coordination of events in different parts of the developing embryo.
The precise timing of developmental events is maintained by the ability of one group of cells to induce or activate differentiation of a second group of cells. Most often induction is mediated by direct cell contact or by soluble factors released by the cells. In a typical case, contact between an aggregate of cells, the mesenchyme, with an overlying epithelial cell layer directs the latter cells to differentiate into an embryonic tissue or in later stages of development into a specific type of tissue. For example, the primitive notochord induces the development of embryonic nervous tissue and brain. Later, an eye forms when contact between a lobe of the developing brain induces the overlying embryonic “skin” to differentiate into a primitive lens.