Spermatogenesis: How the Male Reproductive System Produces Sperm
- 0:06 Meiosis
- 0:40 Sperm Production
- 1:00 Sperm Cell Precursors
- 1:54 Sperm Anatomy
- 3:16 Spermatogenesis
- 4:41 Lesson Summary
In this lesson, you'll learn the series of steps necessary to produce a fully functional sperm cell. You'll also look at the structure of a sperm cell and see how it's equipped to venture forth into new territory.
Spermatogenesis is the process by which the male gametes, called sperm, are created.
Now, you may recall that in order to create sperm that can combine with a female gamete, each sperm must be haploid and contain only one copy of each chromosome. You may also remember that in order to create haploid gametes, a cell must go through the process of meiosis, which involves replicating its genome and then dividing not once, but twice in order to create four haploid gametes from a single diploid cell.
In humans, spermatogenesis takes place in the seminiferous tubules, which are an intricate system of tubules in the testes where spermatogenesis takes place. The seminiferous tubules of an adult human male can sometimes produce over 100 million sperm per day.
Sperm Cell Precursors
In order to produce such a huge number of sperm, the tubules contain a very large and active population of dividing, self-sustaining germ cells, which are the cells that will become gametes. The germ cells that are found in the testes are called spermatogonia. Every day, somewhere around 25 million spermatogonia enter meiosis and become primary spermatocytes. After the first meiotic division, the cells double in number and are called secondary spermatocytes. Once the second meiotic division is completed but before the haploid cells have had a chance to differentiate into actual sperm, the cells are called spermatids. These spermatids now have the correct number of chromosomes to be functional gametes, but they don't have the physical characteristics of sperm that allow them to swim to the ovum and fertilize it.
A functional sperm cell looks a lot like a tadpole, but unlike tadpoles, sperm have only three basic components. The nucleus contains the haploid genome and not much else. The main job of the sperm is to get the haploid genome into the ovum, which will provide the other half of the genome along with pretty much all of the cytosol, cell membrane, and cellular organelles.
The acrosome is a structure that forms a cap over most of the nucleus of the sperm cell. The main job of the acrosome is to penetrate the outer layers of the ovum so that the sperm can get inside.
The flagellum is a long, whip-like cellular appendage that is used for locomotion. A sperm cell uses its flagellum in a whip-like fashion, lashing it back and forth to propel the sperm forward. Now, you may be wondering where the sperm cell gets the energy to whip a long flagellum back and forth so vigorously. After all, the nucleus contains very little besides DNA, and the acrosome doesn't contain any mitochondria to provide energy. That just leaves the flagellum itself. Sure enough, a portion of the flagellum right here is surrounded by mitochondria that provide the ATP that powers the whipping motion.
Okay, now let's see how all of this works in the context of the seminiferous tubules. As you can see from this highly magnified section, the testes are packed with seminiferous tubules that produce millions of sperm and release them into the lumens of the tubules.
The tubules are lined with an epithelial layer of Sertoli cells, which are somatic cells of the seminiferous tubules that support and provide nutrients to the various sperm precursors. The rapidly dividing spermatogonium are located at the base of the Sertoli cells. Often, when a spermatogonium divides by mitosis, one of the daughter cells will take a position between Sertoli cells, enter meiosis, and become a primary spermatocyte.
As meiosis continues, the primary spermatocyte migrates away from the base of the Sertoli cells and divides to form two secondary spermatocytes. The two secondary spermatocytes continue migrating and then divide to form four haploid spermatids. After meiosis is complete and each spermatid has the correct number of chromosomes, they begin the process of differentiating into fully functional sperm with their chromosomes packed tightly in the nucleus, an acrosome capping the nucleus, and a single flagellum to propel the sperm toward the ovum.
Let's review. Spermatogenesis is the process by which the male gametes, called sperm, are created. Spermatogenesis takes place in the seminiferous tubules, which are an intricate system of tubules in the testes where spermatogenesis takes place. The tubules are lined with an epithelial layer of Sertoli cells, which are somatic cells of the seminiferous tubules that support and provide nutrients to the various sperm precursors. The rapidly dividing spermatogonia, or sperm cells that are found in the testes, are located at the base of the Sertoli cells.
When a spermatogonium enters meiosis, it becomes a primary spermatocyte and migrates away from the base of the Sertoli cells. Then it divides to form two secondary spermatocytes, which continue migrating and then divide to form four haploid spermatids. After meiosis is complete and each spermatid has the correct number of chromosomes, they differentiate into fully functional sperm. Mature sperm have a nucleus that contains the haploid genome and not much else. They also have an acrosome cap which will be used to penetrate the outer layers of the ovum so that the sperm can get inside. In addition, they have a single flagellum, which is a long, whip-like cellular appendage that is used for locomotion. A sperm cell lashes its flagellum back and forth to propel the sperm forward.
Chapters in Biology 101: Intro to Biology
- 1. Science Basics (6 lessons)
- 2. Review of Inorganic Chemistry For Biologists (14 lessons)
- 3. Introduction to Organic Chemistry (8 lessons)
- 4. Nucleic Acids: DNA and RNA (4 lessons)
- 5. Enzymatic Biochemistry (4 lessons)
- 6. Cell Biology (14 lessons)
- 7. DNA Replication: Processes and Steps (5 lessons)
- 8. The Transcription and Translation Process (10 lessons)
- 9. Genetic Mutations (4 lessons)
- 10. Metabolic Biochemistry (9 lessons)
- 11. Cell Division (13 lessons)
- 12. Plant Biology (12 lessons)
- 13. Plant Reproduction and Growth (10 lessons)
- 14. Physiology I: The Circulatory, Respiratory, Digestive,... (12 lessons)
- 15. Physiology II: The Nervous, Immune, and Endocrine Systems (13 lessons)
- 16. Animal Reproduction and Development (12 lessons)
- 17. Genetics: Principles of Heredity (10 lessons)
- 18. Principles of Ecology (18 lessons)
- 19. Principles of Evolution (9 lessons)
- 20. The Origin and History of Life On Earth (4 lessons)
- 21. Phylogeny and the Classification of Organisms (7 lessons)
- 22. Social Biology (6 lessons)
- 23. Basic Molecular Biology Laboratory Techniques (13 lessons)
- 24. Analyzing Scientific Data (3 lessons)
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