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AN INTRODUCTION TO CELLS AND TISSUES Domestic animals, like all others, are made up of an amazing aggregation of simple units - the living cells. Each cell exhibits a degree of independence from all others, meaning that it is a living unit in its own right. The real beauty of function of the complex animal, however, results from the cells coexisting as a community. Individual cells support each other's needs, communicate with and protect each other. In some situations, specialized populations of cells serve to protect the whole animal by seeking out and destroying other, abnormal cells. By cooperating with each other, a mixture of cells in a tissue can perform tasks that would be impossible for individual cells in isolation. These functions of the basic units of living matter must be understood to appreciate how the whole animal functions, both in health and in disease. At this time, some descriptive information about cells is both useful and important; much greater detail will follow in later sections. At the simplest level, each cell consists of a mass of jelly-like material called protoplasm , which is made up of an inner nucleus and the surrounding cytoplasm . At the outer surface, the cytoplasm is contained within a cell membrane, or plasmalemma . There is no cell wall in animal cells. In most cells, numerous specialized structures called organelles are found within the cytoplasm (Figure 2-1). In most cases the powerful resolving power of the electron microscope is needed to observe intracellular structures smaller than the nucleus. The organelles are also usually bounded by membranes, and their location within the three- dimensional space of the cell interior is determined by their attachments to a delicate intracellular latticework called the cytoskeleton . Cells are alive and dynamic but are usually killed in preparation for microscopic examination. Figure 2-1. The generalized cell. The cell is bounded by a plasma membrane (PM), which may be specialized at the apical (ap) surface and possess microvilli (Mv). At the base of the microvilli may be pinocytotic vesicles (pv). On the lateral margins, the membrane may form junctions (j) with adjacent cells. The cell may be associated at its basal surface with a basement membrane (bm), an extracellular supporting material. The nucleus (N), containing a nucleolus (nu) and chromatin (ch), is bounded by a nuclear membrane (nm) with characteristic nuclear pores (np). Membranous organelles extending into the cytoplasm include smooth endoplasmic reticulum (ER) and rough endoplasmic reticulum (RER). The latter is decorated with ribosomes (R) that may also be free in the cytoplasm. The Golgi (G), a layered vesicular membrane system, gives rise to membrane-enclosed granules (gr). These are usually secretory granules that can accumulate in the cytoplasm awaiting a secretory episode. Mitochondria (M) vary in number depending upon the metabolic activity level of the cell; they are involved in energy transformation. 2007 version – page 4
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This note was uploaded on 03/23/2009 for the course ANSCI 1110 taught by Professor Brucecurrie during the Fall '08 term at Cornell University (Engineering School).

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