Fine Structure of the Macronucleus of the Gymnostome Ciliate, <i>Chlamydodon pedarius</i>

Abstract

The fine structure of the macronucleus of the gymnostome ciliate Chlamydodon pedarius at young stage and interphase approaching the division has been investigated by means of the electron microscopy.The macronucleus comprises two different structures: the anterior half is less dense structure which consists of the diffuse filamentous matrix material, and contains one endosome composed of much dense filamentous material in its center; on the contrary the posterior half is a very dense structure, and consists of a reticulum made of very dense strands or cords, very dense nucleolar materials and little less dense matrix material. However, the fine structure changes in the stages of the young macronucleus and of the interphase approaching the division.The nucleoli are distributed beneath the envelope with a short distance. The structure is composed of filamentous material, dense particles of various size, and less dense matrix in the young macronucleus, but in the interphase macronucleus approaching the division it shows very loose and vanishing structure of aggregate of granular texture. They do not have any retaining membrane always.The longitudinal stripes of about 100mμ to 200mμ in width are found in the macronucleus approaching the division. They run parallel to the long axis of the macronucleus, seeming to contribute the elongation of the macronucleus, and are inferred to be preliminary form of the chromonemata in the very early prophase macronucleus.The highly electron-scattering threads of the order of 25mμ to 30mμ in thickness and the electron-dense helices of about 15mμ to 20mμ thick are found in the corresponding places to the DNA containing elements of the macronucleus, and the filament making up them seems to correspond to a nucleoprotein complex. It is possible, consequently, to draw the following conclusions: The individual thread appears as a result of duplication of the filament; the strands in the young macronucleus are fragment of the chromonemata; the cords constituting the network of the posterior half represent the chromonemata in the interphase or very early prophase macronucleus.The structure of an envelope enclosing the macronucleus is composed of double membranes of about 50A thick respectively, separated by a space of about 200A wide, and numerous penetrating pores of about 400A diameter in normal area.