A closer look into the cellular and molecular biology of myoepithelial cells across various exocrine glands.

Myoepithelial cells (MECs) are a unique subset of epithelial cells that possess several smooth muscle cell characteristics, such as a high number of actin-myosin filaments and the ability to contract. These cells are primarily located around the secretory cells of exocrine glands, including the salivary, mammary, lacrimal, and sweat glands. Their primary functions involve the construction of the basement membrane and help with secretion of gland products through contraction. So far, no comparative analysis of MECs in different exocrine glands had ever evaluated their differences. In this review, we took advantage of the various publicly available scRNAseq data from mouse exocrine glands to identify their shared and unique characteristics. The aim of this review is to compare the role of MECs in maintaining healthy glandular function, their involvement in disease states, and their regenerative capacity, with a particular emphasis on the latest research findings in these areas.

Exocrine gland-resident memory CD8+ T cells use mechanosensing for tissue surveillance.

Tissue-resident CD8+ T cells (TRM) continuously scan peptide-MHC (pMHC) complexes in their organ of residence to intercept microbial invaders. Recent data showed that TRM lodged in exocrine glands scan tissue in the absence of any chemoattractant or adhesion receptor signaling, thus bypassing the requirement for canonical migration-promoting factors. The signals eliciting this noncanonical motility and its relevance for organ surveillance have remained unknown. Using mouse models of viral infections, we report that exocrine gland TRM autonomously generated front-to-back F-actin flow for locomotion, accompanied by high cortical actomyosin contractility, and leading-edge bleb formation. The distinctive mode of exocrine gland TRM locomotion was triggered by sensing physical confinement and was closely correlated with nuclear deformation, which acts as a mechanosensor via an arachidonic acid and Ca2+ signaling pathway. By contrast, naïve CD8+ T cells or TRM surveilling microbe-exposed epithelial barriers did not show mechanosensing capacity. Inhibition of nuclear mechanosensing disrupted exocrine gland TRM scanning and impaired their ability to intercept target cells. These findings indicate that confinement is sufficient to elicit autonomous T cell surveillance in glands with restricted chemokine expression and constitutes a scanning strategy that complements chemosensing-dependent migration.

Substances in the mandibular glands mediate queen effects on larval development and colony organization in an annual bumble bee.

Social organization is commonly dynamic, with extreme examples in annual social insects, but little is known about the underlying signals and mechanisms. Bumble bee larvae with close contact to a queen do not differentiate into gynes, pupate at an earlier age, and are commonly smaller than siblings that do not contact a queen. We combined detailed observations, proteomics, microRNA transcriptomics, and gland removal surgery to study the regulation of brood development and division of labor in the annual social bumble bee Bombus terrestris. We found that regurgitates fed to larvae by queens and workers differ in their protein and microRNA composition. The proteome of the regurgitate overlaps significantly with that of the mandibular (MG) and hypopharyngeal glands (HPG), suggesting that these exocrine glands are sources of regurgitate proteins. The proteome of the MG and HPG, but not the salivary glands, differs between queens and workers, with caste-specificity preserved for the MG and regurgitate proteomes. Queens subjected to surgical removal of the MG showed normal behavior, brood care, and weight gain, but failed to shorten larval development. These findings suggest that substances in the queen MG are fed to larvae and influence their developmental program. We suggest that when workers emerge and contribute to larval feeding, they dilute the effects of the queen substances, until she can no longer manipulate the development of all larvae. Longer developmental duration may allow female larvae to differentiate into gynes rather than to workers, mediating the colony transition from the ergonomic to the reproductive phase.

Morphology of the Mandibular and Intramandibular Glands of Army Ant Workers of Labidus praedator (Smith 1858) and Labidus coecus (Latreille 1802) (Formicidae: Dorylinae).

