Animal Pole, Vegetal Pole

 The cell mass of an egg is not uniformly distributed, but it exhibits significant differences in terms of morphology and at the molecular level. In order to describe this polarity, the terms animal pole and vegetal pole were invented to describe the two opposite poles of the egg.

Unlike the eggs of insects, which are elliptically shaped, most oocytes of amphibians and mammalians exhibit a less pronounced asymmetry. One element of asymmetry is the location of the cell nucleus, which is normally not right in the center of the oocyte, but is more peripheral, sometimes even adjacent to the egg membrane. Due to gravitational forces, the yolk within settles to the bottom of the egg and forms the vegetal pole; consequently, the polarity becomes more apparent when more yolk is present in the egg. Another component causing asymmetry is that polar bodies extruded during meiosis are frequently located in the region of the animal pole. In ascidians, sperm enters the egg somewhere in the animal hemisphere (1), causing cytoplasmic movements and rotation of the egg cortex. As a consequence, a further distinct region, the gray crescent, becomes visible in the fertilized egg. The gray crescent is also visible in amphibians, but is not very apparent in sea urchins. In the egg of Unio elongatulus, sperm entry occurs only at the vegetal pole. This is attributed to the presence of a 220-kDa binding protein that is concentrated in a restricted region of the crater region within the vegetal pole (2).

 The naming of the two poles, animal and vegetal, is not based on a precise function; rather, the names have arisen from the idea that the “higher” organs evolve in the animal polar region, whereas the vegetal pole was assumed to be destined to form the “lower” organs necessary for reproduction and providing nutrition. The two poles form one of three possible coordinates, and further developmental changes in a number of amphibians correlate with the subsequent dorsal-ventral body axis of the animal. In mammals, the mechanism by which the inner cell mass settles in certain places is not fully understood. However, it was shown in the mouse that the bilateral symmetry of the early blastocyst is normally aligned with the animal-vegetal axis of the zygote. The embryonic-abembryonic axis is oriented orthogonally to the animal-vegetal axis (3).

 After a sperm activates the egg, karyogamy of the male and female pronuclei occurs, and the egg starts to divide by mitosis. Most important, the cell mass does not increase during the first cell divisions; starting from the one cell, two cells are formed, after another round of divisions four cells, then eight cells, 16 cells, and so forth, until a great number of smaller cells are formed at the morula stage. These cells do not all have the same size; the smaller ones are called micromeres, whereas the larger ones are named macromeres. During these cleavage steps, there is relatively little gene expression from the nucleus of the new individual cells. In other words, the genome of the new cell does not determine its own development after fertilization and karyogamy at these very early stages of development. With regard to the new cells, the regulating elements are external, maternally derived gene products. These substances, mostly RNA molecules, are already present in the unfertilized egg, and they are the important factors that determine the fate of the divided cells. This has been shown by a number of experiments. Chemical inactivation or enucleation in embryos has shown that the nucleus is not necessary for the initial rounds of cleavage. Even enucleated egg fragments are able to perform developmental changes, and cross-fertilization experiments revealed that cells follow the maternal pattern of development. These maternally derived gene products are not evenly distributed in the egg, indicating that the animal and vegetal pole are not only a matter of morphological appearance, but are also related to the presence of a concentration gradient of different gene products. This was demonstrated in experiments in which sections of the animal or vegetal pole were excised and recultivated. When micromeres of the vegetal pole at the 16-cell stage were implanted into the animal pole of a donor embryo, a complete second gut developed; the micromeres are capable of changing the fate of neighboring cells (4). Cutting sea urchin eggs at the eight-cell stage into two halves, to produce two embryos each with an animal and a vegetal cell, generates two pluteus capable of normal development. In contrast, cutting the cell into an animal and an vegetal hemisphere causes considerable aberrations from the normal development.

One of the most quoted experiments to show that animal and vegetal pole cells differ significantly involved excision in which the fate of sections at the 64-cell stage were investigated. The individual cells alone were capable of forming only a blastula. However, adding at least four micromeres from the vegetal pole could compensate for this defect and lead to a pluteus. Fewer numbers of micromeres resulted in forms intermediate between blastula and normal pluteus. Interestingly, the vegetalization occurs not only after the addition of micromeres, but also when Li+ ions were provided. At the animal pole, messenger RNA were found that code for cell-surface proteins. These mRNA were found only in the macromeres and mesomeres, not in the micromeres (5). In the vegetal pole region are localized dorsal determinants that are necessary for dorsal axis development in Xenopus. The dorsal determinants move from the vegetal pole to a subequatorial region, where they are incorporated into gastrulating cells (6). However, dorsal development may be also activated by the contact between the cortical dorsal determinant and the equatorial core cytoplasm that are brought together by cortical rotation upon fertilization (7).

Gastrulation is the next morphogenic event, and it starts in the region of the vegetal pole or near to

the gray crescent, depending on the species. At least five different epigenic movements are observable in the further process of differentiation: invagination of cells, immigration, delamination, proliferation, and epiboly. This means that, although the polarity of the egg determines the fate of the cells at different positions, further movements and reorganization take place that are necessary for organogenesis in higher mammals. One predominantly epigenic movement is invagination during gastrulation. It was shown that vegetal egg cytoplasm is responsible for the specification of vegetal blastomeres and promotes gastrulation. Vegetal-deficient embryos of Halocynthia roretzi fail to enter gastrulation. They are animalized and arrested at the blastula stage. Reimplantation of the vegetal pole cytoplasm into vegetal-deficient embryos caused gastrulation at the site where implantation occurred (8).

References

1. J. E. Speksnijder, L. F. Jaffe, and C. Sardet (1989) Dev. Biol. 133, 180–184. 

2. R. Focarelli and F. Rosati (1995) Dev. Biol. 171, 606–614. 

3. R. L. Gardner (1997) Development 124, 289–301. 

4. A. Ransick and E. H. Davidson (1993) Science 259, 1134–1138. 

5. M. Di Carlo, D. P. Romancino, G. Montana, and G. Ghersi (1994) Proc. Natl. Acad. Sci. USA 91, 5622–5626    .

6. M. Sakai (1996) Development 122, 2207–2214. 

7. H. Kageura (1997) Development 124, 1543–1551. 

8. H. Nishida (1996) Development 122, 1271–1279.