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Ultrastructural and autoradiographic studies of non-generative cells in the antheridium of Chara vulgaris L. III. Shield cells.

Shield cells form the antheridium envelopes. At the first stage of spermatogenesis they grow intensively in the tangential direction, which is stopped during the period of spermatozoid differentiation. The increase in shield cell volumes is associated with the increase in DNA level in the nucleus up to 16-32 C. 3H thymidine incorporation occurs in about 30% of shields at younger developmental stages and lasts until the stage in which 16 celled antheridial filaments predominate. At first stage of spermatogenesis the intensity of 3H leucine incorporation increases as DNA amount in the nuclei increases, reaching the maximum value at the end of this period. During spermiogenesis it gradually decreases. Shield nuclei are characterized by low content of condensed chromatin, the presence of numerous nucleoli with nucleolonema-like structure as well as the occurrence of bands of intranuclear microtubules. It has been suggested that these microtubules are associated with cyclical changes in the shapes of nuclei. During DNA replication the nuclei have the form of flat discs which between successive endoreplication cycles become ring shaped. Peripheral zone of shield cells is compartmentalized through incomplete walls. They support the radial walls of shields increasing the contact surface of plasmalemma with a cell wall. During spermiogenesis the increase in plasmalemma surface results from the growth of shields in the radial direction. The shield cells contain plastids placed close to each other at the inner tangential wall. They are orange in colour and have fully formed system of grana and intergrana thylakoids, like the plastids of the thallus. The number and sizes of the plastoglobules increase as the anteridium develops. Dictiosomes are surrounded with numerous smooth and coated vesicles. Mitochondria exhibit poorly condensed structure. Microbodies adjoining the plastids are sporadically encountered. It has been assumed that changes in structural organization as well as growth character of shield cells constitute the factor regulating the exchange with external environment, determine light spectrum penetrating to the antheridium and the volume of antheridial space.