Histology of the Accessory Digestive Organs
The primary digestive track is responsible for transporting and absorbing nutrients from ingested foods. Its actions are supplemented by accessory digestive organs that provide additional enzymes and lubricants to facilitate further breakdown and transport through the gastrointestinal pathway. The salivary glands, liver, gallbladder, and pancreas are all accessory digestive organs that support the digestive system. Their histological composition will be discussed in this article.
The first set of accessory digestive organs to be encountered is 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. A mucous acinuss secretes mucin – lubricant. In a mixed serous-mucous acinus, the serous acinus forms a serous demilune around mucousacinus.
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 - columnar epithelium in the lumen and invaginations at the base of the cell membrane (These 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.
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.
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 say 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 are poorly stained with H&E and consequently show 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 more interlobular connective tissue septa in the sublingual gland than the others.
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 serous von Ebner’s glands found around the circumvallate papillae at the posterior part 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.
The liver is the largest organ in the human body. It is able to filter and process nutrient rich blood from the digestive tract because of its intricate histological arrangement. Its fibrous capsule infiltrates the substance of the organ and divides it into lobules that are almost hexagonal. Each lobule typically has three to six portal areas/canals (also known as the portal triad) – each of which is usually located at the apices of the lobule. Each portal canal contains a branch of the hepatic portal vein, hepatic artery, lymphatic vessels and a bile duct.
Liver cells (hepatocytes) appear to emanate as cords of cells from the center of the lobule and travels towards its periphery. In between the cords are fenestrated capillaries known as sinusoids. The perisinusoidal space of Disse provides some degree of separation between the hepatocytes and the sinusoids. It’s by way of the fenestrations of the sinusoids that mixed blood from both the hepatic portal vein and the hepatic artery gains access to the hepatocytes where they are processed. Additionally, the nutrient rich blood also encounters macrophagic Kupffer cells that degrade worn out cellular structures. The sinusoids travel a convoluted pathway towards the center of the lobule where it drains into the central vein, which then drains to larger hepatic veins that drain directly to the inferior vena cava.
Finally, bile produced by the hepatocytes is released into bile canaliculi, which are adjacent to individual liver cells. The canaliculi then merge at the edge of the lobules to form bile duct in the portal triad. Therefore blood travels centrally to the central vein of the lobule, while bile travels peripherally to the portal triad; thus preventing any mixture of the two substances.
Although the gallbladder is not a true gland (because it doesn’t produce any secretions), it stores and concentrates bile for subsequent release upon hormonal signalling. The muscular sac’s histological layout is similar to that of the gastrointestinal tract. It has a mucosa, muscular layer and an outer serosa. The epithelium of the mucosa is lined with simple columnar cells attached to a lamina propria that is made up of loose connective tissue and vasculature. The smooth muscle fibers of the muscularis layer surround the lamina propria. These muscle fibers aid in the ejection of bile from the gallbladder.
When the gallbladder is empty, it has numerous temporary mucosal folds. In this state, the histological appearance of the gallbladder is similar to that of the small intestines. External to the smooth muscle layer is thick, dense connective tissue – in which large neurovascular and lymphatic structures are embedded. The serosa covers the aspect of the gallbladder that is not attached to the inferior surface of the liver.
The pancreas is functionally and histologically divided into exocrine and endocrine components. 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.
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, somatostatin-secreting delta cells and pancreatic polypeptide cells that inhibit the secretion of alkaline solutions and the production of pancreatic enzymes.