Student Performance Objectives1. Explain 3 functions for body hair.2. Describe the formation of a hair within its follicle.3. Explain why hair and nails are considered structures of epidermal origin.4. Explain the formation of a nail from the nail root.
Lesson OutlineA. Hair - in general. Go to the following link and click on slide 9. 1. Present on most skin surfaces except where gripping power is needed such as the palms of the hands and sides of the fingers, and the soles and sides of the feet. Selected other areas are also hairless like the lips and parts of the external genitals. 2. About 25% of one's 2.5 million body hairs are on the head where they provide insulation and provide UV protection for the skin of the scalp. 3. Hairs are part of our sensory system since the deeper parts of the hair's production unit, the hair follicle, is richly innervated and we are conscious of slight hair movements. 4. Hairs respond to emotion due to smooth muscles attached to each one that can pull the hair into a more upright position from its more angled, relaxed position in relation to the skin surface. The smooth muscle attached to each hair is called an arrector pili muscle. Its contraction can give us goose bumps by pulling down a given skin area. 5. Hair occurs in 3 types: lanugo, very fine, unpigmented hair seen only in a fetus, vellus hair seen in children and on adults as peach-fuzz hair, and terminal hair, which is the mature, pigmented hair arising in response to our hormones at puberty. 6. Hair color is determined in great part by the genetically determined type of melanin present. Some melanin is dark brown, some is yellow-brown, and some is reddish. 7. Grey hair is due mainly to reduced melanin presence in the keratinocytes composing the hairs.B. Hair - the part we see. 1. The exposed part of hair we see is the hair shaft and is composed of dead cells full of keratin. a. When you feel a single strand of your hair, you are touching the hair's outer layer of flattened, keratinized cells called the cuticle. b. Within the cuticle are an outer hair cortex, filled with cells containing hard keratin, and an inner hair medulla, with cells filled with soft keratin. c. The flexible medulla and the stiff cortex, along with the surrounding cuticle give exposed hair shaft the tough, pliable quality we are familiar with.C. Hair - below the skin's surface. 1. Once the hair is below the skin's surface, it is, technically, within the hair follicle: a tube within which the hair forms and in which it grows toward the skin surface. The follicle extends from the skin surface down to the lower portion of the dermis and sometimes even into the hypodermis. a. The follicle can be thought of as having two portions: a slightly swollen hair bulb at its lowest portion and a hair root that extends from the bulb up to the skin surface. (1) The hair bulb is where the hair initially forms. It is a mass of germinative epidermal cells sitting on top of a dermal papilla, a mass of vascular connective tissue that feeds nutrients to the avascular bulb. (2) The hair root is a core of gradually keratinizing cells (gradually becoming a hair) rising toward the skin surface, surrounded by a root sheath of typical epidermal cell layers. The hair is fully dead, keratinized and structured into medulla, cortex and cuticle by the time it is about 2/3 of the way up through the dermis. (3) The root sheath of epidermal cells is, in turn, surrounded by a basement membrane called the glassy membrane. (3) The glassy membrane is in turn surrounded by a wrapping of dense, fibrous connective tissue called the connective tissue sheath (to which the arrector pili muscles attach). b. Hairs grow at a rate of about 2 mm per week and are on 2 to 5 year cycles after which they stop growing and the root detaches from the bulb region. When a new cycle of growth occurs from the bulb, the growing new hair pushes out the old hair which is shed. This applies to all hair including hair on the head, eyebrows and eyelashes.D. Nails 1. Nails consist of the nail body resting on an epithelial surface called the nail bed. 2. The nail originates from the nail root - an epithelium, infolding down from the skin surface that is close to the bone of the finger tip and not visible from the surface. 3. As the nail emerges to the exterior, the part of the nail root epithelium surrounding the nail pushes up over the nail forming the cuticle (also called the eponychium). 4. Dermal blood vessels are visible through the nail and give it its characteristic pinkish color. Near the point where the nail visibly emerges, the thickness of the nail root epithelium produces a whiter appearance to the nail called the lunula.