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Comparative Anatomy, Physiology, and Biochemistry of Mammalian Skin
Published in David W. Hobson, 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.
The Rex (Re), Wavy Coat (Rewc), and Denuded (Reden) Mutations, Chromosome 11
Published in John P. Sundberg, Handbook of Mouse Mutations with Skin and Hair Abnormalities, 2020
In all three alleles of the rex (Re) mutation, all hair types are present; however, the diameter of the hair shaft is irregular, and there are bends and twists in the hairs. Medullary structures within the hair shaft break down. Pigment remains aggregated around cortical ridges or is scattered within the medulla. These observations were confirmed in hairs plucked from the mice shown in Figure 2. The primary change from the control was a general distortion of the hair shaft. This distortion manifested from various shaft diameters with twisting of the hair (Figures 3 and 4). Both of these changes were subtle and appeared not to effect the integrity of the hair. Cuticle scales did appear slightly longer than in the normal. The defect has been reported to be due to abnormalities in all three layers of the inner root sheath that is irregular in shape and unable to support the hair shaft and control its diameter.9
Biology of the hair follicle
Published in Pierre Bouhanna, Eric Bouhanna, The Alopecias, 2015
Ulrike Blume-Peytavi, Varvara Kanti, Annika Vogt
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.
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).
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].
Recent advances in follicular drug delivery of nanoparticles
Published in Expert Opinion on Drug Delivery, 2020
Alexa Patzelt, Juergen Lademann
In contrast to the intercellular penetration pathway, which has meanwhile been well explored, investigations on the follicular penetration pathway and the follicular penetration mechanism are still underrepresented. The pilosebaceous unit itself is a complex and dynamic three-dimensional structure [39] combing the hair follicle, the hair shaft, the musculus arrector pili and the associated sebaceous gland [40]. The hair follicle, as schematically presented in Figure 2, can be divided into five regions: the infundibulum (reaching from the skin surface to the duct of the sebaceous gland), the isthmus (reaching from the sebaceous gland to the bulge region), the bulge region (which is host of stem cells and insertion point of the musculus arrector pili), the suprabulbar region and the hair bulb which is located around the papilla and contains hair matrix cells and germinative cells. The papilla is the only part of the hair follicle supplied with blood vessels [41]. The infundibulum, the isthmus and the bulge region belong to the permanent portion of the hair follicle. The transient part of the hair follicle, which is the region between the bulge and the hair bulb, undergoes a growth cycle which is regulated by different signal transduction pathways, growth factors and cytokines [42] and comprises three major phases. In the anagen phase, the cells proliferate, form the inner root sheath and migrate upwards to form the hair shaft [40]. In the following catagen phase, cell death of the lower follicle segment occurs. The final telogen phase can be considered as the resting phase before the hair is shed [43]. The anagen hair bulb shows a reduced expression of MHC-I molecules and a reduced number of T cells and Langerhans cells [44] and has been characterized as an area of relative and cycle-dependent immune privilege [45]. The anagen status of a hair follicle has been found to be a prerequisite for efficient follicular penetration [46].