# Hair --- **Hair** is an accessory structure in the [[integumentary system]]. It is made up of mostly [[epidermis#Keratinocytes Keratin|keratin]]. Hair is found all over the human body and serves a variety of purposes, such as protection, sensory input, [[body temperature|temperature]] control and even communication. ![[hair.png]] ## The Hair Follicle & Bulb The **hair follicle** is the tunnel-like hole in the [[epidermis]] through which the hair fiber emerges. It is connected to the **hair bulb** in the [[dermis]] which is where the hair filament it self is made when keratinous mush together and grow outward. ![[hair follicle and bulb.png]] The walls of the hair follicle have three main layers, the **inner sheath**, the **outer sheath** and the [[cartilage#Hyaline Cartilage|glassy]] [[basement membrane|membrane]] on the outside. The hair bulb is at the very bottom of the follicle, and it houses the **hair papilla** and the **hair matrix**. The hair papilla is in the center of the bulb and is where the [[capillaries]] and [[nerves]] endings are. The hair matrix is where the mitotically active [[epidermis#Stratum Basale|basal cells]] are dividing and being pushed together to form the hair fiber itself. The matrix also has [[melanocytes|melanocytes]] which give the hair it's coloring. The production of melanin decreases as we age, which is why our hair turns grey or white. The **arrector pili muscle** is a tiny muscle that attaches to each hair follicle that can contract and cause the hair to "stand up straight". ### Furuncles A **furuncle**, or a **boil** is a hair follicle that has a bacterial infection. It's quite painful. The beginning stage of this is called **folliculitis** A **carbuncle** is when the infection affects several follicles at once. ## The Hair Fiber The **hair fiber** itself is comprised of the **hair shaft** and the **hair root**. The hair shaft is mostly exposed above the surface of the skin and not actually anchored down to the hair follicle. It's also the part of the hair that is completely dead, without any nerves or anything, which is why we can cut our hair without discomfort. The hair root is compleatly below the surface of the skin and still attached to the walls of the follicle and bulb. The hair can also be divided longitudially, where it has two or three seperate sections, the **medulla**, the **cortex** and the **cuticle**. Not every hair has a medulla, but if it's present it's the very center of the fiber and is made of soft, flexible keratin. The **cortex** is the middle-inside layer that is comprised of hard squished together [[epidermis#Keratinocytes Keratin|keratinocytes]]. The **cuticle** is the very outside layer and is very thin, very tough keratinized cells. ## Hair Growth The hair growth of each individual hair occurs in three stages, the **anagen**, the **catagen** and the **telogen** stages. We loose and replace about 50 hairs a day. The **anagen** phase last for about 2-7 years and is when there is a lot of dividing of basal cells which pushes the hair fiber up and out. During this phase hair grows at about 0.3 mm per day. The **catagen** phase lasts about 2-3 weeks and is just the transition phase between it actively growing or not. The **telogen** phase lasts about 2-4 months is where there is no new growth and the hair follicle is at rest. At the end of the telogen phase the anagen phase begins again. ## Problems with the Hair - [[alopecia]] - common during pregnancy/post partum. ___