Integumentary system

Modes of hormone delivery

2011-09-24T06:36:00.000-07:00

Endocrine: most common, classical mode, hormones delivered to target cells by blood.
Paracrine: hormone released diffuses to its target cells through immediate extracellular space. Blood is not directly involved in the delivery.

Neuroendocrine: hormone is produced and released by a neuron., delivered to target cells by blood.

Autocrine: hormone released feeds-back on the cell of origin, again without entering blood circulation.

HORMONE TARGET CELL SPECIFICITY

Only target cells, or cells that have specific receptors, will respond to the hormone’s presence. The strength of this response will depend on:
-blood levels of the hormone. – the relative numbers of receptors for that hormone on or in the target cells.
The affinity (or strength of interactions) of the hormone and the receptor.

Half-life onset and duration of hormone activity..

The affinity of hormones to their specific receptors is typically very high. The actual concentration of a circulating hormone in blood at any time reflects. – its rate of release – the speed of its inactiviation and removal from the body.

The half –life is the time required for the hormone to loose half of its original effectiveness(or drop to half of its orginal concentration)

The time required for hormone effects to take place varies greatly, from almost immediate responses to hours or even days.

In addition, some hormones are produced in an inactive form and must be activated in the target cells before exerting cellular responses.

In terms of duration of hormone action, it ranges from about 20 minutes to several hours, depending on the hormone.

CONTROL OF HORMONE RELEASE:

The synthesis and secretion of most hormones are usually regulated by negative feedback systems. As hormone levels rise, they stimulate target organ responses. These in turn, inhibit further hormone release. The stimuli that induce endocrine glands to synthesize and release hormones belong to one of the following major types.
Humoral
Neural
Hormonal

The endocrine system works closely with the nervous system to maintain and steady state of the body.the functions of hormones. Endocrin gland is ductless gland. Target issue. Chemical structure of hormones: protein, peptide, aminoacid derived and steroid hormones. Pheromones

Histology of Integument Video

2010-05-11T08:59:00.000-07:00

Video of basic histology , Histology of Integument

integumentary slides

2010-04-16T13:06:00.000-07:00

Integumentary system info

intergumentary system videos and parts

2010-03-13T04:18:00.000-08:00

Another Video of the Integumentary System, again these are some study notes & pictures to share with classmates who may benefit from them. Video is very nice and good details.

Integumentary System video

2010-03-13T04:13:00.000-08:00

intergumentary system video and very simple integumentary system clip. very usefull for students and learners. Basic integumentary system parts explaining in this video.

The Integumentary System song

2010-03-13T04:11:00.000-08:00

very good song about intergumentary system :) integumentary system video song ..

the skin

2009-05-29T06:28:00.000-07:00

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.

skin color

2009-04-04T06:01:00.000-07:00

Skin color: Skin color results from the presence of melanin, carotene (yellow to orange pigment), and underlying blood reflected through skin. Melanin keeps excessive ultraviolet rays from burning the skin. Exposure to sunlight causes the skin to produce more melanin, causing suntan, a temporary change in skin color. Melanin-rich cells continually move toward the surface and then they are sloughed.

Hiv virus video

2009-03-29T04:55:00.000-07:00

Integumentary System Structure and Function

2009-02-28T03:49:00.000-08:00

Integumentary System Structure and Function video.Brief overview of the major structures and functions of the Integumentary System .

Human skin video

2008-12-12T07:49:00.000-08:00

The Human Skin Animation Video from youtube. Integumentary system's important part is : human skin and in this youtube video you learn great usefull informations about human skin.

Functions of integumentary system

2008-11-11T15:21:00.000-08:00

Integumentary system's main functions are below :

The integumentary system has lots of different functions, it:
Protects the body’s internal organs
Protects the body against bacteria
Protects the body from dehydration
Protects the body against sunburns
Stores water, fat, and vitamin D

hair care tips

2008-10-16T06:43:00.001-07:00

Caring for your hair and some tips

The most important thing is to keep their hair clean. Some people wash their hair every day, but others do it once or twice a week. That depends on the type of hair you have and where activities in which you are involved, such as exercise or swim.

