Skin Appendages: Dermatology for Permanent Makeup Artists

After articles on the structure of the skin , blood vessels and the nervous apparatus, we turn to information about the appendages of the skin. Skin appendages include sebaceous and sweat glands, hair and nails.

The sebaceous glands (glandulae sebaceae) are located throughout the skin, with the exception of the palms and soles, they are usually in close contact with the hair follicles, where their ducts open. Only in the skin of the red border of the lips, the glans penis, the inner layer of the foreskin, the coronary sulcus (the glands of the foreskin are the tysonian glands), the labia minora, as well as in the nipple and areola of the mammary gland, along the edge of the eyelids (the glands of the cartilage of the eyelids are meibomian glands) sebaceous glands open directly on the surface of the skin. Each follicle has one or more sebaceous glands. Larger glands are observed in persons 17-25 years old and are located in the face (nose, cheeks), chest and back.

By structure, the sebaceous glands belong to simple alveolar glands and have a holocrine type of secretion, in which the formation of a secret is associated with cell destruction. Most sebaceous glands are spherical or ovoid in shape. Their secretory sections consist of 1-2 lobules surrounded by connective tissue. The lobules consist of acini or alveoli that open into a common duct. The acini of the sebaceous gland are devoid of gaps; they are compact formations consisting of concentrically located cells lying on the basement membrane. In the alveoli of the sebaceous gland there are poorly differentiated prismatic cells capable of mitotic division and constituting the outermost layer of the glandular epithelium, as well as cells at different stages of fatty degeneration. The cells that form the outer germ layer have large nuclei that occupy most of the cytoplasm. By mitosis, they form cells located inside the alveoli, having a round or polygonal shape and a cytoplasm with drops of fat. In fully differentiated cells, lipid droplets occupy the entire cytoplasm, and the nuclei shrivel, become hyperchromic, and die. As fat accumulates, the cells move towards the excretory duct and disintegrate. The short excretory duct of the sebaceous glands is lined with stratified epithelium, directly passing into the epithelium of the outer epithelial sheath of the hair follicle.

Sweat glands (glandulae suboriferae) are simple tubular glands. Their number in human skin is very large (up to 3.5 million). They can be found in any part of the skin, with the exception of the skin of the glans penis, the inner leaf of the foreskin, the outer surface of the labia minora. Most of the human sweat glands belong to the eccrine (merocrine) glands, the secretion of which is not accompanied by even a partial death of the secerating cellular elements. Apocrine (holocrine) glands are found only in certain areas (in the armpits, around the anus, on the skin of the pubis and areola, as well as in the skin of the labia majora), the secretion of which is associated with partial cell death.

Eccrine (merocrine) sweat glands consist of a secretory section represented by a glomerulus surrounded by a basement membrane and lined with a single-layer epithelium, the cells of which, at rest and participating in secretion formation, are cylindrical in shape and contain secretory granules 1–2 μm in diameter, and after secretion they flatten . On the basement membrane, in addition to secretory, there are also myoepithelial cells containing a large number of myofilaments in the cytoplasm. contracting under the influence of nerve impulses, which is associated with secretion. The excretory duct of the eccrine sweat gland ends in the basal layer of the epidermis, and then continues in the form of a corkscrew-shaped tortuous fissure, opening on the surface of the skin with a sweat pore. Apocrine (holocrine) sweat glands are deeper, have a large size, and their excretory ducts, similar to the excretory ducts of the eccrine sweat glands, flow into the sebaceous hair follicles.

Hair . Each hair (pilus) has two parts: the shaft and the root. The shaft is the part of the hair that protrudes above the surface of the skin. The hair root is embedded in the dermis and sometimes reaches the subcutaneous fatty tissue. The root is surrounded by epithelial root sheaths and is immersed in a connective tissue bag - the dermal sheath, which make up the hair follicle. The hair follicle has a cylindrical shape and opens on the surface of the skin with a kind of extension - a funnel in which the hair shaft is placed. At the border of the upper and middle thirds of the follicle, the excretory duct of the sebaceous gland opens into it. The epithelial part of the hair follicle is formed on the 2-3rd month of intrauterine life by immersion in the connective tissue of the dermis of the processes of the integumentary epithelium. However, only in the region of the funnel, the epithelium retains all its layers. Below the funnel, the epithelium lining the follicle consists only of cells of the basal and spinous layers. This part of the epithelial wall of the follicle is called the outer root sheath. As it deepens and approaches the bulb, the outer root sheath passes into the growth layer of the epidermis and the cells acquire the ability to keratinize. The outer root sheath serves as a source of hair and follicle cells for hair growth and skin wound healing. The connective tissue bag of the hair follicle consists of soft fibrous connective tissue with a large number of elastic and reticular fibers. The latter, on the border with the outer root sheath, form a basement membrane.

