Figure 212

Photomicrograph of secretory cells of the pancreas. Note that secretory vesicles containing protein ready for secretion fill the apical portion of the cells. This process requires an external signaling mechanism for the cell to discharge the accumulated granules. x860.

Endosomes

The TEM reveals the presence in the cytoplasm of membrane-enclosed compartments associated with all the endocytotic pathways described above (Fig. 2.14). These compartments, called early endosomes, are restricted to a portion of the cytoplasm near the cell membrane where vesicles originating from the cell membrane fuse. From here, many vesicles return to the plasma membrane. However, large numbers of vesicles originating in early endosomes travel to deeper structures in the cytoplasm, called late endosomes. The latter typically develop into lysosomes.

Endosomes can be viewed either as stable cytoplasmic organelles or as transient structures formed as the result of endocytosis

Recent experimental observations of endocytotic pathways conducted in vitro and in vivo suggest two different models that explain the origin and formation of the endosomal compartments in the cell:

• The stable compartment model describes early and late endosomes as stable cellular organelles connected by vesicular transport with the external environment of the

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