Index
Basically we speak of appendages of the integument.
There are eccrine and apocrine sweat glands. They differ in embryology, distribution, and function. Eccrine sweat glands are simple, coiled tubular glands present throughout the body, mainly on the soles of the feet.
The thin skin covers most of the body and contains sweat glands, as well as hair follicles, hair-retraction muscles, and sebaceous glands.
The exceptions are the vermilion border of the lips, the external auditory canal, the nail beds, the glans, the clitoris, and the labia minora, which do not contain sweat glands.
The thick skin that covers the palms of the hands and the soles of the feet lacks all skin appendages except the sweat glands.
Apocrine sweat glands, also known as scent sweat glands, are known to produce smelly perspiration.
They are large, branching glands that are primarily confined to the axillary and perineal regions, including the perianal region, the labia majora in women, and the scrotum and foreskin in men.
Apocrine sweat glands are also present on the nipples and the areolar tissue that surrounds the nipples.
Structure and function
The eccrine sweat glands fulfill a thermoregulatory function through the loss of heat by evaporation. When the internal body temperature rises, the sweat glands release water to the surface of the skin.
There, it quickly evaporates, subsequently cooling the skin and blood underneath. This is the most effective means of thermoregulation in humans. The eccrine sweat glands are also involved in the excretion of nitrogenous ions and wastes.
In response to emotional or thermal stimuli, the sweat glands can produce at least 500 ml to 750 ml in a day.
The apocrine sweat glands begin to function at puberty under the stimulation of sex hormones. They are associated with hair follicles in the groin and armpit region.
The viscous, protein-rich product is initially odorless, but may develop an odor after exposure to bacteria.
Modified apocrine sweat glands include the wax-producing ceruminous glands of the external auditory meatus, the Moll glands found on the free margins of the eyelids, and the mammary glands of the breast.
The sweat glands play a regenerative role in skin damage. In second-degree skin burns, which extend into the reticular dermis, regeneration of the epithelium occurs through the appendages of the skin, including hair follicles, sebaceous glands, and sweat glands.
The epithelial cells that surround these appendages produce more epithelial cells that progress to form new epithelium, a process that can take 1 to 3 weeks.
Embryology
Both eccrine and apocrine sweat glands originate in the epidermis. The eccrine glands begin as epithelial cell shoots that grow into the underlying mesenchyme.
The glandular secretory components are then formed by elongation of the gland and twisting of the ends.
The primordial sweat ducts are formed by epithelial junctions of the developing gland. Eventually, the core cells degenerate to form the lumen of the sweat duct.
Cells on the periphery of the gland differentiate into secretory and myoepithelial cells.
Myoepithelial cells are believed to be specialized smooth muscle cells that function to expel sweat from the gland. Eccrine sweat glands first appear on the palms and soles during the fourth month of gestation; they become functional shortly after birth.
On the other hand, apocrine sweat glands do not function until hormonal stimulation during puberty, and their ducts do not open onto the surface of the skin. This is because these glands originate in the germinal layer of the epidermis.
Therefore, the downward growth does not produce an open duct to the surface of the skin. Instead, the ducts open into the hair follicles and sweat is released through the opening of the hair into the skin.
Channels from these apocrine sweat gland ducts enter the superficial hair follicle into the sebaceous gland, resulting in protein-rich sweat rather than the watery sweat associated with eccrine sweat glands.
Blood and lymphatic supply
The sweat glands along with all other skin appendages receive blood supply from skin perforators from the underlying source vessels.
The perforators can branch directly from the source as septocutaneous or fasciocutaneous perforators or from muscle branches as musculocutaneous perforators.
Once these perforators reach the skin, they form extensive networks called dermal and subdermal plexuses. The interconnections between these plexuses are formed through connecting vessels that run perpendicular to the surface of the skin, forming a continuous vascular plexus in the skin.
Lymphatic drainage parallels the blood supply, beginning with blind lymphatic capillaries in the dermal papillae. These drain into the dermal and deep dermal plexuses that eventually coalesce to form larger lymphatic vessels.
Nerves
The eccrine sweat glands receive sympathetic innervation through cholinergic fibers that send impulses in response to changes in core body temperature. The sympathetic innervation of the sweat glands is mediated by the thermoregulatory center of the hypothalamus.
A short preganglionic cholinergic fiber originates in the thoracolumbar region of the spinal cord with synapse of the postganglionic neuron via nicotinic acetylcholine.
Postganglionic fiber releases acetylcholine, which differs from all other sympathetic postganglionic fibers that release norepinephrine.
Cholinergic stimulation of muscarinic receptors induces sweating. The apocrine sweat glands receive adrenergic sympathetic innervation.
Because apocrine sweat glands respond to norepinephrine, they are involved in emotional sweating due to stress, fear, pain, and sexual stimulation.
Clinical significance
Given the role of the sweat glands in thermoregulation, both the eccrine and apocrine glands are associated with various diseases ranging from mild and uncomfortable to life threatening.
Sweating disorders can have emotional, social, and professional implications.
Hyperhidrosis is the excessive excretion of sweat in excess of the amount necessary for thermoregulation. It can be idiopathic or due to other endocrine, neurological, or infectious disorders.
Treatment options include topical medications, oral medications, surgical procedures, or botulinum toxin injection. Bromhidrosis is a similar disorder that presents with excessive smelly perspiration.
It can involve apocrine or eccrine sweat glands; Apocrine bromhidrosis tends to develop after puberty, while eccrine bromhidrosis can develop at any age.
It is caused by excessive perspiration that secondarily becomes smelly by bacterial decomposition. Because poor hygiene most often aggravates bromhidrosis, an effective treatment is to improve personal hygiene.
Surgical approaches, antibacterial agents, and antiperspirants are also treatment options.
The sweat glands of cystic fibrosis (CF) patients are not effective at reabsorbing salt, which has important implications.
CF is an autosomal recessive congenital disease in which the cystic fibrosis transmembrane regulator (CFTR) that normally inhabits the apical membrane of epithelial cells is defective.
CFTR is a transmembrane protein that functions as part of a chloride ion channel regulated by cAMP; in normal sweat glands, the ductal epithelium reabsorbs sodium and chloride ions in response to aldosterone, making sweat hypotonic.
In CF patients, the sweat glands do not reabsorb chloride, which affects sodium reabsorption, resulting in salty sweat and the inability of the sweat glands to participate in ion regulation.
Disruption of the same membrane proteins in the respiratory and gastrointestinal epithelium produces accumulations of thick mucus.
Another autosomal recessive congenital disorder that affects the sweat glands is lamellar ichthyosis. Babies with persistent, flaky skin and hair growth may be reduced.
Impaired sweat gland development often causes babies to suffer in extremely hot climates, as they cannot maintain thermoregulation through sweat.
Hidradenitis suppurativa is a chronic inflammatory disease that affects the hair follicles. This medical condition has been classically associated with apocrine sweat glands, as it manifests itself after puberty in concentrated areas of the apocrine gland.
However, the pathophysiology involves follicular occlusion rather than apocrine disorder as previously thought. Patients often present with tender subcutaneous and suppurative nodules and abscesses in the armpits and groin. Lesions can form extensive sinus tracts and scars.
Hypohidrotic ectodermal dysplasia is a disease characterized by hypotrichosis (decreased growth of the scalp and body hair), hypodontia (congenital absence of teeth), and hypohidrosis.
The term “hypohydrotic” indicates impaired ability to perspire. Patients born with hypohydrotic ectodermal dysplasia have difficulty regulating body temperature and therefore must learn to modify their environment to control heat exposure.
