Short and long term clinical and histological evaluation of medium-depth chemical facial peel with 35°/o trichloroacetic acid on photo damaged skin

Short and long term clinical and histological evaluation of medium-depth chemical facial peel with 35°/o trichloroacetic acid on photo damaged skin

Sherien Metwally  Salama, MDa; Mohamed Saber Mohamed, MDb

a) Plastic  Surgery Department, Ain Shams University, Cairo, Egypt. b) Histology Department, Aim-Shams University, Cairo, Egypt.

Chemical  peeling involves  the topical  application  of a wounding  agent  with the goal of effecting an organized regeneration  of the skin. The histological features of photo aged skin include structural abnormalities that disrupt normal epidermal and dermal architecture.

In the present prospective study we compared short and long term clinical and histological changes that occur after three sessions of enhanced medium depth chemical facial peel with 35% trichloroacetic acid on ten patients with photo aged skin changes. Biopsy specimens were taken pre-peel (control), and 3, 12, 48, months post-peel, for histological, and immunohistochemical analysis  to assess total collagen  type I content  throughout  the  study that  reach up to four years. Histological evaluations during the course of treatment have revealed an array of time­ dependent morphologic changes that were classified into early, intermediate, and late. Clinical resolution of actinic damage corresponded with restoration of epidermal and dermal polarity. Characteristic  histological  and immunohistochemical   analysis  post-peel  include  decreased elastic fibers, increased activated fibroblasts and organized parallel arrays of collagen fibrils. As  treatment  is  continued  beyond  12  months,  the  neocollagen  synthesis  and  organization continues to increase and elastosis continues to decrease. Besides, increases in epidermal and dermal mucin and decreases in epidermal melanin. We concluded that although there was no direct correlation between histological changes and clinical improvement, the major structural changes appear to be directed at restoring the skin to the pre-sun-damaged state.

Key words: Chemical peel- photodamaged skin- histology-long term.

Introduction:

Aging is defined as the process ofbecoming older. This traditional definition was recently challenged   in  the   new   Handbook   of  the Biology  of Aging  (Academic  Press,  2006), as the process of system's  deterioration with time. This definition allows for existence of non-aging  systems (when "old is as good as new"), and anti-aging interventions (when accumulated damage is repaired). Certainly, facial skin deterioration is one of the most apparent examples  of aging. For this reason combined with the social perception of youthfulness as a measure of outer beauty, it is not a surprise that people are seeking more avenues for anti-ageing interventions. This demand has prompted tremendous  growth in

the  cosmetic  industry, with numerous  over­ the-counter products available to "reverse aging   changes."   Whether   to   reverse   the signs of aging or to treat cutaneous lesions, ablative skin  resurfacing  is an integral  part of the practice of facial plastic surgery and dermatology.!

Chemical peeling is a technique that removes   superficial   lesions   and  improves the  texture  of  skin  by  the  application   of a   chemical    defoliant.    Peeling   produces a    controlled,    partial-thickness     chemical bum of the epidermis and the outer dermis. Regeneration  of peeled skin from follicular and  eccrine   duct  epithelium   results   in  a fresh, orderly, organized epidermis. In the dermis,  a  new  2 to  3  mm  band  of  dense,

compact, orderly  collagen  is formed  between the epidermis  and the underlying damaged dermis,  which  results  in effective  ablation  of the fine wrinkles in the skin and a reduction of

pigmentation. 2 These clinical and histological

changes   are  long-lasting  (15  to  20  years) and  may  be  permanent for  some  patients. Medium-depth chemical peels are defined  as the application of a chemical  cauterant  to the skin to produce  wounding to the level  of the upper  reticular  dermis.3  although   medium­ depth chemical  peels have been used to treat a myriad  of cutaneous disorders; they are most commonly used  to  improve   photodamaged skin  by removing solar  keratoses, softening mild     rhytides,    and    evening   pigmentary dyschromias. Reepithelialization of the  skin after  medium-depth wounding occurs  from adnexal  structures and by the stimulation of new collagen formation. Trichloroacetic acid (TCA)  is a chemical cauterant that coagulates the proteins of the skin. During the application of  TCA  to  the  skin,  a transient  gray-white color  change  known  as "frosting" occurs  on the  treated   skin.  Trichloroacetic  acid  peels are  used  to  treat  actinically damaged   skin, resulting in  necrosis   of  the  epidermis  and the  upper  dermis,  followed   by  replacement with  new  epidermis and  dermal  connective tissues.   The  depth   of  penetration  of  TCA into  the  skin  increases  as the  concentration of  TCA  is  increased, as  does  the  potential

