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In man, mutations of the megalin-encoding gene causes the rare Donnai-Barrow/Facio-Oculo-Acoustico-Renal Syndrome, which is partially characterized by high-grade myopia. Previous studies of renal megalin function have established that megalin is crucial for conservation of renal filtered nutrients including vitamin A; however, the role of megalin in ocular physiology and development is presently unknown. Therefore, we investigate ocular megalin expression and the ocular phenotype of megalin-deficient mice. Topographical and subcellular localization of megalin as well as the ocular phenotype of megalin-deficient mice were examined with immunological techniques using light, confocal and electron microscopy. We identified megalin in the retinal pigment epithelium (RPE) and non-pigmented ciliary body epithelium (NPCBE) in normal mouse eyes. Immunocytochemical investigations furthermore showed that megalin localizes to vesicular structures in the RPE and NPCBE cells. Histological investigations of ocular mouse tissue also identified a severe myopia phenotype as well as enlarged RPE melanosomes and abnormal ciliary body development in the megalin-deficient mice. In conclusion, the complex ocular phenotype observed in the megalin-deficient mice suggests that megalin-mediated developmental abnormalities may contribute to the high myopia phenotype observed in the Donnai-Barrow Syndrome patients and, thus, that megalin harbors important roles in ocular development and physiology. Finally, our data show that megalin-deficient mice may provide a valuable model for future studies of megalin in ocular physiology and pathology.

Purpose: Mutations in the megalin-encoding gene, LRP2, cause high myopia as seen in patients suffering from Donnai-Barrow/facio-oculo-acoustico-renal syndrome. Megalin is present in both the nonpigmented epithelium of the ciliary body and in the RPE. In this study, we set out to establish an animal model to study the mechanisms underlying the ocular phenotype and to establish if high myopia/megaophthalmos is induced by postnatal megalin-deficiency in the RPE.Methods: Postnatal RPE-specific deletion of megalin was generated by crossing mice bearing a homozygous loxP-flanked Lrp2 allele with transgenic mice expressing the Cre recombinase driven by the BEST1 promotor. The model was investigated by immunohistologic techniques, and transmission electron microscopy.Results: Mice with postnatal RPE-specific loss of megalin developed a megaophthalmos phenotype with dramatic increase in ocular size and severe retinal thinning associated with compromised vision. This phenotype was present at postnatal day 14, indicating rapid development in the period from onset of BEST1 promotor activity at postnatal day 10. Additionally, RPE melanosomes exhibited abnormal size and morphology, suggested by electron tomography to be caused by fusion events between multiple melanosomes.Conclusions: Postnatal loss of megalin in the RPE induces dramatic and rapid ocular growth and retinal degeneration compatible with the high myopia observed in Donnai-Barrow patients. The morphologic changes of RPE melanosomes, believed to be largely inert and fully differentiated at birth, suggested a continued plasticity of mature melanosomes and a requirement for megalin to maintain their number and morphology.

Congenital hypertrophy of the retinal pigment epithelium (CHRPE). Solitary and grouped CHRPE consist of a single layer of hypertrophied RPE cells packed with large round pigment granules consisting of macromelanosomes.

BACKGROUND: Macromelanosomes are melanin-containing granules characterized by their large size and spherical or ellipsoidal morphology. They are reported to be present in a variety of pigmented skin lesions, including lentigines. However, there is limited information on macromelanosomes in malignant melanocytic proliferations. The margins of a lentigo maligna/malignant melanoma in situ (LM/MMIS) and solar lentigo share morphological similarities, including lentiginous proliferation of melanocytes, increased melanin in basal cell layer keratinocytes, and solar elastosis. This may represent a potential diagnostic pitfall, particularly in small biopsies. We sought to identify whether the presence of macromelanosomes at the lesional margins of LM/MMIS may represent a morphological clue in distinguishing between these 2 entities. METHODS: Data were obtained from 2 different institutions between 2001 and 2010. 619 cases of solar lentigo and 117 cases of LM/MMIS were screened for the presence and distribution of macromelanosomes. RESULTS: 25 of 117 cases (21%) of LM/MMIS had macromelanosomes identified at the lesional margins, compared with 7 of 619 (1%) of solar lentigo cases (P < 0.0001). CONCLUSIONS: Our data demonstrate that the identification of macromelanosomes within a lentiginous melanocytic proliferation should prompt further evaluation to rule out the possibility of a contiguous LM/MMIS, particularly in a small biopsy.

