Topology of the germ plasm and development of primordial germ cells in inverted amphibian eggs

Simulation of the effects of microtubules in the cortical rotation of amphibian embryos in normal and zero gravity

This paper reports the results of computer modeling of microtubules that end up in the cortical region of a one-cell amphibian embryo, prior to the first cell division. Microtubules are modeled as initially randomly oriented semi-flexible rods, represented by several lines of point-masses interacting with one another like masses on springs with longitudinal and transverse stiffness. They are also considered to be space-filling rods floating in a viscous fluid (cytoplasm) experiencing drag forces and buoyancy from the fluid under a variable gravity field to test gravitational effects. Their randomly distributed interactions with the surrounding spherical container (the cell membrane) have a statistical nonzero average that creates a torque causing a rotational displacement between the cytoplasm and the rigid cortex. The simulation has been done for zero and normal gravity and it validates the observation that cortical rotation occurs in microgravity as well as on Earth. The speed of rotation depends on gravity, but is still substantial in microgravity.

Fluid dynamics in developmental biology: moving fluids that shape ontogeny

Human conception, indeed fertilization in general, takes place in a fluid, but what role does fluid dynamics have during the subsequent development of an organism? It is becoming increasingly clear that the number of genes in the genome of a typical organism is not sufficient to specify the minutiae of all features of its ontogeny. Instead, genetics often acts as a choreographer, guiding development but leaving some aspects to be controlled by physical and chemical means. Fluids are ubiquitous in biological systems, so it is not surprising that fluid dynamics should play an important role in the physical and chemical processes shaping ontogeny. However, only in a few cases have the strands been teased apart to see exactly how fluid forces operate to guide development. Here, we review instances in which the hand of fluid dynamics in developmental biology is acknowledged, both in human development and within a wider biological context, together with some in which fluid dynamics is notable but whose workings have yet to be understood, and we provide a fluid dynamicist's perspective on possible avenues for future research.

Rayleigh instability of the inverted one-cell amphibian embryo

The one-cell amphibian embryo is modeled as a rigid spherical shell containing equal volumes of two immiscible fluids with different densities and viscosities and a surface tension between them. The fluids represent denser yolk in the bottom hemisphere and clearer cytoplasm and the germinal vesicle in the top hemisphere. The unstable equilibrium configuration of the inverted system (the heavier fluid on top) depends on the value of the contact angle. The theoretically calculated normal modes of perturbation and the instability of each mode are in agreement with the results from ComFlo computational fluid dynamic simulations of the same system. The two dominant types of modes of perturbation give rise to axisymmetric and asymmetric sloshing of the cytoplasm of the inverted embryos, respectively. This work quantifies our hypothesis that the axisymmetric mode corresponds to failure of development, and the asymmetric sloshing mode corresponds to development proceeding normally, but with reversed pigmentation, for inverted embryos.

Dorsal determinants in the Xenopus egg are firmly associated with the vegetal cortex and behave like activators of the Wnt pathway

The Xenopus egg contains maternal dorsal determinants that are specifically located at the vegetal cortex. To study physical and functional properties of the dorsal determinants, we took advantage of the animal-vegetal reversed embryo. The animal-vegetal reversed embryo is produced by inversion of the fertilized egg, which results in formation of ectoderm and endoderm from the unpigmented and the pigmented halves, respectively [Neff et al. (1983). Dev. Biol. 97, 103-112; Black and Gerhart (1985). Dev. Biol. 108, 310-324]. We demonstrated by cytoplasmic transplantation that the dorsal activity was specifically localized to the unpigmented cortical cytoplasm of the inverted egg, which is segregated into the future ectodermal lineage. This result suggests that the dorsal determinants are associated with the unpigmented cortex and are not dislodged by the inversion. In addition, we found that two vegetally localized transcripts, Xcat2 and Vg1 mRNAs, were present in the reversed animal pole of the inverted egg, suggesting their association with the unpigmented cortex. In order to compare the dorsal determinant activity with known dorsalizing molecules, we examined the expression pattern of Xnr3 and Siamois in the reversed embryo because these two genes are activated by the Wnt-pathway activators (Xwnt-8, beta-catenin, etc.) but not by other dorsalizing molecules (noggin, BVg1, etc.). Animal cap of the reversed embryo, which received the unpigmented cortex of the egg, expressed Xnr3 and Siamois. However, Mix.1, a marker expressed in endoderm and mesoderm in the normal embryo in response to mesodermal inducers, was not detected in the animal cap of the reversed embryo. In addition, we found that beta-catenin protein accumulated in nuclei of unpigmented animal pole cells of the reversed embryo. These results suggest that the maternal dorsal determinants behave more similarly to the Wnt-pathway activators than noggin or BVg1.

Subcellular components of the amphibian egg: insights provided by gravitational studies

Most cytoplasmic regions of fertilized amphibian eggs move with respect to the gravity vector in experimentally gravity oriented eggs. The pattern and extent of this movement varies among different batches of eggs. This variation in apparent cytoplasmic viscosity (or, conversely, cytoplasmic mobility) can be correlated with variations in subsequent morphogenesis of experimental, gravitationally manipulated eggs. Therefore, the proper interpretation of gravity experiments with amphibian eggs requires that one understand the subcellular basis for this variation on cytoplasmic mobility. Variation in the packing of the major cytoplasmic organelle, the yolk platelets, or the organization and amount of cytoskeletal components may explain the variation in cytoplasmic mobility. Evidence is presented that the variation in yolk volume density (fraction of total cytoplasmic volume occupied by yolk platelets) does not account for the variation in cytoplasmic mobility in Xenopus laevis eggs. Experimental evidence from cold-shocked inverted eggs indicates that microtubules may be involved in determining cytoplasmic mobility. However, quantitative evidence that the microtubule levels and state of the microtubules (polymerized vs. non-polymerized) in the whole Xenopus laevis egg does not correlate directly with cytoplasmic mobility is presented. The apparent conflict these data represent regarding the role of the cytoskeleton in determining cytoplasmic mobility is discussed.

The amphibian egg as a model system for analyzing gravity effects

Amphibian eggs provide several advantageous features as a model system for analyzing the effects of gravity on single cells. Those features include large size, readily tracked intracellular inclusions, and ease of experimental manipulation. Employing novel gravity orientation as a tool, a substantial data base is being developed. That information is being used to construct a 3-D model of the frog (Xenopus laevis) egg. Internal cytoplasmic organization (rather than surface features) are being emphasized. Several cytoplasmic compartments (domains) have been elucidated, and their behavior in inverted eggs monitored. They have been incorporated into the model, and serve as a point of departure for further inquiry and speculation.