Social insects are characterized by having a wide diversity of exocrine glands, with highlights for ants with about 85 glands spreading throughout the body. The mandibular and intramandibular glands are associated with the production of pheromones. The army ants (Dorylinae) play an important role in the structure of the invertebrate community because they are efficient predators and provide suitable conditions for various animals following their invasions in the food search. Labidus coecus (Latreille) is an underground-ameliorating ant and Labidus praedator (Smith) is a generalist surface predator which can deplete invertebrate biomass by up to 75%. This work investigated the morphology of the mandibular and intramandibular glands of L. praedator and L. coecus workers. The glands were analyzed by light microscopy, histochemistry, and scanning electron microscopy. The mandibular and intramandibular glands of the two species were classified as class III glands. The data on the morphology of the mandibular glands has revealed that they have characteristics in common with other subfamilies. The intramandibular glands of the two species of Labidus have similar morphology and chemical composition, which indicates that the components of these glands can have the same function despite their different habits.

Morphology and ultrastructure of the mandibular gland in Camponotus japonicus.

The mandibular gland is an important exocrine gland connected to the mandibles. In ants, it plays a crucial role in alarm communication and reproduction. While the overall morphology of the mandibular gland does not vary much between ant species, significant differences exist in the ultrastructural characteristics and gland size in some reported cases. However, there have been few systematic comparative studies on this subject. In this paper, we conducted microscopic observations to analyze the morphological and ultrastructural similarities and differences of the mandibular gland in different castes of Camponotus japonicus. Our findings revealed that the mandibular glands of males were significantly larger than those of females. In terms of relative size, within the female castes the mandibular glands of minor workers were the largest. Additionally, our observations of the ultrastructure showed that secretory substances were most abundant in queens. All female castes and males showed the presence of crystalline structures in the secretory cells. The content of these biocrystals decreased from minor workers to major workers, queens, and males. In males, however, crystalline structures were rare and the most abundant substances are droplet-like secretions. The differences in morphology and ultrastructure between different castes suggest that the function of the mandibular gland varies among castes. In conclusion, our study provides insights into the morphological and ultrastructural characteristics of the mandibular gland in different castes of C. japonicus. Further research is needed to fully understand the specific functions and mechanisms of this gland in ants.

Morphology of the abdominal segmental glands and spinning behaviour of Stenus larvae (Coleoptera, Staphylinidae).

We focus on the morphology of the "segmental glands" and their openings in third instar Stenus larvae. The location of the openings was similar in both studied species, with paired rosette-like structures present on the head, all thoracic segments and abdominal segments 1-9. No such openings could be found on the antennae, the maxillary palps, the urogomphi, and the legs as suggested in some older publications. We presume that the glands up to abdominal segment 7 are "adhesive" glands. They are compound glandular units consisting of a secretory syncytium with a common reservoir and a canal cell. The common reservoir is connected through a single efferent duct with the opening of the gland. Glands of abdominal segments 8 and 9 show differences in their length, number of reservoirs, the orientation of the efferent canal, the inner structures of the gland openings towards the exterior and the shape and content of the secretion vesicles indicating that they are silk glands for cocoon building. The spinning behaviour has been observed during the building of the hatching and pupation retreats. The larva first attaches to the substrate with its pygopod, secretes silk droplets from silk gland openings and pulls out a silk filament from the tip of its urogomphi. Whereas L1 and L2 instars produce an open single-layered net, L3 build a closed bi-layered cocoon.

Morphology of the metapleural gland and its associated novel atrial cone gland in Strumigenys ants.

The metapleural gland is a unique feature of the ant phenotype, but diversity in its anatomy and function across the ants is not well documented or understood. We studied the morphology of the metapleural gland in 20 mainly Oriental Strumigenys species using histology, scanning and transmission electron microscopy, and micro-CT. The gland is formed by a cluster of class-3 cells, their secretion is transported through a bundle of ducts into the sclerotized atrium and is guided along a series of parallel cuticular ridges towards the atrial opening. Among the examined species, queens have more gland cells than conspecific workers, while the examined males do not have the gland. The social parasite S. mutica has the most developed metapleural gland. In addition, we describe a novel class-1 atrial cone gland which is associated with the metapleural gland. The epithelium of this cone gland forms an invagination into the ventral atrium of the metapleural gland. The cuticular cone may be lacking in some Afrotropical and Neotropical species, although these may still contain the epithelial gland. The functional and evolutionary drivers of morphological variation in the exocrine system across species form interesting questions for future work.

