As their name suggests, intraepithelial glands are glands found between epithelial cells. Intraepithelial glands secrete chemicals and substances, such as mucous and saliva. There are a wide variety of intraepithelial glands throughout the body including sebaceous glands, mucous glands, and urethral glands.
Intraepithelial glands can be simple or compound. Compound glands can be tubular, alveolar, or tubuloalveolar. Most glands are alveolar, often referred to as acinar glands. Acinar glands contain acini, which are small sac-like structures surrounded by secretory cells. Acini have a similar appearance to a bunch of grapes. The apical face of the cells secrete product into a small lumen which passes out of the gland via the ducts. If a gland comprises numerous acini, it is known as a racemose gland. Racemose glands are common, and include the salivary glands.
Acini have basal positioned basement membrane, which can be fenestrated, and made up of flat cells. The nature of areolar connective tissue means that it has an affinity for diffusion. The membrane is located on the same side of the gland (outer surface) as the blood and lymph vessels. This loose connective tissue also allows attachment for lymph and blood vessels. The inner surface of the basement membrane is made up of secretory epithelial cells, which are usually polygonal in shape. The basement membrane encapsulates the entire acinus and comprises extracellular material from modified epithelial cells (secretory cells) and loose connective tissue (areolar) for simple diffusion. Acinar cells sometimes share a basement membrane with nearby satellite cells which are involved in muscle repair.
Salivary glands are made up of acini, which are sometimes referred to as alveoli, as they form an alveolus-like sac. Each acinus comprises a number of cells formed around a central globular cavity. This cavity is continuous with the lumen of a duct in the acinus, from which the saliva is drained out. A thin duct draining an acinus is known as an intercalated duct. These combine together to make an intralobular duct. These ducts then combine to form a duct known as an interlobular duct. Interlobular ducts joined together are what form the main duct of the gland. There are three main types of salivary gland, serous, mucous, and mixed.
This type of salivary glands produce thin saliva. Serous glands contain serous acinar cells, which cluster together forming secretory acini. They produce a clear and watery fluid that is isotonic with blood plasma. The fluid contains digestive enzymes such as alpha-amylase. A serous gland has a lumen formed by an acinus of pyramidal glandular cells. Each acinar cell has a circular nucleus at the base, and secretory zymogen granules at the top. These granules can easily be stained using the Hematoxylin and eosin (H&E) stain technique. Squamous epithelial cells, connected to the acinus form an intercalated duct. Cuboidal cells along with striations of basally folded cytoplasmic membrane form a striated duct. Both striated ducts and intercalated ducts are subdivisions of interlobular ducts. A striated ducts connects an intercalated duct to the interlobular duct. The acinar cells and the basement membrane are separated by myoepithelial cells.
These glands contain acini formed of cuboidal cells with a small irregular lumen. The acinar cells are transparent with vacuoles, and small flattened nuclei towards the base. Like in the serous gland, the basement membrane and acini have a thin layer of myoepithelial cells between them. Simple cuboidal and columnar cells form an excretory duct which secretes mucous containing proteins and glycoproteins from the gland.
Mixed seromucous glands
These glands either contain both serous and mucous acini, or both serous and mucous cells in one acinus. Often, a small number of serous cells are arranged in a crescent at the end of the acinus, referred to as a demilune. This results from mucous cells swelling with mucous and forcing the serous cells into a demilune shaped arrangement. Myoepithelial cells surround the acinus, and both intercalated and striated ducts are present. As one might expect, these glands produce a combination of mucous and serous fluids. The submandibular in the mouth are examples of this type of gland.
Parotid glands are large salivary glands found on each side of the ramus of the mandible on top of the masseter muscle, in front of the ears. The parotid gland is a fairly large type of serous gland, weighing around 15 g. It is surrounded by fibroadipose tissue and borders the masseter muscle, zygomatic arch, external auditory canal, and the styloid process.
The main duct of the parotid gland is called the Stensen’s duct, sometimes simply the parotid duct. This stretches along the masseter muscle and buccal fat pad and through the buccinator muscle. It opens into the oral cavity close to the upper second maxillary molar. Saliva that is secreted from the Stensen’s duct initiates starch digestion via the enzyme amylase. The opening of the duct is marked by a small tissue elevation on the inside of the cheek. Cranial nerve 7, referred to as the facial nerve, runs through the parotid glands and innervates other salivary glands, including the sublingual and submandibular glands.
Also referred to as the submaxillary glands, these are a pair of tubuloalveolar glands found beneath the floor of the mouth. This gland is found within the submandibular triangle which is formed from digastric muscle and the mandible.
The gland is divided into lobes by the mylohyoid muscle. These lobes can be divided into two types, the deep lobe and the superficial lobe. The superficial lobe is much larger than the deep lobe, and makes up the majority of the gland. The lobes contain lobules which in turn contain adenomeres, which are involved in secretion. The adenomeres contain acini that secrete their product into the ducts of the gland. The acini of the gland are made up of a mixture of serous and mucous cells, making it a seromucous gland. Sometimes serous demilunes are present. These are crescent shaped collections of lysozyme-secreting serous cells.
