Melanosomes in dermal Schwann cells of human and rodent skin

Alterations in the epidermal-dermal melanin axis and factor XIIIa melanophages in senile lentigo and ageing skin

BACKGROUND

Senile lentigo (SL) is a pigmentation disorder that occurs predominantly on the dorsa of the hands, the forearms and the face; its incidence increases with age. Histological hallmarks of SL lesions are hyperpigmentation of the epidermis and elongation of the epidermal rete ridges. Various factors such as alpha-melanocyte-stimulating hormone, endothelin-1 or stem cell factor are involved in the onset and maintenance of the increased pigmentation. Alterations of the dermal compartment have not yet been analysed in detail in SL.

OBJECTIVES

To study the occurrence and distribution of melanin in the dermis from SL and aged skin, biopsies from 12 subjects were morphologically analysed by light and electron microscopy in comparison with unaffected skin.

METHODS

Punch biopsies of SL and adjacent skin from 12 male or female volunteers aged 52-81 years were prepared for light and electron microscopy and samples were analysed by morphological, morphometric, histochemical and immunohistochemical methods.

RESULTS

The epidermis from SL revealed morphological features such as hyperpigmentation of basal keratinocytes and the formation of elongated rete ridges. S100+ melanocytes in the stratum basale were not markedly increased, indicating that the hyperpigmentation is predominantly due to changes in melanin synthesis, distribution or turnover. Quantification of epidermal cells expressing the proliferation marker Ki67 did not show an increase of this parameter in SL, indicating that at least in the established lesion cell proliferation is not enhanced. We further focused on the dermal compartment and observed granulated cells which were more abundant in SL. Electron microscopic and histochemical analysis revealed that the granulation of these cells is based on melanosomes, mostly present in large melanosomal complexes. Immunohistochemistry using antibodies to CD68 and factor XIIIa (FXIIIa) showed these melanophages to be predominantly FXIIIa+ dermal dendrocytes, which were about six times more abundant than CD68+ macrophages.

CONCLUSIONS

In SL an increased number of melanophages was found compared with unaffected skin from the same subject. These melanophages were identified as FXIIIa+ dermal dendrocytes. Possible functional consequences of the massive melanin uptake by dermal dendrocytes are discussed.

Intradermal melanocytic nevus with prominent schwannian differentiation

Features of peripheral nerve sheath differentiation such as neuroid cords, nerve corpuscles, fascicle-like structures, and, exceptionally, palisading have been reported in melanocytic nevi. We report an intradermal melanocytic nevus with prominent Verocay-like bodies. The upper portion of the neoplasm was composed of typical round intradermal nevus cells, many of which were pigmented. Within the deeper portion, there was a nonpigmented spindle cell proliferation with prominent Verocay bodies, simulating a neurilemmoma. Typical nevus nests merged with neurilemmoma-like areas. The entire lesion stained positively for S-100 and Mart-1 proteins and negatively for HMB-45 stain. Diffuse Mart-1 positivity excluded a collision of a melanocytic lesion with a neurilemmoma. The histopathologic features of this nevus further support a close relation between nevus cells and Schwann cells.

The relationship between melanocytes and peripheral nerve sheath cells (Part I): melanocytic nevus (excluding so-called "blue nevus") with peripheral nerve sheath differentiation

Among thousands of specimens of melanocytic nevi, not including giant congenital melanocytic nevus or blue nevus, 42 melanocytic nevi that showed peripheral nerve sheath differentiation were collected. The patterns of melanocytic nevi with peripheral nerve sheath differentiation may be classified into three groups: 1) "neurotized and neural nevi" with nests of "neuroid cords" and "nevic corpuscles" (the most common pattern); 2) nerve fascicle-like structures with no relation to neurotized and neural nevi; and 3) palisading melanocytes of a nevus in nests of conventional melanocytic nevi (a rare pattern). Each pattern may represent a different expression of nerve sheath differentiation in melanocytic nevi. Some melanocytic nevi with nerve fascicle-like structures show discrete structures closely resembling authentic nerve fascicles, confirming a close relationship between melanocytes and peripheral nerve sheath cells (Schwann cells and probably perineurial cells in part) and suggesting derivation of the two types of cells from common precursor cells of the neural crest and their de novo development in the dermis rather than by Abtropfung of melanocytes from the epidermis. In addition, the high prevalence of Unna, Miescher, and superficial congenital nevi in melanocytic nevi with peripheral nerve sheath differentiation suggests a different character or process for these congenital melanocytic nevi than for Clark and Spitz nevi (junctional and compound types).

