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Hair Follicle Companion Layer: Reacquainting an Old Friend

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Hair Follicle Companion Layer: Reacquainting an Old Friend 42S ROTHNAGEL AND ROOP THE JOURNAL OF INVESTIGATIVE DERMATOLOGY Hair Follicle Companion Layer: Reacquainting an Old Friend Joseph A. Rothnagel and Dennis R. Roop Departments of Cell Biology and Dermatology, Baylor College of Medicine, Houston, Texas, U.S.A. he anagen hair follicle consists of a grouping of companion layer are in intimate contact with cells of the Henle specialized fibroblasts (termed the dennal papilla) layer, through numerous cell membrane interdigitations, desmo­ surrounded by a germinative epithelium (tenned the somes, and gap junctions [3-5]. As the Henle cell matures, T hair matrix) that produces the cells that differentiate differentiates, and migrates away from the follicle bulb, the neigh­ to form the concentric layers characteristic of this boring companion cell maintains these tight associations and ac­ structure (reviewed in [1]). The emerging hair fiber consists of a companies the Henle cell on its upward journey. In addition to medulla (when present) and a cortex protected by a cuticle. The forming numerous contacts with the cells of the Henle layer, the developing fiber is surrounded by the inner root sheath, which companion layer is also in direct communication with the Huxley consists of its own cuticle layer and the Huxley and Henle layers, layer through outgrowths from these cells (termed Fliigelzellen) and an outer root sheath (ORS) that encases the entire follicle and that reach around the intervening Henle cells to contact the cells of is contiguous with the interfollicular epidennis. the companion layer. At the end of terminal differentiation both the A number of studies have shown that the ORS does not consist cells of the inner root sheath and the companion layer cornifY, of a homogeneous cell population. Labeling studies have shown degrade, and slough into the follicle lumen [3-5]. These observa­ that cells of the ORS differ in their mitotic and migrating activity tions led Orwin to speculate that the companion cell may play a depending on their position along the follicle [2]. Furthennore, nutritive role in the maintenance of inner root sheath cells and ultrastructural studies have determined at least two distinct cell perhaps also influence their differentiation and eventual destruction types in the ORS [3-5]. The ORS cells that form above the bulb in the pilary canal [4]. Rogers suggested that because there are few tend to be more cuboidal and usually contain large quantities of cellular connections between companion cells and the cells of the glycogen deposits in their cytoplasm together with numerous ORS that this border provides the slippage plane to allow the mitochondria. In contrast, those cells that arise from progenitor growing fiber to move past the surrounding tissue [3]. cells adjacent to the dennal papilla fonn a single cell layer of As part of a study investigating the regulation of the K6 gene we elongated cells that are closely associated with cells of the periph­ produced transgenic mice expressing mouse K6 in which the C­ eral Henle layer of the inner root sheath. These distinctive flattened terminal domain (E2) was replaced with a human Kl epitope tag to cells contain fewer mitochondria than the cuboidal ORS cells and allow us to follow its expression by double-label immunofluorescence. lack glycogen deposits. Orwin coined the tenn "companion layer" Transgenic animals were completely nonnal at birth and indistinguish­ to distinguish this band of elongated cells from other cell types in able from their non-transgenic littennates in tenns of appearance and the ORS [4]. Later Ito referred to this layer as the innennost cell layer gross morphology. Their epidermis and hair follicles were uuremark­ of the ORS [5]. This layer has been variously referred to in the able for the first few months of life. However, at about 6 months some literature as being either a component of the inner root sheath or part of these transgenic animals showed signs of alopecia. The alopecia of the ORS. Using immunohistochemical analysis we have found that always developed on the dorsal side and first occurred at the nape of the companion layer is composed of cells that are unique, not only in the neck and steadily progressed towards the tail. The denuded areas their morphology but also in their biochemistry. Companion cells can became highly keratotic and were prone to infection. These areas often be distinguished from inner root sheath cells because they lack filled with exudate, never properly healed, and could progress to trichohyalin granules and do not stain with antibodies directed against involve the entire skin surface of the mouse, requiring the animal to be trichohyalin. Moreover, companion cells react with antibodies against sacrificed. Histologic analysis confirmed areas of infection and a keratin 6 (K6) in contrast to both inner root sheath cells and lymphocytic infiltrate. Ultrastructural