Effects of castration and sex steroids on sexually dimorphic development of the mouse submandibular gland

The aims of this study were to characterize sexual dimorphism in the submandibular glands of young adult mice and to determine how sex differences arise during postnatal development. In the mouse submandibular glands, prominent sexual dimorphism was observed at 30 days of age, when the male gland was superior in both the relative occupied area (ROA) and the mitotic rate of the granular convoluted tubules (GCT) to those of the female. By neonatal castration, this sexual dimorphism was abolished, and the intraglandular structures of castrated males were similar to those of normal females. In castrated mice of both sexes, daily treatment with testosterone and 5 alpha-dihydrotestosterone for 10 days from 20 days induced only the ROA of the GCT to increase to the normal male levels but not those of the other three regions of the glands, the acini, intercalated ducts and excretory striated ducts. Testosterone responsiveness of the glands, considering both the glandular weight gain and the mitotic rate of the GCT, was significantly higher in castrated males than in castrated females. On the other hand, 17 beta-estradiol had no effect on the glands of castrated mice. Therefore, the present study suggests that the testicular hormones are responsible for the masculine development of GCT of the glands, but not the ovarian hormones, and that there is a sex difference in the responsiveness of the glands to testosterone, which is more effective in males than in females.     

Effects of a restricted diet on the submaxillary gland of intact and castrated male rats

Effects of diet restriction (50% of normal) during 4 weeks were studied in intact (controls) and castrated rats. A restricted diet led to cessation of animal growth. No differences were observed between controls and castrated rats. However, underfeeding produced a reduction of testicular weight in intact rats. Histochemical observations showed a reduction of tryptophan (a marker of the presence of testosterone-dependent protein) in the submaxillary gland from animals fed a restricted-diet. In animals fed a normal diet, castration reduced significantly the gland weight, as well as the diameter of tubules and acini. When animals fed a restricted-diet are compared, no significant differences were found in the diameter of tubules and acini or in the weight of submaxillary glands.     

Immunocytochemical localization of seminal proteins in salivary and lacrimal glands of the rat

Antibodies against 10 different secretory proteins from the accessory sex glands of the male rat were used for immunohistochemical studies of salivary and lacrimal glands from intact and castrated rats, at the light- and electron-microscopic levels. In the parotid gland, secretory acinar cells showed immunoreactivity with antibodies against prostatic binding protein, cystatin-related peptide and acid phosphatase (isoenzyme pI 8.0; 5.6) typical of ventral prostate, and seminal vesicle secretion VI. Western blotting analysis indicated that immunoreactivity against prostatic binding protein was attributable to a subunit, presumably C3. Acid phosphatase pI 5.6 showed a molecular weight of 66 kDa, which is at variance with the prostatic form. Immunoreactivity for secretory transglutaminase, derived from the coagulating gland, was restricted to myoepithelial and stromal cells. In castrated animals, the immunoreactivity of acinar cells was reduced to the background level, whereas stromal transglutaminase immunoreactivity was unaltered. The distribution pattern of immunoreactivity for the proteins mentioned was almost identical in the lacrimal gland. Significant differences were however observed in the immunoreactivity of the inframandibular gland, where serous glandular cells were non-immunoreactive for seminal proteins, with the exception of acid phosphatase isoenzyme pI 8.0. Granules present in the convoluted granular ducts were immunoreactive particularly for acid phosphatase (isoenzyme pI 5.6)but much less for cystatin-related peptide; immunoreactivity was reduced after castration. The straight portion of the inframandibular duct system was immunoreactive for transglutaminase, but no influence of castration was visible. The distribution of immunoreactivity for seminal proteins present in the salivary and lacrimal glands and the pronounced androgen-dependence of their expression point to functional relationships of the respective proteins at both glandular sites.     

Ultrastructural aspects of cat submandibular glands

Submandibular glands of five adult female cats were examined by conventional electron microscopic techniques. All gland acini are mucous secreting and each acinus is capped with mucous secreting demilunar cells. Secretory product of demilunar cells is more electron lucent than that of acinar cells. The demilunes show intercellular tissue spaces and intercellular canaliculi whereas similar specializations are absent between acinar cells. Mitochondria and arrays of granular endoplasmic reticulum are more numerous in demilunar cells than in acinar cells. In acinar and demilunar cells secretory droplets first appear as enlarged Golgi saccules which subsequently become closely related to cisternae of the granular endoplasmic reticulum. Filamentous structures, interpreted as mucin molecules, are present in secretory droplets of acinar cells. Intercalated ducts are short, consisting of several junctional cells between acini and striated ducts. Striated ducts are long and tortuous and contain light cells, dark cells and basal cells. Light cells contain numerous membrane bound granules in their distal ends whereas dark cells show electron lucent vesicles in the same position. Basal cells contain a paucity of organelles and membrane plications but exhibit hemidesmosomes along their basal plasma membranes. Myoepithelial cells are abundant in relation to acinar and demilunar cells. Nerve terminals are present in some instances between acinar cells or between acinar and myoepithelial cells.     

Prenatal development of the human submandibular gland

The prenatal development of the human submandibular gland has been investigated in 26 fetuses from the 10th week of gestation to full term. At 10-12 weeks, the glandular elements (primitive ducts and acini) were immature and surrounded by a loose mesenchyme. The acinar cell population increased gradually till the age of 32 weeks, and the rate of increase was diminished thereafter. At 16 weeks, intercalated and striated ducts were distinguished and their number increased till the age of 32 weeks when their number seemed to be stabilized. The development of the granular convoluted tubule cells from the proximal segments of striated ducts occupied the later stages of development. They appeared around the age of 20 weeks and proceeded till full term. At birth, the gland appeared devoid of mucous acini and fat cells and the secretory end-pieces were of the serous type. During the second trimester, periodic acid-Schiff- and alcian blue-positive secretory materials appeared in the epithelial cells of both ducts and acini, and in their lumina. This secretory activity was transitory and disappeared around the age of 28 weeks. The possible function of these secretory products is discussed.     

Rate of protein synthesis in rat salivary gland cells after pilocarpine or feeding

In untreated, fasting animals the cells of the serous demilunes of the sublingual gland incorporate [3H]-leucine at a higher rate than any other of the 5 main cell types of the 3 major salivary glands. The acinar cells of the submandibular and the mucous cells of the sublingual gland show intermediate values, while the cells of the granular ducts of the submandibular and the acini of the parotid gland have a low rate of incorporation. In fasting animals extrusion of newly synthesized protein starts early in the cells of the serous demilunes. It starts between 4 and 7 hrs after [3H]-leucine injection in the acinar cells of the submandibular gland, while the other cell types did not lose substantial amounts of labelled (glyco)protein within 7 hrs. The secretion of protein is stimulated by the cholinergic drug pilocarpine in all but one of the 5 types of salivary gland cells studied. The acinar cells of the submandibular gland react strongly, the granular duct cells less strongly. Still less are the reactions of the acinar cells of the parotid and of the nucous cells of the sublingual gland. The cells of the serous demilunes of the latter appear to be insensible to pilocarpine. The effect of food uptake on secretion does not differ from pilocarpine stimulation, with one exception: the acinar cells of the parotid gland react more strongly on food uptake than on cholenergic stimulation.     

Occurrence and Nuclear Localization of cAMP Response Element- Binding Protein in the Post-natal Development of the Rat Submandibular Gland

Cyclic AMP response element-binding protein (CREB) is a 43-kDa polypeptide that binds a cAMP response element located at the 5 promoter region of cAMP regulatory genes. The spatial and temporal distribution of CREB in the post-natal development of the rat submandibular gland was investigated using immunohistochemistry with a specific antibody. At birth, cells of the terminal tubules and ducts in the submandibular gland showed a nuclear CREB immunoreactivity of moderate intensity. At 1–2 weeks after birth, an intense CREB immunoreactivity was localized primarily to acinar cells. When the r352;-adrenergic agonist isoproterenol was administered to 2-week-old rats, a twofold transient increase in the number of immunoreactive acinar cells was induced. Beginning 3 weeks after birth, CREB immunoreactivity shifted from acini to the duct system and showed a clear localization in the cells of the intercalated ducts and distal portions of striated ducts, where the granular convoluted tubule develops after 4 weeks. Immunopositive materials were localized exclusively in the nuclei of both acinar and ductal immunoreactive cells. After the development of the granular convoluted tubules, CREB immunoreactivity was absent in the tubule cells and was gradually reduced in intensity over the entire gland. In order to examine a hypothesis that CREB is involved in the initial differentiation of the granular convoluted tubular cells, testosterone was administered to hypophysectomized adult rats. Whereas the tubular cells of hypophysectomized rats showed a complete regression, and no CREB immunoreactivity was found in any acinar or duct cells, administration of testosterone for a few days induced an intense CREB immunoreactivity in the nuclei of duct cells, followed by their differentiation into the granular convoluted tubular cells. These results suggested that CREB is involved not only in the growth and differentiation of acinar ce lls that are regulated by r352;-adrenergic nerves but also in those of the duct system, and especially in the androgen-regulated differentiation of the granular convoluted tubular cells, during the post-natal development of the rat submandibular gland.     

