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WNT is a secretory glycoprotein with more than 20 related family members. Its secretion is mediated by wntless (Wls), which is a transmembrane transporter. Although the role of Wls in the induction of hair follicle development is still unclear, it has been found to exist in embryonic epithelium and hair follicles after hair formation (Huang et al., 2012). Studies have found that the WNT family can be divided into primary WNT and secondary WNT.
What does my nail matrix do?
This cross-linking gives the HS a characteristic tensile strength and flexibility. The IRS can also be keratinized, which can support and guide the growth of HS in the process of HS differentiation. During anagen, the cell cycle of highly proliferative stromal cells is about 18 h (Harland, 2018). The time of growth cycle determines the length of hair and is related to the continuous proliferation and differentiation of stromal cells at the base of hair follicles (Morgun and Vorotelyak, 2020). Hair follicle melanocytes and their precursors reside in the hair matrix and along the outer root sheath of anagen hair follicles. However, production of hair pigment (black eumelanin and/or the reddish pheomelanin) only occurs in the specialized hair follicle pigmentary unit, located above and around the dermal papilla during anagen III–VI.
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What is a nail matrix?
EDAR contains extracellular ligand binding N-terminal, single transmembrane domain, and intracellular death domain (Schuepbach-Mallepell et al., 2021). Its death domain can specifically bind to the extracellular domain of intracellular binding protein EDARARR, initiate signal transduction, and regulate the transcription of downstream target genes (Wohlfart and Schneider, 2019). At the end of anagen, Eda-A1 was expressed in hair matrix, IRS, and ORS. In the middle stage of catagen, the expression of Eda-A1 decreased sharply and was only expressed in the hair buds of secondary hair follicles. EDAR was expressed in the hair matrix and the IRS at the middle stage of the anagen, but at the end of anagen, the expression of EDAR in the IRS and the ORS increased rapidly.
How does hair grow?
At the beginning of catagen, EDAR was expressed only in the IRS and ORS, but at the end of catagen, EDAR only appeared in the hair buds of secondary hair follicles (Gomez et al., 2013). After the catagen stage, the hair follicle enters telogen, when the biological activity of the hair follicle is the weakest and the HS falls off. However, expression and activity of the relevant regulatory factors in the hair follicle governing its cyclical growth will be significantly enhanced to prepare for the beginning of the next anagen. During telogen, the DPCs migrate to the lower part of the bulge, so that the DPCs can interact directly with the stem cells in the bulge.
Structure
The first is the anagen phase, during which cells divide rapidly at the root of the hair, pushing the hair shaft up and out. The length of this phase is measured in years, typically from 2 to 7 years. The catagen phase lasts only 2 to 3 weeks, and marks a transition from the hair follicle’s active growth. Finally, during the telogen phase, the hair follicle is at rest and no new growth occurs.
2. Structure of the hair
Hair follicles are micro organs-formed by the interaction between epidermis and dermis (Ji et al., 2021). Thus, these structures are composed of cells that arise from two different embryonic tissue sources, ectoderm and mesoderm (Ji et al., 2021). Epidermal stem cells and neural crest stem cells are derived from ectoderm (Yang et al., 2019; Soto et al., 2021), while mesenchymal stem cells arise from mesoderm (Schaefer et al., 2020). Keratinocytes can further differentiate into IRS cells, ORS cells, and hair cells (Morgan et al., 2020). Mesenchymal-derived cell lines include fibroblasts and connective tissue sheath cells (Wang et al., 2018; Hernaez-Estrada et al., 2022). Neural crest cells can form melanocytes, which are the source of hair pigment (Gacem et al., 2020).
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Whether it is straight or curly will depend on the cross-sectional shape of hair. The more oval-shaped the cross-section is, the curlier the hair will be. How much of each hair type you have varies from person to person and also depends on your age and sex.
Nail Matrix
In cases of severe nail matrix damage, some or all of your nail may not grow back. A variety of conditions and injuries can damage your nail matrix. ACell and PRP therapy is an exciting new scientific development to treat temporary or hereditary hair loss in men, women, and children. This is because PRP can cause short-term, limited growth, but will not stop the hair thinning process.
DPCs are essential for the activation of stem cells and the initiation of new hair cycles. After activation, HFSCs proliferate and the number of HFSCs reaches a critical value, the next anagen stage of hair follicles begins (Kim et al., 2022). In the hair cycle of mice, the first telogen is very short, lasting only 1–2 days; the second telogen lasts more than 2 weeks, starting about the 42nd day after birth (Hwang et al., 2021) (Figure 3C). As the first barrier against external environmental damage, the skin is composed of three layers. The second layer underlying the epidermis is the dermis, which contains the skin appendages, including hair follicles, sebaceous glands (SGs), eccrine glands, and apocrine sweat glands.
I very much thank Seda Bilir Aksu for the excellent work of the figures in this chapter. Hair follicles are made up of many different components, but these are the four key structures.
Lanugo, vellus and terminal hairs follow the same basic architectural principles. The first “coat” that is formed is fine, long, variably pigmented lanugo hair, which is shed in an anterior to posterior wave during last trimester of gestation. A second coat of fine, shorter, unpigmented lanugo hair then grows in all areas except the scalp and is shed 3–4 months after birth. After these first two cycles, hair starts to grow in an asynchronous “mosaic” pattern rather than in waves [2]. Due to the presence of the dermal papilla and hair matrix, the bulb of the hair is termed as the active site for hair growth. Above the dermal papilla is the hair matrix which consists of proliferating keratinocytes.
The hair root is enveloped by the hair follicle which is a skin appendage that lies deep in the dermis of the skin. Anagen hairs also vary in size, from long, thick terminal hairs to short, light-colored vellus hairs. Increased hormones during puberty turn vellus hair (almost colorless) into terminal hair (darker and larger).
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