Biology of the hair follicle
Pierre Bouhanna, Eric Bouhanna in The Alopecias, 2015
The hair bulb is defined by the position of the dermal papilla and contains specialized mesenchymal cells with important inductive properties and a capillary loop to provide nutrition. The papilla is surrounded by undifferentiated, actively proliferating hair matrix cells, which give rise to the hair shaft and the inner root sheath. The fibrous sheath and the epithelial outer and inner root sheaths form concentric layers, which ensheathe the hair shaft. The outer root sheath extends from the matrix cells in the hair bulb up to the entry level of the sebaceous duct. Outer root sheath cells contain clear vacuolated cytoplasm filled with large amounts of glycogen. Below the isthmus, the outer root sheath is not keratinized. However, at the level of the isthmus, where the inner root sheath disintegrates, the outer root sheath keratinizes without forming granules. Outer root sheath cells express a large diversity of mediators, hormones, and receptors. The inner root sheath consists of three layers, the Henle, Huxley, and cuticle, all of which keratinize and provide the form to the hair shaft. The mesenchymal sheath is separated from the epithelial root sheaths by a vitreous or basal membrane. This whole complex is surrounded by a dense vascular network. Free nerve endings form a cuff and provide the basis for intensive piloneural interactions.
Comparative Anatomy, Physiology, and Biochemistry of Mammalian Skin
David W. Hobson in Dermal and Ocular Toxicology, 2020
The hair follicle is composed of three primary layers: inner root sheath, outer root sheath, and connective tissue sheath. The first is composed of scale-like keratinized cells which interlock with cuticle cells of the hair. The second is continuous with the epidermis. Structurally, it resembles epidermis but also has glycogen in it. The third layer, the connective tissue sheath layer, is continuous with the papillary layer of the dermis and with the dermal papilla of the hair follicle. At the base of the hair follicle, termed the bulb, the dermal papilla and hair matrix are located. The matrix is composed of the germinative epithelial cells which give rise to the hair proper, and is the region responsible for biochemical regulation of hair growth. They are also the target cells for toxicants directly affecting hair growth.
Transglutaminases
Elling Kvamme in Glutamine and Glutamate in Mammals, 1988
TG and its substrate are present in the granular layer of the epidermis and contribute to the cornified envelope. The inner root sheath and medulla of the hair keratinize in a manner similar to epidermis and contain a hair follicle TG.7 Fetal rat epidermis and hair follicle inner root sheath undergo terminal keratinization late in gestation and the TGs in both structures show a rise in activity concomitant with the onset of keratinization.75 TGs are of importance for normal cell differentiation. Plasma glucocorticoid levels and epidermal TG activity rise around gestational day 14 in the chick embryo and the rise is followed by developmental changes, such as synthesis of epidermal structural proteins, insolubilization of proteins, and envelope formation.76 Thus, glucocorticoid appears to induce TG activity. Involucrin, a cytosolic protein (92 kdaltons), and at least six membrane proteins are cross-linked by a membrane-associated TG during terminal differentiation of cultured keratino-cytes, producing the cornified envelope.77,78 Involucrin appears only in significant quantities in cell culture, whereas keratolinine, another soluble protein (36 kdaltons) acts as a substrate for human and bovine epidermic TG and is a major constituent of the cornified envelope in the native tissue.79
Are hair follicle stem cells promising candidates for wound healing?
Published in Expert Opinion on Biological Therapy, 2019
Bingmin Li, Wenzhi Hu, Kui Ma, Cuiping Zhang, Xiaobing Fu
If HFSCs are intended for wound healing, they have to possess a powerful capability of regeneration. Abundant researches have described the multiple differentiation potential of HF cells. During homeostasis, bulge SCs maintain the outer root sheath (ORS), the inner root sheath (IRS), the hair matrix and the secondary hair germ, contributing to the regenerative cycling of the hair follicle and fueling hair growth [51]. The morphogenesis of hair follicles proceeds through the cycle of proliferation (anagen), destruction (catagen), and stasis (telogen) (Figure 1) [52,53]. In this process, the upper portion of the hair follicle (HF) is retained while the lower portion undergoes repeating degeneration and regeneration. At the onset of anagen, stem cell progenies in the secondary hair germ (sHG) are initially activated and launch the process of the telogen-anagen transition. Subsequently, bulge HFSCs begin to proliferate and give rise to the ORS, and then, they proliferate and migrate downward. In addition, the sHG, which is composed of abundant transient amplifying cells (TACs), develops into the matrix. TACs differentiate into the hair shaft and the IRS, fueling hair growth [54]. With the consumption of TACs, bulge HFSCs gradually return to quiescence [55]. Once catagen is initiated, the lower portion of the ORS and matrix cells die through apoptosis, while the cells of the upper and middle portions of the ORS migrate upward and form a new bulge [56,57].
Low-frequency electromagnetic fields promote hair follicles regeneration by injection a mixture of epidermal stem cells and dermal papilla cells
Published in Electromagnetic Biology and Medicine, 2020
Xinping Li, Yan Ye, Xiaohan Liu, Liming Bai, Pin Zhao, Wenfang Bai, Mingsheng Zhang
None of the mice had visible hair generation when DP cells were injected alone (Nilforoushzadeh et al. 2017), and the mixture of ESCs and DP cells induced new hair follicle formation was investigated in this experiment (Figure 1). After the injection of the mixtures into nude mice for 14 days, the hairs were seen emerging from the dorsal skin (Figure 1B). Comparing to the control group, the hairs erupted at a higher density in the EMF group. Then we next analyzed the feature of the new hair follicle. Results of H&E staining showed that the new hair formed the correct structure comprising hair matrix, hair shaft, and inner root sheath, outer root sheath, and DP (Figure 1C). Comparing with the characteristics of new hair bulbs in the control group, EMF exposure induced a higher density of hair bulbs formation (Figure 2).
Long-term effects of total skin electron beam therapy for mycosis fungoides on hair and nail loss and regrowth
Published in Journal of Dermatological Treatment, 2022
Debra L. Breneman, Alyssa Breneman, Elaine Ballman, John C. Breneman
In some patients, the new hair was finer and curlier than before treatment. Similar changes in hair texture have been reported in association with multiple modalities of cancer treatment (11). A change from straight to curly hair has also been observed as a paraneoplastic syndrome, during treatment with isotretinoin, during treatment with Lithium, and in patients with human immunodeficiency virus on highly active antiretroviral treatment (12–15). The mechanism behind these changes is not fully understood. It has been hypothesized that, in the case of retinoid use, treatment may affect keratinization of the inner root sheath of the hair shaft, causing structural changes which result in kinked hair (16). Pigmentary changes of the hair were also noted among our study’s participants. Hair regrowth was darker in some patients and returned to normal color after a year of growth. It has been suggested that melanocytes reside in the hair bulb and may be activated by TSEBT, causing darkening of the hair (17).
Related Knowledge Centers
- Outer Root Sheath
- Henle'S Layer
- Huxley'S Layer
- Cuticle
- Epithelium
- Cell Nucleus
- Keratin
- Protoplasm