web
analytics

Skin and Hair Structure:
Simple Explanations and Clear Images


What, exactly, is hair?

This page explains basic hair structure in a simple, easy to understand way...


First of all, hair is formed within the skin, so it's important to know the basic structure of skin as well as hair.

Read the bullet points below and quickly learn about the main features of skin and hair structure.

Please refer to the diagrams for those words shown in bold.

Also note that, since hair and skin structure are both quite complex, only the most relevant details have been given.


Structure of Skin

  • Your skin is the largest organ in your body. It has various functions including temperature regulation, excretion, protection against sun, rain, bugs, infection, etc.

  • Skin has two main parts - the epidermis and dermis (see diagram below).

  • The epidermis has five layers. The uppermost layer is called the horny layer and forms the surface of the skin. This is made from dead cells that are continuously being shed (brushed or washed off) and replaced from below.

  • The lowest layer of the epidermis is the germinating layer (or basal layer). This region is constantly growing and dividing into new cells, and pushing the old cells up towards the surface of the skin.

  • Below the epidermis is the dermis. The dermis is the thickest part of the skin and contains blood vessels to supply the nutrients needed for skin cells to grow.

Structure of Hair


Before hair growth can begin, a hair follicle (pouch) must first be created - the germinating layer of the epidermis grows downwards into the dermis, and forms the outside of each hair follicle.

Structure of hair follicle.


  • The dermis grows up into the base of the follicle forming the dermal papilla. Blood vessels can then enter the papilla and provide the nutrients to grow the hair shaft.

  • The bottom of the follicle enlarges into an area of actively growing cells called the hair bulb.

  • At the base of the hair bulb, the germinating layer becomes continuous with the outer root sheath (this forms the inner wall of the follicle).

  • The outer root sheath extends into the germinal matrix (the hair root) which then surrounds the dermal papilla. The germinal matrix contains stem cells and forms the region where hair growth takes place.

  • The peripheral area of the germinal matrix grows the inner root sheath (this is the white thickening at the end of a hair shaft if it is pulled out).

  • The central area of the germinal matrix grows the hair shaft. These cells constantly divide pushing the older ones upwards. Initially, the cells are alike, but as they move up the follicle they begin to change shape and keratin (a protein) develops within the cells.

  • The different types of hair cell are formed (cuticle, cortex and medulla). By the time these cells are one third of the way up the follicle, they are fully hardened (keratinized) and have died.

  • A sebaceous gland lies within each follicle. Sebaceous glands produce an oily substance called sebum from a duct that opens into the hair follicle about halfway down from the skin surface.

  • The follicle has a bulge which lies directly below the sebaceous gland duct in the outer root sheath at the attachment point of the arrector pili muscle. The bulge produces stem cells that regenerate the follicle during the hair growth cycle.


Cross section of a blonde hair shaft showing the hair structure,

  • From the image above, you can see that the hair structure of the shaft (i.e., a strand of hair) has three layers: the cuticle (outer layer), the cortex (middle layer) and the medulla (inner layer).

  • The medulla is a honeycomb keratin structure with air spaces within it.

  • The cortex gives flexibility and tensile strength to hair and contains melanin granules, which give hair its colour (see "Skin and hair pigmentation" section below).

  • The cuticle is made from 6 to 11 layers of overlapping semi-transparent scales (which make the hair waterproof and allow it to be stretched). Someone with thick, course hair will have more overlapping layers of cuticles than someone with fine hair.

Skin and Hair Pigmentation


Skin colour - Inside the germinating layer of the epidermis, cells called melanocytes produce melanosomes (these are small packets that contain melanin granules). The melanosomes then spread upwards through the epidermis and accumulate just above the nucleus within skin cells - this gives protection against UV radiation. Darker skin contains more melanin and so has more protection against the sun.

Hair colour - Within the dermal papilla at the base of a follicle, melanocytes produce melanin granules. Then, as germinal matrix cells are produced, melanin granules pass into the cells which then develop into the cortex (the middle layer) of a hair shaft to give hair its colour.

There are two types of melanin: eumelanin is dominant in brown and black hair, and pheomelanin is dominant in ginger and blonde hair. Without melanin granules, the partly hollow hair appears grey. Hair that is completely absent in melanin is white.

Keratin

Both hair and skin are classed as epithelial tissue (i.e., tissue that lines cavities and surfaces of the body, and also forms several glands). The structure of hair, nails and the outer layer of the skin are largely composed of keratin.

Keratin is a very hard structural protein that is used to protect and support the body by forming tough, insoluble structures that are arranged in layers.

In hair, it forms long filaments that provide hair structure and strength, and is a key component in all three hair shaft layers (cuticle, cortex and medulla).


In mammals, keratin forms hair, hooves, horns, skin and nails. Only
mammals have hair, but amphibians, reptiles, birds and some fish also
use keratin (i.e., for claws, feathers and scales).


Next page - Hair Growth Cycle.

Leave Hair Structure and return to Hair Growth index.


Top of Hair Structure Page.


References:
education.vetmed.vt.edu
wikipedia
nature.com