The human integumentary system contains several types of glands, including sweat glands. Apocrine glands represent a specific class of exocrine structures found within the skin. While often grouped with other sweat glands, the apocrine type possesses distinctive characteristics in its structure, method of secretion, and function. Understanding these differences is necessary to grasp the unique role this gland plays in human biology.
Anatomy and Secretion Method
Apocrine glands are large, coiled tubular structures that extend deep into the dermis and sometimes into the subcutaneous fat layer. The secretory portion is wide, forming a substantial lumen where the product is collected.
The method by which apocrine glands release their product gives the gland its name. This process involves the cell pinching off and releasing the apical portion of the secretory cell along with the accumulated material. This unique mechanism is known as apocrine secretion, resulting in cellular material being discharged alongside the fluid.
The resulting secretion is characteristically thick, viscous, and often described as milky or oily due to its high content of lipids, proteins, and steroids. The fluid is sterile and odorless immediately upon its release from the gland.
Location and Activation
Apocrine glands are confined to specific anatomical regions rather than being universally distributed across the skin surface. They are most densely concentrated in the axilla (armpit) and the anogenital area (groin). Smaller numbers are also found in the areola of the nipples, the eyelids (Moll’s glands), and the external ear canal (ceruminous glands).
A defining feature is the gland’s relationship with the pilosebaceous unit. The secretory duct does not open directly onto the skin surface but instead empties its contents into the canal of a hair follicle. This means the secretion must travel up the hair shaft before reaching the surface.
These glands remain inactive throughout childhood, becoming fully functional only during the onset of puberty. This activation is driven by the rise in sex hormones, specifically androgens, which regulate their growth and secretory activity.
Apocrine Versus Eccrine Glands
The distinction between apocrine and eccrine glands is based on several biological and functional differences. Eccrine glands are distributed across almost the entire body surface and are primarily responsible for thermoregulation through the production of sweat that cools the skin upon evaporation. Apocrine glands, in contrast, are limited to specific areas and have no known role in regulating body temperature.
The method of secretion fundamentally separates the two gland types. While apocrine glands utilize the unique pinching-off method, eccrine glands secrete their product via exocytosis, a process called merocrine secretion. This merocrine method involves releasing contents without any loss of cellular material.
The chemical makeup of the released fluid differs significantly. Eccrine sweat is largely an electrolyte solution, consisting of about 99% water and dissolved salts like sodium chloride. Apocrine secretion, however, is rich in complex organic molecules, including lipids, proteins, ammonia, and sugars, making it thicker and more viscous.
The Role in Body Odor
The thick, organic-rich secretion produced by the apocrine gland is the primary precursor to body odor. Although the fluid is initially odorless upon release, its chemical composition provides an ideal nutrient source for the resident microflora on the skin surface. This interaction is where the characteristic scent originates.
Bacteria, particularly species of the genus Corynebacterium, thrive on the lipids, proteins, and complex molecules in the apocrine fluid. These microbes possess enzymes that break down the larger, odorless molecules into smaller, highly volatile compounds. The warm, moist environment of the axilla promotes this rapid bacterial growth and metabolic activity.
The resulting breakdown products are primarily volatile fatty acids (VFAs), such as 3-methyl-2-hexenoic acid, which are responsible for the pungent smell associated with body odor. Other metabolic byproducts, including certain steroids and thioalcohols, also contribute to the complex mixture of scents.
When this bacterial metabolism becomes excessive, the condition is clinically referred to as bromhidrosis. This highlights the direct link between the gland’s unique output and the chemical transformation caused by the skin’s microbial ecosystem.