analysis suggested that hair loss non-induced ORS cells, which do not. On the basis of these obser­ was due to the destruction of d1e ORS with the keratosis, subsequent vations we suggest that the companion layer should be considered as infiltrate and infection occurring as secondary events. a histologic distinct component of the hair follicle. These prelin1inary observations suggest that disturbances in the Another ultrastructural feature of companion cells is the distinc­ K6/K16 filament system of companion cells can result in the tive clustering of keratinfilaments located along the cell boundary that destruction of this layer and ultimately to the loss of the hair follicle. apposes the inner root sheath. These filaments have a circumferential Not only do these studies underscore the importance of the orientation, perpendicular to the orientation of intennediate filaments companion layer to the biology of the follicle but they also suggest within the Henle layer and the growth axis of the hair fiber. It has been that genetic defects or epigenetic events that result in an imbalance suggested that these filaments may act like the "hoops of a barrel" to in the ratio of K6 and K16 subunits could be the basis of certain support and stabilize the growing hair follicle [5]. We believe that fonns of cicatricial alopecias. these filaments are composed ofK6 and K16 subunits but this remains to be verified by immunoelectron microscopy. The biologic function of the companion layer has yet to be elucidated. However, it has been noted that the cells of the We thal1kJalleile Laminackfor the preparatiol1 of this J11a1l1/sC/ipt. rfTe thank MOlY AliI! Lallgley, Jackie Bickenbach, alld Toshihiko Seki for access to their ullpublished data. This 1V0rk lVas s"ppO/ted b}, NIH research grallt HD25479.JAR is a recipient Reprint requests to: Dr. Joseph A. Rothnagel, Baylor College of of a dermatology career developmwt award spoll5ored by 0,1/10 Pharmaceuticals. Medicine, One Baylor Plaza, Houston, TX 77030. 0022-202X/95/$09.50 • SSDI0022-202X(95)00133-6 • Copyright © 1995 by The Society for Investigative Dermatology, Inc. VOL. 104, NO. 5, SUPPLEMENT, MAY 1995 THE COMPANION LAYER 43S REFERENCES W, Lobitz WC (eds.). Tlte Epidermis. Academic Press, New York. 1964, pp 179-236 4. Orwin DFG: Cell dilferentiation in the lower outer sheath of the romney wool 8:55-61,1992 1. Hardy MH: The secret life of the hair follicle. Trends Genet follicle: A companion cell layer. AusI] Bioi Sci 24:989-999,1971 2. Chapman RE: Cell migration in wool follicles of sheep.] Cell Sci 9:791-803, 1971 5. Ito M: The innermost cell layer of the outer root sheath in anagen hair follicle: 3. Rogers GE: Structural and biochemical features of the hair follicle. In: Montagna light and electron microscopic study. Arch Dermatol Res 279:112-119, 1986 Hair Follicle Development and Hair Growth from Defined Cell Populations Grafted onto Nude Mice Ulrike Lichti, Aline B. Scandurro, Tonja Kartasova, Jeffrey S. Rubin,* William LaRochelle,* and Stuart H. Yuspa Laboratory of Cellular Carcinogenesis and Tumor Promotion and *Laboratory of Cellular and Molecular Biology, National Cancer Institute, Bethesda, Maryland, U.S.A. air growth depends on prior development of a DPCs or a need for supportive contributions by non-DPCs in the functioning hair follicle (HF). Hence knowledge FDC preparation. Thus this minimal component graft cell mixture, of the signals that control HF development is consisting of immature HF buds and immortalized DPC lines, essential to understanding defects in hair forma­ provides an assay for positive or negative influences on HF Htion. We are using a nude mouse graft procedure development exerted by added selected cell types, growth factors, [1,2] to study factors that influence HF development from defined or other substances. cell populations in a wound-healing environment. In attempts to improve HF development in grafts of HF buds In this grafting procedure, the test cell mixture is transferred as a combined with positive immortalized DPC lines, we included the thick slurry onto the connective tissue exposed by removal of a following cell lines in the cell mixture: keratinocyte growth factor piece of skin of about 1 em in diameter on the back of the nude (KGF)-overproducing NIH 3T3 cells, KGF-overproducing Swiss mouse. Contact with host skin is minimized during the first week 3T3 cells, hepatocyte growth factor (HGF)-overproducing NIH post-grafting by a silicone chamber. Hair growth is evaluated at 5 3T3 cells, NIH 3T3 cells producing an HGF antagonist peptide (for weeks after grafting. Hair density in the graft site approaching that a review, see [3]), and a clonally derived cell line, LF24, derived of mouse pelage results from transplanting a mixture of immature from Balb 3 T3-A31 cells (see Kartasova, et aI, this issue, p 21). Both HF buds, isolated from the epidermal fraction of trypsin-treated KGF-overproducing cell lines increased the number ofHFs devel­ skin of 1-2-day-old mice, and a fresh dermal cell (FDC) prepara­ oping in the graft site by about 50%.
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