The spongiform tissue in Strumigenys ants contains exocrine glands.

The insect cuticle is multifunctional and often includes projections used for support, communication or protection. Ants in the genus Strumigenys exhibit a peculiar honeycomb-like spongiform tissue that covers their petiole, postpetiole and sometimes also the posterior mesosoma and anterior part of the first gastral segment. The tissue is abundantly developed in workers and queens, and much reduced in males. We found this spongiform tissue is associated with a novel exocrine gland that is made up by class-3 secretory cells that are clustered underneath the major pillars of the cuticular extensions, their associated narrow ducts enter these extensions and open at the surface through small pores. The chemical nature and function of the secretion are still unknown. The honeycomb texture may act in the storage and dispersion of the glandular secretions. In addition to the spongiform tissue gland, the posterior region of the petiole and postpetiole also contain intersegmental petiole and postpetiole glands, of which the ducts open through the intersegmental membrane that forms the connection with the next segment. Future work aimed at identifying the chemicals secreted by these glands will shed light onto the function of these unusual structures.

Comparative morphology and ultrastructure of the labial gland among castes of Camponotus japonicus (Hymenoptera: Formicidae).

The labial gland is a major exocrine gland in the thoracic cavity of ants, which is connected to the mouth area and opens at the base of the labium. The labial gland in the various castes and males of Camponotus japonicus were examined using dissection, light microscopy, scanning and transmission electron microscopy. Our findings reveal clear caste differences in the appearance of the labial gland, and suggest a relationship between gland structure and its behavioral role in queens, males and workers: queens possess the largest reservoir sac; males have the smallest; workers have abundant secretion droplets in the gland cells of the tubules; epithelial thickness of gland cells varies considerably between workers and reproductive ants. The apical cell region has a high density of microvilli and polymorphic mitochondria, whereas the central cell region is rich in rough endoplasmic reticulum (RER) which means its main secretions are proteinaceous compounds. We believe that the labial gland has a different function in the castes of C. japonicus: the gland of workers and queens may be used to feed the larvae; workers participate in trophallaxis in the nest, while the males are not involved in these activities. Calculations of the relative size of the labial gland for each caste and males indicated that minor workers have the relative highest developed labial gland, supporting a role in trophallaxis. In addition, we found a phenomenon of enlarged labial gland in minor workers, which accounted for almost 22%, but the reason for this is unknown.

Aquaporins in Glandular Secretion.

Exocrine and endocrine glands deliver their secretory product, respectively, at the surface of the target organs or within the bloodstream. The release of their products has been shown to rely on secretory mechanisms often involving aquaporins (AQPs). This chapter will provide insight into the role of AQPs in secretory glands located within the gastrointestinal tract, including salivary glands, gastric glands, duodenal Brunner's glands, liver, gallbladder, intestinal goblets cells, and pancreas, as well and in other parts of the body, including airway submucosal glands, lacrimal glands, mammary glands, and eccrine sweat glands. The involvement of AQPs in both physiological and pathophysiological conditions will also be highlighted.

Morphology and ultrastructure of the prepharyngeal and pharyngeal glands in the ant Camponotus japonicus.

