Inhibitory effects of bFGF, VEGF and minoxidil on collagen synthesis by cultured hair dermal papilla cells

Mesenchymal cell replacement corrects thymic hypoplasia in murine models of 22q11.2 deletion syndrome

22q11.2 deletion syndrome (22q11.2DS) is the most common human chromosomal microdeletion, causing developmentally linked congenital malformations, thymic hypoplasia, hypoparathyroidism, and/or cardiac defects. Thymic hypoplasia leads to T cell lymphopenia, which most often results in mild SCID. Despite decades of research, the molecular underpinnings leading to thymic hypoplasia in 22q11.2DS remain unknown. Comparison of embryonic thymuses from mouse models of 22q11.2DS (Tbx1neo2/neo2) revealed proportions of mesenchymal, epithelial, and hematopoietic cell types similar to those of control thymuses. Yet, the small thymuses were growth restricted in fetal organ cultures. Replacement of Tbx1neo2/neo2 thymic mesenchymal cells with normal ones restored tissue growth. Comparative single-cell RNA-Seq of embryonic thymuses uncovered 17 distinct cell subsets, with transcriptome differences predominant in the 5 mesenchymal subsets from the Tbx1neo2/neo2 cell line. The transcripts affected included those for extracellular matrix proteins, consistent with the increased collagen deposition we observed in the small thymuses. Attenuating collagen cross-links with minoxidil restored thymic tissue expansion for hypoplastic lobes. In colony-forming assays, the Tbx1neo2/neo2-derived mesenchymal cells had reduced expansion potential, in contrast to the normal growth of thymic epithelial cells. These findings suggest that mesenchymal cells were causal to the small embryonic thymuses in the 22q11.2DS mouse models, which was correctable by substitution with normal mesenchyme.

Lichen planopilaris and frontal fibrosing alopecia: review and update of diagnostic and therapeutic features

Lichen planopilaris and frontal fibrosing alopecia are primary scarring alopecias where diagnosis can be suggested by clinical and trichoscopy features, especially in the early stages, but scalp biopsy is the standard exam for definitive diagnosis. Frontal fibrosing alopecia is considered a variant of lichen planopilaris, as the histopathological findings are similar, with a perifollicular lymphohistiocytic infiltrate, sometimes with a lichenoid pattern. A thorough clinical examination, trichoscopy and photographic documentation are essential to assess the evolution and therapeutic response. To date, there are no validated treatments or guidelines for these diseases, but there are recommendations that vary with the individual characteristics of each patient. This article presents a comprehensive review of the literature, including an update on topics related to the diagnosis, follow-up, histopathological aspects and available treatments for lichen planopilaris and frontal fibrosing alopecia, highlighting their similarities, differences and peculiarities.

Retinal artery occlusion as a probable idiosyncratic reaction to topical minoxidil: a case report

BACKGROUND

Minoxidil hair formulation is commonly used for the treatment of male or female androgenic alopecia. This over-the-counter product is considered safe, but should be used with caution. Ocular side effects following topical minoxidil use are rarely reported. In this paper, we report a rare case of inferior hemiretinal artery occlusion possibly caused by topical 5% minoxidil treatment.

CASE DESCRIPTION

A 21-year-old Asian Indian male presented to the retina clinic with sudden onset blurring of vision and superior visual field loss in the right eye since morning. He was diagnosed with androgenic alopecia and was on treatment with topical 5% minoxidil spray twice a day for the last 3 years. On examination, his corrected distance visual acuity was 6/6, N6 in both eyes. Anterior segment examination and intraocular pressure in both eyes and left eye fundus were within normal limits. Right eye fundus examination showed features suggestive of inferior hemiretinal artery occlusion, which were confirmed on fluorescein angiography and optical coherence tomography. A detailed systemic evaluation and investigations (blood pressure, random blood sugar, hematological and coagulation profile, serum homocysteine level, Mantoux test, chest x-ray, cardiac two-dimensional echography, thyroid function test, and immunological profile) did not detect any abnormalities. The ocular condition and its visual prognosis were explained to the patient, and he was asked to review after 4 weeks.

CONCLUSION

Though there is no definite cause-outcome relationship between topical minoxidil use and retinal artery occlusion development, this possibility should be kept in mind when observing retinal vascular occlusion cases with concurrent use of topical minoxidil.

