Spermatogenesis



Spermatogenesis is the process of producing sperm with only half the number of chromosomes of regular cells. At fertilization, the sperm recombines with the female haploid gametes. This allows genetic contributions to be obtained from both parents. Spermatogenesis occurs in seminiferous tubules, which are part of the male testes. After spermatogenesis occurs, more cells are produced than normally presented. It results in haploid gametes with genetic variability.

Sperm Formation
Spermatogenesis occurs in the seminiferous tubules of the testes, beginning with special cells called spermatogonia. Spermatogonia undergoes the cell division process called mitosis instead of meiosis. One of cells is the result from mitosis that remains as a spermatogonium (singular spermatogonia), and the other is a primary spermatocyte. The primary spermatocyte, which has forty six similar chromosomes, undergoes meiosis I to produce two secondary spermatocytes. The secondary spermatocytes undergo meiosis II to separate the duplicates, since they have only one chromosome from each homologous pair, which are the same. Therefore, there are total of four spermatids, each with twnety three chromosomes, none duplicated. Also, they only have one allele for every gene. After they form, the spermatids develop specific features. When they develop the features, the sperm enter the epididymides through the umen of the seminiferous tubules to become fully mature. It takes about seventy days for the entire process of spermatogenesis. When fully mature, the sperm can live up to several weeks inside the male, but it only lives a few days once in the vagina.

There are two types of spermatogenesis. The first type, Sertoli cells, form a liquid-tight enclosure called the blood-testes barrier around the developing sperm. The sperm may create antigens that will be attacked by the lymphatic system, and the blood-testes barrier helps it not to happen. The blood-testes barrier seals off the sperm which is crucial during the development. However, since the developing sperm are sealed off from the blood supply, they cannot receive nutrients and oxygen. Therefore, the Sertoli cells support them with the nutrients and oxygen they need. The Sertoli cells also produce a hormone called inhibin. The inhibin is involved in a complex sequence of hormone secretions that controls the rate of spermatogenesis. The other type cell is Leydig cells, which secrete testosterone. Leydig cells are located outside of the seminiferous tubules in the testes.

Sperm Structure
Sperm is divided into three sections, the head, the midpiece, and the tail. An acrosome, which contains digestive enzymes, is located on the end of the head. The digestive enzymes bteak down the covering of the egg cell. Many sperm will embed their heads in an egg cell to digest the covering. The sperm needs many mitochondria because it needs a lot of energy to move. The tail exists to swim towards the egg. In order to do its major job (get to an egg cell and fertilize it) the sperm must be highly movable. And the length of the tail illustrates that the sperm is very mobile. The mitochondria in midpiece provides the energy to do so. When the sperm reaches an egg, it gets inside after breaking through the outer protective barrier of the egg. The acrosome does this job. God has designed the sperm with what it needs to reach and fertilize the egg. The sperm is the smallest cell in the human cell. A healthy man ejaculates about sixty to ninety million sperm in a single milliliter. When the number of sperm per milliliter is below 30 million, the man has a low sperm count. Furthermore, when the number drops to ten million, the man is infertile.

Meiosis
The cellular basis of sexual reproduction is called meiosis, which is very similar to mitosis, but different in results.

In a normal human cell, there are twenty three homologous pairs of chromosomes. In other words, there are total of forty six chromosomes. The number of homologous pairs is called the haploid number of the cell, and the total number of chromosomes is called diploid number. Meiosis only produce cells that have one chromosome from each homologous pair, called haploid cells. Also they may be called gametes because the cells are used for reproduction.

Meiosis Meiosis I 1. Early prophase I - The chromosomes and centriole duplicate and the centrioles begin to move. 2. Late prophase I - The chromosomes line up in homologous pairs and the spindle begins to form. 3. Metaphase I - The chromosomes line up on the equatorial plane. The microtubules attach to one homologue each. 4. Anaphase I - The microtubules of the spindle pull the homologues apart. 5. Telophase I - The plasma membrane constricts, and the chromosomes become surrounded by nuclear material. 6. End of meiosis I - Two haploid cells result. These cells still have duplicated chromosomes however. They must enter Meiosis II.

Meiosis II 1. Prophase II- Both cells duplicate centrioles. 2. Metaphase II - The chromosomes of each cell line up on the equatorial plane. 3. Anaphase II - The microtubules pull the duplicates from the originals. 4. Telophase II - The plasma membranes constrict, and nuclei form. 5. End of Meiosis II - Four haploid cells (gametes), each with no duplicate chromosomes, result.