Distribution Of S-100 Protein Research Articles

A new approach to discovery of S100 protein heterodimers.

S100 proteins are a sub-family of EF-hand calcium binding proteins that form very similar, unique dimer structures. Although the possibility has been known for many years, no systematic study of the formation of S100 protein heterodimers has been reported. Shaw and coworkers demonstrate the application of a new approach that provides an in-depth evaluation of the distribution of S100 protein homo- and heterodimer and show that the prevalence of heterodimers may be much higher than previously anticipated.

Progressive macular hypomelanosis pathogenesis and treatment: a randomized clinical trial

BackgroundProgressive macular hypomelanosis (PMH) is a cosmetically disturbing skin disorder that is poorly understood with indefinite treatment. The aim of the present study was to clinically outline PMH and study its pathogenesis. Based upon the literature suggesting improvement of PMH with antibiotic therapy to decrease Propionibacterium acnes (P. acnes) colonization, different treatment modalities were tried to reach the best treatment option. Patients and methodsThis study included 12 newly diagnosed PMH selected patients who attended Tanta University Hospital outpatient clinic of Dermatology and Venereology from June 2009 to March 2010. Patient's lesions were subjected to wood's lamp and potassium hydroxide (KOH) examination as well as biopsies used in histological, immunohistochemical, bacteriological and electron microscopic examination. A randomized clinical trial with different treatment modalities was applied. ResultsThe patient's mean age was 25±10.8 with significant female predominance (P=0.0001). Reduction of epidermal melanosomes and tyrosine activity together with difference in distribution of S100 proteins in hypopigmented skin was demonstrated. Abnormal distribution of tonofilaments was noticed inside keratinocytes with increased apoptosis. P. acnes were detected in hair follicles of 83.3% hypopigmented lesions. Administration of local and systemic antimicrobial treatment with narrow band ultraviolet B (NBUVB) phototherapy for 3 months was the best treatment modalities. ConclusionThe etiology of PMH is multifactorial where genetic predisposition, the presence of P. acnes and hormonal imbalance play the main role. Administration of local and systemic antimicrobial treatment with NBUVB phototherapy for 3 months is an effective treatment regimen for PMH.

Innervation of the Circumvallate Papilla of the Cat Tongue

Immunohistochemically, the distribution of S-100 protein and acetylated tubulin-positive nerve fibres was studied in the circumvallate papilla and its taste buds (TB) in the adult cat. The immunostaining for acetylated tubulin demonstrated an extensive innervation of the circumvallate papilla. Vegetative ganglionic cells were found in the central area of papilla, whereas fine nerve fibres were concentrated under epithelium. Individual positive axons were seen in relation to TB. Nerve fibres enter the TB and branch out among the supporting and sensory cells. Some nerve fibres reach the apical surface of the TB, including the taste pore. Nerve fibres positive for S-100 protein were observed as dense nerve plexus located in the core of the papilla. Satellite cells localized inside the papilla were seen to surround the ganglionic cells. Bands of fine nerve fibres were present under lining epithelium, mainly at the base of the TB. A weak reaction was displayed by taste bud cells and surrounding epithelial cells as well as by the epithelial cells of the papillary side of the moat. A dense network of the nerve fibres was present among the glandular acini and surrounding the ducts of the serous Ebner's glands.

An immunohistochemical study of S-100 protein in the intestinal tract of Chinese soft-shelled turtle, Pelodiscus sinensis

The present work describes the distribution of S-100 protein in the intestinal tract of a Chinese soft-shelled turtle specimen ( Pelodiscus sinensis). S-100 protein positive cells were located in the intestinal tract, from the proximal small to distal large intestine. S-100 protein positive dendritic cells had irregular shape and were positive in both cytoplasm and nucleus. Most of them were located both lamina propria and submucosa in the small intestine, while few were found in the large intestine. S-100 protein, C-kit positive ICCs and Silver staining glial cells were predominantly observed in three locations: (1) in the interspace between the submucosa and circular muscle layer; (2) in the circular muscle layer; and (3) between the circular and longitudinal muscle layers of the intestine. Fewer were found in the large intestinal lamina propria and submucosa. Three types of positive cells (S-100 protein positive cells, C-kit positive ICCs and Silver staining glial cells) with 1–2 long or 2–3 short processes were distributed as lace-like or surrounding blood vessels in the different locations mentioned above. In the lamina propria, all the positive cells with irregular processes were connected with each other and formed a network. In the submucosa, all the positive cells were found surrounding the blood vessels.

