Preparations for Fertilization
The fusion of female and male gametes is called fertilization. For this process to occur species-specifically without multiple fertilizations, close interaction between male and female cells is necessary. Prior to fertilization, male gametes or gametophytes need to be activated. In human sperms, this activation (known as sperm capacitation) is caused by bicarbonate ions in the genital duct of females. The bicarbonate ions enter the sperm and promote the production of cAMP, which changes the composition of lipids and glycoproteins in the plasma membrane, lowers membrane potential and increases intracellular metabolic activity and motor activity, thereby making the sperm fertile.
Since spermatids (male gametes) in plants do not have motor ability, they are carried by pollen tubes (male gametophytes) (Fig. 12-14A). The activation of pollen tubes and their extension to the embryo sac (a female gametophyte) have been studied in torenia, whose embryo sac is denuded, making experimental procedures easy. Pollen tubes acquire the ability to extend toward an egg cell by passing through the style - a female tissue (Fig. 12-14B). This alone, however, does not guide pollen tubes correctly to the ovum; such tubes can send spermatids to the embryo sac only in the presence of species-specific elements secreted from synergids in the embryo sac (Fig. 12-14C). The pollen tube enters the embryo sac via a synergid and sends one spermatid to the egg cell and the other spermatid to the diploid central cell, thus allowing fertilization (double fertilization). Studies on species related to torenia have shown that the response of pollen tubes to the elements secreted from synergids is species-specific, indicating that these elements lead to the species-specificity of fertilization.
Fig. 12-14. Pollen-tube guidance and fertilization in plants
A) Pollens attached to the stigma (1) extend pollen tubes (male gametophytes). These tubes reach the embryo sac (a female gametophyte) after passing through the style (2), the placenta (3), the funiculus (4) and the micropyle (5). Since pollen tubes sometimes stray in the regions circled by the broken lines, it is believed that signals that guide the tubes exist in these areas.
B) The extension of pollen tubes is promoted by elements contained in the style. Pollen tubes that do not pass through the style therefore do not elongate significantly.
C) A pollen tube that has reached the embryo sac enters one synergid via the filiform apparatus in response to inducers secreted from the synergid. The pollen tube breaks down in the synergid, and two spermatids fuse with the central cell and the egg cell, respectively (double fertilization).
When activated animal sperms approach the ovum surface, they are blocked by special matrices called the zona pellucida or the vitelline membrane. The zona pellucida mainly consists of three glycoproteins, Z1, Z2 and Z3. The Z3 protein species-specifically binds to Z3 receptors on the plasma membrane of sperms (Fig. 12-15). This binding triggers the acrosome reaction (Fig. 12-15), in which hydrolase stored in the acrosomal vesicle of a sperm is released to the zona pellucida and degrades it, allowing the sperm to penetrate. Fusion of the ovum plasma membrane and the sperm plasma membrane then occurs, and the sperm nucleus enters the ovum.
In addition to introducing a paternal gene set to an ovum, the sperm also activates the ovum. In mammals, when a sperm fuses with the surface of an ovum, there is a local increase in the level of intracellular Ca2+, which is then transmitted to the entire ovum in a wave-like motion (known as a calcium wave; see Fig. 9-4A in Chapter 9) and activates the ovum. In the activated ovum, hydrolase is secreted from the cortical granules in the cell to the zona pellucida, thus changing its nature to block the entry of other sperms. On the other hand, the Ca2+ signal drives the developmental program of the ovum, leading the fertilized egg to the next stages of development. These sperm roles can be replaced with physical or chemical treatment. As an example, a frog ovum can be activated by a pinprick. In some organisms, the ovum is activated without a sperm and develops into a complete organism. This type of development is called parthenogenesis.
When only the sperm involved in fertilization enters the ovum, the sperm nucleus undergoes nuclear protein conversion and swells, thus forming the male pronucleus (Fig. 12-16). The centrosome located at the base of the flagellum also plays a pivotal role in creating the asteroid body*5 in the ovum. Using the microtubules of the asteroid body as rails, the female pronucleus is then carried to the center of the asteroid body. In sea urchins, the male and female pronuclei fuse together, forming a 2n fertilized nucleus. In mammals, ovulation occurs in the metaphase of meiosis II, leading to fertilization. After fertilization, meiosis II resumes, and the male and female pronuclei approach each other using the same mechanism seen in sea urchins. However, they do not fuse together, and chromosomes derived from the two nuclei start to exhibit the same behavior following the breakdown of the nuclear membrane during division. The cell that thus becomes the 2n type again performs cleavage repeatedly and undergoes the developmental process.
Asteroid body: Structure of microtubules growing radially from both poles of the mitotic spindle during cell division