S U M M A R Y Neural stem cells proliferate in liquid culture as cell clusters (neurospheres).This study was undertaken to characterize the epidermal growth factor (EGF)-expanded free-floating neurospheres derived from rat fetal striatum. We examined the ultrastructural and antigenic characteristics of these spheres. They consisted of two cell types, electron-dense and electron-lucent cells. Lucent cells were immunopositive to actin, vimentin, and nestin, whereas dense cells were immunopositive to actin, weakly positive to vimentin, and nestin-negative. Neurospheres contained healthy, apoptotic, and necrotic cells. Healthy cells were attached to each other by adherens junctions. They showed many pseudopodia and occasionally a single cilium. Sphere cells showed phagocytic capability because healthy cells phagocytosed the cell debris derived from dead cells in a particular process that involves the engulfment of dying cells by cell processes from healthy cells. Sphere cells showed a cytoplasmic and a nuclear pool of fibroblast growth factor (FGF) receptors. They expressed E-and N-cadherin, ␣ -and ␤ -catenin, EGF receptor, and a specific subset of FGF receptors. Because sphere cells expressed this factor in the absence of exogenous FGF-2, we propose that they are able to synthesize FGF-2.

The intermediate filament protein nestin is predominantly expressed in some stem/progenitor cells and appears to be a useful molecular tool to characterise tumours originating from precursor cells of neuroectodermal and mesenchymal lineages. Leydig cells originate in the adult testis by differentiation from stem cells and express a variety of neural and neuroendocrine markers. The possible expression of the neural stem cell marker nestin in Leydig cells and testicular tumour cells was determined by analysing the patterns of nestin expression in normal and pathological human testes by Western blot and immunohistochemical methods. In normal testis, nestin was found in some vascular endothelial cells, a subset of peritubular spindle-shaped cells and some Leydig cells; spermatogenic and Sertoli cells were unstained. In normal Leydig cells, nestin was distributed in the perinuclear cytoplasm and accumulated in the crystalloids of Reinke with ageing. In non-tumour pathologies (cryptorchidism, impaired spermatogenesis), the seminiferous tubules were immunonegative, whereas hyperplastic Leydig cells showed cytoplasmic immunolabelling. In testicular malignancies, nestin was localised in the Sertoli cells of the seminiferous tubules affected with intratubular germ cell neoplasia, in the hyperplastic Leydig cells associated with these tumours and in some components (mesenchymal and neuroepithelial cells) of teratomas; spermatocytic and non-spermatocytic seminomas were unstained. Some vascular endothelial cells were immunolabelled in all tumour samples. Thus, nestin is expressed in a population of normal and hyperplastic Leydig cells and in Sertoli cells in the presence of intratubular germ-cell neoplasia. Nestin may be a good marker for identifying components of testicular teratomas.

S U M M A R YThe amino acid taurine has been implicated in several aspects of reproductive system physiology. However, its localization in these organs has not been previously analyzed. The aim of this study was to characterize its distribution in male rat reproductive organs by immunohistochemical methods. Taurine was localized in the smooth muscle cells of the tissues studied and in the skeletal fibers of the cremaster muscle. In the testis, taurine was found in Leydig cells, vascular endothelial cells, and other interstitial cells. No immunoreactivity was observed in the cells of the seminiferous tubules, either in germ cells at all spermatogenic stages or in Sertoli cells. However, peritubular myoid cells were immunostained. Most epithelial cells of the efferent ducts were immunolabeled, whereas the epithelial cells of the rete testis (extratesticular segments), epididymis (caput, corpus, and cauda regions), and ductus deferens were unstained. However, most epithelial cells from the intratesticular segments of the rete were immunopositive. Some cells identified as intraepithelial macrophages and lymphocytes, apical cells, and narrow cells were intensely immunolabeled. Regional differences in the distribution of these cell types along the ducts studied were also noted. The possible functional roles for taurine in these cells are discussed.

Previous reports have dealt with the hypoglycemic properties of taurine and its effects on insulin secretion by adult and fetal isolated islets. We have studied the presence and cellular distribution of taurine in rat islets, the conditions to evoke its release, and its possible modulatory action on insulin secretion. We localized taurine by techniques of double immunolabeling in most glucagon-positive cells and in some somatostatin-positive cells, whereas insulin-positive cells were not labeled with the taurine antibody. Although high-glucose stimulation did not evoke any taurine release, a hyposmotic solution (17% osmolarity reduction) induced a specific phasic release of taurine and GABA (34 and 52% increase on their basal release rate). On the other hand, taurine (10 mmol/l) application slightly reduced the second phase of insulin secretion induced by glucose stimulation. In conclusion, taurine is highly concentrated in glucagon-containing cells of the islet periphery. It is not liberated by glucose stimulation but is strongly released under hyposmotic conditions. All of these data suggest that taurine plays an osmoregulatory role in alpha-cells.

The content and distribution of the amino acid taurine in squamous epithelia were studied using highperformance liquid chromatography and immunohistochemical methods. Quantitative analysis demonstrated that taurine was highly concentrated in the epidermis (5.49 µmol/g fresh tissue in the hairless skin of the hind footpad of the rat), although the values in the isolated stratum corneum were extremely low (<0.073 µmol/g in the horny layer of the same skin area). No other analysed amino acid (such as glutamate, glutamine, glycine or alanine) showed this specific pattern of distribution. The immunohistochemical study revealed that in the dog and rat epidermis, taurine was present in the keratinocytes of the granular and upper spinous layers. The basal layer, lower spinous layer and stratum corneum were immunonegative. A similar immunostaining pattern was found in the epithelia of the different organs studied: the mouth, tongue and oesophagus of the dog and rat, the rat forestomach and the rat corneal epithelium. Other cell types, such as sebaceous and muscle cells, were immunolabelled. The existence of a circulating pool of taurine in the epidermis (via taurine release from keratinocytes before they reach the horny layer and its uptake by nearby cells) and its possible roles in these cells are discussed.