Sea squirts, also known as ascidians, are marine organisms that undergo fascinating evolutionary changes during their development. A recent study conducted by scientists from the Heisenberg group at the Institute of Science and Technology Austria (ISTA) has revealed that friction forces play a crucial role in driving these changes in sea squirt development .
Sea squirts start their lives as free-moving larvae but eventually settle down and attach themselves to solid surfaces like rocks or corals. They then undergo a transformation, developing tubes known as siphons, which are their defining feature. Despite their adult appearance as rubbery blobs, sea squirts are actually the most closely related invertebrate relatives to humans. This close relationship makes them an ideal model organism for studying early vertebrate development.
The researchers focused on the embryonic development of sea squirts, particularly the process of cytoplasmic reorganization in oocytes after fertilization. They discovered that friction forces within the oocytes play a crucial role in reshaping and reorganizing their insides, leading to the formation of a bell-like protrusion called the contraction pole (CP). The CP is a crucial structure where essential materials gather to facilitate the embryo’s maturation .
The study revealed that the actomyosin cortex, a dynamic structure found beneath the cell membrane in animal cells, undergoes contraction and flow, resulting in the initial changes in the cell’s shape. However, the actomyosin flow stops during the expansion of the contraction pole, indicating the involvement of additional factors. The researchers identified the myoplasm, a layer composed of intracellular organelles and molecules, as a key player in this process. The myoplasm behaves like a stretchy solid and changes its shape along with the oocyte during fertilization. Friction forces between the actomyosin cortex and the myoplasm cause the myoplasm to fold and form buckles. When the actomyosin movement stops, the friction forces disappear, leading to the expansion of the contraction pole and the resolution of the myoplasm buckles into a well-defined bell-like shape .