Neurogenesis and neuronal circuit formation in the Drosophila visual center.

The Drosophila optic lobe is composed of a wide variety of neurons that form laminar structures and columnar units. The fly optic lobe shares structural features with the mammalian brain, and fly genetics allow precise genetic manipulations. Thus, the Drosophila visual center is an excellent model for studying the mechanisms underlying the establishment of a functional neuronal circuit during brain development. However, little is understood about the developmental mechanisms that produce neuronal diversity and establish neuronal circuits in the medulla, the largest component of the optic lobe. Our recent research revealed key features of medulla development, such as birth-order-dependent specification of neuronal types and the subdivision of the medulla primordium into concentric zones, which is characterized by the expression of four transcription factors. Here, we review recent investigations into the development of the medulla and discuss the mechanisms that establish functional neuronal circuits.