Different 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-defined borders in collecting duct (CD), squamous cells in the thin segment (TS) of nephron, and even more flattened cells lining blood vessels, the vasa recta in the kidney (VR). x380. b. Dorsal root ganglion cells. Note the large size of these nerve cell bodies and the large, pale (eu-

chromatic) nuclei (N) with distinct nucleoli. Each ganglion cell is surrounded by flattened 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 |rER| visible as darker granules within the cytoplasm) reflect the extensive synthetic activity required to maintain the exceedingly long processes (axons) of these cells. x380. 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. x380.

Inclusions are materials in the cytoplasm that may or may not be surrounded by a membrane. They comprise such diverse materials as secretory granules, pigment, neutral fat, glycogen, and stored waste products.

The cytoplasmic ground substance was called cytosol in older texts because it was believed to be an amorphous fluid. The term cytoplasmic matrix is now used to emphasize that it is a concentrated aqueous gel of different-size molecules and has an organized structure.

Intracellular membranes increase surface area and delimit compartments

Many organelles and inclusions are membrane-limited structures; i.e., they are surrounded by a membrane. The membranes form vesicular, tubular, and other structural patterns that may be convoluted (as in the case of the smooth-surfaced endoplasmic reticulum [sERJ) or plicated (as in the case of the inner mitochondrial membrane). These membrane configurations greatly increase the surface area on which essential physiologic processes and biochemical reactions take place.

Moreover, the spaces enclosed by membranes constitute intracellular microcompartments in which substrates, products, or other substances are segregated or concentrated. For example, the enzymes of lysosomes are separated by a membrane from the cytoplasmic matrix because their hy-drolytic activity would be detrimental to the cell.

Organelles are described as membranous (membrane-limited) or nonmembranous

All cells have the same basic set of intracellular organelles, which can be classified into one of 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 membranous organelles include

• Plasma (cell) membrane, a lipid bilayer that forms the cell boundary as well as the boundaries of many organelles within the cell.

• Rough-surfaced endoplasmic reticulum (rER), a region of endoplasmic reticulum associated with ribosomes. It is the site of protein synthesis and modification of newly synthesized proteins.

• Smooth-surfaced endoplasmic reticulum (sER), a region of endoplasmic reticulum involved in lipid and steroid synthesis. It is not associated with ribosomes.

• Golgi apparatus, a membranous organelle composed of multiple flattened cisternae responsible for modifying, sorting, and packaging proteins and lipids for intracellular or extracellular transport.

• Endosomes, membrane-bounded compartments interposed within endocytotic pathways. Their major func tion is sorting proteins delivered to them via endocytotic vesicles and redirecting them to different cellular compartments for their final destination.

• Lysosomes, small organelles containing digestive enzymes; their derivatives include phagosomes, phagolysosomes, autophagosomes, and autophagolysosomes.

• Transport vesicles, which include pinocytotic vesicles, endocytotic vesicles, and coated vesicles. These vesicles are involved in both endocytosis and exocytosis and vary in shape and the material that they transport.

• Mitochondria, organelles that provide most of the energy to the cell by producing adenosine triphosphate (ATP) in the process of oxidative phosphorylation.

• Peroxisomes, small organelles involved in the production and degradation of H202 and degradation of fatty acids.

The nonmembranous organelles include

• Microtubules, which together with actin and intermediate filaments form elements of the cytoskeleton. Microtubules continuously elongate (by adding tubulin dimers) and shorten (by removing tubulin dimers), a property referred to as dynamic instability.

• Filaments, which are also part of the cytoskeleton. In general, filaments can be classified into one of two groups: actin filaments, which are flexible chains of globular actin molecules, and intermediate filaments, which are rope-like fibers formed from a variety of proteins. Both provide tensile strength to withstand tension and confer resistance to shearing forces.

• Centrioles, short, paired cylindrical structures found in the center of the centrosome. Derivatives of centrioles give rise to basal bodies of cilia.

• Ribosomes, structures composed of ribosomal RNA (rRNA) and ribosomal proteins (including proteins attached to membranes of the rER and proteins free in the cytoplasm). Ribosomes are essential for protein synthesis.

An outline of the key features relating to the identification of cell organelles and inclusions is provided at the end of the chapter (see Table 2.3, page 76). The normal function and the related pathologies are also summarized (see Table 2.4, page 77).


The plasma membrane is a lipid-bilayered structure visible with transmission electron microscopy

The plasma membrane (cell membrane) is a dynamic structure that actively participates in many physiologic and biochemical activities essential to cell function and survival.

When the plasma membrane is properly fixed, sectioned, stained, and viewed on edge with the transmission electron

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