Meiosis and Gametogenesis
Objective 9
Compare and contrast the homologous endocrine events between spermatogenesis and oogenesis, thoroughly review the similarities and differences between mitosis and meiosis, and relate the phases of meiosis with the processes of spermatogenesis and oogenesis.
Homologous Endocrine Events
As we’ve just covered, in females, gonadotropin releasing hormone (GnRH) from the hypothalamus directs the secretion of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) from the anterior pituitary. FSH stimulates follicle development and estrogen production. LH stimulates ovulation, corpus luteum formation, and progesterone and estrogen production (by the corpus luteum).
Although named for their actions in the female reproductive system, FSH, and LH are also released in males. However, there are no follicles and no corpora lutea. Instead, FSH stimulates the spermatogenic cells and the sustentacular cells of the testes to produce sperm. A negative feedback loop controlled by inhibin ensures sperm are not over-produced. LH stimulates the interstitial cells of the testes to secrete testosterone.
Mitosis
Mitosis and meiosis – you’ve heard these terms before, back in Unit 6, The Molecular Biology of the Cell. We need to review mitosis and meiosis, including the differences between the two processes, so we can understand how these concepts apply directly to male and female gametogenesis.
The vast majority of our body cells are somatic cells. When somatic cells divide they undergo mitosis, a process where the DNA content is doubled (DNA replication during S phase), and then divided equally (M phase), resulting in two daughter cells that are genetically identical to each other and to the parent cell. In doing so, we preserve a principle called ploidy, which refers to the number of complete sets of chromosomes in each cell of the organism.
To better understand ploidy, let’s take that same somatic cell and describe its chromosomes a little bit differently. Rather than saying it has 23 pairs (of two) chromosomes, lets describe it as having two sets of 23 chromosomes (one set came from Mom and has a copy of chromosome 1, a copy of chromosome 2, …a copy of chromosome 22, and a sex chromosome; the other set came from Dad and contains the same list of chromosomes). Because there are two complete sets, we say the cell is diploid or 2N. All somatic cells are diploid, 2N.
N counts the number of chromosomes; C counts the number of DNA molecules. During the cell cycle, those 46 DNA molecules briefly become 92 DNA molecules before the cell divides into two daughter cells and restores the diploid number (C=46) again.
Meiosis
The only cells in the body that are not somatic cells are gametes (sperm, ova). Each gamete has just one set of 23 chromosomes, which makes them haploid, or N (C=23).
Think about the two gametes (haploid) – one sperm and one ovum – that united to form the zygote (diploid) that became a fetus that became you. Wher