Berndt E. Hagström

Electron microscopic studies of unfertilized and fertilized eggs from marine teleosts

Abstract Ripe eggs of several species of marine teleostean fishes were studied in the unfertilized state and after insemination. The ultrastructure of the fish egg was investigated and several species-specific differences were discovered in the surface layers of the ripe egg. The electron microscopic changes found upon fertilization of the egg are discussed.

Experimental studies of Strongylocentrotus droebachiensis and S. pallidus

Abstract The problem as to whether Strongylocentrotus pallidus (G. O. Sars, 1871) is a distinct species or only a colour-variety of S. droebachiensis was subjected to a renewed investigation. For the first time simultaneous experiments on homologous and heterologous fertilization of S. droebachiensis and S. pallidus were carried out. These experiments revealed the existence of clear specificity barriers between the two forms. Studies of mitosis in hybrid embryos indicated that chromosomal aberrations are frequent. Electron microscopic investigations demonstrated that the ultra-structure of the cortical granules is different in the two species. The differences observed supply strong evidence in support of the opinion that Strongylocentrotus pallidus is a good species.

Morphological and experimental studies on the genus Echinus

Abstract The biological variation in the population of the genus Echinus from the vicinity of Bergen was studied. The morphological characters previously used in distinguishing between the species were discussed and new species-specific characters were established. The number of adult hybrid animals was found to be considerable. An experimental investigation of fertilization, cross fertilization and development in the genus Echinus was performed.

Time-lapse and electron microscopic studies of sea urchin micromeres

SummaryThe cleavage pattern of the young sea urchin embryo was studied by means of light and electron microscopy.The micromeres, which are known to have a strong organizing effect on the embryo, were found to form a syncytium with their neighbouring micromeres and with the macromeres. The cell walls between these cells were observed to be incomplete while there were interphase nuclei with intact nuclear membranes in the micro- and the macromeres. Similar phenomena with a break down of the cell membranes were not observed between macro- and mesomeres while there were intact interphase nuclei in these cells. Micromeres implanted on macromeres or mesomeres were found to coalesce with these latter cells in the course of a few minutes. During interphase, when the nuclei of both micro- and mesomere (macromere) had intact nuclear membranes, there also was a break down of the cell walls and a syncytium was formed by the “host cell” and the implanted micromere (see Fig. 6).The primary mesenchyme cells, which are regarded as the descendants of the micromeres, were also studied and were likewise found to form true syncytia.The importance to embryogenesis of this unique formation of syncytia is discussed.

The effects of Esso Corexit 9527 on the fertilizing capacity of spermatozoa

Abstract The water-soluble oil dispersant Esso Corexit 9527 has earlier been found to interfere, even in low concentrations, with fertilization and development. Further studies of the effect on sea urchin spermatozoa demonstrate that Corexit 9527 gives negative biological effects in concentrations down to 0.0003 ppm.

Scanning electron microscope studies of the surface of the sea urchin egg.

SummaryThe surface of the egg of the two sea urchin species,Strongylocentrotus droebachiensis andS. pallidus, was studied with SEM. Clear differences were observed between the two species, in both the number and form of the microvilli. The changes in the egg surface which follow insemination are described.

Studies of cleavage and development of isolated sea urchin blastomeres

Abstract Isolated blastomeres from young larvae of Echinocyamus pusillus and Psammechinus miliaris were studied by means of time-lapse cinematography. The isolated mesomeres and macromeres cleave at about the same rate as in situ in intact larvae. It was found that single meso- and macromeres were able to develop into blastulae. Isolated micromeres cleave somewhat more slowly than the micromeres in situ and as isolated cells they were not able to pass through more than three cleavages. They are characterized by the sending out of cytoplasmic protrusions. The rate of cleavage is different for meso-, macro- and micromeres and the meso- and macromeres cleave, both in situ and as isolated cells, considerably faster than the micromeres. Owing to the different rates of cleavage during the blastula stage the cells of the prospective ectoderm multiply more rapidly than the cells, which later form the endoderm and mesoderm and therefore the early development is characterized by an “animal” trend. Even minor change...

Cytological and morphological studies of the action of lithium on the development of the sea urchin embryo

SummaryThe effect of lithium ions on cleavage and development of sea urchin larvae was investigated. Lithium was found to interfere with the movements of the chromosomes at mitosis, which is also very delayed in the presence of lithium. The disturbances inflicted by lithium were observed during the course of development up to the pluteus stage.The “lithium sensitive period” coincides with the period in which the mitotic activity reaches its maximum.The rate of cleavage is reduced by lithium. In the normal untreated larva there is during the early blastula stage an “animal” trend in which the formation of cells of prospective ectodermal significance preponderates. The mesomeres were found to be relatively more affected by lithium than were the macromeres. This shift in the cleavage pattern influences the numbers of cells of ectodermal and endo-mesodermal significance.Studies were also made of morphological changes following treatment with lithium.

The rate of development in isolated halves of sea urchin embryos

Abstract Sea urchin larvae were at the 16-cell stage incised through the meridional or equatorial plane. The rate of development of the divided embryos was studied and compared with that of control larvae. It was found that the animal half develops faster than the vegetal half. Sea urchin larvae divided through the meridional plane display the same rate of development as control larvae.

Studies of the species specificity of echinoderms

Abstract Experiments in which oocytes were inseminated with heterologous spermatozoa showed that Qocytes are susceptible to the same species of sperm, as are the ripe eggs. Immediately after the spermatozoon has entered an egg or an oocyte, material appearing dark in the phase contrast microscope is ejected from the maternal nucleus; a ring-shaped corona is formed around the nucleus. No aster formation takes place in oocytes which have not yet developed the cortical reaction.