I. Skin structure and function
The skin is the largest organ of the human body, with a total weight of 5%-15% of body weight, a total area of 1.5-2 square meters, and a thickness of 0.5-4 mm that varies from person to person or from site to site. The skin covers the entire body and it protects various tissues and organs in the body from physical, mechanical, chemical and pathogenic microbial attacks. The skin serves as a barrier in two ways: on the one hand, it prevents the loss of water, electrolytes and other substances from the body; on the other hand, it prevents the invasion of harmful substances from outside. It maintains the stability of the body’s internal environment and plays an important protective function physiologically, while the skin is also involved in the metabolic processes of the body. The skin has several colors (white, yellow, red, brown, black, etc.) and varies mainly according to race, age and location. The skin of children is much thinner than that of adults, and the skin of the extremities and trunk is thicker on the extensor side than on the flexor side; the skin is thickest on the back of the occiput, the neck, the buttocks, and the palms and soles of the feet, and the skin is thinnest on the eyelids, vulva, and breasts.
The skin is composed of epidermis, dermis and subcutaneous tissue, and contains accessory organs (sweat glands, sebaceous glands, nails, toenails) as well as blood vessels, lymphatic vessels, nerves and muscles.
The epidermis is the outermost layer of the skin, with an average thickness of 0.2 mm. It can be divided into five layers from the outside inward according to the different developmental stages and morphological characteristics of the cells.
1, the stratum corneum: composed of several layers of keratinized cells, containing keratin. It resists friction and prevents extravasation of body fluids and internal invasion of chemicals. Keratin absorbing power is strong, the general water content is not less than 10%, in order to maintain the softness of the skin, such as below this value, the skin is dry, scaling or cracking. Due to the different parts, its thickness varies greatly, such as eyelids, foreskin, forehead, abdomen, elbow fossa and other parts of the thin, palm and plantar parts of the thickest. The cells of the stratum corneum have no nucleus, if there is a nuclear remnant, it is called incomplete keratinization.
2.Transparent layer: consists of 2-3 layers of flat transparent cells whose nuclei have disappeared, containing keratin. It can prevent water, electrolytes and chemical substances from passing through, so it is also called the barrier zone. This layer is most obvious in the palm and metatarsal areas.
3.Granular layer: composed of 2-4 layers of flattened spindle cells, containing a large number of basophilic hyaline keratin granules. When the number of granular layer flattened spindle cells layer increases, it is called granular layer hypertrophy, and often accompanied by hyperkeratosis; granular layer disappears, often accompanied by incomplete keratinization.
4, spiny cell layer: composed of 4-8 layers of polygonal spiny cells, from the bottom upward tends to flatten, the cells are connected to each other by bridge particles, forming the so-called intercellular bridge.
5, basal layer: consists of a layer of cylindrical cells arranged in a fenestrated pattern. This layer of cells constantly divides (often 3-5% of cells divide), gradually pushing upward, keratinization, deformation, the formation of other layers of the epidermis, and finally keratinized and shed. The time between the division of the basal cells and their shedding is generally considered to be 28 days, which is called the turnover time, of which 14 days is from the division of the basal cells to the uppermost layer of the granular layer, and 14 days is from the formation of the stratum corneum to the final shedding. The basal cells are interspersed with a kind of melanocytes (also called dendritic cells) from the neural crest, accounting for 4% to 10% of the entire basal cells, which can produce melanin (pigment granules) and determine the shade of skin color.
Second, the dermis comes from the mesoderm and is composed of fibers, stroma and cells. Close to the epidermis of the dermal papillae called the papillae layer, also known as the superficial dermis; under it is called the reticular layer, also known as the deep dermis, there is no strict boundary between the two.
1, fiber: there are collagen fibers, elastic fibers and reticular fibers three.
(1) collagen fibers: the main component of the dermis, accounting for about 95%, the collection consists of bundles. In the papillary layer fiber bundles are thin, closely arranged, travel in different directions, and do not intertwine. In the reticular layer, the fiber bundles are thicker, more loosely arranged, interwoven into a network, and more parallel to the skin surface. Because the fiber bundles are spiral, so there is a certain stretchability.
