The border cells are a cluster of 6–8 cells that migrate in the

ovariole An ovariole is a tubular component of the insect ovary, and the basic unit of egg production. Each ovariole is composed of a germarium (the germline Stem-cell niche, stem cell niche) at the anterior tip, a set of developing Oocyte, oocytes containe ...

of the fruit-fly ''

Drosophila melanogaster ''Drosophila melanogaster'' is a species of fly (an insect of the Order (biology), order Diptera) in the family Drosophilidae. The species is often referred to as the fruit fly or lesser fruit fly, or less commonly the "vinegar fly", "pomace fly" ...

'', during the process of

oogenesis Oogenesis () or ovogenesis is the differentiation of the ovum (egg cell) into a cell competent to further develop when fertilized. It is developed from the primary oocyte by maturation. Oogenesis is initiated before birth during embryonic devel ...

. A fly ovary consists of a string of ovarioles or egg chambers arranged in an increasing order of maturity. Each egg chamber contains 16 central

germline In biology and genetics, the germline is the population of a multicellular organism's cells that develop into germ cells. In other words, they are the cells that form gametes ( eggs and sperm), which can come together to form a zygote. They dif ...

, nurse cells surrounded by a monolayer epithelium of nearly 1000 follicle cells. At stage 8 of oogenesis, these cells initiate invading the neighbouring nurse cells, and reach the

oocyte An oocyte (, oöcyte, or ovocyte) is a female gametocyte or germ cell involved in reproduction. In other words, it is an immature ovum, or egg cell. An oocyte is produced in a female fetus in the ovary during female gametogenesis. The female ger ...

boundary by Stage 10.

Mechanism of action

The border cells are specified by the activation of the JAK-STAT pathway in response to the cytokine, Unpaired. The signaling initiates by binding of Unpaired ligand to the Domeless receptor. The migratory fate is acquired by the activation of ''slbo'', a transcription factor that leads to the transformation of stationary follicle cells into the migratory border cell fate by regulating the expression of various genes. Once the cluster detaches from the anterior end of the

, a gradient of chemokines, PDGF and VEGF (PVF1) aids in the directional migration of border cells towards the oocyte. The timing of the border cell migration along with egg chamber development is coordinated by

ecdysone Ecdysone is a prohormone of the major insect molting hormone 20-hydroxyecdysone, secreted from the prothoracic glands. It is of steroidal structure. Insect molting hormones (ecdysone and its homologues) are generally called ecdysteroids. Ecdy ...

, a steroid hormone, which functions through Taiman, ecdysone receptor and ultraspiracle receptors. These cells migrate as a free group by means of transient cell-cell adhesions, further initiating signaling cascades that regulate expression of

cytoskeletal The cytoskeleton is a complex, dynamic network of interlinking protein filaments present in the cytoplasm of all Cell (biology), cells, including those of bacteria and archaea. In eukaryotes, it extends from the cell nucleus to the cell membrane ...

components and triggers cellular extensions enabling forward movement. The invasive movement of the cluster towards the posteriorly located oocyte is followed by the off-centred movement towards the dorsal side of the egg chamber. The second kind of movement is mediated by the binding of Gurken, Spitz and Keren ligands to the EGFR on the border cells.

Significance

The directional and collective migration of border cells aids in the formation of micropyle, a specialised passage through which the sperm enters during

fertilisation Fertilisation or fertilization (see spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes to give rise to a zygote and initiate its development into a new individual organism or of ...

. Studying cell migration is important from the point of view of

metastasis Metastasis is a pathogenic agent's spreading from an initial or primary site to a different or secondary site within the host's body; the term is typically used when referring to metastasis by a cancerous tumor. The newly pathological sites, ...

, and for this reason, several

model organisms A model organism is a non-human species that is extensively studied to understand particular biological phenomena, with the expectation that discoveries made in the model organism will provide insight into the workings of other organisms. Mo ...

are used to determine what molecules are important in initiating, maintaining and guiding moving cells. ''

'' 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 migrating phenotype. After identifying the genes that are important for this

phenotype In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology (physical form and structure), its developmental processes, its biochemical and physiological propert ...

, their homologs can be investigated for putative roles in turning non-invasive cancerous

tumors A neoplasm () is a type of abnormal and excessive growth of tissue. The process that occurs to form or produce a neoplasm is called neoplasia. The growth of a neoplasm is uncoordinated with that of the normal surrounding tissue, and persists ...

into metastatic ones.

References

{{Reflist Insect developmental biology