When you wash your hair, use a mild shampoo and lukewarm water. Apply shampoo and massage with the fingertips, rather than nails, to make foam. You can use a conditioner or a shampoo that already contains. The conditioner helps untangle the hair and leave it soft and silky. But according to the type of hair, it may also be able to crush a lot and leave it oily. Rinse your hair with plenty of clean water. Secale gently with a towel and use a wide-tooth comb to untangle.

Try either your hair, either when wet or dry, brushing or combing carefully. Do not try pulling of desenredarte knots strong and they do not use a pony tail or braids too tight. You can irritate the scalp. And if you're using rizadores or hair dryer, be careful and ask an adult to help, if needed. If not careful, you can burn.

Here we give you a tip to showcase a beautiful hair: Follow a healthy diet. It sounds weird, but it is not. A diet rich in nutrients helps your body to look beautiful from the inside out!

The variety of hair color

2008-10-16T06:42:00.000-07:00

What kind of hair you have? ¿Black and wavy?, Blond and Lazio? Or some other combination? The color of the hair is given melanin, the substance that gives color to the skin. The more clear is the hair, you have less melanin. A person with black or brown hair has a lot more melanin than a hair blond or red. As people get older, decreases the amount of melanin and therefore begin to exit the gray hairs.

Usually, the skin color of a person is in agreement with the color of his hair. For example, most of the blond have light skin, while people with darker skin is brown or black hair. And do not forget the genes (the genes inherited from your parents): usually the color of the hair is determined by the color of the hair of one or both parents.

Hair follicles play a key role in determining the type of hair. Some hair follicles are structured in a way that produces wavy hair, while others generate hair Lazio. Follicles also determine whether your hair is thick or thin.

Where does your hair?

2008-10-16T06:41:00.000-07:00

The hair always grows through the skin in the same way, no matter where it comes out (of your head, arm or ankle). Start at the root capillaries beneath the skin, where cells are grouped to form keratin (the protein from being formed nails). The root is within a follicle, which is a kind of small tube into the skin.

The hair grows from the root, leaves the follicle and goes through the skin, making it visible. The small blood vessels that are at the base of each follicle feed the roots of the hair to permit their growth. But once the hair emerge from the surface of the skin, the cells that form it are no longer alive. The cells of one of the hairs that you see in your body are dead. That's why when you cut your hair does not hurt us.

Virtually every hair follicle is connected to a Sebaceous gland. These sebaceous glands produce a substance, tallow, which gives the skin a shiny appearance and protects making it water resistant. Sometimes, for example during puberty, these glands can produce too much sebum (fat), which causes the hair of a person is oily. If that's the case, it is time to wash your hair!
What took the wind!

You have more than 100,000 hairs on your scalp, but every day many of them you fall. Every day, you lose about 50 to 100 hairs. This happens while you wash your hair, you comb, brush or just while you're immobile. But do not worry, constantly grow new hair to replace those that are going to fall.

Every hair on your scalp grows for about 2 to 6 years. Then, resting for a few months and finally falls. It replaced a hair again, it starts to grow in the hair follicle. This cycle of growth, rest, fall and replacement to maintain an adequate amount of hair on your head.

hairs biology

2008-10-16T06:40:00.000-07:00

The "hair" is not just that you have hair on the head. You have "hair" (and hair) in almost all parts of the body. (The places where you have no hair are the lips, the palms and soles).

Most of the hair that you have in your body is easy to see, such as the eyebrows, head, arms or legs. But the hair you have on, for example, the cheeks of the face is almost invisible. Depending on the location, hair fulfills different functions. The hair that you have in your head keeps the body heat of this and gives some protection against your skull blows. Eyelashes protect your eyes by reducing the amount of light and dust that can penetrate them; and eyebrows protect the eyes of sweat that can drip on your forehead.

Integument

2008-09-19T15:50:00.000-07:00

The Integumente are an integral part of the plant seeds of seed plants. There are Hüllschichten which encircle the Nucellus. After fertilization, the Integumente to Testa.

They leave a small opening, the Mikropyle freely through which the scar on the subject with the pollen in the pollen tube fertilization hindurchwächst.

