In the animal kingdom, the first developmental determination of the germ cells is the presence of a polarity . This is necessary to determine early coordinates for further development and organogenesis. In other words, there are one or more mechanisms to determine which part of the cell is supposed to form the posterior, anterior, dorsal, and ventral poles. The morphological appearance of the egg before fertilization, and also shortly after, fertilization is not uniform but shows a more or less distinct polarity, which is termed the animal-vegetal polarity. In certain species, sperm can only enter at the animal pole, indicating that there must be differences in molecular composition of the opposite poles.
There are various reasons for the development of the animal-vegetal polarity. In many cases, the feeder cells of oocytes are not uniformly located around the oocytes. Thus, they are fed from one side, which results in an asymmetrical concentration of certain molecules within the oocyte plasma. Another influence to produce polarity is probably light and gravity. However, after fertilization in space or in a fast rotating clinostate, Xenopus exhibited normal embryonic development and normal axis formation (1). In eggs containing a considerable amount of yolk, gravity forces the yolk to settle down. This event takes place in amphibia after sperm entry, which enables a free rotation of the egg cytoplasm. This causes the formation of the vegetal and animal polarity, in which the vegetal pole contains larger amounts of yolk than the animal pole.
The other polarity of the cell is not as easily determined in the unfertilized egg. In many cases, the posterior-anterior axis is determined by the site of sperm entry. On egg activation, a number of cytoplasmic reorganizations take place. In some species a region becomes visible, the gray crescent, which is located opposite to the site of the sperm entry. This is the prospective area in which gastrulation will take place. In amphibia, this region marks the prospective backside and distal side of the embryo. In birds, the dorsal-ventral polarity is determined by the structure of the egg, and the caudal distal polarity is assumed to be based on the influence of gravity. In mammals, there are contrary observations regarding where the inner cell mass settles. This embryonic event determines at least the dorsal side, whereas the mechanism that determines the prospective axis of the head and tail is still the subject of investigation.
Nusslein-Volhard and others have provided more information about the situation in Drosophila. In this species, the presence of a genetically determined polarity of the anterior and posterior regions of the egg was demonstrated. In the anterior region, a product of the gene bicoid was found to determine the development of the head and thorax (2, 3). In Drosophila bcd – / – mothers, which could not produce the bicoid gene product, larvae with no heads appeared. The product of the gene nanos is responsible for the formation of an abdomen and germline development (4), and its highest concentration occurs in the posterior end of the egg (5). In mothers homozygous negative for the nanos gene, no abdomen is formed. Compensation of the missing gene products by injection of bicoid messenger RNA or nanos mRNA leads in each negative variant to the development of a normal phenotype. Nanos and bicoid form concentration gradients from the anterior to the posterior and activate in a concentration-dependent manner a number of zygotic segmentation genes (6, 7). Bicoid controls transcription of the hunchback gene (6). In this cascade, the gene staufen is important for the localization of maternal mRNA to the posterior pole and for bicoid mRNA to the anterior pole in the Drosophila egg (8). In contrast, the nanos protein suppresses translation of hunchback mRNA. Hunchback is a gene responsible for the formation of anterior structures; on inhibition of these anterior structures, posterior ones are formed. The hunchback gradient controls caudal , which is concluded to be a region-specific activator of abdominal segmentation genes (9). Furthermore, injection of nanos mRNA into the anterior end induced the development of larvae with two abdomen and no head. Two heads and no abdomen develop in larvae on injection of bicoid mRNA into the posterior end of the egg.
In Drosophila, axis formation involves a series of interactions between the oocyte and the surrounding somatic follicle cells. The dorsal and ventral sides is determined by the localization of the oocyte nucleus and gurken mRNA to the dorsal-anterior corner of the oocyte. Gurken protein is believed to act as a ligand for the Drosophila epidermal growth factor (EGF) receptor (10).
It must be admitted that the naming of the above genes is somewhat peculiar. Many of the genes are not named according to their function, but instead according to the phenotype produced when the gene is defective. In this respect, this means that the gene dorsal determines the development of ventral structures, not dorsal. The dorsal protein migrates into the nuclei of the ventral side and acts, similar to nanos or bicoid, as a transcription factor.