The prepharyngeal gland (prePG) and pharyngeal gland (PG) make up the largest exocrine structures in the head of the ant Camponotus japonicus. We used microscopy to study the histological and ultrastructural features of both glands in different castes. The number of secretory units in the prePG is considerably higher than in other ant species and shows a complex duct system which is made up by duct cells, secondary ducts and a main duct. These lead the secretions of hundreds to thousands of secretory cells into the prepharynx through a modified sieve plate at each side. The glove-shaped PG shows clear caste differences in tubule number. The ultrastructure of both the prePG and PG shows abundant mitochondria and secretion vesicles. Moreover, the prePG is loaded with rough endoplasmic reticulum (RER) which means its main secretions are proteinaceous compounds, while the PG is dominated by smooth endoplasmic reticulum (SER) which means the main secretions are lipids. The morphological differences like cell number of the prePG and tubule number of the PG indicate different secretory abilities of each caste. We for the first time introduce histology-based relative size to indicate secretory activity. The proportionally high development of the prePG in minor workers supports a role in trophallaxis.

Sensorineural Hearing Loss in Sjögren's Syndrome.

Sjögren's syndrome is a chronic autoimmune disease characterized by systemic dysfunction of exocrine glands, mainly the salivary and lachrymal glands. Sjögren's syndrome consists of two forms: primary Sjögren's syndrome, which is characterized by dry eyes and dry mouth without autoimmune diseases; and secondary Sjögren's syndrome, which is characterized by symptoms associated with other autoimmune diseases, such as systemic lupus erythematosus. Disease severities vary considerably from mild glandular dryness to severe glandular involvement with numerous extraglandular and systemic features. Sensorineural hearing loss is sometimes observed in both primary and secondary Sjögren's syndrome. This review article consists of (1) Pathology of Sjögren's syndrome, (2) Clinical manifestation of Sjögren's syndrome, (3) Autoimmune inner ear disease, (4) Histoanatomical features of the inner ear, (5) Immunological characteristics of the inner ear, (6) Pathophysiology of autoimmune inner ear disease, (7) Treatment for sensorineural hearing loss in Sjögren's syndrome, and (8) Future direction. Finally, we introduce a recently developed disease model of salivary gland inflammation and discuss future expectations for the treatment of sensorineural hearing loss in Sjögren's syndrome.

Nutritional condition affects tergal gland secretion and courtship success of male cockroaches.

An integral part of the courtship sequence of the German cockroach (Blattella germanica) involves the male raising his wings to expose tergal glands on his dorsum. When a female cockroach feeds on the secretion of these glands, she is optimally positioned for mating. Core chemical components have been identified, but the effect of male diet on the quality of the tergal gland secretion remains unexplored. After validating the pivotal role of tergal feeding in mating, we starved or fed reproductively mature males for one week. We then paired each male with a sexually receptive female and observed their interactions through an infrared-sensitive camera. While starvation had no effect on male courtship behavior, it did influence the duration of female tergal feeding and mating outcomes. Females fed longer on the gland secretion of fed males, and fed males experienced greater mating success than starved males (73.9% vs. 48.3%, respectively). These results suggest that the quality of the tergal gland secretions, and by association mating success, are dependent on the nutritional condition of the male.

Abnormal Epigenetic Regulations in the Immunocytes of Sjögren's Syndrome Patients and Therapeutic Potentials.

Sjögren's syndrome (SjS), characterized by keratoconjunctivitis sicca and dry mouth, is a common autoimmune disease, especially in middle-aged women. The immunopathogenesis of SjS is caused by the sequential infiltration of T and B cells into exocrine glands, including salivary and lacrimal glands. Effector cytokines produced by these immunocytes, such as interferons (IFNs), IL-17, IL-22, IL-21, IL-4, TNF-α, BAFF and APRIL, play critical roles in promoting autoimmune responses and inducing tissue damages. Epigenetic regulations, including DNA methylation, histone modification and non-coding RNAs, have recently been comprehensively studied during the activation of various immunocytes. The deficiency of key epigenetic enzymes usually leads to aberrant immune activation. Epigenetic modifications in T and B cells are usually found to be altered during the immunopathogenesis of SjS, and they are closely correlated with autoimmune responses. In particular, the important role of methylation in activating IFN pathways during SjS progression has been revealed. Thus, according to the involvement of epigenetic regulations in SjS, target therapies to reverse the altered epigenetic modifications in auto-responsive T and B cells are worthy of being considered as a potential therapeutic strategy for SjS.

