DNA replication in fertilized eggs of the starfish Asterina pectinifera." Early starfish embryos were labeled with 5-bromo-2''-deoxyuridine (BrdU; the thymidine analogue) for 3-4 min of pulse, and the BrdU incorporated into newly replicated DNA was immunocytochemically detected with anti-BrdU antibody. Series of optical sections of the stained embryos observed with a confocal laser scanning microscope (CLSM) were collected, and three-dimensional images of the chromatin and the incorporated BrdU were reconstructed on a computer display. The distribution of the DNA replication sites and the chromatin structure were investigated from a three-dimensional point of view. The oocytes fertilized during maturation and the eggs fertilized after maturation were examined for the timing of S phase, DNA replication pattern, and chromatin distribution in the nucleus. Duration of the first S phase between the two types of embryos differed in the first S phase, but was the same in the second and third S phases. We observed a successive change in the distribution of DNA replication sites from a ""uniform"" to a ""condensing"" pattern during a single S phase traverse, except in the first S phase of the eggs fertilized after maturation. These two patterns of DNA replication almost coincided with chromatin visualized by anti-single-stranded DNA antiserum. The apparent change in the pattern of DNA replication sites thus simply reflects the change in the spatial distribution of chromatin. The spatial pattern of DNA replication at the first S phase of the eggs fertilized after maturation was unique in that the whole nucleus was a mosaic of two distinct regions, each supposedly derived from the female and male pronuclei. The putative female region had a fibrillar pattern of DNA replication sites confined to the periphery of the nucleus, while the male region showed condensed sites of DNA replication, which persisted longer than the sites in the female pronuclear region. We discuss such a singularity of the first S phase in starfish eggs on the basis of the temporal relation between the progress of maturation and the time of activation."