Haploid cells generated by meiosis do not immediately become gametes such as ova and sperms. For example, in angiosperms, a female haploid cell undergoes three more divisions to form eight cells, one of which becomes an egg cell (Figure18-8A). Through a single division, a male haploid cell becomes a reproductive cell (generative cell) and a vegetative cell to support the generative cell, which is further divided into two cells (Figure 18-8B). The two resulting sperm cells later fertilize an egg cell and a central cell of the female, respectively (double fertilization) (see Selection 7 of Chapter 18).
Compared with plants, higher animal forms such as mammals, haploids exist for only a very short period of time. In mammals, although meiosis for oogenesis is initiated in the early stages, the process is arrested at the primary oocyte stage in the prophase of meiosis I when the oocyte is still a diploid (Figure 18-9). In humans, the process then remains dormant for many years. Once individuals mature and hormone secretion is initiated, meiosis is resumed and ova will rapidly complete maturation when prompted by fertilization. Unfertilized ova are promptly removed.
In mammalian males, spermatogenesis is initiated after sexual maturation. In humans, it takes 24 days for a spermatocyte to complete meiosis and become four spermatids, and approximately 9 weeks for a spermatid to become a mature sperm (Figure 18-10). Unlike ova, much of the sperm differentiation process occurs after they become haploids. Sperms compensate for the disadvantage of being haploid by forming a special structure called a syncytium*6. Spermatogonia do not undergo cytokinesis during the first somatic cell division and the subsequent meiosis, and the resulting cells continue to share the cytoplasm. Haploid spermatids therefore inherit the cytoplasm from diploid cells, and this cytoplasm controls the differentiation of sperms. The syncytium also contributes to synchronous spermatogenesis.
*6 Apocyte formed by fusion of cells and mixing of cytoplasms.
Figure 18-8 Gametogenesis in plants
A) Female gametes. The figure shows a pattern common to many angiosperms. A megaspore mother cell is divided into four haploid cells through Meiosis I and II. Of these cells, only one matures into a megaspore. Through three mitoses, this megaspore becomes an embryo sac consisting of eight cells. B) Male gametes. A pollen mother cell undergoes meiosis to become a pollen tetrad, which dissociates and produces four microspores. The nucleus of each microspore moves to the side wall before Mitosis I. This mitosis has unequal cell division, producing a large vegetative cell and a small generative cell having a nucleus with condensed chromatin structure. The generative cell moves into the vegetative cell and divides into 2 spermatids via Mitosis II.
Figure 18-9 Oogenesis in mammals
A primordial germ cell that moves into an ovary in early embryogenesis becomes an oogonium. After several mitoses, the oogonium starts Meiosis I and becomes a primary oocyte. In mammals, primary oocytes are formed in very early stages (during fetal stage in humans), and their development is arrested in the early stage of Meiosis I until the individual becomes sexually mature. Once the individual matures, a small number of cells periodically mature under the influence of hormones, complete meiosis I to become secondary oocytes, and become mature ova via Meiosis II. The stage at which ova or oogoniums are released from the ovary for fertilization differs by species. In humans, they are released during the middle stage of Meiosis II and mature after fertilization.
Figure 18-10 Spermatogenesis in mammals
Progeny cells derived from the same spermatogonium are connected through the cytoplasmic bridge until they are differentiated into mature sperms. The structure is called syncytium. To aid understanding, the figure shows how two connected spermatogonia become eight connected haploid spermatids through meiosis. The actual number of connected cells simultaneously differentiated through meiosis is much higher than shown in the figure.