1. Morphogenesis of the Salivary Glands
Salivary glands originate from an epithelial placode during embryonic development (from E11 to E16 in mice and between the 4th and the 12th embryonic weeks in humans). The initial placode grows and extends into the underlying mesenchyme, acquiring a bud shape. The growing epithelial bud progressively stratifies with concentric layers each formed by a specialized cell type. During branching morphogenesis, the initial salivary bud divides into additional, independent buds that grow and cleave again, until the formation of an extensive arborization typical for the mature salivary gland
[1].
A portion of the cells forming the outer epithelial layer of the buds differentiates into myoepithelial cells. They will then acquire smooth muscle characteristics and locate in direct contact with the acinar structure to regulate the release of secretion
[2]. Inner epithelial cells differentiate further to acquire a distal (tips) or proximal (stalk) identity, which, in turn, evolves into acini or ducts, respectively
[3].
Both epithelial and mesenchymal cells produce the basement membrane and stromal extracellular matrix. The composition of the extracellular matrix varies from region to region during branching, and bundles of collagen I, IV and fibronectin are thought to directly control the maturation process. Low concentrations of fibronectin and glycosaminoglycans facilitate the area of bud growth, while accumulation of fibronectin, collagen IV and glycosaminoglycans limit epithelial activity of the peripheral nervous system also participates in directing salivary growth and stabilizing the basal lamina, determining sites where branching and clefting occurs
[4][5]. The peripheral nervous system participates in directing salivary gland maturation. The primordial epithelial structure is innervated by cholinergic neurons, whose axonal growth follows the ramified pattern of the developing gland
[6]. Local release of acetylcholine induces proliferation of epithelial progenitor which positively regulate epithelial branching
[7][8][9].
A network of capillaries derived from terminal arterioles develops in parallel with the acini-duct systems and has an instructive role in the establishment of the epithelial patterning
[10].
2. Histological and Anatomical Features
The three major salivary glands have a similar anatomical structure, with a main secretory duct extending from the main body of the gland to the oral cavity. The secretory duct of the SMG is the Wharton’s duct, which reaches the oral cavity under the tongue at the sublingual caruncula. The Bartholin duct is the major duct of the SL and it connects with the Warthon’s duct at its extremity before the opening in the oral mucosa. SL also have smaller ducts called Rivinus’s ducts that release secretion beneath the tongue onto the floor of the mouth. Finally, the PG has an independent duct called Stensen’s duct, opening on the upper portion of the oral cavity.
The secretory duct branches up into striated ducts, composed of columnar epithelial cells whose appearance is due to infoldings of the basal membrane. Striated ducts extend further into progressively smaller intercalated duct, characterized by a wall of flat cuboidal epithelial cells. Finally, the structure ends into a secretory unit of acinar cells grouped as end-pieces specialized in producing and releasing the primary secretion.
3. Innervation and Trophic Support
Salivary glands are densely innervated by the autonomic nervous system.
The parasympathetic nerves release acetylcholine, which activates the muscarinic receptors stimulating fluid secretion. The sympathetic nerves, on the other hand, control salivation through release of noradrenaline and activation of α-and β-adrenoreceptors, the first stimulating fluid-rich secretion and the latter protein-rich secretion
[11][12]. This suggests that the serous population of cells is innervated by the parasympathetic system, while the mucous population mainly depends on the sympathetic stimulation
[13][14][15]. The distribution of the secretory nerves highly depends on species, age and type of gland
[16].
Innervation of the salivary glands starts during embryonic development and progress in parallel with the organ definition. Neural crest-derived cells migrate to their appropriate location in the oral epithelium to instruct the thickening for the placode formation. The neural crest-derived precursors differentiate to form the parasympathetic submandibular ganglion (PSG) surrounding the epithelial primordia of the major secretory duct. As branching proceeds further with the developmental process, axons from the PSG extend along the epithelium to envelop the secretory end-pieces. By E14 in the mouse, the gland is highly branched and fully innervated
[17]. The instructive role of the PSG is currently gathering interest, as ablation of the PSG reduces expression of epithelial progenitor markers such as Krt5 and Krt15, and might, therefore, be implicated in the maintenance of endogenous stem cells
[18]. Similarly, acetylcholine induces proliferation of Krt5-expressing progenitor cells via regulation of the EGF pathway, suggesting that the parasympathetic activity coordinates the maintenance of the undifferentiated pool and their balance during organogenesis
[18].
The development of the sympathetic innervation proceeds conjointly with acinar and ductal maturation, which suggests a role in the final specification of the salivary gland.
The primary sympathetic salivary centres are located in the upper thoracic spinal cord, and reach the salivary gland via the superior cervical ganglion. Sympathetic axons enter the SMG in parallel with the vascular system, and vascular-derived guidance cues are needed for sympathetic neurons to grow. Mice lacking endothelin3 or endothelin-receptor type-A have reduced sympathetic innervation of the salivary glands and defects in SMG secretion
[19][20].
Despite developing with a similar timing and being surrounded by the same mesenchymal cap, the SL gland contains only a few sympathetic nerves, while the SMG has rich innervation. This dichotomy is probably to be associated with the different levels of NGF (high in the SMG and low in SL)
[21]. After submandibular gland removal, the NGF levels drop dramatically in the plasma, to then go back to a normal level after several weeks. It has, therefore, been postulated that the submandibular gland in mice might work as source of NGF
[22]. Nevertheless, removal of the SMG in mice has no deleterious systemic effect, indicating more of an accessory role in NGF secretion rather than a primary one
[23].
Once reaching the salivary gland, the independent innervation of parasympathetic and sympathetic efference bundles up together surrounded by Schwann myelinating cells
[24]. Dual innervation can be found in myoepithelial cells, acinar end-pieces and local blood vessels, all of which plays a functional role in salivary gland secretion
[25].