for   scarring.4   Traditionally, medium-depth

chemical peels were performed using 40-60% trichloroacetic acid  (TCA).   However,  these higher  concentrations of  TCA  carried   with them  an increased  risk of scarring.s In 1986, Brody   and  Hailey   introduced  the   concept of   combination  medium-    depth   chemical peels  whereby two  superficial  agents,  solid carbon   dioxide   and  35%  TCA,   were  used in succession to reach  the same  histological depth   as  a  single   agent   while   decreasing the   risk   of  scarringl,3.   In   1989,   Monheit described the  use  of Jessner's solution (JS: resorcinol, salicylic  acid,  and  lactic  acid)  as a keratolytic agent to increase  the absorption and   penetration  of   35%   TCA   and   thus produce  a medium-depth chemical peel  for

photodamaged  skin.6  Coleman   and  Eutrell

described  a   new   medium-depth  chemical peel:  the   glycolic   acid-trichloroacetic  acid (GA-TCA) peel.  This peel  employed 70% glycolic   acid   instead   of  Jessner's  solution as the initial  agent prior to the application of

35%  TCA.  It was  theorized by the  authors

that  pretreatment of  the  skin  with  glycolic acid would produce  more even penetration of the  TCA  35%.  Although  the use  of glycolic acid   in  superficial   chemical  peels   is  now well  established, its  role  in  medium-depth chemical peels has yet to be fully elucidated.7

Tretinoin (Retin-A), or all-transretinoic acid, is a synthetic  vitamin A analogue  used to treat a variety  of dermatoses including photoaged skin.s  Interest  has focused on tretinoin as a wound-healing promoter.  In  animal  studies, topical   retinoid   have  enhanced the  healing of  full-thickness  skin  wounds   and  corneal epithelial wounds_9,10 In an open non placebo­ controlled study, tretinoin pretreatment of the skin  for  2 weeks  prior  to  dermabrasion was shown to accelerate  healing.ll Since tretinoin may   alter   the   epidermal  barrier   to   allow enhanced  percutaneous absorption  of  other chemicals,12  tretinoin pretreated skin  might be  expected   to  increase  the  depth  of  TCA penetration into the  skin.  Since,  all patients in our study were pretreated  for 2 weeks with retina  -A  0.1%  prior  to  chemical peel  with

35%   TCA  to  achieve   safe   medium   depth peel.  The  histological changes  of  the  aging skin  are typical  of actinic  changes  which  are the photochemical effects of solar radiation exposure. These  changes  include  a loss of orderly  differentiation in the epidermis and degeneration ofthe elastic network, along with some  mottled  pigmentation and lymphocytic infiltration. There is a decrease  in collagen as well as disordered degeneration of the dermal fibers, a flattening of the dermal-epidermal junction, and multiple  actinic  keratoses with atypia seen.

The number of melanocytes was increased

in this  actinic  skin,  but they  were  unevenly distributed and  contained variable   amounts of   melanin.l3  Collagen   I  predominates   in human  dermis,  accounting for  85  percent  of the  total,  whereas  collagen III  accounts  for

only  10  percent.l4 Thus,  studies  of collagen

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in human  skin  are more easily  accomplished if directed  at collagen  I. The biosynthesis of collagen I in skin begins with the formation of procollagen I within  dermal  fibroblasts_15,16

After  its  secretion from  fibroblasts, procollagen I is enzymatically cleaved  of its aminopropeptide and carboxypropeptide in a one-to-one ratio;  the  presence  of propeptide provides  an index  of collagen  I synthesis.17

Therefore,   an  antibody that  can  recognize

the  aminopropeptide portion  of  procollagen I or its distal derivatives18  will both identify precursors  of   collagen   I   and   provide   an indirect   measure   of   collagen  I  formation Figure(l).    Using    such    an   antibody    we determined whether the formation of collagen I is reduced in photodamaged human skin and is increased by intermediate depth  peel with

35% TCA. The finding  of increased collagen

I formation in photodamaged human skin treated  with  chemical peel  suggests that peeling  promotes clinical  improvement by repairing dermal  collagen.