Expression and distribution of S-100, CD83 and apoptosis-related proteins (Fas, FasL and Bcl-2) in tissues of thyroid carcinoma

Previous studies have shown that dendritic cells (DCs) and apoptosis play a critical role in the pathogenesis of thyroid carcinoma (TC). This study was designed to investigate the expression and distribution of S-100 protein, CD83 and apoptosis-related proteins (Fas, FasL and Bcl-2) in the thyroid tissues of thyroid papillary carcinoma (TPC) and their role in TPC pathogenesis. Immunohistochemical staining techniques and other methods were used on pathological tissues of 30 patients with Thyroid papillary carcinoma (TPC) and 30 cases of thyroid follicular adenoma (TFA,as control) to detect the expression and distribution of S-100 protein, CD83, Fas, FasL and Bcl-2. A higher expression of S-100 protein in TPC (4.6+/-3.2%) vs.TFA (0.95+/-0.64%) (p<0.001) was observed as well as a higher expression of CD83 in the peri-cancerous tissues (PCT) (32.51+/-22.32) vs. TFA (5.19+/-8.08) (p<0.001), oppositely, CD83 was negative in the cancerous net.TPC showed greater increases in levels of Fas, FasL and Bcl-2 than did the TFA. Our findings suggest that impaired immune function, absence of CD83-positive mature and activated dendritic cells in cancer nodules may have a role in the pathogenesis of thyroid papillary carcinoma.The regulation of Fas, FasL and Bcl-2 in TPC may help them evade the immune system.

Expression and distribution of S-100 protein, CD83 and apoptosis-related proteins (Fas, FasL and Bcl-2) in thyroid tissues of autoimmune thyroid diseases

Previous studies have shown that dendritic cells (DCs) and apoptosis-related proteins play a critical role in the pathogenesis of autoimmune thyroid diseases (ATD). This study was designed to investigate the expression and distribution of S-100 protein, CD83 and apoptosis-related proteins (Fas, FasL and Bcl-2) in the thyroid tissues of ATD and their role in ATD pathogenesis as determined by immunochemical staing techniques and other methods. Pathological tissues of 30 patients with Hashimoto's thyroiditis (HT), 30 patients with Graves' disease (GD) and 30 cases of thyroid follicular adenoma (TFA, as control) were used for this study. A higher expression of S-100 in HT (4.2+/-3.1%) and GD (3.9+/-2.8%) vs TFA (0.95+/-0.64%) (p<0.001). was observed as well as a higher expression of CD83 in HT (22.58+/-13.96% and GD (29.92+/-14.43%) vs TFA (5.19+/-8.08%) (p<0.001). HT thyrocytes adjacent to thyroid infiltrating lymphocytes (TILs) showed greater increases in the levels of Fas and FasL than did the GD thyrocytes while HT TILs exhibited lower expression of Fas and FasL than did the GD TILs. GD thyrocytes expressed increased levels of the antiapoptotic protein Bcl-2 as compared to the low levels detected in HT thyrocytes. An opposite pattern was observed in the TILs in GD (low expression of Bcl-2) and HT (high expression of Bcl-2). The findings suggest that the high expression of DC markers is related to the pathogenesis of HT and GD. Up-regulation of both the number and matured functions of DCs may lead to the presentation of more antigens to lymphocytes which are related to the development of autoimmune thyroid diseases. The regulation of Fas/FasL/Bcl-2 in GD favors apoptosis of infiltrating lymphocytes and thyrocyte survival. The regulation of Fas/FasL/Bcl-2 in HT may promote thyrocyte apoptosis leading to hypothyroidism.

Expression and distribution of S100 protein in the nervous system of the adult zebrafish (Danio rerio)

S100 proteins are EF-hand calcium-binding protein highly preserved during evolution present in both neuronal and non-neuronal tissues of the higher vertebrates. Data about the expression of S100 protein in fishes are scarce, and no data are available on zebrafish, a common model used in biology to study development but also human diseases. In this study, we have investigated the expression of S100 protein in the central nervous system of adult zebrafish using PCR, Western blot, and immunohistochemistry. The central nervous system of the adult zebrafish express S100 protein mRNA, and contain a protein of approximately 10 kDa identified as S100 protein. S100 protein immunoreactivity was detected widespread distributed in the central nervous system, labeling the cytoplasm of both neuronal and non-neuronal cells. In fact, S100 protein immunoreactivity was primarily found in glial and ependymal cells, whereas the only neurons displaying S100 immunoreactivity were the Purkinje's neurons of the cerebellar cortex and those forming the deep cerebellar nuclei. Outside the central nervous system, S100 protein immunoreactivity was observed in a subpopulation of sensory and sympathetic neurons, and it was absent from the enteric nervous system. The functional role of S100 protein in both neurons and non-neuronal cells of the zebrafish central nervous system remains to be elucidated, but present results might serve as baseline for future experimental studies using this teleost as a model.