NAILS

2008-08-30T08:13:00.003-07:00

nThese plates of highly keratinized cells are analogous to, but harder than, the stratum corneum.
A. Nail Development: The formation of the nails is similar to that of hair, but involves produc ing plates rather than cylinders. At the end of the third month of embryonic development, a narrow plate of epidermis on the dorsal surface of the terminal phalanges invades the underlying dermis of each finger and toe. This invasion continues proximally, forming a furrow called the nail groove. Epithelial cells beneath the groove proliferate to form the nail matrix, whose composition and function are similar to those of the hair's germinal matrix. Proliferation in the nail matrix pushes the upper cells toward the surface. These cells differentiate, becoming highly keratinized to form the nail plate. The plate is gradually pushed out of the groove by further cell proliferation and differentiation in the nail matrix. The growing plate slides distally on the dorsal surface of the digit. The epidermis over which it slides becomes the nail bed.

B. Nail Complex Structure: The nail plate (or nail) consists of 2 parts: the nail body (the visible part of the nail) and the nail root--(the part hidden in the nail groove). The nail and its supporting structure are surrounded by papillary dermis. The nail matrix is a thickened region of epidermis containing proliferating cells in the layer that directly contacts the dermis, and keratinizing cells between this basal layer and the nail plate. The nail matrix surrounds the root and extends beyond the nail groove. The nail bed lies beneath the nail body, distal to the nail matrix. It consists of only the deeper epidermal strata, for which the nail serves as a stratum corneum. The eponychium (or cuticle) is a thick keratinized layer extending from the upper surface of the nail groove over the most proximal part of the nail body. The hyponychium is a local thickening of the stratum corneum underlying the free (distal) end of the tail. The lunula is the whitish, opaque, crescent-shaped region on the proximal nail body, adjacent to the nail groove. Its distal border corresponds roughly to the underlying nail matrix

Apocrine Sweat Glands

2008-08-30T08:13:00.001-07:00

nB. Apocrine Sweat Glands
n1. Distribution. Less numerous than the eccrine type, these glands occur mainly in the axilla, pubic and anal regions, and the areolae of the breasts. 2. Structure. Apocrine sweat glands are also simple coiled tubular glands, but are generally larger than eccrine glands. a. Ducts. These coiled ducts are lined with low cuboidal epithelium and open into hair follicles . b. Secretory portions. Coiled and embedded in the dermis, each has a wide lumen lined by cuboidal to columnar cells. Myoepithelial cells are present between the secretory cells and the basal lamina.
n3. Secretory Product. Apocrine sweat is a viscous, odorless fluid that, once secreted, acquires a distinctive odor as a result ofbacterial degradation. The term apocrine derives from early evidence that the secretory cells of these glands released their apical cytoplasm along with the secretory product. Recent evidence, however, argues against apical shedding. Therefore, although the secretory products ofapocrine and eccnne sweat glands do differ, their mode of secretion-merocrine-is similar.

SWEAT GLANDS

2008-08-30T08:12:00.003-07:00

nTwo types of sweat glands, eccrine (or merocrine) and apocrine, occur in human skin. Both develop as epidermal invaginations into the dermis, and they differ mainly in their size, distribution, and secretory products.
nA. Eccrine Sweat Glands:
n1. Distribution. The most numerous sweat glands in humans, these average about 3 million per individual. They occur over most of the body, except for the glans penis, glans clitoridis, and the vermiliion border of the lips. They are most abundant in thick skin, such as the palms, where there are about 3000 per square inch. 2. Structure. They are simply coiled tubular glands. a. Ducts. The slightly coiled ducts are lined with simple to stratified cuboidal epithelium; their lining cells are smaller than those in the secretory portions and stain darker. Each duct opens directly onto the skin surface. b. Secretory portions. These highly coiled parts of the sweat glands are located in deep reticular dermis or shallow hypodermis. Surrounding connective tissue condenses to form a sheath around the basal lamina, and there are numerous myoepithelial cells between the basal lamina and the secretory cells. The secretions are released via ex ocytosis (merocrine secretion). Secretory cells are larger and stain lighter than the duct lining cells. Two secretory cell types are seen. Dark (mucoid) cells are pyramidal and line most of the gland's secretory portion; their bases do not reach the basal lamina. They contain rodlike mitochondria, a well-developed Golgi complex, RER, many free ribo somes, and dark glycoprotein-containing granules. Clear cells are also pyramidal. They lack secretory granules, contain abundant glycogen, and surround the inner layer of dark cells. Their basal plasma membranes, which do contact the basal lamina, are highly infolded, suggesting a role in ion and water transport.3. Secretory product. Eccrine sweat is a watery secretion whose main components (besides water) include NaCI, urea, ammonia, and uric acid. The glands thus assist in excreting by-products of protein metabolism. In addition, evaporation of water from the skin surface reduces body temperature by cooling the blood in the papillary capillaries.