(2) Elastic fibers: more in the lower part of the reticular layer, mostly coiled under the collagen fiber bundles and around the skin appendages. In addition to giving the skin elasticity, they also form the scaffold of the skin and its appendages.
(3) Reticular fibers: considered immature collagen fibers, which surround the skin appendages and blood vessels.
(2) Matrix: It is an amorphous, homogeneous gel-like substance that fills the interfibrillar bundles and intercellular spaces, providing material support for the various components of the skin and providing sites for material metabolism.
3.Cells: there are mainly the following kinds.
(1) fibroblasts: can produce collagen fibers, elastic fibers and matrix.
(2) Histiocytes: are a component of the reticuloendothelial system and have the ability to phagocytose microorganisms, metabolites, pigment particles and foreign bodies, playing an effective role in clearance.
(3) Mast cells are found in the dermis and subcutaneous tissues, with the papillary layer of the dermis being the most abundant. The granules in their cytoplasm can store and release histamine and heparin, etc.
Subcutaneous tissue is derived from mesoderm, in the lower part of the dermis, consisting of loose connective tissue and fatty lobules, which is immediately below the muscle membrane. The thickness of the subcutaneous tissue varies according to age, sex, location and nutritional status. It has the function of preventing heat dissipation, reserving energy and resisting external mechanical impact.
D. Accessory organs
1.Sweat glands.
(1) small sweat glands: that is, the general sweat glands. Located in the dermal reticular layer of the subcutaneous tissue. In addition to the lips, glans, inner surface of the foreskin and clitoris, distributed throughout the body. They are more numerous in the palm, metatarsal, armpit and groin. Sweat glands can secrete sweat and regulate body temperature.
(2) Sweat glands: mainly located in the armpits, areola, umbilicus, perianal and external genitalia and other parts. After puberty, the secretion is vigorous, and its secretions are decomposed by bacteria to produce a special odor, which is one of the causes of oedema.
2, sebaceous glands: located in the dermis, near the hair follicles. In addition to the palm, plantar, distributed throughout the body, with more scalp, face, chest, scapula and mons pubis. The sebaceous glands in the lips, nipples, glans and labia minora open directly on the skin surface, while the rest open in the upper 1/3 of the hair follicle. Sebaceous glands can secrete sebum, lubricate the skin and hair, prevent dry skin, and secrete vigorously after puberty.
3, hair: divided into three kinds of long hair, short hair and milli hair. The part of the hair above the skin surface is called the hair stem, the part inside the hair follicle is called the hair root, the enlarged part of the lower part of the hair root is called the hair ball, and the part protruding into the bottom of the hair ball is called the hair papilla. The hair papilla contains rich blood vessels and nerves to maintain the nutrition and generation of hair, and if atrophy occurs, hair loss occurs. The hair grows and rests periodically, but all hairs are not in the same cycle, so human hair is shed and grows at any time. The length of the cycle varies for different types of hair. The anagen phase of hair is about 5 to 7 years, followed by the regression phase, which is about 2 to 4 weeks, then the resting phase, which is about several months, and finally the hair falls out. After that, the hair grows back to a new growth period. Therefore, it is a normal physiological phenomenon to find a small amount of hair loss when washing or combing hair.
V. Blood vessels, lymphatic vessels, nerves and muscles
1.Vascular: There is no blood vessel in the epidermis. Arteries enter the subcutaneous tissue and then branch up to the junction of subcutaneous tissue and dermis to form a deep vascular network, supplying nutrition to hair papillae, sweat glands, nerves and muscles.
2.Lymphatic vessels: The blind end of capillary lymphatic vessels starts in the papillary layer of the dermis and travels along the blood vessels, forming a network of lymphatic vessels at the superficial and deep vascular network, which gradually converge into thicker lymphatic vessels and flow into the lymph nodes to which they belong. The lymphatic vessels are an auxiliary circulatory system that prevents the invasion of microorganisms and foreign bodies.
Skin classification
I. Dry skin.
1, performance characteristics: skin moisture, oil are not normal, dry, rough, lack of elasticity, skin PH value is not normal, pores are fine, facial skin is thin, easy to be sensitive. The facial skin is dull, lusterless, easy to break, flaky, long spots, and not easy to apply makeup. However, the appearance is relatively clean, the skin mound is flat, the skin furrow is in a straight line, shallow, messy and wide. The skin is loose, easy to produce wrinkles and aging phenomenon; dry skin can be divided into two kinds of lack of oil and lack of water.