Clinical Patterns of Hair Loss in Men: Is Dihydrotestosterone the Only Culprit?

A number of pathways and factors including oxidative stress, inflammation, prostaglandins, vasculogenesis, Wnt/β-catenin, and transforming growth factor-β have been shown to be important in male androgenetic alopecia. There is limited but increasing evidence of the potential usefulness of antioxidants, anti-inflammatory agents, prostaglandins, and growth factors for treating of androgenetic alopecia. Lifestyle factors and comorbidities including cardiovascular risk factors have been shown to be associated with male androgenetic alopecia. Further study of these pathways, factors, and comorbidities is needed to better understand the pathophysiology, find potentially useful therapeutic targets, and ensure a comprehensive approach to the management of androgenetic alopecia in men.

Minoxidil induced central serous Chorioretinopathy treated with oral Eplerenone - a case report

Background

Minoxidil solution has routinely been used for decades for the treatment of androgenic alopecia. Central serous chorioretinopathy (CSCR) is a rare side-effect noted following prolonged topical minoxidil therapy for androgenic alopecia. In this report, we describe a case of a 41-year-old young man who developed CSCR following prolonged therapy with topical Minoxidil solution and was treated with oral eplerenone.

Case presentation

A 41-year-old male presented to the retina clinic with complaints of seeing a black spot, blurred vision and metamorphopsia involving the right eye for the past 4 months. He was on treatment for androgenic alopecia with topical 5% Minoxidil application on scalp two times a day. He noticed the symptoms 8 months after starting the treatment and had stopped the medication since the past 2 months. On examination, best-corrected visual acuity was 20/20 in both eyes. Fundoscopic examination of the right eye with +78D lens on slit lamp revealed the presence of subretinal fluid and few focal spots of retinal pigment epithelial alterations. Optical coherence tomography scan evaluation showed the presence of subretinal fluid (SRF) and pachychoroid supporting the diagnosis of CSCR. Indocyanine green angiography revealed dilated hyperpermeable choroidal vasculature on the nasal side of the fovea in the early and later phases of the angiogram. The patient was diagnosed with CSCR as a possible consequence of the topical minoxidil solution. Patient was asked to avoid future use of Minoxidil and was started on oral eplerenone therapy 50 mg/day for 4 consecutive weeks. One month later, there was complete resolution of his symptoms and SRF. At the final follow-up visit, 2 months after starting the therapy, there was no recurrence of SRF.

Conclusion

CSCR is a rare side-effect noted following prolonged topical minoxidil therapy for androgenic alopecia. While we found oral eplerenone to be safe and effective, further studies would be required before it can be routinely used in the population.

Effect of High-Dose Topical Minoxidil on Erythrocyte Quality in SKH1 Hairless Mice

Simple Summary

In any animal species, involuntary exposure to unknown agents may increase genetic material damage. This genetic damage can also induce the appearance of diseases such as cancer or other pathologies, including problems that can be passed on to the offspring of the damaged individual. For instance, living organisms may be affected due to the use of medications or exposure to certain chemical, physical, or biological agents which cause cell failure. This impaired function acts as an indicator that helps identify and evaluate damage in order to avoid or minimize it. In this work, excessive doses of a cosmetic drug for topical use in dermatological treatments, known as minoxidil, produced defects in the blood of hairless mice, particularly in red cells, indicating loss of DNA, a situation that may compromise life or the offspring by causing damage to their genetic material. It is important to consider that compounds may be tissue- or species-specific, although we cannot rule out the possibility that similar damage could occur in other animal species. Thus, excessive exposure to this compound should be prevented.

Abstract

SKH1 hairless mice are widely used in carcinogenesis and dermatology research due to their bare skin, as exposure to different agents is facilitated. Minoxidil is a cosmetic drug that is recognized as a mitogenic agent, and mitogens are suggested to have carcinogenic and mutagenic potential by inducing cell division and increasing the possibility of perpetuating DNA damage. Therefore, we hypothesized that the application of high doses of minoxidil to the skin of hairless mice would increase the number of micronucleated erythrocytes (MNEs) in peripheral blood. The objective of this study was to evaluate the topical administration of high doses of minoxidil on peripheral blood erythrocytes of SKH1 mice by means of micronucleus assay. Minoxidil was administered on the entire body surface of mice every 12 or 24 h. Minoxidil dosing every 24 h increased the number of micronucleated polychromatic erythrocytes (MNPCEs), and dosing every 12 h increased the number of MNEs and MNPCEs, as compared to baseline and the negative control group. No decrease in polychromatic erythrocyte frequencies was observed in the minoxidil groups. Therefore, topical application of high minoxidil doses to mice can produce DNA damage, as observed through an increase in the number of MNEs, without producing cytotoxicity, possibly due to its mitogenic effect.

