Biology of Hair and Nails

Biology of Hair and Nails • Trichinosis • Molecular and Cellular Pathology • Dr Tim Scott-Taylor • Health and Human Sciences

Topics Topics covered; • the nature of hair follicles • structure of keratin • normal patterns of hair growth • causes of hair lost Tutorial; • review of skin structure and function • light microscopy

Learning Objectives • to know the nature of skin follicles and hair growth • to know the function of hair in animals and humans • to know the structure of hair fibres • to understand the causes and treatment of hair loss

Hair • Longitudinal filamentous growth from the skin • Composed of keratin • Found mainly in mammals; definition of phylum • Wool, fur, pelage Function: insulation protection protective coloration ornamentation / communication sensory (vibrissae)

Skin and Keratin • Keratinized stratified squamous epithelium is found only in the skin • Keratin accumulation within maturing cells effectively waterproofs the cells, blocking diffusion of nutrients and wastes. The cells subsequently die. Hair is specialised adaptation of epithelial production of keratin

Keratocytes • keratocytes are cells that make keratins • >40 highly insoluble proteins forming a network in the cytoplasm of keratocytes. • transform from undifferentiated basal cells to cornified skin cells. • four continuous layers basal spinous granular cornified recognizable histologically In the course of approximately 13 days columnar basal cells mature through a polygonal spinous cell, a diamond-shaped granular cell to a flat cornified cell that covers 25 basal cells.

Parts of Hair • Hair follicle; involution of epidermis • Root; implanted in skin • Shaft or scapus; projecting from surface • hair Bulb; root enlargment, soft, white • Papilla; dermal, blood and nerves • Sebaceous glands; open into follicle • Arrector muscle; attached to epidermis

Nails • Longitudinal section through nail and its nail groove

Claws, Nails, Hooves and Spurs Keratin structures found in all mammals except cetaceans (whales). Derived from reptile claws. Consist of dorsal unguis = nail plate ventral subunguis = sole plate a) claws - unguis is curved and encloses the subunguis retractable in some carnivores. b) hooves - Unguis completely surrounds subunguis. cloven hooves in even-toed ungulates (Artiodactyls) c) spurs - in male monotremes a hollow keratin spur on hind legs poison gland associated with it. d) nails - only in primates. Flat unguis + subunguis greater precise manipulation of food or tools. Some Horns and antlers • rhino horn - mass of keratin threads produced by epidermis • bovid horn - bony core, horny sheath, no shedding • pronghorn - bony core, horny sheath shed annually

Reptiles Mammals scales few epidermal glands no mammary glands 3-chambered heart ectothermy little cerebral development indeterminate growth egg laying hair many epidermal glands mammary glands 4-chambered heart endothermy much cerebral development determinate growth live birth Hair is a Defining Feature Comparison Reptiles - Mammals Hair is characteristic of the phylla Temperature regulation was a fundamental development in mammalian evolution

Animal Hair • Fur, coat, pelage, wool: furry insulating covering • Special adaptations; Spines; porcupine Bristles; lion’s mane UV transporter; polar bear • dolphins, whales; aquatic adaptation (otters and seals) • Size and hair; mass  1/ follicle per cm2 eg elephant, rhinoceros heat loss

Types of Animal Hair • Vibrissae; elongated + stiffened associaed with many nerve fibers mainly tactile receptors facial and carpal especially in carnivores primates rodentia

Types of Animal Hair Underhair; main function is insulation. a) wool long soft and curly angora b) fur fine relatively short hair definitive growth c) velli down or fuzz with velvety appearance

Types of Animal Hair • Guard hair; long, straight and pointed overhairs present on most mammals a) awns stiff intermediate sized hair, protects down b) spines enlarged, stiff guard hairs with finite growth evolved parallel hedgehogs ~6 times in mammals spiny anteaters tenrecs barbs porcupines poisonous montremes c) bristles long firm hair eg. horse and lion manes# hair serves to insulate, to conceal, to signal, to protect, and to sense the immediate surroundings. Insulation serves to conserve heat, but can protect against excessive heat eg camel

Tenrec • Tenrec ecaudatus(tailless tenrec) • The common tenrec occurs on Madagascar and on the Comoro Islands, between Madagascar and Africa. • Pelage is not dense and is a combination of hairs and blunt spines.