Existence and distribution of melanocytes and HMB-45-positive cells in the human minor salivary glands

The existence and distribution of melanocytes in the human minor salivary glands were investigated in a series of autopsy and biopsy materials. The cells with the following characteristics were regarded as melanocytes; spindle-shaped or dendritic cells with fine granular pigments: (i) stained brownish-black by hematoxylin-eosin stain, and black by Masson-Fontana's silver impregnation method; and (ii) disappeared after treatment with peroxide and potassium permanganate solution. In addition, the expression of antigen identified by anti-HMB-45 antibody in serial sections with melanocytes was examined. Melanocytes were found in eight (1.8%) of 445 cases, and there was no relationship between the existence of melanocytes and significant diseases of the subjects. Various numbers of melanocytes were distributed in fibrous tissue around the interlobular ducts, intralobular ducts and acini, but were not in direct contact with the epithelia. Neither melanocytes nor melanin granules were found in the salivary gland epithelia. HMB-45-positive cells without intracytoplasmic fine granules were found solitarily or in small groups in periductal and periacinar fibrous tissues with or without slight infiltration of small mononuclear cells.

Pathology and incidence of amelanotic melanomas of the skin in F-344/N rats

A total of 121 spontaneous amelanotic melanomas of the skin were identified in 70 of 11,171 male and 51 of 10,927 female Fischer-344/N rats in 63 2-yr carcinogenicity studies conducted by the National Cancer Institute's Carcinogenicity Testing Program/National Toxicology Program. Amelanotic melanomas had characteristic anatomical locations and histologic features distinguishable from Schwann cell tumors. Of the 121 tumors, 84, 19, 10, and 8 cases occurred in the pinna, eyelid, scrotum, and perianal region (anus and tail), respectively. Amelanotic melanomas originated from the dermis and consisted of spindle cells arranged in an interlacing fascicular pattern often with a perivascular orientation; epithelioid cells were rarely seen. Only the tumors arising in the pinna metastasized to the lung and/or mandibular lymph nodes. The metastatic rate was 19% (16/84) of the tumors and was clearly increased with an increase in tumor size. Most metastasizing tumors had focal areas consisting of anaplastic spindle cells with an increased number of mitosis. The tumor cells stained positive for S-100 protein but negative for melanin. Ultrastructurally, the tumors were diagnosed as amelanotic melanomas based on the identification of numerous, intracytoplasmic premelanosomes without melanin formation in the tumor cells which were not enveloped by pericytoplasmic basal laminae. One localized amelanotic melanoma of the pinna was successfully transplanted to the subcutaneous tissue in the flank of 3 Fischer-344/N rats.

Foci of stromal melanocytes (so-called blue naevus) of the uterine cervix in Japanese women

Foci of stromal melanocytes (FSM) of the uterine cervix have been known as extra-cutaneous "blue naevus". However macroscopic and histological findings suggest that FSM of the cervix are analogous to dermal melanocytosis, rather than to cutaneous blue naevus and the lesions are more appropriately called stromal melanocytosis. FSM of the cervix have been considered rare, but our study showed that they are not uncommon in Japanese women occurring in 8.6% (42/486). The lesions were initially observed in the third decade of life and became most prevalent in the fifth decade (15/86 cases, 17.4%). In stroma of the cervix, stromal melanocytes (SM) were present where many peripheral nerve fibres were seen. SM of the cervix were positive for S-100 protein in immunohistochemical studies and were sometimes observed close to peripheral nerve fibres. Melanocytes were never observed in the ectocervical and endocervical epithelium, but only in the stroma of the cervix. We suggest that malignant melanoma of the uterine cervix may originate from SM.

