Yasuo Maeda

CELL DIFFERENTIATION AND FINE STRUCTURES IN THE DEVELOPMENT OF THE CELLULAR SLIME MOLDS1

Changes in fine structures during the development of the cellular slime molds D. discoideum and D. mucoroides were studied, with emphasis on the regional differentiation between the prestalk and prespore cells of the slug.

Influence of ionic conditions on cell differentiation and morphogenesis of the cellular slime molds.

Effects of various ionic conditions on the development of the cellular slime molds D. discoideum and D. mucoroides were studied.

Heterogeneity of the cell population of the cellular slime mold Dictyostelium discoideum before aggregation, and its relation to the subsequent locations of the cells

Abstract Interphase amoebae of D. discoideum were fractionated into light (L) and heavy (H) cells by isopycnic centrifugation, and the two types of cells were compared, with special emphasis on their behavior during slug formation. It was found that the L and H cells became localized in the anterior and posterior regions of the slug, respectively. Other differences between these two types of cells were: (1) The L cells formed cell aggregates and slugs more rapidly than H cells. (2) The L cells contained about twice as much as calcium and 10 times more 3′,5′-cyclic AMP than H cells. (3) The L cells were far more sensitive to 3′,5′-cyclic AMP than H cells. The biological significance of these differences is discussed in connection with the establishment of polar organization in the slug.

The calcium content of the cellular slime mold, Dictyostelium discoideum, during development and differentiation.

Abstract A high calcium concentration is known to induce stalk differentiation of the cellular slime mold D. discoideum. Therefore, the change in the calcium content of this organism during differentiation was studied and found to vary during development, more calcium being found in the anterior prestalk cells of the pseudoplasmodium (slug) than in the posterior prespore cells. It is concluded from the results that calcium is of importance in the cell differentiation of this organism and particularly in stalk formation.

ROLE OF CYCLIC AMP IN THE POLARIZED MOVEMENT OF THE MIGRATING PSEUDOPLASMODIUM OF DICTYOSTELIUM DISCOIDEUM

Cyclic AMP is known to act as a chemotactic agent that directs the movement of aggregating Dictyostelium discoideum cells. Its role in the multicellular organization of this organism was studied with special reference to the polarized movement of the migrating pseudoplasmodium (slug). The results showed that the tip of the slug has the ability to function as an aggregation center, and that slug cells are chemotactically sensitive to cyclic AMP. The addition of calcium or magnesium appeared to enhance formation of cell streams, thus facilitating detection of chemotactic response of slug cells, but this addition was not required for the response itself. These indicate that the polar movement of the slug may be principally controlled by cyclic AMP.

Formation of a prespore specific structure from a mitochondrion during development of the cellular slime mold, Dictyostelium discoideum.

The origin of a unique vacuole (PSV), which was specifically present in the prespore cell of the cellular slime mold Dictyostelium discoideum. was investigated electronmicroscopically. A considerable number of PSV‐mitochondrion complexes was found in the intermediate fraction between a pure PSV and a pure mitochondria fractions, which were obtained by isopicnic centrifugation of cellular components of the prespore cell. Similar complexes were also observed in the differentiating prespore cells. Furthermore, the activity of succinic dehydrogenase, a typical mitochondrial enzyme was found cytochemically to be localized in the PSV as well as in mitochondria. From these results, it was concluded that the PSV was formed from the mitochondrion through some intermediate steps.

Ultrastructural Changes of the Two Types of Differentiated Cells during the Migration and Early Culmination Stages of Dictyostelium discoideum

Changes of fine structure during prolonged migration of Dictyostelium discoideum slugs were studied by electronmicroscopy. Prespore specific vacuoles of cells located near the substratum gradually degenerated and the prespore antigen contained in them was lost. During the process, mitochondria in the prespore cells were transformed dramatically: as the mitochondrion elongates, its central part becomes thinner and the cristae become localized at its two ends. Then it bends and its two ends fuse to segregate part of the cytoplasm. The cristae then accumulate in the original ends. Similar mitochondrial transformation was observed in prespore cells of cell masses induced to culminate after a long period of migration.

Fractionation of the differentiated types of cells constituting the pseudoplasmodia of the cellular slime molds

The observation of Milleret al. (1969) that the two types of cells (the prestalk and prespore cells) constituting the slug ofDictyostelium are separated by isopicnic centrifugation was reexamined by using more reliable methods both for dissociation of the slug and for identification of the cell type. Dissociated cells of slugs which had been grown on a standard culture medium formed two distinct bands after centrifugation through a Urografin density gradient. Contrary to Millers findings, however, the light band consisted of the prestalk cells and the heavy band of the prespore cells.When the culture medium was modified, a population of spores of different buoyant density newly appeared during the subculture. Slug cells derived from such a spore had different buoyant densities and formed extra bands in a Urografin gradient. However, the prespore fraction was always heavier than the prestalk fraction derived from the same type of spores.

CHANGES IN CHARGED GROUPS ON THE CELL SURFACE DURING DEVELOPMENT OF THE CELLULAR SLIME MOLD DICTYOSTELIUM DISCOIDEUM: AN ELECTRON MICROSCOPIC STUDY

The distribution of charged groups on the surface of Dictyostelium cells and their change during development were examined by electronmicroscopy using cationic and anionic ferritins. The number of anionic sites on the cell surface decreased greatly during the course of development. The whole surface of vegetative cells stained strongly with cationic ferritin (CF). On the other hand, the surface of aggregation‐competent cells had fewer negative charges and these were unequally distributed, the surface of the advancing area (lobopodial region) being devoid of anionic sites. The number of anionic sites on the cell surface decreased progressively during further development, and the suface of slug cells did not stain at all with CF. The cell surface did not stain with anionic ferritin at any developmental stage, indicating the absence of detectable cationic sites. The biological significance of these findings is discussed in connection with cell adhesiveness and movement.