摘要
The best known cases of cell autotomy are the formation of erythrocytes and thrombocytes(platelets) from progenitor cells that reside in special niches. Recently, autotomy of stem cells and its enigmatic interaction with the niche has been reported from male germline stem cells(GSCs) in several insect species. First described in lepidopterans, the silkmoth, followed by the gipsy moth and consecutively in hemipterans, foremost the milkweed bug. In both, moths and the milkweed bug, GSCs form finger-like projections toward the niche, the apical cells(homologs of the hub cells in Drosophila). Whereas in the milkweed bug the projection terminals remain at the surface of the niche cells, in the gipsy moth they protrude deeply into the singular niche cell. In both cases, the projections undergo serial retrograde fragmentation with progressing signs of autophagy. In the gipsy moth, the autotomized vesicles are phagocytized and digested by the niche cell. In the milkweed bug the autotomized vesicles accumulate at the niche surface and disintegrate. Autotomy and sprouting of new projections appears to occur continuously. The significance of the GSC-niche interactions, however, remains enigmatic. Our concept on the signaling relationship between stem cell-niche in general and GSC and niche(hub cells and cyst stem cells) in particular has been greatly shaped by Drosophila melanogaster. In comparing the interactions of GSCs with their niche in Drosophila with those in species exhibiting GSC autotomy it is obvious that additional or alternative modes of stem cell-niche communication exist. Thus, essential signaling pathways, including niche-stem cell adhesion(E-cadherin) and the direction of asymmetrical GSC division- as they were found in Drosophila- can hardly be translated into the systems where GSC autotomywas reported. It is shown here that the serial autotomy of GSC projections shows remarkable similarities with Wallerian axonal destruction, developmental axon pruning and dying-back degeneration in neurodegenerative diseases. Especially the hypothesis of an existing evolutionary conserved "autodestruction program" in axons that might also be active in GSC projections appears attractive. Investigations on the underlying signaling pathways have to be carried out. There are two other well known cases of programmed cell autotomy: the enucleation of erythroblasts in the process of erythrocyte maturation and the segregation of thousands of thrombocytes(platelets) from one megakaryocyte. Both progenitor cell types- erythroblasts and megakaryocytes- are associated with a niche in the bone marrow, erythroblasts with a macrophage, which they surround, and the megakaryocytes with the endothelial cells of sinusoids and their extracellular matrix. Although the regulatory mechanisms may be specific in each case, there is one aspect that connects all described processes of programmed cell autotomy and neuronal autodestruction: apoptotic pathways play always a prominent role. Studies on the role of male GSC autotomy in stem cell-niche interaction have just started but are expected to reveal hitherto unknown ways of signal exchange. Spermatogenesis in mammals advance our understanding of insect spermatogenesis. Mammal and insect spermatogenesis share some broad principles, but a comparison of the signaling pathways is difficult. We have intimate knowledge from Drosophila, but of almost no other insect, and we have only limited knowledge from mammals. The discovery of stem cell autotomy as part of the interaction with the niche promises new general insights into the complicated stem cell-niche interdependence.
The best known cases of cell autotomy are theformation of erythrocytes and thrombocytes （platelets）from progenitor cells that reside in special niches.Recently, autotomy of stem cells and its enigmaticinteraction with the niche has been reported from malegermline stem cells （GSCs） in several insect species.First described in lepidopterans, the silkmoth, followedby the gipsy moth and consecutively in hemipterans,foremost the milkweed bug. In both, moths and themilkweed bug, GSCs form finger-like projectionstoward the niche, the apical cells （homologs of thehub cells in Drosophila）. Whereas in the milkweedbug the projection terminals remain at the surfaceof the niche cells, in the gipsy moth they protrudedeeply into the singular niche cell. In both cases, theprojections undergo serial retrograde fragmentationwith progressing signs of autophagy. In the gipsy moth,the autotomized vesicles are phagocytized and digestedby the niche cell. In the milkweed bug the autotomizedvesicles accumulate at the niche surface and disintegrate.Autotomy and sprouting of new projectionsappears to occur continuously. The significance of theGSC-niche interactions, however, remains enigmatic.Our concept on the signaling relationship betweenstem cell-niche in general and GSC and niche （hubcells and cyst stem cells） in particular has been greatlyshaped by Drosophila melanogaster. In comparingthe interactions of GSCs with their niche in Drosophilawith those in species exhibiting GSC autotomy itis obvious that additional or alternative modes ofstem cell-niche communication exist. Thus, essentialsignaling pathways, including niche-stem cell adhesion（E-cadherin） and the direction of asymmetrical GSCdivision - as they were found in Drosophila - can hardlybe translated into the systems where GSC autotomy was reported. It is shown here that the serial autotomyof GSC projections shows remarkable similarities withWallerian axonal destruction, developmental axonpruning and dying-back degeneration in neurodegenerativediseases. Especially the hypothesis of anexisting evolutionary conserved ＂autodestructionprogram＂ in axons that might also be active in GSCprojections appears attractive. Investigations on theunderlying signaling pathways have to be carried out.There are two other well known cases of programmedcell autotomy： the enucleation of erythroblasts in theprocess of erythrocyte maturation and the segregationof thousands of thrombocytes （platelets） from onemegakaryocyte. Both progenitor cell types - erythroblastsand megakaryocytes - are associated with a niche inthe bone marrow, erythroblasts with a macrophage,which they surround, and the megakaryocytes withthe endothelial cells of sinusoids and their extracellularmatrix. Although the regulatory mechanisms may bespecific in each case, there is one aspect that connectsall described processes of programmed cell autotomyand neuronal autodestruction： apoptotic pathways playalways a prominent role. Studies on the role of maleGSC autotomy in stem cell-niche interaction have juststarted but are expected to reveal hitherto unknownways of signal exchange. Spermatogenesis in mammalsadvance our understanding of insect spermatogenesis.Mammal and insect spermatogenesis share some broadprinciples, but a comparison of the signaling pathways isdifficult. We have intimate knowledge from Drosophila,but of almost no other insect, and we have only limitedknowledge from mammals. The discovery of stemcell autotomy as part of the interaction with the nichepromises new general insights into the complicatedstem cell-niche interdependence.