The    finding    of   increased      collagen  I

formation in  photodamaged human  skin treated     chemical     peel    suggests   that    it promotes clinical improvement by repairing dermal collagen.

The purpose of our study was to compare short term, long term relationship between, clinical   improvement  and   histological changes  that  occur  after  medium-depth chemical   facial   peel   with   35%   TCA   for patients  with photoaging skin.

Patients and methods:

The present  work was conducted in the Plastic  Surgery  and  Histology Departments of  Ain  Shams  University during  the  period from   January   2007   to  January   2012.  Ten patients (three men and seven women) with photodamaged  facial   skin   were   evaluated. All patients  had Fitzpatrick skin types  IV-V. Their  age ranged from  40 to 65 years (mean age 52 years). No patient had a previous chemical    peel    or   dermabrasion,  or   used topical tretinoin in the year before entering the study. Informed  consent  prior to entering the study was obtained after the risks and benefits were   explained.   Each   subject    underwent

three sessions  of 35% TCA peel preceded  by pretreatment with  Tretinoin  (Retin-A) 0.1% for 2 weeks.

The entire face was cleansed  with alcohol

followed by acetone to degrease the skin vigorously until  it  became   very  dry  to  the touch.  35  %  TCA  was  then  applied  using two cotton  tipped  applicators until a uniform white frost  was achieved. This was followed by tap water  soaks  for  10-20 minutes.  Post­ peel, patients were instructed to use sunscreen

1-2 weeks  after the procedure, retin-A  was resumed   after   3months   and  continued for one  year  and  beyond.  None  of  the  patients was  treated  with  hydroquinone pre  or  post­ peel except for those who developed post­ inflammatory hyperpigmentaion. Clinical improvement was assessed by three independent evaluators using  photographic analysis.  Right  and  left  profile  photographs of each patient  were taken  pre and post peel at the time of taking  biopsy specimen. All photographs were taken with the same camera under similar  parameters, and magnification. Clinical  parameters evaluated included, clearing   of  actinic  keratoses, lightening of solar lentigines, and lessening of rhytides. Complications such  as  postinflammatory hyper or hypopigmentation, persistent erythema,   persistent    erosions,   and   milia were  also  assessed.  A numerical scale  was

used  to  grade  clinical  improvement (0 =no                     D

response;   1 =fair;  2 =good;  3 =excellent)

and    complications  {0 =none;   1 =slight;

2 =moderate; 3 =significant). The  data were                          D

tabulated.

To  evaluate   histological  changes,   Full­

thickness 5mm punch biopsy were taken from                          u

the preauricular or temporal area one month pre-peel  (control) and at 3 month,  12 month,

48 month  post-peel. The  biopsy  specimens

were divided  into two parts: one was fixed in

10% formalin and stained  with haematoxylin and eosin  stains,  Verhoeff's  elastic  stain  and Masson  trichrome  stain  and  processed   for histological  analysis   by  light   microscopy. The  second  group  of specimens was frozen in liquid  nitrogen  and  stored  at -80  °C. The samples   were   processed  for  staining   with an  immunoperoxidase technique (Vectastain

ABC  Kit, Vector  Laboratories, Burlingame, Calif).  For the  immunohistologic analysis  to detect monoclonal antibody oftype I collagen, we used the term "collagen I immunostaining" to  refer to tissue  staining observed  with this antibody.  The intensity and extent of staining were assessed  with a six-point scale in which a score  of  0  indicated   no  staining,  a  score of  1 minimal  staining, a score  of  2 low-to­ moderate   levels   of  staining,  a  score   of  3 moderate  staining, a score  of 4 high levels of staining, and  a score  of 5 maximal staining. The  patterns   of  collagen I  immunostaining were   extracellular,  in   a   papillary    dermal band  (a  narrow   zone  immediately beneath the epidermis)         and       intracellular,         within fibroblasts. Four high-power fields from each section  were graded  under light microscopy.

