The development of the granular convoluted duct in the rat submandibular gland

The development of the granular convoluted duct in the submandibular gland of male rats, 4 to 12 weeks of age, was investigated. During this period, the average weight of the gland increased from 213 to 526 mg, the total DNA and RNA contents doubled, and the protein content tripled. Radioautographs were prepared from Epon embedded sections of the gland of the rats given ³H-thymidine and stained with toluidine blue. The glands of 4-week-old rats consisted mainly of acinar cells (45%), intercalated ductal cells (20%) and striated ductal cells (16%). A few granular convoluted ductal cells were seen in the striated duct close to the intercalated duct. The frequency (and absolute number) of granular convoluted ductal cells increased linearly from 1% (3 × 10⁶) at four weeks to 26% (68 × 10⁶) at eight weeks, while the calculated number of striated ductal cells remained stationary. The absolute number of acinar cells and intercalated ductal cells nearly doubled between four to eight weeks of age. The proliferative activity of all cell types declined with age but between six and ten weeks of age the rate of proliferation of ductal cells was relatively higher than the rate of proliferation of the acinar cells. Morphologically the size and number of granules in the granular convoluted ductal cells increased with age. Based on the above data it is concluded that the granular convoluted ductal cells developed from that segment of the striated ductal cells which is in close proximity with the intercalated ductal cells. The heterogeneity of the granules in the granular convoluted ductal cells observed from six weeks of age might denote the functional diversity of the cells.     

The development of the granular convoluted duct in the rat submandibular gland.

The development of the granular convoluted duct in the submandibular gland of male rats, 4 to 12 weeks of age, was investigated. During this period, the average weight of the gland increased from 213 to 526 mg, the total DNA and RNA contents doubled, and the protein content tripled. Radioautographs were prepared from Epon embedded sections of the gland of the rats given 3-H-thymidine and stained with toluidine blue. The glands of 4-week-old rats consisted mainly of acinar cells (45%), intercalated ductal cells (20%) and striated ductal cells (16%). A few granular convoluted ductal cells were seen in the striated duct close to the intercalated duct. The frequency (and absolute number) of granular convoluted ductal cells increased linearly from 1% (3 X 10-6) at four weeks to 26% (68 X 10-6) at eight weeks, while the calculated number of striated ductal cells remained stationary. The absolute number of acinar cells and intercalated ductal cells nearly doubled between four to eight weeks of age. The proliferative activity of all cell types declined with age but between six and ten weeks of age the rate of proliferation of ductal cells was relatively higher than the rate of proliferation of the acinar cells. Morphologically the size and number of granules in the granular convoluted ductal cells increased with age. Based on the above data it is concluded that the granular convoluted ductal cells developed from that segment of the striated ductal cells which is in close proximity with the intercalated ductal cells. The heterogeneity of the granules in the granular convoluted ductal cells observed from six weeks of age might denote the functional diversity of the cells.     

A granular cell in the proximal intercalated duct of human parotid and submandibular glands

In human parotid and submandibular gland unusual granulated cells are observed at the acinar-intercalated duct junction. These cells, which show a well developed Golgi apparatus, greatly differ from the typical elements of the intercalated ducts mainly due to the presence of abundant secretory granules of unknown nature. A complex substructure distinguishes these granules from those of conventional ductal cells and from those of acinar cells as well. In addition, some differences exist in the morphology of the granules observed in parotid with respect to those of submandibular gland.     

