Cells within the bulge region of mouse hair follicle transiently proliferate during early anagen: heterogeneity and functional differences of various hair cycles

Differentiation (1994) 55: 127-136 Differentiation Ontogen!, \eopla\ia and Differentiation lherap)

( Springer-Verlag 1994

Cells within the bulge region of mouse hair follicle transiently proliferate during early anagen : heterogeneity and functional differences of various hair cycles Caroline Wilson I, George Cotsarelis I , Zhi-Gang Wei I , Eric Fryer I, Jennifer Margolis-Fryer I , Matt Ostead , Robert Tokarek I , Tung-Tien Sun2, Robert M. Lavker Department of Dermatology, University of Pennsylvania School of Medicine, Philadelphia, PA 19104, USA Epithelial Biology Unit, The Ronald 0. Perelman Department of Dermatology and Department or Pharmacology, Kaplan Comprehensive Cancer Center, New York University School of Medicine, New York, NY 10016. USA

Accepted in revised form July 12, 1993

Abstract. Based on cell kinetic, morphological and several biological considerations, we have recently proposed that hair follicle stem cells reside in the bulge area of the upper follicle. We predicted that during early anagen the normally slow-cycling bulge stem cells may be activated by the abutting dermal papilla cells to undergo transient proliferation giving rise to keratinocytes of the lower follicle. In the present work, we performed tritiated thymidine-labeling of DNA-synthesizing cells and col- cemid-arrest of mitotic figures on the skins of 20-23 and 75-80 day old SENCAR mice, when the follicles entered the anagen phase of the 2nd and 3rd hair cycles. The results clearly indicate that the normally slow-cycling bulge cells indeed undergo transient proliferation during early anagen. Similar results were obtained when the telogen follicles are experimentally induced to enter the 3rd hair cycle by plucking and by topical applications of phorbol ester or tretinoin. These results support the no- tion that bulge cells are follicular stem cells, and that transient proliferation of these cells is a critical feature of early anagen. However, the long duration of the 2nd telogen (> 30 days in mouse) suggests that a new anagen phase does not automatically result from the physical proximity of dermal papilla to the bulge cells, and that another factor is required for the initiation of the 3rd anagen. The tremendous difference in the durations of the first and second telogen (lasting for 2-3 days and > S O days, respectively) suggests that follicles can exist in a non-cycling state that may be conceptually equivalent to the G,, state of the cell cycle. Our results also under- score the fact that the first hair cycle is distinct from all the subsequent hair cycles in their cellular origin and morphological sequence, and thus should be regarded as a neogenic event.

Correspondence lo: R.M. Lavker, Department of Dermatology, University of Pennsylvania School of Medicine, Clinical Research Building, Rm. 235A, 422 Curie Boulevard, Philadelphia, PA 19104, USA

Introduction

The hair follicle is an epidermal derivative which under- goes cycles of growth, involution and rest [4, 12, 13, 321. During the growing phase (anagen), the matrix keratinocytes of the bulb proliferate rapidly, differentiating into inner root sheath as well as the cuticle, cortex and medulla of the hair shaft [3, 8, 17, 311. At the end of anagen, following a time period that is species- and body site-dependent, matrix keratinocytes abruptly cease proliferating and undergo terminal differentiation so that the lower follicle involutes and regresses (catagen; [7, 12, 13, 261). In catagen, the specialized mesenchymal cells of the dermal (follicular) papilla round up, and they remain contained in the connective tissue sheath, which contracts thus pulling the condensed dermal papilla towards the bottom of the epithelial portion of the regressing follicle [12, 131. At the end of catagen, the dermal papilla comes to rest at the bottom of the permanent portion of the hair follicle, and remains there while the follicle is in the resting (telogen) phase. Eventually a new growing phase occurs, and the cycle is repeated.

We have recently proposed a bulge-activation hypothesis which helps to explain some of the paradoxical aspects of the hair cycle [S, 14, 341, particularly the cellular origin of the new growth initiated by the follicular papilla. This hypothesis was based on our finding in the mouse hair follicle of a subpopulation of outer root sheath cells, located in the bulge region at the insertion site of the arrector pili muscle, that have properties consistent with their being the epithelial stem cells. These cells are slow-cycling and thus are experimentally identifiable as the label-retaining cells (LCRs): they have a relatively undifferentiated cytoplasm ; and they are responsive to growth stimulation by phorbol ester [5].

A bulge location of follicular stem cells is a significant deviation from the widely held earlier belief that follicular stem cells are located in the matrix [12. 13. 14, 351. This earlier view was based on (1) the rapid proliferative rates of matrix keratinocytes during anagen, (2) the known interactions between the dermal papilla and ma-

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trix keratinocytes in regulating hair growth [lo, 22, 29, 301, and (3) the multipotency of the matrix cells which can give rise to several terminal differentiation pathways (hair medulla, cortex and inner root sheath). However, this view that stem cells reside in the lower-most portion of the anagen follicle poses several major conceptual and biological difficulties : A. If matrix keratinocytes were the follicular stem cells, these cells would have to be preserved in catagen and migrate upwards; this is contrary to the current concept that stem cells reside in a relatively fixed micro-environ- ment or niche within a tissue [18, 271. B. The transection experiments by Oliver and co-workers [23-251 showed that rat vibrissa could regenerate even when the lower follicle (below the bulge equivalent region) was surgically removed, as long as a new dermal papilla was supplied. This clearly demonstrated that keratinocytes of the lower follicle were dispensable. Simi- larly, surgical removal of the lower portion of human axillary hair follicles did not prevent hair follicle regeneration [9]. C. Since stem cells are known to be slow-cycling [18, 271, rapidly proliferating matrix cells with a growth frac- tion of approaching 1.0 [35], by definition, cannot be the stem cells. Consistent with this, slow-cycling cells were found exclusively in the bulge and none were found in anagen bulbs [5]. D. Multipotency is not a property ascribed exclusively to stem cells. In a hematopoetic system, for example, some progenitor cells with limited life span are multipo- tent [28].

One of the major tenants of the bulge-activation hypothesis is that during early anagen, the normally slow- cycling bulge cells become activated . They then undergo transient proliferation, giving rise to a population of transient-amplifying cells which later form the matrix portion of the new (lower) follicle. To test this aspect of the hypothesis, we further investigated the hair cycles of the SENCAR mouse. We observed that the first anagen was equivalent to follicular neogenesis, followed by regular hair cycles. We report here that at the onset of the second and third (regular) anagen, bulge cells indeed replicate spontaneously. We also report that bulge cells can be stimulated to proliferate in response to both physical and chemical stimuli causing telogen follicles to commence anagen. Finally, we present evi- dence that while the interactions between dermal papilla and bulge cells appear to play a central role in bulge activation, it is not sufficient to initiate the 3rd hair cycle suggesting that another factor may be needed for initiating this and possibly subsequent hair cycles.

Methods

Determination of the stage of the hair cycle by dyeing. SENCAR (albino) mice were used in all experiments, ranging in age from birth to 84 days. The hair cycle was followed using the shaving and hair-dyeing technique described by Borum [2]. Briefly, the dorsum was. stained with a black hair dye (Clairol) diluted 2 : l with 4% hydrogen peroxide, and the animals were examined daily. The onset of anagen was heralded by a change in skin color from

pink to white, followed a day later by the emergence of white stubs at the base of black hairs. When the white portion of the hair reached a stable length, this indicated that hair growth had ceased and mice were in telogen. The phase of hair cycle was con- firmed by examining the histology of skin specimens from the interscapular area of the back, taken from the mice at daily intervals during the first week of life, and then at 3-4 day intervals over the next 3 months. In all instances, the histological examination was consistent with the clinical observation.

Determination of proliferative activity. In order to examine the proliferative activity of follicular cells, mitotic activity in the skin was examined. Mice were treated with intra-peritoneal colchicine (10 mg in 0.1 ml phosphate-buffered-saline (PBS) per animal; Sig- ma Chemical), and skin samples were taken 8 h later (this time interval had previously been found to yield the maximum number of mitotic figures) [36] for histological examination. An additional group of mice was used to examine DNA synthesis; one hour prior to sampling, mice received an intradermal injection of 10 pCi tritiated-thymidine (3H-TdR; specific activity 82.7 Ci/mmol; New England Nuclear, Boston, Mass., USA) in 0.1 ml of sterile PBS to the inter-scapular portion of the back. Skin biopsies were taken from the injected area, immediately fixed in formalin, and pro- cessed for histological, histogeometric and autoradiographic analy- ses.

Histology and autoradiography. After fixation in formalin, tissues were dehydrated through a graded series of ethyl alcohol and infil- trated overnight in a monomer solution of JB-4 water-soluble plas- tic media (Polysciences, Fort Washington, Pa., USA). The tissues were embedded in JB-4 embedding medium, and 3 pm sections were cut with a glass knife on a Reichert-Jung 2050 Supercut micro- tome (Cambridge Instruments, Buffalo, N.Y., USA). Autoradiog- raphy was performed as described earlier [6].

