The immediate and delayed darkening of skin exposed to a single sunlight exposure is an acute response to UV radiation. On the other hand, freckling, mottled pigmentation, melasma and lentigo on the sun-exposed skin of the elderly are responses to chronic solar exposure. The most common pigmented lesions in sun-exposed skin include ephelides (freckling), melasma, lentigo, mottled pigmentation and pigmented seborrheic keratosis, etc. It has been reported that the patterns of pigmentary change that occur are dependent upon gender in Koreans (6). Lentigo increases with age, and is more frequent in women than men. On the other hand, seborrheic keratosis also increases with age, and this is more common in men (6).
Changes of melanocytes and pigmentation in photoaged skin
The number of dopa-positive melanocytes in human skin decreases with age by approximately 10–20% with each decade in both habitually sun-exposed and protected skin (56, 57). Despite decreased melanocyte density with aging, photoaged skin shows irregular pigmentation and, frequently, hyperpigmentation. This may be due to a higher level of dopa activity in chronically irradiated melanocytes. Heterogeneity of skin color in exposed areas of elderly skin is due to an uneven distribution of pigment cells, a local loss of melanocytes, and modified interactions between melanocytes and keratinocytes (58).
In the photoaged skin of Caucasians, the density of dopa-positive melanocytes was roughly two-fold higher than in the nonexposed skin at all ages (from 28 to 80 years), suggesting the irreversible effect of sun exposure. In the case of the melanocyte population, chronic sun exposure does not accelerate aging, but rather appears to have a net stimulatory effect on exposed melanocytes (59). This phenomenon may be an adaptive response of habitually exposed skin against the damage caused by cumulative sun exposure.
In Korean brown skin, we have also found that the number of melanocytes in sun-exposed facial skin is greater than that of sun-protected buttock skin (Fig. 4b). In Koreans, the number of melanocytes decreased with aging in sun-protected buttock skin, as in Caucasians. In Korean photoaged facial skin, DOPA activity was greater and the number of melanocytes higher with aging (Fig. 4b). The amount of melanin pigment in the sun-exposed Korean skin is greater than that in sun-protected skin on an individual basis (Fig. 4c). Melanin pigment tends to decrease slightly in sun-protected buttock skin with aging, and tends to localize mainly in the basal cell layer. On the other hand, in sun-exposed skin, the melanin pigment appears to increase with aging, and to extend to the upper spinous layers beyond the basal cell layer (Fig. 4c).
Figure 4. Aging- and photoaging-dependent changes of melanocytes and melanin pigments in human skin in vivo: (a) wrinkle grade, (b) dopa stain, (c) Fontana-Masson stain.
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Melasma is an acquired irregular brown or gray-brown hypermelanosis of the face and occasionally of the neck and forearm. The etiology is attributable to sunlight, genetic predisposition and pregnancy. Different clinical forms of melasma are recognized, and facial patterns may be either centrofacial, malar or mandibular. Histologically, increased pigment may be situated epidermally, dermally or at both sites (61).
In East Asia, males and females seem to be equally affected (3), and although all races are affected, there is a particular prominence among Latinos, particularly of Caribbean origin, and among Asians.
Seborrheic keratoses (SKs) are common in white races, and males and females are equally affected (65, 66). It has also been reported that SKs are less common in populations with dark skin, such as the Africans and brown-skinned Asians (67, 68). These results suggest the influence of racial and ethnic differences in the prevalence of SKs. However, these lesions are a common problem in Asians. The color of these lesions can vary from a pale brown with pink tones to dark brown or black. In brown skin, most lesions seem to be brown to dark brown. Seborrheic keratosis is not a pigmentary disorder, but a benign tumor. However, because most seborrheic keratosis in Asians is usually pigmented, patients and some investigators regard them as a pigmentation problem. SKs have diverse clinical and histopathological characteristics, and are observed in 80–100% of those over 50 years (65, 69–72).
The etiology of SKs is not well understood, though they are known to show familial traits with an autosomal dominant pattern in those with a large number of lesions (73). Human papilloma virus has been suggested to be a possible cause, because of the verrucous appearance of the lesions, but no definite causal relationship has been established despite a significant amount of research (74).
Although there is still some debate, exposure to sunlight has been suggested to be a risk factor for SKs (69), and they have been reported to show a higher prevalence on exposed areas than intermittently exposed areas (66, 69). Exposure to sunlight has been suggested to play a role in the development of lesions in those predisposed to develop SKs (66, 69).
However, no well-designed study has been undertaken to investigate the clinical characteristics of SKs in Asians. Recently, Kwon et al. (66) investigated the clinical characteristics of SKs in Korean males and the relationship between SKs and sunlight exposure. SKs are believed to be less common in more pigmented skin populations (67, 68). However, such a belief may be the result of a lack of a systematized clinical study in the Asian or black-skinned populations. It has been reported that 81–100% of Caucasian males over the age of 40 with white skin have at least one SK (69, 70, 75). Similarly, the prevalence of SKs in Korean males has also been related to age; i.e., rising from 78.9% at 40 years, to 93.9% at 50 years and to 98.7% in those over 60 years (66). These results show that SKs are as common in Korean males as in the white population. Although the prevalence of SK may be similar in Asians and Caucasians, the number of SKs in Korean males appears to be less than in Caucasian males on an individual basis.
On exposed areas, body surface sites such as the face, neck and dorsum of the hands show significant increases in the prevalence of SKs by decade, while, on partly exposed areas, it was found to increase with age, but without statistical significance (66). When the estimated body surface area (BSA) is taken into account, the numbers of SKs on both the face and on the dorsum of hands were over-represented compared with the trunk. They were also concentrated on the neck and v-area. The outer forearms also showed three-fold more SKs, in terms of BSA, than neighboring the inner forearms, which are classified as partly exposed areas (66). These findings suggest that the development of SKs concentrates at exposed areas.
After controlling for many variables including age, smoking and skin types, subjects with a sun-exposure history of more than 6 h per day showed a 2.3-fold increased risk of severe SKs compared with those exposed for less than 3 h per day (66). SK lesions on sun-exposed skin were found to be usually smaller than on partly exposed skin, especially in the younger age group. Moreover, SKs on exposed skin grow significantly with aging, while those on partly exposed skin tend to remain the same size. On the other hand, the number of SKs was greater on exposed skin than on partly exposed skin. Therefore, the total area occupied by SKs was greater on exposed skin than that on partly exposed skin (66). These results suggest that sunlight causes multiple small-sized SKs, and that it also plays a role in the development and growth of SKs (66).
In summary, SKs are very common in Korean males and may be one of the major pigmentary problems. SKs are concentrated on the sun-exposed skin, especially on the face and on the dorsum of the hands. Both aging and lifetime cumulative sunlight exposure are independent contributory factors and may work synergistically.