Organ Morphogenesis

All multicellular organisms embrace an intricate set of organs with unique architecture and functionality. Organogenesis, i.e. formation of an organ, involves a coordination of biochemical signals and mechanical cues. Although a significant advancement has been made in understanding the genetic regulation of different cell and tissue behaviors during 3D organ development, the role of mechanical force is only beginning to emerge. We use Caenorhabditis elegans reproductive organ, i.e. gonad as a model, to explore the physical basis of organ development.
Morphogenesis of the nematode Caenorhabditis elegans gonad is guided by two somatic cells known as distal tip cell (DTCs). The C. elegans 

hermaphrodite has two U-shaped gonadal arms which form post-embryonically. After hatching, the gonad primordium consists of two somatic precursors – Z1 and Z4 – giving rise to somatic structures (uterus, spermatheca, sheath cells and DTCs) and two germline precursors – Z2 and Z3 – which form the entire germline. The trajectory of the two DTCs, present at the two ends of the gonad primordium, determines the final morphology of the mature hermaphrodite gonad (See image below). DTCs function as leader cells guiding several follower germ cells in three distinct phases. In the first phase, during larval stage 2 (L2) and early larval stage 3 (L3) the two DTCs move away from each other. The second phase involves a turn of DTCs from ventral to dorsal surface. Finally, during early L4 stage DTCs elongates on the dorsal surface towards the middle position of the body, ultimately forming two U-shaped gonadal arms.

The transparent worm body allows visualization and analysis of gonad morphology using a dissecting microscope. Other advantages of using it as a model for studying gonad morphogenesis include its invariant developmental pattern, simpler anatomy, and ease of genetic manipulation.

Despite a significant advancement in the identification of molecular signals, the role of mechanics in gonad development has remained unexplored. Gonad morphogenesis is analogous to the phenomena of branching of tubular organs in vertebrates and cancer metastasis. Hence, understanding the molecular and physical aspects of gonad development might help us to gain insights into organogenesis and invasive tumors.