Histometric analysis. For determination of hair follicle length, sections were cut perpendicular to the skin surface. Histometric measurements of follicle length were made from follicle base along the follicle to the skin surface using a computer-assisted image analysis system (Southern Micro Systems, Chattanooga, Tenn., USA). Measurements were made from at least four specimens, with each section separated by 50 pm.

Physical and chemical stimulation of resting hair follicles. Telogen hairs were plucked from the interscapular skin, the mice received

Fig. 1. Histology of follicular neogenesis in SENCAR mouse. Por- tions of skin from Newborn (a), 2 day (b), 4 day (c), 9 day (d), 14 day (e), 17 day (0, 18 day (9) and 20 day (h) old SENCAR mice depicting various stages of follicular neogenesis. a Newborn mice possess only small bud-like structures (arrows) which by day 2 (b) evolve into thin columns of cells analogous to primitive follicles (F). These follicles progressively extend deeper into the dermis reaching maturity by day 14 (e) of post-natal life. The follicular bulb consisting of matrix keratinocytes (M) surrounded by mesenchymal cells (arrow) is recognizable by day 4 (c), whereas the bulge region (*) is recognizable between 7-9 days (d). The lower two- thirds of the hair follicle rapidly degenerates during day 17-18 (catagen; f, g) and by day 20 (h) hair follicles are in resting phase (telogen). i Graphic representation of the sequences of follicular neogenesis and the second and third hair cycles in SENCAR mice. Measurements of hair length encompassed the distance from the base of the hair follicle (bulb) to the epidermal surface. The first 1 6 1 7 days of postnatal life represent follicular neogenesis. After a short catagen/telogen period, the second anagen begins and lasts for 7-9 days. The second telogen phase begins around days 3&35 and lasts for at least 40 days. The third hair cycle is less synchronized, beginning around days 70-80

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Fig. 2. Transient activation of the bulge cells. Portions of hair follicles in telogen (a), early (b, c), and full-anagen (d) showing either cells in 'S' phase of DNA synthesis as depicted by tritiated- thymidine ('H-TdR) incorporation (a, b, d), or after colchicine- arrest to detect mitotic activity (c). Numerous mitotic figures (arrows; c), and 'H-TdR-labeled nuclei (arrows; b) are observed in the bulge region during early anagen. Note relationship of arrector pili muscle (up) to bulge region. Once follicles become further differentiated, labeled cells are no longer observed in the bulge but concentrate in the bulb region (arrows; d)

an intradermal injection of 'H-TdR 1, 2, 3, 4, 5, 7 and 10 days later, and the tissues were harvested and prepared for histology and autoradiography as described above.

Other groups of mice in telogen received either topical application of 0.01 % 0-tetradecanoylphorbol-13-acetate (TPA) in petro- latum or 0.05% tretinoin (Retin-A Cream; Ortho Pharmaceuticals,

istered in the treated area and tissues were prepared for histology and autoradiography.

Results Raritan, N.J., USA). Applications were made once daily to the interscapular &in of the back for days and then twice week[y for a total of 35 days. For the first seven days of treatment and

Although the hair cycle in mice has been described earlier [31, little attention was directed towards the bulge

weekly thereafter. an intradermal injection of 3H-TdR was admin- and it was unclear at what stage the bulge structure

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is formed. The skin of newborn mice contains only rudi- mentary follicles; thus the first wave of hair growth real- ly represents follicular neogenesis (Fig. 1). While epithelial buds forming thin columns of cells can be seen ex- tending deeply into the dermis by 2-3 days of post-natal life, and the follicular bulb consisting of matrix keratinocytes surrounding mesenchymal cells is easily recognized by 4 days of post-natal life, a discrete bulge region be- came morphologically recognizable, just below the seba- ceous gland, between 7-9 days of age. At this time many hair follicles are fully differentiated.