Parenchymal cell proliferation and mechanisms for maintenance of granular duct and acinar cell populations in adult male mouse submandibular gland

To evaluate proliferation as a factor in maintenance of parenchymal cell populations in adult male mouse submandibular glands, a variety of surveys were conducted following a pulse with ³H-thymidine. Striated granular duct (SGD) cells had the highest labeling index, followed by intercalated duct (ID) cells, then acinar (AC) cells, and granular duct (GD) cells had the lowest. These cell types showed from 30% to 60% completion of mitosis by 24 hr, with SGD, AC, and GD showing a likely second wave of mitosis sometime between 2 and 7 days after the pulse. About 40% of the pulse-labeled cells still remained as single cells at 42 days after the pulse. Repeat divisions in daughter cells of the primary labeled cells were very rare. A shift in the pattern of labeled cells at the ID-GD junction indicates that ID and SGD cells in this compartment are differentiating to GD cells. Further comparison of the magnitude of this conversion with the amount of noncompartmental GD cell proliferation provided a basis for calculating that ～70% of GD cell population maintenance occurs by selfproliferation, and the remaining 30% is contributed by differentiation from ID and SGD cells. A similar survey at the ID-acinus junction showed no evidence of conversion of ID cells to AC cells indicating that most, if not all, proliferative activity leading to AC cell population maintenance occurs by self-proliferation. Finally, based in part on structural changes at the ID-GD junction during the survey period, a pattern of cell conversion described as “in situ differentiation” is proposed. When this pattern is carried to fruition, this explains several structural features of the secretory complex typical to the male pattern submandibular gland. The proposed mechanism is supported by a three-dimensionally reconstructed sequence of likely intermediate structures. © 1993 Wiley-Liss, Inc.     

Elastic fibers in the duct system of the rat submandibular salivary gland.

The submandibular salivary gland originates from the floor of the mouth whose mucosa contains elastic fibers. Therefore, such fibers were sought in the duct system of the derivative organ. In adult rats, light microscopy has indeed revealed fine, circumferential, elastic fibers near the basement membrane of the duct epithelium. In the larger extralobular ducts, they were separated from several layers of longitudinal elastic fibers by a capillary-rich zone sparse in elastic fibers except for fine angular ones. More peripherally, larger angular-appearing fibers were frequently present near the submandibular parasympathetic ganglia in the duct wall. As duct diameter decreased, elastic fiber size and number diminished. Intralobularly, the smaller striated ducts, granular and intercalated ducts, and acini generally lacked such fibers. Electron microscopy of the extraglandular portion of the main duct revealed fibrils extending from both fibroblasts and elastic fibers that were close to the epithelium. Microfibrils coursed from them toward the lamina densa. Anchoring filaments joined the lamina densa to the basal plasma membrane of the epithelium. Elastic fibers also appeared to connect to both capillaries and collagen via finer intermediate structures. These associations might permit dynamic interactions of fibroblasts, fibers, smaller fibrillar components, vasa, and the duct epithelium. This interplay could occur during feeding and grooming when tongue protrusion and neck extension stretch the submandibular duct and the gland itself. As a result, the tensile forces engendered could modify cell geometry and the calibers of the larger ducts' lumens and intercellular spaces, thus affecting the flow and composition of salivary secretion.     

Simple method to isolate the intact duct system of the rat submandibular gland

The excurrent duct system of the rat submandibular gland consists of a number of distinct segments. Using the direction of salivary flow as a reference point, these segments are, in order, intercalated duct, granular convoluted tubule, striated duct, excretory duct, main excretory duct (MED), and salivary bladder (which is an expanded portion of the MED). Because these ducts (with the exception of the MED and the salivary bladder) are encased in secretory endpieces, they are difficult to locate and to observe by scanning electron microscopy. A simple method has been devised to rid the gland of these obscuring endpieces so that the detailed architecture of the duct system can be examined. Rat submandibular glands were fixed initially by vascular perfusion with half‐strength Karnovsky's fixative. The connective tissue capsule was removed from extirpated glands and the glands remained in fixative for varying lengths of time. For our purposes, a 30‐minute immersion in the aldehyde mixture was optimum. After the sublingual gland was removed, the submandibular gland was softy struck with forceps having rounded tips, then shaken in fixative or buffer. The tissue that remained was postfixed in osmium tetroxide. This method results in the complete divestment of nonductular parenchyma from the rat submandibular gland, leaving the duct system clean and ready for microscopic examination. Anat Rec 254:74–75, 1999. © 1999 Wiley‐Liss, Inc.     

Intraoral duct ligation without inclusion of the parasympathetic nerve supply induces rat submandibular gland atrophy

The atrophic effect of ligating the main duct of the right submandibular gland was examined in rat using a novel intraoral approach that did not include the chorda lingual (CL) nerve. Comparison was made with the effect of duct ligation including the attached CL nerve as carried out in previous studies. In all animals, the contralateral, unligated left submandibular gland was used as a control. At different times (1, 2, 7, 14 and 21 days) after ligation, glands were removed and weighed. Tissue was fixed for morphological analysis and homogenized for biochemical assay of secretory proteins. After 21 days, ligated glands showed a significant decrease in wet weight compared with unligated glands. Weight loss was the greatest (P < 0.05) in glands ligated with the CL nerve included. Light microscopy revealed that following ligation, an initial inflammatory reaction was followed by severe atrophy of acini and granular ducts. The atrophy was less severe when the CL nerve was not ligated. Secretory proteins were decreased from day 1 onwards following duct ligation in both groups. It can be concluded that most of the atrophy induced by duct ligation is independent of damage caused to the parasympathetic nerve supply, although the latter causes a greater atrophy presumably due to denervation.     

Uptake of cationized ferritin by the epithelium of the main excretory duct of the rat submandibular gland

Previous studies demonstrated that the main excretory duct (MED) of the rat submandibular gland can internalize exogenous protein in addition to reabsorbing and secreting electrolytes. However, more precise studies have not been conducted. The aim of this study was to elucidate the cell types responsible for endocytosis of an exogenous protein (ferritin) and to follow the movements of the endocytosed protein in the ductal epithelial cells. The MEDs of the right submandibular gland of male Wistar rats were exposed near the glands proper and cationized ferritin solution was injected into each MED through a fine glass cannula. The MEDs were removed at intervals after ferritin injection, fixed and examined by transmission electron microscopy. The epithelium of the MED of the rat submandibular gland was pseudostratified and consisted of light (types I and II), dark, tuft and basal cells. Uptake of ferritin by the light (types I and II) and dark cells occurred frequently. Small vesicles and multivesicular bodies containing ferritin particles were observed in the supra-nuclear and lateral nuclear cytoplasm. Endocytosis of tracers by tuft cells was rare. Some of the small vesicles and the multivesicular bodies were acid phosphatase-positive. By 60 min after treatment, ferritin-containing small vesicles and multivesicular bodies appeared in the basal cytoplasm. Ferritin particles were also observed in basal extracellular spaces. The light (types I and II), dark and tuft cells (latter rarely) participated in endocytosis of exogenous proteins in the epithelium of the MED of the rat submandibular gland. Almost all of the internalized proteins appeared to be processed by the lysosomal system, and some proteins were released into the extracellular spaces.     

Ultrastructure of the secretory response of male mouse submandibular gland granular tubules

The organization and fine structure of granular convoluted tubule cells (GCT) from male mouse submandibular glands have been examined in controls and in animals injected with adrenergic and cholinergic secretagogues. Control submandibular glands exhibited a single population of GCT cells with numerous homogeneous granules filling the apical two-thirds of the cytoplasm. A zone of transition cells, exhibiting characteristics of both GCT and striated duct cells, was found between the agranular intercalated duct and GCT segments. These transition cells possessed apical granules of variable size as well as prominent basal striations. Dramatic changes in the morphology of GCT cells followed administration of the alpha-adrenergic agent, phenylephrine. The extensive degranulation involved formation of "secretory pools" of fused granules and release of secretory material into the lumen. The appearance of numerous smooth vesicles near luminal membranes suggested extensive membrane retrieval. Intracellular membrane-limited aggregates of membrane fragments suggested that much of the retrieved membrane was destined for degradation. Rough endoplasmic reticulum was highly dilated but there was no indication of increased size or activity of the Golgi complex. Ultrastructural evidence indicated that the secretory responses to isoproterenol, a beta-adrenergic agent, and to pilocarpine, a cholinergic agent, were much more modest, but it is clear that some secretory response to these agents does occur. The other cell types of the duct and tubule system did not exhibit comparable morphological changes in response to the agents used.     

Reaction of the rat submandibular salivary gland to injury to the contralateral gland

After burns of resection of the submandibular salivary gland the intact contralateral gland in rats responds by increased proliferative activity. The number of mitoses reached a maximum 72 h after injury in the case of burns and 48 h after resection. Burns of the salivary gland cause lasting but weak compensatory hypertrophy of the contralateral gland. Hypertrophy of the gland is accompanied by an increase in size of the cells and nuclei, the area of which rises by 10 and 17% respectively. Resection of the salivary gland causes an increase in weight of the intact gland only in the early period of observation; by the 30th and 45th days after the operation the weight of the experimental glands was not significantly different from the control. Differences in compensatory growth of the intact glands observed after two types of injury of the contralateral gland evidently depend on the quantity of tissue breakdown products and the duration of their presence in the body.Department of Biology and General Genetics, Moscow Medical Stomatological Institute. (Presented by Academician of the Academy of Medical Sciences of the USSR A. P. Avtsyn.) Translated from Byulleten' Éksperimental'noi Biologii i Meditsiny, Vol. 82, No. 9, pp. 1108–1110, September, 1976.     

Effects of irradiation on the submandibular gland of the rat

Summary Single-dose cervical irradiation by cobalt 60 in rats induced lasting functional disturbances of the submandibular gland which were excessive when compaired with the relative integrity of the gland as seen under the light microscope. Enzyme histochemical and ultrastructural studies revealed severe damage shortly after exposure with appearance of karyolytic bodies and autophagosomes accompanied by increased hydrolase activity. Mitochondrial alterations were concomitant with diminished ductal oxidative enzyme activity. Although most of these alterations resolved rapidly as a result of acinar and ductal cell repair and regeneration originating in the intercalated ducts, secretory abnormalities were still observed two months after exposure as evidenced by the accumulation of granules in acinar cells and the heterogeneity of ductal cell granules. These anomalies, comparable to those observed in sialadenoses, probably result from persistent alterations of intralobular nerve endings.The authors wish to thank M.F. Baucher, A. Lesot and M. Tacnet for their technical assistance     

Perivascular nerves in the rat submandibular salivary gland.

Perivascular nerves in the rat submandibular salivary gland have been studied using a variety of histochemical procedures coupled with electron microscopy. Two principal nerve types, adrenergic and cholinergic, appear to predominate and are localized principally around arterioles. Venules are rarely innervated. The possibility that a non-adrenergic non-cholinergic nerve population might influence blood flow is discussed critically in the light of anatomical and physiological findings.     

Myoepithelial cell ultrastructure in the submandibular gland of man

Summary In human submandibular glands, two types of myoepithelial cells can be distinguished in serial, ultrathin sections. The dark myoepithelial cell type was stellate in shape and exhibited a pronouneced electron density due to numerous myofilaments with focal densities. Dark cell types accounted for the greater part (76%) of the myoepithelial cells and furthermore showed adenosine triphosphatase activity. This type of myoepithelial cell is considered to be that previously observed in mammalian salivary glands. Occasionally, desmosomes could be found between the processes of adjacent dark myoepithelial cell types, which is appropriate with respect to the strong compression of acinar or intercalated duct cells. The light myoepithelial cell type was large and ellipsoid with a few short-thick processes, and was characterized by an electron lucent cytoplasm which included scant and unevenly distributed myofilaments. Light cell types showed positive adenosine triphosphatase activity and accounted for only a small part (17%) of the myoepithelial cell number. Transitional forms between these two types were also observed. The light myoepithelial cell type may mature into the dark myoepithelial cell type by means of the transitional form. In addition, clear cells were sometimes encountered between the myoepithelial cell and the acinar or intercalated duct cells.