Publisher Summary This chapter describes the cytology of salivary glands. The salivary glands are divided into two groups—major and minor. The parotid, submandibular (submaxillary), and sublingual glands are major salivary glands. The gross anatomical relationships of the salivary glands of mammals vary greatly between species. The nature of the secretory products of various salivary glands is widely investigated by histochemical staining methods. Staining methods for mucosubstances demonstrate that gross anatomically comparable glands from different species may secrete entirely different mucosubstances. The chapter discusses the secretory functions of the salivary glands. Salivary glands possess systems that allow these organs to secrete remarkable amounts of proteins, glycoproteins, and water. The secretory capacity of the salivary gland acinar cells has been well investigated, particularly the acinar cells of parotid glands. The synthesis, intracellular transport, storage, and discharge of exportable protein by exocrine-secreting cells are also discussed.

The cytology of salivary glands.

The role of natural salivary defences in maintaining a healthy oral microbiota.

Origin, structure, and biological activities of peroxidases in human saliva

The seminal vesicles originate in embryos of about 58 mm crown-rump-length from the Wolffian duct under the influence of testosterone. Along with the ampulla of the vas deferens and the ejaculatory duct, they form a functional unit that develops slowly until the onset of puberty. Developmental malformations occur as uni- or bilateral agenesis, aplasia, cysts, or ureterovesicular fistules. After puberty, the glands form sac-like structures which have a capacity of about 3.4-4.5 ccm and contribute about 70% of the seminal fluid. In addition to secretion, they are capable of reabsorption of fluids or dissolved substances, and of spermatophagy (ingestion and degradation of damaged spermatozoa by epithelial cells). Secretory activity of the glands is a measure of testosterone supplementation to the epithelium. Nervous regulation of secretion is realized by cholinergic post-ganglionic, sympathetic (and perhaps parasympathetic) fibres, derived from pelvic plexus. Contraction of the muscular wall occurs under the influence of excitatory adrenergic and modulatory NPY-encephalin-peptidergic nerve fibres. The secretory products of the seminal vesicles encompass (1) ions (K+: 1.1 mM ml-1) (2) low molecular weight substances (fructose: above 1.2 mg ml-1; prostaglandins above 250 microliters ml-1, (3) peptides (endorphin: 330 pg ml-1), and (4) proteins. In addition to plasma protein related forms such as transferrin, lactoferrin, and fibronectin, specific proteins such as semenogelin (52 kDa) are synthesized, the scaffold protein of semen coagulate forming the substrate for PSA (prostate specific antigen), sperm motility inhibitor (ca. 18 kDa), and others (placental protein 5, protein kinase inhibitor, carboanhydrase, 5'-nucleotidase), some of which are immunosuppressive. Therefore, functions of the seminal vesicles concern (a) formation of seminal coagulum, (b) modification of sperm functions (motility, capacitation), and (c) immunosuppression. Additional functions within the female genital system, perhaps during pre-implantation period, are likely, but remain to be proven experimentally.

Morphology and functions of the human seminal vesicle

Oral streptococci are able to produce growth-inhibiting amounts of hydrogen peroxide (H2O2) as byproduct of aerobic metabolism. Several recent studies showed that the produced H2O2 is not a simple byproduct of metabolism but functions in several aspects of oral bacterial biofilm ecology. First, the release of DNA from cells is closely associated to the production of H2O2 in Streptococcus sanguinis and Streptococcus gordonii. Extracellular DNA is crucial for biofilm development and stabilization and can also serve as source for horizontal gene transfer between oral streptococci. Second, due to the growth inhibiting nature of H2O2, H2O2 compatible species associate with the producers. H2O2 production therefore might help in structuring the initial biofilm development. On the other hand, the oral environment harbors salivary peroxidases that are potent in H2O2 scavenging. Therefore, the effects of biofilm intrinsic H2O2 production might be locally confined. However, taking into account that 80% of initial oral biofilm constituents are streptococci, the influence of H2O2 on biofilm development and environmental adaptation might be under appreciated in current research.

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The Role of Hydrogen Peroxide in Environmental Adaptation of Oral Microbial Communities

The localization of endogenous peroxidase was studied in human parotid and submandibular glands using the medium of Strum & Karnovsky either at pH 7 or at pH 8.3, after a short fixation of the tissues with a low concentration of glutaraldehyde. At pH 7 reaction product was found in seromucous cells of both glands in the cisternae of the rough endoplasmic reticulum, in the elements of the Golgi apparatus, and in secretory products. However, while in the parotid gland all secretory granules showed peroxidase activity, in the submandibular gland some of the granules were left unstained. At pH 8.3 the pattern of reactivity was practically unchanged in the submandibular gland, whereas the amount of precipitate noticeable in the secretory granules of the parotid gland was greatly reduced. The method of Novikoff et al. for catalase revealed the presence of a number of small bodies having in the characteristics of peroxisomes. Acid phosphatase was not demonstrable in the secretory granules of either parotid or submandibular glands. The lead precipitate indicating enzyme activity was observed only in lysosomes and, occasionally, in the elements of the Golgi apparatus of some sermucous cells.

