Migration and division of cleavage nuclei in the gall midge,Wachtliella persicariae: III. Pattern of anaphase-triggering waves altered by temperature gradients and local gas exchange

Calcium waves

Waves through living systems are best characterized by their speeds at 20 degrees C. These speeds vary from those of calcium action potentials to those of ultraslow ones which move at 1-10 and/or 10-20 nm s(-1). All such waves are known or inferred to be calcium waves. The two classes of calcium waves which include ones with important morphogenetic effects are slow waves that move at 0.2-2 microm s(-1) and ultraslow ones. Both may be propagated by cycles in which the entry of calcium through the plasma membrane induces subsurface contraction. This contraction opens nearby stretch-sensitive calcium channels. Calcium entry through these channels propagates the calcium wave. Many slow waves are seen as waves of indentation. Some are considered to act via cellular peristalsis; for example, those which seem to drive the germ plasm to the vegetal pole of the Xenopus egg. Other good examples of morphogenetic slow waves are ones through fertilizing maize eggs, through developing barnacle eggs and through axolotl embryos during neural induction. Good examples of ultraslow morphogenetic waves are ones during inversion in developing Volvox embryos and across developing Drosophila eye discs. Morphogenetic waves may be best pursued by imaging their calcium with aequorins.

Calcium waves and development

Those calcium oscillations which go deep into cells take the form of 'fast' calcium waves. In fully active cells at room temperature, these move at 15-30 microns/s and are propagated by a reaction-diffusion mechanism governed by the Luther equation in which calcium ions are the only propagators and calcium-induced calcium release is the only reaction. However, they may be initiated by a second mode of Ca(2+)-induced Ca2+ release within the lumen of the endoplasmic reticulum (ER). In sea urchin fertilization, this second mode of Ca(2+)-induced Ca2+ release is in turn begun by calcium entering the sperm and thence the ER. Subsurface calcium waves include an important class of surface contraction waves which move at 0.3-3 microns/s and are called 'slow' waves. Their prototype is the 0.5 micron/s wave which accompanies and controls cytokinesis in large eggs. Slow waves may be propagated by mechanical tension rather than by diffusion. Recent work with Dictyostelium transfected with apoaequorin has provided the first views of free calcium patterns within a developing, multicellular organism. During most or all of development, those regions which will differentiate into stalk or stalk-like cells (as opposed to spores) exhibit frequent calcium pulses. These pulses are believed to be fast calcium waves and to feed back on these regions so as to favour non-spore differentiation.

Calcium transients accompany ooplasmic segregation in zebrafish embryos

Through the injection of f-aequorin (a calcium-specific luminescent reporter), and the use of an imaging photon detector, transient localized elevations of free cytosolic calcium in the forming blastodisc (BD) and animal hemisphere cortex were visualized that correlated with ooplasmic segregation. The introduction of an appropriate concentration of the weak (KD = 1.5 micromol/L) calcium buffer 5,5'-dibromo-BAPTA results in the dissipation of these calcium domains, and inhibits cytoplasmic streaming and the subsequent formation of a BD at the animal pole. These inhibitory actions are dependent on the final cytosolic concentration of buffer within the egg: > or = 1.3 mmol/L blocks ooplasmic streaming; < 1.3 mmol/L eggs segregate normally. Injection of 5,5'-dimethyl-BAPTA (KD = 0.15 micromol/L) to a final concentration of 1.5 mmol/L as a control has no effect on ooplasmic streaming. These results suggest that localized domains of elevated free cytosolic calcium are essential for ooplasmic segregation in zebrafish. Furthermore, a hypothetical model is presented linking these calcium transients to the contraction of a cortically located actin microfilament network as a possible mechanism providing the driving force for segregation.

Developmental asynchrony caused by steep temperature gradients does not impair pattern formation in the wasp, Pimpla turionellae L

Generating developmental gradients by temperature gradients established within a developing organism is an easy, non-invasive technique to study physiological interdependencies between locally separated subsystems. A linear temperature gradient of about 10° C/mm was maintained up to 5 h in either direction along the long axis of a long-germ-type hymenopteran egg, which was simultaneously filmed by the 16 mm timelapse technique. The result was a dramatic desynchronization of development, which between the egg poles could reach up to 9.3 h relative to normal development. Within the same egg, up to seven mitotic waves (i.e. eight different nuclear generations) were observed at the same time, and the subsequent cellularization process was extremely asynchronous. The "initial regions" of the mitotic waves, the "fountain flow" of the ooplasm, and the gastrulation process were shifted towards the egg pole kept at higher temperatures. Developmental processes occurring successively in normal development now took place simultaneously, with either direction of the temperature gradient. For instance, while gastrulation had started in the warm region, midblastula transition and cellularization were in progress in the middle of the egg, and intravitelline nuclear multiplication occurred at the cold pole, by rapid and still biphasic cell cycles. In some respects, development resembled that of a short-germ-type insect egg. Nevertheless, the developmental processes were resynchronized after the temperature gradient was switched off. Surprisingly, the extreme desynchronization during early development did not affect the segment pattern of the resulting embryos. The technique of inducing well-defined developmental asynchronies might be applied in Drosophila to analyse the subtle interplay between maternal and zygotic gene activities described in this species.