2, maintenance focus: more massage care, promote blood circulation, pay attention to the use of nourishing, whitening, active repair cream and nutrition cream. Pay attention to replenish the skin’s moisture and nutrients, regulate the water-oil balance of care.
3, skin care products selection: drink more water, eat more fruits, vegetables, do not too frequent bathing and excessive use of cleansers, pay attention to the week care and use to maintain the nutritional products, choose non-foaming, less alkaline cleaning products, with moisturizing lotion.
Second, normal skin.
1, performance characteristics: water, oil moderate, skin pH moderate, smooth and soft skin, rich in elasticity, red and lustrous, small pores, no blemishes, neatly arranged lines, skin furrow vertical and horizontal direction, is the most ideal beautiful skin. Neutral skin is mostly found in children, usually in young girls under 14 years old before development. Younger people, especially after puberty, rarely maintain normal skin. This kind of skin is generally hot summer easy on the oil side, winter easy on the dry side.
2, maintenance focus: pay attention to cleaning, toning, moisturizing and massage of the week care. Pay attention to daily hydration, adjust the oil balance of care.
3, skin care products selection: according to age, seasonal selection, hydrophilic summer, winter nourishing selection, a wide range of choices.
Third, oily skin.
1, performance characteristics: high oil secretion, T parts of the oil is obvious, large pores, touch blackheads, thick hard cortex is not smooth, deeper skin lines; dark yellow appearance, darker skin tone, alkaline skin, better elasticity, not easy to wrinkles, aging, not sensitive to external stimuli. The skin is easy to absorb ultraviolet light easy to darken, easy to take off makeup, easy to produce acne, acne.
2, maintenance focus: keep the skin clean and fresh at all times, eat less sugar, coffee, stimulating food, eat more vitamin B2/B6 to increase skin resistance, pay attention to hydration and deep cleaning of the skin, control the excessive secretion of oil, regulate the balance of the skin.
3, skin care products selection: use less oil, refreshing, inhibit sebum secretion, astringent effect of the stronger skin care products. Use warm water to wash your face during the day and choose a cleanser suitable for oily skin to keep the pores open and the skin clean. Makeup should not be applied to acne areas, and oily skin care products should not be used, and makeup appliances should be washed or replaced frequently. What’s more, you should pay attention to moderate moisturizing.
Fourth, mixed skin.
1, performance characteristics: a skin presents two or more appearances (with the characteristics of both oily and dry skin). Mostly seen as the face T-zone area is prone to oil, the rest of the dry, and sometimes acne, 80% of men are mixed skin. Combination skin mostly occurs between the ages of 20-35.
2, maintenance focus: according to the piece of oily, dry, neutral skin respectively focus on processing, in the use of skin care products, first moisturize the drier parts, and then in other parts with the remaining amount of wipe. Pay attention to timely hydration, supplemental nutrients, and adjust the balance of the skin.
3, skin care products selection: summer reference to the third choice of oily skin, winter reference to the first choice of dry skin.
V. Sensitive skin.
1, performance characteristics: the skin is more sensitive, thin sebaceous film, the skin’s own protective ability is weak, the skin is prone to redness, swelling, prickling, itching, pain and peeling, dehydration phenomenon.
2, maintenance focus: frequent maintenance of the skin; wash the face when the water can not be too hot and cold, to use a mild cleanser to wash the face. In the morning, you can use sunscreen to avoid sun damage to the skin; in the evening, you can use nutritious make-up water to increase the moisture of the skin. Pay attention to foods that cause allergies in your diet. After the skin allergy, immediately stop using any cosmetics, observe and maintain the skin care.
3, skin care products selection: should first be adapted to the test, in the case of no reaction before use. Do not use poor quality cosmetics or use multiple cosmetics at the same time, and pay attention not to frequently replace cosmetics containing too much fragrance and too acidic and alkaline skin care products can not be used, but should choose cosmetics suitable for sensitive skin.