Hair follicles are entwined with a large number of nerve fibers. The deepest expanded part of the hair root is called the hair follicle; the lower part of the bulb - the matrix - consists of undifferentiated pluripotent cells, which are characterized by a very high mitotic activity and ensure hair growth. Here are melanocytescapable of synthesizing melanin. At the base of the follicle, the hair (dermal) papilla protrudes into the hair follicle, containing vessels that feed the hair follicle. The hair follicle consists of polygonal cells, constantly multiplying and containing a large amount of pigment. Bulb cells form both the hair itself and several rows of cells located between the hair root and the outer root sheath, forming an internal root sheath that is interrupted at the top of the follicle, usually at the level of the sebaceous gland duct. It consists of three layers: the cuticle of the internal vagina is located inside, the granular layer of Huxley and the pale layer of Henle lie outside of it. In the hair root, the medulla, cortex, and cuticle can be distinguished. The medulla is determined only in long hairs and consists of one or more layers of polygonal cells containing remnants of the nucleus and pigment. They originate from a stem cell located directly above the papilla and differentiate as they move upward. The cortical substance, representing the bulk of the hair, consists of one or more layers of cells that differentiate from the cambial cells of the bulb, located lateral to the cambial cells of the medulla: as they differentiate, these cells lengthen in the vertical direction; the cortical substance, formed from spindle-shaped horn elements, contains a large amount of pigment.

The hair cuticle adjacent to the cortical substance consists of 6-10 layers of cells (horny plates), located in a tile-like manner and not containing pigment. The cuticle of the hair root, connecting with the cuticle of the inner root sheath, creates a strong bond between the hair and the walls of the hair follicle. The hair root without a sharp boundary passes into its shaft, in which all processes of differentiation are completed. The shaft contains a cortical substance and a cuticle; the medulla in thick hair disappears at the level of the funnel. Approximately at the level of the middle of the follicle, the muscle that lifts the hair is attached to the connective tissue bag at an acute angle. Its other end is woven into the fibrous framework of the dermis. When the muscle contracts, not only the erection of the hair occurs, but also the secretion of the sebaceous glands is squeezed out. The sebum that has fallen on the surface of the skin, having a low thermal conductivity, prevents heat loss.

Nails . Nail (unguis) is a horny plate covering the back surface of the distal phalanx of the fingers. It is located on the nail bed. Distinguish between the body and the root of the nail. The body of the nail - its visible part, has a pink color due to translucent capillary blood. Behind and from the sides, it is covered with skin folds - nail rollers. The roller, arcuately covering the proximal part of the nail, forms a thin horny - supraungual plate (eponychium). The part of the body of the nail, adjacent to the root and having the appearance of a white-colored crescent, is called the hole of the nail. The free edge of the nail (margo liber) protrudes forward. The backmost part of the nail - the root of the nail (radix unguis) - goes deep under the nail fold. The epidermal cells of the proximal part of the nail bed, located under the root of the nail, are called the nail matrix. Due to the matrix, the nail grows in length. Matrix cells of epidermal origin, characterized by larger sizes and light homogeneous cytoplasm, are called onychoblasts. The lower cells of the matrix proliferate, due to which the nail grows and thickness; the upper cells of the matrix differentiate into the stratum corneum of the nail. The nail consists of a dense compact horny mass containing 89% solid keratin, 10% water and about 1% fat. The outer surface of the nail is smooth, the inner ~ uneven due to the formation of horny protrusions and grooves, due to which the nail is tightly adjacent to the nail bed. The growth rate of nails is on average 0.5-1 mm per week. Renewal of the entire nail plate occurs in 170-230 days. Nails grow faster on the hands than on the feet.