Hair follicle-Structure, Functions, and Growth Phases
The hair follicle is a structure which appears like a bag or sac and the hair grows from it. The term hair follicle is derived from the latin word “follis”. Follis means a bag like structure. Hair follicles and hair are seen all over the body except some regions like our palms, soles, and lips.
The rate of growth of hair follicles differs in different people. But, the average rate at which the hair or the hair follicles grow is about half an inch per month. Do you want to know what is the structure of a hair follicle, all the parts present in the hair follicles complex and their cell cycle? Come let us discuss all the hair follicles.
The structure of a hair follicle:
There are many parts of a hair follicle. Let us start discussing the main parts of the follicles of hair.
Hair root is the part of the hair follicle that is buried beneath the skin. There are much more parts of the hair root. They are the papilla, germinal matrix and the bulb.
The papilla is the main part of the hair root. It is made up of connective tissue, which is the tissue which connects or binds many different types of cells and tissues. The papilla is also rich in many capillaries, which is called the capillary loop. There is no division of cells in the papilla, yet it is the biggest part of the hair root.
Germinal matrix is the part of the hair root that surrounds the papilla completely. It consists of many epithelial cells. It also contains another type of cells called the melanocytes.
Melanocytes are the cells which are responsible for the production of melanin, a pigment which is responsible for the color of our skin.
The bulb is the base of the hair root. There is an active division of the cells in the bulb. The division of the cells in the bulb leads to the production of the cells of the hair shaft.
This is the part of the hair which is visible above the skin or the layer called epidermis. The hair or the hair shaft is made up of many dead cells called keratinocytes or keratin.
Hair shaft also has many different layers called the cuticle, cortex, and the medulla. The cuticle is the outermost layer of the hair or hair shaft. Medulla is the middle layer of the hair shaft or hair. The innermost layer of the hair shaft is the cortex.
Cuticle or the outer layer of the hair shaft protects the hair from the damages caused by environmental and pollution. Cortex or the innermost layer of the hair shaft is responsible for the appearance of the hair. Different people have different color and texture of the hair based on the characteristics of their cortex.
Root sheath is the part of the hair follicle that covers the hair root completely. Root sheath is made up of epithelial cells. There are still two layers of the root sheath called the internal root sheath and the external root sheath.
The internal root sheath is further divided into three more layers from outside to inside called the internal cuticle, Huxley’s layer, and Henle’s layer.
Root sheath is surrounded by a bulge. It contains many connective tissue cells and also the stem cells. The stem cells are the cells which act as a precursor cells for the formation of all the types of cells in the body.
Other structures associated with hair follicles:
Arrector pili muscle:
Arrector pili muscles are the bundle of muscles which are attached to the hair follicles at their base. These muscles contract and then lead to the erection of hair follicles when we experience any sudden exposure to cold or when we are surprised. This erection of the hair follicles and the hair caused by arrector pili muscles is called goosebumps.
Sebaceous glands are modified sweat glands. These glands produce a substance called sebum. This sebum is oily and lubricates the hair follicle and hair. The number of sebaceous glands is directly proportional to the number of the hair follicles or the density of hair in a particular region of skin.
There are two types of sweat glands in the body called the apocrine sweat glands and the eccrine sweat glands. Apocrine sweat glands are attached to the hair follicles. These Apocrine sweat glands are only found in certain parts of the body like the axillae, areola, nipples, ear, eyelids, near genital regions, etc.
Some types of receptor cells are attached to the hair follicles. These receptor cells sense the position of the hair and follicles and signal our nervous system and brain about it.
The growth cycle of the hair follicles:
There are three phases in the growth of the hair follicles or the hair. They are the anagen phase, catagen phase, and the telogen phase. Let us discuss all the different growth phases of the hair follicle.
Every hair follicle needs to grow actively to allow the hair to grow and increase in length. The anagen phase is the active growth phase of the hair follicle. In this phase, there is the growth of about 1 cm every month.
The duration for which the hair follicles of each region of the body stays in Anagen phase is different. Also, the duration of Anagen phase is different in different people. This time period of Anagen phase is based on genetic factors. This phase lasts almost for years.
Due to some unknown impulse in the body, the hair follicle is suddenly shifted from Anagen phase to the Catagen phase. This phase lasts only for a short duration, less than 1 month. This Catagen phase is just a transition phase from Anagen phase to the Telogen phase.
This is the resting phase of the hair follicle. It lasts for only a few months. During this phase, hair loss or hair fall occurs. If you are stressed, anxious or depressed, hormonal changes in the body lead to extreme hair loss during Telogen phase. This condition is called Telogen Effluvium.
You, people, have seen all about what is a hair follicle, the structure of the hair follicle, the structures associated with the hair follicle and the growth phases of the hair follicle. We will discuss in depth about all the hair disorders in our next posts.