Showing posts with label skin. Show all posts
Showing posts with label skin. Show all posts
the skin
The skin is an important sense organ, and as such includes a number of nerves that are mainly in the dermis, with a few reaching the epidermis. Nerves carry impulses to and from hair muscles, sweat glands, and blood vessels, and receive messages from touch, temperature, and pain receptors. Some nerve endings are specialized such as sensory receptors that detect external stimuli. The nerve endings in the dermal papillae are known as Meissner's corpuscles, which detect light touch, such as a pat, or the feel of clothing on the skin. Pacinian corpuscles, located in the deeper dermis, are stimulated by stronger pressure on the skin. Receptors near hair roots detect displacement of the skin hairs by stimuli such as touch or wind. Bare nerve endings throughout the skin report information to the brain about temperature change (both heat and cold), texture, pressure, and trauma.
HAIR
nHair occurs only in thin skin; its color, size, shape, and distribution vary according to race, age, sex, and body region. The structures in skin that form hairs and maintain their growth are called hair follicles.
nA. Follicle and Hair Development:
1. Follicles. Early in the third month of human development, local epidermal thickenings form at the sites of future hairs: first on the eyebrows, chin, and upper lip and then over the rest of the thin skin. Cells at the base of each thickening invade the dermis, and a small dermal papilla invades the leading edge of the epidermal downgrowth. Interactions between the papilla and the invaginating epidermis induce the differentiation of the hair follicle. Hair begins to form in the hair bulb at the base of the hair follicle as a result of the keratinization of the bulb's epithelial cells. These cells are pushed toward the surface by the mitosis in the germinal matrix (hair bulb epithelium). Some epithelial cells in the walls of the developing follicle divide, forming bulges that differentiate into sebaceous glands. integumentary system
2. Hairs. By the fifth or sixth month of gestation, the fetus is covered by fine hairs (lanugo). Just before birth, most of the lanugo is shed, except for the scalp, eyebrows, and eyelashes. A few months after birth, the remaining lanugo has been replaced by coarser mature terminal hairs; the rest of the body is covered with a coat of fine short hairs, called vellus. At puberty, coarse terminal hairs replace the vellus in specific body areas. In males, terminal hairs develop in the axilla and pubic region, on the face, and, to some extent, over the rest of the body. In females, they develop mainly in the axilla and pubic regions.
nA. Follicle and Hair Development:
1. Follicles. Early in the third month of human development, local epidermal thickenings form at the sites of future hairs: first on the eyebrows, chin, and upper lip and then over the rest of the thin skin. Cells at the base of each thickening invade the dermis, and a small dermal papilla invades the leading edge of the epidermal downgrowth. Interactions between the papilla and the invaginating epidermis induce the differentiation of the hair follicle. Hair begins to form in the hair bulb at the base of the hair follicle as a result of the keratinization of the bulb's epithelial cells. These cells are pushed toward the surface by the mitosis in the germinal matrix (hair bulb epithelium). Some epithelial cells in the walls of the developing follicle divide, forming bulges that differentiate into sebaceous glands. integumentary system
2. Hairs. By the fifth or sixth month of gestation, the fetus is covered by fine hairs (lanugo). Just before birth, most of the lanugo is shed, except for the scalp, eyebrows, and eyelashes. A few months after birth, the remaining lanugo has been replaced by coarser mature terminal hairs; the rest of the body is covered with a coat of fine short hairs, called vellus. At puberty, coarse terminal hairs replace the vellus in specific body areas. In males, terminal hairs develop in the axilla and pubic region, on the face, and, to some extent, over the rest of the body. In females, they develop mainly in the axilla and pubic regions.
DERMIS
nThe dermis, which contains the hair follicles (found only in thin skin) and sebaceous and sweat glands, consists of 2 layers of vascular connective tissue that blend at their common border.
nA. Papillary Layer: This layer of loose connective tissue, rich in elastic fibers, lies directly beneath the epidermal basement membrane. Its projections--dermal papillae-interdigitate with the epidermal ridges, increasing the area of contact. Special collagen fibers, anchoring fibrils, extend from this layer into the epidermal basal lamina to reinforce the dermal-epidermal junction. The papillary layer contains immunoprotective cells, a rich capillary net work, and abundant free nerve endings, some of which penetrate the epidermis. The tips of many dermal papillae contain encapsulated touch receptors called Meissner's corpuscles.
nB. Reticular Layer: Beneath the papillary layer is a thicker layer of dense irregular connective tissue. Also richly vascularized, this layer contains many arteriovenous anastomoses, or shunts, that control the amount of blood reaching the papillary capillaries and thus aid in regulating heat loss and blood pressure. The reticular layer also contains a rich supply of nerves in both free and encapsulated endings leg, Pacinian corpuscles).
nA. Papillary Layer: This layer of loose connective tissue, rich in elastic fibers, lies directly beneath the epidermal basement membrane. Its projections--dermal papillae-interdigitate with the epidermal ridges, increasing the area of contact. Special collagen fibers, anchoring fibrils, extend from this layer into the epidermal basal lamina to reinforce the dermal-epidermal junction. The papillary layer contains immunoprotective cells, a rich capillary net work, and abundant free nerve endings, some of which penetrate the epidermis. The tips of many dermal papillae contain encapsulated touch receptors called Meissner's corpuscles.
nB. Reticular Layer: Beneath the papillary layer is a thicker layer of dense irregular connective tissue. Also richly vascularized, this layer contains many arteriovenous anastomoses, or shunts, that control the amount of blood reaching the papillary capillaries and thus aid in regulating heat loss and blood pressure. The reticular layer also contains a rich supply of nerves in both free and encapsulated endings leg, Pacinian corpuscles).
