An Electron Microscope Study on Regeneration of Trichocysts in Paramecium caudatum*

Abstract

SYNOPSIS. A study of regeneration of trichocysts in Paramecium caudatum was made in the electron microscope. Organisms, from which trichocysts were removed en masse with electric current, were allowed opportunity to regenerate their lost organelles before they were prepared for microscopical examination of developing stages.Thin sections of organisms from various regeneration periods demonstrate clearly the endoplasmic origin and development of trichocysts. The first stage seen is a membrane‐limited, endoplasmic vesicle enclosing an homogeneous mass, the primordium, within which appears a minute, point condensation of greater electron density. This is subsequently elaborated as a closely packed system of linearly oriented fibrous elements (30–50 Å)—the first definitive sign of the presumptive trichocyst. By progressive utilization of the intra‐vesicular primordial matter in association with apparently continuous provision of the building material by extra‐vesicular biosynthetic activity, the form and structure of the juvenile trichocysts recognizable as a system of closely packed, multi‐layered, fibrous sheets become increasingly evident. The basic structural patterns extend also into the now developing tip. Further growth and differentiation involving the transition of its gross shape from oval to oblong effect a second notable morphological transformation: the formation of a dense U‐shaped cortex enclosing a lightly stained center. Full maturation of the trichocyst includes the formation of the cap, disappearance of the limiting membrane, reduction and ultimate loss of the dense cortex with simultaneous assumption of light electron stain homogeneity of the body, and elaboration of the characteristic periodic striation of the tip. These neo‐formative events, which also occur—although with reduced frequency—in normal vegetative forms, in conjugating pairs, and in dividing organisms, are concluded prior to the final positioning of the organelle in the bipolar ciliary meridian.The implications of these findings are discussed in light of current hypotheses concerning the role of the nucleus and ciliary basal bodies in the genesis of cortical organelles.