Skin appendages types, function (hair, nails, sweat glands & sebaceous glands)
Skin appendages are epidermal & dermal derived components of the skin that include hair, nails, sweat glands, and sebaceous glands. The skin, along with hair and nails, is the protective covering of the body, the skin prevents germs from entering the body and damaging the internal organs, the skin manufactures vitamin D when exposed to the sun’s ultraviolet rays, and vitamin D is an essential vitamin for healthy skin.
Skin appendages are skin-associated structures, they serve a particular function including sensation, contractility, lubrication, and heat loss. Skin appendages (or adnexa) are derived from the skin, and they are adjacent to it, The skin supports the life of all other body parts and plays a role in maintaining the immune system.
Types of appendages include hair, glands, and nails. hairs are used for sensation, heat loss, filter for breathing & protection, arrector pilli (smooth muscles that pull hairs straight), sebaceous glands secrete sebum onto hair follicle, which oils the hair, sweat glands can secrete sweat with a strong odour (apocrine) or with a faint odour (merocrine or eccrine), and nails (protection).
Hairs are horny threads varying in length, thickness, color, and distribution according to the race, sex, and site of skin.
- The hair shaft develops from the hair follicle which is a cylindrical downgrowth of the epidermis into the dermis, hair follicle ends by the hair bulb.
- The hair bulb is invaginated from below by avascular connective tissue dermal papilla, which if destroyed, the hair follicle dies and no hair will grow again.
Nails are horny plates present on the dorsal surfaces of the terminal phalanges of fingers and toes, they consist of closely-packed hard keratin scales.
These are simple branched alveolar, holocrine glands that develop from the upper third of the hair follicle; therefore, they are usually present where the hair is present. They are most numerous over the head region and the anogenital area.
1- The secretory portion:
- It is a pale flask-shaped structure formed of one or more alveoli with a common duct.
- The alveolus is lined with a layer of flattened germinal cells lying on a basement membrane, these cells proliferate by mitosis, enlarge by accumulating fatty material, become polyhedral, and are pushed to the center, where they degenerate and discharge sebum (holocrine mode), thus; sebum is a mixture of fatty substances, cell debris, and keratin.
2- The excretory duct is short and wide, it opens obliquely into the upper third of the hair follicle or directly into the epidermal surface. The duct is lined by stratified squamous epithelium.
The function of the sebaceous gland
Sebaceous glands secrete sebum which lubricates the skin surface and hair, also has a bactericidal effect. Sebaceous glands are relatively inactive until puberty when they are stimulated by the rising levels of sex hormones.
A disturbance in the normal flow of sebum is one of the reasons for the development of acne during adolescence, Acne is a chronic inflammation of obstructed sebaceous glands.
They are simple coiled tubular glands, they are found deep in the dermis or the hypodermis, there are two types of sweat glands; the eccrine and the apocrine glands.
The eccrine sweat glands
They are distributed all over the body, they secrete a watery secretion rich in sodium chloride.
1- The secretory portion appears in histological sections the form of small rounded acini with a narrow lumen and lined by stratified cuboidal epithelium composed of three cell types:
- The dark pyramidal cells secreting glycoprotein mucoid substance.
- The clear cuboidal cells secreting a watery secretion.
- The myoepithelial cells to move the secretion into the ducts.
2- The long excretory duct is lined by stratified cuboidal cells, It ascends in a helical course to the epidermis where it opens on the skin surface.
The apocrine sweat glands
They are present only in the skin of the axilla, areola of the breast, and perianal region, they begin their secretory function at puberty, their secretion is viscous and has a characteristic odor.
- The acini of the secretory portion are larger in size and of wider lumen than the eccrine glands, the secretory portion is composed of only two cell types; large cuboidal cells with apical secretory granules and myoepithelial cells.
- The excretory duct is lined by stratified cuboidal cells and opens into the hair follicle.
- The secretory cells undergo merocrine, not apocrine, secretion, thus; the glands are misnamed.
Functions of the integumentary system
- Protection of the body against mechanical, chemical, thermal, and bacterial agents, this occurs through the heavily keratinized cells of the stratum corneum.
- Protection of the body against water loss or gain (waterproof-barrier function), this occurs through the lipid-rich extracellular material in stratum granulosum and corneum.
- Screening against sun ultraviolet radiation through melanin pigments.
- Perception of different stimuli from the environment through the presence of various receptors (sensory nerve endings) in the skin.
- Regulation of the body temperature:
– In hot weather, cooling of the body is enhanced by increase sweat secretion and evaporation with vasodilation of the dermal capillaries and closure of the arteriovenous (A-V) shunts, these mechanisms allow maximum cutaneous blood flow and heat loss.
– In cold weather, constriction of dermal blood vessels and opening of the A-V shunts to reduce cutaneous blood flow and reduce heat loss.
- Excretion of nitrogenous products and sodium chloride in sweat.
- Formation of vitamin D in the epidermis (mainly in stratum basale and spinosum) when the skin is exposed to sunlight.