SEBACEOUS GLANDS

2008-08-30T08:12:00.001-07:00

A. Structure and Location: These exocrine glands occur in all thin skin, most often in association with hair follicles into which their ducts empty, but are most numerous in the skin of the face, forehead, and scalp. In hairless skin, they open directly onto the surface. Their acinar secretory portions contain many large lipid-filled cells that appear pale-staining and foamy.

B. Function: The acinar cells of sebaceous glands fill with lipid droplets containing a mixture of triglycerides, waxes, squalene, and cholesterol and its esters. Their nuclei become pyknotic, and the cells eventually burst, releasing their contents and other cell debris (together termed sebum) into the ducts. The entire cell is shed, a type of secretion known as bolocrine secretion. The oily sebum moves through the ducts and into the hair follicle. It covers the hair and moves out onto the surface. Here, it lubricates the skin and may have some antibacterial or antifungal effects. The secretory activity of these glands, which begin functioning at puberty, is continuous and is increased by androgens.

Hair Growth

2008-08-30T08:11:00.002-07:00

Hair growth is not continuous but cycles through repeated growing and resting phases. In the growing phase, the proliferation and differentiation of cells in the germinal matrix cause the hair to elongate. In the resting phase, the germinal matrix becomes inactive and may atrophy. The hair detaches from the bulb, moving upward as the external root sheath retracts toward the surface. Eventually, the hair is shed. During the next growing phase, the lower part of the external root sheath grows downward again, either forming a new germinal matrix over the old papilla or stimulating formation of a new papilla. The bulb re-forms, and the next phase of the cycle--proliferation in the matrix and renewed hair growth--begins. Hair growth cycles do not occur synchronously over the entire body surface. Rather, they occur in patches, a pattern called growth in mosaic. Several hormones, especially androgens, influence the pattern of terminal hair distribution and growth rate.

Keratinization of Hair

2008-08-30T08:11:00.001-07:00

Although both the hair and the epidermis contain keratin, there are differences in their keratinization. For example, the keratin of the hair's cortex and cuticle is harder than that of the epidermis; keratinized hair cells remain tightly attached to one another, whereas those of skin are continuously sloughed; keratinization of the hair is intermittent and is restricted to the bulb, whereas that of skin is continuous and occurs over the entire surface; and keratinized cells of the epidermis are identical, whereas those in hairs differ in structure and function depending on their position in the hair.

Associated structures

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Found near the neck of the root sheath, sebaceous glands always accompany hairs. They empty their secretions via a short duct into the follicular canal. Arrector pill muscles are small bundles of smooth muscle fibers that originate in the papillary dermis and extend obliquely toward the hair follicle to insert into the follicle's connective tissue sheath below the sebaceous glands. When they contract, these muscles cause the hairs to stand upright, giving the appearance of gooseflesh. Their contraction also compresses the sebaceous glands, pushing their secretions into the neck of the follicular canal and out onto the surface of the skin.

Root sheaths

2008-08-30T08:10:00.003-07:00

The concentric sheaths surrounding the hair shaft are more clearly distin- guished in the area between the bulb and the skin surface. a. Internal root sheath. The layer closest to the hair shaft, it extends only from the bulb to the level of the sebaceous gland ducts. At this point the soft keratin-filled cells are shed into the follicular canal. There are 3 component layers: the cuticle of the internal root sheath is a layer of flat cells separated from the hair shaft cuticle only by the follicular canal; the middle layer is Huxley's layer, comprising one to 3 layers of low cuboidal cells; the outermost layer is Henle's layer, a translucent layer of flattened to cuboidal cells resembling the epidermal stratum lucidum. b. External root sheath. This surrounds the internal root sheath and is continuous with the epidermis. Above the level of the sebaceous glands, it includes all the epidermal layers. Below this level, it retains only the granulosum, spinosum, and basale. The granulosum is also lost near the follicle's base, where the spinosum and basale become continuous with the layers of the germinal matrix. c. Glassy membrane. This is the thickened basal lamina underlying the stratum basale of the external root sheath and separating it from the surrounding connective tissue sheath. d. Connective tissue sheath. A layer of condensed connective tissue, this surrounds the entire follicle, including the bulb. It extends along the follicle to the surface, where it blends into the looser papillary dermis

Hair shaft layers

2008-08-30T08:10:00.001-07:00

These 3 concentric layers are formed by the germinal matrix. The cell borders are indistinct, however, and cross sections through hair follicles near the skin surface often do not show the cellular nature of these layers. In addition, the hair itself may be dislodged from the canal during tissue processing, leaving only the open space (follicular canal) originally occupied by the shaft. The medulla forms the shaft's thin central core. It is composed of poorly keratinized and often vacuolated cells. The cortex surrounds the medulla and is composed of several layers of well-keratinized polygonal cells. The cuticle is the shaft's outermost layer. Within the bulb, its cells are cuboidal; farther up the shaft they become tall columnar, fill with keratin, and finally change their orientation to become a few layers of flattened, highly keratinized cells. These cells form the hard, shinglelike cuticle that covers the hair's outer surface.

1.Germinal matrix

2008-08-30T08:09:00.002-07:00

This cluster of epithelial cells capping the dermal papilla can be divided into 4 indistinct zones that are arranged concentrically around the papilla. The zone closest to the papilla resembles the stratum basale of the epidermis in both structure and function. It contains both columnar epidermal cells and the melanocytes that give the hair its color. This germinal layer gives rise to the poorly keratinized cells of the medulla of the hair shaft and to the cells in the other 3 zones of the germinal matrix. Around the base of the bulb, this layer is continuous with the external root sheath that surrounds the entire bulb and shaft; near the surface, it is continuous with the stratum basale. Cells in the next layer form the cuticle. The most peripheral layer of the germinal matrix forms the poorly keratinized cells of the internal root sheath.

Follicle and Hair Structure

2008-08-30T08:09:00.001-07:00

nHair follicles extend from the surface deep into the dermis or hypodermis. The follicle's broad base, or hair bulb, consists of a cap of rapidly dividing epithelial cells (the germinal matrix) overlying a dermal papilla that harbors the nerve and blood supply. Cells from the germinal matrix keratinize, forming the concentric layers of the hair shaft as they move toward the surface. Near the surface, distinct layers can be seen ensheathing the canal that contains the hair shaft.

B. Follicle and Hair Structure

2008-08-04T04:54:00.000-07:00

Hair follicles extend from the surface deep into the dermis or hypodermis. The follicle's broad base, or hair bulb, consists of a cap of rapidly dividing epithelial cells (the germinal matrix) overlying a dermal papilla that harbors the nerve and blood supply. Cells from the germinal matrix keratinize, forming the concentric layers of the hair shaft as they move toward the surface. Near the surface, distinct layers can be seen ensheathing the canal that contains the hair shaft. Integumentary system

HAIR

2008-07-16T04:21:00.000-07:00

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.

DERMIS

2008-05-07T06:25:00.000-07:00

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).

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2008-04-04T05:54:00.000-07:00

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D. Merkel's cells

2008-04-03T09:57:00.003-07:00

D. Merkel's cells: Scattered in the stratum basale, these cells are most numerous in thick skin. They resemble basal keratinocytes but have a clearer cytoplasm containing many small dense granules. Free nerve endings form a disklike expansion (Merkel's disk) that covers the basal surface of each Merkel's cell. This arrangement suggest that the cells function as sensory mechanoreceptors, but other evidence suggests that they may have DNES-related functions

C. Langerhans' cells

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C. Langerhans' cells: These star-shaped cells lack tonofilaments and occur mainly in the stratum spinosum (400-1000 cells/mm2 of skin surface). They stain selectively with gold chloride and contain numerous rodlike or racket-shaped cytoplasmic granules (Birbeck's granules). They are thought to be antigen-presenting cells that process and present to the lymphocytes any antigenic material that penetrates the skin's surface. Of mesodermal origin, they arise in bone marrow and may belong to the mononuclear phagocyte system. Langerhans' cells also occur in oral and vaginal epithelia as well as in the thymus.

