The living-tissue microscope: the importance of studying stem cells in their natural, undisturbed microenvironment

Protecting and Diversifying the Germline

Gametogenesis represents the most dramatic cellular differentiation pathways in both female and male flies. At the genome level, meiosis ensures that diploid germ cells become haploid gametes. At the epigenome level, extensive changes are required to turn on and shut off gene expression in a precise spatiotemporally controlled manner. Research applying conventional molecular genetics and cell biology, in combination with rapidly advancing genomic tools have helped us to investigate (1) how germ cells maintain lineage specificity throughout their adult reproductive lifetime; (2) what molecular mechanisms ensure proper oogenesis and spermatogenesis, as well as protect genome integrity of the germline; (3) how signaling pathways contribute to germline-soma communication; and (4) if such communication is important. In this chapter, we highlight recent discoveries that have improved our understanding of these questions. On the other hand, restarting a new life cycle upon fertilization is a unique challenge faced by gametes, raising questions that involve intergenerational and transgenerational epigenetic inheritance. Therefore, we also discuss new developments that link changes during gametogenesis to early embryonic development-a rapidly growing field that promises to bring more understanding to some fundamental questions regarding metazoan development.

Cell lineage tracing in human epithelial tissues using mitochondrial DNA mutations as clonal markers

The study of cell lineages through heritable genetic lineage tracing is well established in experimental animals, particularly mice. While such techniques are not feasible in humans, we have taken advantage of the fact that the mitochondrial genome is highly prone to nonpathogenic mutations and such mutations can be used as clonal markers to identify stem cell derived clonal populations in human tissue sections. A mitochondrial DNA (mtDNA) mutation can spread by a stochastic process through the several copies of the circular genome in a single mitochondrion, and then through the many mitochondria in a single cell, a process called 'genetic drift.' This process takes many years and so is likely to occur only in stem cells, but once established, the fate of stem cell progeny can be followed. A cell having at least 80% of its mtDNA genomes bearing the mutation results in a demonstrable deficiency in mtDNA-encoded cytochrome c oxidase (CCO), optimally detected in frozen tissue sections by dual-color histochemistry, whereby CCO activity stains brown and CCO deficiency is highlighted by subsequent succinate dehydrogenase activity, staining the CCO-deficient areas blue. Cells with CCO deficiency can be laser captured and subsequent mtDNA sequencing can ascertain the nature of the mutation. If all cells in a CCO-deficient area have an identical mutation, then a clonal population has been identified; the chances of the same mutation initially arising in separate cells are highly improbable. The technique lends itself to the study of both normal epithelia and can answer several questions in tumor biology. WIREs Dev Biol 2016, 5:103-117. doi: 10.1002/wdev.203 For further resources related to this article, please visit the WIREs website.