Ultrastructural localization of endogenous peroxidase in human parotid and submandibular glands.

An intralobular duct of a human parotid gland has two parts, an intercalated part and a striated part. Intercalated ducts are lined with low cuboidal cells endowed with scanty cytoplasmic organelles. Striated ducts are lined with columnar cells rich in mitochondria and glycogen particles, and are characterized by extensive infoldings of the basal plasma membrane. The apical cytoplasm of the cells of the striated ducts shows a number of membrane-bound granules having a diameter of about 0-15 mum. These granules contain material of varying electron density which does not react with silver or with the histochemical reagents employed in the present study. Thus, on the basis of their small size and histochemical characteristics, they are distinct from the large and dense secretory granules observed in the so-called granular striated ducts of some animals. In addition, cells of striated ducts contain lysosomes, peroxisomes, and large lipoid bodies which give histochemical reactions typical of lipofuscins. Bodies of myoepithelial cells have been observed only in intercalated ducts. Their processes, however, extend into the proximal parts of striated ducts.

Fine structure and cytochemistry of the intralobular ducts of the human parotid gland

The duct of the human submandibular gland (Wharton) is lined by a pseudostratified epithelium consisting of principal and basal cells. Scattered among them are a few goblet and ciliated cells. The principal cells are columnar in shape with many mitochondria, numerous dense bodies and a central nucleus with some indentations. Their apical cytoplasm shows a number of clear vesicles, some of which are reactive to silver and are extruded by exocytosis into the lumen. Other vesicles, which are unreactive, may represent the products of the absorption process. A mechanism of apocrine secretion is also observed in the principal cells. Thus, with regard to its functions, the duct of the human submandibular gland modifies the composition of saliva by adding a secretory component to it. This latter material is derived from the goblet cells but chiefly from merocrine (exocytosis) and apocrine secretion of principal cells.

The epithelium of the excretory duct of the human submandibular gland: a transmission and scanning electron microscopic study.

Two secretory mechanisms (eccrine and apocrine) were observed by transmission and scanning electron microscopy in the same secretory cells of the human ceruminous glands. The eccrine secretion occurs as a typical exocytosis by fusion of the limiting membrane of the secretory granule with the apical plasmalemma. The apocrine secretion is more complex and takes place by sequential steps: bulging of the cellular apex into the lumen, constriction of the projection, and detachment of it from the cell. This mechanism generally causes the removal of the entire projection all at once by decapitation at its base; some variations of this process have been found, however, and are described in the present work. A double membrane, apparently separating the apical protrusion from the rest of the cell, is noticeable in our photographs. Nevertheless, even if this membrane is very similar to a demarcation layer, we consider it as an intercellular double membrane separating two obliquely sectioned cells.

Secretory mechanisms of human ceruminous glands: a transmission and scanning electron microscopic study.

The epithelium of the human ampulla consists of rare basal cells and a single type of principal (secretory) cells. The principal cells, which vary in height from cuboidal to columnar, often have conical or club shaped apical portions projecting and bulging into the lumen. Their cytoplasm contains an abundant rough-surfaced endoplasmic reticulum, numerous mitochondria, and many lipopigment granules. The Golgi apparatus is well developed and originates the secretory vacuoles which are discharged by exocytosis. Though the majority of nuclei are ovoid, deeply fissured or even lobated nuclei are not uncommon. The basal cells are small, nonsecretory cells, whose cytoplasm shows numerous filaments having a diameter of 55 A. The lumen contains masses of amorphous secretions and some spermatozoa. The human ampulla is a gland endowed with the same morphological features as those of the seminal vesicles.

Francesca Testa-Riva

Papers

Ultrastructural localization of endogenous peroxidase in human parotid and submandibular glands.

Fine structure and cytochemistry of the intralobular ducts of the human parotid gland

The epithelium of the excretory duct of the human submandibular gland: a transmission and scanning electron microscopic study.

Secretory mechanisms of human ceruminous glands: a transmission and scanning electron microscopic study.

The ampulla ductus deferentis in man, as viewed by SEM and TEM.