Skin refers to the surface of the body wrapped in muscle outside the tissue, is the largest organ of the human body, the total weight of the skin accounted for 5% to 15% of body weight, the total area of 15 to 20,000 square meters, the thickness varies from 0.5 mm to 4 mm depending on the person or site. human and higher animals skin consists of epidermis, dermis, subcutaneous tissue three layers, and contains accessory organs (sweat glands, sebaceous glands, nails, toenails), as well as blood vessels, lymphatic vessels, nerves and muscles. Lymphatic vessels, nerves and muscles.
The thickest skin is at the bottom of the foot, with a thickness of 4 mm, while the skin on the eyelid is the thinnest, with less than 1 mm.
I. Epidermis
The epidermis is the outermost layer of the skin, with an average thickness of 0.2 mm, and can be divided into five layers from the outside in according to the different developmental stages and morphological characteristics of the cells.
1.Keratin layer: It is composed of several layers of keratinized cells and contains keratin. It resists friction and prevents extravasation of body fluids and internal invasion of chemicals. Keratin is strong in absorbing water, and generally contains no less than 10% water to maintain the skin’s softness. If it is lower than this value, the skin will be dry and scaly or cracked. 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 keratinization incomplete.
2, the transparent layer: by 2 to 3 layers of nuclear has died of flat transparent cells, containing keratin. It can prevent the passage of water, electrolytes and chemical substances, so it is also called the barrier zone. This layer is most obvious in the palm and metatarsal areas.
3.Granular layer: It is composed of 2 to 4 layers of flattened pyknotic cells and contains a large number of basophilic hyaline keratin granules. When the number of flattened spindle cells in the granular layer increases, it is called granular layer hypertrophy, and is often accompanied by hyperkeratosis. The granular layer disappears, often accompanied by hyperkeratosis.
4.Echinocyte layer: It is composed of 4-8 layers of polygonal echinocytes, which tend to flatten from the bottom to the top, and the cells are interconnected by bridge grains, forming the so-called intercellular bridge.
5, basal layer: also known as the hair growth layer, composed of a layer of cylindrical cells arranged in a fence. This layer of cells constantly divides (often 3% to 5% of the 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 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 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 melanocytes (also known as dendritic cells) that originate from the neural crest and account for 4% to 10% of the entire basal cells, which can produce melanin (pigment granules) and determine the shade of skin color.
In addition, the latest findings: from the perspective of skin care epidermis is not the outermost skin component, there is a protective sebaceous membrane outside.
Second, the dermis
Originated from the mesoderm, composed of fibers, stroma, cells. Close to the epidermis of the dermal papillae called the papillae layer, also known as the superficial dermis; underneath 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, 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 bundle is thin, closely arranged, travel in different directions, and not intertwined.
(2) Elastic fibers: more in the lower part of the reticular layer, more coiled under the collagen fiber bundles and around the skin appendages. In addition to giving the skin elasticity, they also constitute the scaffold of the skin and its appendages.
(3) Reticular fibers: Considered immature collagen fibers, they surround the skin appendages and blood vessels. In the reticular layer, the fiber bundles are thicker, more loosely arranged, interwoven into a network, and more parallel to the skin surface. Since the fiber bundles are spiral, they have some stretchability.
2.Matrix: It is an amorphous, homogeneous gel-like substance, which is stuffed between fiber bundles and cells, providing material support for various components of the skin and providing a place for material metabolism.
3.Cells: there are mainly the following.
(1) Fibroblasts: can produce collagen fibers, elastin fibers and matrix.
(2) Histiocytes: a component of the reticuloendothelial system, with the ability to phagocytose microorganisms, metabolites, pigment particles and foreign bodies, playing an effective role in clearance.
(3) Mast cells: present in the dermis and subcutaneous tissues, with the dermal papillae being the most abundant. The granules in their cytoplasm can store and release histamine and heparin, etc.
Examples of the significance of the distinction between the epidermis and the dermis: acne and the like are prone to secondary infection if scratched with the hands. Generally speaking, skin infection to the epidermis, what scar does not fall, a skin disease is very strong, it does not fall scar, it is in the epidermis. If the infection has reached the dermis, it must be scarred. The skin is a protective barrier, so do not destroy it.
Third, the subcutaneous tissue
Originated from the mesoderm, in the lower part of the dermis, consisting of loose connective tissue and fatty lobules, under which is adjacent to 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. 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 to lubricate the skin and hair and prevent dryness of the skin, and the secretion is strong after puberty.
(1) Distribution of sebaceous glands: Except for the palm, it is almost all over the body, so in winter, the skin of hands will be especially dry, and there are few sebaceous glands distributed around the eyes, and the skin around the eyes is extremely thin, so it is easy to produce fine lines.
