Gametogenesis

Sexual reproduction is the predominant mode of reproduction in animals. Sexual reproduction involves the production of gametes: the eggs and sperm. In most animals, these gametes are produced in specialized organs called gonads (ovaries and testes). The sex cells in most animals are separate—that is, each individual animal contains either testes or ovaries, but not both. Such animals are said to be dioecious. In dioecious animals, the sex cells from two different individuals (one male and one female) will fuse together in a process known as fertilization to form offspring. The advantage of sexual reproduction seems to be in its potential to produce variability in the gametes and, therefore, in the new organism.

Gametes are highly specialized cells that are adapted for reproduction. These egg and sperm cells develop by a process of gametogenesis, or gamete formation. Sperm cells are relatively small cells that are specialized for motility (movement); egg cells are larger, nonmotile cells that, in many species, contain considerable amounts of stored materials that are used in the early development of the zygote (fertilized egg).

In animals, gametogenesis consists of two major events. One involves the structural and functional changes in the formation of the gamete. The other involves the process of meiosis. Animal body cells normally contain the diploid amount of genetic material. Each species of animal has a characteristic diploid number that remains the same from generation to generation. Because fertilization involves the fusion of the egg and the sperm, bringing together each cell’s set of genetic material, some mechanism must reduce the amount of genetic information in the gamete, or it would double every generation. Meiosis is a special nuclear division whereby the genetic material is reassorted and reduced to form haploid cells. Therefore, gametes are haploid, and gamete fusion during fertilization reestablishes the diploid content in the zygote.

Sperm

Sperm are highly motile cells that have reduced much of their cellular contents and are little more than a nucleus. Sperm are produced in the testes from a population of stem cells called spermatogonia. Spermatogonia are large diploid cells that reproduce by an equal division process called meiosis. Spermatogenesis is the process by which these relatively unspecialized diploid cells become haploid cells; it is a continuous process that occurs throughout the sexually mature male’s life. When a spermatogonium is ready to become sperm, it will stop dividing mitotically, enlarge, and begin the reduction division process of meiosis. These large diploid cells that begin to divide meiotically are known as primary spermatocytes.

The first step in the division process involves each primary spermatocyte dividing to form two secondary spermatocytes. Each secondary spermatocyte continues to divide, and each forms two spermatids. These spermatids are haploid cells. For each primary spermatocyte that undergoes spermatogenesis, four spermatids are formed. The spermatids are fairly ordinary cells; they must undergo a process to form them into functional sperm. The transformation process of a spermatid into a sperm is called spermiogenesis and involves several changes within the cell. The genetic material present in the nucleus begins to condense, while much of the cytoplasm and its subcellular structures are lost. The major exception to this latter event is the retention of mitochondria, cytoplasmic structures involved in energy production. The mature sperm has three main structural subdivisions: the head, the neck (or midpiece), and the tail. All are contained within the cell’s membrane. The oval head has two main parts, the haploid nucleus and the acrosome. The acrosome comes in various shapes but generally forms a cap over the sperm nucleus. The acrosome functions differently in various animals, but generally, its functions are associated with the fertilization process (union and subsequent fusion of egg nucleus and sperm nucleus). Acrosomes contain powerful digestive enzymes (organic substances that speed the breakdown of specific structures and substances), allowing the sperm to reach the egg’s membrane. The midpiece of the sperm contains numerous mitochondria, which provide the energy for the sperm’s movement. The tail, which has the same general organization as flagella or cilia (subcellular structures used for locomotion or movement of materials), uses a whiplike action to propel the sperm forward during locomotion. The structural changes that occur during spermiogenesis are meant to streamline and pare down the sperm cell for specific action of a limited duration. The sperm’s function is to “swim” to the egg, to fuse with the egg’s surface, and to introduce its haploid nucleus into the egg’s interior.

Eggs

The female gamete, the egg or ovum, is produced by a process known as oogenesis. This process occurs in the female gonads, the ovaries. At first glance, oogenesis and spermatogenesis appear to be very similar, but there are some striking differences. The major similarity is that both processes form gametes, which contain genetic material that has been reduced to the haploid condition. To understand oogenesis, one must consider that its goal is to produce a cell that is capable of development. The mature egg in all animal cells is larger than other cells, particularly the sperm. Two important features of the egg must be considered: the presence of a blueprint for development and the means to construct an embryo from that blueprint. In other words, the egg must be programmed and packaged during oogenesis. The programming refers to the information that is coded within the structure of the egg. This information includes the genetic material as well as the cytoplasmic information. Together, the nucleus and cytoplasm provide the egg with the potential to transform a simple cell into a complex, preadult form. Since this transformation occurs within the egg, the programming must be within the organization of the egg, and the directions for development must be within that organization. The packaging refers to the presence of all the material necessary to build embryonic structures, to nourish this developing embryo, and to provide its energy until it can obtain nourishment on its own.

As happens in spermatogenesis, in oogenesis, the potential eggs are formed from unspecialized stem cells called oogonia. Oogonia contain the diploid amount of genetic material and divide by the process of mitosis. At some point in their life, oogonia stop dividing mitotically, enlarge, and prepare to become eggs—that is, they begin meiosis. The cell that begins this reduction division process is called the primary oocyte. Each primary oocyte divides into two cells—one large cell, the secondary oocyte, and a very small cell, the first polar body. The secondary oocyte continues the final reduction phase of meiosis and forms two cells, one large one (the ovum) and one very small one (the second polar body). The first and second polar bodies are nonfunctional by-products of meiosis. The one functional cell, the mature egg, contains most of the cytoplasm of the primary oocyte and one-half of its genetic material. In many animals (primarily the vertebrates), all oogonia in the ovaries enter meiosis simultaneously; the initial events of oogenesis are synchronous within the animal. Unlike spermatogenesis in the male, oogenesis in many animals is not a continuous process. Rather, the primary oocytes in the first stages of reduction division may remain inactivated for a long time—in some cases, for several decades. Therefore, in female animals with this format of oogenesis, a primary oocyte population is maintained, and eggs will mature as they are needed.

Thus far, it appears that the egg’s formation differs from the sperm’s in three ways. First, in many female animals, there is a limited number of primary oocytes capable of going on to form eggs; second, this egg formation is not necessarily a continuous process; third, one primary oocyte yields one mature egg at the end of meiosis. Although these are three very important differences, there are other distinct egg events that deal with the developmental programming and the packaging of materials in this potential gamete.

Eggs and RNA

There is still much to learn about the egg’s storage of developmental directions or the actual programming of information, but developmental and molecular biologists are beginning to elucidate events that occur during oogenesis that are concerned with the function of the egg. One such event, fairly widespread among the animal kingdom, is the formation of so-called lampbrush chromosomes during oogenesis. The chromosome’s backbone unravels at many sites so that regions, composed of specific genes, loop outward from the backbone. These loops give the chromosome its distinctive lampbrush-like appearance. Large amounts of nucleic acid known as messenger ribonucleic acid (mRNA) are being made on each loop. This mRNA is then processed and sent into the developing eggs’s cytoplasm, where most of it will be stored for use during early development. It is interesting to note that maternally provided mRNAs are the only expressed genes in initial stages of development. After fertilization, these mRNAs can be used to make specific proteins necessary for the embryo.

Another event present in some developing eggs is the mass production of another type of RNA known as ribosomal RNA (rRNA). Most of this rRNA will also be stored until fertilization. After fertilization, these rRNA particles will help form cytoplasmic structures called ribosomes (the sites of protein synthesis). In addition to these egg products, many animal eggs must become filled with yolk. Yolk is the general term that covers the major storage of material in the egg.

Because the maternal proteins (yolk and other protein components) and nucleic acids (various RNAs) form the bulk of the egg cytoplasm, they profoundly influence the embryo's development. In particular, the positions of maternal mRNAs, ribosomes, and proteins affect the organization of the embryo. It is evident, then, that the maternal genetic information and the arrangement of the products of this information provide crucial developmental information that will control much of the course of embryonic development. Therefore, the egg contributes considerably more than a haploid nucleus to the zygote.

Studying Gametogenesis

There are several approaches to the study of gametogenesis. Early biologists employed cytological techniques (methods of preparing cells to study their structure and function) and microscopy to study gamete formation. These early studies were, in fact, observations of the actual events themselves. Although these early descriptive approaches gave much information about the cells involved at each stage of gamete formation, they did not provide any information about the control mechanisms for this process. Biochemical studies have contributed to the understanding of certain regulatory substances and how they function in gametogenesis. By enhancing or inhibiting the presence of these regulatory substances in the organism, investigators have been able to elucidate many of the normal events of gametogenesis.

Beginning at puberty, the hormones (substances released from endocrine glands, generally functioning to regulate specific body activity) of the hypothalamus, the pituitary gland, and the gonads interact to establish and regulate gametogenesis in the organism. gonadotropin-releasing hormone (GnRH) from the hypothalamus stimulates the release of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) from the anterior portion of the pituitary gland. All three of these hormones are necessary for spermatogenesis and oogenesis.

Surgical removal of the mammalian pituitary gland (hypophysectomy) in the male leads to degeneration of the testes. Testicular function can be restored in these hypophysectomized animals by administering the hormones FSH and LH. These studies suggest that FSH and LH are necessary for the normal functioning of the testes. LH appears to stimulate the release of testosterone (male hormone) by certain cells (Leydig cells) of the testes. Both testosterone and FSH are necessary for spermatogenesis, but the exact role that each of these hormones plays in male sexual physiology has yet to be determined.

Oogenesis in the female has been the subject of intense investigation. At the beginning of each ovarian cycle, from puberty to menopause, one primary oocyte present in the female’s ovaries is activated to continue the process of gamete formation. The release of GnRH from the hypothalamus at the beginning of each cycle stimulates the anterior portion of the pituitary gland to release FSH. FSH, in turn, affects the ovaries: It stimulates a primary oocyte to mature to the point that it can be released from the ovary as a secondary oocyte, and it causes certain cells (follicle cells) in the ovary to produce estrogens, female hormones. High estrogen levels will cause the pituitary to inhibit FSH release, a negative feedback mechanism, and stimulate LH release. These estrogen-mediated events occur at approximately the middle of the ovarian cycle. LH also affects the ovaries. LH, however, is responsible for ovulation (the release of the oocyte from the ovaries) and for forming a cellular structure called the corpus luteum. LH also stimulates the corpus luteum to produce progesterone, another female hormone. Eventually, high progesterone levels will inhibit LH release from the pituitary gland, and the cycle begins anew.

Along with the study of oogenesis, the study of the lifespan of the oocyte, from formation within the fetus to the end of its lifespan at menopause, has also been the subject of researchers. Particular attention has been paid to maternal age in regard to oocyte fertilization. Early research suggests that the longer the oocyte waits to undergo ovulation, the longer the process from oogenesis to fertilization takes, and in turn, the lower the chances to form a viable embryo. Studies such as this are important to better understand causes of and solutions to infertility among humans.

Principal Terms

Diploid: the number of chromosomes or the amount of genetic material normally found in the nucleus of body cells; this number is constant for a particular species of animal

Gamete: a sex cell; the egg or ovum in the female and the sperm in the male

Haploid: one-half of the diploid number; the number of chromosomes or the amount of genetic material found in a gamete

Meiosis: reduction division of the genetic material in the nucleus to the haploid condition; it is the process used by animal cells to form the gametes

Oogenesis: gamete formation in the female; it occurs in the female gonads, or ovaries

Spermatogenesis: gamete formation in the male; it occurs in the male gonads, or testes

Spermiogenesis: the structural and functional changes of a spermatid that lead to the formation of a mature sperm cell

Bibliography

Epel, D. “The Program of Fertilization.” Scientific American 237 (November, 1977): 128-140.

Harry, Nathan D., and Christina Zakas. “Maternal Patterns of Inheritance Alter Transcript Expression in Eggs.” BMC Genomics, vol. 24, no. 1, 2023, p. 191. doi:10.1186/s12864-023-09291-8. Accessed 14 Sept. 2024.

Kinne, Rolf K. H., ed. Oogenesis, Spermatogenesis, and Reproduction. New York: Karger, 1991.

Krajnik, Kornelia, et al. “Oogenesis in Women: From Molecular Regulatory Pathways and Maternal Age to Stem Cells.” International journal of molecular sciences, vol. 24, no. 7, 2023. doi:10.3390/ijms24076837. Accessed 14 Sept. 2024.

Larose, Hailey, et al. “Gametogenesis: A Journey From Inception to Conception.” Current Topics in Developmental Biology, vol. 132, 2019, pp. 257-310. doi:10.1016/bs.ctdb.2018.12.006.

Sadler, R. M. The Reproduction of Vertebrates. New York: Academic Press, 1973.

Van Blerkom, Jonathan, and Pietro M. Motta, eds. Ultrastructure of Reproduction: Gametogenesis, Fertilization, and Embryogenesis. Boston: Kluwer, 1984.