FIGURE 2.1 • Histologic features of different cell types. These three photomicrographs show different types of cells in three different organs of the body. The distinguishing features include size, shape, orientation, and cytoplasmic contents that can be related to each cell’s specialized activity or function. a. Epithelial cells in the kidney. Note several shapes of epithelial cells: columnar cells with well-defned borders in the collecting duct (CD), squamous cells in the thin segment (TS ) of the nephron, and even more-fattened cells lining blood vessels, the vasa recta in the kidney (VR). 380. b. Dorsal root ganglion cells. Note the large size of these nerve cell bodies and the large, pale (euchromatic) nuclei (N) with distinct nucleoli. Each ganglion cell is surrounded by fattened satellite cells (S ). The size of the ganglion cell and the presence of a euchromatic nucleus, prominent nucleolus, and Nissl bodies (rough-surfaced endoplasmic reticulum visible as darker granules within the cytoplasm) refect the extensive synthetic activity required to maintain the exceedingly long processes (axons) of these cells. 380. C. Smooth muscle cells of the small intestine. Note that these cells are typically elongated, fusiform shaped, and organized in a parallel array. The nuclei are also elongated to conform to the general shape of the cell. 380.
of solutes within the matrix, which inﬂuences the rate of metabolic activity within the cytoplasmic compartment. The nucleus is the largest organelle within the cell and contains the genome along with the enzymes necessary for DNA replication and RNA transcription. The cytoplasm and nucleus play distinct functional roles but also work in concert to maintain the cell’s viability. The structure and function of the nucleus is discussed in Chapter 3.
Organelles are described as membranous (membranelimited) or nonmembranous.
Organelles include the membrane systems of the cell and the membrane-limited compartments that perform the metabolic, synthetic, energy-requiring, and energy-generating functions of the cell, as well as nonmembranous structural components. All cells have the same basic set of intracellular organelles, which can be classified into two groups: (1) membranous organelles with plasma membranes that separate the internal environment of the organelle from the cytoplasm, and (2) nonmembranous organelles without plasma membranes.
The membranes of membranous organelles form vesicular, tubular, and other structural patterns within the cytoplasm that may be convoluted (as in smooth-surfaced endoplasmic reticulum) or plicated (as in the inner mitochondrial membrane). These membrane configurations greatly increase the surface area on which essential physiologic and biochemical reactions take place. The spaces enclosed by the organelles’ membranes constitute the intracellular microcompartments in which substrates, products, and other substances are segregated or concentrated. In addition, each type of organelle contains a set of unique proteins; in membranous organelles, these proteins are either incorporated into their membranes or sequestered within their lumens. For example, the enzymes of lysosomes are separated by a specific enzyme-resistant membrane from the cytoplasmic matrix because their hydrolytic activity would be detrimental to the cell. In nonmembranous organelles, the unique proteins usually self-assemble into polymers that form the structural elements of the cytoskeleton.
Besides organelles, the cytoplasm contains inclusions, structures that are not usually surrounded by a plasma membrane. They consist of such diverse materials as crystals, pigment granules, lipids, glycogen, and other stored waste products (for details, see page 71).