Border cells

For the cells produced by plant roots that are also known as border cells, see root.

The Drosophila melanogaster border cells are a cluster of 6-8 migratory cells, found in the ovary, and derived from the follicular epithelium.

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Studying cell migration is important from the point of view of metastasis, and for this reason, several model organisms are used to determine what molecules are important in initiating, maintaining and guiding moving cells. Drosophila melanogaster is one of the favorite organisms for studying biological phenomena. The border cells of the Drosophila ovary are a genetically tractable system for studying diverse aspects of moving phenotype. After identifying the genes that are important for this phenotype, their homologs can be investigated for putative roles in turning non-invasive cancerous tumors into metastatic ones.

The egg chamber of the Drosophila ovary contains 16 central germline cells surrounded by a monolayer epithelium of about 1000 follicle cells. At stage 9 of Drosophila oogenesis, these cells perform a stereotypical invasive migration on the intervening nurse cells, and reach the oocyte. The migration consists of an initial posterior migration, followed by another in the dorsal direction. It is now known that the transcription factor Slbo (slow border cells) is a master regulator of the migratory phenotype, and the race is on to identify its targets.

The migratory cluster is initiated by JAK-STAT pathway and is guided by the growth of long cellular extensions. The directional cues are provided by receptor tyrosine kinases.

Cells that undergo such a change in fate as migratory border cells have two distinct alternatives: reorganize the existing proteins or bring about massive changes in gene expression. In our case, the latter case seems to hold true. The transcription factor slbo which commandeers the transcription of several genes is believed to be central to this phenomenon. slbo was identified in a P-element screen for female sterility several years ago – and since then several proteins downstream of it have come up. The slbo locus encodes the Drosophila homolog of the mammalian CCAAT Enhancer Binding Protein (C/EBP) - a basic region-leucine zipper transcription factor, and it was thus expected that one or more of its targets would be essential in migration.

Importance

This migration of the border cells appears to be important for two reasons – the formation of a structure called a micropyle, which is important for sperm entry, and for expressing the torso-like gene, which is an anterior-posterior patterning signal. Laser ablation of the border cells or their failure to migrate lead to improper morphogenesis of the micropyle.

References

Ambra Bianco, Minna Poukkula, Adam Cliffe, Juliette Mathieu, Carlos M. Luque, Tudor A. Fulga & Pernille Rørth.
Two distinct modes of guidance signalling during collective migration of border cells.
Nature 448, 362-366 (2007).
http://dx.doi.org/10.1038/nature05965

Pernille Rørth. Initiating and guiding migration: lessons from border cells.
Trends Cell Biol. 12, 325–331 (2002).
http://dx.doi.org/10.1016/S0962-8924(02)02311-5