The first and second hair cycles are highly synchronized, starting in a wave-like fashion on the ventral side and progressing from the head toward the tail, over a period of about 24 h (Fig. 1 i). Follicular neogenesis in the first 16.days of post-natal life is succeeded by an extremely short catagen/telogen period lasting only 3-4 days. The second anagen phase, lasting about 9 days,

Fig. 3. Plucking induces transient activation of the bulge cells. Transient activation of normally quiescent bulge cells 24 h after plucking telogen follicles from mice 45 days of age (a) is indicated by the incorporation of 'H- TdR (arrows). Seven days after plucking (b) 'H-TdR labeling is restricted to the matrix keratinocytes (arrows); cells of the bulge region ( B ) no longer incorporate 'H-TdR at this time. Graphic representation of the effects of plucking (c) on resting follicles indicates the time course of the pluck-stimulated anagen is similar to the 2nd growth cycle

is slightly shorter than the first. Follicles enter the second telogen phase around day 30 and then remain in this quiescent phase for at least 40 days. While the length of the hair follicles in the 3rd anagen phase were similar to those of the first two cycles, hair growth often ap- peared in patches or islands, and distributed somewhat randomly over the body. This breakdown in the wave pattern of hair growth made accurate determination of the length of the 3rd anagen phase difficult.

Transient activation of the bulge region

To study the proliferative status of the bulge during the hair cycle, we used tritiated thymidine-labeling to detect cells in the 'S' phase of the cell cycle, and used colchicine-arrest techniques to observe mitotic activity. Both methods yielded similar findings. When follicles were

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Fig. 4. Tretinoin induces transient activation of the bulge cells. After 3 days of daily topical application of tretinoin during the telogen phase (45 day old mice), proliferation of bulge cells is detected by 3H-TdR incorporation (arrows; a). At this time endotheli- al cells (orrowheads) are also observed to be proliferating (a). Con-

tinued treatment results in the formation of a mature hair by 14 days (b) with proliferation confined to the matrix keratinocytes (M). After 28 days of treatment (c) most hair follicles entered telogen. This sequence is graphically depicted in panel (d)

in telogen, 3H-TdR labeled nuclei and arrested mitoses were seen scattered in the epidermal basal layer and se- baceous gland but were rarely seen in the lower telogen follicles (Fig. 2a). However, at the onset of the second and third anagen phase, numerous 3H-TdR-labeled nuclei as we14 as mitotic figures were observed in the bulge region (Fig. 2b, c). Many of the 3H-TdR-labeled cells

have irregular nuclei (as shown in Fig. 2b, c), thus, clearly demonstrating their bulge nature. Once follicles had developed to the anagen IV stage, labeled nuclei or mitotic figures were no longer observed in the bulge (Fig. 2d). These results established that, as we have predicted earlier [ 5 ] , bulge cells indeed undergo transient proliferation during early anagen.

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Fig. 5. Phorbol ester induces transient activation of the bulge cells. After 3 days of daily topical application of 0-tetradecanoylphorbol- 13-acetate (TPA) during the telogen phase (45 day old mice), proliferation of bulge cells is detected by 3H-TdR incorporation (arrows; a). Continued treatment results in the formation of a mature hair

by 14 days (b) which is maintained after 28 days of treatment (c). Graphic representation of the effects of TPA (d) indicates that TPA-treated follicles have an abbreviated catagen and immediately re-enter a 4th anagen

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Physical und chemicul uctivation of telogen follicles

The prolonged second telogen of mouse hair cycle of greater than 40 days is not only conceptually important (see below), but also experimentally convenient since it provides a window of time in which one can experimentally manipulate the quiescent bulge cells. We have therefore used physical and chemical stimuli to shorten this telogen and to study the behavior of the bulge cells during the artificially induced anagen.

Plucking

I t has been well-established that the removal of the club hair from a telogen follicle can induce new hair formation [32]. However, little was known about the proliferative sequence of the bulge region of these plucked hairs. Within twenty-four hours after plucking, 3H-TdR-incor- porating cells were detected in the bulge area (Fig. 3a), as well as in some epidermal basal cells. By forty-eight hours, epidermal labeling returned to the normal (pre- plucking) level, whereas thymidine-incorporation was still extensive in the bulge and upper infundibular regions. Seven to ten days after plucking, new hair fibers emerge from the skin surface, and a morphologically normal anagen follicle is formed. At this stage, all the 3H-TdR-labeled cells were restricted to the matrix region of the bulb (Fig. 3b), and cells of the bulge region had returned to their dormant state (Fig. 3b). The time course of the plucking-stimulated anagen is similar to the 2nd growth cycle, and an anagen follicle of normal- length is formed (Fig. 3c). Contrary to chemical stimulation (see below), plucking did not produce histological signs of dermal inflammation (e.g., accumulation of neu- trophils, lymphocytes and edema).