Differential distribution of S100 protein and calretinin in mechanosensory and chemosensory cells of adult zebrafish ( Danio rerio)

Calcium-binding proteins play a critical role in vertebrate sensory cells, and some of them have been detected in mechanosensory and chemosensory cells of bony and cartilaginous fishes. In this study immunohistochemistry and Western blot were used to investigate the occurrence and the distribution of S100 protein and calretinin in mechanosensory (neuromasts of the lateral line system; maculae and cristae ampullaris of the inner ear) as well as chemosensory (superficial and oral taste buds; olfactory epithelium) cells in adult zebrafish ( Danio rerio). Specific protein bands with an estimated molecular weight of around 10 kDa and 30 kDa were detected by Western blot and were identified with S100 protein and calretinin, respectively. S100 protein and calretinin were observed segregated in mechanosensory and chemosensory cells, and the presence of S100 protein in a cell excluded that of calretinin, and vice versa. As a rule, the mechanosensory cells were S100 protein positive, whereas the chemosensory ones displayed calretinin immunoreactivity. Calretinin was also detected in nerve fibers supplying some of the investigated organs. In the olfactory epithelium, S100 protein immunoreactivity was present in the crypt olfactory sensory neurons, whereas calretinin immunoreactivity was widespread in olfactory sensory neurons and probably other olfactory cells. In this localization the co-expression of S100 protein and calretinin cannot be excluded. These results demonstrate the cell segregation of two specific calcium-binding proteins, and they enable to selectively label these cells by using easily reproducible immunohistochemical techniques associated to well-known antibodies.

Serum and mucosal S100 proteins, calprotectin (S100A8/S100A9) and S100A12, are elevated at diagnosis in children with inflammatory bowel disease

Objective. Various markers characterize the complex inflammatory processes seen in chronic inflammatory bowel disease (IBD) including calprotectin, a complex of two S100 proteins, which has been evaluated and validated as a faecal marker of inflammation. However, the systemic and mucosal expression patterns of calprotectin and related S100 proteins are not well characterized in this disease. The objective of this study was to assess serum and mucosal levels of calprotectin, S100A12 and soluble receptor for advanced glycation end products (sRAGE), a putative S100 ligand, in a paediatric population with IBD. Material and methods. Children were enrolled at diagnosis of IBD, along with groups of children without IBD. Standard inflammatory markers and disease activity scores were collated. Calprotectin, S100A12 and sRAGE levels in serum and biopsy culture supernatants were measured by ELISA and tissue distribution of S100 proteins was investigated by immunohistochemistry. Results. Serum and mucosal calprotectin and S100A12 levels were increased in children with IBD as compared with non-IBD controls. Serum calprotectin levels correlated with S100A12 levels and with disease activity scores in children with IBD. sRAGE levels were not increased in IBD. S100A8, S100A9 and S100A12 were abundantly expressed throughout the lamina propria and epithelium in inflamed mucosa. In contrast, these proteins were present in the lamina propria, but not the epithelium, in non-inflamed mucosa. Conclusions. Serum calprotectin and S100A12 are increased in children with IBD and indicate disease activity. Elevated levels of these proteins are present in the colonic mucosa and may contribute to the pathogenesis of IBD. Furthermore, an imbalance between sRAGE and S100A12 may contribute to inflammatory changes present in IBD.

