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Lesson 4 - Integument Accessory Structures - Hair and Nails

Student Performance Objectives
1. 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 Outline
A. 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   
        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.