**5. Etiology of pigment cell disorders**

Skin color is a very important social and cultural human characteristic. Even a minor alteration of cutaneous pigmentation can result in a major esthetic concern, with psychological implica‐ tions. Skin color is primarily due to the pigment melanin, which is produced by melanocytes, and normal pigmentation is dependent on the normal structure and function of these cells. Any defect affecting the complex process of skin pigmentation may result in pigmentary disorder, which may be either (a) hyperpigmentary or (b) depigmentary/hypopigmentary.

#### **5.1. Pathology and ultrastructure of hyperpigmentary disorders**

Hyperpigmentary skin disorders may be defined as enhanced pigmentation of the skin above the normal level. Such increase in melanin production may occur in the epidermis, dermis, or both. Epidermal hyperpigmentary disorders exclusively occur due to the increase of melanin pigment and are characterized by "brown hyperpigmentation" because of their brown color. They may be associated with (melanocytic epidermal hyperpigmentation) or without (mela‐ notic epidermal hyperpigmentation) an actual increase in the melanocytes number. On the other hand, in dermal hyper pigmentary diseases, collectively known as "blue hyperpigmen‐ tation" or circuloderma, either melanin pigments (melanin dermal hyperpigmentation) or nonmelanin pigments (non-melanin dermal hyperpigmentation) are involved [95]. Generally hyperpigmentary disorders can be congenital or acquired [96].

#### **a. Melanotic Melanoma**

the locus coeruleus, and ultimately gets accumulated in the human substantia nigra with age [88]. Several researches have favored the view that neuromelanins have a defensive role by binding/elimination of reactive oxygen species (ROS) along with metals that would otherwise be extremely toxic to neurons [89]. In a recent study, it has been reported that nearly all brain tissues hold significant quantity of neuromelanin, which is supposed to play crucial tasks in

Interestingly, it has been observed that melanocytes are also situated in the valves and septa of heart [91]. It was further hypothesized that the origin of cardiac melanocytes may take place from the same precursor population as that of skin melanocytes because of its dependency on the same signaling molecules which is needed for the appropriate development of skin melanocytes [92], but their exact function in this location is still unclear. The synthesis of melanin is not always beneficial, either in heart or in other tissues, for example, in the lungs where in an unusual ailment known as Lymph angioleio myomatosis (LAM) [93], the muscle cells slip back into their developmental stages along with the expression of melanocytes like specific markers Pmel17, tyrosinase, etc. The resulting production and accumulation of

Randhawa *et al.* [94] reported that the biosynthesis of melanin also occurs in the adipose tissue of morbidly obese humans. It was also assumed that the ectopic synthesis of melanin in the cytosol of obese adipocytes possibly provide a balanced mechanism to work as anti-inflam‐ matory factor as well as it decreases the oxidative damage. In the course of enhanced deposition of cellular fat, adipocytes become more exposed to endogenous apoptotic signals especially with ROS, which could be counteracted by ectopically produced melanin. In addition, adipocytic melanin may also suppress the secretion of proinflammatory molecules [3].

Skin color is a very important social and cultural human characteristic. Even a minor alteration of cutaneous pigmentation can result in a major esthetic concern, with psychological implica‐ tions. Skin color is primarily due to the pigment melanin, which is produced by melanocytes, and normal pigmentation is dependent on the normal structure and function of these cells. Any defect affecting the complex process of skin pigmentation may result in pigmentary disorder, which may be either (a) hyperpigmentary or (b) depigmentary/hypopigmentary.

Hyperpigmentary skin disorders may be defined as enhanced pigmentation of the skin above the normal level. Such increase in melanin production may occur in the epidermis, dermis, or both. Epidermal hyperpigmentary disorders exclusively occur due to the increase of melanin pigment and are characterized by "brown hyperpigmentation" because of their brown color. They may be associated with (melanocytic epidermal hyperpigmentation) or without (mela‐ notic epidermal hyperpigmentation) an actual increase in the melanocytes number. On the other hand, in dermal hyper pigmentary diseases, collectively known as "blue hyperpigmen‐

reducing organ toxicity [90].

274 Muscle Cell and Tissue

melanin in lung tissues is eventually lethal.

**5. Etiology of pigment cell disorders**

**5.1. Pathology and ultrastructure of hyperpigmentary disorders**

The incidence of malignant melanoma is continuously increasing worldwide. The detailed fine structure of melanotic melanoma cells studied by Wellings *et al.*[97] was of the pale melanocyte variety. It has been reviewed by Polnikorn [98] that there is increased number of epidermal and follicular melanocytes (approx. 30%) in the melanotic lesions. Melanotic melanocytes were either round, elliptical or at times irregular and possessed numerous cytoplasmic microvilli. The ratio of the nucleus to the cytoplasm was low. The nuclei, which contained light chromatin, were usually ovoid, with a definite indentation on one side, thus giving the cell an appearance of polarity. Some of the nucleoli contained masses of fine granules and had clear round areas within them. The endoplasmic reticulum consisted of slender sac-like forms with lamellae and vesicles, which infrequently were slightly swollen. The endoplasmic reticulum often was found surfaced with ribosomes, but some de-granulated forms of it were also observed. The Golgi apparatus and centrioles occupied an extensive part of the cytoplasm of most cells and usually were located immediately adjacent to the indented part of the nucleus, when such was present. In the immediate vicinity of the Golgi area, numerous vesicles, premelanosomes and immature melanosomes at various stage of development, dense areas of ribosomes and many mitochondria were present; however, melanosomes in various stages of development also were observed in areas of the cytoplasm far from the Golgi apparatus [99].

