HAIR ANATOMY

Hair Growth

There are 3 stages of hair growth:

Anagen – the active growth phase 2-6 years

Catagen – the transition stage 1-2 weeks

Telogen – the resting phase   5-6 weeks

Hair grows through the narrow and tube-like depressions in the skin called the hair follicle. The root of the hair is surrounded by the dermal papilla which has an abundant supply of blood vessels which provide nutrients to the area. New hair is formed in the papilla and created by hair germ cells. Hairs are soft at the base and eventually harden and die as they get towards the surface.

Hair is made up of 3 levels.

The outer layer, the Cuticle is a tough outer protective layer of the hair shaft. The cells are translucent so allows for the colour underneath to show through. The cells form scales that overlap each other like that of roof tiles.

The cortex lies beneath the Cuticle and is the main part of the hair. The Cortex contains the colour pigments that make up the hair colour. The cells of the Cortex contain bundles of fibres and gives the hair its strength and flexibility.

Keratin is also formed in this layer.

The middle core of the hair is called the Medulla, which is not always present, and this section does not appear to have any function.

Eyelash Function

Eyelashes are not just a pretty feature on the body; they have a specific and important function. Eyelashes are designed to prevent objects from getting in the eyes. Each eyelash is a sensory hair that reflexively shuts the eyelid whenever it is touched by dirt, dust, or anything else that could possibly get in the eye. The upper lid typically has about 90 to 150 lashes on it, while the bottom has between 70 and 80 lashes. Most eyelashes grow to be about 10 mm long (just over 3/8 inch)

Only about 40 percent of the upper lashes and 15 percent of the lower lashes are in the anagen phase at any one time. Each lash will grow to a specific length and then stop.

During the catagen phase the lash stops growing and the hair follicle shrinks. If an eyelash falls out or is plucked out during this phase, it will not grow back right away because the follicle needs to complete the catagen phase before it can move on to the next one. This phase lasts between two and three weeks.

The telogen phase can last more than 100 days before then and an eyelash falls out new one begins to grow. Because each individual lash is in its own phase of the growing cycle, it is normal for a few lashes to fall out most days. It typically takes between four and eight weeks to fully replace an eyelash.

Anatomy of the Eye

The eye is an organ that detects light and sends signals along the optic nerve to the brain. In humans, the eye is a valuable sense organ that gives us the ability to see. It allows for light perception and vision, including the ability to differentiate between colours and depth.

Although small, the eye is an overly complex organ. The eye is approximately 1 inch wide, 1 inch deep and 0.9 inches tall. The human eye has a 200-degree viewing angle and can see 10 million colours and shades. Humans have two eyes which allows us to have better depth perception and binocular stereopsis.

Cornea – The cornea is the clear, dome-like structure on the front part of the eye. The cornea delivers 2/3 of the refracting power to the eye.

Conjunctiva – The conjunctiva is a mucus membrane that covers the surface of the eye and the inner part of the eyelids.

Sclera – The sclera is the white, tough, outer covering of globe of the eye. The sclera is continuous with the cornea.

Iris – The iris is a pigmented tissue with two muscles that control pupil constriction and pupil dilation. The iris acts like a diaphragm that controls the amount of light allowed into the eye. The iris is the coloured part of the eye.

Pupil – The pupil is the hole in the middle of the iris in which light pass through to the retina. The pupil is black because the light that is allowed into the eye is absorbed in the retina.

Anterior Chamber – The anterior chamber is the fluid-filled chamber between the iris and the cornea’s inner surface which is comprised of the endothelium. Aqueous humour is the fluid that fills the anterior chamber.

Trabecular meshwork – The trabecular meshwork is a meshwork of tissue located around the base of the cornea, in the angle of the eye. The trabecular meshwork is continuous with the ciliary body. It is responsible for draining the aqueous humour into a Schlemm’s canal (drainage tubes) and into the blood system.

Crystalline Lens – The Crystalline lens delivers 1/3 of the refracting or focusing power to the eye. It is a fibrous tissue that can change shape to increase or decrease its power. Because it can change shape, it allows the eye to focus on intermediate and near objects.

Ciliary body – The Crystalline lens is attached to the ciliary body by lens zonules. The ciliary body is a muscle that can contract to change the shape of the lens. This allows humans to carefully focus on near objects. The other function of the ciliary body is to produce aqueous humour that flows into the anterior chamber.

Retina – The retina is a light-sensitive tissue that captures light energy and transfers that energy to the brain as nerve impulses.

Optic nerve – The optic nerve is a network of nerve cells which receives impulses from the nerve fibre layer on the retina. It transfers nerve impulses to the brain.