2
Structure and Function
of Cell Organelles
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Fig. 1. Overview of cell organization. (A) Diagram of the major cell organelles, including the cytoskeleton
and nucleus. This drawing depicts a single idealized cell, and so does not include the cell-cell and cell-ECM
junctions elaborated by cells in tissues. (B–D) Examples of low-power electron micrographs of thin sections of
rat tissues showing how cell organization reflects cell function. (B) Shows intestinal epithelial cells, which are
modified to aid in the digestion and absorption of food. The apical membranes of these cells develop highly
organized microvilli (MV), which are supported by bundles of microfilaments, to increase the surface area of
these cells. The epithelial cells are bound to each other by junctional complexes (JC) consisting of a cluster of
tight junctions, zonulae adherens junctions, and desmosomes. The tight junctions prevent material in the
intestinal lumen from diffusing between cells into the body cavity, and the adherens and desmosomal junctions firmly anchor cells to each other. A migrating immune cell (LYM, lymphocyte) is also present in this
section. (C) Shows salivary gland cells, which are specialized to produce and release large amounts of secretory glycoproteins. These cells contain extensive arrays of rough endoplasmic reticulum (RER), and become
filled with secretory vesicles (SV). Two nuclei are present in this section, and both display prominent nucleoli
(NU). (D) Shows cells of the esophageal epithelium, which are specialized to accomodate and resist mechanical stresses. These cells are constantly renewed by the mitotic activity of a basal layer of cells (MC, mitotic
cell). As cells are produced and differentiate, they move toward the lumen of the esophagus, become flattened, and form extensive desmosomal connections (D) with neighboring cells.
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Fig. 2. Electron micrographs illustrating the structural features of various cell organelles. (A) Epithelial cells
from a tadpole (Rana pipiens) tail; (B), (F), (G), and (H) show endocrine cells from a rat pituitary gland; (C) sea
urchin coelomocyte; (D) and (E) secretory cells from digenetic trematodes (Halipegus eccentricus and
Quinqueserialis quinqueserialis). (A) Low-power electron micrograph of an epidermal cell, showing a number
of cell-cell and cell-extracellular matrix junctions. These cells elaborate numerous desmosomes (D) and
hemidesmosomes (H), and the cytoplasm is filled with prominent bundles of intermediate filaments (IF), which
interconnect these junctions. ECM, extracellular matrix. (B) Cytoplasm of an endocrine cell, showing a mitochondrion (M), smooth endoplasmic reticulum (SER), and flattened cisternae of the Golgi apparatus (G). Also
present are small clusters of free ribosomes, secretory vesicles, and part of the nucleus (upper left corner). (C)
Low-voltage scanning electron micrograph of an extracted cell (the plasma membrane and soluble cytoplasmic proteins were removed by detergent), showing a few tubular extensions of SER embedded in a network of
microfilaments (MF). This type of microscopy shows the surface features of organelles. (D) A dense array of
RER from a secretory cell; note the high level of organization in the parallel alignment of cisternae. Attached
ribosomes (R) appear as small granular bodies. (E) Section of cytoplasm containing a number of mitochondria.
Their striped appearance is due to the invagination of the inner mitochondrial membrane, which forms cristae
(C). (F) Endocrine cell cytoplasm, showing two small lysosomes (L), secretory vesicles (SV), and a centriole
pair of a centrosome. In this section, pericentriolar material and attached microtubules are not clearly displayed. (G) Exocytosis in an endocrine cell. This micrograph shows the periphery of the cell, and the deep
invagination of the plasma membrane indicates the site where a secretory vesicle has undergone exocytosis
(arrow). A mitochondrion (M), some strands of RER, and clusters of free ribosomes (R) are also shown. (H)
Pinocytosis in an endocrine cell. The plasma membrane of this cell displays numerous small, smooth invaginations (arrows), characteristic of non-clathrin mediated internalization of material.
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