Vulva Formation in C. elegans by Dr.M.Jothimuniyandi

Vulva Formation in C.elegans - Dr.M.Jothimuniyandi - Organogenesis-Caenorhabditis elegans -Vulval Precursor Cells (VPCs) and Cell Lineage-Inductive Signaling from the Anchor Cell -RTK/Ras/MAPK Pathway-Lateral Signaling via Notch Pathway-Cell Fates and Pattern Formation-6.Gene Regulation and Feedback Mechanisms-Morphogenesis of the Vulva-Additional Pathways-Vulval Development and Evolution

1. Organogenesis – Vulva formation in Caenorhabditis elegans ( C. elegans ) Dr.M.Jothimuniyandi Assistant Professor

2. Organogenesis Organogenesis is the process during embryonic development where organs form from the three primary germ layers: ectoderm, mesoderm, and endoderm. It is a critical phase that follows gastrulation and continues throughout fetal development, eventually leading to the formation of fully functional organs in multicellular organisms.

3. Caenorhabditis elegans Caenorhabditis elegans ( C. elegans ) is a small, free-living, transparent nematode (roundworm) that has become one of the most extensively studied organisms in biology. It is particularly popular in developmental biology, neurobiology, genetics, and molecular biology due to its simplicity, well-mapped genetic structure, and clear developmental stages.

4. Vulva formation in Caenorhabditis elegans Vulva formation in Caenorhabditis elegans ( C. elegans ) is a well-studied model for understanding the mechanisms of organogenesis and cell fate determination. The process is primarily driven by intercellular signaling , cell-cell communication, and gene regulation.

5. 1. Vulval Precursor Cells (VPCs) and Cell Lineage: Vulva formation begins with six vulval precursor cells (VPCs) known as P3.p, P4.p, P5.p, P6.p, P7.p, and P8.p . These VPCs are located in the ventral epidermis and are descendants of the Pn.p cells . Out of these six, three VPCs (P5.p, P6.p, and P7.p) are induced to form the vulva. The rest (P3.p, P4.p, and P8.p) typically fuse with the surrounding hypodermis and do not contribute to vulva formation.

6. 2. Inductive Signaling from the Anchor Cell (AC): The anchor cell (AC) , a specialized cell in the somatic gonad, plays a key role by providing an inductive signal to the VPCs . The anchor cell releases LIN-3 , a ligand of the epidermal growth factor (EGF) family, which is crucial for the induction of vulval cell fates . LIN-3/EGF binds to the receptor tyrosine kinase (RTK) LET-23 on the VPCs, initiating the RTK/ Ras signaling pathway.

7. 3. RTK/ Ras /MAPK Pathway: Binding of LIN-3 to LET-23 on P6.p activates a cascade involving the Ras protein LET-60 and MAP kinase (MPK-1 ) . This pathway activation leads P6.p to adopt the primary (1°) cell fate , which is central to vulval development . As a result, P6.p divides and differentiates into cells that form the central part of the vulva.

8. Vulva formation in Caenorhabditis elegans

9. Vulva formation in Caenorhabditis elegans Ref: https://littleflowercollege.edu.in/upload/e_contents/files/345d6fa88efd898b0e76bdd3d9ca1089.pdf

10. 4. Lateral Signaling via Notch Pathway: The cells adjacent to P6.p (P5.p and P7.p) are induced to adopt the secondary (2°) cell fate through lateral signaling . P6.p, having adopted the primary fate, sends lateral signals to P5.p and P7.p via the LIN-12 Notch pathway. This pathway is activated when the ligand LAG-2 binds to the Notch receptor LIN-12 on the neighboring cells . Activation of LIN-12 in P5.p and P7.p inhibits them from adopting the 1° fate and directs them to follow the 2° fate.

11. 5. Cell Fates and Pattern Formation: The establishment of primary (1°) and secondary (2°) cell fates creates a precise pattern: P6.p : Primary (1°) fate – central cells of the vulva. P5.p and P7.p : Secondary (2°) fate – cells that form the lateral parts of the vulva . The remaining VPCs (P3.p, P4.p, P8.p) generally adopt a tertiary (3°) fate and do not contribute to vulval development.

12. 6.Gene Regulation and Feedback Mechanisms: Several key regulatory genes are involved in the vulva formation process, controlling cell signaling , fate determination, and feedback mechanisms: lin-3 : Encodes the EGF-like ligand responsible for primary fate induction. let-23 : Encodes the EGF receptor on VPCs. let-60 ( Ras ) : A key regulator of the Ras signaling pathway, activated by the LET-23 receptor. lin-12 : The Notch receptor responsible for lateral signaling , ensuring that neighboring cells adopt secondary fates. mpk-1 (MAPK) : Involved in executing the fate decision in VPCs. Feedback loops ensure that the pattern is maintained and that the correct number of cells adopts each fate. For example, overactivation or inhibition of the RTK/ Ras pathway leads to abnormal patterns, such as multivulva ( Muv ) or vulvaless ( Vul ) phenotypes

13. Mechanisms of Vulva formation in Caenorhabditis elegans Ref: https://link.springer.com/protocol/10.1007/978-1-4939-7219-7_3

14. 7. Morphogenesis of the Vulva: After the fate of VPCs is determined, the cells undergo multiple rounds of cell division to generate a total of 22 cells that form the vulva . These cells differentiate into specific vulval rings (termed vulA to vulF ) that contribute to different parts of the vulval structure, such as the opening, lumen, and supporting tissues . Proper morphogenesis depends on precise cell movements, adhesion, and signaling between cells.

15. 8. Additional Pathways: Other pathways, such as the Wnt signaling pathway , also play a role in regulating vulval development, particularly in the orientation and positioning of VPCs and their daughter cells . Mutations in components of the Wnt pathway can disrupt the spatial arrangement of vulval cells.

16. 9. Vulval Development and Evolution: Vulval formation in C. elegans serves as a powerful model for studying evolution and developmental biology. The mechanisms of vulva formation are highly conserved in related nematode species, although variations in signaling and timing occur, providing insights into evolutionary developmental biology ( evo-devo ).

17. Summary Vulva formation in C. elegans is a precisely orchestrated process regulated by intercellular signaling pathways, especially the RTK/ Ras /MAPK pathway for primary induction and the Notch pathway for lateral signaling . The combination of these pathways ensures that cells adopt the correct fate and contribute to the proper formation of the vulva. This model has provided critical insights into the molecular mechanisms of organogenesis and cell fate determination.

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