What you need to know about the hair follicle: morphogenesis, structure and regeneration of hair

In modern society the beauty of hair has acquired a special meaning, because now hair performs not so much a physiological function as an aesthetic one.

The more the doctor knows about morphogenesis, structure and hair growth cycle, the easier it is for him to choose the right treatment.

The fact is that different preparations act, respectively, on different phases of the hair growth cycle and on different structures of the hair itself.

On estet-portal.com we figured out in detail how hair develops, starting from the prenatal period, what are the types of human hair, what are the features of the structure of the hair, and how does it regenerate.

• Hair growth: morphogenesis and differentiation
• Features of the hair structure
• Human Hair Types
• Hair follicle regenerationa

Hair growth: morphogenesis and differentiation

The regulation of hair follicle development(HF) in the embryonic period and the cycling of HF growth in the postnatal period of life are highly conserved processes that are regulated by a similar series of signals between epithelial and mesenchymal cells.

Epithelial-mesenchymal interactions are the basis for the formation of epidermal appendages (hair, nails and sweat glands).

The initial morphological event in the development of the HF is the thickening of the surface ectoderm with the formation of the placode (Fig. 1A).

This initial event is a response to a signal from the underlying mesenchyme.

Progression to the placode stage occurs with a grouping of dermal cells adjacent to the placode resulting in the formation of a hair germ.

The hair germ proliferates and invaginates into the dermis, forming the hair shaft where the condensed mesenchyme becomes the dermal papilla (Figure 1A) .

Fig. 1: Morphogenesis, differentiation and structure of hair.

(A) Sections of fetal hair follicles at the stages of placode, hair germ, hair shaft formation and VF differentiation (80 to 170 days).

(B) Complete anagen WF on development day 126.

(C) 1. Left drawing: the lower part of the hair follicle and the region of the human hair bulb, depicting the various sections of the hair.

2. Right figure: Scanning electron microscopy of a human hair shaft. 

Sequential signal from the dermal papilla to adjacent epidermal cells induces differentiation of the inner root sheath, in which the future hair shaft will develop.

When the inner root sheath is formed, the epidermal cells that surround the dermal papilla, known as hair matrix cells, begin to differentiate into different lineages to form different hair shaft structures that will grow inside the inner root sheathand.

T cells: hair growth activators

Features of hair structure

The hair follicle, which is usually associated with the sebaceous gland, can be divided into four main areas:

• infundibulum (funnel), which covers the hair from the surface of the skin to the junction with the sebaceous gland;
• isthmus - from the attachment of the muscle that raises the hair to the entrance to the lower part of the sebaceous duct;
root sheath
• ;
bulb
• , inside the concavity of which is the dermal papilla.
The type of hair depends on the volume and shape of the dermal papilla.

Onion

– it is a stem cell population store for the follicle. These bulging stem cells are essential for normal

hair regeneration

, but can also colonize the interfollicular epidermis during wound healing.

Mechanism of stem cell activation during hair follicle injury The hair follicle consists of

concentric sheaths

, each with its own functions and characteristics. This includes the

support sheaths

that surround and stabilize the growing hair.

The outer root sheath

, which is continuous with the basal layer of the epidermis, is the outermost layer of non-keratinizing follicle cells. Further, from outside to inside are

companion level

and inner root shell level. The inner root sheath has several compartments, the most peripheral being the

layers of Henle

, then the layers of Huxley and finally the cuticle.

The inner root sheath

ends at the junction between the isthmus and infundibulum. At the center of the follicle is the

hair shaft

, which consists of the following three layers:

hair cuticle
• which forms the surface of the hair;
bark
• which forms the majority of the hair and is composed of keratinized cells that contain pigment from bulb melanocytes; The innermost elongated
medulla
• , characterized by the presence of cells that differentiate and arrange themselves in one or more columns.
Around the HF is

a sheath of connective tissue

that supports the dermal papilla at the base of thefollicle.

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Telegram! Human Hair Types

Hair covers almost the entire human body, with the exception of the skin on the palms, soles and lips. A person has different types of hair.

Hair density is determined during fetal development and varies from person to person.

The first type of hair is formed in utero and is called

lanugo

. This is fine, unmedulated and lightly pigmented hair.

There are specialized types of hair, such as the hair found on the eyebrows, eyelashes, ears and nose, called

bristly

.

Vlus hair

replaces lanugo at the end of pregnancy and is short, non-medulated and non-sebaceous hair. On the scalp, vellus hairs are replaced by

pigmented terminal hairs

before or shortly after birth. On the body, vellus hair is present throughout childhood and is replaced by terminal hair in certain areas of the body (arms, legs) during and after puberty.

The pituitary gland secretes hormones that trigger the production of androgens in the ovaries and testicles, promoting the replacement of vellus hair with coarse hair in the pubic and axillary regions.

In addition, adult males develop terminal hair on the face and growl

di.

Human stem cells for hair regeneration Hair follicle regeneration

Following initial follicular morphogenesis, the hair follicle cycles through periodic stages that include

growth phase (anagen), regression phase (catagen) and resting phase (telogen)

. This cyclical regeneration is a

stem cell dependent

process that occurs throughout a person's life. The molecular signals underlying hair cycle reversal are similar to the formation of the placode during embryogenesis.

During

anagen
• matrix cells proliferate and differentiate to form the inner root sheath and hair shaft. During
catagen
• the epithelial compartment of the inferior follicle, including matrix cells and differentiated layers, undergoes apoptosis while the dermal papilla remains intact. The follicle then enters a resting
telogen
• phase, after which the dermal papilla cells stimulate stem cell activation and hair matrix cell division for a new hair growth cycle.

On the scalp, most follicles are

in the anagen phase,

which lasts 2–6 years, while the catagen and telogen phases > only last 2–3 weeks and 3 months respectively. Human hair growth

is not synchronized

, so different stages occur simultaneously in different follicles.

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