In gametes, but they don’t have the physical

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.

Meiosis

Spermatogenesis is the process by which the male gametes, called sperm, are created.

Definition of spermatogenesis
Spermatogenesis Definition

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.

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Diagram of the meiosis process
Meiosis Illustration

Sperm Production

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.

Sperm Anatomy

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.

Illustration of sperm anatomy
Enlarged image of seminiferous tubules
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.

The spermatogenesis process
Spermatogenesis Illustration

Lesson Summary

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.

Lesson Objective

After viewing this lesson, you should be able to:

  • State the term for the process by which the male gametes are made and explain where this process occurs
  • Define what Sertoli cells are and describe the process that takes place in the Sertoli cells
  • Enumerate the steps of meiotic division, identify the cells produced at each step, and explain the purpose of this process
  • Classify the parts of a mature sperm cell and explain the function of each part
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