Tretinoin and TPA

Although it is known that topical application of tretinoin and TPA can induce epidermal hyperplasia, their effects on hair follicles were unknown. After 3 days of daily topical application, during the second telogen phase (starting on day 35), of either tretinoin (Fig. 4a) or TPA (Fig. 5a), proliferation of bulge cells was detected by 3H-TdR-incorporation. Both agents resulted in increased dermal cellularity consisting mainly of acute inflammatory cells. Occasional 3H-TdR-labeled endo- thelial cells were noted in the superficial venules around the follicles and beneath the epidermis, indicative of en- dothelial proliferation (Fig. 4a, b). Continued tretinoin (Fig. 4b) and TPA (Fig. 5b, c) treatment resulted in the formation of full-length anagen follicles by 14 days. At this stage, proliferation was primarily confined to the matrix kerdtinocytes, and occasional outer root sheath cells were labeled; bulge cells did not incorporate 3H- TdR (Fig. 4b, 5b).

Although the initial time course of tretinoin- and TPA-induced anagen was similar, continued treatment of these two agents produced very different results. The

tretinoin-induced follicles (Fig. 4c, d) entered telogen around the 28th day of treatment. They remained in telogen despite continued tretinoin treatment, even though control animals a t this time (- 89 days of age) commenced the third anagen. Conversely, the TPA- treated follicles entered a brief telogen (lasting 1-2 days), but then immediately re-entered anagen (the fourth; Fig. 5c, d).

Discussion

Are all hair cycles equivalent ?

Previous investigations paid relatively little attention to distinguishing different hair cycles. Consequently one can easily get an impression from the literature that all hair cycles are the same. However, our results on mouse follicles, which develop synchronously, strongly suggest that different hair cycles may have very different functional meaning, and revealed that the regulation of hair cycle is even more tightly controlled than we have previously appreciated. Thus, the first hair cycle is initiated from an ectodermal downgrowth in which the bulge structure is not morphologically identifiable until day 9 when the follicle is completely formed. This means that the initiation of the hair follicle in newborn mice is, from a developmental viewpoint, equivalent to the neogenesis of follicles which is largely completed in utero in man. Unlike all the subsequent hair cycles, which can be inhibited by corticosteroid [19, 331, this first hair cycle is insensitive to steroid inhibition (unpublished ob- servations). This indicates that at least one regulatory step, which is corticosteroid-sensitive and is involved in subsequent hair cycles, is absent in the first hair cycle. Fundamental differences between the first and subsequent murine hair cycles were also apparent in a recent investigation utilizing transgenic mice to study the role of transforming growth factor-beta (TGF-/I) related growth factors [I]. In these transgenic animals, ectopic expression of bone morphogenic protein-4 (a member of the TGF-p superfamily) had no affect on the first hair cycle. However subsequent hair cycles were inhibited, leading to progressive balding. The first hair cycle is therefore clearly distinct from all the subsequent cycles, and provides unique opportunities for studying or- ganogenesis, in particular the inductive interactions between dermal papilla cells and follicular epithelium, as well as the mechanisms underlying the formation of follicular stem cells.

The second hair cycle, which follows immediately after the completion of the first (neogenesis) hair cycle, has several important features. The initiation of this second hair cycle can be blocked when the dermal papilla fails to attach to the lower part of the telogen follicle, as happens in the Hr/Hr mouse mutant [4, 211. This indicates that the proximity of the dermal papilla to the bulge area (which is well-formed by now) is crucially important for anagen-initiation. Another important feature is that, unlike the first cycle, the second cycle has an extremely long telogen lasting for more than 40 days

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(ages between 35-70 days). This long telogen provides a convenient window of time for testing the ability of various pharmaceutical agents to initiate a new hair cycle. Perhaps even more importantly, this long telogen strongly implies that the physical proximity of the dermal papilla to the follicle bulge cells, although clearly necessary, i s insufficient for the initiation of the subsequent hair cycle. The nature of this signal is not yet clear. As mentioned earlier, the third hair cycle becomes much less synchronized than the preceding ones and is therefore more difficult to study.

Huir cycles ure not contiguous

Previous studies on human scalp follicles established that neighboring follicles traverse through their cycles inde- pendently (or asynchronously). Furthermore, most of human scalp hair follicles appear to be in anagen, because only relatively few catagen/telogen follicles can be seen in routine histological sections. Based on the average growth/elongation rate of the hair and the ratio of follicles found in different phases of the cycle, und by assuming that all follicles traverse continuously through the hair cycle, it has been estimated that a typi- cal human scalp hair spends 1 year, 10 days and 1 month in anagen, catagen and telogen, respectively [12, 131. However, the assumption that follicles traverse regularly through hair cycles is unsubstantiated. In fact, our data on mouse follicles clearly indicate that, at least in this species, this is not the case, because after the second cycle all the follicles apparently remain in telogen for a prolonged period of time. Considering the tremendous energy requirement for follicular growth during anagen, the arrest of the hair cycle in a long resting stage, much like the Go state of a cell cycle, makes biological sense. Although our data are derived from studies of murine follicles by taking advantage of their synchrony, our findings raise a cautionary note as to whether the cycle of human follicles may also be discontinuous and/or heterogeneous.

Dgferences between the bulge cells and germ cells

In some earlier studies, Montagna described a population of germ cells located at the very bottom of telogen follicles which, as the name implies, were thought to be the germ of the lower follicle [20]. Can these cells be the stem cells? We do not believe so because these cells, located at the very bottom of telogen follicles represent, by definition, a transient structure. Studies from a number of self-renewing tissues including hematopoeit- ic system [28], corneal epithelium [6, 361, spermatogenic [27] and intestinal epithelium [27] strongly suggest that stem cells constitute a permanent cell population resid- ing in a rather fixed tissue location (a stem cell niche that can nurture and maintain the sternness of the oc- cupant cells). If so, what is the relationship between these germ cells and the bulge cells? Our detailed histological analysis of telogen follicles indicate that in a great major- ity of these follicles, the bulge area, as defined by their

characteristic multi-lobular nuclear morphology and by their proximity to the insertion site of arrector pili muscle, are extremely close to the so-called germ cells. In these situations, many of the germ cells have multi- lobular nuclei and are morphologically similar to the bulge cells. In a minor population of follicles, which are clearly telogen as evidenced by the morphological features of a condensed, half-moon shaped dermal papilla, there is a considerable distance between the bulge and these germ cells. In these structures, the germ cells tend not to have multi-lobular nuclei. Moreover, we found that, even in these follicles, label-retaining cells are located exclusively in the bulge, and never in the area occupied by the germ cells. These results indicate that the germ cells are not a constant cell population. In most cases where the germ cells and the bulge cells are morphologically similar, the former appears to be a part of, or is derived from, the latter. In the exceptional cases, where the bulge is not immediately next to the so-called germ cells, the latter may be a left-over of some outer root sheath cells of the previous lower follicle.

Where are the follicular stem cells?

We have proposed previously that follicular stem cells reside in the bulge area based on the fact that these bulge cells are slow-cycling and are ultrastructurally primitive, and that cells in the lower follicle are clearly dispensable [5, 14, 341. The assignment of follicular stem cells to the bulge region satisfies the requirement that stem cells should be a permanent subpopulation of cells occupying a fixed position. The close proximity of the dermal papilla to the bulge cells during telogen suggests the possible involvement of the dermal papilla in anagen- initiation. The hypothesis also provides an adequate ex- planation for the dispensability of the lower follicle. This hypothesis predicts that bulge cells should undergo transient proliferation during early anagen, and should go back to a dormant state during the rest of the hair cycle [5, 14, 341. Our present data (Fig. 2) clearly estab- lish that this is indeed the case, and thus provides additional support to the thesis that follicular stem cells reside in the bulge area.

Is the length of unagen fixed?

In the bulge-activation hypothesis, we proposed that matrix cells of the hair bulb are transient amplifying cell [5, 14, 341. The definition of a transient amplifying cell is that it has a significant but finite proliferative potential, and it is actively cycling (versus the slow-cycling state.of the stem cells [15, 161). The question arises as to what is the proliferative potential of these transient amplifying cells. Is it a precisely fixed number of cell divisions or is there room for regulation within a certain range? In this regard, it is interesting to note that plucking-induced third hair cycle lasts for about 15 days (Fig. 4), while the tretinoin- and TPA-induced cycles last about twice as long. This was further reflected by an

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increased length of the visible hair shaft in tretinoin- and TPA-treated mice versus plucked animals, suggesting that the proliferative potential of the matrix cells can be regulated to a significant extent [ l l ] . However, neither tretinoin nor TPA maintained anagen indefinite- ly, demonstrating a limited proliferative potential of the transient amplifying matrix keratinocytes of the matrix.

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Documents

Cells within the bulge region of mouse hair follicle transiently proliferate during early anagen: heterogeneity and functional differences of various hair cycles