Cells: A common denominator
All of the physiological systems, for example, digestive, respiratory, or cardiovascular, depend on the actions and activities of cells. Groups of cells and their products coalesce to create the four basic tissue types (epithelial, neural, muscular, and connective tissues).
Attributes of these tissues will be discussed in detail in Chapter 4. Combinations of these tissues produce organs. Functionally related organs are arranged into physiological systems. To illustrate, the digestive system includes the mouth and oral cavity, esophagus, stomach, small intestine and large intestine, and related accessory organs (liver, pancreas, gall bladder). This tube-within-a-tube organization allows for acquisition of food, physical mastication, chemical digestion, and ultimately, absorption of nutrients across the lining of the gastrointestinal (GI) tract into the bloodstream. The mature GI tract has elements of each of the four major tissue types.The internal lining, the mucosa (an example of epithelial tissue), is composed of a layer of specialized epithelial cells called enterocytes. The enterocytes rest upon a thin layer of extracellular proteins, the basement
membrane. The mucosal layer also includes other specialized connective tissue elements, including the structurally important proteins collagen and elastin, as well as protein-carbohydrate hybrid molecules called proteoglycans. The mucosa also has a population of scattered smooth muscle cells, the muscularis mucosa, and a distinctive connective tissue called the lamina propria. The submucosa appears between the enterocytes and the next major tissue layer, the muscularis. This region provides a passageway for capillaries and lymphatic vessels. Exocrine glands, which produce secretions destined for the lumen of the GI tract, also reside in this location. Closer to the outer circumference of the tract, there are two closely aligned, dense layers of smooth muscle cells called the muscularis externa.
The innermost layer of smooth muscle cells are arranged around the circumference of the GI tract, while the outer layer is oriented along the longitudinal axis of the GI tract. The coordinated contraction and relaxation of these two smooth muscle cell layers provide for mixing and movement of gut contents. A thin layer of epithelial cells called the serosa covers the outside of the GI tract that is adjacent to the internal body cavity. The serosa is continuous with the mesentery, which provides a means for entrance of veins, arteries, and nerve fibers into the muscularis externa and submucosa and for general support via attachment to ligaments.Despite the complexity of tissue and cell types in the GI tract, and requirement of multiple cell and tissue types for maximum efficiency, the essential function of the GI tract depends on the actions of the enterocytes. Consequently, understanding physiological systems and principles ultimately should begin with an appreciation of cellular physiology and function. A common theme that we will emphasize repeatedly is that structure and function go together. This idea will become apparent at multiple levels of organization—molecular, cellular, organ, and system. Our story begins with a discussion of the cell.
Once past the primordial stem cell stage of the embryo, cells acquire varying degrees of structural and functional differentiation. Differentiation of cells equips them for their particular function. For many years, the dogma was that once cells became differentiated, it was impossible to reprogram them cell so that these cells or their daughters could be induced to follow a different path. Under usual circumstances, this likely is true; however, it is also evident that advances in cell and molecular biology have called this dogma into question. For example, development of the cloned sheep Dolly in 1996 was achieved using cultured fibroblasts. Other examples of animals cloned from fully differentiated cells have been recently reported.
It is now known that virtually all tissues harbor populations of undifferentiated cells that serve as stem cells capable of being induced to proliferateBox 2.1 Practical anatomy and physiology
There is a lot in the news lately about stem cells. Much of this involves efforts to produce replacement tissues or organs or combat ravages of cancer and other diseases. What about general physiology? Are there examples? The answer is yes. One of the most elegant demonstrations for the existence of stem cells is for the mammary gland. Using genetically similar mice, researchers first prepared recipient mice which had their rudimentary mammary epithelium surgically removed before puberty. This resulted in animals with an intact mammary fat pad (so-called cleared mammary gland), that is, no remaining mammary parenchymal tissue. Using cell separation and isolation techniques, scientists recreated the entire epithelial (parenchymal) portion of the mammary gland and induced milk synthesis following the injection of a single stem cell into a cleared mammary fat pad (Kordon and Smith, 1998; Bussard and Smith, 2011).
Other animal scientists are working to identify and manipulate stem cells in domestic animals to improve meat and milk production.
and thereby create new lineages of cells that can repopulate those tissues. Box 2.1 provides some examples of mammary stem cells.
These examples serve to emphasize an unexpected plasticity of tissues and cells. It may be possible in the future to bioengineer replacements for damaged or diseased organs or tissues as more is learned about the rules governing cell growth and differentiation.