Histiocytoma cutis: a tumour of dermal dendrocytes (dermal dendrocytoma)

The histogenesis of histiocytoma (dermatofibroma) was investigated using new antibodies that demonstrate factor XIIIa (FXIIIa) positive cells and the monocyte macrophage cell series (MAC 387), in formalin fixed tissue. The distribution of S100 protein, vimentin and Ulex europaeus agglutinin I (UEA-I) were also studied. The antibody against FXIIIa labelled the normal dermal population of fixed connective tissue cells (dermal dendrocytes) emphasizing their dendritic processes; cells that are widely distributed, but are most numerous in the papillary dermis. In contrast, the antibody, MAC 387 against monocyte derived macrophages, did not label this cell population. In 30 histiocytomas, intense labelling for FXIIIa was found peripherally, but labelling was rather weaker in cells situated centrally. Only a few cells labelled for S100 protein and with the monoclonal antibody MAC 387, but vimentin positivity was universal. UEA-I labelled only vessels in histiocytomas. However, FXIIIa labelling was negative in 16 dermatofibrosarcomas, three keloids and several other fibroproliferative lesions. These contrasting results with FXIIIa labelling have practical implications for diagnosis of benign and malignant spindle cell tumours of the skin. We conclude that histiocytomas (dermal dendrocytomas) take their origin from dermal fixed connective tissue cells, (dermal dendrocytes), but other spindle cell tumours may differ in histogenesis.

Epidermal growth factor (EGF) expression in lateral line system and in taste buds of adult zebrafish ( Brachidanio rerio)

The mechano and chemosensory organs of adult teleosts undergoes a continuous cell renewal and turnover which is regulated in part by growth factors. Here, we investigated the occurrence and the cell localization of epidermal growth factor (EGF) in the lateral line system and taste bud of adult zebrafish, using Western blot and immunohistochemistry associated to a polyclonal antibody against mammalian EGF. Furthermore, the distribution of S100 protein was studied in parallel to label hair sensory cells in the lateral line system. Western blot revealed one unique protein band with an estimated molecular weight of about 13 kDa, equivalent to the EGF of mammals. Specific immunoreactivity for EGF was observed in the epithelial basal and/or supporting cells of the neuromasts of the lateral line system and taste buds. Conversely, the sensory cells in both sensory structures were devoid of immunostaining. Present results demonstrate the occurrence of EGF in mechano and sensory system of adult zebrafish, suggesting a role for this molecule in the cell renewal and turnover of these structures.

Immunohistochemical distribution of S-100 protein in the cerebral cortex with regard to the cause of death in forensic autopsy

S-100 protein (S100) is an acidic calcium-binding protein, which is abundantly found in the brain. The aim of the present study was to investigate the immunohistochemical distribution of S100 in the cerebral cortex in forensic autopsy cases with regard to the cause of death and relationship to its serum levels. The cases ( n=286, 3–48 h postmortem) included fatal head injuries ( n=89), acute death from other blunt injuries ( n=29), sharp instrument injuries ( n=20), asphyxiation ( n=29: strangulation/hanging, n=22; aspiration, n=7), drownings ( n=22), fire fatalities ( n=68), cerebrovascular diseases ( n=9) and acute myocardial infarction/ischemia (AMI, n=20). S100-immunopositivity was mainly observed in the gliacytes, in part, in the neurons and myelins. For S100B-immunostaining, the gliacytes and myelins were positive, whereas the neurons were negative. Positivity in astrocytes was dependent on the cause of death, and was significantly lower in acute deaths due to strangulation/hanging and drowning, and mildly low in those due to injuries. Positivity in neurons and myelins was frequently observed in delayed head injury deaths and fire fatalities, showing an inverse relationship with the positivity in astrocytes in head injury cases. For cases of acute death, there was an inverse relationship between S100-positivity in the astrocytes and the serum S100B level. These observations suggest that astrocytes are more rapidly and severely injured than neurons during fatal brain damage, thereby causing an elevation in the serum S100B level.

S-100 proteins in the human peripheral nervous system.

This article reviews the distribution of S100 proteins in the human peripheral nervous system. The expression of S100 by peripheral glial cells seems to be a distinctive fact of these cells, independently of their localization and their ability to myelinate or not. S100 proteins expressing cells include satellite cells of sensory, sympathetic and enteric ganglia, supporting cells of the adrenal medulla, myelinating and non-myelinating Schwann cells in the nerve trunks, and the Schwann-related cells of sensory corpuscles. In addition, S100 proteins are expressed in peripheral neurons. Most of them express S100alpha protein, and a subpopulation of sensory neurons in dorsal root ganglia contains S100beta protein or S100alpha plus S100beta proteins.

