As an embryo about 5 million hairs developed over your entire body. These hairs were fine and nearly transparent and called lanugo hair. These hairs were shed before birth.
As an adult you have two basic types of hair. The typical hair on your scalp that grows long is called terminal hair. There can be great variation in the colour, waviness and coarseness of hair from one person to the next. On average, terminal hair measures 65 to 75 microns (0.065 to 0.075 mm) in diameter (width) but the average hair shaft diameter may range from 50 microns in someone with very fine hair to 100 microns in someone with very coarse hair. You also have short, fine, light-colored hairs called vellus hair. At puberty, the increase in hormone levels causes vellus hair in the pubic, armpit and male beard areas to grow deeper and longer and become terminal hairs.
Before hair loss, an average person has 100,000 scalp hairs. Each hair goes through a growth cycle. A terminal scalp hair will typically grow Â½ inch per month for 2-6 years in the “Anagen” phase then over 2-4 weeks it will go through a transition, called the “Catagen” phase, at the end of which begins a resting phase, called the “Telogen” phase, which lasts for 2—4 months. These Telogen hairs are loose and can be shed or pulled out easily or pushed out by the new Anagen hair that grows to replace it.
In some animals all the hairs are in the same phase at the same time and all the hairs go through regular, usually seasonal, growing and molting/shedding phases at the same time but in humans there is asynchronous growth because each hair has its own different independent growth cycle. At any given time, 85-90% of hairs are in the Anagen Phase and 10-15% of hairs are in the Telogen Phase. 1 in 1000 hairs (or about 100 hairs) are shed each day as part of the normal growth cycle of hair but those hairs will be replaced by new hairs. In some conditions such as Telogen Effluvium more hairs go into the Telogen phase and more hairs are shed each day than usual.
Body hair, eyebrow hair, eyelashes etc. all have shorter Anagen phases and longer Telogen phases so they don’t grow as long as scalp hair.
In Androgenetic Alopecia, the commonest cause of permanent hair loss, the hormone, DHT, causes each generation of genetically sensitive hairs to have a shorter Anagen phase. The affected hairs get progressively more fine and shorter and lighter in colour with each growth cycle until they finally don’t grow back.BACK