LOCALIZATION AND QUANTIFICATION OF CYCLIN-A AND CYCLIN-B MESSENGER-RNA DURING THE EMBRYONIC-DEVELOPMENT OF PATELLA-VULGATA

As the first five cleavages of the Patella vulgata embryo are synchron ous, they are well suited to determine the mRNA level of cyclin A and B genes in an embryo. During the third and fourth cleavage cycle the q uantity of A and B mRNA is regulated in a cell-cycle-dependent way, re aching a high level between cleavages and a lower level just after mit osis. This implies that transcription of the cyclin genes occurs befor e the overall transcription increases directly after the fifth cleavag e. During the first cleavages cyclin A and B mRNA is localized in dist inct parts of the cytoplasm. Between two successive cell devisions it is found as a crescent-shaped domain at the peripheral side of the nuc leus. At cytokinesis it is present between two separating nuclei and a t newly formed cell membranes. At the fifth cleavage this localization disappears. Changes in the expression pattern of cyclin A and B may b e expected after the fifth cleavage, when the first cells become arres ted in cell division and differentiate. The mechanism causing cell div ision arrest of these primary trochoblasts is still un known. Cell div ision arrest caused by the absence of cyclin A and/or B mRNA could be conditional for further differentiation. However, a decrease in cyclin A and B mRNA level in the trochoblasts is not detectable until 4 h af ter their last division. Later in development no cyclin A and B mRNA c an be detected in these cells, whereas cyclin A and B mRNA is present in other cells of the embryo. Thus, the absence of cyclin A and B mRNA in primary trochoblasts, and in the later differentiating secondary a nd accessory trochoblasts is not obligatory for cell division arrest o r cell differentiation.