Skin 8, Hair

we now want to think about hair because

hair is an appendage of the dermis and

hair is out of hair follicles so let's

draw a picture of this so again we've

got the surface of the the skin there

this is a surface of the epidermis and

the hair follicles go deep down into the

dermis and there's a wider part at the

bottom there like that and as always we

have the epidermis on top and the area

of the dermis below and as we've noted

already when we looked at the epidermis

epidermal keratinocytes line all of the

inside of the hair follicle all the way

down to the base and this is very

important if this partial thickness

injuries if the top part of the skin is

lost because this means there can be

epidermal regeneration from the

keratinocytes within the lower parts of

the dermis in preserved deep dermal

structures so here we have the hair

follicle and the hair is inside the hair

follicle so here we have a hair in here

now there's actually a projection of the

way like that that's called the hair

papillae the hair papillae inside the

hair and the hair is going to grow

inside here like this now what we

actually have here in this bottom area

is the German ative layer of the hair so

there's ongoing mitosis here with

epidermal type cells and these cells are

pushed away from the German ative layer

up the way and they

die and they become compacted cells so

as the hair grows its compacted cells

compacted keratin producing cells and

the hair grows up the hair follicle

forming a shaft like this coming out

onto the surface so all this is the hair

this is the hair shaft this bottom part

at the bottom is called the bulb the

wider part at the bottom is called the

hair bulb then this is the shaft

compacted dead keratin yielding cells

held together by some protein

extracellular matrix so that's the hair

and the color of the hair is also

determined by this German ative layer

because in this German ative layer there

are cells which will add pigment to the

hair so there's pigment producing cells

here that give the hair its color and if

you have got gray hair when you get

older that's because there's a lack of

pigment there's less pigment present in

the hair now we mentioned that there's a

dermal papilla here this is important

because we have a arterial branch coming

along here and there's going to be a

branch a small arteriole going into that

taking nutrients in and then there's

going to be some capillaries in here

small area of capillaries and then the

venule will drain out the way connecting

up to a venous vessel

so blood containing oxygen and nutrients

is going to go into this dermal papillae

this area in here and that's going to

nourish the tissue fluids in that area

meaning that the german ative layer of

the hair is going to receive the oxygen

and the nutrients that it requires now

also associated with the hair follicles

there are other structures fairly near

the surface that open onto the hair

follicle and these are called sebaceous

glands and the sebaceous glands are

glandular they produce a product so

round about the edge of the sebaceous

gland here there's going to be cells and

these cells are going to excrete sebum

into the gland let's draw sebum as a

green color today it's not really today

so there's the sebum being produced by

the sub sebum producing cells in the

sebaceous gland and the sebum is an oily

material its waxy and it gives the skin

waterproofing it contains triglycerides

and cholesterol and it contains some

protein and also some mineral salts as

well and it makes the surface of the

skin oily and supple and it stops it

from drying out and it improves the

waterproofing of the skin as well and if

you just rub your finger between your

nose and your face there can you feel

all that sort of oily stuff there

that's sebum and it's good because it's

keeping the surface of the skin moist

and also it has some antibacterial

properties so paradoxically if you wash

a lot and you watch a lot of the sebum

away from the surface of your skin that

can actually

some of the antibacterial properties

that sebum naturally has so sebum

produced by sebaceous glands and in

nearly all parts of the body the

sebaceous glands are opening into a hair

follicle and the sebum travels up the

hair follicle and onto the surface of

the skin there's the situation in most

parts of the body the hair follicle is

the medium for taking the sebum from the

sebaceous gland at the neck of the hair

follicle out onto the surface of the

body now also in this unit with hairs

hairs also have what's called a hair

erector muscle or an erector pili a

hairy rectum muscle I guess I wrecked or

pill is kind of Latin some people still

use that term but what it is the hair

erector muscle is a muscle that's

connected to the hair follicle so it's

connected here and it connects up onto

some adjacent epidermis as well connects

up there and there's a muscle here a

small muscle so this muscle is called

the hair erector muscle or the erector

pili now what's it doing here well when

this muscle contracts what it does is it

pulls the bottom of the hair follicle

that way and that means the hairs going

to move up that way so when that

contracts that's going to move over and

how is going to become more direct it's

exactly what it says it's a hair erector

muscle it makes the hair stand erect so

what's the purpose of that well the idea

is that if you have lots of hairs

standing erect on the surface

what you're actually doing is trapping a

layer of air next to the surface of the

skin so this would be trapped air next

to the surface of the skin and air is a

particularly good insulator of heat it

keeps the heat in so the hair erector

muscles are going to contract when the

person's too cold and as the hair

erector muscle contracts it's going to

pull that part of the dermis or that

part of the epidermis down making an

indentation in that area of the

epidermis where it pulls down and in

English we call this goose flesh or

goose pimples so when you've got goose

pimples it's because you're cold but the

idea is that it increases the boundary

layer of air and it would only be

effective if you were a particularly

hairy person but that's that's what

happens so bit to the hair follicle

really we've got the hair follicle blood

supply to the hair pili the growing hair

bulb the hair shaft coming out of the

hair follicle the associated sebaceous

gland and the associated hair erector

muscle but there's not the component as

well remember when we talked about

sensation well you can actually become

aware of touch if someone just brushes

the surface of the hairs without

actually touching the surface of the

epidermis so to become aware of touch

you don't actually need your skin to be

touched just the surface of the hair to

be flicked and this is particularly true

with eyelashes and it's particularly

true with mice and rats around whiskers

that are very sensitive that they can

detect their way through tunnels in the

dark well we also have this as humans

because there's an arrangement of nerve

fibres around the hair follicle called

hair root plexus so around about the

nerve follicle with a fiber of a sensory

nerve going off towards the central

nervous system there's a plexus of

nerves a collection of nerves around

about the follicle and these are very

sensitive to light touch so if the hair

is flicked on the surface that's going

to make the hair flick inside and just a

very light touch on the hair on the

surface will actually stimulate this

because you've kind of got a leave Ridge

effect you've got that whole hair as a

leverage effect so if someone flicks the

hair on top here like that that's going

to move the hair date deeper down in the

follicle that's going to slightly move

the follicle and that's going to be

detected by this sensory hair root

plexus and here we have the dendrite of

the sensory neuron taking that

information off towards the central

nervous system where we will experience

it as sensation