B. Pigmentation System

2008-04-03T09:57:00.000-07:00

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.

layers of the epidermis

2008-03-05T10:19:00.003-08:00

1. Stratum basale (stratum germiuativum). This single layer of columnar basophilic keratinocytes rests on the basal lamina that separates epidermis from dermis. These cells divide continuously and give rise to the keratinocytes in all other layers. They attach to their neighbors by desmosomes and to the basal lamina by hemidesmosomes. Cytokeratin intermediate filaments (tonofilaments) are important components of both junctions. The cytokeratin content increases as these cells approach the stratum corneum, where it constitutes about 50% of their total protein. The basophilia of the basal layer is caused by ribosomes.

2. Stratum spinosum. This comprises several layers of large keratinocytes overlying the stratum basale. The cells are cuboidal or polygonal in the deeper layers and slightly flattened in the upper layer. Tonofibrils (tonofilament bundles) fill the cytoplasm, extend into the numerous cell processes that give these cells their spiny appearance, and insert into the desmosomes that attach the tips of these processes to those of adjacent cells. The mitotic rate here is lower than in the stratum basale. Mitosis occurs only in the malphighian layer, which includes the stratum basale and stratum spinosum.

3. Stratum granulosum. This lies above the stratum spinosum and, in thick skin, consists of 3-5 layers of flattened polygonal (often diamond-shaped) cells that contain numerous mem braneless keratohyalin granules. The intense basophilia of these granules is caused by their content of a phosphorylated histidine-rich precursor of the protein filaggrin. Cells in this layer also contain small ovoid or rodlike lamellar granules. These fuse with the plasma membrane and release their contents (glycosaminoglycans and phospholipids) into the inter cellular spaces. This material may be important in sealing the deeper layers of the skin from the external environment and in protection from dehydration.

4. Stratum lucidum, This layer overlies the stratum granulosum and is apparent only in thick skin. It is a narrow, acidophilic, translucent band of flattened keratinocytes whose nuclei, organelles, and intercellular borders are not visible. The cytoplasm contains dense cytokeratin aggregates embedded in an amorphous electron-dense matrix derived from the keratohyalin granules. This intracellular mixture of intermediate filaments and matrix con stitutes the immature keratin, sometimes called eleidin,

5. Stratum corneum, The outermost layer, this consists of many layers of dead, platelike enucleate keratinocytes with thickened plasma membranes. These cells represent the final stage of keratinization and are filled with mature keratin, a birefringent scleroprotein consisting of at least 6 polypeptides. The molecular weights of the polypeptides of mature keratin in the stratum corneum is higher than those of immature keratin in deeper, less differentiated cells. Keratin's substructure includes tonofilament subunits formed by 3 coiled and intertwined polypeptide chains. Nine of these subunits coil together to form each IO-nm thick intermediate filament. As they aggregate end to end, the tonofilament increases in length. Tonofilaments are embedded in and bound together by the amorphous matrix first found in keratohyalin granules. Dead cells are continuously sloughed (exfoliated) from the surface and replaced, through successive waves of mitosis and differentiation, by cells from the deeper waves.

EPIDERMIS

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The epidermis contains 2 major and 2 minor cell populations specialized for specific functions. Major populations include the keratinocytes and melanocytes. Minor populations include Langerhans' and Merkel's cells.

A. Keratinizing System: The keratinocytes make up most of the epidermis. They participate in the continuous turnover (renewal) of the skin surface by passing through 4 overlapping processes: cell renewal, or mitosis; differentiation, or keratinization; cell death; and exfoliation (the sloughing of dead cells from the skin surface). The entire process takes 15-30 days and occurs in waves. A cell layer produced by a mitotic wave in the basal layer undergoes keratinization in synchrony. Each wave pushes the cell layers produced in earlier waves toward the surface. The layers from several waves, each at a different depth and step in the process, give a stratified appearance to vertical sections of the epidermis. The 5 layers of the epidermis are distinguished by the shape, staining properties, contents, and orientation of the keratinocytes they contain.

GENERAL FEATURES OF THE SKIN

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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