Frontal Fibrosing Alopecia: An Update on Pathogenesis, Diagnosis, and Treatment

Frontal fibrosing alopecia (FFA), first described by Kossard in the early 1990s, is a form of primary lymphocytic cicatricial alopecia characterized by selective involvement of the frontotemporal hairline and eyebrows. Since the original description, an increasing number of cases have been reported worldwide and the clinical aspects of the disease have been better characterized. However, the pathogenesis is still unknown and several hypotheses have been made about possible triggering factors, including hormones, neurogenic inflammation, smoking, UV filters, and ingredients in leave-on facial products. A genetic basis has also been hypothesized as the disease can occur in siblings and members of the same family. Besides its pathogenesis, research is also focused on treatment; FFA is a chronic condition and at present there is no validated or approved treatment for this disorder. Commonly prescribed topical treatments include corticosteroids, minoxidil, and calcineurin inhibitors. Systemic treatments include 5α-reductase inhibitors, hydroxychloroquine, and retinoids. Intralesional triamcinolone acetonide is also utilized, especially for the eyebrows. Other possible treatments include pioglitazone, naltrexone, tofacitinib, and lasers.

Basic fibroblast growth factor regulates phosphate/pyrophosphate regulatory genes in stem cells isolated from human exfoliated deciduous teeth

Background

Basic fibroblast growth factor (bFGF) regulates maintenance of stemness and modulation of osteo/odontogenic differentiation and mineralization in stem cells from human exfoliated deciduous teeth (SHEDs). Mineralization in the bones and teeth is in part controlled by pericellular levels of inorganic phosphate (P_i), a component of hydroxyapatite, and inorganic pyrophosphate (PP_i), an inhibitor of mineralization. The progressive ankylosis protein (gene ANKH; protein ANKH) and ectonucleotide pyrophosphatase phosphodiesterase 1 (ENPP1/ENPP1) increase PP_i and inhibit mineralization, while tissue-nonspecific alkaline phosphatase (ALPL; TNAP) is a critical pro-mineralization enzyme that hydrolyzes PP_i. We hypothesized that regulation by bFGF of mineralization in SHEDs occurs by modulation of P_i/PP_i-associated genes.

Methods

Cells were isolated from human exfoliated deciduous teeth and characterized for mesenchymal stem cell characteristics. Cells were treated with bFGF, and the osteogenic differentiation ability was determined. The mRNA expression was evaluated using real-time polymerase chain reaction. The mineralization was examined using alizarin red S staining.

Results

Cells isolated from primary teeth expressed mesenchymal stem cell markers, CD44, CD90, and CD105, and were able to differentiate into osteo/odontogenic and adipogenic lineages. Addition of 10 ng/ml bFGF to SHEDs during in vitro osteo/odontogenic differentiation decreased ALPL mRNA expression and ALP enzyme activity, increased ANKH mRNA, and decreased both P_i/PP_i ratio and mineral deposition. Effects of bFGF on ALPL and ANKH expression were detected within 24 h. Addition of 20 mM fibroblast growth factor receptor (FGFR) inhibitor SU5402 revealed the necessity of FGFR-mediated signaling, and inclusion of 1 μg/ml cyclohexamide (CHX) implicated the necessity of protein synthesis for effects on ALPL and ANKH. Addition of exogenous 10 μm PP_i inhibited mineralization and increased ANKH, collagen type 1a1 (COL1A1), and osteopontin (SPP1) mRNA, while addition of exogenous P_i increased mineralization and osterix (OSX), ANKH, SPP1, and dentin matrix protein 1 (DMP1) mRNA. The effects of PP_i and P_i on mineralization could be replicated by short-term 3- and 7-day treatments, suggesting signaling effects in addition to physicochemical regulation of mineral deposition.

