Life is composed of cells. The largest cell in the human body is an egg cell. Then what is the smallest cell? Is it the dermal papilla cells responsible for hair growth? After searching for a lot of information, no answer to this question was found.

Hair growth is promoted by hair follicles. The source of growth for hair follicles and hair is the dermal papilla cell, which is still unknown if it is the smallest cell.

Before focusing on the dermal papilla cells, let us strengthen our knowledge on hair follicles. In a sentence, hair follicles are skin appendages responsible for hair growth. Without hair follicles, there would be no hair. Hair follicles have the characteristics of cyclical growth, controlling the three stages. The growth phase, transitional phase, and resting phase of hair. These three stages repeat and form the hair growth cycle.

The growth phase of hair is 2 to 7 years. Based on different ages, it can grow as long as 1.2 cm every month. Generally, it grows fastest for young people, reaching 1.2 cm. It grows slower for older people, reaching maximum of 0.8 cm per month. In this way, hypothetically the hair of a young girl during the growth period is limited to 7 years and can grow more than 1 cm per month. But what if it is for those that are older? It is estimated that the growth period is just 2 years, 3 years, or 4 years? And hair grows very slowly.

The resting phase of hair is 2 to 4 weeks. During this period, the hair follicles and melanocytes stop working, and the hair bulbs of the hair follicles leave the micro-circulation (blood capillaries) at the bottom, and move upwards. Hair loses its nutrient supply, followed by the close arrival of the resting phase, which lasts from 1 to 4 months. It is characterized by hair loss.

On the surface, hair follicles are regulating the three cycles of hair, but in fact, it is the dermal papilla cells that regulate the hair follicles in the end. The dermal papilla cells are in the recesses of the hair bulb at the root of the hair follicles. Its regulation process is very detailed. It can be roughly divided into two stages, the pre-growth period and post-growth period. The pre-growth period occurs under the scalp, at the hair shaft inside the hair follicle; in the post-growth period, the hair roots have emerged from the scalp, penetrated the pores, and became visible hair shafts.

The logic is very clear all along, the hair follicles are the driving force of hair growth, but in fact they are dermal papilla cells, this group of human cells seems inconspicuous, but in fact it is the important driving force in charge of the “forest on our head”.

In addition to having a close relationship with nutrition, hair is also closely tied to the human endocrine system. The human body's endocrine system secretes various hormones and regulates the body's metabolism and physiological functions within the nervous system,

Under normal circumstances, various hormones are comprehensively balanced. If the secretion of a certain hormone is too much or too little, once this balance is broken, it will cause endocrine disorders, and corresponding signs will appear in various parts of the body. Signs manifested in the hair are thickening and thinning, and shedding and whitening of various body hairs on the body.

The endocrine system affecting hair includes the function of the adrenal cortex, anterior pituitary gland, sex hormones, thyroid function and others. The abnormal secretion of the adrenal cortex and the anterior pituitary gland can cause hypertrichosis and hypotrichosis, but the most important factor of the endocrine system affecting the thickness and dryness of hair are sex hormones.

We know that be it male sex hormones or female sex hormones, during high levels of secretion, they can promote the growth of dense hair. During childbirth in females, due to the increase of estrogen, they can prolong the growth phase of hair and delay the transitional phase of hair. After childbirth, estrogen is restored to its normal levels, resulting in a large amount of accumulated hair loss for a certain period of time, which is clinically called postpartum alopecia.

The chronic disease of alopecia is androgenetic alopecia, also known as male alopecia. The main cause of this type of alopecia is also related to male sex hormones. When the 5α-reductases activity on the top of the head of some men is higher than normal, testosterone acts on the 5α-reductases producing dimethyltestosterone, dimethyltestosterone. The killer of hair follicles, it directly hinders the metabolism of hair follicles, shortens the growth phase of hair, prolongs the resting phase, shrinks the hair follicles, directly leading to a large area of alopecia.

Finally, let us talk about the thyroid gland. Normal thyroid function is necessary for hair growth. It directly or indirectly affects the gonads and acts on the hair. However, a breakdown of the thyroid gland balance, whether it is hypothyroidism or hyperactivity, can cause hair loss. Especially for people with hyperthyroidism, about 40% of people will have diffuse hair loss or total baldness, 8% will have alopecia areata, few people will have white hair prematurely.