| Popis: |
This chapter explores ultrastructural aspects of crustacean oogenesis. It focuses on various cellular processes associated with female germline development in selected crustacean groups. Oogenesis in crustaceans comprises four stages: proliferation of germline cells, previtellogenesis, vitellogenesis, and formation of egg coverings. The greater part of oogenesis occurs in the ovary. In Crustacea, two structurally and functionally distinct types of ovary are recognized: panoistic and meroistic. In panoistic ovaries, all germline cells differentiate into oocytes, and this type of ovarian organization occurs in a great majority of crustaceans, including Malacostraca. In contrast, in the meroistic ovaries, oogonial cells are connected by intercellular bridges and form characteristic linear cysts. Within each cyst, only one cell becomes an oocyte, and the remaining cells differentiate into nurse cells. Meroistic ovaries are typical for Branchiopoda and Ostracoda: Podocopida. Ultrastructural studies reveal that the nucleus and cytoplasmic organelles of the oocyte are highly synthetically active in the panoistic ovary, whereas in the meroistic type, oocyte development is supported, to some extent, by accompanying nurse cells. During previtellogenesis, oocytes accumulate large numbers of various organelles, e.g. ribosomes, mitochondria, and cisternae of endoplasmic reticulum. The oocyte cytoplasm also contains characteristic disc-shaped bodies and cortical granules. A comparative analysis of the proteinaceous yolk formation in different crustaceans reveals two distinct types of vitellogenesis (autosynthesis and heterosynthesis), and indicates that a mixed type prevails in these arthropods. In most crustacean species, germline cells associate with somatic follicle cells that may fulfill several functions during oogenesis. |