S-100 protein is a selective marker for sensory hair cells of the lateral line system in teleosts

The distribution of S100 protein in the neuromast of the lateral line system (LLS) was investigated immunohistochemically in alevins of three species of teleosts ( Salmo trutta, Salmo salar and Dicentrarchus labrax), using a polyclonal antibody. In both the neuromasts of the canals, as well as in the pit organs, the hair cells, regarded as the specific sensory cells, displayed cytoplasmic immunoreactivity for S100 protein. Conversely, the supporting cells, mantle cells and basal cells were devoid of immunoreaction. These results demonstrate for the first time the occurrence of S100 in the LLS of teleosts. Due to the cell specific localization, this protein might serve as a marker for sensory hair cells in neuromasts.

Transient changes in cortical distribution of S100 proteins during reorganization of somatotopy in the primary motor cortex induced by facial nerve transection in adult rats.

In adult rats, the primary motor cortex (MI) comprises a somatotopic map of muscle representations. This somatotopy is modified after transection of the facial nerve (N7x). Mapping with cortical stimulation revealed that the underlying cortical reorganization is biphasic. Primary changes cause a transient disinhibition of long cortico-cortical connections in both hemispheres. While the first reaction vanishes within a few hours, short intra-areal connections are disinhibited within MI contralateral to N7x. The resulting co-operation between adjacent parts of MI persists as long as peripheral reinnervation is prevented. Cellular mechanisms underlying this cortical reorganization are largely unknown. Here, we utilized changes in immunoreactivity of S100 proteins (S100-IR) known as a sensitive indicator of astroglial reactions during plastic reactions in the central nervous system. Within 1 h of N7x, zones with enhanced S100-IR appeared in both hemispheres. Between 3. 5 and 18 h, reaction patterns with changing topography were transiently prominent in many cortical areas including parts of MI which surrounded the facial muscle representation fields. After 24 h, the facial muscle representation contralateral to N7x became labelled while S100-IR enhancement disappeared in most of the cortex. S100-IR-enhancement vanished completely during the next day of survival. Data presented suggest that (i) enhancement of S100-IR labels cortical tissue during the functional reorganization that is induced by N7x, (ii) large parts of the cerebral cortex participate in the reorganization, before it is finally focused on the representation field of MI that corresponds with contralateral N7x, and (iii) temporo-spatial patterns of astrocytic reactions apparently play a role in the underlying plasticity reaction.

Different distribution of S-100 protein and glial fibrillary acidic protein (GFAP) immunoreactive cells in the human fetal small intestine

International Journal of Gynecology & ObstetricsVolume 70, Issue S1 p. A131-A131 Physiological of reproduction 1 Different distribution of S-100 protein and glial fibrillary acidic protein (GFAP) immunoreactive cells in the human fetal small intestine B.A. Resch, B.A. ReschSearch for more papers by this authorE. Fekete, E. FeketeSearch for more papers by this author B.A. Resch, B.A. ReschSearch for more papers by this authorE. Fekete, E. FeketeSearch for more papers by this author First published: 10 December 2003 https://doi.org/10.1016/S0020-7292(00)83438-4AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume70, IssueS12000Pages A131-A131 RelatedInformation

The distribution of S-100 protein in hyperplastic and neoplastic prostatic epithelium.

Specimens from 30 cases of benign prostatic hyperplasia and 75 cases of prostatic carcinoma obtained during suprapubic prostatectomy, transurethal resection of the prostate and radical prostatectomy, were stained immunohistochemically for S-100 protein, prostatic acid phosphatase (PAP), prostatic specific antigen (PSA), neuron specific enolase (NSE) and polyclonal keratin. S-100 protein was positive in 9.3% of prostatic carcinomas and negative in all cases of prostatic hyperplasia. PAP and PSA were positive in all cases, while NSE was positive in 16% of the carcinoma cases. Polyclonal keratin was positive in both cell layers of the double layered hyperplastic prostatic epithelium with a more intense staining pattern in the outer cell layer. The authors believe that the S-100 protein immunoreactivity observed in some prostatic carcinomas, reflecting the change in the functional status of the neoplastic cells, might be of prognostic significance. They also emphasize the non-myoepithelial nature of the outer cell layer of the double layered prostatic epithelium.

Different distribution of S-100 protein and glial fibrillary acidic protein (GFAP) immunoreactive cells and their relations with nitrergic neurons in the human fetal small intestine.

