Skin Introduction .
[1]. Skin is the largest organ of the body. It is not uniformly thick.
[2]. At some places it is thick and at some places it is thin.
[3]. The average thickness of the skin is about 1 to 2 mm.
[4]. In the sole of the foot, palm of the hand and in the interscapular region, it is considerably thick, measuring about 5 mm. In other areas of the body, the skin is thin.
[5]. It is thinnest over eyelids and penis, measuring about 0.5 mm only.
Layer of Skin .
Skin is made up of two layers:
I. Outer epidermis
II. Inner dermis.
Epidermis .
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| Structure of skin . |
[1]. Epidermis is the outer layer of skin.
[2]. It is formed by stratified epithelium.
[3]. Important feature of epidermis is that, it does not have blood vessels .
[4]. Nutrition is provided to the epidermis by the capillaries of dermis.
Layers of Epidermis .
Epidermis is formed by five layers:
1. Stratum corneum .
2. Stratum lucidum ,
3. Stratum granulosum ,
4. Stratum spinosum ,
5. Stratum germinativum.
1. Stratum corneum .
[1]. Stratum corneum is also known as horny layer.
[2]. It is the outermost layer and consists of dead cells, which are called corneocytes.
[3]. These cells lose their nucleus due to pressure and become dead cells.
[4]. The cytoplasm is flattened with fibrous protein known as keratin. Apart from this, these cells also contain phospholipids and glycogen.
2. Stratum lucidum .
[1]. Stratum lucidum is made up of flattened epithelial cells.
[2]. Many cells have degenerated nucleus and in some cells, the nucleus is absent.
[3]. As these cells exhibit shiny character, the layer looks like a homogeneous translucent zone. So, this layer is called stratum lucidum (lucid = clear).
3. Stratum granulosum .
[1]. Stratum granulosum is a thin layer with two to five rows of flattened rhomboid cells.
[2]. Cytoplasm contains granules of a protein called keratohyalin.
[3]. Keratohyalin is the precursor of keratin.
4. Stratum spinosum .
Stratum spinosum is also known as prickle cell layer because, the cells of this layer possess some spinelike protoplasmic projections. By these projections, the cells are connected to one another .
5. Stratum germinativum .
[1]. Stratum germinativum is a thick layer made up of polygonal cells, superficially and columnar or cuboidal epithelial cells in the deeper parts. Here, new cells are constantly formed by mitotic division. [2]. The newly formed cells move continuously towards the stratum corneum.
[3]. The stem cells, which give rise to new cells, are known as keratinocytes. Another type of cells called melanocytes are scattered between the keratinocytes.
[4]. Melanocytes produce the pigment called melanin.
[5]. The color of the skin depends upon melanin.
[6]. From this layer, some projections called rete ridges extend down up to dermis. These projections provide anchoring and nutritional function.
Dermis .
[1]. Dermis is the inner layer of the skin.
[2]. It is a connective tissue layer, made up of dense and stout collagen fibers, fibroblasts and histiocytes.
[3]. Collagen fibers exhibit elastic property and are capable of storing or holding water.
[4]. Collagen fibers contain the enzyme collagenase, which is responsible for wound healing.
Dermis is made up of two layers .
1. Superficial papillary layer .
2. Deeper reticular layer.
Superficial papillary layer .
[1]. Superficial papillary layer projects into the epidermis.
[2]. It contains blood vessels, lymphatics and nerve fibers. This layer also has some pigment containing cells known as chromatophores.
[3]. Dermal papillae are fingerlike projections, arising from the superficial papillary dermis.
[4]. Each papilla contains a plexus of capillaries and lymphatics, which are oriented perpendicular to the skin surface.
[5]. The papillae are surrounded by rete ridges, extending from the epidermis.
Reticular layer .
[1]. Reticular layer is made up of reticular and elastic fibers.
[2]. These fibers are found around the hair bulbs, sweat glands and sebaceous glands.
[3]. The reticular layer also contains mast cells, nerve endings, lymphatics, epidermal appendages and fibroblasts.
[4]. Immediately below the dermis, subcutaneous tissue is present. It is a loose connective tissue, which connects the skin with the internal structures of the body.
[5]. It serves as an insulator to protect the body from excessive heat and cold of the environment.
[6]. Lot of smooth muscles called arrector pili are also found in skin around the hair follicles.
Appendages of Skin .
Hair follicles with hair, nails, sweat glands, sebaceous glands and mammary glands are considered as appendages of the skin.
Color of Skin .
Color of skin depends upon two important factors:
1. Pigmentation of skin .
2. Hemoglobin in the blood.
Pigmentation of Skin .
[1]. Cells of the skin contain a brown pigment called melanin, which is responsible for the color of the skin.
[2]. It is synthesized by melanocytes, which are present mainly in the stratum germinativum and stratum spinosum of epidermis. After synthesis, this pigment spreads to the cells of the other layers.
Melanin .
[1]. Melanin is the skin pigment and it forms the major color determinant of human skin.