(2) Formation of acidic sebum film: The sebum secreted by sebaceous glands forms a film on the skin, which is weakly acidic and is a natural cream for the skin and has a good protective effect.
(3) Antibacterial effect of sebum film: The weak acidic film (PH5.2 or so) can inhibit the growth of microorganisms on the skin. Normal skin often hosts a variety of bacteria and other microorganisms, but does not cause disease. It depends on factors such as the body’s resistance and the skin’s intact structure and acidic membrane to maintain. When these factors are destroyed, bacteria and other microorganisms can invade the body and cause disease.
(4) Acidic sebaceous film prevents water loss: The sebaceous film has the function of locking water and does not allow water loss from the skin to the air. And for the sebaceous membrane incomplete dry skin, to give it a special supplement some oil, such as night cream, etc..
3, hair: divided into three kinds of long hair, short hair, milli hair. The part of hair above the surface of the skin is called hair stem, the part inside the hair follicle is called hair root, the enlarged part of the lower part of the hair root is called hair ball, and the part protruding into the bottom of the hair ball is called 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 and new hair is grown. Therefore, when you wash your hair or comb your hair, you will find a small amount of hair loss, which is a normal physiological phenomenon.
4, finger (toe) nail: nail, is a human and ape finger (toe) end of the back of the flat nail-like structure, belongs to the connective tissue. For the deformation of the claw, also known as the flat claw, its main component is keratin. It is homologous with the claw, and the claw plate forms a rectangular sheet, which is the product of epidermal keratinization of the finger (toe) end and plays a role in protecting the finger (toe) end. Both human and primate species of the suborder Ape have nails. Some species of the suborder Protossaurus do not have nails, some species only part of the finger (toe) end of the nail, such as the bee monkey (sloth monkey) the second toe for the claw, the rest of the nail, while the finger monkey only the first finger (toe) have nails, the rest of the finger (toe) end of the claw. The main component of both rhinoceros horn and nail is keratin.
Blood vessels, lymphatic vessels, nerves and muscles
1, blood vessels: no blood vessels in the epidermis. The dermis and below have. Arteries enter the subcutaneous tissue and branch up to the junction of subcutaneous tissue and dermis to form a deep vascular network, supplying nutrients 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 auxiliary circulatory systems that prevent the invasion of microorganisms and foreign bodies.
Functions of the skin
I. Protective functions
(a) Defense against mechanical stimuli The skin covers the surface of the body, and the epidermal layers of cells are closely connected. The dermis contains a large number of collagen fibers and elastic fibers, making the skin both tough and soft, with a certain degree of tensile and elastic. When rubbed or pulled by external forces, it can still remain intact and recover its original state after external forces are removed. The subcutaneous tissue is loose and contains a large number of fat cells, which has a cushioning effect and can slow down the impact of external forces and protect the internal tissues from damage.
(B) Defense against physical stimuli Blocking electric current, blocking ultraviolet rays, preventing evaporation of water from the body and infiltration of water from outside the body. The stratum corneum is a bad conductor and has the ability to insulate against electric currents, which can prevent a certain amount of current from damaging the body. The stratum corneum and melanin particles can reflect and absorb some of the ultraviolet rays, preventing them from entering the body and harming the internal tissues. With prolonged sun exposure, the stratum corneum of the skin will thicken accordingly, melanin particles will increase, and the appearance of the skin will become rough and the skin tone will deepen. The sebaceous glands secrete sebum and the sweat glands secrete sweat, which mixes to form an emulsified skin film on the surface of the skin. It moisturizes the stratum corneum, prevents the skin from drying and cracking, and prevents the evaporation of water from the body and the penetration of water from outside the body.
(C) Defense against chemical stimulation The main component of the cells of the stratum corneum is keratin, which is resistant to corrosion by weak acids and bases. Sweat can dilute the acidity of chemical substances to a certain extent and protect the skin.
(D) Defense against biological stimuli The sebaceous film on the surface of the skin is weakly acidic, which can prevent the invasion of bacteria and fungi on the surface of the skin, and has antibacterial and bactericidal effects.
Second, the skin’s regulation of body temperature function
The human body needs a relatively constant body temperature to ensure the normal conduct of various vital activities, the normal body temperature in the range of 36 to 37 ℃. The skin plays an important role in thermoregulation. The skin regulates body temperature in two ways.
