Biology Reference
In-Depth Information
a quiescence phase. They are then reactivated during the larval stage and
continue to proliferate during larval and pupal stages. These postembryonic
NBs generate
90% of neurons that constitute the adult CNS (
Prokop &
Technau, 1991; Truman & Bate, 1988
). Relatively less is known about
the postembryonic lineages as compared to embryonic lineages, but several
recent studies have shed some light on the temporal control of larval lineages
(see below).
1.2. Olfactory system
Another well-characterized system is the olfactory system, including
the antennal lobe, which receives olfactory inputs from olfactory receptor
neurons (ORNs), and the mushroom body, which receives inputs from
projection neurons (PNs) in the antenna lobe. Systematic clonal analysis
of mushroom body neurons using the mosaic analysis with a repressible cell
marker (MARCM) technique has demonstrated that each of the four mush-
room body NBs sequentially generates at least three types of neurons,
g
neu-
rons first, followed by
a
0
b
0
, and then
ab
neurons (
Lee et al., 1999
), suggesting
birth-order-dependent neuronal specification. Similarly, the antennal lobe
PNs that are derived from three NBs (an anterodorsal, a lateral, and a ventral
NB) are specified by lineage and birth order to connect with specific classes of
ORN axons (
Jefferis et al., 2001
). These three lineages have been extensively
characterized (
Das, Reichert, & Rodrigues, 2010; Das et al., 2008; Lai,
Awasaki, Ito, & Lee, 2008; Lin, Kao, Yu, Huang, & Lee, 2012; Lin et al.,
2010; Yu et al., 2010
). For example, the anterodorsal lineage produces
40 types of PNs in a stereotypic order, while the lateral lineage sequentially
generates 48 pairs of local interneurons paired with distinct PNs.
1.3. Medulla in the optic lobes
The
Drosophila
optic lobes, composed of lamina, medulla, and lobula
complex, are the processing centers of visual information in the brain.
Among them, the medulla is the largest neuropil. It contains approximately
40,000 neurons, belonging to more than 70 different cell types (
Fischbach &
Dittrich, 1989; Morante & Desplan, 2008
). These neurons are generated
by NBs derived from a crescent-shaped single-layered neuroepithelium
(NE) in the larval brain called the outer proliferation center (
Green,
Hartenstein, & Hartenstein, 1993; White & Kankel, 1978
). During devel-
opment, a wave of neurogenesis progresses from the edge of the crescent
toward its center, and sequentially converts NE cells into medulla NBs.
