The antennae are a crucial sensory organ in crayfish that plays vital roles in their survival, communication, and navigation within their environment. These appendages are equipped with sensory hairs that enable crayfish to detect various stimuli. Crayfish possess the unique ability to regenerate their antennae. Hemocytes are specialized cells within crayfish and play a crucial role in their immune system and contribute to the process of regeneration. Understanding the mechanisms underlying antenna regeneration provides detailed insights into crayfish biology and has implications for regenerative studies in other organisms. Studying crayfish antenna regeneration could inspire advancements in regenerative medicine and tissue engineering, potentially leading to the development of therapies for sensory organ injuries in advanced veterinary and human medical sciences. We applied cryo-scanning and Transmission electron microscopies to study sensory organ morphology and regeneration in crayfish.In Chapter 2, cryo-scanning electron microscopy was used to identify potential differences in antennal morphology in six different crayfish species including marbled crayfish Procambarus virginalis, Mexican dwarf crayfish Cambarellus patzcuarensis, red swamp crayfish Procambarus clarkii, signal crayfish Pacifastacus leniusculus, common yabby Cherax destructor, and spiny-cheek crayfish Faxonius limosus. We observed significant differences among the six crayfish species in the ratios of antenna length, segment length and width to carapace length, and the number of segments. Examination of ultrastructural features unveiled differences in the distribution patterns of sensory hairs along the antenna and the morphology of the antennal surface. The varying morphology of antennae among studied species possibly indicates an adaptation to the specific conditions of their respective habitats. Furthermore, the outcomes demonstrated that a combination of variations in both antennal morphological characteristics and biometric measurements could effectively help us to distinguish the different studied crayfish species.In Chapter 3, the ultrastructural behaviour of hemocytes during coagulation and phagocytosis in the initial phases of injury in marbled crayfish was explored using transmission electron microscopy. During the coagulation process, hemocytes experienced marked transformations in morphology. The cytoplasmic granules exhibited a change from electron-dense to electron- lucent forms with the progress of coagulation. The transformed granules containing amorphous, electron-lucent material were observed to combine and release their contents into the extracellular space as part of the coagulation process. Additionally, it was observed that the nucleus contents also play a role in the coagulation process. Furthermore, the amputation of the leg led to substantial muscle degeneration, and phagocytic hemocytes start to take up the necrotic tissues. Besides, we observed the digested remains from phagocytized necrotic tissues merged into granules and other cellular components, thereby enhancing the granularity of the hemocytes and altering their cellular morphology. However, it is essential to note that hemocyte degranulation during coagulation could potentially reduce their granularity. Since morphological features are critical for classifying hemocytes, these morphological changes during coagulation and phagocytosis must be considered significant factors. The degenerated material that stored inside phagocytic hemocytes may have application in immunological and regeneration processes.In Chapter 4, we employed transmission electron microscopy to explore the ultrastructural aspects of potential immune cell involvement in nerve regeneration within crayfish antennae post-amputation. The findings indicated that, during nerve regeneration, all three forms ofhemocytes were present. However, the granules within semi-granulocytes and granulocytes mainly contributed to the generation of new organelles such as mitochondria, the Golgi apparatus, and nerve fibers in the regenerated nerves of crayfish antennae. We revealed the transformation of granules of hemocytes into diverse organelles during the process of nerve regeneration at the ultrastructural level. In conclusion, these granules function as compact repositories of adaptable materials carried by immune cells, capable of transformation into various organelles during nerve regeneration in crayfish antennae.To sum up, the antennae of crayfish as a sensory organ have diverse morphology to let them sense their specific environments. The immune cells, as the major drivers of regeneration in crayfish, can absorb material from decomposing tissues via phagocytosis and reuse them as a source for the production of new organelles in the regenerating nerve of crayfish antenna. This research would not have been possible without the financial support from the following projects: - The Grant Agency of the University of South Bohemia in České Budějovice (GAJU) projectsNo. GAJU 035/2021/Z, GAJU 063/2021/Z, GAJU 055/2022/Z and GAJU 012/2022/Z. - Laboratory experiments were performed using the facilities of the CENAKVA research center(LM 2023038) in accordance with the objectives of the European consortium DANUBIUS RI.