and Jürgen Roth2
Medical University of Vienna, Vienna, Austria
University of Zurich, Zurich, Switzerland
Structural Organization of a Mammalian Cell
All eukaryotic cells present qualitatively a similar structural organization that can vary quantitatively depending on their degree of differentiation and specialization as well as functional state. The electron micrograph shows at low magnification highly specialized acinar cells of rat pancreas, which represent the prototype of a polarized exocrine secretory cell. Several such acinar cells form a functional unit, named the secretory acinus.
Each cell consists of two major compartments, the nucleus and the cytoplasm, and these are the focus of the first chapter. The cytoplasm of different cell types contains a common set of membrane-bound organelles and the various structural components of the cytoskeleton. They are both embedded in the cytosol, which houses the intermediate metabolism and is the location where protein synthesis commences and proteasomal degradation of (misfolded) proteins as well as protein O-GlcNAcylation occur. The rough endoplasmic reticulum (RER) is involved in translocation of secretory and membrane proteins and protein quality control, is the site of initiation of protein N-glycosylation, and is a major Ca2+ store. Lipids are also synthesized in the endoplasmic reticulum. Many proteins and lipids are transported to the Golgi apparatus, where they receive different post-translational modifications and are sorted to their final destinations. The protein polypeptide:GalNAc O-glycosylation is initiated in the Golgi apparatus. The pre-Golgi intermediates are implicated in anterograde and retrograde transport of cargo between the endoplasmic reticulum and the Golgi apparatus. In acinar pancreatic cells and other types of secretory cells, secretory proteins are sorted and packed into immature secretory granules, so-called condensing vacuoles (CV) forming in the trans Golgi apparatus. They mature into zymogen granules (ZG), which are stored in the apical cytoplasm and undergo secretion on stimulation. The endoplasmic reticulum and its transitional elements, the pre-Golgi intermediates, the Golgi apparatus, and secretory granules constitute the secretory pathway. Further cellular organelles are the mitochondria (M), which mostly generate energy; peroxisomes, which perform oxidative reactions; lysosomes and autophagosomes/autolysosomes, which have degradative functions; and endosomes, which are involved in cellular uptake of extracellular substances.
The plasma membrane is the cell’s boundary with its environment. In epithelial cells such as the acinar cells, two plasma membrane domains can be distinguished. The apical plasma membrane domain forms the acinar lumen (AL) and is separated by junctional complexes (arrows) from the lateral (LPM) and basal (BPM) plasma membrane domain.
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