Neural crest origin of clear cell sarcoma of tendons and aponeuroses. Ultrastructural and enzyme cytochemical study of human and nude mouse-transplanted tumours

In order to clarify the histogenesis of clear cell sarcoma of tendons and aponeuroses (CCS), two cases of human and one nude mouse-transplanted CCS line were studied using an ultrastructural and enzyme cytochemical approach. Most of the tumour cells obtained from the primary and transplanted CCS demonstrated melanosomes in various stages of development within the cytoplasm, whereas no melanosomes could be identified in the metastatic CCS. However, cholinesterase and tyrosinase activities could be demonstrated not only in the melanotic primary and transplanted CCS but also in the amelanotic metastatic CCS. The results therefore support the hypothesis that CCS is a soft tissue tumour derived from the neural crest.

Melanin-forming Schwann cells in some oral mucosal lesions

Normal oral mucosa and some mucosal lesions show melanin pigmentation, and it is usually assumed that melanin-containing cells in the lamina propria are melanophages. In a study of oral mucosa from two cases of lichen planus and tow of sublingual keratosis, many brown, intracellular, Masson-Fontana-positive granules were seen. At the ultrastructural level some of the melanin-containing cells displayed characteristics of Schwann cells rather than melanophages. These cells were surrounded by a basal lamina and contained unmyelinated axons. Although the majority of the melanin granules in these cells were compound melanosomes, single melanosomes and premelanosomes were also seen. These findings show that in sublingual keratosis and oral lichen planus lesions some of the melanin-containing cells in the connective tissue may be melanin-forming Schwann cells.

Induction of melanogenesis in Schwann cell and perineural epithelium by 9,10-dimethyl-1,2-benzanthracene (DMBA) and 12-o-tetradecanoylphorbol-13-acetate (TPA) in BDF1 mice

Six-week-old female BDF1 mice were treated with a single topical application of DMBA followed by repeated application of TPA on the clipped dorsal skin. Several weeks after DMBA application, the intradermal melanocytes of perifollicular melanocytic network began to proliferate to form melanocytic tumors in all treated mice skin. Besides these changes, single membrane-bound melanosomes and premelanosomes were found in the cytoplasm of perineural epithelia and Schwann cells with mesaxons of the nerve bundles involved, and the melanogenic activity of the Schwann cell and the perineural epithelium of the dermal peripheral nerve bundle was discussed in terms of a common feature of neuro-ectodermal cells.

Melanocytes in the nasal cavity and paranasal sinus. Incidence and distribution in Japan

Random, nonselected tissue specimens from 99 Japanese-20 cylindrically cut nasal blocks removed during autopsy (A.C., Autopsy Cases) and 79 cases removed during surgery, consisting of 32 chronic sinusitis cases (C.S.) and 47 nasal polyp cases (N.P.)-were examined histopathologically and electronmicroscopically with respect to distribution and frequency of melanocytes in the nasal cavity and paranasal sinus. Malignant melanoma cases were excluded. The overall incidence of positive cases for melanocytes revealed 21.2% (21 of 99 cases), with an incidence ratio of male to female of 0.9:1.0. Melanocytes were found beginning in the under 19 age group with incidence increasing proportionately with age. Peak incidence was in the 50-year age group at 50%. Melanocytes and melanotic cell foci were distributed in the stroma of the propria mucosa beneath the pseudostratified columnar epithelium and focused around the nasal and paranasal glands and sinuses. In 2 of the 21 cases positive for stromal melanocytes, intraepithelial melanocytes with dendritic processes were found. The histogenesis of malignant melanoma arising from the nasal cavity and paranasal sinus are also discussed in this study.

Distribution of laminin and collagen type-IV in benign and malignant lesions of melanocytic origin

Using antisera for two specific basement membrane (b.m.) antigens such as laminin and collagen type-IV, together with electron microscopy, we have shown that fully antigenic b.m. and morphologically typical basal lamina (b.l.) are associated with normal and transformed cells of the melanocyte lineage in different ways. Thus, while b.m. and b.l. surround individual choroidal melanocytes, intradermal nevus cells and cells of blue nevi are not detectable at the periphery of resting and proliferating epidermal melanocytes. They have a low degree of expression with a heterogeneous pattern of distribution in primary and metastatic melanoma. This heterogeneity is present within single metastases and among autologous metastases. These findings indicate that the presence of b.m. can be an additional marker for cells of the melanocyte lineage and should be considered when applying serological means for the detection and control of neoplasms of melanocytic origin.

