Pituitary endocrine cells are supplied by Sox2-expressing stem/progenitor cells in the anterior lobe of the adult pituitary. In relation to their microenvironment ("niche"), SOX2-positive cells exist in two types of niches; the marginal cell layer-niche and the parenchymal-niche. Recently, we isolated dense stem/progenitor cell clusters from the parenchymal-niche as parenchymal stem/progenitor cell (PS)-clusters. We classified these PS-clusters into three subtypes based on differences in S100β-expression (S100β-positive, -negative, and -mixed type), and reported that S100β-positive PS-clusters exhibited the capacity for differentiation into endocrine cells under 3-dimensional cultivation system. In the present study, we further characterized S100β-positive PS-clusters using an in vitro 2-dimensional cultivation system. The results demonstrated that S100β-positive PS-clusters in the 2-dimensional cultivation system proliferated more actively than S100β-negative clusters. Moreover, in 2-dimensional cultivation conditions, S100β-positive PS-clusters showed differentiation capacity into non-endocrine cells (Myogenin-, αSMA-, NG2-, or SOX17-positive cells) but not into endocrine cells, whereas S100β-negative PS-clusters did not. Collectively, PS-clusters were heterogeneous, exhibiting different proliferation and differentiation properties based on the difference in S100β-expression. Specifically, a part of SOX2-positive cells in the parenchymal-niche had capacities for differentiation into non-endocrine cells, and S100β-positive PS-clusters may be in more progressive stages toward differentiation than S100β-negative clusters.