(a) over the vascular regulation of body temperature When the external temperature is high, the skin capillary network a large number of open, the body surface blood flow increases, skin heat dissipation increases, so that the body temperature is not too high. When the temperature is low, the skin capillary network is partially closed, and part of the blood flow from the arteries does not go through the body surface, directly from the arteriovenous anastomosis branch into the veins, so that the body surface blood flow is reduced, reducing heat dissipation and maintaining body temperature.
(B) regulate body temperature through sweat gland evaporation When the temperature is high, the body sweats a lot, the process of evaporation of sweat can take away part of the body heat, play a role in reducing body temperature.
Third, the sensory function of the skin The skin contains a wealth of sensory nerve endings, can feel a variety of external stimuli, producing a variety of different sensations, such as touch, pain, pressure, heat, cold, etc..
Fourth, the skin’s secretion and excretion function
(a) secretion function sweat glands can secrete sweat, sebaceous glands can secrete sebum. Sebum mixes with sweat on the skin surface to form an emulsified sebum film, which moisturizes and protects the skin and hair. There are many factors that affect the secretion function of sebaceous glands, mainly in the following aspects.
1, the influence of endocrine androgen and adrenocorticotropic hormone can make the sebaceous gland gland hypertrophy, secretion function is enhanced. So generally male skin is oily than female skin, pores are large.
2, the influence of external temperature When the temperature is high, the sebum secretion is more; when the temperature is low, the sebum secretion is reduced. Therefore, our skin is more oily in summer, and in winter, the skin becomes dry.
3.The influence of skin surface humidity The humidity of skin surface can affect the secretion of sebum diffusion. When the moisture on the skin surface is high, sebum is easy to emulsify and diffuse; while when the skin is dry, the secretion and diffusion of sebum will become slow.
4.The influence of age The secretion of sebum is less in childhood; it starts to increase in adolescence; it starts to decrease gradually after 35 years old. Therefore, the skin of children and middle-aged people is dry, while the skin of adolescence is oily.
5, the influence of diet oily food, spicy stimulating food can make sebum secretion increase. Therefore, people with oily skin, especially those with acne, should not eat sweet, greasy and stimulating foods.
(B) Excretory function The skin excretes waste products produced by metabolism in the body, such as uric acid and urea, through sweating.
V. Respiratory function of the skin
The skin can also breathe through sweat holes and pores, absorbing oxygen directly from the air while expelling carbon dioxide from the body. Its respiratory capacity is about 1% of the lungs. The facial stratum corneum is thin, capillary rich, and directly exposed to the air, its respiratory function is more prominent than other parts of the body, usually too thick makeup or with makeup for too long, will affect the breathing of the skin, which is not good for the health of the skin.
Sixth, the absorption function of the skin
The skin is not absolutely dense and impermeable, it can selectively absorb nutrients from outside.
(A) the absorption pathway of the skin The skin directly from the outside world to absorb nutrients in the following ways.
1.Nutrients penetrate through the cell membrane of the stratum corneum and enter the keratinocytes.
2, a small amount of large molecules and water-soluble substances can be absorbed through the pores, sweat pores.
3, a small amount of nutrients through the surface cell interstitial penetration into the dermis.
(B) The absorption capacity of skin for various substances The absorption capacity of skin for substances is related to the physical and chemical properties of the absorbed substances. Fat-soluble substances are easily absorbed. The skin has a strong absorption capacity for animal fats, so mink oil, lanolin and guano have good nourishing effects on the skin. The skin has the second highest absorption capacity for vegetable oils, and the worst absorption capacity for mineral oils. The skin has a certain absorption capacity for vitamin classes. Fat-soluble vitamins are easily absorbed, such as vitamin A, vitamin D, vitamin E, etc. The absorption capacity for water-soluble vitamins is poor, such as vitamin B and vitamin C. The skin has a certain ability to absorb certain metal elements, such as lead and mercury. Some cosmetics contain lead and mercury, which will cause poisoning and dark spots and rashes after absorption and accumulation in the skin.
(C) factors affecting the absorption function of the skin The absorption function of the skin is affected by the following factors.
1, the thickness of the stratum corneum The thinner the stratum corneum, the easier it is for nutrients to penetrate and be absorbed. When you do skin care, you can use the de-flaking method to make the stratum corneum thinner.
2, the amount of water content of the skin The more water content of the skin, the stronger the absorption capacity. The use of steam spray surface can supplement the water content of the colloid layer, and the skin is dissolved soft can increase the penetration and absorption ability.
3.Pore state When the pores are dilated, the nutrients can reach the dermis through the pores and be absorbed.
4.Local skin temperature When the local skin temperature is high and the sweat pores are open, the nutrients can enter the dermis through the sweat pores and be absorbed. Skin massage, hot film, steam spray surface, etc. can increase the local temperature and promote the absorption of nutrients.