Biomedical Engineering Reference
In-Depth Information
Figure 4
. In all figures, the broken circle indicates the position of the oocyte nucleus. (
A
) The
overlap between the dorsal-ventral gradient of Gurken-mediated EGFR signaling and the ante-
rior-posterior gradient of TGF
signaling defines the dorsal-anterior region. (B) The dorsal-
anterior is patterned further by EGFR signaling into three different fates: the operculum-
producing dorsal midline cells (black), the dorsolateral cells that position the future dorsal
appendages (dotted), and the appendage progenitors (striped). The arrows indicate the orienta-
tion in all figures (A = anterior, P = posterior, D = dorsal, V = ventral).
released locally from the oocyte induces a gradient of EGFR signaling in the
overlying follicle cells. The follicle cells closest to the oocyte nucleus, hence
receiving a high level of EGFR signaling, adopt the dorsal fate; the cells far
from the nucleus receive a low level of EGFR signaling and express the ventral
marker genes.
A dorsal-anterior domain in the follicular epithelium is defined by the over-
lap between the gradient of the Gurken-mediated high EGFR activity and a
transverse gradient of activity of the TGF
pathway (Figure 4A) (24). Over
time, the EGFR signaling further divides the dorsal-anterior domain into three
distinct groups of cells: the dorsal midline cells, which will contribute to the
production of operculum; the dorsolateral cells, which will specify the position
of the dorsal appendages; and the dorsal appendage anlagen, which will the se-
crete the appendage materials (Figure 4B).
The mechanism by which EGFR signaling mediates the subsequent refine-
ments in the dorsal cells is not well understood. The currently accepted mecha-
nism (Figure 5), proposed by Wasserman and Freeman (11), is by no means the
complete story. Expression of
rhomboid
is induced through a crosstalk between
the EGFR and TGF
signaling in the dorsal-anterior domain (Figure 4A) (24).
Rhomboid cleaves the Spitz transmembrane precursors, leading to localized
secretion of Spitz. The released Spitz acts as a positive feedback that amplifies
the initial EGFR activity induced by Gurken.
