The Protozoan Nucleus

Publisher Summary This chapter discusses the protozoan nucleus. The nuclei of tissue cells of multicellular organisms are of rather uniform appearance. As a rule, chromosomes are not recognizable in them. In contrast, chromocentric nuclei are absent in protozoa. Aside from two exceptions, heterochromatin, including sex chromosomes, is unknown in unicellular organisms. The appearance of resting nuclei in protozoa is determined to a high degree by the variability of the chromosomal material. In protozoa, the chromosomes undergo changes of form and structure in connection with the coiling cycle similar to those of higher organisms. The chromosomes are often quite uncoiled and extended in the interphasic type of resting nuclei and are more or less tightly coiled in the non-interphasic or chromosomal type. the quantity and distribution of the nucleolar material may also be responsible for the appearance of protozoan nuclei. One frequently encountered type is the karyosome nucleus, which is characterized by a single centrally located nucleolus. It is found in interphasic and in chromosomal resting nuclei of euglenoids and foraminiferans.