Sexual reproduction relies on a specialized form of cell division called meiosis. This division is fundamentally different from the mitosis used by body cells for growth and repair. Meiosis serves a dual purpose: it reduces the number of chromosomes by half, and it introduces genetic variation into the resulting cells. It transitions a cell from a diploid state (containing two sets of chromosomes) to a haploid state (containing only one set). This reduction is necessary to ensure that when two reproductive cells fuse during fertilization, the resulting offspring has the correct, full number of chromosomes.
Identifying the Germline Cells
Meiosis is strictly reserved for a specific population of cells known as the germline. This cell lineage is distinct from the somatic cells, which make up the vast majority of the organism’s tissues and organs and reproduce only through mitosis. The germline is set aside early in development with the sole purpose of producing gametes (sperm and eggs).
The diploid cells that directly enter meiosis are specifically named based on the sex of the organism. In males, these precursors are called primary spermatocytes, which reside within the seminiferous tubules of the testes. For females, the corresponding cells are the primary oocytes, which are housed within the ovaries. These cells initiate the reduction division, starting with Meiosis I, to ultimately yield the haploid gametes required for the next generation.
The Diverse Outcomes in Humans
Once the primary spermatocytes and primary oocytes begin the meiotic process, the events that follow are markedly different between males and females in a process known as gametogenesis.
Spermatogenesis
In males, the process of spermatogenesis is continuous, beginning at puberty and persisting throughout the lifespan. A single primary spermatocyte undergoes two rounds of division to produce four functional, genetically unique haploid cells. The cytoplasmic division during this process is equal, resulting in four small, mobile sperm cells.
Oogenesis
The timing of oogenesis in females is fundamentally different, characterized by prolonged periods of arrest. Primary oocytes are formed during fetal development and enter a state of arrest in Prophase I of meiosis. They remain dormant in the ovary until adolescence, at which point a primary oocyte will periodically complete Meiosis I. This first division results in one large secondary oocyte and a much smaller cell called a polar body.
The division of the cytoplasm is highly unequal, ensuring that the single ovum receives the vast majority of the cellular resources and organelles. The secondary oocyte then arrests again at Metaphase II and is released during ovulation. Meiosis II will only be completed if the cell is fertilized, yielding a mature ovum and a second polar body. Thus, one primary oocyte results in one large, viable ovum and two or three non-functional polar bodies.
Meiosis in the Plant Kingdom
The location and timing of meiosis in the plant kingdom differ significantly from the animal model due to a life cycle called the alternation of generations. Plants possess both a multicellular diploid stage, known as the sporophyte, and a multicellular haploid stage, the gametophyte. Meiosis occurs within specialized structures of the sporophyte generation, such as the sporangium.
The specific cells within the sporophyte that undergo meiosis are called sporocytes. Unlike in animals, the products of this meiotic division are not gametes; instead, they are haploid cells called spores. These spores are reproductive cells that can grow into a new organism without fusing with another cell.
Each haploid spore then divides repeatedly by mitosis, leading to the formation of the multicellular gametophyte. Since the gametophyte is already haploid, it produces its gametes through the process of mitosis, completing the cycle of generations.