The intermediate vesicle, which originated in the Golgi apparatus, underwent changes which ultimately led to the fully-developed melanosome. The most striking feature of pigmented melanoma cells of humans was the occurrence of numerous melanosomes that varied consid‐ erably in size and shape, particularly the spheroidal melanosomes with a diameter ranging between 200 nm and 500 nm. Some of the melanosomes showed ellipsoidal profiles and melanosomes showing a granular structure were common, whereas lamellar melanosomes were seen less frequently. As the melanosome became completely electron dense due to the deposition of melanin granules, the pigment tended to disturb the arrangement and to leak into the cytoplasm [100]. The cultures of pigment cell lines were observed to have lower growth rate and viability in comparison to the non-pigmented cell lines. Extremely pigmented cells died and discharged their distinct melanin granules into the medium. The "amelanotic" melanoma cells were examined to have minimal synthesis as well as faulty development of premelanosomes. Also, it was noticed that they did not possess the distinct fibrillar structure and had less deposition of melanin as reported in the "melanotic" melanoma cells. It seems that the formation of discrete fibrillar pattern may be the fundamental step for the normal synthesis and deposition of melanin along with the development of melanosomes [101].

#### **5.2. Pathology and ultrastructure of hypopigmentary disorders**

Hypopigmentary/depigmentary disorder of the skin is characterized by the reduction/ complete loss of skin pigmentation. Usually they may be of two types: (a) melanopenic (melanocytopenic), which is related to the melanin pigment, and (b) non-melanopenic, which is not related to the melanin pigment. Melanopenic hypopigmentation mainly occurs due to the disruption in the complex process of skin pigmentation. Non-melanopenic hypopigmen‐ tary disorders may be associated with anemia, edema, and Raynauld's phenomenon.

#### **a. Vitiligo**

Vitiligo is a hypopigmentary disorder of the skin in which cutaneous and ocular melanocytes are selectively destroyed that result in loss of pigmentation. It affects 1–2% of the population including both sexes and all races equally [102]. Multiple theories have been proposed, including genetic, neural, biochemical, viral, and autoimmune mechanisms. However, an autoimmune mechanism has been proposed as the most accepted cause of vitiligo. In previous studies, it has been demonstrated that most important feature in vitiligo is alternation of the melanocyte ratio at the dermal-epidermal junction [103,104]. The long dendritic, melanin granules filled, dopa positive melanocytes were found to be prominent in the outer peripheries of vitiligo lesions [105].

Study conducted by Mohamed and El-Saman [106] on vitiligo patients using light and electron microscopy, showed complete loss of melanin pigment granules in the epidermis, mononu‐ clear cellular infiltration in the dermis, and marked positive ICAM-1 expression over kerati‐ nocytes in the epidermis and around endothelial and inflammatory cells in the dermis of the vitiligo sections. The above authors have reported that the biopsies of vitiligous skin samples exhibited significant ultrastructural changes of degenerative nature in the keratinocytes. This was conspicuous by the presence of electron dense cytoplasm with vacuolization and frag‐ mentation of keratin tonofilaments. Large, irregular indented nuclei were also observed in the biopsies. Besides, the cytoplasm of keratinocytes becomes completely deficient of melano‐ somes in contrast with the control group. Dilated intercellular spaces (edema) and loss of intercellular junctions with or without remnants were also observed. Complete absence of melanocytes and obvious presence of lymphocytes in the basal layer and dermis were evident. These findings give strong fine structural evidence that the vitiligo leads to the degenerative changes in the structure of melanocytes.

We have recently shown that active ingredients of plants like *psoralea corylifolia,* (psoralen), *nigella sativa* (thymoquinone), *piper nigrum* (piperine), and *withania somnifera* (withaferin), *berberis vulgaris* (berberine) all have powerful melanogenic (skin darkening) properties and are excellent activators of melanosomal receptors, which when properly stimulated can cause skin darkening in some of the animal models studied in our laboratory [107–112]. Whether these have a similar role in mammalian melanocytes and specific cell lines is yet to be determined at the cellular level. Therefore, it could be an interesting possibility to further explore the herbal ingredient-activated melanosomal receptor signaling cross talk within the melanogenetic pathways, where different receptors participate in skin pigmentation and its dysfunctions.