Conclusion

This study reveals for the first time the effects of bFGF on P_i/PP_i regulators in SHEDs and implicates these factors in how bFGF directs osteo/odontogenic differentiation and mineralization by these cells.

Electronic supplementary material

The online version of this article (10.1186/s13287-018-1093-9) contains supplementary material, which is available to authorized users.

Review of Hair Follicle Dermal Papilla cells as in vitro screening model for hair growth

Hair disorders such as hair loss (alopecia) and androgen dependent, excessive hair growth (hirsutism, hypertrichosis) may impact the social and psychological well-being of an individual. Recent advances in understanding the biology of hair have accelerated the research and development of novel therapeutic and cosmetic hair growth agents. Preclinical models aid in dermocosmetic efficacy testing and claim substantiation of hair growth modulators. The in vitro models to investigate hair growth utilize the hair follicle Dermal Papilla cells (DPCs), specialized mesenchymal cells located at the base of hair follicle that play essential roles in hair follicular morphogenesis and postnatal hair growth cycles. In this review, we have compiled and discussed the extensively reported literature citing DPCs as in vitro model to study hair growth promoting and inhibitory effects. A variety of agents such as herbal and natural extracts, growth factors and cytokines, platelet-rich plasma, placental extract, stem cells and conditioned medium, peptides, hormones, lipid-nanocarrier, light, electrical and electromagnetic field stimulation, androgens and their analogs, stress-serum and chemotherapeutic agents etc. have been examined for their hair growth modulating effects in DPCs. Effects on DPCs' activity were determined from untreated (basal) or stress induced levels. Cell proliferation, apoptosis and secretion of growth factors were included as primary end-point markers. Effects on a wide range of biomolecules and mechanistic pathways that play key role in the biology of hair growth were also investigated. This consolidated and comprehensive review summarizes the up-to-date information and understanding regarding DPCs based screening models for hair growth and may be helpful for researchers to select the appropriate assay system and biomarkers. This review highlights the pivotal role of DPCs in the forefront of hair research as screening platforms by providing insights into mechanistic action at cellular level, which may further direct the development of novel hair growth modulators.

A bald statement - Current approaches to manipulate miniaturisation focus only on promoting hair growth

Hair plays a large part in communication and society with its role changing through time and across cultures. Most people do not leave the house before combing their hair or shaving their beard and for many hair loss or irregular hair growth can have a significant impact on their psychological health. Somewhat unsurprisingly, according to GMR Data, today's global hair care industry is worth an estimated $87 Billion, with hair loss estimated at $2.8 Billion. Considering that no current hair loss-related products can completely reverse hair loss, it is reasonable to believe this market could expand significantly with the discovery of a comprehensive therapy. As such, a great deal of research focuses on overcoming hair loss, and in particular, a common form of hair loss known as androgenetic alopecia (AGA) or male pattern baldness. In AGA, hair follicles miniaturise in a large step change from a terminal to a vellus state. Within this viewpoint article, we discuss how influx and efflux of cells into and out from the dermal papilla (DP) can modulate DP size during the hair cycle. As DP size is positively correlated with the size of the hair fibre produced by a follicle, we argue here that therapies for treating AGA should be developed which can alter DP size, rather than just promote hair growth. We also discuss current therapeutics for AGA and emphasise the importance of using the right model systems to analyse miniaturisation.

Potassium channel openers increase aortic elastic fiber formation and reverse the genetically determined elastin deficit in the BN rat

Hypertension is a cardiovascular disorder that appears in more than half of the patients with Williams-Beuren syndrome, hemizygous for the elastin gene among 26 to 28 other genes. It was shown that the antihypertensive drug minoxidil, an ATP-dependent potassium channel opener, enhances elastic fiber formation; however, no wide clinical application was developed because of its adverse side effects. The Brown Norway rat was used here as an arterial elastin-deficient model. We tested 3 different potassium channel openers, minoxidil, diazoxide, and pinacidil, and 1 potassium channel blocker, glibenclamide, on cultured smooth muscle cells from Brown Norway rat aorta. All tested potassium channel openers increased mRNAs encoding proteins and enzymes involved in elastic fiber formation, whereas glibenclamide had the opposite effect. The higher steady-state level of tropoelastin mRNA in minoxidil-treated cells was attributable to an increase in both transcription and mRNA stability. Treatment of Brown Norway rats for 10 weeks with minoxidil or diazoxide increased elastic fiber content and decreased cell number in the aortic media, without changing collagen content. The minoxidil-induced cardiac hypertrophy was reduced when animals simultaneously received irbesartan, an angiotensin II-receptor antagonist. This side effect of minoxidil was not observed in diazoxide-treated animals. In conclusion, diazoxide, causing less undesirable side effects than minoxidil, or coadministration of minoxidil and irbesartan, increases elastic fiber content, decreases cell number in the aorta and, thus, could be suitable for treating vascular pathologies characterized by diminished arterial elastin content and simultaneous hypertension.

