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2005 ; ä (ä): 1-28
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Vulval development
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2005[Jun]; ä (ä): 1-28
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C. elegans vulval development is an intensively studied example of animal
organogenesis. A network of intercellular signaling, signal transduction, and
transcriptional regulation underlies the precise formation of this organ, which
is the connection between the hermaphrodite uterus and the outside of the
nematode. A single cell of the somatic gonad, the anchor cell, organizes the
development of the vulva from epidermal precursors as well as the physical
connection of the epidermis with the uterus. WNT signaling acting via the HOX
gene lin-39 renders six epidermal precursor cells competent to respond to other
developmental signals. The anchor cell induces nearby epidermal precursor cells
to generate vulval cells via an epidermal growth factor (EGF) signaling pathway.
The precise pattern of vulval precursor cell fates involves the graded action of
the EGF signaling and LIN-12 (Notch) mediated lateral signaling. EGF promotes the
primary fate while LIN-12 promotes the secondary fate. Both EGF and LIN-12
prevent precursor cells from adopting the tertiary fate, which generates
non-specialized epidermis. EGF-receptor and Notch signaling are antagonistic:
EGF-receptor signaling leads to down-regulation of the Notch-like receptor
LIN-12, while LIN-12 signaling induces negative regulators of EGF-receptor
signaling such as MAP kinase phosphatase LIP-1 and the tyrosine kinase ARK-1. The
primary precursor cell generates vulE and vulF mature vulval cells; the pattern
of vulE and vulF cells requires an additional signal from the anchor cell as well
as WNT signaling. The two secondary precursor cells generate vulA, vulB1, vulB2,
vulC and vulD cells but in mirror symmetric polar patterns: ABCD and DCBA. The
reversed polarity of the posterior secondary precursor cell lineage requires WNT
signaling mediated by both Frizzled class and Ryk class WNT-receptors LIN-17 and
LIN-18, respectively. A network of transcription factors controls the seven
mature adult cell types; these include the LIM domain protein LIN-11, the
Pax2/5/8 protein EGL-38, the zinc finger protein LIN-29, and the Nkx6.1/6.2
protein COG-1. The anchor cell also patterns nearby uterine cells, via the DSL
ligand LAG-2 and LIN-12, to generate the four uv1 cells that form the tight
connection with the vulva. This connection is initiated by the anchor cell, which
invades between the vulF cells in a process analogous to invasive behavior of
metastatic tumor cells. During this invasion process, the basement membranes
between the gonad and body wall are degraded. The extensive information about
vulval development in C. elegans has helped it become a paradigmatic case for
identifying and studying a variety of regulatory pathways.
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