Initiating the Embryonic Body Plan: Specification of the anteroposterior axis in Caenorhabditis elegansHow does a radially symmetrical egg form a bilaterally symmetrical embryo? |
After the blastula stage, the cells that were generated by cleavage are rearranged according to the organism's body plan. This process executes prior determinative events. The eggs of most species have radial symmetry and are polarized along the animal-vegetal axis. The symmetry of the embryo is usually established after oogenesis. Most embryos develop with bilateral symmetry and become polarized along anterior-posterior, dorsal-ventral and left-right axes. Exquisite genetic and cell manipulation studies using the nematode Caenorhabditis elegans have examined the establishment of the anteroposterior axis. The following essay, which was prepared by students in my class in 1996, discusses a pivotal paper by Goldstein and Hird that analyzed this process.
A central question of developmental biology is how the different cell types and organization of the adult are generated from a single cell. Although much is known about Caenorhabditis elegans, a nematode that is often used as a model system for studying development, the source of the initial asymmetry in its development was not understood until the work of Goldstein and Hird (1996).
The C. elegans embryo shows distinct embryonic anteroposterior (AP) asymmetries even before the first cell division. This embryonic polarisation, including the localisation of critical regulatory proteins, is associated with cytoplasmic rearrangement during the first cell cycle. Specifically, after fertilisation of the oocyte, the cortical cytoplasm flows towards the anterior pole, and the central material flows in the opposite direction. Nevertheless, what causes the cytoplasmic rearrangement was not known until Goldstein and Hird determined the cue responsible for specifying the anteroposterior axis and explained how this cue generates initial asymmetry.
Normally, C. elegans oocytes are fertilised at their leading edge as they pass from the oviduct into the spermatheca. This results in the sperm entering the oocyte at the opposite pole to the one where the egg nucleus is located. Normally, the site of sperm entry becomes the posterior pole, and the anterior pole forms in the region that contains the egg nucleus at the time of fertilisation. In their experiment, Goldstein and Hird modified the location at which sperm entry occurred and followed embryo development with Nomarski microscopy. When the sperm entered at the opposite pole from normal, the AP axis was reversed, but the embryo developed normally in all other aspects. They concluded that the sperm pronucleus, or a component associated with it, directs the cytoplasmic rearrangement responsible for the establishment of asymmetries.
Of course, under manipulated conditions C. elegans sperm could also enter the egg laterally, rather than at a pole. When this occured, the sperm pronucleus always traveled to the closest pole, which then became the posterior. The fact that either end of the egg could become the posterior region indicated that unlike the oocytes of many animals, including Drosophila melanogaster, the C. elegans oocyte is constructed with no axis prespecified in the form of asymmetrically localised determinants. Instead, it is the site of sperm entry that is key to establishing the AP axis. This idea also appears to hold true in two species closely related to C. elegans, which have naturally varying points of sperm entry.
In summary, the site of sperm entry in C. elegans dictates the pole at which the sperm pronucleus will become established. The sperm pronucleus, in turn, establishes the AP axis through the control of cytoplasmic rearrangement, with the posterior pole developing at its (i.e. the sperm pronucleus's) end of the oocyte. But how exactly does the sperm direct cytoplasmic rearrangement in C. elegans?
According to Goldstein and Hird, the sperm brings the centrosome into the oocyte. The poles of the oocyte meiotic spindle lack centrioles, and do not play a mitotic role. The sperm centrosome duplicates and, when cytoplasmic rearrangement starts, both centrosomes nucleate astral arrays of microtubules, which stay linked to the sperm pronucleus. Since microfilaments are usually involved with cytoplasmic rearrangement, Goldstein and Hird proposed that the microtubule asters control the movement of cortical actin away from their vicinity. The flow of cortical actin toward the presumptive anterior pole causes cortical material to be moved away from the sperm nucleus, thus driving the central cytoplasmic material in the opposite direction, toward the sperm pronucleus. Consequently, cytoplasmic determinants in the embryo are redistributed, and the AP axis is established.
Gene products that may become segregated by the sperm directed movements include PAR-3 and MEX-3. The former is a novel protein required for several aspects of AP polarity and is localised to the anterior pole, perhaps by cytoplasmic streaming. Similarly, MEX-3 is involved in the establishment of some AP asymmetries, but is localised in the posterior region. As shown by immunofluorescence, MEX-3 is associated with P granules and is localised as a result of their localisation. Note that P granules are cytoplasmic granules in C. elegans that are destined to enter the primordial germ cells. Goldstein and Hird demonstrated, by confocal time lapse recordings of embryos containing fluorescently-tagged antibody recognising P granules, that their segregation to the posterior end is sperm directed.
It is worth pointing out that it is not yet known whether the above mechanism is responsible for all the generated asymmetries. Perhaps certain asymmetries are caused by the sperm through an as yet unknown mechanism. Furthermore, it is still to be established to what extent the C. elegans AP axis specification system can be applied to other nematodes, in view of their diverse characteristics. One group, for example, lacks sperm completely and develops parthenogenically, yet the AP axis is specified, nonetheless. These points, however, do not undermine the significant contributions made by Goldstein and Hird.
| Gametogenesis | |||||
| The Foundations of Developmental Biology | |
The Developmental Biology Journal Club | |||
Regulating blastomere identity: spatio-temporal interactions between maternal gene products (from Zygote)
Goldstein, B. and S. N. Hird. 1996. Specification of the anteroposterior axis in Caenorhabditis elegans. Development 122: 1467-1474.
Copyright © Anna Barbasiewicz, Rogy Masri, Leigh Maximuk and
Christopher Prusinkiewicz1996. This material may be reproduced for educational
purposes only provided credit is given to the original source.
September 29, 1996