External nasal gland morphology of Eurycea bislineata (Amphibia, Urodela, Plethodontidae).

Plethodontid salamanders possess numerous courtship glands. Previous studies have shown that the glands are more prominent in male individuals than females, and often experience periods of atrophy and hypertrophy throughout the year that correlate to the nonmating and mating seasons, respectively. We sampled male and female Eurycea bislineata throughout the year to test the hypothesis that external nasal glands are courtship glands. External nasal glands are paired, branched tubular glands that extend from excretory ducts dorsal to the nares to terminal secretory units posterior to the eyes. We found that the glands hypertrophy and stain/react more intensely with histochemical procedures during the mating season. Hypertrophy of the glands is more pronounced in males, and seasonal variation in epithelial height of external nasal glands of males is significantly correlated to that of seasonal variation in mental gland epithelial height, a known courtship gland found in males, when compared throughout the year. This correlation was not as strong in females, confirming sexual dimorphism of external nasal glands in terms of seasonal variation. We found no ultrastructural differences between male and female external nasal glands. In all specimens, the glandular tubules were lined by a simple, columnar epithelium that was packed with secretory granules that often obscured other cytoplasmic contents.

Histology and ultrastructure of glands associated with the sting apparatus of the swarm-founding wasp Protopolybia exigua (Hymenoptera: Vespidae: Polistinae).

Social insects present a great diversity of exocrine glands, which are linked to fundamental roles of social life, and their morphological characterization represents the first step toward the knowledge of their function. We here describe the exocrine glands present in the sting apparatus of queens and workers of the wasp Protopolybia exigua. Histological analysis of serial sections under light microscopy revealed a glandular epithelium in the spiracular plate, the quadrate plate, and the gonostyli, while the last two in addition also contain clusters of class-3 cells. A big cluster of class-3 cells was also found in the oblong plate of some workers and queens. These findings reveal that a variety of glands, in addition to the common venom gland and Dufour gland, are present in association with the sting apparatus, of which three glands are novel reports for the exocrine repertoire of social insects. The position of the glands suggests a role in lubrication to provide mobility of the sting and venom ejection and/or participation in alarm behavior, while a function related to reproduction is also possible. The peculiar characteristics displayed by swarm-founding wasps are linked with exocrine gland secretions, making their investigation in Epiponini wasps a vast and interesting field.

Venom production and secretion in reptiles.

The venom glands of reptiles, particularly those of front-fanged advanced snakes, must satisfy conflicting biological demands: rapid synthesis of potentially labile and highly toxic proteins, storage in the gland lumen for long periods, stabilization of the stored secretions, immediate activation of toxins upon deployment and protection of the animal from the toxic effects of its own venom. This dynamic system could serve as a model for the study of a variety of different phenomena involving exocrine gland activation, protein synthesis, stabilization of protein products and secretory mechanisms. However, these studies have been hampered by a lack of a long-term model that can be propagated in the lab (as opposed to whole-animal studies). Numerous attempts have been made to extend the lifetime of venom gland secretory cells, but only recently has an organoid model been shown to have the requisite qualities of recapitulation of the native system, self-propagation and long-term viability (>1 year). A tractable model is now available for myriad cell- and molecular-level studies of venom glands, protein synthesis and secretion. However, venom glands of reptiles are not identical, and many differ very extensively in overall architecture, microanatomy and protein products produced. This Review summarizes the similarities among and differences between venom glands of helodermatid lizards and of rear-fanged and front-fanged snakes, highlighting those areas that are well understood and identifying areas where future studies can fill in significant gaps in knowledge of these ancient, yet fascinating systems.

The ant abdomen: The skeletomuscular and soft tissue anatomy of Amblyopone australis workers (Hymenoptera: Formicidae).

