parotid gland Dr.Jastrow's electron microscopic atlas

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H. Jastrow

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Overview parotid gland (Glandula parotidea):
Labelling is in preparation all specimens are from a rat


Overview 1	Overview 2	Acinus and initial intercalated duct	Acinus 2	Detail thereof	epithelial cell with large nucleus	Detail: extracellular space "inside" nucleus


basal cut of an epithelial cell	epithelial cell with 2 nuclei	Detail: RER + vesicles	Detail: partly dilated RER	other cell base with RER + mitochondria	paralell oriented RER	similar detail with partly dilated RER


longitudinally cut epithelial cells	X-section of an epithelial cell + myoepithelial cell	Other myoepithelial cell	Cytoplasm of acinar epithelial cells	Detail with heterolysosome	apical region with secretory vesicles	Detail: confluating vesicles	X-section of a glandular acinus


intercalated duct in longitudinal section	intercalated duct X-section	Detail thereof with junctional complex	intercalated duct 2	Detail: dilated nuc- lear membrane (bleb):	intercalated duct 3 with innervation	Detail: non-myelinated nerve + synapse en passant

The parotid gland (Terminologia histologica: Glandula parotidea) is a truely serous salivary gland. Its secretion is non-viscous and contains mainly water, ions and further substances released by the merely electron-dense secretory vesicles: alpha-amylase (splitting alpha-1,4-glycosidic junctions of amylose, of amylopectins and glycogen and serves for digestion of starch, immunoglobulin-A (IgA) antibodies for defense of foreign proteins, lysozyme (attacks cell-walls of Gram-positive bacteries). The only slight electron-density of the vesicles points to a low protein but a high water content. The content of water in the secretion may rais up to 99% after stimulation, e.g., after swallowing a sour drop. The main factor in this context is a paracellular water flow through the pore between the tight-junctions connecting the epithelial cells. The epithelial cells (acinous cells) have a Na⁺-K⁺-ATPase in their basal cell membranes which transports 1 Na⁺ out of the cytoplasm in exchange with 1 K⁺ which is transported inwards. Further, a connected co-transporter pumps 1 Na⁺ 1 K⁺ and 2 Cl^- into the cell additionally 1 K⁺ is transported outwards passively in the basal membrane. Since the chloride is quickly moved out of the cell at the apical (luminal) membrane an osmotic gradient is produced which results in a passive Na⁺ and water current through the tiny pores between the tight-junctions of the apical intercellular space. The resulting primary saliva is isotonic. Flat to cuboid epithelial cells belonging to the intercalated duct lead from the glandular acinus into the excretory system. Long modified smooth muscle cells called myoepithelial cells are located above the basal lamina wind around such intercalated ducts as well as the glandular acini. Thei contraction results in an active transport of secretions. The intercalated ducts continue into the intralobular ducts which are called striated ducts since they show a characteristic order of their mitochondria which lie in rows paralell to the infolded basal cell membrane (no image here). The mostly columnar cells of the straited ducts draw NaCl out of secretion located in the lumen of the duct by high amounts of Na⁺-K⁺-ATPase and passive chloridechannels in their infolded and thus extremely enlarged basal cell membrane. This causes a cytoplasmatic lack of salt which is refilled by channels of the apical cell membrane from the lumen. Since there are virtually no pores between the tight-junctions in striated ducts water cannot follow resulting in a hypotonic secundary saliva. The latter is further transported via larger interlobular ducts with a two to multilayered epithelium to the larger excretory ducts with a multilayered squamous epithelium into the main excretory duct (Ductus parotideus) which openes into the oral cavity opposite to the 2nd upper molar.

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