The yolk syncytial layer of Fundulus: its origin and history and its significance for early embryogenesis

Because of its importance in early embryogenesis, the developmental history of the yolk syncytial layer (YSL) of Fundulus has been investigated in detail. As in other teleosts, the Fundulus YSL forms mainly by collapse of certain marginal blastomeres which then merge with the cytoplasm of the yolk cell peripheral to the blastoderm. Nuclei enter the yolk cell from these open blastomeres variably during cleavages 8-11, but most frequently at cleavages 9 and 10. After entry, the first nuclei divide five times and later nuclei divide with them. Thus, nuclei that have invaded at the next cleavage divide four times, etc. When the first YSL nuclei cease dividing, all other YSL nuclei cease dividing with them. These YSL mitoses occur in metachrony. Two or more metachronous waves progress through the YSL cytoplasm at each mitosis. After each nuclear division, the YSL increases in width and its nuclei are quite evenly spaced. After the 5th and last mitosis, when the YSL is at its widest, it contracts in its animal-vegetal axis. This slow contraction has two major effects: 1) narrowing of the YSL, accompanied by crowding of its nuclei and their disappearance beneath the blastoderm to nucleate the internal YSL; 2) epibolic expansion of the I-YSL and the blastoderm, followed soon after by other cell movements of gastrulation. This YSL transition, therefore, sets the stage for the onset of gastrulation. It is preceded by increased duration and variability of succeeding mitoses and, in particular, duration of their interphases, a decrease and deceleration in the rate of the last metachronous waves, and, finally, by the complete cessation of mitosis and the entry of YSL nuclei into permanent interphase.

Pattern formation fails after blastoderm formation by rapid cell cycles in an artificially activated insect egg

Oocytes explanted from adult ovaries of the arrhenotokous Hymenopteron Pimpla turionellae remain in an inactive state, because development has not been initiated by mechanical deformation during natural oviposition. However, they could be induced to enter development by injecting cleavage energids into the posterior pole. After lag phases of up to 32 h, the implanted nuclei initiated a normal cleavage process, except that the polarity of its progress was reversed. In other oocytes, the injected energids congregated in a ring-shaped region at the egg surface to form a superficial "nuclear front", which slowly advanced towards the anterior egg pole, thereby successively stimulating portions of the quiescent ooplasm to take part in development. Up to 41 rapid cell cycles started from that front, each of them with an anaphase wave running backwards into the region already peripherally occupied by nuclei. Thus, the blastoderm was formed extremely metachronously and by rapid obviously biphasic cell cycles, which never occur at the egg surface during normal cleavage. A germ band, however, was only formed under the following conditions: (1) that cleavage did not follow the nuclear front mode, and (2) that ooplasm from the donor's posterior pole was co-injected with the graft nuclei. We conclude that embryonic differentiation requires some of the events which had been omitted in eggs where development failed, especially the exponential increase of the cell cycle length, and the activity of some posterior factor(s) during egg activation.

Early embryonic development of the dipteran insect Heteropeza pygmaea in the presence of cytoskeleton-affecting drugs

Embryos of the paedogenetically reproducing gall midge Heteropeza pygmaea develop floating in the haemocoel of a so-called mother larva. The egg membranes remain permeable and the embryos increase in size during embryonic development by taking up nutrients from the haemolymph. Such embryos can be cultured in vitro, i.e. in haemolymph drops obtained from mother larvae. We tested the effects of several drugs known to interact with cytoskeletal elements on different stages of embryonic development, including cleavage and gastrulation. The drugs were added to the in vitro cultures and the effects were studied with time-lapse cine-micrography. Colchicine and vinblastine blocked cleaving eggs in metaphase stage and arrested yolk globule oscillation. In spite of such a block blastoderms once formed continued development through germ band formation and extension and also increased in size. Cytochalasin B did not affect the stage of cleavage; however, it inhibited gastrulation and subsequent morphogenetic processes and also prevented size increase. We conclude that (1) the functioning of microtubules is needed for yolk globule oscillation during cleavage interphases but not for the gastrulation processes subsequent to blastoderm formation and (2) microfilaments do not play an important role in cleavage, at least not for the orderly succession of the cleavage divisions, but are essential for the morphogenetic movements associated with gastrulation. We suggest that during cleavage a limited stock of microtubules and their precursors is responsible for both transport of chromosomes during mitoses and translocation of organelles during interphase. Yolk oscillation seems to be a secondary effect and of minor or no importance for the normal course of embryonic development.