B. Pigmentation System
B. Pigmentation System: Skin color is conferred mainly by the pigments melanin and carotene, the thickness of the epidermis, the number of dermal blood vessels, and the color of the blood in those vessels.
1. Melanins contribute to skin, eye, and hair color. Synthesized by melanocytes, they include the dark brown pigment eumelanin, found in the epidermis, iris, and brown and black hair; and the cysteine-rich pigment pheomelanin, found in red hair. 2. Melanocytes derive from the neural crest and migrate into the epidermis during em bryogenesis. Although they are scattered among the keratinocytes of the stratum basale, they are not attached to them by desmosomes. They have round cell bodies, central nuclei, and long cytoplasmic processes that pass between the cells of the strata basale and spinosum and terminate in small indentations on the keratinocyte surfaces. Melanocytes make up 10-25% of this layer's cells but do not participate in keratinization.
1. Melanins contribute to skin, eye, and hair color. Synthesized by melanocytes, they include the dark brown pigment eumelanin, found in the epidermis, iris, and brown and black hair; and the cysteine-rich pigment pheomelanin, found in red hair. 2. Melanocytes derive from the neural crest and migrate into the epidermis during em bryogenesis. Although they are scattered among the keratinocytes of the stratum basale, they are not attached to them by desmosomes. They have round cell bodies, central nuclei, and long cytoplasmic processes that pass between the cells of the strata basale and spinosum and terminate in small indentations on the keratinocyte surfaces. Melanocytes make up 10-25% of this layer's cells but do not participate in keratinization.
GENERAL FEATURES OF THE SKIN
A. General Functions: The skin is the largest and heaviest organ. It protects against microorganisms, toxic substances, dehydration, ultraviolet radiation, impact, and friction. It also acts as a sensory receptor and has a role in excretion, vitamin D metabolism, and regulation of blood pressure and body temperature.
B. General Organization: Human skin (the integument) is of 2 types. Thick skin, restricted to the palms of the hands and soles of the feet, lacks hairs and has abundant sweat glands. Thin skin, which has hairs, covers the rest of the body. Thick or thin, the skin consists of 2 distinct but tightly attached layers, the epidermis and dermis, which are underlain by the hypodermis.
1. Epidermis. This outer (superficial) layer of skin, composed of keratinized stratified squamous epithelium, derives from embryonic surface ectoderm. It is avascular, receiving nourishment from vessels in the underlying dermis. Its only innervation is by unencapsulated (free) nerve endings. The epidermal layer is further divided into 5 stratea; these layers, in order from superficial to deep, are the stratum corneum, stratum lucidum, stratum granulosum, stratum spinosum, and stratum basale. The thickness of these layers differs in thick and thin skin.
2. Dermis, This inner (deeper) layer is a vascular connective tissue of mesodermal origin. It can be further divided into a superficial papillary layer and a deeper reticular layer. The papillary layer contains extensive capillary networks, which nourish the epidermis. The reticular layer contains many arteriovenous anastomoses that help regulate blood pressure and body temperature. It is richly supplied with free nerve endings, a variety of encapsulated sensory receptors, and autonomic fibers that control the vascular smooth muscle. Even in thick skin, the dermis is much thicker than the overlying epidermis.
3. Hypodermis, Although not a part of the skin, this layer of mesoderm-derived loose connective and adipose tissue underlying the dermis flexibly binds the skin to deeper structures. Its thickness varies, depending on nutritional status, level of activity, body region, and gender. The hypodermis is also called the subcutaneous fascia and, where thick enough, the panniculus adiposus
Structures Associated With the Skin: Glands (sebaceous and sweat), hairs, and nails arise from epidermal downgrowths into the dermis during embryonic development. These structures, which are mainly of epithelial origin, require epitheliomesenchymal interactions between the epidermis and dermis for their formation and maintenance
B. General Organization: Human skin (the integument) is of 2 types. Thick skin, restricted to the palms of the hands and soles of the feet, lacks hairs and has abundant sweat glands. Thin skin, which has hairs, covers the rest of the body. Thick or thin, the skin consists of 2 distinct but tightly attached layers, the epidermis and dermis, which are underlain by the hypodermis.
1. Epidermis. This outer (superficial) layer of skin, composed of keratinized stratified squamous epithelium, derives from embryonic surface ectoderm. It is avascular, receiving nourishment from vessels in the underlying dermis. Its only innervation is by unencapsulated (free) nerve endings. The epidermal layer is further divided into 5 stratea; these layers, in order from superficial to deep, are the stratum corneum, stratum lucidum, stratum granulosum, stratum spinosum, and stratum basale. The thickness of these layers differs in thick and thin skin.
2. Dermis, This inner (deeper) layer is a vascular connective tissue of mesodermal origin. It can be further divided into a superficial papillary layer and a deeper reticular layer. The papillary layer contains extensive capillary networks, which nourish the epidermis. The reticular layer contains many arteriovenous anastomoses that help regulate blood pressure and body temperature. It is richly supplied with free nerve endings, a variety of encapsulated sensory receptors, and autonomic fibers that control the vascular smooth muscle. Even in thick skin, the dermis is much thicker than the overlying epidermis.
3. Hypodermis, Although not a part of the skin, this layer of mesoderm-derived loose connective and adipose tissue underlying the dermis flexibly binds the skin to deeper structures. Its thickness varies, depending on nutritional status, level of activity, body region, and gender. The hypodermis is also called the subcutaneous fascia and, where thick enough, the panniculus adiposus
Structures Associated With the Skin: Glands (sebaceous and sweat), hairs, and nails arise from epidermal downgrowths into the dermis during embryonic development. These structures, which are mainly of epithelial origin, require epitheliomesenchymal interactions between the epidermis and dermis for their formation and maintenance
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