Results:

Clinically,   the   enhanced  medium-depth chemical  peel   with   35%   TCA   peel   was effective  in   treating   photodamaged   skin. The   peeling   effect   occurred   between   the third   and  fourth   days  after  the  application of the peeling  agent.  Erythema, crusting  and swelling occurred  within  24 h with  crusting and  swelling  subsiding by  7 days  post-peel. The   time   to   complete   re  epithelialization occurred within  7-10 days. Once the scaling subsided, the  skin  had  a  more  turgent   and juvenile  appearance, the skin  color  appeared to   be  more   uniform   and   improvement   in surface  texture  was  noted  in  most  patients. All    treated       patients                   showed    statistically significant improvement and lessening of fine rhytides,  pinkness, roughness and lightening of solar lentigines after a period  of 6 months. When  the  study  period   was  extended to  a total  of 12  months  and  beyond,  the  clinical improvements in  wrinkling were  sustained, and   most   of   the   change   occurred   during the  first  6-12  months,   with  little  additional improvement  discernible  after  this   period. No  or  little  improvement of  deep  rhytides. No     complications,     except    for    rebound hyperpigmentation that  occurred in 35%  of cases especially with type V skin but subsided after  use  of  bleaching creams.   The  clinical

results  and complications are summarized in

Table(l), Figures(2-5).

Histologically, before peeling, structural changes    varies    in   severity    according  to the degree of photodamage but overall parameters showed   that  photodamged skin had solar elastosis, strands  of elastic fibers coursed  through the papillary  dermis  derma­ epidermal junction  showed  a clear flattening, resulting  in a decrease  in the junction  surface area with a corresponding decrease  in interdigitating papillae and loss of rete ridges, vascular  ectasia, and a fragmented array of disordered and poor striated  collagen  bundle Figures(6a,7a).

At     3    months    post-peel    biopsy,    the

histological  changes   were  most   noticeable in the  epidermis, as there  were  increases   in epidermal thickness, decreased melanocytes, compaction   of   the   basket-weave   pattern of the stratum  corneum, and deposition of alcian-blue staining/material (mucin) in the stratum   corneum   of  90%  of  specimens.  A more  organized epidermis overlay  a thicker papillary  dermis  composed of  more  parallel homogenized layers  of  collagen showed   in

20%   of  specimens  The  Verhoeff's  elastic

stain  showed  minimal  development of  mid­ dermal   elastic  fibers   and  papillary   dermal elastic  fibers  were decreased compared with the skin before peeling in 70% of specimens Figures(6b,7b).

12 month post peel biopsy:  Compaction of the stratum  corneum was found  in 30% of 12 months biopsies, which is less than the 90% observed at 3 months biopsies. The conversion ofthe stratum corneum from the basket-weave pattern to the compact appearance, melanin pigment   continued  to  decrease   as  well  as other  epidermal changes,   was  presumed   to have  been a major  contributing factor  to the clinical  improvement. However, these results bring  into question the  degree  of correlation between  clinical  and histological changes  in the epidermis.  Organized  bundles of collagen were  evident   in  the  papillary and  reticular dermis  in 70% of specimens Figure(7c).

48 months post peel biopsy: There was a striking return  to normal  appearance in two­ thirds   of  the  samples,  although   there   had

been  no  change  after  only  6  months,  yet  at

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Procollagen I

Amlnopropeptide                         Collagen I

Carboxypropeptide

Triple Helix

Fibroblast

J Secretion

'-     C-Protelnase

Figure (2): 60 year patient 2 years post 3

pN Collagen I            I            Carboxypropeptide

N·                                                                                           Fragment

Amlnopropeptide

Fragment

times peel showed marked improvement of fine wrinkles, skin texture and lightness of solar lentigins.

Mature

Collagen 1

tNmloo-regiOn rewgnl.leO oy monoclOnal antilm)' $PI ,DB

Extracellular

Figure (1): Biosynthesis of collagen I.

Figure (3): 55 years male patient after 1 year post peel show improvement of photoaging changes with no response on deep rhytides.

Figure (5): 65 year female patient 24 months post peel show marked improvement of solar lentigens andfine wrinkles of the forehead

21 months the epidetmal thickness retumed to baseline, the granular cell layer number declined, and the basket-weave pattem of the stratum comeum retumed to baseline in 80% of samples. Melanin continued to decrease

Figure (4): 40 years patient 6 months post peel show good response of solar lentigens with improvement of skin texture.

--...c·... --' . --

Figures (6a, 6b): The histology taken before chemical peel treatments shows loose, haphazard collagen bundles, typical of photodamaged skin (above left). 3 months post-peel treatments increases in epidermal thickness, more organized epidennis with ret­ ridges thicker papillary composed of more parallel homogenized layers of collagen. (Hematoxylin and eosin stained, 450X).

and conelate with an observed improvement in pigmentation in addition to persistent and more organized increase in papillaty and reticular detmal collagen in 70% of samples Figw·e(7d).

Figure (7): Facial biopsies of the same pre-peel (a), 3months (b), 12 months (c), and 48 months

(d) post-peel. (Hematoxylin and eosin stained, 340X).

Figure (8): Collagen I immunostaining in photodamaged skin from two patients after 12 months with intennediate  depth peel. In Panel A, there is minimal  (grade  1) extracellular  staining within the papillary dennis of photodamaged skin at base line (left- panel) and moderate (grade

3) staining after 12 months  of treatment with chemical peel (right panel) (x90). In Panel B, there is minimal (grade 1) extracellular staining within the papillary dennis of photodamaged skin at base line (left panel) and slightly more staining (grade 2) after 12 months of treatment (right-panel) (x90). There is also an increase infibroblast stainingfrom grade 1 at base line to grade 2 after 12 months of treatment with chemical peel.

Figure (9): Collagen  I immunostaining  of fibroblasts  in   photodamaged   skin  from  a patient treated with intermediate depth peel (x100). Fibroblast staining within the dennis of photodamaged skin is virtually absent prepeel (left panel) but is markedly increased (grade 5) after 12 months post-peel (right panel).

Immunohistochemical analysis of dennal collagen:

The     short   term    immunohistochemical

Figure (10): Collagen I immunostaining (x60) from a single patient skin 12 month, 48months post-peel shows increase of extracellular collagen I staining form grade 1 (left panel) to grade 5 (right panel) extracellular staining within the papillary dennis.

study (3months) showed no evidence of qualitative or quantitative changes in detmal collagen after 4 months oftreatment in80% of

Tablel. Clinical Efficacy of enhanced intermediate depth peel at different time interval.

Table 2: Histological  results  of intermediate depth  peel with 35%  TCA  on  photodamaged skin. NR, not reported.

samples, only 20% showed grade 1 increase in collagen immunostain in the papillary  dermis. After  12-24 months  collagen  synthesis  was significantly increased  as there was increase in  extracellular collagen  I  immunostaining within   the  papillary  dermis        and  reticular dermis  by 75 % (figure  8) and 80 %increase in  intracellular  collagen  I  immunostaining in  fibroblasts Figure (9)  as  compared with

pre-peel    specimens   in   70%   of   samples.

The  histological results  are  summarized  m

Table (2).

Discussion:

Whether for transient cosmetic improvement or for  prophylaxis against  skin malignancy, the  chemical  peel stimulates the repair  process   so  that  defects  are  replaced with  organized tissue.l9 Histological studies

have reported the presence  of a subepidermal

Am-ShamsJSurg2014; 7(2):415-424

band of new collagen or 'repair  zone' in the papillary dermis following  chemical peels.20

The  enhanced  medium  depth  peel includes the use of glycolic  acid (70% solution) as a dyscohesive  agent to further  the absorption and penetration  of a 35% TCA peel. In our study   we  used  tretinoin   (Retin  -A)   two weeks before  35% TCA peeling  as it alters the   epidermal   barrier  to   allow   enhanced percutaneous  absorption  of other chemicals to increase the depth of TCA penetration into the skin. The results of our study showed that Retin-A when used 2 weeks before 35% TCA, produced   a  medium   depth  chemical   peel that was effective in treating  photodamaged skin,  evening  pigmentary  dyschromias  and lessening  fine  rhytides.  This  finding  is  in accord with that ofTse et aPl  who found that a GA-TCA peel was efficacious in removing actlmc               keratoses          and       lightening            solar lentigines but was not successful in lessening coarse   rhytides.   Most  clinical   studies   on chemical  peel had been conducted  on light­ skinned after single session of peeling for each patient, with the exception of two published studies  on black people.22 Information from other  racial  groups,  such  as  Orientals,   is limited.  Our study have been conducted  on type  IV, V skin so we enforced  to  perform three sessions of35% TCA peel with 3 months apart  for  each  patient  to  achieve  satisfied clinical results, in spite of that rebound hyper pigmentation was the only complication that occurred up to 60% of cases. So patients with that  type  of skin  must  use sunscreen  post­ peel of  high  SPF and  avoid prolonged  sun exposure.  Many  clinical  and animal  studies showed  detailed  histological  improvements in  the  epidermis,  pigmentation  and  dermal matrix of photodamaded  skin after chemical peel,     either    light       microscopic,   electron microscopy and even immunohistochemistry, laser  electrophoresis  to  asses  improvement in dermal matrix collagen,  elastic fibers and melanin.   But  all  these  studies   have  been conducted   on   early   phase   (3-6   months) after   single  cession   of  chemical   peel   by whatever            chemical.        In  a    double-blind, vehicle-controlled  study  on  photo  damaged skin after chemical  peel, all treated  patients

showed statistically  significant improvement in wrinkling, pinkness, and roughness after a period of 16 weeks.9 The histological changes that were most noticeable were increases in epidermal thickness, decreased melanocyte hypertrophy hyperplasia, compaction of the basket-weave pattern of the stratum corneum, and deposition of (mucin) in the stratum corneum. There were no dermal changes observed  by  light microscopy.  This finding is in accord with our results in early period (3-6 months) post peel I as we concluded that the majority of the clinical improvement in hyper pigmentation  pinkness, and wrinkling/ texture  was  a  direct  result  of  the  changes in  epidermal  components,  and that the absence  of dermal changes  was  due to the short treatment duration. The first studies attempted to correlate clinical improvement with underlying histological changes after chemical peel specifically designed for the treatment of acne. No reported study explored long term(>12 months) relationship between clinical   improvement   and  histological changes after multiple cessions of chemical peel on oriental races. So little knowledge had been accumulated on: (I) whether clinical and histological improvement could be sustained; (ii) what underlying factors were responsible for  the  clinical  changes;  and  (iii)  to  what extent these were reversible or permanent. In our study we tried to answer these questions to  achieve the  goal  of our study. We found that,  after  12 months  of treatment,  some  of early changes were no longer evident.

The     epidermal   thickness    returned    to

baseline,   the   granular   cell   layer   number declined,  and  the  basket-weave   pattern  of the  stratum  cornea  returned  to  baseline  in many  samples.  There  were  also  observed changes in the structural  organization  of the derma-epidermal  junction   and  appearance of  ret-ridges  although  the  fiat  profile  was unchanged   at   3   month   biopsy.   Melanin continued  to  decrease  and to  correlate  with an  observed  improvement  in  pigmentation. Additional  changes also became apparent at this stage, epidermal and dermal cumin were increased, elastic tissue decreased, and dermal

collagen increased with organized bundles of

_  :       -4A

collagen  evident in the papillary  and reticular dermis as shown by light microscope and immunohistochemical analysis  of dermal collagen.   After   12  months,   these   findings were  inconsistent with epidermal and dermal alterations being  responsible for the  clinical improvements  that   were   sustained    during that  period  of treatment as lessening of fine wrinkles, lightening of solar lentigens and improvement of skin texture.

In   48   months   biopsies,    there   was   a

striking    return   to   normal    appearance  in two-thirds  of  the   samples,  although   there had  been  no change  after  only 6 months,  at

21  month,  the  area  of  dermal  elastosis   was

decreased by  34%,  consistent with  a trend noted at earlier time points,  epidermal mucin continued to  increase,  melanin  continued to decrease,   and  other  histological alterations, such   as  epidermal thickness continued to revert  to  baseline  levels  with  restoration of basket-weave pattern  ofthe stratum  corneum in 70%  of samples.  The  whole  mark  of this stage is persistent increase  of well organized papillary and reticular  collagen dermal as compared with 3 month and 12month biopsy in 80% of samples.

The  subdivision into  early,  intermediate,

and late is, and is based largely on the results of the  largest  clinical  trials  conducted up to this point.

Conclusion:

Many  histological markers  of  photo damaged  skin  have  been  identified. Intermediate depth  chemical  peel treatment induces a continuous course of clinical  and improvement that  is  most  rapid  in  the  first

6-12  months.    Some   histological  changes

(increase   in  epidermal mucin  and  decrease in  melanin)  parallel   clinical   changes,   and seem  to  be  responsible for  the  continuing improvements in,  for  example, skin  texture and  pigmentation.             Other   changes   (in  the epidermis)  are   transient,  and   still   others (dermal  collagen synthesis) occur  only  after prolonged periods,  and  their  relationship to clinical parameters is not clear. Nevertheless, the  major  histological changes   induced   by

tretinoin appear to be specific and directed  at

restoring the skin structure  and function.

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