Canine malignant melanotic schwannomas: a light and electron microscopic study of two cases

Malignant, melanotic schwannoma of the spinal cord was diagnosed in two two-year-old dogs. The neoplastic spindle cells were arranged in a herringbone or storiform pattern; the intracytoplasmic melanin varied in amount in different areas of the two neoplasms. Ultrastructural features including cytoplasmic lamellar premelanosomes and melanosomes, rudimentary cell junctions, scattered pinocytotic vesicles, and remnants of external lamina supported the diagnosis of melanotic schwannoma.

Primary melanocytic schwannoma of the stomach. Evidence of dual melanocytic and schwannian differentiation in an extra-axial site in a patient without neurofibromatosis

Pigmented neoplasms of the peripheral nervous system are uncommon. Such lesions, while well documented in the para-axial region, are infrequently encountered in more peripheral extra-axial sites. The authors present a case of a melanocytic schwannoma arising in the gastric antrum of a 51-year-old woman. Ultrastructural features attesting to the nerve sheath origin of the neoplasm included the presence of a well-developed basal lamina, complex infoldings of the plasma membrane, and intercellular bundles of long spaced collagen ("Luse bodies"). Premelanosomes and melanosomes in all stages of development were identified within the cytoplasm of many of the neoplastic cells, providing morphologic support for a close relationship between Schwann cells and melanocytes. Immunoperoxidase studies with S-100 protein disclosed intense cytoplasmic staining throughout the tumor. The S-100 protein antigen has been documented in both nerve sheath elements and melanocytes; this shared antigenicity lends further support to the close relationship between melanocytic and schwannian elements suggested by ultrastructural observations. Additional immunoperoxidase studies employing antibody to glial fibrillary acidic protein revealed focal cytoplasmic staining, an uncommon phenomenon in peripheral nerve sheath neoplasms. A review of the literature failed to provide any previous well documented cases of melanocytic schwannoma of the stomach. The patient was treated with en bloc resection of the tumor and gastric antrum, and was free of evidence of recurrent or metastatic disease 22 months postoperatively. Theories regarding the histogenesis of pigmented nerve sheath neoplasms and prognostic implications are discussed, based on experience with this and previously reported pigmented nerve sheath tumors.

Pigmentary incontinence in fixed drug eruptions. Histologic and electron microscopic findings

Pigmentary incontinence is a phenomenon observed in some inflammatory skin disorders. Clinically it may be seen as a slate-colored pigmentation. Histologically it is seen as an accumulation of melanin in the upper dermis. The possible mechanism for development of pigmentary incontinence is discussed based on a review of the literature and electron microscopic studies of fixed drug eruption.

The ultra-structure of a melanotic schwannoma of the skin

The case is described of a 53-year-old female with a pigmented tumour of the skin of the foot. The light and electron-microscopic findings established that the tumour was a melanotic Schwannoma. The ultra-structural features indicated that the Schwann cells, which contained melanosomes in varying stages of maturation, were producing melanin. A second type of cell within the tumour with features of a mesenchymal cell also contained melanosomes. The interrelationships of nerve sheath tumours, their relationship to pigment cells of neural crest origin and to other pigmented tumours of uncertain histogenesis, is discussed in the light of the findings described in the paper.

Spinal melanotic clear-cell sarcoma: a light and electron microscopic study

We report a melanotic spindle-cell tumor that arose from a thoracic spinal nerve root and metastasized to both lungs. The bulk of the tumor consisted of glycogen-rich clear cells and bore a striking resemblance on light and electron microscopy to at least one variant of the clear-cell sarcoma of tendons and aponeuroses. The presence of schwannoma-like areas noted in the primary tumor on light microscopy and the formation of a highly developed basal lamina noted on ultrastructural examination suggest that the tumor may be a partially dedifferentiated malignant melanotic schwannoma. This tumor is discussed in the context of a simple histogenetic classification of melanotic tumors.