SKIN ATTACHMENTS: HAIR, MUSCLES, NAILS, GLANDS - ANATOMY AND PHYSIOLOGY - 2024

The skin annexes, also known as cutaneous annexes, are tegumentary structures of mammals that have specialized functions, such as thermal insulation, mechanical protection, extension of the sense of touch and production of various types of secretions.

Human skin attachments include hair (cephalic hair; body hair), the muscles that produce capillary erections, the fingernails and toenails, the breasts, the sebaceous glands, and the apocrine and eccrine sweat glands.

Source: Jan Homann

Within the class Mammalia (mammals), humans are classified within the order Primates. Compared to other mammals, primates are distinguished by having a single pair of pectoral breasts and by lacking certain skin attachments, such as horns and antlers, as well as various types of scent glands.

Compared to other primates, humans are distinguished by having cephalic hair (hair, beard) of continuous growth, and underdeveloped body hair (hair).

Human skin or skin attachments

Hair

Like other skin attachments, it is derived from the epidermis. It is found on the entire skin surface, with the exception of the palms of the hands, the soles of the feet and parts of the genitals. There are three types of hair:

- Lanugo, which are long, fine hairs that cover the fetus until shortly before birth (seen in premature babies).

- Body hair, which are short, fine hairs that cover most of the body surface.

- Terminal hair, which are long hairs on the scalp, face, armpits and genital region.

Externally, the hairs consist of fine, flexible tubes made up of fully keratinized (dead) epithelial cells. Internally, they are surrounded by hair follicles, invaginate within the dermis and hypodermis, which contain fat and are lined by living epithelial cells.

In most mammals, the fur forms an insulating layer that promotes thermoregulation, protects the skin from friction and extends the sense of touch. The latter is exemplified by the vibrisas ("whiskers" of mice, cats and other animals).

With the exception of terminal hairs, which form an insulating layer (head), or reduce friction (armpits; genital region), these functions have disappeared in humans.

Erector hair muscles (arrector pili)

They are small bundles of smooth muscle that join hairs at their roots to the upper layer of the dermis. They are autonomously controlled by adrenergic sympathetic nerves. They act collectively. By contracting, they make the hairs rise relative to the skin.

In non-human mammals, the simultaneous erection of the body hairs causes the fur to fill with cavities with still air, that is, it becomes more voluminous and fluffy. Typically, this is a response to cold and wind to conserve heat.

In some animals, such as wolves and dogs, the erection of the dorsal coat is a visual signal that indicates the readiness to defend or attack.

In humans, the erector hair muscles are vestigial and do not contribute to thermoregulation. However, they retain the ancestral ability to contract in response to cold, fear, and fury, producing horrifying (popularly called "goose bumps"). This reaction is usually accompanied by tremors that raise body temperature.

Nail

In the first fully terrestrial vertebrates, the nails served to exert traction on the substrate during locomotion. This function has been preserved in their descendants, which include reptiles, birds and mammals, in which the nails have also been adapted for grooming, defense, and attack.

In humans, the nails have lost their original locomotive function, but they retain the grooming function, protect the fingertips, have tactile functions, and serve as tools for manipulating, separating and piercing objects.

Like hair, nails are epithelial structures made up of keratinized dead cells. They consist of: 1) sheet; 2) matrix; 3) basement; 4) surrounding folds.

The lamina, or visible part of the nail, is made up of multiple flattened layers of keratinized cells (oncocytes).

The matrix is ​​a thick specialized epithelium located under the posterior part of the lamina. It is composed of proliferating living cells (keratinocytes) that give rise to oncocytes.

The basement is formed by the basal and spiny layers of the epidermis. It is located under the anterior part of the lamina. It is continuously keratinized to keep the nail attached.

The surrounding folds are composed of the epidermis that covers the root and the lateral edges of the lamina.

Mammary glands or breasts

They are present and functional in the females of all mammals. They can be present without being functional (monotremes; placental mammals), or absent (marsupials), in males. The accumulation of fatty tissue under them that begins during puberty produces the characteristic breasts of human females.

They are highly specialized epidermal glands. They have a branched structure that makes them much larger and more complex than other skin glands.

Due to similarities in the mode of secretion and in some aspects of development, it has been proposed that the mammary glands are derived from sebaceous glands or basal apocrine sweat glands.

In the skin of the embryo, they develop along two parallel ventrolateral lines, in which the epidermis invaginates into the dermis and hypodermis to form ducts. These end in basal alveoli grouped in lobes and surrounded by milk-producing cells.

The ducts converge on the surface under a raised nipple in which, when there is milk production, they open to the outside.

During breastfeeding, nerve impulses that travel from the nipple to the mother's brain cause the hypothalamus to release oxytocin. This hormone stimulates the contraction of the alveoli, forcing the milk into the ducts and the nipple.

Sebaceous glands

They are found in the dermis, usually closely associated (as lateral protrusions) with hair follicles, in which they discharge their secretions. They consist of pear-shaped alveoli with projecting ducts connected to these follicles.

They are present under all skin surfaces, with the exception of the palms of the hands and the soles of the feet. They are very abundant on the face, chest and back.

Its internal cells contain lipids (triglycerides, cholesterol, cholesterol esters, fatty acids), collectively called sebum, which they release when disintegrating under the stimulation of testosterone.

Because your cells are the secreted product, the endocrine glands fall into a larger category called the holocrine glands.

The oily nature of sebum has a softening and waterproofing effect on hair and skin.

In some places on the skin (eyelids, lips, areolas, parts of the female and male genitalia), and in some mucous membranes (mouth and lips), the sebaceous glands are not associated with hair follicles, opening directly to the outside.

Examples of sebaceous glands include those that produce, together with the apocrine glands, the earwax of the external auditory canal and the eyelid secretions that lubricate the conjunctiva.

Apocrine sweat glands

Apocrine sweat glands are mainly present in the armpits, pubis, anogenital region, foreskin, and around the nipples.

They are large, tubular and convoluted glands. Its secretory component is housed in the lower dermis and hypodermis, surrounded by fat cells and blood vessels.

Its secretions, which consist of a milky and viscous yellowish or whitish liquid, rich in lipids, are discharged into the hair follicles under adrenergic autonomic control. When drying on the skin they form a shiny film.

They appear around the sixth month of fetal development, but are not fully functional until puberty, when the production of sex hormones increases. To the taste of modern humans, in part due to bacterial action, their secretions have an unpleasant odor that we try to eliminate through the use of soaps and deodorants.

In the case of humans, a definite and important function is usually not recognized for apocrine secretions.

They are certainly not involved in dissipating body heat. However, in other mammals, its production is correlated with reproductive cycles, and its aroma is used as a sexual attractant and to mark the territory.

Eccrine sweat glands

Eccrine sweat glands are present throughout the skin of the body at densities of 100–600 / cm ². Its maximum abundance is reached on the palms of the hands and on the soles of the feet.

As with the apocrine glands, its secretory component is housed in the lower dermis and hypodermis, and its secretions are discharged into the hair follicles. However, they are smaller in size and simpler in structure, and are released under both cholinergic and adrenergic autonomic control.

They produce a colorless aqueous sweat, in which salts of sodium, ammonia and urea are excreted. The evaporation of this sweat significantly dissipates heat from the body, which is why it is considered that the eccrine sweat glands have an eminently thermoregulatory function. The process is called active evaporative cooling.

In addition to humans, horses, camels, and kangaroos have active evaporative cooling capabilities.

However, rodents, rabbits, dogs, and pigs lack it. In the case of humans, when activity and heat are extreme, the loss of water can reach 2 liters / hour and, therefore, is not sustainable for long periods of time.

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