Integumentary System. Integumentary System

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1 1. General aspects a. The integumentary system consists of several organs major organ of the system is the skin other organs are relatively small and they can be considered as specialized structures of the skin rather than organs as such include fingernails (or claws), hairs, feathers, sebaceous glands, sweat glands, oil glands, etc. b. The skin has the greatest mass of all organs of the body. 1 c. Two layers: (1) Epidermis: outer layer of the skin; consists of stratified squamous epithelium. (2) Dermis: inner layer of the skin; generally composed of dense interwoven connective tissue. (derm: skin) 2

2 d. Underneath the skin there is usually a hypodermic layer. (hypo: under; hypodermic: under the skin) (1) The hypodermis is not part of the skin itself. (2) The hypodermis is composed of connective tissue that is typically looser that that of the dermis--often typical loose (areolar) C.T. 3 e. The dermis of the skin, and often even the hypodermic layer below the skin, usually contain special structures that are derived from the epidermal epithelium by tubular indentations during embryonic development and remain connected to the epidermal epithelium proper these include sweat glands and hair follicles. 4

3 b. Epidermis: general features (1) consists of stratified squamous epithelium. (2) generally thicker than the lumenal stratified squamous epithelium of the mouth, pharynx, esophagus, anus, vagina, etc. (3) keratinized and has the complex composition of keratinized examples of this tissue. (a) Degree of keratinization is variable with body area and with time and use very thick stratum corneum is usually present on finger and toe pads and any other surface area frequently experiencing contact and abrasion increase in contact and abrasion will result in thicker stratum corneum. 5 (4) The lower (basal) surface of the epidermis is very irregular in almost all cases, with dermal papillae projecting up above the deepest level of this surface some of these papillae may be obliquely sectioned or even cross-sectioned, as they are not all oriented perfectly perpendicular to the outer surface of the skin. 6

4 Layers of the epidermis corenum (a) Basal: stratum germinativum Basal cells are cuboidal to low columnar. (b) S. spinosum prickel cells; desmosomes (c) S. granulosum keratin granules (d) S. lucidum thin, transparent, often absent (e) S. corenum Lumenal cells are squamous. dead cells lumen granulosum spinosum germinativum 7 c. Layer of melanocytes (1) Tan, brown, or black skin colors are due to the presence of a layer of melanocytes, at the interface of the epidermis and the dermis and among the stratum germinativum cells. (2) Melanocytes produce melanin pigment molecules many different molecular forms of melanin 8

5 (a) Melanocytes are not the only cell type that produces melanin (another example: cell of pigmented layer of retina), but they are the most common and generally the most important cell type that produces melanin. (b) In humans, melanin is produced in the skin mainly to absorb ultra-violet radiation from the sun, to protect the more delicate and vulnerable cells of the skin from the destructive effects of that radiation. These cells include those of the stratum germinativum of the epidermis Light-skinned races of humans have generally developed in colder climates where clothing was necessary for thermoregulation need for some ultra-violet radiation in humans for adequate production of vitamin D (for absorption of dietary Ca 2+ ). 10

6 (4) The melanin granules are released by the melanocyte, to some extent, and adjacent epithelial cells take up those granules. In humans the cells of the superficial layers do not retain the melanin. 11 d. Dermis: general features (1) Composed of dense interwoven connective tissue; usually at least 95% of the volume of the matrix is occupied by collagen fibers. (a) It is this layer that is preserved and softened to make leather tiny thread-like components of leather are collagen fibers. 12

7 (2) Vascularization is low to moderate in general, but moderate to moderately high immediately under the epidermis. (b) The epidermis tends to be more metabolically active than the connective tissue of the dermis, of course, and so it has a higher need for 0 2, nutrients, etc. (c) A major role of the sub-epidermal vascularization is for increasing or decreasing the rate of body heat loss, in thermoregulation, via control of blood flow through that capillary bed. tissues of the skin can survive low blood flow for minutes to hours if necessary for conservation of body heat. 13 (3) Cells present in the C. T. are mostly fibroblasts also present are immune defense cells (macrophages, T-lymphocytes, etc.) adipocytes, in clumps (restricted to the hypodermis). 14