[2]. Skin becomes dark when melanin content increases.
[3]. It is protein in nature and it is synthesized from the amino acid tyrosine via dihydroxyphenylalanine (DOPA).
[4]. Deficiency of melanin leads to albinism (hypopigmentary congenital disorder).
Hemoglobin in the Blood .
[1]. Amount and nature of hemoglobin that circulates in the cutaneous blood vessels play an important role in the coloration of the skin.
[2]. Skin becomes:
i. Pale, when hemoglobin content decreases
ii. Pink, when blood rushes to skin due to cutaneous vasodilatation (blushing)
iii. Bluish during cyanosis, which is caused by excess amount of reduced hemoglobin.
Function of Skin .
Primary function of skin is protection of organs. However, it has many other important functions also.
1. Protective Function .
Skin forms the covering of all the organs of the body and protects these organs from the following factors:
i. Bacteria and toxic substances .
ii. Mechanical blow .
iii. Ultraviolet rays.
i. Protection from Bacteria and Toxic Substances .
[1]. Skin covers the organs of the body and protects the organs from having direct contact with external environment. Thus, it prevents the bacterial infection.
[2]. Lysozyme secreted in skin destroys the bacteria.
[3]. Keratinized stratum corneum of epidermis is responsible for the protective function of skin. This layer also offers resistance against toxic chemicals like acids and alkalis.
[4]. If the skin is injured, infection occurs due to invasion of bacteria from external environment.
During injury or skin infection, the keratinocytes secrete:
a. Cytokines like interleukins, α-tumor necrosis factor and γ-interferon, which play important role in inflammation, immunological reactions, tissue repair and wound healing
b. Antimicrobial peptides like β-defensins, which prevent invasion of microbes.
ii. Protection from Mechanical Blow .
[1]. Skin is not tightly placed over the underlying organs or tissues.
[2]. It is somewhat loose and moves over the underlying subcutaneous tissues. So, the mechanical impact of any blow to the skin is not transmitted to the underlying tissues.
iii. Protection from Ultraviolet Rays .
[1]. Skin protects the body from ultraviolet rays of sunlight.
[2]. Exposure to sunlight or to any other source of ultraviolet rays increases the production of melanin pigment in skin.
[3]. Melanin absorbs ultraviolet rays.
[4]. At the same time, the thickness of stratum corneum increases. This layer of epidermis also absorbs the ultraviolet rays.
2. Sensory Function .
[1]. Skin is considered as the largest sense organ in the body. It has many nerve endings, which form the specialized cutaneous receptors .
[2]. These receptors are stimulated by sensations of touch, pain, pressure or temperature sensation and convey these sensations to the brain via afferent nerves. At the brain level, perception of different sensations occurs.
3. Storage Function .
[1]. Skin stores fat, water, chloride and sugar.
[2]. It can also store blood by the dilatation of the cutaneous blood vessels.
4. Synthetic Function .
Vitamin D3 is synthesized in skin by the action of ultraviolet rays from sunlight on cholesterol.
5. Regulation of Body Temperature .
[1]. Skin plays an important role in the regulation of body temperature.
[2]. Excess heat is lost from the body through skin by radiation, conduction, convection and evaporation.
[3]. Sweat glands of the skin play an active part in heat loss, by secreting sweat.
[4]. The lipid content of sebum prevents loss of heat from the body in cold environment.
6. Regulation of Water & Electrolyte Balance .
Skin regulates water balance and electrolyte balance by excreting water and salts through sweat.
7. Excretory Function .
Skin excretes small quantities of waste materials like urea, salts and fatty substance.
8. Absorptive Function .
Skin absorbs fat-soluble substances and some ointments.
9. Secretory Function .
[1]. Skin secretes sweat through sweat glands and sebum through sebaceous glands. By secreting sweat, skin regulates body temperature and water balance.
[2]. Sebum keeps the skin smooth and moist.
Glands of Skin .
Skin contains two types of glands, namely sebaceous glands and sweat glands.
Sebaceous Glands .
Sebaceous glands are simple or branched alveolar glands, situated in the dermis of skin.
Structure .
[1]. Sebaceous glands are ovoid or spherical in shape and are situated at the side of the hair follicle.
[2]. These glands develop from hair follicles. So, the sebaceous glands are absent over the thick skin, which is devoid of hair follicles.
[3]. Each gland is covered by a connective tissue capsule.
[4]. The alveoli of the gland are lined by stratified epithelial cells.
[5]. Sebaceous glands open into the neck of the hair follicle through a duct.
[6]. In some areas like face, lips, nipple, glans penis and labia minora, the sebaceous glands open directly into the exterior.
Secretion of Sebaceous Gland – Sebum .
[1]. Sebaceous glands secrete an oily substance called sebum.
[2]. Sebum is formed by the liquefaction of the alveolar cells and poured out through the ducts either via the hair follicle or directly into the exterior.
Composition of Sebum .
