Tissue Differentiation

cell internally, usually by changing their conformation or permitting a molecule to enter a channel that is part of the receptor.

The next part is transduction. This is the process by which there are relay molecules that often involves the addition or subtraction of phosphate groups from larger molecules in what involves multiple steps. The last part is called the “response phase”. It can involve any possible cellular activity that is present in the body, triggered by the transduction phase. The response phase can involve the turning on or turning off of specific genes.

Cellular communication can happen locally or from a distance. Locally, there can be communication via gap junctions between local cells. Plants themselves are connected through plasmodesmata. These kinds of communications are seen in embryonic development. Cell communication happens locally through paracrine signaling, which is signaling that happens over short distances. Autocrine signaling happens when cells act on identical cells. Endocrine signaling happens over large distances within the organism. All of these types of signaling mechanisms can happen in both plants and animals.

TISSUE DIFFERENTIATION

Cells that are of the same type form tissues, which together form a specific function. In this case, we are talking about multicellular animal organisms. There are four types of tissues in the human body, each of which forms a different function. These are connective tissue, epithelial tissue, nervous tissue, and muscle tissue.

In general, connective tissue is supportive to many other tissue types, while epithelial tissue creates protective barriers and is involved in ion and molecule diffusion. Nervous tissue will both transmit and integrate information and muscle tissue initiates movement.

Epithelial tissue is the tissue that forms glands. It forms the coverings over body surfaces and lines body cavities. The tissues that form the receptors for the special senses (like smell and taste) is also epithelial tissue. These cells are closely connected to one another and have junctions that connect each other. These are cells that have an

apical surface or outer surface and a basal surface or inner surface. The apical surface is the lining of ducts, tubes, and the outer surface of the body. This is tissue that is innervated but not vascularized. Figure 19 shows what the different epithelial tissues look like:

Figure 19.

In fact, there are three sides to epithelial tissue: a basal side, an apical side, and a lateral side. The basal surface is closest to the basement membrane. The basement membrane is a thin substance that acts as a barrier between connective tissue and the basal layer of the epithelial tissue. There are hemidesmosomes, which will be discusses, which are specialized junctions that connect the basal surface to the basement membrane.

The apical surface is nearest the free space or ductal lumen. There may be extensions into the lumen, called microvilli. Microvilli are intended to increase the surface area of the apical surface. These are found, for example, in the small intestinal lumen and in the kidneys, where absorption is important. Cilia are found in the female reproductive

system and the respiratory tract of humans. Cilia cause movement of things like mucus across the apical surface. Stereocilia are similar to cilia but are not motile. Stereocilia are found in the male reproductive system.

The lateral surfaces are the surfaces between adjacent epithelial cells. This is where epithelial cells have most of their junctions. Desmosomes are connectors between neighboring cells, forming spots where the cells are connected to one another. These desmosomes use the cytoskeleton of the cell to interconnect the cells. On the other hand, tight junctions between the cells will form a solid barrier that does not allow substances to cross the epithelium. Gap junctions between cells allow molecules to pass between adjacent cells. These allow for coordinated movement of heart muscle cells, for example.

As you can see by figure 19, there are different shapes of epithelial cells, which can be layered or non-layered. Squamous cells are flattened and can be either keratinized or nonkeratinized. These are protective cells that sometimes are used for diffusion. Cuboidal cells are cube-shaped, involved in absorption and secretion. Columnar cells are rectangular and sometimes have cilia. They have multiple functions. The epithelial cells can be simple (with one layer), stratified (or layered), or pseudostratified (which have one layer that looks stratified under the microscope).

Connective tissue is the most common tissue type in the body. It consists of connective tissue cells and extracellular matrix, made from protein fibers and ground substance. The connective tissue is also what makes up blood and lymph cells, which do not have ground substance or fibers. All connective tissue arises from embryonic mesenchyme cells.

Connective tissue has several possible functions. It can be structural, as is seen in chondroblasts, osteoblasts, and fibroblasts. It can be immunological, such as is seen in leukocytes and plasma cells. It can be related to defense, as is seen in macrophages and mast cells. It can be an energy reservoir, as is seen in fat cells.

There are three major types of connective tissue fibers that make up the ground substance in connective tissue. The most abundant is collagen fibers. These are flexible but have a high tensile strength. There are several types of collagen fibers, that vary

according to their function. Reticular fibers are similar to collagen fibers but are thinner. They form a structural framework in connective tissue, usually invisible when connective tissue is stained. Elastic fibers are also thin. These are highly stretchable, able to stretch without breaking. They are found in the lungs, in blood vessels, and in skin tissue.

Connective tissue can be described as proper, specialized, or embryonic. Proper connective tissue is loose connective tissue or dense connective tissue. Loose connective tissue is also called areolar tissue and has loosely-arranged collagen fibers in it. Dense connective tissue can be regular or irregular. Regular dense connective tissue is what makes ligaments or tendons. Irregular dense connective tissue is linked to things like the hollow intestinal organs.

Embryonic connective tissue is the precursor tissue to other types of connective tissue. It is divided into mucous connective tissue and mesenchyme. Mesenchyme is embryonic tissue. Mucous connective tissue is found in the umbilical cord and is associated with the Wharton’s jelly that surrounds the cord.

Specialized connective tissue is that which is seen in adipose tissue, cartilage, blood, and bone. Adipose tissue stores fat energy and makes certain molecules, like hormones and growth factors. These cells are mixed with loose connective tissue individually or in clusters. Bone has mineralized extracellular matrix, making it very strong. Blood is specialized connective tissue that has plasma as its extracellular matrix.

Muscle tissue is distensible and elastic, able to be stretched and contracted. These cells are capable of being contractile because of the action of actin and myosin filaments within the cells. The cells are highly organized into bundles. While there are three types of muscle fibers (cardiac, smooth, and skeletal), all are arranged parallel to one another to some degree. Figure 20 shows the three different muscle types:

Figure 20.

Skeletal muscle is the type of muscle fiber that is voluntary. The cells are long and cylindrical. They are multinucleated from embryonic myoblasts that fuse together in the embryo. The muscles appear striated because of the arrangement of actin and myosin. Cardiac muscle is found in the heart and also appears striated. The movement is involuntary. Gap junctions coordinate these cellular functions. The cardiac muscle cells have just one nucleus.

Smooth muscle tissue is that found in the GI tract and arterial walls. The action of these muscle cells is involuntary and relatively weak compared to other muscle types. They are shaped like spindles and have a single central nucleus. The cells do not appear striated because the cells are irregularly arranged.

The main cells of the nervous system are the neurons and the glial cells. Neurons transmit electrical signals and have a large soma or body, with long projections called axons or dendrites that send information from one neuron to another. In most cases, it is the axon that sends signals away from the soma and dendrites that receive information for the cell. A group of neurons is called a nucleus in the central nervous system or a ganglion in the peripheral nervous system.

Glial cells are the supportive cells of the nervous system. There are astrocytes, oligodendrocytes, and Schwann cells, for example. The oligodendrocytes and Schwann cells make myelin, forming the myelin sheath around white matter in the nervous system. Microglia, on the other hand, are the main macrophages of the nervous tissue. It is microglia that destroy pathogens and cellular debris in the nervous system.