Expression and localization of immunoreactive-sialomucin complex (Muc4) in salivary glands

Sialomucin Complex (SMC; Muc4) is a heterodimeric glycoprotein consisting of two subunits, the mucin component ASGP-1 and the transmembrane subunit ASGP-2. Northern blot and immunoblot analyses demonstrated the presence of SMC/Muc4 in submaxillary, sublingual and parotid salivary glands of the rat. Immunocytochemical staining of SMC using monoclonal antisera raised against ASGP-2 and glycosylated ASGP-1 on paraffin-embedded sections of parotid, submaxillary and sublingual tissues was performed to examine the localization of the mucin in the major rat salivary glands. Histological and immunocytochemical staining of cell markers showed that the salivary glands consisted of varying numbers of serous and mucous acini which are drained by ducts. Parotid glands were composed almost entirely of serous acini, sublingual glands were mainly mucous in composition and a mixture of serous and mucous acini were present in submaxillary glands. Since immunoreactive (ir)-SMC was specifically localized to the serous cells, staining was most abundant in parotid glands, intermediate levels in submaxillary glands and least in sublingual glands. Ir-SMC in sublingual glands was localized to caps of cells around mucous acini, known as serous demilunes, which are also present in submaxillary glands. Immunocytochemical staining of SMC in human parotid glands was localized to epithelial cells of serous acini and ducts. However, the staining pattern of epithelial cells was heterogeneous, with ir-SMC present in some acinar and ductal epithelial cells but not in others. This report provides a map of normal ir-SMC/Muc4 distribution in parotid, submaxillary and sublingual glands which can be used for the study of SMC/Muc4 expression in salivary gland tumors.     

Active participation of apoptosis and mitosis in sublingual gland regeneration of the rat following release from duct ligation

Summary This study was designed to establish how mitotic cell proliferation and apoptotic cell death participate in the regeneration of atrophied rat sublingual glands. To induce atrophy to the sublingual gland of rats, the excretory duct was ligated unilaterally near the hilum, and after 1 week of ligation (day 0) the duct ligation was released to enable gland regeneration. The regenerating glands were examined with routine histology, immunohistochemistry for proliferating cell nuclear antigen (PCNA) as a marker of proliferating cells, terminal deoxynucleotidyl transferase-mediated dUTP-digoxigenin nick end labeling (TUNEL) as a marker of apoptotic cells, and transmission electron microscopy. At day 0, a few acini and many ducts remained in the atrophic sublingual glands, and newly formed immature acini were observed at day 3. Thereafter acinar cells progressively matured and increased in number, although the number of ducts decreased. Many PCNA- and some TUNEL-positive cells were seen in acini and ducts during regeneration. The labeling indices for both cell types were statistically significantly different from that of the control at several time points of the regeneration. Apoptotic and mitotic cells were also confirmed to be present in the experimental sublingual glands by electron microscopy. These observations suggest that apoptosis as well as mitosis of duct and acinar cells actively participate in and play important roles in sublingual gland regeneration.     

Sexually dimorphic duct system of the submandibular gland in mouse with testicular feminization mutation (Tfm/Y).

The X-linked testicular feminization mutation (Tfm/Y) in the mouse is characterized by androgen insensitivity of the target cells. The aim of this study was to examine sexually dimorphic development of the submandibular gland of Tfm/Y mutant mice in comparison with those of wild-type male, wild-type female and heterozygous Tfm female mice. In either 30- or 90-day-old wild-type male mice, the granular convoluted tubules (GCT) of the glands were more developed, and the relative occupied areas (ROA) of GCT were superior to those of the age-matched wild-type and heterozygous Tfm females. In androgen-insensitive Tfm/Y mice, the glandular structures rather resembled the female glands, showing lower values of the ROA of the GCT. Sex differences in the mitotic rate were observed at 30 days of age, being significantly higher in the wild-type male GCT than in the female GCT. Thereafter, the mitotic rate of the wild-type male GCT declined to the female levels by 90 days of age. The mitotic rate of GCT in Tfm/Y mutants was as low as those of the females during observation periods. An other three regions, the acini, the intercalated ducts and the excretory striated ducts, were not significantly different in either the ROA or the mitotic rate among wild-type males and females, and Tfm/Y. On the other hand, either the ROA or the mitotic activity of GCT of the glands in Tfm/Y mutants was completely unaffected by 5 alpha-dihydrotestosterone (DHT).(ABSTRACT TRUNCATED AT 250 WORDS)     

