The fruit fly, Drosophila melanogaster, is a model organism for genetic research, offering insights into biological processes. Its utility comes from structures called imaginal discs, which are pouches of cells in the larva that develop into adult body parts during metamorphosis. The wing imaginal disc is one of the most studied, forming the adult wing and part of the fly’s thorax.
The wing disc begins as a small cluster of around 30 cells in the embryo and proliferates to approximately 35,000 cells by the end of the larval stage. Its predictable development and accessibility make it a valuable system for investigating how genes control the formation of complex structures.
Anatomy of the Wing Imaginal Disc
The wing imaginal disc is a flattened sac composed of epithelial cells, often visualized as a deflated and intricately folded balloon. This structure consists of two layers. The main layer is a convoluted columnar epithelium, which contains the majority of the disc’s cells and is responsible for forming the adult structures. The other is a much thinner sheet of squamous cells known as the peripodial membrane, which covers the reverse side of the disc.
Within the columnar epithelial layer, distinct territories are allocated to form specific parts of the adult fly, their fates determined long before the physical transformation begins. The central region of the disc is the wing pouch, which will develop into the blade of the wing. This area is surrounded by the hinge region, a territory of cells fated to form the complex structures that articulate the wing. The outermost region is the notum, which will give rise to the dorsal portion of the adult thorax where the wings are attached.
The Metamorphosis of the Wing Disc
The transition from a folded internal sac to a functional adult appendage is a process of morphogenesis that occurs during the pupal stage. The sequence unfolds over a few hours after puparium formation, when the larva commits to metamorphosis. A primary event is eversion, where the disc turns itself inside out. This movement shifts the structure from its internal position to an external one, unfurling the future wing.
Following eversion, the dorsal and ventral surfaces of the wing pouch are pressed together in a process called fusion. This apposition of the two epithelial sheets eliminates the internal lumen and creates the thin, two-sided blade of the adult wing. Once the wing has achieved its final shape, the epithelial cells differentiate to form specialized structures, including the hardened veins for support, sensory bristles, and the wing membrane.
Cell Signaling and Genetic Patterning
The architecture of the adult wing results from a system of cell-to-cell communication that patterns the disc during its growth. This system relies on morphogens, signaling molecules that emanate from a source and form a concentration gradient. This gradient provides cells with positional information based on the amount of signal they receive. Two orthogonal axes are established by different signaling pathways, creating a coordinate system that guides development.
The anterior-posterior (A/P) axis is organized by the Hedgehog (Hh) signal. Cells in the posterior compartment produce Hh protein, which diffuses into the anterior compartment. There, it instructs a narrow band of cells along the A/P boundary to secrete a different morphogen, Decapentaplegic (Dpp). Dpp spreads throughout the compartments, and cells interpret their position along the A/P axis based on the Dpp concentration, which activates genes that define features like longitudinal wing veins.
A second boundary is the dorsal-ventral (D/V) axis. The dorsal identity of the disc is established by the expression of the gene Apterous. This leads to the activation of the signaling molecule Wingless (Wg) in a narrow stripe of cells along the D/V boundary. Wg functions as a morphogen, spreading into both the dorsal and ventral compartments to pattern features across that axis, including the placement of sensory bristles along the wing margin.
Relevance in Human Disease Research
The study of the Drosophila wing disc offers insights into human health because many of its signaling pathways are highly conserved in humans. Pathways initiated by molecules like Wingless (homologous to the human Wnt family), Hedgehog, and Dpp (a member of the human TGF-β family) are employed during human embryonic development. They play analogous roles in guiding the formation of limbs, organs, and other complex structures.
The connection is direct in the context of cancer research. The genes and proteins that regulate cell growth and identity in the wing disc are the same ones that, when misregulated, can lead to disease in humans. For instance, mutations that cause the Hedgehog or Wnt signaling pathways to become overactive can lead to uncontrolled cell division, a characteristic of tumor formation.
This makes the wing disc a model for understanding what goes wrong in human cancers. Basal cell carcinoma, the most common human cancer, is frequently caused by mutations in the Hedgehog pathway. By studying how these signals are controlled in the genetically manageable wing disc, researchers can uncover the mechanisms that maintain normal tissue architecture and learn how their failure contributes to the development of tumors.