&p.1:Abstract Four cases of trichoblastoma rich in Merkel cells (MCs) are reported. They occurred in two men and two women, with ages ranging from 58 to 76 years (mean 67.5 years). MCs were detected immunohisto- chemically with antibodies to keratin 20, chromogranin A and neuron-specific enolase (NSE). In an attempt at better definition of the nature and role of MCs in tricho- blastoma, the distribution of MCs in normal adult and fe- tal skins obtained at autopsy was studied. In addition, ten cases of sebaceous naevus of Jadassohn (NSJ) were eval- uated along similar lines. MCs made up 2–20% of the tu- mour cells in trichoblastomas; they were present in nor- mal fetal skin and were rare in normal adult skin. All but one of the cases of NSJ showed numerous positive cells in the epidermal component of the lesion with all three antibodies. Six basal cell carcinomas and one syringo- cystadenoma papilliferum associated with NSJ were neg- ative with keratin 20, chromogranin A and NSE antibod- ies, whereas a minute trichoblastoma arising against the same background was positive for these markers. Hair follicle cell tumours may recapitulate the skin embryo- genesis, as numerous MCs are present in fetal follicles, but only occasional such cells are seen in adult skin. &kwd:Key words Merkel cell · Trichoblastoma · Fetal skin · Normal adult skin&bdy: Introduction Trichoblastomas are rare tumours of the skin, which may occur as solitary dermal neoplasms or in association with sebaceous naevus of Jadassohn (NSJ) [1]. These were subdivided by Headington [8] into four different catego- ries, trichoblastoma, trichoblastic fibroma, trichogenic trichoblastoma and trichogenic myxoma, according to the appearance of the stromal component. Subsequently, Ackerman et al. [1] proposed the unifying term of tricho- blastoma to encompass cutaneous tumour types with cir- cumscribed margins and a predominance of follicular germinative cells. It has been reported that trichoblastoma can be associ- ated with NSJ [1], and these lesions have been shown to be composed of follicular germ cells intermingled with Merkel cells (MCs) [21]. The aim of the present study is to describe four cases of trichoblastoma rich in Merkel cells (propositus cases). In addition, and in an attempt at better definition of the nature and the role of MCs in these lesions, the presence and distribution of MCs were evaluated in normal fetal and adult skins and also in cases of NSJ (reference cases). Materials and methods Propositus cases The cases studied were obtained from the files of the Institutes of Anatomic Pathology of the University of Bologna at Bellaria Hos- pital and of Modena (1 case). All tissue specimens were fixed in 4% formaldehyde and em- bedded in Paraplast. Sections were stained with haematoxylin and eosin (H&E), PAS with diastase control, and Alcian blue. One case was studied ultrastructurally (case 1) after retrieval from for- malin-fixed tissue. The material was post-fixed in osmium tetraox- ide, dehydrated, and embedded in epoxy resin. Ultrathin sections were stained with uranyl acetate and lead citrate and viewed with a Zeiss M10 electron microscope. Immunohistochemistry was performed in all cases using the avi- din–biotin–peroxidase complex [9]. Commercially available anti- bodies were used. Their sources and dilutions are listed in Table 1. G. Collina · V. Eusebi ( ✉ ) 1 Department of Oncology, Section of Anatomic Pathology “M. Malpighi”, at Bellaria Hospital, University of Bologna, Bologna, Italy C. Capella Department of Clinical and Biological Sciences, University of Pavia at Varese, Italy J. Rosai Department of Pathology, Memorial Sloan-Kettering Cancer Center, New York, USA Mailing address: 1 Sezione di Anatomia, Istologia e Citologia Patologica “M. Malpighi”, Ospedale Bellaria, Via Altura, 3, I-40139 Bologna, Italy Tel.: +39-51-6225750, Fax: +39-51-6225759&/fn-block: Virchows Arch (1998) 433:291–296 © Springer-Verlag 1998 ORIGINAL ARTICLE &roles:Guido Collina · Vincenzo Eusebi · Carlo Capella Juan Rosai Merkel cell differentiation in trichoblastoma &misc:Received: 26 February 1998 / Accepted: 22 April 1998