Harderianization is another sexual dimorphism of rat exorbital lacrimal gland

The exorbital lacrimal glands (ELG) of rats were examined for both sexes to determine what degree of harderianization occurred as a function of age and after castration, and to investigate its time course and origin in ELG. Light microscopically, very small Harderian foci were seen in the ELG of both sexes at 3 weeks of age. As the male rats became older, the relative volume of the Harderian gland (HG) cells in the ELG increased. At age 6 months, the value was 1.25 +/- 0.31% in males and 0.13 +/- 0.05% in females (p less than 0.05). After castration, a significant decrease (0.21 +/- 0.01%, p less than 0.05) was observed in that of male ELG. In contrast, in female ELG, HG cells were inconspicuous and the relative volume of those did not vary during this experimental period or after castration. It appeared that the HG cells had developed from undifferentiated basal cells of the acini and the intercalated ducts in the ELG at age 2-6 months. Then, at age 22 months, they also probably developed from those of the excretory ducts of the ELG.     

Properties of Go cells: variations in the proliferative response following isoprenaline.

The proliferative behaviour induced in the acinar cells of the rat submaxillary gland in response to isoprenaline has been used to examine the transit time of cells from a quiescent (Go) state into the S phase. Cumulative 3H-TdR labelling index curves were constructed to determine the mean time interval (Gis time) between stimulation with isoprenaline and entry into the S phase. Data were collected for the proliferative wave induced by three sequential injections of isoprenaline, and the effects of varying the interval between the second and third injections of isoprenaline, and of changing the dose of the drug, were examined. Intervals of 28, 52 and 76 hr between isoprenaline injections resulted in mean Gis times of 16-2, 20-9 and 25-6 hr respectively. It was concluded that the Gis time depended on the recent history of cells with respect to stimulation, but not division. The results are considered in terms of two models, in one of which the time to leave Go is variable, whilst in the other the cells leave Go immediately the stimulus is applied.     

Compensatory proliferative response of the rat submandibular salivary gland to unilateral extirpation

The present experiment was conducted in order to identify the progenitor compartment of the submandibular salivary gland (SSG) and to explore the proliferative activity of this gland in response to unilateral extirpation. Left submandibular and retrolingual glands were extirpated in 30 rats (B.W. 200 +/- 12 g). The rats were killed 0, 1, 2, 3, 5 and 7 days after surgery. Five intact rats served as controls. The animals were given intraperitoneal injections of 3HTdR (0.5 microCi/grB. W) 1 h before they were killed. The contralateral SSG's were subjected to routine histological procedures and embedded in glycol methacrylate. Selected sections (2 micron thickness) were processed for autoradiography. In each gland, labelled and unlabelled nuclei were counted in 50 random microscopic fields and sorted according to their parenchymal histomorphological features and "nuclear class" (number of nuclei/cross section/feature). In the control glands the total labelling index (LI) was 0.18%; during acute compensatory stimulation, however, the total LI reached a maximum of 0.86% on day 3 after surgery. suggesting that the SSG, which normally undergoes a slow turnover, is capable of elevated proliferation in response to a stimulus. In both normal and stimulated glands, the LI was higher in the intercalated ducts (1.1%-5.85%) than in the granular ducts (0.17%-0.93%) and acini (0.05%-0.36%). This consistency of LI ratio between the various histomorphological features in the normal and experimental glands indicates that the glandular progenitor compartment is located in the intercalated ducts, which supply cells to both the ductal system and acini.(ABSTRACT TRUNCATED AT 250 WORDS)     

Sex-related expression of sialic acid acceptor sugars in the mouse submandibular gland. Simultaneous visualization by confocal laser scanning microscopy.

