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1. Vulva formation in Caenorhabditis elegans

2. Single cells can induce changes in their neighbors The tiny nematode Caenorhabditis elegans is used as a model organism in many biological studies, but it is especially useful for studying development. It normally lives in the soil, where it feeds on bacteria, but can also grow in the laboratory if supplied with its food source. ‘ The process of development from fertilized egg to larva takes only about 8 hours, and the worm reaches the adult stage in just 3.5 days. The process is easily observed using a low-magnification dissecting microscope.

3. The adult nematode is hermaphroditic, containing both male and female reproductive organs. It lays eggs through a pore called the vulva on the ventral (belly) surface. During development, a single cell, called the anchor cell , induces the vulva to form. If the anchor cell is destroyed by laser surgery, no vulva forms.

4. The anchor cell controls the fates of six cells on the animal’s ventral surface through two molecular switches. Each of these cells has three possible fates. It may become a primary vulval precursor cell, a secondary vulval precursor cell, or simply part of the worm’s surfaces- an epidermal cell. The anchor cell produces an inducer that diffuses out of the cell and interacts with adjacent cells. Cells that receive enough of the inducer become vulval precursor cells; cells slightly farther from the anchor cell become epidermis.

5. The first molecular switch, controlled by the inducer from the anchor cell, determines whether a cell takes the “track” toward becoming part of the vulva or the track toward becoming epidermis. The cell closet to the anchor cell, having received the most inducer, differentiates into the primary vulval precursor cell. It produces its own inducer, which acts on the two neighboring cells and directs them to become secondary vulval precursor cells. Thus, the primary vulval precursor cell controls a second molecular switch, determining whether a vulval precursor will take the primary track or the secondary track. The two inducers control the activation or inactivational specific genes in the responding cells.

6. Much of development is controlled by molecular switches that allow a cell to proceed down one of two alternative tracks. The primary inducer released by the C. elegans anchor cell appears to be a growth factor (EFG). The nematode growth factor called LIN-3 binds to a receptor on the surface of a potential vulval precursor cell. This binding sets in motion a signal transduction cascade involving the Ras protein and MAP kinases. The end result is increased transcription of the genes involved in the differentiation of vulval cells. The development of the vulva in C. elegans offers several examples of induction on the cellular level. The formation of the anchor cell is mediated by the lin-12 gene, the C. elegans hermaphrodites two adjacent cells, Z1.ppp and Z4.aaa, have the potential to become the anchor cell. They interact in a manner that causes one of them to become the anchor cell while the other one becomes the precursor of the uterine tissue.

7. Studies using genetic mosaics and cell ablations have shown that this decision is made in the second larval stage and that the LIN-12 gene only needs to function in that cell destined to become the uterine precursor cell. The presumptive anchor cell does not need it. Seydoux and Greenwald (1989) speculate that these two cells originally synthesize both the signal for uterine differentiation (the LAG-2 protein,) and the receptor for this molecule (the LIN-12 protein). The cell secreting LAG-2 becomes the gonadal anchor cell while the cell receiving the signal through its LIN-12 protein becomes the ventral uterine precursor cell. Thus the two cells are thought to determine each other prior to their respective differentiation events.