Spiny Anteaters • Echidnas, genus Tachyglossus, subclass Holotheria. • monotreme • found only in Australia and New Guinea. • lays a single egg that is kept in her pouch for seven to ten days until spines begin to harden • Pelage a mixture of spines and quills

Hair on Humans • Filamentous growth from the skin • Mainly in mammals; genus definition • Function head: protective, ornamental eyelashes: protect eyes from dust eyebrows: shield eyes axillae; wicking of sweat • scalp palms axillae soles pubes penis chest labia beard fingers mucus membranes • Naked ape everywhere else covered in vellus hair terminal hair bare

Naked Ape Great apes have sparse and bare areas but none extensive Why is man different???? 1). Adaptation; cool forest to hot savana bipedal locomotion upright stance but other cursorial apes, eg gorilla, chimpanzee, no loss ape pelage shields from UV, wind, radiant heat 2). Eco-parasites; fitness cost high cohabitation, communal living but other gregarious apes, eg chimpanzee, no loss 3). Aquatic ape; 5-7 e6 yr BC, Danakil region Ethiopia extended period wading lifestyle hair loss and subcut. fat more efficient theory not account for eccrine gland, density same as gorilla an chimp but developed for sweat production

Neoteny • paedomorphism • slowing of certain aspects of physiological development • retention by adults of juvenile physical characteristics • dogs share many physical features of immature wolves • Louis Bolk 1926: "man is a primate fetus that has become sexually mature" • advocated by; Desmond Morris: The Naked Ape Stephen Jay Gould: chimp bone brain • 4 fold increase in hominid brain size over past 3e6 years • loss of body hair a side effect

Types of Hair • Languno: fine hair covers nearly entire embryo • Vellus: poorly pigmented, short, peach fuzz growing most areas • Terminal: fully developed, longer, coarser thicker and darker • Middle ear hair Although vellus hair continues to develop and accounts for 6% to 25% of scalp hair, it is less noticeable because of its light pigmentation

Depth • vellus follicles situated high in the dermis • large terminal follicles rooted deep in the subcutaneous fat Dermis

Variable Skin • The density of hair and the numbers of associated eccrine glands varies with different situations in the body

Hair Growth Hair elongates from the dermal papilla

Length And Growth Rate Length of hair and growth rate per day vary for each site on the body • hairs on the head 70cm / 0.35mm • Eyebrows 3cm / 0.15mm • beards and whiskers 28cm / 0.4mm • armpit hairs 8cm / 0.3mm • pubic hairs 10cm / 0.2mm

Normal Hair Growth • Cyclical pattern Anagen = growth phase 2.5 – 3 years Catgen = short Telogen = resting 100 days • asynchronous anagen 1000 days catgen Telogen 100 days

Growth Cycle • 100 to 150,000 follicles on the scalp • ~95% of them are in an active growth phase called Anagen • ~5% completed their growth phase • Telogen hairs typically remain in the scalp for about three months before they are shed. • shampooing, combing or brushing typically releases hair at a rate of about 50 to 80 per day.

Functions of Human Hair • head hair for insulation and protection; trauma UV • eyelashes blink to prevent entry into eye • eyebrows, like awnings, protect the eyes from sunlight and sweat that might be hazardous • body hair for insulation; goose bumps traps air residual? • axilla; wicking of sweat • long hair; marker of health? secondary selection

Inner Ear Hair • Hairs in the ampullae of the semi-circular canals are responsible for balance • Otoliths, crystals of calcium carbonate, floating in gelatinous medium of the saccule brush against the stereocilia of the inner ear. This movement generates stereotypic awareness

Hair Colour Two types of melanin account for all possible hair colors: Eumelanin: colors hair brown to black Pheomelanin: an iron-rich pigment colours hair yellow-blonde to red • Both made from amino acid tyrosine, by tyrosinase. Increased tyrosinase activity results in darker hair color • Melanocytes in the hair bulb contain subcellular organelles melanosomes; synthesize and store melanin. • Melanocytes deliver the melanosomes to keratinocytes, where they are incorporated into the growing hair shaft

Long Hair • anthropologists speculate functional significance of long head hair may be adornment • byproduct of secondary natural selection once other somatic hair had been lost • or Fisherian runaway sexual selection; lustrous hair is a visible marker for a health waist length hair = ~1 m or 39 inches long ~80 months, 7 years, to grow long period of nutrition stability hygiene • explain why long hair attractive both sexes

Shape Affects Curls straight hair is round curly hair is flattened

Hair Structure • medulla = central core of cuboidal cells • cortex = highly packed cells • cuticular scale = outer layer

Follicle • The hair follicle consists of two coats—an outer or dermic, and an inner or epidermic. • The outer or dermic coat is formed mainly of fibrous tissue; it is continuous with the dermis, is highly vascular, and supplied by numerous minute nervous filaments. • Dermal coat consists of three layers The most internal is a hyaline basement membrane, well-marked only in the larger hair follicles, it is limited to the deeper part of the follicle • Outside this is a compact layer of fibers and spindle-shaped cells arranged circularly around the follicle; this layer extends from the bottom of the follicle as high as the entrance of the ducts of the sebaceous glands. • Externally is a thick layer of connective tissue, arranged in longitudinal bundles, forming a more open texture and corresponding to the reticular part of the dermis; in this are contained the blood vessels and nerves.

