Animal Reproduction and Development

Gamete Formation

Sexually reproducing organisms use gametes (sex cells) to reproduce.
Bacteria and other single-celled organisms reproduce simply by copying their genetic material, then splitting in half. The benefit of this method is that these organisms do not need to find a mate. The downside, however, is that each offspring has the same genetic makeup as the parent. While asexual reproduction has its benefits, the transfer of genetic material that occurs during sexual reproduction allows for genetic variation in offspring. The genetic variation achieved with sexual reproduction allows the potential for some offspring to have an increased chance to survive their environment.

Sexual reproduction involves the combination of genetic material from two parents. This happens through the production and transmission of gametes. A gamete is the sex cell of a sexually reproducing organism, with a haploid set of chromosomes. Gametes are produced from cells called germ cells. They are different from the rest of the body cells in their functions, and they stay that way for their entire lives. When an animal is of reproductive age, the germ cells are activated in the gonads, the areas of the body where gametes are produced (in humans, this is the testes for males and the ovaries for females). It is here that the sex cells are formed. Spermatogenesis is the formation of sperm through the process of meiosis. Spermatogenesis produces four sperm cells from a single existing precursor cell. Oogenesis is the process of egg cell formation during meiosis. Oogenesis produces one viable egg cell from one existing precursor cell. Once these cells have been made, they undergo the process of meiosis, a process in cell division during which the number of chromosomes is reduced to half the original number by two divisions of the nucleus, resulting in the production of gametes. This reduction of chromosomes (bundles of genetic material) present in each cell is referred to as haploid (1n or n), half the total of the parent. Gametes must be haploid in sexually reproducing organisms in order to combine with the gamete from the parent of the opposite sex and form an offspring with a complete complement of chromosomes.

Sperm production happens in the male in specialized organs called the testes. In most mammals, the testes are located in an external sac called the scrotum because normal sperm production requires temperatures slightly lower than the organism's body temperature. Special cells, called Sertoli cells, provide nutrition for the developing sperm. Meiosis in sperm cells begins with a spermatogonium, which is the diploid (2n) cell that is the origin cell in spermatogenesis. Diploid means having a double set of chromosomes in homologous pairs; the diploid condition is designated by 2n. Sperm production involves the following processes:

Step 1. The spermatogonium divides, producing two diploid primary spermatocytes. A primary spermatocyte is a diploid (2n) cell that is the first cell formed in spermatogenesis.

Step 2. The primary spermatocytes then divide, reducing the chromosome number to form two haploid secondary spermatocytes. A secondary spermatocyte is the haploid (1n) cell that is the second cell formed in spermatogenesis.

Step 3. The secondary spermatocytes then divide into spermatids, which are immature sperm cells. These stay connected to each other via cytoplasmic bridges, areas that connect the cells via their cytoplasm. This attachment coordinates development. Many genes required for sperm maturation are coded on the X chromosome, but not all sperm have an X chromosome due to being haploid. By staying connected via the cytoplasm (the cell's watery interior), the new spermatocytes are able to share this genetic information.

Step 4. When fully developed, haploid (1n) male sex cells, the mature spermatozoa, separate.

Sperm production results in four individual sperm cells being produced from each precursor germ cell. It begins when the organism reaches reproductive age and continues throughout the male's life span. By producing far more sperm than are needed (each ejaculation can contain 20 million sperm), the chances that at least one will make it to the female egg cell and fertilize it increase.
In spermatogenesis, the spermatogonia divide to produce the primary spermatocytes, which then further divide to produce secondary spermatocytes. These form the spermatids, which remain connected to each other until they are fully developed as spermatozoa, or haploid male sex cells.
The formation of an oocyte, the female haploid (1n) sex cell, is called oogenesis. It begins with oogonia, which are the diploid (2n) cells that will further divide to form the egg cells. This process is complete before birth, so female organisms are born with all the oogonia they will ever have. For this reason, the formation of oogonia is not considered part of oogenesis proper, but it is vital to the process of forming oocytes. The formation of oocytes involves the following events:

Step 1. The first true step of oogenesis is oocytogenesis, the process by which oocytes develop, which happens after a female is born. During oocytogenesis, the oogonia undergo mitosis to form the primary oocytes. A primary oocyte is a diploid (2n) cell that undergoes meiosis to produce the secondary oocyte (egg cell). It is thought that oocytogenesis is complete shortly after birth, but this idea has recently been challenged and is under investigation by researchers.

Step 2. Ootidogenesis is the part of meiosis that produces the secondary oocyte from the primary oocyte. In this step, the primary oocyte undergoes meiosis, splitting its genetic material between two daughter cells. However, this process is halted in prophase I. The oocytes remain in this stage until triggered to continue during the female's fertile period. During each fertile cycle, only some cells move forward in the process and are available for fertilization (in humans, just one or two; hundreds may be available for fertilization during a single cycle in species that lay many eggs at once, such as sunfish or seahorses).

Step 3. The oocyte continues through the end of meiosis I, forming one viable cell (the secondary oocyte) and a polar body (a non-viable cell produced at the end of meiosis in females which contains no genetic material). However, this secondary oocyte is halted at metaphase II of meiosis II until fertilization occurs.

Step 4. Upon fertilization , the cell completes meiosis II, producing an ootid (a fertilized egg cell) and another polar body.
Oogenesis begins when the diploid (2n) oogonium undergoes mitosis to produce the diploid primary oocyte (2n). The primary oocyte enters meiosis but is halted early until the female enters her fertile cycle. The fertile cycle triggers the cell to finish meiosis I, producing the haploid secondary oocyte (1n) and a polar body (a nonviable cell). The secondary oocyte is halted in meiosis II unless fertilization occurs. Upon fertilization, the cell completes meiosis II, producing a single viable cell and other polar bodies.