The appearance, distribution and some histochemical features of non-neuronal cells (NN cells) associated with the myenteric plexus of human fetal small intestine have been studied by means of S-100 protein and GFAP immunocytochemistry between the 10th and 17th week of gestation. In addition, double labelling immunocytochemistry using an antibody raised against a constitutive isoform of nitric oxide synthase (bNOS) in combination with an S-100 protein antibody was applied to investigate the morphological relations between NN cells and nitrergic neurons in the developing gut wall. Cells with immunoreactivity for both glial-specific proteins are already present in the 10th week of gestation. While cells with S-100 protein immunoreactivity are located within the circular muscle layer as well as in the myenteric, and submucous plexuses, cells with GFAP immunopositivity are mainly restricted to the side of the myenteric plexus adjacent to the longitudinal muscle layer. In contrast to the dense network formed by S-100 protein immunopositive structures the GFAP immunopositive cells appear singly and do not connect into a network. Double-labelling immunocytochemistry reveals nitrergic fibers (NOS-IR) in close relation to the S-100 protein immunoreactive glial network. NOS-IR varicosities are in close association with the surface of those cells both in the circular muscle layer (CM) and in the tertiary plexus. It is concluded that two populations of NN cells with different locations and different immunohistochemical characters appear and develop together with the enteric ganglia in the human fetal intestine. The close morphological relation of NOS-IR fibers with S-100 protein immunopositive cellular network indicate a functional relationship between S-100 protein immunopositive cells and the nitrergic nerves during the early development of human enteric nervous system (ENS).

Subcellular distribution of S100 proteins in tumor cells and their relocation in response to calcium activation.

S100 proteins, a subgroup of the EF-hand Ca2+-binding protein family, regulate a variety of cellular processes via interaction with different target proteins. Several pathological disorders, including cancer, are linked to altered Ca2+ homeostasis and might involve the multifunctional S100 proteins, which are expressed in a cell- and tissue-specific manner. The present work demonstrates a distinct intracellular localization of S100A6, S100A4, and S100A2 in two tumor cell lines derived from metastatic epithelial breast adenocarcinoma (MDA-MB231) and cervical carcinoma (HeLa). Treatment of the cells by thapsigargin, the ionophore A23187, or cyclic ADP-ribose, to increase [Ca2+]i via different pathways, led to relocation of S100A6 and S100A4 but only partially of the nuclear S100A2, as demonstrated by confocal laser scanning microscopy. These findings support the hypothesis that S100 proteins could play a crucial role in the regulation of Ca2+ homeostasis in cancer cells.

Comparative immunohistochemical study of stellate cells in normal canine and equine adenohypophyses and in pituitary tumours

The presence and distribution of S100 protein (alpha and beta subunits), cytokeratin polypeptides, glial fibrillary acidic protein, neurofilaments, vimentin, neuron specific enolase, synaptophysin, HLA class II DR antigen, and pituitary hormones (prolactin, adrenocorticotropic hormone and human chorionic gonadotrophin) in stellate cells were studied immunohistochemically in four normal canine pituitary glands, five canine pituitary adenomas, two canine pituitary carcinomas and two equine pituitary adenomas (with surrounding normal glandular tissue). Stellate cells of the pars distalis and pars intermedia of canine and equine adenohypophyses showed a strong reaction with antibodies against S100 protein subunits alpha and beta. They also reacted with antibody against high and low molecular weight cytokeratins, but not with those against other intermediate filament proteins, neuroendocrine markers, the HLA-class II DR antigen or the pituitary hormones. Other populations of cells expressing both subunits of the S100 protein were polygonal cells of the pars distalis of the adenohypophysis (horse) and marginal epithelial cells of the pars intermedia of the adenohypophysis (dog and horse). Some pituitary tumours had S100-immunoreactive cells with a distribution of alpha and beta subunits that differed between the two species. Some canine tumours (one adenoma and one carcinoma) expressed only the alpha subunit, but both of the equine adenomas expressed alpha and beta protein subunits. Some of the S100-immunoreactive tumour cells reacted with RCK-102 (cytokeratins 5+8) antibody in the dog but not in the horse. The results suggested that canine and equine stellate cells of the adenohypophysis are more closely related to epithelial than to glial cells, as is the case in cattle, sheep and goats but not human beings or mice. No subpopulation of cells of bone marrow origin could be identified among canine stellate cells, as they lack MHC class II antigen. The results also suggested that the presence of S100-immunoreactive cells is more striking in canine and equine tumours than in human tumours.