Lecture Notes: The Layers of Skin

The skin has 2 layers separated by a basement membrane.

1. Epidermis = outer most covering

a. Stratified squamous epithelium

b. Basement membrane = anchors epithelium to underneath tissue

2. Dermis = connective tissue

a. Alive & contains blood vessels, glands, nerve cells, and hair roots

b. Grab bag tissue

c. Cells widely spaced apart w/ matrix btwn them

d. Contain collagen & elastic fibers

3. Hypodermis (sub Qlayer, subcutaneous layer) = below dermis

a. Contains aerolar connective tissue

b. addipose tissue (does not pick up stain therefor looks like empty space)

I. Epidermis

§ stratified squamous epithelium

§ No blood vessels!

§ Cells:

i. The epidermis contains keratinocytes (96 % of cells in skin), cells that have hardened in a process called keratinization

i. This process involves the production of keratin, a protein which makes the cells physically tough and waterproof.

ii. The epidermis also contains melanocytes = cells which secrete a pigment called melanin which spreads over keratinocytes

a. provides protection against UV waves

§ UVB à mutate DNA in cells (cancer)

§ UVAà break down elastic fibers in dermis therefor wrinkles!

b. gives the skin its color.

c. All people have the same # melanocytes, just produce different amts of melanin

§ Epidermis has 5 layers (strata) born in germinativum and migrate toward the surface to die in the stratum corneum

i. stratum basale (germinativum)

a. cells are born (called keratinocytes or melanocytes)

b. near blood vesselsin

ii. stratum spinosum

a. spiny desmosomes form a barrier to prevent fluid loss

b. keratinocytes are O line man (filled w. keratin or lipids), “link arms” to prevent things going in between

iii. stratum granulosum

a. lipid filled cells which also serve as a barrier to h2o loss

iv. stratum lucidum

a. rarely seen

v. stratum corneum

a. dead or dying cells filled w/ keratin which get sloughed off

b. # layers depends on need

c. most have lost nuclei

II. Dermis = layer of connective tissue beneath the epidermis

§ made of collagenous (strength) and elastic (elasticity & extensibility) fibers

§ contains blood vessels, nerve fibers, hair follicles and glands (sweat and sebaceous)

a. Fibroblasts

a. type of cell in the dermis

b. produces collagen à hard & not flexible, elastinà less hard, more flexible and other substances which give the skin support and elasticity

b. Blood vessels (vasculated)

a. supply the epidermis with nutrients

b. have a role in thermoregulation by dilating or constricting

c. Nerve fibers

a. provide sensory input for the organism

b. free nerve endings signal pain and temperature

c. encapsulated nerves signal pressure, vibration and position

f. Hair follicles: serves no vital fnc (may wish you were dead, but won’t die)

a. FNC: insulation, protection, scent trap, sensor

b. FACTS:

i. Hair grows 1000 inches per day

ii. Avg head has 100,000 hairs

iii. Each hair grows 1/100 inch daily

iv. Every hair has a lifespan of 2 to 4 years

v. Eyelash lives about 150 days

c. derived from epidermal cells that invaginate into the dermis

d. root t(he actively growing portion of hair) = epidermal cells which lie in dermis

e. shaft = composed of dead, keratinized cells that are outside the skin (otherwise go “ouch, ouch, ouch” every time you cut it

f. hair color is determined by the secretions of melanocytes (amt pigment prdcd in the follicle)

g. arrector pili muscle is attached to the hair follicle and is responsible for "goose bumps"

i. trap an insulating layer of air (works well for bears or birds, but not humans with sparse hair) to preserve heat

ii. stick up to lose heat

g. Sebaceous glands

1. holocrine gland à errupts & secretes an oily mixture of fats and dead cells called sebum into the hair follicle

a. keeps the hair and skin soft and waterproof

b. protects against bac & funcgus

c. may clog hair follicle & cause acne

i. bacteria love sebum & eat it!

2. secretion is caused by androgens and decreased by estrogen

h. Sweat glands

i. thermoregulation!

ii. several million glands can perspire up to 10L per day!

iii. open directly into the skin surface through a pore

iv. secretes a hypotonic solution

1. plasma filtrate, but glands reabsorb Na+ so sweat is hypotonic

v. perpetual = palms & soles

vi. thermal = all surfaces

vii. emotional = palms, soles, axillae (underarm) & forehead

viii. axillary (armpit) and genital sweat is mix of sweat & sebum

1. bacteria action causes odor prdcd

….Like the Back of Your Hand

Did you know that a 3/4-inch-square patch of skin, which is only about 1/20^th of an inch thick contains: