Experimental pigment donation in vivo

Ultrastructural findings in progressive macular hypomelanosis indicate decreased melanin production

BACKGROUND

The pathogenesis of progressive macular hypomelanosis (PMH) is unknown. Recently, Westerhof et al. (Arch Dermatol 2004; 140: 210-214) hypothesized that Propionibacterium acnes produces a depigmenting factor that interferes with melanogenesis in the skin, resulting in hypopigmented spots. The purpose of the study is to gain an insight into the pathogenesis of PMH.

MATERIALS AND METHODS

We took a biopsy of 2-mm diameter from normal and lesional skin in eight PMH patients. Using electron microscopy, we compared melanization of melanosomes, melanosome transfer and amount of epidermal melanin in normal and lesional skin.

RESULT

Compared to non-lesional skin, we observed a decrease of epidermal melanin and less melanized melanosomes in lesional skin of all patients. When comparing normal and lesional skin of patients with skin type V and VI, we observed a difference in melanosome size and maturation and a switch of transferred melanosomes from single stage IV transferred melanosomes to aggregated stage I, II and III transferred melanosomes, as seen in healthy skin of skin type I to IV.

CONCLUSION

Hypopigmentation in PMH seems to be the result of an altered melanogenesis based on a decrease in melanin formation and a change in the distribution of melanosomes. In lesional skin of PMH patients with skin type V and VI less melanized, aggregated melanosomes in stead of single, mature melanosomes are transferred from melanocytes to keratinocytes. This results in a decrease of epidermal melanin. Further investigations are needed to determine the precise role of Propionibacterium acnes in this alteration of melanogenesis.

Requirement of Dynactin p150Glued Subunit for the Functional Integrity of the Keratinocyte Microparasol

The keratinocyte microparasol, composed of a perinuclear microtubular/melano-phagolysosomal complex, protects the nucleus from UV-induced DNA damage. We have previously demonstrated that cytoplasmic dynein is the motor involved in the perinuclear-directed aggregation of phagocytosed melanosomes. Dynactin, of which p150(Glued) is the major subunit, can link directly to microtubules and links organelles to dynein at different domains. To further define the mechanism of the microparasol, we transfected siRNA targeted against p150(Glued) into human keratinocytes cultured with 0.5 mm fluorescent microspheres and performed time-lapse analysis, confocal immunolocalization, and Western immunoblotting after 24 and 48 hours. Western blots revealed a significant knockdown of the p150(Glued) subunit. The knockdown decreased p150(Glued) colocalization with microtubules and decreased perinuclear positioning of the convergent microtubular framework. It also inhibited perinuclear aggregation of phagocytosed fluorescent microspheres and reduced mean centripetal microsphere displacement. The findings provide evidence that dynactin p150(Glued) plays an important role in the functional integrity of the keratinocyte microparasol.

The Quest for the Mechanism of Melanin Transfer

Skin pigmentation is accomplished by production of melanin in specialized membrane-bound organelles termed melanosomes and by transfer of these organelles from melanocytes to surrounding keratinocytes. The mechanism by which these cells transfer melanin is yet unknown. A central role has been established for the protease-activated receptor-2 of the keratinocyte which effectuates melanin transfer via phagocytosis. What exactly is being phagocytosed - naked melanin, melanosomes or melanocytic cell parts - remains to be defined. Analogy of melanocytes to neuronal cells and cells of the haemopoietic lineage suggests exocytosis of melanosomes and subsequent phagocytosis of naked melanin. Otherwise, microscopy studies demonstrate cytophagocytosis of melanocytic dendrites. Other plausible mechanisms are transfer via melanosome-containing vesicles shed by the melanocyte or transfer via fusion of keratinocyte and melanocyte plasma membranes with formation of tunnelling nanotubes. Molecules involved in transfer are being identified. Transfer is influenced by the interactions of lectins and glycoproteins and, probably, by the action of E-cadherin, SNAREs, Rab and Rho GTPases. Further clues as to what mechanism and molecular machinery will arise with the identification of the function of specific genes which are mutated in diseases that affect transfer.

Melanosome transfer to and translocation in the keratinocyte

Complexion coloration in humans is primarily regulated by the amount and type of melanin synthesized by the epidermal melanocyte. However, additional and equally contributing factors consist of (1) efficient transfer of melanin from the melanocytes to the neighboring keratinocytes and (2) distribution and degradation of the transferred melanosomes by the recipient keratinocytes. Once synthesized in the cell body of the epidermal melanocyte, pigmented melanosomes are translocated down the dendrites and captured at the dendritic tips via various cytoskeletal elements. Molecules recently identified that participate in this process consist of Rab27a, myosin-Va and melanophilin. Eventually, these peripherally localized melanosomes are transferred to keratinocytes by a presently undefined mechanism. The protease-activated receptor-2 (PAR-2) and unidentified surface lectins and glycoproteins facilitate this transfer process. Once incorporated into the keratinocytes, melanosomes are distributed individually or as clusters, aggregated towards the apical pole of the nucleus, and degraded as the keratinocytes undergo terminal differentiation and desquamation. Ultraviolet irradiation (UVR) can modulate the process of melanosome transfer from the melanocytes to the keratinocytes. UVR can upregulate expression of PAR-2 and lectin-binding receptors and increase phagocytic activity of cultured keratinocytes. Therefore, many cellular and molecular events that occur after melanogenesis contribute to skin color.

Role of cytoplasmic dynein in perinuclear aggregation of phagocytosed melanosomes and supranuclear melanin cap formation in human keratinocytes

Cytoplasmic dynein is a microtubule-associated motor molecule involved in the retrograde transport of membrane-bound organelles. To determine whether the supranuclear melanin cap of transferred, phagocytosed melanosomes in keratinocytes is associated with cytoplasmic dynein, we performed immunofluorescent confocal microscopy on human keratinocytes in situ. We identified the intermediate chain of cytoplasmic dynein by immunoblotting and examined its distribution by confocal microscopy in relation to microtubules and melano-phagolysosomes in vitro. We also used antisense and sense oligonucleotides of the cytoplasmic dynein heavy chain 1 (Dyh1) and time-lapse and microscopy. The intermediate chain of cytoplasmic dynein was identified in extracts of human foreskin epidermis and in isolated human keratinocytes. The intermediate chain localized with the perinuclear melano-phagolysosomal aggregates in vitro and the supranuclear melanin cap in situ. Antisense oligonucleotides directed towards Dyh1 resulted in dispersal of the keratinocyte perinuclear melano-phagolysosomal aggregates after 24 to 48 h, whereas cells treated with diluent or sense oligonucleotides maintained tight perinuclear aggregates. Taken together, these findings indicate that in human keratinocytes, the retrograde microtubule motor cytoplasmic dynein mediates the perinuclear aggregation of phagocytosed melanosomes, participates in the formation of the supranuclear melanin cap or "microparasol" and serves as a mechanism to help protect the nucleus from ultraviolet-induced DNA damage.

Influx and efflux of amphetamine and N-acetylamphetamine in keratinocytes, pigmented melanocytes, and nonpigmented melanocytes

To establish an in vitro model of drug incorporation into hair and to elucidate the potential roles of hair cell selectivity and hair color in the incorporation of certain drugs into hair, the basic drug amphetamine and its nonbasic analog N-acetylamphetamine (N-AcAp) were analyzed for influx and efflux into and out of keratinocytes, pigmented melanocytes (PM), and nonpigmented melanocytes (NPM) as a model for incorporation and efflux of these drugs from hair cells. NPM were of the same melan-a cell line as PM, but cultured in the presence of the tyrosinase inhibitor phenylthiocarbamide. Results show that PM take up large amounts of the basic drug amphetamine (levels of uptake dependent on melanin content), whereas keratinocytes and NPM take up only small amounts of amphetamine. None of the cells take up N-AcAp above background levels. Interestingly, whereas keratinocytes and NPM quickly efflux most of the influxed drug, PM are slow to efflux and only efflux approximately 65% of influxed drug, if efflux media is not refreshed. (If efflux media is periodically refreshed, PM will eventually redistribute essentially all influxed drug back into the media.) These results demonstrate that pigmented cells take up greater amounts of the basic drug amphetamine, and efflux it more slowly than nonpigmented cells. Also, these results are consistent with previous data for in vivo incorporation of amphetamine in animal hair. In combination with previous data, an overall comparison of the amphetamine and N-AcAp incorporation data support a non-diffusion mediated model for drug incorporation into hair cells.

Influence of alpha-melanocyte-stimulating hormone and ultraviolet radiation on the transfer of melanosomes to keratinocytes

The epidermal melanin unit in human skin is composed of melanocytes and keratinocytes. Melanocytes, located in the basal layer of the epidermis, manufacture melanin-loaded organelles called melanosomes. Through their dendritic processes, melanocytes distribute melanosomes to neighboring keratinocytes, where their presence confers to the skin its characteristic color and photoprotective properties. In this study, we used murine melanocytes and keratinocytes alone and in coculture to characterize the processes involved in melanosome transfer. Ultraviolet (UV) radiation induced an accumulation of melanosomes in melanocytes, whereas treatment with a-melanocyte-stimulating hormone (MSH) induced exocytosis of melanosomes accompanied by ruffling of the melanocyte membrane. We found that keratinocytes phagocytose melanosomes and latex beads equally well and that this phagocytic process was increased by exposure of keratinocytes to UV radiation or to MSH. Coculture of melanocytes and keratinocytes resulted in an increase in MSH released to the medium. Gene array analysis of MSH-treated melanocytes showed up-regulation of many genes associated with exocytosis. In our studies, we never observed cytophagocytosis of melanosome-filled processes. This result, together with the other findings, suggests that a combination of signals that increase melanosome production and release by melanocytes and that stimulate phagocytosis by keratinocytes are the most relevant mechanisms involved in skin tanning.

Keratinocytes play a role in regulating distribution patterns of recipient melanosomes in vitro

Melanosomes in keratinocytes of Black skin are larger and distributed individually whereas those within keratinocytes of Caucasian skin are smaller and distributed in clusters. This disparity contributes to differences in skin pigmentation and photoprotection, but the control of these innate distribution patterns is poorly understood. To investigate this process, cocultures were established using melanocytes and keratinocytes derived from different racial backgrounds and were examined by electron microscopy. Melanosomes transferred to keratinocytes were categorized as individual or in various clusters. Melanosome size was also determined for individual and clustered melanosomes. Results indicate that, in our model system, melanosomes in keratinocytes from different racial backgrounds show a combination of clustered and individual melanosomes. When keratinocytes from dark skin were cocultured with melanocytes from (i) dark skin or (ii) light skin, however, recipient melanosomes were individual versus clustered in (i) 77% vs 23% and (ii) 64% vs 36%, respectively. In contrast, when keratinocytes from light skin were cocultured with melanocytes from (iii) dark skin or (iv) light skin, recipient melanosomes were individual versus clustered in (iii) 34% vs 66% and (iv) 39% vs 61%, respectively. These results indicate that recipient melanosomes, regardless of origin, are predominantly distributed individually by keratinocytes from dark skin, and in membrane-bound clusters by those from light skin. There were also differences in melanosome size from dark or light donor melanocytes. Melanosome size was not related to whether the melanosomes were distributed individually or clustered, however, in cocultures. These results suggest that regulatory factor(s) within the keratinocyte determine recipient melanosome distribution patterns.

Role of cytoplasmic dynein in melanosome transport in human melanocytes

Cytoplasmic dynein is a microtubule-associated retrograde-directed motor molecule for transport of membrane-bound organelles. To determine whether cytoplasmic dynein is expressed in melanocytes, we performed reverse transcriptase polymerase chain reaction using melanocyte cDNA and primers complementary to human brain cytoplasmic dynein heavy chain. A polymerase chain reaction product of the expected molecular size was generated and the identity was confirmed by sequence analysis. Western blotting of total melanocyte proteins reacted with an anti-intermediate chain cytoplasmic dynein antibody identified the appropriate 74 kDa band. To determine whether cytoplasmic dynein plays a role in melanosome transport, duplicate cultures were treated with cytoplasmic dynein antisense or sense (control) oligodeoxynucleotides and the cells were observed by high-resolution time-lapse microscopy, which allows visualization of melanosomal aggregates and individual melanosomes. Antisense-treated melanocytes demonstrated a strong anterograde transport of melanosomes from the cell body into the dendrites, whereas melanosome distribution was not affected in sense-treated melanocytes. To determine whether ultraviolet irradiation modifies cytoplasmic dynein expression, melanocyte cultures were exposed to increasing doses of solar-simulated irradiation, equivalent to a mild to moderate sunburn exposure for intact skin. Within 24 h, doses of 5 and 10 mJ per cm2 induced cytoplasmic dynein protein, whereas doses of 30 mJ per cm2 or more were associated with decreased levels of cytoplasmic dynein compared with sham-irradiated controls. Our data show that cytoplasmic dynein participates in retrograde melanosomal transport in human melanocytes and suggest that the altered melanosomal distribution in skin after sun exposure is due, at least in part, to decreased cytoplasmic dynein levels resulting in augmented anterograde transport.

Chimeric human epidermal reconstructs to study the role of melanocytes and keratinocytes in pigmentation and photoprotection

Chimeric epidermal reconstructs made with Negroid melanocytes and Caucasoid keratinocytes (or vice versa) were studied before and after UVB irradiation to understand the respective roles of these cells in tanning and photoprotection, especially lipoperoxidation and enzymatic defences against free radicals. Using this approach, we have confirmed overall the theory of the epidermal melanin unit. We have also shown that melanocytes of poorly tanning Caucasoids, which have a comparatively higher content of unsaturated fatty acids in their cell membrane, are more prone to the peroxidative effects of UV light, and that keratinocytes participate in photoprotection via phototype-dependent antioxidant enzyme activities, especially for catalase.

Melanin

Melanin is an irregular light-absorbing polymer containing indoles and other intermediate products derived from the oxidation of tyrosine. Melanin is widely dispersed in the animal and plant kingdoms. It is the major pigment present in the surface structures of vertebrates. The critical step in melanin biogenesis is the oxidation of tyrosine by the enzyme tyrosinase. In vertebrates this enzyme is active only in specialized organelles in retinal pigment epithelium and melanocytes. In mammals melanin is formed as intracellular granules. Melanin granules are transferred from melanocytes to epithelial cells and form the predominant pigment of hair and epidermis. Melanin has many biological functions. Reactive quinone intermediates in the melanin biosynthetic pathway exhibit antibiotic properties and the polymer is an important strengthening element of plant cell walls and insect cuticle. Light absorption by melanin has several biological functions, including photoreceptor shielding, thermoregulation, photoprotection, camouflage and display. Melanin is a powerful cation chelator and may act as a free radical sink. Melanin is used commercially as a component of photoprotective creams, although mainly for its free radical scavenging rather than its light absorption properties. The pigment is also a potential target for anti-melanoma therapy.

Ex vivo study of skin phototypes

We studied skin phototypes ex vivo to validate a model of epidermal reconstruction with melanocytes. We made autologous epidermal reconstructs with keratinocytes and melanocytes of healthy donors of skin phototypes I to VI. Keratinocytes and melanocytes were seeded on a dead de-epidermized dermis (Pruniéras type) at a 1:20 melanocyte/keratinocyte ratio. Reconstructed epidermis was grown for 15 d at the air-liquid interface with or without ultraviolet B irradiation. A macroscopic, chromometric. histologic, and ultrastructural evaluation was performed. Reconstructs reproduced the initial phototype with few modifications. The intensity of melanin transfer correlated with the in vivo situation and was stimulated after ultraviolet B irradiation in reconstructs of all categories of skin phototypes.

Three modes of melanosome transfers in Caucasian facial skin: hypothesis based on an ultrastructural study

The transfer mechanism of melanosome from the melanocyte into the keratinocyte was investigated in mildly photodamaged Caucasian facial skin by electron microscopy. Three ways of transfer are suggested by our observations. The first mechanism probably occurs through the following process: 1) protrusion and insertion of the thick dendrite of the melanocyte into the basal keratinocyte, 2) formation of sac-dendrite complex in the subnuclear region, 3) digestion and segregation of the enclosed dendrite, 4) formation of the cistern in the paranuclear region, and 5) pinching-off of the melanosomes in single or aggregated form from the tip of the cistern. The second mechanism probably takes place through a membrane fusion between the melanocyte and the keratinocyte. Such a membrane fusion possibly forms a passage way for release of the melanosome from the former cell to the latter. The third mechanism is considered to include exocytosis of the single melanosome from the melanocyte followed by the endocytosis through the formation of coated-pit in the keratinocyte.

Neural-tube-derived melanocyte subsets undergo commitment to their distinct lineages in culture

Neural-crest-derived melanocytes populate two anatomical sites in the chicken, the epidermis of regenerating feathers and the uveal tract of the eyes. These two anatomical populations of melanocytes differ morphologically and functionally. Morphologically, feather and uveal melanocytes synthesize structurally different pigment granules (melanosomes). Feather melanosomes are rod-shaped, 0.2 x 0.8 micron, whereas uveal melanosomes are larger and more oval, 0.6 x 0.9 micron. Functionally, feather melanocytes continuously synthesize melanosomes during feather regeneration, and transfer these melanosomes to neighboring keratinocytes. Ocular melanocytes, on the other hand, synthesize melanosomes until their cytoplasm becomes congested with melanosomes, at which time the melanocytes become melanogenically dormant and do not transfer granules to neighboring cells. Cultures of melanocytes established from neural tubes of Light Brown Leghorn chick embryos produce two populations of melanocytes containing small (0.45 micron) or larger (0.90 micron) melanosomes which resemble the two types described in situ. Both types of melanocytes emigrate from along the entire length of the neural tube during several embryonic stages. Melanocyte cultures developed from neural tubes of the Recessive White breed of chicken, which has tyrosinase-negative, feather melanocytes and pigmented, functionally normal uveal melanocytes, also develop a mixture of amelanotic and pigmented melanocytes which maintain their respective characteristics even after separation by flow cytometry and reculture. These findings suggest that epidermal and uveal melanocytes are two distinct sub-populations of melanocytes whose commitment to separate lineages can occur in culture in the absence of their respective target tissue environment.

Molecular basis of mouse Himalayan mutation

Many different coat-colors result from the c-locus mutation in the mouse. One of these interesting mutants is a Himalayan, which produces temperature sensitive tyrosinase, and the basis of this sensitivity remains unknown. We cultured Himalayan mouse melanocytes from the skin and constructed a cDNA library; then, we isolated the Himalayan tyrosinase cDNAs and determined the nucleotide sequence. The tyrosinase gene in the Himalayan mouse contains an A----G change at nucleotide 1259 that alters a histidine residue to an arginine residue at amino acid 420. This histidine residue and the surrounding amino acids are conserved in their evolution from mouse to human. Interestingly, the residue with its surrounding eight amino acids are aligned between mouse b-protein and human tyrosinase. These results indicate the possibility that the altered residue at amino acid 420 of mouse tyrosinase may be important in stabilization of the tyrosinase molecule, or in interaction with other molecules, such as tyrosinase inhibitors.

Morphologic alterations of epidermal melanocytes and melanosomes in PUVA lentigines: a comparative ultrastructural investigation of lentigines induced by PUVA and sunlight

Ultrastructural studies were conducted in order to determine morphologic and functional differences in melanocytes and melanosomes in PUVA lentigines and solar lentigines, and light-protected buttock skin. Compared to melanocytes in solar lentigines from 7 subjects and light-protected buttock skin from 5 subjects (none of these subjects had received UV radiation therapy), melanocytes in PUVA lentigines from 6 subjects generally had longer and more numerous dendrites, and showed more active melanogenesis. Basal keratinocytes in PUVA lentigines had a significantly increased frequency of large, single melanosomes, and revealed significantly larger individual melanosomes within compound melanosomes. Other findings in some PUVA lentigines included the close apposition of Langerhans cells to melanocytes, and atypical nuclear, cytoplasmic and melanosomal alterations, including melanosomal pleomorphism and melanin macroglobules. The presence of relatively large and predominantly single melanosomes in basal keratinocytes of PUVA lentigines suggests more active melanogenesis and/or an irreversible somatic alteration. It will be important to determine the clinical course and ultrastructural findings of PUVA lentigines that persist long after PUVA is discontinued.

Control of melanin synthesis and secretion by B16/C3 melanoma cells

In culture, B16/C3 murine melanoma cells grown in the presence of serum undergo melanogenesis at a specific time after plating. At this time, melanin is synthesized intracellularly and then secreted into the extracellular culture fluid. We have found that melanin secretion is dependent on the presence of serum in the growth medium. When confluent cultures are deprived of serum, that is, refed with serum-free medium, cells remain viable but do not undergo melanogenesis. Addition of serum-free medium supplemented with either melanocyte-stimulating hormone (MSH) or dibutyryl cAMP induced melanogenesis in these cells but did not result in melanin secretion. Furthermore, when B16/C3 cells are grown in serum-free, hormone-supplemented medium, they also undergo melanogenesis but fail to release melanin. The addition of serum, however, to B16/C3 cells induced to undergo melanogenesis with MSH, dibutyryl cAMP, or hormone-supplemented medium promotes melanin secretion. Fractionation studies hence revealed that serum contains specific factors capable of inducing melanin secretion. These results demonstrate that factors that regulate melanin synthesis are distinct from those that induce cells to release melanin into their extracellular environment. Furthermore, the ability to induce melanogenesis with single factors will permit us to study the precise sequence of events leading to differentiation in B16/C3 cells under chemically defined conditions.

The epidermal melanocyte system in individuals of Scandinavian origin, determined by DOPA-staining and TEM

The quantitative evaluation of DOPA-positive epidermal melanocytes in 16 patients of Scandinavian origin showed both individual and regional differences in the melanocyte count. Our data is in agreement with other published studies. The distribution in the number of melanocytes varies significantly in some specimens. This is due partly to the preparation procedure and partly to normal biological variations. We believe that we have demonstrated a cyclic function of the melanocyte in the epidermis. The varying density of cells in epidermal sheets as well as their varying morphology support the theory concerning the presence of the epidermal melanin unit.

Hyperpigmentation after bleomycin therapy. Ultrastructural study

Pigmentation in a Causasian male, resulting from bleomycin therapy for Hodgkin's disease, has been studied ultrastructurally. The melanocytes, though present in normal numbers, showed several abnormalities; the endoplasmic reticulum and the Golgi apparatus were were well developed and the mitochondria were enlarged. Lipid inclusions in the endoplasmic reticulum and numerous autophagocytic vacuoles, some containing lipids were observed. Transfer of melanosomes to keratinocytes appeared to be increased. The melanosomes, which measured less than 0.55 mu were dispersed in the cytoplasm and did not form complexes, as has been observed with nitrogen mustard. The increase of melanocytic activity and the disturbance of melanosome transfer are discussed.

Use of scanning electron microscopy for the study of human epidermal melanocytes

A method is presented which makes it possible to study melanocytes in situ in the human epidermis by means of scanning electron microscopy. Melanocytes are located both under and wedged between the basal epidermocytes. The dendrites describe a short course in the dermo-epidermal (d-e) junction and then ascend and disappear into the spaces between the epidermocytes. As a rule, the surface of the cells is smooth. However, proliferations such as blebs and microvilli can sometimes be observed on the surface, particularly on that of the round, adendritic cells. Contact between the melanocytes in the d-e junction is only occasionally seen.

Transfer mechanism of melanosomes in epidermal cell culture

The mode of melanosome transfer from melanocytes to keratinocytes in epidermal cell cultures has been examined with time-lapse cinematography and electron microscopy. A tip of a melanocyte dendrite containing melanosomes became enfolded by a recipient keratinocyte. It was then pinched off to form a cluster of melanosomes which initially seemed to be surrounded by two layers of membranes. The phagocytized dendrite was gradually decomposed and became an aggregate of melanosomes surrounded by a single membrane of the keratinocyte. The individual melanosomes were dispersed from the aggregate into the keratinocyte cytoplasm, depending on the size of melanosomes. The larger ones were single and smaller ones were complex. The mechanism of melanosome transfer in vitro is a type of cytophagocytosis. The entire process consists of two steps: the first is a cytophagic process and the second a melanosome dispersion process. The process is influenced by various exogenous factors.

Trends in electron microscopy of skin

Some trends in electron microscopy of skin have emerged and should be pursued in the future. The fine structure and some basic cellular reaction patterns of epidermal cells are discussed to illustrate the interplay of morphologic, cytochemical, and tracer studies. Intracytoplasmic membranes and secretory granules, lysosomes and endocytic mechanisms, cytomembranes and cell surface specialization are discussed to show how these can be used to arrive at a more meaningful interpretation of structure. Despite all advances, however, a great deal more needs to be done before the details of skin structure are completely elucidated.

Some aspects of melanin biology: 1950-1975

Recent advances in the biology of mammalian pigmentation are reviewed. The multicellular epidermal melanin unit (melanocyte and associated pool of keratinocytes) rather than the melanocyte alone forms the focal point for melanin metabolism within mammalian epidermis. Within an epidermal melanin unit, melanosomes are synthesized by melanocytes and transferred to keratinocytes where they are degraded as they ascend to the epidermal surface. During the past 25 years, technical advances in biology and biochemistry have frosted a multidisciplinary approach to research on mammalian pigmentation. Emphasizing this perspective, we have examined the current state of knowledge of the form and function of epidermal melanin units from the levels of biologic organization ranging from the molecules relevant to melanin synthesis through the skin as a totally intergrated system. To an unusual degree, advances in melanin pigmentation have resulted from the integration of clinical medicine and basic science.

Ultrastructural localization of acid phosphatase in normal and X-irradiated epidermis

Acid phosphatase activity has been studied in normal and X-irradiated rat epidermis. A remarkable reaction increment was found in irradiated epithelium at the granular layer keeping the same pattern of distribution observed in the normal tissue. No reactive lysosomes or keratinosomes were observed in the cytochemical sections, while many keratinosomes but no lysosomes appeared in the electron microscopic preparations of irradiated animals. Irradiated epithelium is discussed as a suitable model to analyze the role of acid phosphatase in the keratinization mechanisms.

Congenital circumscribed hypomelanosis: a characterization based on electron microscopic study of tuberous sclerosis, nevus depigmentosus, and piebaldism

Subcellular defects of hypomelanosis in tuberous sclerosis (TS) (28 subjects) were compared by light and electron microscopy with oThere forms of congenital circumscribed hypomelanosis that occur in nevus depigmentosus (ND) (8 subjects) and in piebaldism (PB) (4 subjects), respectively. On the light microscopic level in both TS and ND, the population density of functioning melanocytes was normal but each perikaryon was small, and dopa activity was decreased. On the ultrastructural level, the hypomelanotic skin and hair of TS were associated with a decrease in the synthesis, melanization, and size of melanosomes; the decrease in the size of melanosomes resulted in the aggregation of melanosomes (i.e., a melanosome complex) in the keratinocytes in all the specimens examined. In ND, ther were no obvious changes in the size and melanocytes. the hypomelanosis of ND is related to the decreased synthesis and also, perhaps, abnormal transfer of melanosomes. In PB the hypomelanosis of the skin and hair results from the absence of functional melanocytes. The hypermelanotic areas of PB, however, characteristically contain melanocytes that synthesize abnormal (sperical and granular) as well as normal (ellipsoidal and lamellar) melanosomes.