The dermis is thicker and deeper than the epidermis. Elastic and collageous fibers are arranged in definite patterns to produce lines of tension and provide skin tone. There are many more elastic fibers in the dermis of a young person than an old one. The extensive network of blood vessels in the dermis provides nourishment to the living portion of the epidermis. The dermis also contains many sweat glands, oil-secreting glands, nerve endings, and hair follicles.

Innervation and Vascular Supply

• Nerve supply: specialized integumentary effectors consist of smooth muscles or glands within the dermis that respond to motor impulses from central nervous system. Several types of sensory receptors respond to various tactile, pressure, temperature, tickle or pain stimuli.
• Vascular supply: blood vessels within dermis supply nutrients to mitotically active stratum basale, and to structures of the dermis. Dermal blood vessels also play an important role in temperature regulation.

Layers of the Dermis

The dermis is composed of two layers. The outermost dermal layer is listed first:

• Stratum papillarosum: in contact with epidermis, and accounts for 1/5 of entire dermis. Papillae form the base for friction ridges on fingers and toes.
• Stratum reticularosum: fibers within this layer are more dense and organized to forma tough, flexible meshwork. It is very distensible, but can be torn when stretched too far. Linea albicans, which appear as temporary white streaks, form when a torn dermis repairs itself.

The presence of hair is a distinguishing feature of mammals. Men and women have the same density of hair, but testosterone makes the hair more apparent on men. The primary functions of hair are protection and attraction.

Each hair consists of a diagonally positioned shaft, hair and bulb. The shaft is the visible, but dead, portion of the hair projecting above the surface of the skin. The bulb is the enlarged base of the root within the hair follicle. Each hair develops from stratum basale cells within the bulb of the hair, where nutrients are received from dermal blood vessels. As the cells divide, they are pushed away from the nutrient supply toward the surface, and cellular death and keratinization occur.

In a healthy person, hair grows at the rate of approximatel 1 mm every 3 days. As the hair becomes longer, however, ti enters a resting period where there is minimal growth. The life span of a hair varies from 3 to 4 months for an eyelash to 3 to 4 years for a scalp hair. Each hair lost is replaced by a new hair that grows from the base of the follicle and pushes the old hair out. Between 10 and 100 pairs are lost daily.

3 layers can be observed in hair that is cut in cross section. The inner medula is composed of loosely arranged cells separated by numerous air cells. The thick cortex surrounding the medula consists of hardened, tightly packed cells. A cuticle covers the cortex and forms the toughened outer layerof the hair. Cells of the cuticle have serrated edges that give it a scaly appearance under the microscope.

Hair color is determine by type and amount of pigment produced in stratum basale at base of hair follicle. Varying amounts of melanin produce from blond to brunette to black. More melanin, darker. Trichosiderin, a pigment with an iron base, produces red hair. Gray or white hair is lack of pgiment and air spaces within layers of shaft of hair. Texture of hair is based on cross-sectional shape: straight hour round, wavy hair oval, kinky hair flat.

Sebaceous glands and arrectores pilorum are attached to hair folicle. Arrectores pilorum muscles involuntary, responded to thermal or pyschological stiumli. When contract, hair pulld into more vertical position to make goose bumps. Three kinds of hair:

Lanugo: fine, silky fetal hair appears during last trimester of development seen only on premature infants
Vellus: shrort, fine hair replacing lanugo. Abundant in children and women just barely extended from the hair follicules.
Terminal hair: coarse, pigment (except in elderly people) and sometimes curly. It includes scalp, pubic, eyelash hair.
Angora hair is terminal hair growing continually as in scalps and faces of mature males.
Definitive hair grows to a certian lengths and stops, including eyelashes.

Introduction & Layers

The epidermis is the superficial protective layer of the skin. It is derived from the ectoderm, and is composed of stratified squamous epithelium that varies in thickness from .007 to .12 mm. All but the deepest layers are composed of dead cells. Areas exposed to high friction have 5 layers; areas not exposed to high friction have 4 layers. Beginning with the innermost layer, the epidermis is composed of the following layers:

• Stratum basale consists of a single layer of cells in contact with the dermis. Four types of cells constitute this layer:
  • Keratinocytes are specialized cells producing keratin. As keratinocytes are pushed away from the vascular nutrient and oxygen supply of the dermis, they undergo keratinization: their nuclei degenerate and their cellular content becomes dominated by keratin.
  • Melanocytes are specilized epithelial cells which produce melanin.
  • Tactile cells are sparse relative to keratinocytes and melanocytes, and are involved in tactile (touch) reception.
  • Nonpigmented granular dendrocytes are scattered throughout the stratum basale. They are protective macrophagic cells which ingest bacteria and other foreign debris.
• Stratum spinosum is oftentimes grouped with the stratum basale. This complex is called the stratum germinativum
• Stratum granulosum contains 3 or 4 flattened alyers of cells. These cells contain granules filled with keratohyalin, a chemical precursor of keratin.
• Stratum lucidum appears clear because the nuclei, organelles and cell membranes are no longer visible. It exists only in the lips, soles, and palms.
• Stratum corneum is composed of 25 to 30 layers of flattened, scalelike, dead cells. It has been cornified, which is a process brought on by keratinization, which means that it has been dried and flattened.

Below are brief descriptions of the major integumentary structures: