SPHEROIDAL AND RING NUCLEOLI IN AMPHIBIAN OOCYTES : Patterns of Uridine Incorporation and Fine Structural Features

In maturing oocytes of the newt Triturus viridescens, the nucleoli undergo a series of morphological changes that are very similar to those described by Callan for the axolotl, Ambystoma mexicanum. The nucleoli first assume the form of spheroids which then become extended into ring or necklace shapes that are DNase-sensitive; in mature oocytes the nucleoli revert to a spheroidal form. Short term in vitro incorporation studies with uridine-³H on both species show that RNA synthesis occurs in a restricted, eccentric portion of the spheroidal nucleoli, thereby producing an asymmetrical pattern of labeling. In the ring forms, however, the localization of the radioactivity suggests that synthesis takes place symmetrically throughout their entire length. The changes in nucleolar morphology apparently reflect the fact that the component DNA has undergone a redistribution from a localized region in the spheroidal nucleoli to an extended circle in the rings; the patterns of uridine-³H incorporation, therefore, parallel the distribution of DNA in both the spheroidal and the ring nucleoli. Ultrastructurally, the nucleoli contain a fibrillar component that corresponds in position to that of the DNA. The typical spheroidal nucleolus consists of a fibrillar core situated eccentrically and surrounded by a hull of granular, ribonucleoprotein material. The ring nucleoli are composed of a central fibrous region that is ensheathed all around its circumference by a layer of similar granular material. This granular substance is thicker at intervals along the length of the rings, representing the "beads" of the necklaces.     

System for the Investigation of the Bacteriophage-directed Synthesis of Diphtherial Toxin

Lytic corynebacteriophage betahv64(tox+) has been characterized, and methods for studying the expression of its tox(+) gene in nontoxinogenic Corynebacterium diphtheriae strain C7(s)(-)(tox-) described. During one cycle of viral growth there was a 1 million-fold increase in extracellular toxin. Both the conditions of the experiment and the use of purified phage, free from toxin, support the conclusion that all of the toxin was newly formed. This toxin was immunochemically indistinguishable from standard toxin produced by the PW8(r)(Pdi)(tox+) strain. Chloramphenicol was found to be an effective agent for synchronizing the initiation of viral growth. Once chloramphenicol was removed, intracellular toxin appeared and continued to increase throughout the latent period. Proflavine, added early in the latent period, blocked phage maturation without similarly affecting yields of toxin. Iron exerted a limited inhibitory effect on final toxin levels attained.     

ALTERATIONS IN THE CYTOLOGIC DETAIL OF INTESTINAL SMOOTH MUSCLE CELLS IN VARIOUS STAGES OF CONTRACTION

The fine structure of the longitudinal layer of the tunica muscularis of the mouse jejunum was studied in various stages of mechanically stimulated contraction. The relaxed cell is long and narrow with smooth cytoplasmic and nuclear contours. As contraction progresses, the cell becomes ellipsoid and its borders exhibit invaginations at the points of myofilamentous attachment to the plasma membrane and vesicle-containing projections of the intervening membrane. These changes are interpreted as representing the deforming forces applied by the myofilaments to the plasma membrane. The nucleus of the contracted cell is shortened and widened, with convolution of its limiting membranes. This alteration, as well as progressive changes in the alignment of cytoplasmic organelles, is thought to be due to forces exerted on the internal structure of the cell by the contractile elements. The myofilaments form a network of oriented bundles during contraction. Aggregates of filaments of two different diameters are noted. The two sizes of filaments intermingle only in small areas of increased density. These dense areas increase in length and number during contraction. A model of the functional organization of the cell is proposed.