In the last chapter we talked about the importance of meiosis in sexual reproduction and diversity, and saw how haploid cells are formed. In males, meiosis occurs during spermatogenesis, in which spermatogonia in the testes become spermatozoa.
The germ cells that will form the male gametes (spermatozoa) are derived from germ cells that migrate from the yolk sac into the site of early gonad formation (see Chapter 38).
Spermatogonia are diploid germ cells in the testes that maintain their numbers by mitosis, thus maintaining spermatozoa numbers through life. Spermatogonia contain both Xand Ysex chromosomes. At a certain point a spermatogonium will stop its other duties and begin meiosis. The cells that result will then pass through more stages of maturation and development and will become mature spermatozoa capable of travelling to and fertilising an ovum.
The testis is made up of very long, tightly coiled tubes called the seminiferous tubules that are surrounded by layers of connective tissue, blood vessels and nerves (Figure 8.1). The seminiferous tubules are linked to straight tubules and a network of tubes called the rete testis which lead to the epididymis. The epididymis is another collection of tubes on the posterior edge of the testis that passes inferiorly and is continuous with the ductus deferens (also known as the vas deferens). The ductus deferens carries mature spermatozoa from the testis to the urethra.
Spermatogonia are found in the walls of the seminiferous tubules, and as they progress through spermatogenesis they pass towards the lumina of those tubules. Leydig cells within the testes produce testosterone. Sertoli cells are also found in the seminifer- ous tubules, and produce a number of hormones.
The spermatogonia that we begin the process with are called spermatogonia A cells (Figure 8.2). These are the stem cells that proliferate and replenish the root source of all spermatozoa. The cells that are about to begin meiosis are called spermatogonia B cells, and can be recognised partly because they are connected to one another by cytoplasmic bridges. They continue to divide by mitosis until they become primary spermatocytes. The cytoplasmic bridges will maintain connections between a group of cells during spermatogenesis, synchronising the process and batch producing groups of spermatozoa.
The primary spermatocytes enter meiosis I. Homologous recombination of chromosomes occurs in this stage. One primary spermatocyte becomes two secondary spermatocytes. These cells are now haploid. Each secondary spermatocyte may contain an X or a Y sex chromosome.
Secondary spermatocytes enter meiosis II and again divide, forming spermatids. As the DNA was not replicated in meiosis II these cells have half their original DNA. During fertilisation this DNA will be combined with the DNA of the maternal ovum. This is the end of the first stage of spermatogenesis, known as spermatocytogenesis.
During spermiogenesis the rounded spermatid cell changes shape, becoming elongated and developing the familiar head and tail. The cell loses cytoplasm, the nucleus is packed into the head, mito- chondria become concentrated in the first part of the tail and an acrosome forms around the tip of the head. The acrosome contains enzymes that will help the sperm penetrate the outer layers of the ovum during fertilisation.
At the end of spermiogenesis the spermatids have become spermatozoa (Figure 8.3).
Spermatogenesis takes around 64 days to produce spermatozoa from germ cells in the above processes. The spermatozoa are then passed in an inactive state to the epididymis, where they continue to mature. During the next week they descend within the epididymis and become motile and ready to be passed into the ductus deferens during ejaculation.
Abnormalities in spermatogenesis are common, and during fertility investigations the number and concentration of spermatozoa, and the proportion of abnormal sperm, are counted in a semen sample. A number of biological and environmental factors will affect the sperm count and fertility, such as smoking, sexuall smitted diseases, toxins, testicular overheating and radiation.