The adrenal cortex is classically divided into three morphologically and biochemically distinct zones, covered by a thin, cellular capsule. The adult adrenal cortex is a dynamic tissue in which distinct regions of cell proliferation, movement and death have been identified. Several models for stem cell maintenance of the adult adrenal cortex have been proposed, but adrenocortical stem cells have not yet been identified. Adrenal cortices of 21OH/LacZ transgenic mice show similar mosaic patterns of β-galactosidase staining to X- inactivation mosaics and LacZ ↔ wildtype chimeras. 21OH/LacZ mice provide a tool for lineage analysis, which may help to i) identify clones of cells produced by stem cells in the adult, ii) determine when stem cells begin to function and iii) evaluate different models of how stem cells maintain the adrenal cortex. Analysis of 21OH/LacZ transgenic adrenal cortices showed that the randomly orientated clusters of fetal patches change progressively during the perinatal period to adult radial stripes. Correlation of changes in mosaic patterns and the locations of cell proliferation suggests that the stripes arise by edge-biased growth during the perinatal growth period. Although stem cells may not be involved in the initial formation of stripes, it seems likely that stem cells later maintain the stripes by producing clones of cells that move centripetally to displace the earlier fetal patterns and later replace aging cells. Various combinations of BrdU labelling and chase periods demonstrated that most cell division occurred in the outer 40% of the adrenal cortex, confirmed that cells moved towards the medulla and identified a population of label-retaining cells near the capsule, which could include stem cells. (Stem cells have been recognised as BrdU label-retaining cells in other tissues because they divide less frequently than their daughter cells so dilute the incorporated BrdU more slowly.) Stripe patterns in adult 21OH/LacZ transgenic adrenal cortices were examined to try to distinguish between various models proposed for stem cell maintenance of the adrenal cortex. The observed continuous radial stripe pattern favours the general hypothesis that a single population of stem cells in the periphery maintains the entire adrenal cortex, although other explanations are possible. Quantitative analysis of adult stripe patterns did not show the reduction in stripe number that might be predicted if an age-related decline in adrenocortical stem cell function occurs, as may happen in some other tissues.

The regenerative tissues such as hair follicles, intestine and teeth have a particular microenvironment known as “stem cell niche” which houses stem cells and act as a signaling center to control stem cell fate. The precise and timely regulation of stem cell renewal and differentiation is essential for tissue formation, growth and homeostasis over the course of a lifetime. However, the molecular underpinning to control this regulation is poorly understood. To address this issue, we use the continuously growing mouse incisor as a model to study the gene regulatory network which controls dental epithelial stem cell (DESC) maintenance, growth and craniofacial morphogenesis. We found FoxO6, a transcription factor mainly expressed in the brain and craniofacial region, control DESC proliferation by regulating Hippo signaling. FoxO6 loss-of-function mice undergo increases in cell proliferation which finally leads to lengthening of the incisors, expansion of the face and skull and enlargement of the mandible and maxilla. We have screened three human FOXO6 single nucleotide polymorphisms which are associated with facial morphology ranging from retrognathism to prognathism. Our study also reveals that Sox2 and Lef-1, two markers for early craniofacial development, are regulated by Pitx2 to control DESC maintenance, differentiation and craniofacial development. Conditional Sox2 deletion in the oral and dental epithelia results in severe craniofacial defects, including ankyloglossia, cleft palate, arrested incisor development and abnormal molar development. The loss of Sox2 in DESCs leads to impaired stem cell proliferation, migration and subsequent dissolution of the tooth germ. On the other hand, conditional overexpression of Lef-1 in oral and dental epithelial region increases DESC proliferation and creates a new labial cervical loop stem cell compartment in dental epithelial stem cell niche, which produces rapidly growing long “tusk-like” incisors. Interestingly, Lef-1 overexpression rescues the tooth arrest defects but not the ankyloglossia or cleft palate in Sox2 conditional deletion mice. Our data also reveal that miRNA and histone remodeler are involved in regulating DESC proliferation and craniofacial morphogenesis. We describe a miR-23a/b:Hmgn2:Pitx2 signaling pathway in regulating dental epithelial cell growth and differentiation. Pitx2 activates expression of amelogenin which is the major protein component for enamel deposition. This activation can be repressed by the chromatin-associated factor Hmgn2. miR-23a and miR-23b directly target Hmgn2, leading to the release of the Hmgn2 inhibition of Pitx2 transcriptional activity and thus enhance Amelogenin production. Phenotypically, ablation of Hmgn2 in mice results in an overgrowth of incisors with increased Amelogenin expression. The findings in this study increase our current understanding of the molecular regulation of dental epithelial stem cell fate. It not only highlights new gene regulatory network that controls dental stem cell maintenance, growth and craniofacial morphogenesis, but also sheds new light on developing novel stem cell therapy or gene therapy for tooth regeneration and dental diseases.