Hair is a common feature of determining characteristics of mammals. It grows on most of the limbs of human being and animals. The study of hair and scalp is known as trichology.
It grows out of the center called hair follicle. Morphologically it is composed of root, shaft, and tips, which gives the characteristic shapes and size and help in identification of an individual. The shaft is a visible part that comes out from the skin.
The root consist of white bulb like structure called as follicles that grows from an epidermis layer of the skin containing oil producing gland to lubricates the hair and the arrestor Pilli muscles. It consists of bundle of fibrils each of which is composed of a protein known as keratin. It also contain pigment granules within the cortex and medullary material in the medulla. Other compounds present in it are uric acid, cholesterol, vitamins and some elements like lead, iron, arsenic.http://modernforensic.in
The cross section of hair shaft is consists of 3 layer :
Cuticle
Cortex
Medulla
CUTICLE: The outermost covering of the shaft is cuticle which consists of translucent scales, that laid one over the opposite pointing towards the tip of the each hair. It is highly kerantized, composed of cell shaped scales which is impermeable. There are three basic different patterns of the cuticular scales: coronal, spinous, and imbricate. The coronal or crown like scales resembles like stacks of paper. These type of scales are rarely found in human beings . The spinous or petal like scales are found in cats, seals but never found in human beings. The imbricate patterns or flattened scales are commonly found inn human beings and many animals. This layer provides stability and resistance to the hair.
CORTEX: The cortex is layer between the cortex and medulla. This consist of spindle shaped cells aligned parallel to the length of the layer. It is a source of mechanical support and helps in water uptake. The cortex comprises filament melanin, made by specialized cells known as melanocytes that gives color to the hair.
MEDULLA: The innermost layer of the shaft is termed as medulla. The nature of the medulla and its diameter concerning the whole diameter of it gives us the thought about the species of an individual . Medulla may be classified as continuous, interrupted, fragmented or absent. Animals have continous medulla whereas human beings have fragmented medulla.
Medullary index: diameter of the medulla/ diameter of the whole shaft.
For human medullary index is smaller amount less than 1/3
For animals it’s ½ or more.
4 STAGES OF HAIR GROWTH
The growth and loss of hair is a complex process and is divided into 4 stages of hair :
1. Anagen
2. Catagen
3. Telogen
4. Exogen
ANAGEN: the growing phase- it is the longest phase that last upto 3-5 years. The stage start from the anagen phase. At any point of the life, the 90% of the hair are in anagen phase. During these phase, the follicles are actively producing hair until it cuts or they reach the end of the life span. Some hair have follicular tags that contains the DNA that helps in individualizing person.
CATAGEN: transition phase- the phase between the anagen and telogen phase is a transition phase that last upto 2-3 weeks. During this stage, it starts to shrink and hair growth slows. The hair also separates from the bottom of the hair follicle ,yet remain in place during its final days of growing. At any time, 5% of hairs are in catagen phase. In catagen phase, the basis takes an elongated appearance because the root bulb shrinks and is pushed out of the follicle.
TELOGEN: resting phase – this phase typically last around 3 months. As estimates 10-15% are in this stages. In this phase, the hair not generally sheds out but remain in the head until the new is formed in the follicle.
EXOGEN: shedding phase- exogen phase is normally an extension to telogen phase. Normally 50-100 hairs shed per day. The exogen phase last upto 2-5 months, during this new hairs are growing in the follicle as old hair sheds off.
