Structure and function of the sweat glands

Secretory segment

The secretory segment consists of three types of cells that lie on the basal lamina: clear, dark and myoepithelial. The clear and dark cells are the secretory cells, while the myoepithelial are the contractile cells. Given the different sizes of cells, the lining of the secretory segment resembles to the pseudostratified epithelium.

• The clear cells contain large amounts of glycogen, numerous smooth endoplasmic reticula and mitochondria, as well as a small Golgi apparatus. The function of these cells is to produce the electrolytes watery component of sweat.
• The dark cells contain numerous rough endoplasmic reticula and a well-developed Golgi apparatus. This is because they produce the glycoprotein component of the sweat. These components are packed within secretory granules which are usually found aggregated in the apical cytoplasm.
• The myoepithelial cells are contractile cells, usually dispersed between the clear and dark cells. They are filled with contractile filaments and their contraction promotes fast excretion of sweat.

Duct segment

The duct segment continues from the secretory segment and goes towards the skin surface. After leaving the dermis, it exhibits slight coiling in course. It is lined by the stratified cuboidal epithelium which has two layers: basal or peripheral layer which is deep, and luminal or apical layer which lies on top of the basal layer. It is important to note that the duct segment does not contain myoepithelial cells.

The basal cells have an ellipsoid nucleus with a prominent nucleolus, as well as a large amount of ribosomes and mitochondria. The luminal cells have smaller nucleus than basal. Their most distinguishing feature is the presence of glassy or hyalinized structure in their cytoplasm, which represents accumulated tonofilaments.

Function

The eccrine glands are responsible for thermoregulatory sweating. Simply said, when the ambient temperature is too high, these glands release sweat which then evaporates and conveys the heat from the body. This type of physiological sweating first starts in the skin of the scalp and forehead, then to the face and the rest of the skin. The last regions to exhibit the thermoregulatory sweating are palms and soles.

In contrast to thermoregulatory sweating, there is also sweating induced by the emotional stress. In this situation, the palms, soles and axillae are the first regions to sweat.

The nervous stimulation for thermoregulatory and stress sweating is different. The thermoregulatory sweating is stimulated by the cholinergic input, whereas the stress sweating is promoted by the adrenergic input.

Secretory segment

The secretory portion is usually located in the superficial layer of the subcutis, however it can sometimes be found in the deep dermis. There are a couple of histological characteristics which differ the secretory segment of apocrine glands from that of the eccrine glands:

• The secretory part of apocrine glands is much wider than that of the eccrine glands.
• The apocrine glands store their product in their lumina, while as we noted before, the eccrine glands store them in the granules within the apical cytoplasm of their cells.

The secretory segment of apocrine glands is lined by simple epithelium which consists of only one cell type. These cells appear eosinophilic and they feature a protrusion on their apical region. It was believed that the separation of this protrusion from the cell is the mechanism of excretion of cellular product. This is why the glands bear the name apocrine, derived from the word apex which means "the tip", suggesting that the pinching of the tip is the way of excretion. However, recent studies have proven that this is not true. Instead, the cells pack their products in small granules, which then reach the lumen through the process of exocytosis. This way of secretion is called merocrine secretion.

The cells are also rich in lysosomes, mitochondria and lipofuscin and feature a prominent Golgi apparatus, especially when the secretion phase is active.

Myoepithelial cells are also dispersed throughout the lining of the secretory segment. Their function is to aid excretion with their contractions.

Duct segment

The duct of apocrine glands has a narrow lumen and a relatively straight course towards the hair follicle where it opens. It is lined by the stratified cuboidal epithelium which is devoid of myoepithelial cells.

The epithelium usually consists of 2-3 cell layers. The cells of the luminal layer, which is the most superficial one, contain glassy aggregation in their apical cytoplasm, which, similar to that of eccrine glands, represents the aggregation of tonofilaments.

Functions

The activity of the apocrine glands heavily depends on sex hormones. In other mammals, these glands release a significant amount of pheromones which affects the odour and 'chemical fingerprint' of an animal. It is considered that the odour plays an important role in sexual pairing and social treatment of an animal. 

Although in humans the product of apocrine glands also affects the body odor, its social and sexual significance is consider as rudimentary to non-existent.