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) at the anterior tip, a set of developing oocytes contained within follicles, and ...

of the fruit-fly ''

Drosophila melanogaster ''Drosophila melanogaster'' is a species of fly (the taxonomic 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" or "pomace fly". Starting with Ch ...

,'' during the process of

oogenesis Oogenesis, ovogenesis, or oögenesis 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 in the embryonic stage. O ...

. A fly ovary consists of a string of

s 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 pass on their genetic material to the progeny (offspring). In other words, they are the cells that form the egg, sperm and the fertilised egg. They ...

nurse cell In general biology or reproductive physiology the term nurse cell is defined as a cell which provides food, helps other cells and provides stability to their neighboring cells. The term nurse cell is used in several unrelated ways in different scien ...

s surrounded by a monolayer epithelium of nearly 1000 follicle cells. At stage 8 of

, these cells initiate invading the neighbouring

s, 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 ...

boundary by Stage 10.

Mechanism of Action

The border cells are specified by the activation of the

JAK-STAT The JAK-STAT signaling pathway is a chain of interactions between proteins in a cell, and is involved in processes such as immunity, cell division, cell death, and tumour formation. The pathway communicates information from chemical signals out ...

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

chemokine Chemokines (), or chemotactic cytokines, are a family of small cytokines or signaling proteins secreted by cells that induce directional movement of leukocytes, as well as other cell types, including endothelial and epithelial cells. In additio ...

PDGF Platelet-derived growth factor (PDGF) is one among numerous growth factors that regulate cell growth and division. In particular, PDGF plays a significant role in blood vessel formation, the growth of blood vessels from already-existing blood v ...

and

VEGF Vascular endothelial growth factor (VEGF, ), originally known as vascular permeability factor (VPF), is a signal protein produced by many cells that stimulates the formation of blood vessels. To be specific, VEGF is a sub-family of growth factors, ...

(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, which is secreted from the prothoracic glands. It is of steroidal structure. Insect molting hormones (ecdysone and its homologues) are generally called ecdysteroids. ...

, a steroid hormone, which functions through Taiman,

Ecdysone receptor The ecdysone receptor is a nuclear receptor found in arthropods, where it controls development and contributes to other processes such as reproduction. The receptor is a non-covalent heterodimer of two proteins, the EcR protein and ultraspiracle ...

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 cells, including those of bacteria and archaea. In eukaryotes, it extends from the cell nucleus to the cell membrane and is compo ...

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 mRNA localization is a common mode of posttranscriptional regulation of gene expression that targets a protein to its site of function. Proteins are highly dependent on cellular environments for stability and function, therefore, mRNA localiza ...

Spitz Spitz (derived from the German word ''spitz'' 'pointed') is a type of domestic dog characterized by long, thick, and often white fur, and pointed ears and muzzles. The tail often curls over the dog's back or droops. While all of the breeds rese ...

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 Micropyle may refer to: * Micropyle (botany) In seed plants, the ovule is the structure that gives rise to and contains the female reproductive cells. It consists of three parts: the ''integument'', forming its outer layer, the ''nucellus'' (o ...

, 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 new individual organism or offspring and initiate its development. Proce ...

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

metastasis Metastasis is a pathogenic agent's spread 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, then, ...

, and for this reason, several

model organisms A model organism (often shortened to model) 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 working ...

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 or physical form and structure, its developmental processes, its biochemical and physiological proper ...

, their

homologs A couple of homologous chromosomes, or homologs, are a set of one maternal and one paternal chromosome that pair up with each other inside a cell during fertilization. Homologs have the same genes in the same locus (genetics), loci where they pr ...

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