Inhibitory effect of tianeptine on catagen induction in alopecia areata-like lesions induced by ultrasonic wave stress in mice

BACKGROUND

Alopecia areata (AA) is characterized by rapid and complete hair loss in one or multiple areas of the scalp. Stress is an important triggering factor in AA.

AIM

To identify the inhibitory effect of tianeptine on catagen induction in C57BL/6 mice with AA-like lesions induced by ultrasonic wave stress (UWS).

METHODS

The mice were divided into four groups. Group 1 received oral tianeptine before and after UWS; group 2 received oral tianeptine only after UWS; group 3 was given UWS treatment only; and group 4 (negative control group) was not given any treatment. Phototrichigraphy and dermatoscopy were used for assessment. Histological analysis was performed using haematoxylin and eosin, toluidine blue, Masson trichrome and Verhoeff-van Gieson stains. Immunohistochemical analysis was also performed. The level of apoptosis and expression of neuropeptides in the skin were assessed by terminal deoxynucleotidyl transferase dUTP nick end labelling and immunofluorescence assays.

RESULTS

Mice in group 1 had an increased rate of hair growth and greater hair-shaft thickness compared with mice in groups 2 and 3. In addition, mice in group 1 had a higher number of anagen hair follicles, increased synthesis of collagen and elastic fibres, decreased mast-cell degranulation, reduction in cell apoptosis in hair follicles, and recovery of vitamin D receptor expression. Expression of neuropeptides (substance P, calcitonin gene-related peptide) was not altered.

CONCLUSIONS

Tianeptine might play a role in suppressing catagen induction in a stress-induced AA mouse model.

Prostaglandin-induced hair growth

Latanoprost, used clinically in the treatment of glaucoma, induces growth of lashes and ancillary hairs around the eyelids. Manifestations include greater thickness and length of lashes, additional lash rows, conversion of vellus to terminal hairs in canthal areas as well as in regions adjacent to lash rows. In conjunction with increased growth, increased pigmentation occurs. Vellus hairs of the lower eyelids also undergo increased growth and pigmentation. Brief latanoprost therapy for 2-17 days (3-25.5 microg total dosage) induced findings comparable to chronic therapy in five patients. Latanoprost reversed alopecia of the eyelashes in one patient. Laboratory experiments with latanoprost have demonstrated stimulation of hair growth in mice and in the balding scalp of the stumptailed macaque, a primate that demonstrates androgenetic alopecia. The increased number of visible lashes is consistent with the ability of latanoprost to induce anagen (the growth phase) in telogen (resting) follicles while inducing hypertrophic changes in the involved follicles. The increased length of lashes is consistent with the ability of latanoprost to prolong the anagen phase of the hair cycle. Correlation with laboratory studies suggests that initiation and completion of latanoprost hair growth effects occur very early in anagen and the likely target is the dermal papilla.

[Hair growth effect of minoxidil]

