Exocrine System

Exocrine glands are glands that produce and secrete substances onto an epithelial surface by way of a duct. Examples of exocrine glands include sweat, salivary, mammary, ceruminous, lacrimal, sebaceous, and mucous.

The salivary glands in mammals are exocrine glands that produce saliva through a system of ducts. Humans have three paired major salivary glands (parotid, submandibular, and sublingual) as well as hundreds of minor salivary glands.Salivary glands can be classified as serous, mucous or seromucous (mixed).

In serous secretions, the main type of protein secreted is ptyalin(alpha-amylase), an enzyme that breaks down starch into maltose and glucose,whereas in mucous secretions, the main protein secreted is mucin which acts as a lubricant

In health, between 0.5 and 1.5 litres of saliva are produced every day.The secretion of saliva (salivation) is mediated by parasympathetic stimulation; acetylcholine is the active neurotransmitter and binds to muscarinic receptors in the glands, leading to increased salivation.

Sweat glands, also known as sudoriferous or sudoriparous glands, from Latin sudor, meaning ‘sweat’, are small tubular structures of the skin that produce sweat. Sweat glands are a type of exocrine gland, which are glands that produce and secrete substances onto an epithelial surface by way of a duct. There are two main types of sweat glands that differ in their structure, function, secretory product, mechanism of excretion, anatomic distribution, and distribution across species:

  • Eccrine sweat glands are distributed almost all over the human body, in varying densities, with the highest density in palms and soles, then on the head, but much less on the trunk and the extremities. Its water-based secretion represents a primary form of cooling in humans.
  • Apocrine sweat glands are mostly limited to the axilla (armpits) and perianal areas in humans. They are not significant for cooling in humans, but are the sole effective sweat glands in hoofed animals, such as the camels, donkeys, horses, and cattle

Sebaceous glands are microscopic exocrine glands in the skin that secrete an oily or waxy matter, called sebum, to lubricate and waterproof the skin and hair of mammals. In humans, they occur in the greatest number on the face and scalp, but also on all parts of the skin except the palms of the hands and soles of the feet. The type of secretion of the sebaceous glands is referred to as holocrine. In the eyelids, meibomian glands, also called tarsal glands, are a type of sebaceous gland that secrete a special type of sebum into tears. Fordyce spots are ectopic (misplaced) sebaceous glands found usually on the lips, gums and inner cheeks, and genitals. Areolar glands surround the female nipples.

Several related medical conditions involve sebum—including acne, sebaceous cysts, hyperplasia, and sebaceous adenoma. These are usually attributable to overactive sebaceous glands, which produce excess sebum.

A mammary gland is an exocrine gland in mammals that produces milk to feed young offspring. Mammals get their name from the Latin word mamma, “breast”. The mammary glands are arranged in organs such as the breasts in primates (for example, humans and chimpanzees), the udder in ruminants (for example, cows, goats, and deer), and the dugs of other animals (for example, dogs and cats). Lactorrhea, the occasional production of milk by the glands, can occur in any mammal, but in most mammals lactation, the production of enough milk for nursing, occurs only in phenotypic females who have gestated in recent months or years. It is directed by hormonal guidance from sex steroids. In a few mammalian species, male lactation can occur.

Ceruminous glands are specialized sudoriferous glands (sweat glands) located subcutaneously in the external auditory canal, in the outer 1/3. Ceruminous glands are simple, coiled, tubular glands made up of an inner secretory layer of cells and an outer myoepithelial layer of cells They are classed as apocrine glands. The glands drain into larger ducts, which then drain into the guard hairs that reside in the external auditory canal.Here they produce cerumen, or earwax, by mixing their secretion with sebum and dead epidermal cells. Cerumen keeps the eardrum pliable, lubricates and cleans the external auditory canal, waterproofs the canal, kills bacteria, and serves as a barrier to trap foreign particles (dust, fungal spores, etc.) by coating the guard hairs of the ear, making them sticky

These glands are capable of developing both benign and malignant tumors. The benign tumors include ceruminous adenoma, ceruminous pleomorphic adenoma, and ceruminous syringocystadenoma papilliferum. The malignant tumors include ceruminous adenocarcinoma, adenoid cystic carcinoma, and mucoepidermoid carcinoma

The lacrimal glands are paired, almond-shaped exocrine glands, one for each eye, that secrete the aqueous layer of the tear film. They are situated in the upper lateral region of each orbit, in the lacrimal fossa of the orbit formed by the frontal bone. Inflammation of the lacrimal glands is called dacryoadenitis. The lacrimal gland produces tears which then flow into canals that connect to the lacrimal sac. From that sac, the tears drain through the lacrimal duct into the nose.

Anatomists divide the gland into two sections. The smaller palpebral portion lies close to the eye, along the inner surface of the eyelid; if the upper eyelid is everted, the palpebral portion can be seen.

Mucous gland, also known as muciparous glands, are found in several different parts of the body, and they typically stain lighter than serous glands during standard histological preparation. Most are multicellular, but goblet cells are single-celled glands.

They are situated on the under surface of the apex of the tongue, one on either side of the frenulum, where they are covered by a fascicle of muscular fibers derived from the styloglossus and inferior longitudinas muscles. They produce a glycoprotein, mucin that absorbs water to form a sticky secretion called mucus.

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