Planarians, a tale of stem cells.

Planarians possess amazing abilities to regulate tissue homeostasis and regenerate missing body parts. These features reside on the presence of a population of pluripotent/totipotent stem cells, the neoblasts, which are considered as the only planarian cells able to proliferate in the asexual strains. Neoblast distribution has been identified by mapping the cells incorporating bromodeoxyuridine, analyzing mitotic figures and using cell proliferation markers. Recently identified molecular markers specifically label subgroups of neoblasts, revealing thus the heterogeneity of the planarian stem cell population. Therefore, the apparent totipotency of neoblasts probably reflects the composite activities of multiple stem cell types. First steps have been undertaken to understand how neoblasts and differentiated cells communicate with each other to adapt the self-renewal and differentiation rates of neoblasts to the demands of the body. Moreover, the introduction of molecular resource database on planarians now paves the way to renewed strategies to understand planarian regeneration and stem cell-related issues.

An MCM2-related gene is expressed in proliferating cells of intact and regenerating planarians.

The minichromosome maintenance (MCM2-7) gene family encodes conserved proteins, which are essential for DNA replication licensing in eukaryotes. They are abundant in proliferating cells, and specific MCM transcripts undergo cell cycle-dependent oscillations. Here we report the characterization of a planarian MCM2 homologue, DjMCM2, which represents the first molecular marker for detecting proliferating cells in planarians. DjMCM2-expressing cells are broadly distributed in the mesenchymal space of the body, with the exception of the cephalic region, and are preferentially accumulated in the peripheral area of the dorso-lateral mesenchyme, along the anteroposterior axis. During regeneration, no DjMCM2 transcripts are observed within the blastema, according to the current view that this structure is not a proliferation site in planarians. Spatio-temporal changes in DjMCM2 RNA expression pattern in the stump parallel blastema growth, coordinately with the orientation of the cut. X-ray irradiation results in the disappearance of DjMCM2 expression, thus confirming that these transcripts are detected specifically in proliferating cells, visualized as neoblasts by in situ hybridization in dissociated cells. In addition to neoblasts, rare large DjMCM2-expressing cells are observed in macerates of tissues excised just below the wound, suggesting that cell types other than neoblasts may be sporadically recruited for proliferation in planarians.

Expression of DjY1, a protein containing a cold shock domain and RG repeat motifs, is targeted to sites of regeneration in planarians.

Planarians are well-known for their exceptional regenerative abilities. This investigation focuses on the involvement of a Y-box protein, defined by the presence of a cold-shock domain, in regeneration-specific processes. Previous studies have shown that developmentally expressed Y-box proteins bind to mRNA molecules and regulate the timing of their translation. We have isolated and characterized a planarian Y-box gene, DjY1, which is specifically expressed at the site of regeneration, the blastema. DjY1 transcripts appear rapidly at the site of cutting and increase in number as the blastema grows. The timing and level of expression is similar irrespective of the orientation of the cut: in anterior, posterior, and lateral regenerative tissue. As regeneration nears completion, there is a general decrease in transcript level except in structures which are still differentiating, specifically in the auricles where new DjY1 transcripts are produced. A similarly modulated temporal pattern of expression throughout regeneration is seen in assaying the DjY1 protein. Within the population of blastemal cells, a subset of differentiating cells is specifically immunostained using antibodies to DjY1. The DjY1 protein contains a cold-shock domain and RG-repeat motifs, both of which are associated with RNA-binding properties: in vitro binding studies using recombinant DjY1 show that the preferred template is single-stranded RNA of heterogeneous sequence. These data provide the first direct evidence that a Y-box protein is involved in the regeneration process in planarians and implicate DjY1 in the translational regulation of differentiation-specific mRNAs.

Cytochemistry of late ovarian chambers of Drosophila melanogaster.

Late ovarian chambers of Drosophila melanogaster have been examined by ultrastructural cytochemistry in an attempt to characterize some of the transformations which precede the completion of oogenesis. From stage 11 onward peroxidase activity is present in the endoplasmic reticulum of both nurse cells and oocyte, as well as in the egg-covering precursors of the columnar follicle cells. Catalase activity is restricted to the very last stages of oogenesis (stage 13-14) and appears to be located in membrane-bound organelles of the ooplasm which are continuous with the endoplasmic reticulum. Because of the presence of catalase as well as by their structural appearance, these organelles are to be identified as microperoxisomes. Catalase activity becomes cytochemically detectable in the ooplasm somehow in coincidence with the formation of glycogen. Furthermore, glycogen is first formed in intimate association with alpha-1 yolk platelets. On the basis of these findings it is suggested that glycogen synthesis occurs by a process of gluconeogenesis.

Cell death in ovarian chambers of Drosophila melanogaster.

An ultrastructural analysis has been made of certain ovarian chambers undergoing abnormal development. The earliest morphological change in these chambers consists of the alteration of the nuclear material which is then followed by engulfment of portions of the nurse cell cytoplasm, including the nuclear fragments, into the overlying follicle cells. The continuation of this process leads to the progressive disappearance of nurse cells with the concomitant formation of huge dense vacuoles in the follicle layer. The morphological features described in the present investigation are similar to those found in other tissues and interpreted as leading to cell death. It is suggested that certain ovarian chambers undergo cell death as a result of the incapability of furthering their development. The role played by cell death in oogenesis is also discussed on the basis of the current literature.