Chapter 6 (3)

• Hair, nails, and cutaneous glands are accessory organs (appendages) of the skin. Hair and nails are composed of mostly dead, keratinized cells. Pliable soft keratin makes up the stratum corneum of skin. Compact hard keratin makes up hair and nails. They are tougher and more compact due to numerous cross-linkages between keratin molecules.
• Pilus is another name for a hair and pili is the plural of pilus. Hair is a slender filament of keratinized cells growing from a tube in the skin called a hair follicle. Hair covers most of the body surface and is used for protection, insulation, and sensation. BUT it does NOT cover the palms, or soles, the palmar, plantar, and lateral surfaces and distal segments of fingers and toes, or lips, nipples, and parts of the genitals.
• The limbs and trunk have 55 to 70 hairs per cm2. The face has about 10 times as many. There are 100,000 hairs on an average person’s scalp. The differences in hairiness across individuals are mainly due to differences in the texture and pigment of hair.
• Three types of hair:

  • LANUGO: fine, downy, unpigmented hair that appears on the fetus in the last 3 months of development
  • VELLUS: fine, pale hair that replaces lanugo by the time of birth
  • Makes up two-thirds of women's hair, one-tenth of men's hair, and all of children's hair except for eyebrows, eyelashes, and hair of the scalp
  • TERMINAL: longer, coarser, and more heavily pigmented
  • Forms eyebrows, eyelashes, and the hair of the scalp
  • After puberty, forms axillary and pubic hair, male facial hair, and some hair on the trunk and limbs
• Hair is divisible into three zones along its length. (1)Bulb: a swelling at the the base where hair originates in the dermis or hypodermis. Only living hair cells are in or near the bulb. (2)Root: the hair that is remaining in the follicle. (3)Shaft: the portion above the skin surface.
• The dermal papilla is the bud of vascular connective tissue encased by a bulb. It is the only source of nutrition for hair. The hair matrix is the region of mitotically active cells immediately above the papilla. It is the hair’s growth center.
• Three layers of the hair in cross-section are: (1)The medulla: It is a core of loosely arranged cells and air spaces. (2) The cortex: It constitutes the bulk of the hair and it consists of several layers of elongated keratinized cells. (3)The cuticle: It is composed of multiple layers of very thin, scaly cells that overlap each other, and its free edges are directed upward.
• A follicle is a diagonal tube that extends into the dermis and possibly the hypodermis. The epithelial root sheath is an extension of the epidermis lying adjacent to the hair root. It widens at the deep end into the bulge (which is the source of stem cells for follicle growth). The connective tissue root sheath is derived from the dermis but is a bit denser. It surrounds the epithelial root sheath.
• Hair receptors are sensory nerve fibers entwining follicles. The piloerector muscle (arrector pili) is a smooth muscle attaching a follicle to the dermis. It contracts to make hair stand on end (goose bumps).
• The texture is related to the cross-sectional shape of hair. Straight hair is round. Wavy hair is oval. Curly hair is relatively flat. Hair color is due to pigment granules in the cells of the cortex. Brown and black hair is rich in eumelanin. Red hair has a high concentration of pheomelanin. Blond hair has an intermediate amount of pheomelanin and very little eumelanin. Gray and white hair have little or no melanin because air is present in the medulla.
• Three stages of the hair cycle: Anagen, Catagen, and Telogen. (1)Anagen: This is the growth stage. 90% of scalp follicles at a given time. Stem cells multiply. Root sheath cells above the papilla form hair cells of the hair matrix and hair cells make keratin and die as they are pushed upward. (2)Catagen: This is the degeneration stage. Mitosis in the hair matrix ceases and sheath cells below the bulge die. The base of hair keratinizes into a hard club (club hair) these are easily pulled out by brushing. (3)Telogen: This is the resting stage and is when the papilla reaches the bulge.
• We lose about 50-100 hairs daily. A club hair may fall out during catagen or telogen or be pushed out in the next anagen phase. In young adults, scalp follicles spend 6–8 years in anagen, 2–3 weeks in catagen, and 1–2 months in telogen. With hair growth the scalp hairs grow 1 mm per 3 days.
• Alopecia is the thinning of the hair or baldness. Pattern baldness is when the hair is lost from select regions. The baldness allele is dominant in males and expressed when testosterone levels are high. Testosterone causes terminal hair on top of the scalp to be replaced by vellus hair. Hirsutism is excessive or undesirable hairiness in areas that are not usually hairy.
• Hair serves various functions in the human body. Most hair present on the trunk and limbs is vestigial and has little function now, but it used to keep our ancestors warm. Hair receptors present on the skin, notify us of any parasites crawling on it. The scalp retains heat and protects against sunburn. Pubic and axillary hair signify sexual maturity and aid in the transmission of sexual scents. Guard hairs, also known as vibrissae, guard the nostrils and ear canals. Eyebrows and eyelashes are used in nonverbal communication.
• Fingernails and toenails are clear, hard derivatives of stratum corneum. They are composed of thin, dead cells packed with hard keratin. They improve grooming, picking apart food, and other manipulations. They also provide a counterforce to enhance the sensitivity of fleshy fingertips to tiny objects.
• Nail plates are the hard part of the nail. Free edge: overhangs the fingertip. Nail body: is the visibly attached part of nail. Nail root: extends proximally under overlying skin.
• Parts of the nail include:

  • Nail fold: surrounds the skin and rises above the nail
  • Nail groove: separates the nail fold from the nail plate
  • Nail bed: skin beneath the nail plate
  • Hyponychium: epidermis of the nail bed
  • Nail matrix: growth zone of thickened stratum basale located at the proximal end of the nail
• Fingernails grow at a rate of 1 mm per week, while toenails grow slightly slower
• Lunule: opaque white crescent at the proximal end of the nail caused by the thickness of the matrix
• Eponychium, or cuticle: narrow zone of dead skin that overhangs the proximal end of the nail