Histology of the accessory organs of the digestive system

General features

The first set of accessory digestive organs to be encountered are the salivary glands. The three major salivary glands to be discussed are the parotid, submandibular and sublingual glands. Apart from the gross anatomical differences of size, parasympathetic innervation and arterial blood supply, the glands share several histological similarities.

Salivary glands are made up of secretory acini (acini - means a rounded secretory unit) and ducts. There are two types of secretions - serous and mucous.

The acini can either be serous, mucous, or a mixture of serous and mucous:

• a serous acinus secretes proteins in an isotonic watery fluid (spherical shape).
• a mucous acinus secretes mucin – a lubricant (tubular shape).
• in a mixed serous-mucous acinus, the serous acinus forms a serous demilune around a mucous acinus.

Firstly, the glands are lobulated by infiltrating septae formed by the connective tissue capsule that surrounds each gland. The functional components of the glands are secretory compound tubuloacini in which the salivary mixture is prepared.

Being exocrine glands, all three salivary glands utilize duct systems for secretion. The acini secrete their contents into intercalated ducts (small ducts lined by cuboidal epithelium and encircled by myoepithelial cells).

A few intercalated ducts fuse and subsequently drain their contents into striated ducts, which are lined with tall columnar epithelium. These ducts have a folded basal membrane, to enable active transport of substances out of the duct. Water resorption, and ion secretion takes place in the striated ducts, to make saliva hypotonic (reduced Na,Cl ions and increased carbonate, and potassium ions).The striated ducts then secrete their contents into the larger interlobular ducts (surrounded by more connective tissue) and finally to interlobar ducts that release the saliva into the oral cavity and vestibule.

Specific glands

Parotid gland 

The parotid gland is a purely serous organ. The cells of these serous acini are pyramidal in shape, with their apex surrounding a lumen and their nuclei basally displaced. The cells stain deeply with hematoxylin and eosin (H&E), which gives the gland its characteristic appearance. Occasionally, fat cells can be observed throughout the gland. The clear, electrolyte-rich, watery secretion of the gland also contains ptyalin (amylase) that is responsible for initiating starch metabolism.

Sublingual glands

The type of acini found in the sublingual glands is the subject of some dispute. Some individuals believe that it is a mixed gland, containing both serous and mucous acini, while others stipulate that it is purely mucous. In the instance where it is believed to be mixed, authors have stated that the gland is predominantly mucous in nature. As a result, the mucous present in the cells is poorly stained with H&E and consequently shows up as pale clusters of pyramidal cells with basally displaced nuclei.

Another distinguishing feature between the sublingual gland and the other two major salivary glands is that there are comparatively more interlobular connective tissue septa in the sublingual gland than the others.

Submandibular glands

The histology of the submandibular gland is a mixture of both the parotid and the sublingual glands. It has purely serous and purely mucous acini, as well as seromucous glands known as serous demilunes. The serous demilunes are comprised of pyramidal mucous cells arranged around its central lumen and crescent shaped serous cells around its periphery.

The size disparity between the acini can be better appreciated in the mixed submandibular gland. The mucous glands are larger, have larger lumen, and have larger ducts than the serous glands.

In addition to the three major salivary glands, there are several hundred other minor salivary glands scattered throughout the proximal digestive tract. In particular, these minor salivary glands can be found along the regions of the tongue, lips, palate and the mucosa of the mouth, to name a few. An example of the minor salivary glands is the group of serous gustatory glands (von Ebner’s glands) found around the vallate papillae on the posterior third of the tongue. The minor salivary glands have a simplified duct system where long intercalated ducts empty their serous, mucous or mixed saliva directly into the oral cavity.

Exocrine component

In the exocrine component, there are zymogenic cells that are pyramidal and form the serous acini. The gland is also lobulated by fibrous septae that contains general neurovascular structures as well as specialized nerves called Pacinian corpuscles. The Pacinian corpuscle provides specialized sensory perception to the gland. Because the exocrine portion is made up of serous glands that are similar to those found in the parotid glands, one can be easily mistaken for the other. However, the presence of paler staining, isolated clusters of cells (islets of Langerhans) in the pancreas should safeguard the observer from such an error. The absence of myoepitheilal cells in the pancreatic acini is also another good indication to the observer that the specimen being reviewed is the pancreas and not the parotid gland.

Centroacinar cells form the initial segment of the duct system for the pancreatic serous acini. They continue with the simple cuboidal epithelium of the intercalated ducts that drain the acini to their interlobular ducts (also covered by simple cuboidal epithelium), which terminate in the pancreatic ducts.

Endocrine component

The endocrine component of the gland is formed by the islets of Langerhans. The highly vascular islets are surrounded by a connective tissue capsule.

The endocrine unit is made up of:

• alpha cells that produce glucagon,
• beta cells that are responsible for the secretion of insulin,
• delta cells that secrete somatostatin and
• pancreatic polypeptide cells that inhibit the secretion of alkaline solutions and the production of pancreatic enzymes.