Keratins • A long-chain protein of >300 residues • contain a high proportion glycine H smallest amino acids alanine methyl • sterically-unhindered hydrogen bonding between the amino and carboxyl groups peptide bonds on adjacent protein chains • closely aligned and tightly bound fibres • tertiary structure: fibres twisted in helical filaments • second in stength to chitin in organic molecules side chains tight -helices

Keratin Secondary Structure • keratin molecules > 300 residues • high proportion of smallest of amino acids; glycine H side group alanine, methyl side group. • sterically-unhindered hydrogen bonding between the amino and carboxyl groups of peptide bonds on adjacent protein chains • close alignment and strong binding between chains • keratin molecules twist around each other to form helical intermediate filaments.

Keratin Tertiary Structure • keratin molecules twine around each other in a left-hand  helix coil structure • two helices in turn form another left rotating helical fibre, called a protofibril • eight protofibrils form a circular or square structure, called a microfibril, that is the basis of the structure of hair. • This structure is stretchy and flexible and can be compared to a rope containing various threads that are twined together.

The Arrangement of Fibres • The a-helix coiled keratin molecules are wound together in protofibrils. • protofibrils are bound together into microfibrils • Bundles of microfibrils are stitched together to make macrofibrils • Each hair is wrapped in a cuticle made up of protein scales and contains macrofibrils embedded in matrix proteins

Interchain Bonding • In addition to intra- and intermolecular hydrogen bonds, keratins have large amounts of the sulfur-containing amino acid cysteine ~14% • disulfide bridges confer additional strength • rigid, permanent, thermally-stable crosslinking similar to sulfur bridges in vulcanized rubber • disulfide bonding contributes to insolubility of keratins • pungent smells of burning hair and rubber due to the sulfur compounds formed

Strength • Intra- and intermolecular hydrogen bonds • disulfide bridges that confer additional strength and rigidity by permanent, thermally-stable crosslinking • It can be made to curl ’permanently’ or set temporarily. The breakage and reseting of the interchain bonds allow hair to regain its shape. Hair can stretched about 70%. • Cross-linking creates strength. Hair is equivalent in strenght to iron wire. A wound strand of 5 mm can withstand a weight of 60kg

Static • The close connections of chains is associated with the hair's ability to become charged with static electricity. • keratin is a good insulator of heat and electricity • The capacity to hold an electric charge is refered to as triboelectric

Styling • hair heated with steam and pulled breaks the relatively weak hydrogen bonds the a-helices • The a-helices can elongate without breaking the molecules • matrix proteins form a tangled supporting mechwork linked by many disulphide bridges. These bridges are also disrupted with moist heat • new interchain hydrogen bonds and disulphide bridges are formed after styling to keep the hair in its new style

Nail Nails Growth Growth occurs from a nail bed over a keratin matrix layer

Nail Keratin • Nailbed The nailbed is an essentially parallel epidermal structure located directly beneath the fingernail formed with parallel lines spaced at intervals. During normal growth, the fingernail travels over the nailbed. The nail is also composed of keratin microfibrils but the molecules are arranged in flat B-pleated sheets

Nails Arranged in Stacks • The molecules of b-keratin in finger and toe nails are organised in sheets stacked in layers. The proteins still fall into two groups; the helical and matrix, but in nail there is less sulphur in the matrix than in the hair matrix.

Hair Length • Sexual selection; visible marker of health • 1.25 cm / 0.5 inch per month: good hygiene 39 cm = 7 years nutrition health stabilty

Dimorphism • male androgens at puberty • testosterone • vellus - terminal hair • affects growth rate • weight • length androgen sensitivity

Hair and Culture Much variation in style and length in different times and places • Egypt head hair shaved, especially childrenlong single lock of hair growing • Middle ages: shaved head + wig • 17 to 18th C: long hair eg Cromwell Napoleon Washington • WWI: lice and fleas brought typhus, short hair adopted Maasai warrior

Body Hair • attitudes towards hair on the human body also vary between different cultures and times • some cultures excessive chest hair on men is a symbol of virility and masculinity, other societies display a hairless body as a sign of youthfulness • ancient Egypt, people regarded a completely smooth, hairless body as the standard of beauty • adopted Greeks; hairless body = youth and beauty (strigyl) • Islam; many hair tenets, five traits of fitrah • Western societies; removal, bikini fashion, ‘manscaping’

Hirsutism • Hormonal changes alter hair presence and thickness • Vellus hairs on face of women, invisible/inconspicuous • Oestrogen, antagonist of DTH • Menopause; thicker and darker • Diseases; Polycystic ovary syndrome Cushing's disease ovarian or adrenal gland tumors

Hair Loss • Whales & porpoises, and walrus use blubber for insulation. • Elephants, rhinos, hippos - warm climate, favorable mass/surface ratio -> retention of heat no problem. • molt = continuous or seasonal (1 or 2x /year) hair replacement • seasonal fur color change: arctic fox, hares, some weasels

Biology of Hair and Nails