The length and size of hair are depend on the anagen term in its hair cycle. It has been reported that the some cell growth factors, such as VEGF, FGF-5S, IGF-1 and KGF, induce the proliferation of cells in the matrix, dermal papilla and dermal papillary vascular system and increase the amount of extra cellular matrix in dermal papilla and then maintain follicles in the anagen phase. On the other hand, negative factors, like FGF-5, thrombospondin, or still unknown ones, terminate the anagen phase. If the negative factors become dominant against cell proliferation factors according to fulfilling some time set by the biological clock for hair follicles, TGF beta induced in the matrix tissues evokes apoptosis of matrix cells and shifts the follicles from anagen to catagen. Androgenetic alopecia is caused by miniaturizing of hair follicles located in the frontal or crown part of scalp and are hereditarily more sensitive to androgen. In their hair cycles, the androgen shortens the anagen phase of follicles and shifts them to the catagen phase earlier than usual. The mode of action of hair growth effect of minoxidil is not completely elucidated, but the most plausible explanation proposed here is that minoxidil works as a sulfonylurea receptor (SUR) activator and prolongs the anagen phase of hair follicles in the following manner: minoxidil (1) induces cell growth factors such as VEGF, HGF, IGF-1 and potentiates HGF and IGF-1 actions by the activation of uncoupled SUR on the plasma membrane of dermal papilla cells, (2) inhibits of TGF beta induced apoptosis of hair matrix cells by opening the Kir 6.0 channel pore coupled with SUR on the mitochondrial inner membrane, and (3) dilates hair follicle arteries and increases blood flow in dermal papilla by opening the Kir 6.0 channel pore coupled with SUR on the plasma membrane of vascular smooth muscle cells.

Minoxidil and male-pattern alopecia: a potential role for a local regulator of sebum secretion with vasoconstrictive effects?

Regulation of the hair cycle takes place at the pilo-sebaceous unit with the sebaceous gland as a sex hormone-dependent part. Although minoxidil stimulates proliferation of follicular cells and activation of prostaglandin endoperoxide synthase-1, it was suggested that other mechanisms, such as an increase in the local blood flow, might mediate the drug effect on hair growth. If that is the case, it is possible that minoxidil counteracts some vasoconstrictive mediator of male-pattern alopecia. This hypothetical vasoconstrictive mediator X would have to meet some criteria: (I) vasoconstriction both in the general circulation and in the hair-growing skin; (II) local vasoconstrictive activity in the hair growing skin should be related to the circulating testosterone level; (III) only an increase in the local mediator X activity causes male-pattern alopecia, since hypertensive patients are not balder than expected. The sebaceous gland is a possible place of the mediator X secretion since it is a sex-hormone-dependent part of the pilo-sebaceous unit. ET-1 might be a suitable candidate for the mediator X, since male hormones raise ET-1 plasma levels and female hormones lower them. The speculation presented here is that ET-1, beside vasoconstriction in the general circulation, might also regulate the sebum secretion, by triggering contractions of the myoepithelial cells. This hypothetical mechanism would normally remain confined to the sebaceous gland. During puberty, sex hormones stimulate growth of sebaceous glands in both sexes. In women hypertrophied sebaceous glands under estrogen control would not increase its ET-1 content, while in men, testosterone would increase ET-1 secretion that might affect the neighboring arterioles. Induced vasoconstriction might reduce the hair growth and promote hair loss. If ET-1 plays the described role, then an ET-1 antagonist, i.e. bosentane, should also have some hair-growing properties.

Serine protease activity is essential for thrombin-induced protein synthesis in cultured human dental pulp cells: modulation roles of prostaglandin E2

Irritations and injuries to the dental pulp usually lead to different degrees of pulpal inflammation. To investigate the roles of thrombin and prostaglandins in the healing and inflammatory processes of dental pulp as well as their effects on pulpal protein synthesis, human dental pulp cell cultures were established and their protein production was measured with or without the presence of exogenous thrombin and prostaglandins. At concentrations of 1-25 U/ml, alpha-thrombin increased the protein synthesis to 1.4-2.3 fold over the vehicle control. On the contrary, 0.1 microg/ml of prostaglandin E1 (PGE1) suppressed protein synthesis by 60%. Prostaglandin E2 (PGE2) also inhibited protein synthesis with an IC50 of 0.4 microg/ml. The stimulatory effects of thrombin (10 U/ml) can be inhibited by antithrombin III (2 U/ml) (a natural thrombin inhibitor) with heparin (2 U/ml), PPACK (D-Phe-Pro-ArgCH2Cl) (20-50 microg/ml) (a serine protease inhibitor), and PGE2 (0.5-1.0 microg/ml). Moreover, TRAP (20-40 microg/ml), a thrombin receptor agonist peptide, also exerted a stimulatory effect (1.21-1.37 fold). In conclusion, thrombin-induced protein synthesis by pulp cells is dependent on proteolytic activity, but not on binding to receptors. Both PGE1 and PGE2 exert suppressive effects on protein synthesis, indicating that interactions between thrombin and prostaglandins are important in regulating the inflammation, repair and regeneration of pulp tissue following injury.