Abstract:
Objective This paper describes the structural and functional characteristics as well as current classification of mollusc hemocytes, and summarizes the molecular mechanisms governing their proliferation, differentiation, and death. These insights will enhance the systematic understanding of mollusc immune cells and their fate determination, offering valuable support for future research in mollusc immunology.
Progress Hemocytes, as the primary immune cells in mollusks, exhibit considerable diversity and complexity in their morphology, function, classification, proliferation and differentiation pathways, as well as modes of cell death. Significant variations exist in the overall cellular morphology and the content of intracellular organelles and granules among hemocytes from different mollusks, reflecting their functional diversity and adaptability. Hemocytes are crucial in defending against pathogenic invasion, primarily through phagocytosis, the synthesis and secretion of immune active substances. During phagocytosis, a respiratory burst occurs, catalyzing the production of various reactive oxygen intermediates (ROIs) that kill pathogens engulfed within the cells. Beyond direct pathogen defense, hemocytes also participate in tissue repair and the maintenance of immune homeostasis. Based on cellular morphology and functional characteristics, mollusc hemocytes are mainly classified into three types: agranulocytes, semigranulocytes, and granulocytes. Agranulocytes typically lack distinct granular structures and can participate in wound repair; semigranulocytes contain a small number of granules and possess certain phagocytic and bactericidal capabilities; granulocytes, on the other hand, are rich in granules that store various bactericidal substances and enzymes. These three types of hemocytes collaborate to form a complex immune defense system in molluscs. The proliferation, differentiation and death of hemocytes represent a complex and dynamic process involving the interaction of multiple molecules and the regulation of various signaling pathways. Regarding proliferation, hemocytes are regulated by multiple transcription factors, growth factors, and cytokines. During differentiation, hemocytes can differentiate into various types of immune cells under specific microenvironments and molecular regulation to meet different immune demands. Hemocytes undergo various forms of programmed cell death, including apoptosis, pyroptosis, autophagy, ferroptosis, and cuproptosis. The proliferation, differentiation, and death of hemocytes play crucial roles in the immune defense and maintenance of immunological homeostasis in molluscs.
Prospect Although significant progress has been made in the research on mollusc hemocytes, there still remain numerous challenges. To address the complexities of their structure, function and classification, advanced techniques are needed to establish a unified and scientific classification system. Furthermore, systems biology approaches integrating multi-omics data should be utilized to reconstruct the molecular regulatory networks governing hemocyte proliferation and differentiation. Regarding cell death, compare the mechanistic differences in various types of cell death in molluscs and clarify the interrelationships among different forms of cell death. Integrating these findings on cell death with the established hemocyte classification and regulatory networks will be instrumental in systematically elucidating the fate-determination mechanisms of various immune cells. These fundamental theoretical advances will not only enhance our understanding of the evolution and mechanisms of the mollusc immune system but also provide crucial theoretical support for the prevention and control of diseases in mollusc aquaculture.