8 e. Special structures (1) Sweat glands (a) Sweat glands develop by tubular invaginations from the surface-epidermis, into the dermis and sometimes into the hypodermis epithelial structures. (b) Sweat glands are tubular and unbranched a straight tubular sweat gland duct leads from the stratum germinativum down into the dermis and/or hypodermis gland consists of a very convoluted dense mass of duct tubules, with dense to loose C.T. 15 (c) Typically simple cuboidal epithelium or stratified cuboidal epithelium. (d) The lumen of the duct and the gland proper is usually very narrow: no greater than the thickness of the epithelial wall. 16

9 (e) Sweat glands produce and secrete a liquid called sweat; the process of secretion of sweat is called perspiration; the main function of perspiration is evaporative cooling of the skin for thermoregulation. 1. The mechanism of thermoregulation is most effective when the skin is bare 2. Water is the critical component for evaporative cooling contains: K +, Na +, Cl -, HC0 3 -, and urea. 17 (f) The sweat gland duct proper ends in the deeper layers of the epidermis, and the sweat reaches surface of the stratum corneum via a non-cellular sweat (gland) channel through the upper layers. 18

10 (2) Hair follicles and associated structures (a) Hair follicles develop by tubular extensions into the dermis and sometimes the hypodermis, from the (stratum germinativum of the) epidermis. (b) Hair follicles are structurally rather complex and contain dermal elements as well as epidermal elements 19 (c) Histological components of hair follicle 1. External root sheath (ERS) epidermal tube running full length of follicle down to hair bulb; usually 2-3 cells thick; cells cuboidal and very small. 2. External C.T. sheath (ECT) thin layer of dense C.T. immediately around external root sheath and hair bulb. 3. Internal root sheath (IRS) epidermal layer lying immediately inside external root sheath, in deeper part of follicle only; cells larger than those of external root sheath. IRS ERS ECT ECT ERS Hair Hair IRS 20

11 4. Hair bulb: cup-shaped end of epithelial (epidermal) portion of hair follicle called a bulb cells of inner wall of cup produce the hair, by active mitotic cell division. a. Melanocytes typically abundant among basal cells of inner wall highly active in melanin production granules of melanin released to the epithelial cells and retained by cells becoming cells of the hair itself b. Cells of deeper part of the hair follicle are alive but then die as that part of the hair nears the skin surface the part of the hair extending out from the skin surface is dead cells only. 21 c. Hair typically has two regions differing in cell size and arrangement, and in thickness: (1) Hair shaft (medulla): inner part of hair; composed of larger cells (2) Hair cuticle (cortex): outer part of hair--usually relatively thin 22

12 (d) Sebaceous gland 1. Sebaceous gland is an epithelial extension off side of upper part of the hair follicle. 2. Sebaceous gland produces and secretes an oil: sebum; this oil may have value in providing softness to the stratum corneum; it may also have value in waterproofing the hair The glandular epithelial cells are very large and slightly to moderately eosinophilic; their cytoplasm looks somewhat granular. 4. The oil is secreted to the lumen of the hair follicle, where it coats the hair. 24

13 e) Erector pili (pilium: hair) 1. Erector pili is a very small, thin smooth muscle, composed of smooth muscle tissue, whose origin is at the basement membrane of the epidermis and whose insertion is on the external C.T. sheath of the hair follicle near hair bulb. 2. Function of erector pili: contraction pulls lower part of hair bulb to elevate the angle of hair follicle and of portion of hair extending out from surface of epidermis 25 a. Two main functions or purposes of this (homologous system with feather follicle in birds) in mammals in general (system relatively vestigial in humans). (1) Increasing thickness of fur layer, for increased thermal insulation by fur layer, for conserving body heat, in thermoregulation (probably the fundamental function). (a) Erector pili contraction in human skin due to exposure to cold produces "goose bumps". (2) Increasing fur thickness increases apparent size of body or part of body, for social signaling in social interactions. 26

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