金沢大学医薬保健研究域保健学系, Melanosomes in sporadic dysplastic nevi were examined by transmission electron microscope. In particular the structure of macromelansomes was observed. Macromelanosomes were present in both nevus cells and keratinocytes. They were round and consisted of a core structure and a cortex structure. The core was composed of electron-dense amorphous material and electron-lucent microvesicles. The cortex contained grains of different sizes and electron densities. Occasionally small melanosomes were observed in the cortex. Electron-lucent microvesicles were also present in small spherical and ellipsoid melanosomes. Melanosome complexes observed in nevus cells contained degrading melanosomes with electronlucent microvesicles, dispersed microvesicles and grains. Melanosome complexes with structures resembling those found in macromelanosomes were also present. The result of the investigation suggests that melanosome complexes found in nevus cells might become macromelanosomes through autophagy.

A patient developed an Addison-like melanoderma over the past decade. An abnormal neuropeptide related to, but bigger than pro-opiomelanocortin was found in large amount in the serum. It has most likely a stimulative action upon melanogenesis with formation of macromelanosomes.

As part of a combined genetic, electrophysiological, clinical and biochemical study in albinism, the ultrastructure of the epidermis of 47 albinos was investigated. On account of the amount of pigmented granules in the keratinocytes, the presence of distinct maturational stages of melanosomes, the occurrence of abnormal melanosomes in the melanocytes and keratinocytes and the packing of melanosomes in the keratinocytes, three groups can be distinguished. (1) Non-pigmented: the keratinocytes of these patients contain only premelanosomes stage I and II and completely lack pigmented granules. (2) Variably pigmented: the keratinocytes of these patients contain stage IV melanosomes in variable number. Premelanosomes stage II and Iii can be present. (3) Pigmented and macromelanosomes: the epidermis of these patients is characterized by the occurrence of macromelanosomes. In addition, the keratinocytes contain aggregated and non-aggregated melanosomes.Comparison of the ultrastructural observations with the clini...

Multiple pigmented lesions including benign lentigines, typical nevi, and dysplastic nevi from 3 patients with the dysplastic nevus syndrome contained giant melanosomes (GM). Ultrastructurally the GM were composed of microvesicles, but lacked limiting membrane and filamentous internal structure. As such, they resembled the GM found in 4 other patients with dysplastic nevus syndrome, leopard syndrome and ocular albinism, but differed from those found in the café-au-lait spots of neurofibromatosis, nevus spilus, vitiliginous achromia, and certain animals.

Two cases of autoptically observed congenital hypertrophy of the retinal pigment epithelium (CHRPE) in a newborn and in an infant, one with electron photomicrographs, are presented. In contrast to the adjacent RPE cells, the apical cellular surface of CHRPE cells shows only rare small microvilli, mostly at the cell border. The prominent findings are large pigment granules from 3.5 to 8 microns protruding towards the apical cellular surface. These macromelanosomes do not exhibit coalescence as noted by others, but show a concentric lamellar structure that may be the result of a disturbed development sequence in melanin granule formation. The authors' observation of CHRPE in a newborn and in an infant suggests that the term congenital HRPE is appropriate. The presence of these changes at this early age is consistent with the condition being a primary failure of differentiation. Finally, the authors present evidence of a disturbed melanogenesis in CHRPE cells resulting in macromelanosomes formation.

Unusual changes in the melanin pigmentary system were observed on a warty papule biopsy taken from a patient with xeroderma pigmentosum (XP). Degenerated melanocytes full of lipids were observed from the basal layer up to the middle of the epidermis. The melanosomes were polymorphous, variable in size and shape with very strange aspects, such as spider-like and whirling configurations. The structure of the macromelanosomes suggests that they are large autophagosomes. These findings are discussed and compared with previous studies.

