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THE INTEGUMENTARY SYSTEM

INTRODUCTION

1. Organs are combinations of tissues, and organ systems are combinations of organs that perform one or more specific functions.

THE INTEGUMENT

1. The integument, or skin,consists of the cutaneous membrane and the accessory structures.

Integumentary Structure and Function

The Epidermis

1. Mitoses in the stratum germinativum replace the superficial cells lost at the surface of the epidermis.

2. As epidermal cells age they pass through the stratum spinosum, the stratum granulosum, the stratum lucidum (if thick skin), and the stratum corneum. In doing so they accumulate large amounts of keratin, a tough, water-resistant protein.

3. The outermost layer of the epithelium, the stratum corneum, consists of dead cells.

4. The thickness and contour of the surface varies according to the location sampled and the environmental stresses encountered.

5. Epidermal ridges provide better gripping abilities and increase the sensitivity of the skin.

6. The color of the skin depends upon its thickness, vascularity, and the presence of melanocytes. Melanocytes are pigment cells that protect underlying tissues from the effects of ultraviolet radiation.

7. the densuty of melanocytes varies according to the region of the body considered, but not between individuals or even races.

The Dermis

1. The papillary layer of the dermis contains blood vessels, lymphatics, and sensory nerves. This layer supports and nourishes the overlying epidermis.

2. The reticular layer consists of a meshwork of collagen and elastic fibers oriented to resist tension in the skin.

The Accessory structures

1. Hairs project above the surface of the body at almost every location. Hair follicles extend deep into the dermis or hypodermis (subcutaneous layer).

2. Each hair has a root and a shaft. The shaft contains an inner, soft medulla surrounded by a stiffer cortex and covered by a tough cuticle.

3.The shape of a hair determines its curliness.

4. There are vellus hairs (fine), intermediate hairs, and terminal hairs (coarse) on the body. Some hair follicles respond to hormonal alterations at puberty or later in life by changing the character of the hairs they produce.

5. The position of the hairs can be changed by contraction of the arrector pili muscles of the dermis.

6. Hairs provide limited protection but provide important sensory information.

7. Color results from trapped air and structural pigments.

8. Hair growth is not constant, but occurs in cycles. A single hair grows for 2-5 years, and is subsequently shed.

9. Sebaceous glands discharge into hair follicles or directly onto the surface of the skin at sebaceous follicles.

10. These glands produce a lipid mixture, sebum, by holocrine secretion of glandular cells.

11. Sebum lubricates the epidermis and inhibits the growth of bacteria.

12. Sebaceous glands secrete in response to neural or hormonal stimulii.

13. Apocrine sweat glands produce an odorous secretion in response to neural and hormonal stimuli.

14. Merocrine, or eccrine, sweat glands produce a watery secretion important in the regulation of body temperature.

15. The activity of merocrine sweat glands is closely regulated

as part of the negative feedback mechanism that controls body temperature.

16. Merocrine glandular secretions may also present an important route for the excretion of water, electrolytes, wastes, and foreign compounds.

17.Ceruminous glands of the ear produce a waxy cerumen, known as "ear wax."

18. Nails are composed of keratin produced by epidermal cells of the nail root.

INTEGRATION WITH OTHER SYSTEMS

Functional Integration

1. Examples of integumentary independence include the response of the germinative layer to mechanical stresses and the initiation of the inflammatory response.

2. The secretion of accessory glands, hair growth, and blood flow through the skin can also be regulated by the nervous or endocrine systems. Receptors in the skin monitor the temperature of the environment and provide information concerning pressure and pain.

Structural Integration

1. The fibers of the reticular layer of the dermis are continuous with those of the hypodermis (subcutaneous layer). The distribution of fat in the hypodermis provides padding, protection, and insulation.

2. Subcutaneous injection using a hypodermic needle introduces drugs into the loose connective tissue of the hypodermis. From there the drugs can slowly enter the general circulation.

3. The epidermis provides mechanical protection and keeps microorganisms outside of the body. Penetration of the epidermis triggers an inflammatory response.

4. The integument can heal even after considerable damage has occurred.

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