The novel combination of sialidase digestion with simultaneous PNA and DBA binding yielded marked differences on sialoglycoconjugate occurrence and distribution in the mouse submandibular gland acinar cells of the two sexes. Striking differences in the structure of terminal disaccharides within stored secretory sialoglycoconjugates were also found. High content of sialic acid, characterized by the terminal sequence sialic acid-alpha-N-acetylgalactosamine, was established to only occur in the male acini where secretory cells appeared to be differently stained; indeed, some cells exhibited codistribution of sialic acid-alpha-N-acetylgalactosamine and sialic acid-beta-galactose terminal disaccharides, whereas other ones exclusively contained one of the two kinds of terminal sequences. In the female acinar cells, the secretory products were found to be almost exclusively composed by glycoconjugates having sialic acid subtended to beta-galactose without appreciable differences between acinar cells. Our finding of such extensive differences in the acinar cells of male and female mice adds new insights into the submandibular gland sexual dimorphism, commonly attributed to the androgen responsiveness of the granular convoluted tubule portion of the gland.     

Antimitotic activity of EA21b mammary-carcinoma extract

Many tumors produce factors that affect cell-cycle and cell proliferation. In the present study we have analyzed the effect of a mammary-tumor extract injection on the mitotic activity of several organs in young male C3H/S mice previously standardized for circadian periodicity. One-half of the animals received an intraperitoneal EA21b tumor extract dose at 16:00 h, while the other half received saline. Animals were sacrificed on the following day at 08:00, 12:00 or 16:00 h. 4 h after receiving an injection of colchicine by the same route. Samples of duodenum, kidney, liver, and submaxillary gland were excised and processed for hematoxylin-eosin staining. Mitotic indices, expressed as the number of colchicine-arrested metaphases per 1,000 nuclei, were assessed in convoluted tubule epithelium, duodenal crypt enterocytes, hepatocytes and submaxillary gland ductal and acinar sialocytes. All values were expressed as mean ± SEM. Statistical analyses were performed by ANOVA, Bonferroni and Student’s t-tests. In contrast to the mitotic indices reductions observed in renal convoluted tubules cells and duodenal crypt enterocytes, neither the submaxillary gland nor the liver were found to contain cell types whose mitotic activity was affected by the tumor extract. We conclude that EA21b mammary carcinoma contains one or more factors that inhibit the proliferation of selected populations of normal cells.     

The streaming of the submandibular gland II: parenchyma and stroma advance at the same velocity

Abstract Thirty young male rats aged 7 weeks, weighing 200 g, were injected with 18.5 kBq g⁻¹ (0.5 μCi g⁻¹) body weight tritiated thymidine [³H]TdR (specific activity 185 GBq). The rats were then killed in groups of five, at the following times: 1 hour, and 14, 30, 60, 90, and 120 days. Autoradiograms of sections through the submandibular gland were prepared, and the location of labelled cells in tubular and acinar cross sections was recorded. The nuclear content of each cross section was defined as its 'class'. In this numbering system, narrow tubuli, e.g. intercalated ducts are of low class, and wider tubuli, e.g. striated ducts, of high class. One hour after labelling most labelled tubular cells were found in low class cross sections, i.e. intercalated ducts and narrow granular ducts. Striated ducts were not labelled. From then onward labelled cells entered wider tubuli, e.g. striated ducts. The advancing labelled epithelium was accompanied by labelled stroma. Both cell types traversed 0.089 classes per day. In acini, labelled cells advanced in the oposite direction, starting from acinar cross sections of high class and ending in class-1 cross sections.     

Localization and in situ Quantification of 5-hydroxytryptamine and its Receptor in Rat Submaxillary Gland

5-Hydroxytryptamine (5-HT) and its receptor have been localized and quantified in the submaxillary gland of rats of various ages, using immunohistochemistry, in situ hybridization and in situ quantification. In male rats, the epithelial cells of serous acini, intercalated ducts, secretary tubes and excretory ducts all showed 5-HT and 5-HT receptor (5-HTR) immunoreactivity. Both 5-HT and 5-HTR reactive sites were found in the same cells of adjacent sections. 5-HT1A receptor mRNA hybridized signals could be detected in cytoplasm of these cells. The parasympathetic ganglia cells and endothelial cells of small vessels also showed 5-HT and 5-HTR immunoreactivity in the cytoplasm. However, in female rats, only the epithelial cells in excretory tubes showed 5-HT and 5-HTR immunoreactivity. The immunoreactivity was present in the same cells of adjacent sections. The relative content of 5-HT and its receptor increased during the first 60 postnatal days but remained constant from day 60 to day 90 postnatum. These results suggest that the submaxillary gland of rats possess autocrine 5-HT, which may regulate the function and development of the gland.     

Ultrastructure of bovine parotid glands

Bovine parotid glands exhibit outstanding structural differences when compared with those of non-ruminant mammals. The acini are tortuous, branched and lined with cells of different heights, imparting a scalloped appearance to acinar lumina. Numerous microvilli, ca. 1.5 μ in length, extend into the lumina and intercellular canaliculi. Intercellular canaliculi measure ca. 3 μ in diameter and interweave in close association with intercellular tissue spaces. Intercellular tissue spaces are separated from the extraacinar spaces across a basal lamina only, whereas junctional complexes guard canaliculi from direct continuity with tissue spaces and/or extraacinar spaces. Flattened cytoplasmic lamellae extend from adjacent acinar cells and loosely interdigitate with one another across the tissue spaces. Acinar cells contain more mitochondria and less granular endoplasmic reticulum than parotid glands of non-ruminant mammals. Two types of secretory material, in the form of inclusions which vary in size and electron density, are present in the acinar cells. Intercalated ducts connect acini with striated ducts which in turn, empty into collecting ducts located between gland lobules. In terms of frequency of “basal infoldings” and numbers of mitochondria, striated ducts of calf parotid glands are not as well developed as those of certain other salivary glands. Myoepithelial cells are most often present at junctions of acini and intercalated ducts where they may attach to both acinar and ductal epithelium. Nerve “terminals” were not observed on the epithelial side of basement membranes in relation to the secretory cells.     

Expression of transforming growth factor beta 1 in the submandibular gland of the rat

We employed immunocytochemical and in situ hybridization techniques to study the expression of transforming growth factor beta 1 (TGF-beta 1) in rat submandibular gland. Immunoreactivity for TGF-beta 1 was observed in the cells of granular convoluted tubules (GCTs), striated ducts, and excretory ducts, whereas it was absent in the intercalated ducts and secretory acini in both male and female rats. Immunoelectron microscopy revealed the ultrastructural localization of TGF-beta 1 in the secretory granules of GCT cells. On the other hand, signals for rat TGF-beta 1 mRNA were abundant in the GCT and striated duct cells but were lacking in the excretory duct cells. These results provided evidence for the production of TGF-beta 1 in the GCTs and striated ducts of rat submandibular gland.     

Effects of double ligation of Stensen's duct on the rabbit parotid gland

Salivary gland duct ligation is an alternative to gland excision for treating sialorrhea or reducing salivary gland size prior to tumor excision. Duct ligation also is used as an approach to study salivary gland aging, regeneration, radiotherapy, sialolithiasis and sialadenitis. Reports conflict about the contribution of each salivary cell population to gland size reduction after ductal ligation. Certain cell populations, especially acini, reportedly undergo atrophy, apoptosis and proliferation during reduction of gland size. Acini also have been reported to de-differentiate into ducts. These contradictory results have been attributed to different animal or salivary gland models, or to methods of ligation. We report here a bilateral double ligature technique for rabbit parotid glands with histologic observations at 1, 7, 14, 30, 60 days after ligation. A large battery of special stains and immunohistochemical procedures was employed to define the cell populations. Four stages with overlapping features were observed that led to progressive shutdown of gland activities: 1) marked atrophy of the acinar cells occurred by 14 days, 2) response to and removal of the secretory material trapped in the acinar and ductal lumens mainly between 30 and 60 days, 3) reduction in the number of parenchymal (mostly acinar) cells by apoptosis that occurred mainly between 14–30 days, and 4) maintenance of steady-state at 60 days with a low rate of fluid, protein, and glycoprotein secretion, which greatly decreased the number of leukocytes engaged in the removal of the luminal contents. The main post- ligation characteristics were dilation of ductal and acinar lumens, massive transient infiltration of mostly heterophils (rabbit polymorphonuclear leukocytes), acinar atrophy, and apoptosis of both acinar and ductal cells. Proliferation was uncommon except in the larger ducts. By 30 days, the distribution of myoepithelial cells had spread from exclusively investing the intercalated ducts pre-ligation to surrounding a majority of the residual duct-like structures, many of which clearly were atrophic acini. Thus, both atrophy and apoptosis made major contributions to the post-ligation reduction in gland size. Structures also occurred with both ductal and acinar markers that suggested acini differentiating into ducts. Overall, the reaction to duct ligation proceeded at a considerably slower pace in the rabbit parotid glands than has been reported for the salivary glands of the rat.