Sebum contains:
1. Free fatty acids .
2. Triglycerides .
3. Squalene .
4. Sterols .
5. Waxes .
6. Paraffin.
Functions of Sebum .
[1]. Free fatty acid content of the sebum has antibacterial and antifungal actions. Thus, it prevents the infection of skin by bacteria or fungi
[2]. Lipid nature of sebum keeps the skin smooth and oily. It protects the skin from unnecessary desquamation and injury caused by dryness
[3]. Lipids of the sebum prevent heat loss from the body. It is particularly useful in cold climate.
Activation of Sebaceous Glands at Puberty .
[1]. Sebaceous glands are inactive till puberty.
[2]. At the time of puberty, these glands are activated by sex hormones in both males and females.
[3]. At the time of puberty, particularly in males, due to the increased secretion of sex hormones, especially dehydroepiandrosterone, the sebaceous glands are stimulated suddenly. It leads to the development of acne on the face.
Acne .
[1]. Acne is the localized inflammatory condition of the skin, characterized by pimples on face, chest and back.
[2]. It occurs because of overactivity of sebaceous glands.
[3]. Acne vulgaris is the common type of acne that is developed during adolescence.
[4]. Acne disappears within few years, when the sebaceous glands become adapted to the sex hormones.
Sweat Glands .
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| Differences between eccrine and apocrine sweat glands . |
Sweat glands are of two types:
1. Eccrine glands .
2. Apocrine glands.
Eccrine Glands .
Distribution of Eccrine glands .
Eccrine glands are distributed throughout the body. There are many eccrine glands over thick skin.
Structure .
Eccrine sweat gland is a tubular coiled gland.
It consists of two parts:
1. A coiled portion lying deeper in dermis, which secretes the sweat
2. A duct portion, which passes through dermis and epidermis.
[1]. Eccrine sweat gland opens out through the sweat pore.
[2]. The coiled portion is formed by single layer of columnar or cuboidal epithelial cells, which are secretory in nature.
[3]. Epithelial cells are interposed by the myoepithelial cells.
[4]. Myoepithelial cells support the secretory epithelial cells.
[5]. The duct of eccrine gland is formed by two layers of cuboidal epithelial cells.
Secretory Activity of Eccrine Glands .
[1]. Eccrine glands function throughout the life since birth.
[2]. These glands secrete a clear watery sweat.
[3]. The secretion increases during increase in temperature and emotional conditions.
[4]. Eccrine glands play an important role in regulating the body temperature by secreting sweat.
[5]. Sweat contains water, sodium chloride, urea and lactic acid.
Control of Eccrine Glands .
[1]. Eccrine glands are under nervous control and are supplied by sympathetic postganglionic cholinergic nerve fibers, which secrete acetylcholine.
[2]. Stimulation of these nerves causes secretion of sweat.
Apocrine Glands .
Distribution of Apocrine glands .
Apocrine Glands are situated only in certain areas of the body like axilla, pubis, areola and umbilicus.
Structure of Apocrine glands .
[1]. Apocrine Glands are also tubular coiled glands.
[2]. The coiled portion lies in deep dermis. But, the duct opens into the hair follicle above the opening of sebaceous gland.
Secretory Activity of Apocrine Glands .
[1]. Apocrine sweat glands are nonfunctional till puberty and start functioning only at the time of puberty.
[2]. In old age, the function of these glands gradually declines. The secretion of the apocrine glands is thick and milky. At the time of secretion, it is odorless.
[3]. When microorganisms grow in this secretion, a characteristic odor develops in the regions where apocrine glands are present.
[4]. Secretion increases only in emotional conditions.
[5]. Apocrine glands do not play any role in temperature regulation like eccrine glands.
Control of Apocrine Glands .
[1]. Apocrine glands are innervated by sympathetic adrenergic nerve fibers. But, the secretory activity is not under nervous control.
[2]. However, adrenaline from adrenal medulla causes secretion by apocrine glands.
[3]. Glands of eyelids, glands of external auditory meatus and mammary glands are the modified apocrine glands.
Pheromones .
[1]. Pheromones are a group of chemical substances that are secreted by apocrine glands. Some scientists call this substance as vomeropherins.
[2]. When secreted into environment by an organism, pheromones produce some behavioral or physiological changes in other members of the same species.
[3]. Pheromones are mostly present in urine, vaginal fluid and other secretions of mammals and influence the behavior and reproductive cycle in these animals.
[4]. Details of pheromones in lower animals are well documented. However, human pheromones are not fully studied.
[5]. Recently, it is found that the pheromones excreted in axilla of a woman affects the menstrual cycle of her room-mate or other woman living with her. These substances stimulate receptors of vomeronasal receptors.
[6]. Vomeronasal receptors are distinct from other olfactory receptors and detect specially the odor of pheromones.
[7]. Impulses from these receptors are transmitted to hypothalamus, which influences the menstrual cycle via pituitary gonadal axis. This effect of pheromones on the menstrual cycle of other individuals is called dormitory effect.
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