The main duct of a submandibular gland is known as the Wharton’s duct, which transmits secretions into the oral cavity at the anterior base of the mouth. The mucous glands produce lubricating mucins which allows food to be swallowed more easily. The serous cells produce amylase-containing saliva, which is involved in the digestion process.
These glands are found in the sublingual fossa of the mandible, connected by the genioglossus and mylohyoid muscle. The glands are drained by up to 20 ducts known as the ducts of Rivinus. These ducts drain into the Bartholin's duct, which drains into the Wharton’s duct of the submandibular glands, and through the sublingual caruncle. This is a papilla near the center of the floor of the mouth, situated next to the lingual frenum. The sublingual glands are mainly made up of mucous acini with serous demilunes attached to them.
Eccrine sweat glands
There are two types of sweat glands - eccrine and apocrine. These are sometimes referred to as ordinary and odoriferous sweat glands, respectively. Eccrine sweat glands are lined by a layer of columnar secretory cells, supported by a distinct basement membrane. Within the acinus, both large glycogen-containing pale or clear cells and smaller mucopolysaccharide-containing dark cells are found. The dark cells tend to line the luminal surface, whereas the pale cells are situated peripheral to the dark cells and, for the most part, do not abut the lumen itself. Clear cells secrete a fluid containing potassium chloride, sodium chloride, ammonia, urea and uric acid, whilst the dark cells produce glycoproteins. Peripheral to the outer row of myoepithelial cells is a basement membrane that separates glandular epithelium from the richly vascular connective tissue of the periadnexal dermis.
Apocrine sweat glands
Apocrine sweat glands comprise a layer of cuboidal secretory cells. The nucleus is circular and elliptical, located near the base of the cell. The apocrine acinus has a larger lumen than that of the eccrine sweat gland, and contains secretory product. The secretory product is a thick fluid, and becomes odoriferous as a result of degradation by bacteria on the skin. Myoepithelial cells are found between the basement membrane and secretory cells.
Sebaceous glands are found in the skin and produce an antibacterial, waterproof, and lubricating product known as sebum. Some sebaceous glands are linked to hair follicles and others are not. They are acinar glands and the sebum they secrete reaches the skin via the hair shaft. The overall structure comprising a hair shaft, hair follicle, arrector pili muscles, and sebaceous gland is referred to as a pilosebaceous unit. Sebaceous glands are also found in hairless areas such as the internal surface of the eyelids, Meibomian glands, and the penis, a type of Fordyce spot. Since the previously named locations lack hair, these glands are therefore not part of a pilosebaceous unit.
The acinus of a sebaceous gland is made up of many cells surrounded by a basement membrane. The mass of cells comprises both basal and secretory cells. The basal cells are cuboidal and have circular nuclei located at the peripheries of the secretory area. The secretory cells are polygonal and are found near the centre of the acinar secretory portion. These cells have a tiny shrunken nucleus and contain lipid droplets in their cytoplasm. When the secretory cells die, a fatty product called sebum is secreted, lubricating the skin and hair. This type of secretion is known as holocrine secretion.
Within the urethra, there are acinar mucous secreting glands. In males, they are small epithelial invaginations found in the dorsal wall of the urethra. The mucous cells penetrate between the veins in the spongy tissue of the penis. The mucus they produce contains a substance called glycosaminoglycans, which protect the epithelium from being damaged by urine.
In the male urethra, there are also small tubular mucous glands known as Littre’s glands. These open out into the spongy tissue of the urethra, but not in the terminal fossa. They are exclusive to males, and contribute mucous to semen. They are most abundant in the part of the spongy urethra that passes through the penis.
In females, small mucous glands are found in the lamina propria of the wall of the urethra. These also produce mucous containing glycosaminoglycans to protect the epithelial walls of the urethra against urine.
Goblet cells are unusual as they are unicellular intraepithelial glands. They are modified epithelial cells that secrete mucous from the epithelia of many organs, particularly those of the intestines and airways. Goblet cells are aptly named, as they are shaped like tiny goblets within the epithelium, with narrow bases and a wide apex. Their anatomy is polarised, with organelles such as the nucleus, mitochondria, and endoplasmic reticulum at the base of the cell. The rest of the cell contains mucin filled secretory granules bound by membranes. They can be stained histologically using mucin stains, such as periodic acid-Schiff staining (PAS), which colours them purple.
Goblet cells start to produce mucus product by exocytosis of the granules when triggered by irritating stimuli such as by smoke in the lungs. As well as this type of secretion, known as stimulated secretion, goblet cells also continually produce small amounts of mucous. This is known as basal, or constitutive secretion. The mucous inside goblet cell granules expands when released. This is a result of calcium ions diffusing out of the cell, causing a phase change in the negatively charged mucins. This causes them to hydrate and expand, forming mucous gel.