The nature and significance of macromelanosomes, or giant melanosomes, in various pigmented skin lesions are characterized in this paper in terms of a) their morphological features, b) the specificity for their occurrence, and c) the possible mechanisms responsible for their formation. These granules occur most frequently in the skin and eyes of persons with x-linked ocular albinism of the Nettleship-Fall type, in the café-au-lait macules of neurofibromatosis, and in the pigmented macules of xeroderma pigmentosa. In these genetic disorders, skin biopsy for detection of the macromelanosomes could be valuable for confirming the diagnosis and for establishing who in asymptomatic families are carriers of the disorders. The macromelanosomes seem not to be formed solely by autophagic degradation of melanosomes, as has been proposed recently in studies of melanocytic nevi and lentigo simplex where aggregation of the macromelanosomes and melanosomes often do occur through autophagocytosis. The macromelanosomes in these genetic disorders may be formed by a more complex process in which the melanocytes are coded for aberrant melanogenesis with formation of precursor vacuoles similar to those of pheomelanosomes and with continuous accumulation of vesiculoglobular bodies. We suggest that vesiculoglobular bodies are crucial to the formation of macromelanosomes.

Skin biopsies of café-au-lait macules from 34 patients with a clinical diagnosis of classical neurofibromatosis were examined histologically and ultrastructurally to determine the presence or absence of macromelanosomes in the epidermal melanocytes and keratinocytes. Sixteen of the 34 patients had macromelanosomes. The presence of macromelanosomes varied with age and ethnic background; they were detected in nine of 12 Whites, six of 10 persons of mixed ancestry, and one of two Blacks. In these populations skin biopsy is useful in early diagnosis of neurofibromatosis. However, none of 10 persons of Indian stock had macromelanosomes. Their total absence in this group may be indicative of genetic heterogeneity.

This study compared the fine structure of macromelanosomes with that of giant melanosome complexes formed through melanosomal autophagocytosis in nevocytes and melanocytes of nevocellular nevi, lentigo simplex and malignant melanoma. While macromelanosomes were found only on rare occasions in these pigmentary disorders [2 of 79 nevocellular nevi (2 junctional nevi), 3 of 12 lentigo simplex and 2 of 93 malignant melanoma], the giant autophagic melanosome complexes were always present, indicating the macromelanosomes are not synthesized simply through melanosomal autophagocytosis. Although both macromelanosomes and giant melanosome complexes exhibited acid phosphatase activity similar to melanosomes, they showed many different ultrastructural features. Characteristically, macromelanosomes contained numerous vesiculoglobular bodies, whereas these bodies were absent in giant melanosome complexes. In those tissues where the presence of macromelanosomes had been ruled out by light microscopy, none of the giant melanosome complexes revealed ultrastructural features indicative of macromelanosomes. Various phases of melanosomal degradation were seen, indicating that they were not simply end-products of lysosomal degradation of melanosomes. It was thought that the key process in the development of macromelanosomes was the accumulation of vesiculoglobular bodies.

A Japanese male with Nettleship-Falls X-linked ocular albinism (NXOA) and associated with Axenfeld's anomaly is described here. The diagnosis of X-linked ocular albinism was difficult because of moderately pigmented fundus and lack of iris translucency, but macromelanosomes recognized by histological study of skin biopsy were helpful for diagnosis. This is the first report identifying the macromelanosomes in Japanese NXOA. A combination of ocular albinism and Axenfeld's anomaly is very rare. There might be a common defective factor for the development of these two congenital disorders.

We noted nonmelanized and partially melanized macromelanosomes in a cellular blue nevus and studied their light microscopic and ultrastructural features. Numerous intracytoplasmic eosinophilic inclusions were found in the lesion; individual cells contained up to nine, although most cells demonstrated two or three. The "macromelanosomelike" inclusions ranged in size from 1 to 15 microns. Most of them were partially melanized, and some were nonmelanized. These globules were periodic acid-Schiff positive and diastase resistant, with a centrally melanized core, best seen with the Fontana-Masson technique. S100 protein stained positively the cytoplasm of some nevus cells but not the inclusions. Electron microscopy confirmed the centrifugal melanization of these structures and their targetlike morphologic characteristics. Nine other cellular blue nevi from our files failed to show similar intracytoplasmic inclusions.

Recent reports have suggested that the diagnosis of neurofibromatosis can be made by finding giant pigment granules in dihydroxyphenylalanine-incubated epidermal whole mounts from cafe au lait spots. Our studies indicate that these "macromelanosomes" may not be found in the hyperpigmented macules in all cases of neurofibromatosis and that they may be less common in children. Failure to find such granules should not preclude a diagnosis of neurofibromatosis.