Body covering {skin}| has epidermis and dermis layers. Skin glands make oil and water and eliminate salt wastes. Most animals have drier skins than humans.
skin color
Skin color depends on epidermis yellow tinge, epidermis translucence, and skin-cell pigment amount and type: yellow, brown, or red. Melanin absorbs ultraviolet light and scavenges free radicals. Ultraviolet-B light causes melanocyte increase.
Chimpanzee skin is light colored, with pink skin at face, hands, and feet. Homo ergaster probably had little or no hair and had many more sweat glands.
High ultraviolet-A light reduces blood folic-acid concentration, needed for spermatogenesis and avoiding neural-tube defects. Ultraviolet-B light starts skin vitamin-D production. Vitamin D is for calcium absorption and for skeleton and immune-system development. Skin color balances these, with women having slightly lighter skin to absorb more calcium.
Spinal nerves receive sensory signals from skin areas {dermatome}.
Skin {glabrous skin}| can be hairless. Hairy skin is thinner than glabrous skin.
about tears {lacrimal}|.
Skin regulates heat loss by constricting or opening capillaries and regulating fluid {sweat}| production from glands {sweat gland}. 90% of heat loss is through skin. After temperature reaches 25 C, eccrine sweat glands affect temperature regulation. Anterior hypothalamus affects eccrine sweat glands.
Outer connective-tissue layers {epidermis, skin}| can have surface dead cells, which come from living and dividing cells below dead cells. Hairs and gland tubes go through epidermis.
Inner connective-tissue layers {dermis}| can have nerves, blood vessels, sense receptors, mechanoreceptors, sweat glands, oil glands, pigments, fat cells, and hair follicles.
Birds have contour feathers {penna}|. They also have down feathers and plumes.
Growing feathers {pinfeather}| have hard sheaths.
Bird wings or outer rear-wing edges contain main feathers {pinion feather}|.
Birds have feathers {plumage}.
Skin {foreskin}| {prepuce} can cover glans penis. Circumcision removes prepuce.
skin color {pigmentation}|.
Soft mounds {comb, bird}| can be on domesticated-fowl crowns.
Mounds {crest, bird}| can be on bird-head tops.
Finger and toe tips have fingerprints that typically have swirls {whorl}|.
Meissner's corpuscles lie in rows just below epidermis in fingertip surface ridges {dermal papillae}.
Collagen makes skin firm, and lack makes skin thin {wrinkle}. Elastin makes skin flexible. Glycosaminoglycans absorb water. Retinol, retin-A, or retinoin can prevent wrinkles by stimulating collagen production.
Wrinkles {crow's feet}| can be at outer eye corners.
Loose skin {dewlap}| can hang from neck fronts.
Skin {wattle}| can hang from chicken, turkey, and lizard neck or throat.
At birth, skin can have discoloration or unusual texture in small regions {birthmark}|.
People can have red birthmarks {strawberry mark}| on faces or scalps.
mole or birthmark {nevus}|.
Thick skin {scab, wound} can cover healed wounds.
Thick hard coverings {callus, skin}| can be over plant or animal wounds.
Gland cells {apocrine} can release hormones by shedding apex.
Exocrine glands {eccrine} include sweat glands.
Glands {merocrine} can secrete hormones without shedding cell parts.
Skin glands {sebaceous gland} can secrete sebum, which bacteria eat to make bad odors.
Sebaceous glands secrete carbohydrate {sebum}, which bacteria eat to make bad odors.
Modified sweat glands {mammary gland}| {teat} produce milk for infants.
Mammary glands produce sweet protein-rich and fat-rich liquid {milk}|, for infants.
Female cows, sheep, and goats have rear-underside bags {udder}| that contain mammary glands.
Heads have 100,000 hairs {hair, human}. Hairs grow several years, five inches a year. Follicles then become dormant, and hairs fall out. Later, new hair grows. Humans have less hair, because they have many more sweat glands with which to stay cool.
Hair holders {hair follicle}| have growing and non-growing phases. Dermal papilla are deep in dermis, organize hair follicles, and start hair matrix. Wnt signaling proteins induce hair follicles and control hair-growing cycle. Hairs fall out after bottom cells die.
region
Hairs have inner and outer sheathes. Hair follicles have attached muscles, which can form goosebumps, and nearby sebaceous glands.
curl
Flat follicles make curly hair.
color
Hair, fur, and feathers are dead skin cells. Pigments, air bubbles, and rough or smooth surfaces cause color.
Head hair {cowlick}| can grow in another direction than other hair.
Hair {forelock}| can grow from or fall over forehead. Horse manes typically have forelocks.
fur {pelt}.
short unshaven facial hair {stubble}.
woman's long hair {tress}.
Fingernails and toenails {nail, finger}| come from nailbeds.
Fingernail and toenail edges have hard skin {cuticle, fingernail}|. Annelids and other invertebrates have hard thin coverings.
Fingernails and toenails come from digit-tip skin-pad cells {nail bed}.
claw {talon, claw}|.
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Date Modified: 2022.0225