Recent studies of insect anatomy evince a trend towards a comprehensive and integrative investigation of individual traits and their evolutionary relationships. The abdomen of ants, however, remains critically understudied. To address this shortcoming, we describe the abdominal anatomy of Amblyopone australis Erichson, using a multimodal approach combining manual dissection, histology, and microcomputed tomography. We focus on skeletomusculature, but additionally describe the metapleural and metasomal exocrine glands, and the morphology of the circulatory, digestive, reproductive, and nervous systems. We describe the muscles of the dorsal vessel and the ducts of the venom and Dufour's gland, and characterize the visceral anal musculature. Through comparison with other major ant lineages, apoid wasps, and other hymenopteran outgroups, we provide a first approximation of the complete abdominal skeletomuscular groundplan in Formicidae, with a nomenclatural schema generally applicable to the hexapod abdomen. All skeletal muscles were identifiable with their homologs, while we observe potential apomorphies in the pregenital skeleton and the sting musculature. Specifically, we propose the eighth coxocoxal muscle as an ant synapomorphy; we consider possible transformation series contributing to the distribution of states of the sternal apodemes in ants, Hymenoptera, and Hexapoda; and we address the possibly synapomorphic loss of the seventh sternal-eighth gonapophyseal muscles in the vespiform Aculeata. We homologize the ovipositor muscles across Hymenoptera, and summarize demonstrated and hypothetical muscle functions across the abdomen. We also give a new interpretation of the proximal processes of gonapophyses VIII and the ventromedial processes of gonocoxites IX, and make nomenclatural suggestions in the context of evolutionary anatomy and ontology. Finally, we discuss the utility of techniques applied and emphasize the value of primary anatomical research.

Anatomy and histology of the metapleural gland in the giant tropical ant Paraponera clavata (Fabricius, 1775) (Formicidae: Paraponerinae).

Ants are social insects with about 85 exocrine glands with different morphologies. The metapleural gland is exclusive to ants and its compounds have antimicrobial action, indicating a function in the defense against pathogens. Within ants, Paraponera clavata is the only living species representative of Paraponerinae. This study describes the anatomy and histology of the metapleural gland in workers of P. clavata. The metapleural gland is formed by secretory units that release secretion in a reservoir through collecting canaliculi. Secretory units are bicellular with a well-developed globular secretory cell, with 145 µm in diameter, and a duct cell forming the collecting canaliculi characterizing a class III gland. The metapleural gland is pointed out as a synapomorphy of ants. Paraponerinae constitutes one of the six subfamilies of the basal and paraphyletic Poneroide clade and knowledge of the metapleural gland morphology contribute to the understanding of evolutionary history of the basal ants' groups.

A Gland of Many Uses: a Diversity of Compounds in the Labial Glands of the Bumble Bee Bombus impatiens Suggests Multiple Signaling Functions.

Communication in social insect colonies depends on signals accurately reflecting the identity and physiological state of the individuals. Such information is coded by the products of multiple exocrine glands, and the resulting blends reflect the species, sex, caste, age, task, reproductive status, and health of an individual, and may also contain caste-specific pheromones regulating the behavior and physiology of other individuals. Here we examined the composition of labial gland secretions in females of the bumble bee Bombus impatiens, of different castes, social condition, age, mating status, and ovarian activation. We show that active queens, gynes, and workers each produce caste-specific compounds that may serve different communicative functions. The composition and amounts of wax esters, mostly octyl esters produced by active queens, differed significantly between castes, mating, and social conditions, suggesting a social signaling role. Farnesyl esters were predominant in gynes and peaked at optimal mating age (6-10 days), suggesting their possible roles as sex pheromone components. Reproductive status of females and age across castes was reflected by the ratio between short- and long-chain hydrocarbons, suggesting that these compounds may serve as fertility signals. Our findings overall suggest that the labial gland composition in B. impatiens reflects different facets of female physiology. While further bioassays are required to determine the functions of these compounds, they are likely to have important roles in communication between individuals.