Structure and Function of Sweat Glands
Sweat glands are small, coiled, simple tubular glands that produce sweat. They are found in the dermis of the skin, throughout the body. Sweat glands are sometimes referred to as sudoriferous or sudoriparous glands. These names are derived from the latin word ‘sudor’ which means ‘sweat’.
There are two types of sweat gland in the body: apocrine and eccrine. These two types of gland differ in a number of ways, from distribution and structure, to their excretory mechanism and secretory product. Eccrine glands are found all over the body and secrete a watery product that cools the skin. Apocrine sweat glands are mainly found in the armpits and perianal area, and secrete a more viscous, odorous product.
In general, sweat glands tend to comprise a secretory unit and a duct through which sweat or secretory product is passed. Sweat glands are situated in the dermis and are surrounded by adipose tissue. At the base of each sweat gland there is a structure known as the secretory coil. This is surrounded by contractile myoepithelial cells which act to help secrete the gland’s product. The contraction of these cells are either controlled by hormones or nerve action.
Sweat glands open into the skin’s surface, or in the case of apocrine sweat glands, into hair follicles. The part of the gland that opens onto the skin or hair follicle is known as the acrosyringium. Whilst sweat glands share a basic structure, apocrine and eccrine sweat glands have many differences which are outlined during the rest of this article.
Eccrine Sweat Glands
Eccrine glands are simple glands that have a coiled tubular structure. They are tiny, and invisible to the naked eye, ranging from between 0.05mm and 0.1mm in size. They consist of two portions - a secretory portion and excretory duct portion. Eccrine glands have cholinergic innervation, allowing them to be thermoregulatory, releasing sweat to maintain a constant body temperature through evaporative cooling. Eccrine glands also play a role in ion excretion, and can produce up to 750 milliliters of sweat a day in response to both temperature and emotional stimuli. Unlike apocrine glands, eccrine glands are found throughout the body, with the exception of the external auditory canal, lips, glans penis, labia minora, and clitoris. The concentration of eccrine glands can vary depending on location in the body.
The secretory portion of the gland is found deep within the dermis of the skin, and comprises two types of secretory cells known as clear cells and dark cells. These are either cuboidal or pyramidal in shape. The clear cells mainly produce electrolytes and water, whilst dark cells release the macromolecular substances found in sweat.
The clear and dark cells are identifiable using hematoxylin and eosin (H&E) staining, but are become most clearly visible using the periodic acid Schiff (PAS) stain. The clear and dark cells fit together within a tubule, and can be visually differentiated from the geometrically arranged layers of cuboidal cells found in the eccrine duct lining.
Situated around the tubule, and found between the secretory cells and basement membrane is a layer of smooth muscle-like cells. These are known as myoepithelial cells. These cells are arranged spirally along the tube, and are between 40 and 100 μm in length.
The secretory coil can be divided into:
- a coiled segment
- a long and straight segment that extends up to the epidermis
- an intraepidermal segment, which joins up with a cork-screw shaped cleft between epithelial cells, opening via a circular aperture
The secretory and duct portions have differing histological structures, however, the entirety of an eccrine gland does consist of a double layer of basophilic cuboidal cells, with an eosinophilic cuticle which is thought to provide structural support. The eosinophilic cuticle can act as a microscopic marker for the eccrine duct, allowing it to be visually distinguished from the secretory tubule.
The duct portion of eccrine glands comprise two layers of cuboidal epithelial cells, which appear dark when histologically stained. The cells of the duct portion of the eccrine gland have a system of microvilli connecting the basal cells together. The duct cells contain smooth endoplasmic reticulum, golgi apparatus, and microtubules.
The basal cells have opaque granules along with small numbers of vesicles. In the duct cells closer to the epidermis, the number of mitochondria decreases and the number of tonofilaments increases. The luminal cells of the duct also contain desmosomes.
Moreover, eccrine secretory cells contain vacuoles, granules, and lipid droplets, which are autofluorescent allowing them to be visualised. Clear cells are also linked to one another via intercellular canaliculi, which extend into the secretory tubule. The clear cells also contain mitochondria, smooth endoplasmic reticulum, and golgi apparatus. Lipid granules and smaller pigment granules are also present.
The dark cells have tiny microvilli present on their luminal side. The cytoplasm is dominated by rough endoplasmic reticulum and golgi apparatus. The dark cells also have short, rod-shaped mitochondria, in lower numbers than the clear cells. Lipid droplets are also present in dark cells, along with tonofilaments and small numbers of lysosomes.
Apocrine Sweat Glands
Apocrine sweat glands are sometimes referred to as odoriferous sweat glands. They are relatively large, measuring from 1 to 5mm, and found only in certain regions of the body. For example, they can be found in the armpits (axilla), beard, scrotum, labia minora, nipples, and perianal regions. The glands are large and spongy, and found within the subcutaneous fat of the dermis. The apocrine glands are not as coiled as eccrine glands, and many of their coils come together to form a tubular network.
Apocrine glands have secretory tubules that form a sac-shaped lumen. This lumen is lined with simple cuboidal epithelial cells and is wider and larger than the eccrine lumen. The size of apocrine secretory cells can vary depending on the state of secretion that they are at. The cells secrete a viscous, oily substance, that is yellow in color, and has an acrid odor. This secretion is produced in response to the presence of bacterial decomposition. Apocrine sweat glands are controlled by sex hormones and therefore only become active at puberty.
The excretory ducts of apocrine sweat glands open into hair follicles, and comprise a body and an excretory duct. The body is formed from coiled tubuloalveolar with sac-shaped outpockets. The tubulo-alveoli are surrounded by both epithelial cells and basal lamina. They are also lined by cuboidal and columnar epithelial cells.
Meanwhile the excretory duct of an apocrine sweat gland is lined by simple cuboidal epithelium. The myoepithelial cells which surround the tubulo-alveoli help to expel secretory product into the excretory duct. The myoepithelial cells are long and spindle-shaped and are found between the bases of apocrine cells, as well as within grooves on their basal surfaces. Both the secretory epithelium and myoepithelium of the glands contain basal lamina on their surfaces. Apocrine glands can be clearly stained using the H&E staining technique. These glands only begin to function when puberty is reached.
Sweat gland dysfunction can lead to a variety of abnormalities and diseases. For example, inflammation of the apocrine sweat glands can lead to the rare diseases known as Fox-Fordyce disease. This causes a persistent rash in the pubic regions and armpits, and is more common in women. Moreover, if the eccrine glands become exhausted due to excessive activity, this can contribute to heatstroke. This in turn can result in hyperpyrexia, which is categorised as an extreme rise in body temperature that can potentially be fatal.
Additionally, the sweat glands can be linked to cystic fibrosis. Eccrine sweat glands usually reabsorb sodium and chloride ions through their ductal epithelium, which causes sweat to be hypotonic. This process is controlled by the hormone aldosterone. However, if defective chloride ion reabsorption occurs, this can lead to parents noticing that a child has salty skin when kissing the child. This acknowledgement leads them to the doctor which usually ends up in a diagnosis of cystic fibrosis. The sweat glands of people with cystic fibrosis appear to be histologically normal, however, they have saltier sweat due to overproduction of sodium and chloride. This issue is related to a protein known as cystic fibrosis transmembrane regulator (CFTR), which is is genetically coded for. Cystic fibrosis a result of a recessive gene meaning that both parents must supply a copy of the defective gene to their child for the disease to occur.