Sperm

From New World Encyclopedia
Sperm
A sperm cell attempts to penetrate an ovum coat to fertilize it.
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Human spermatozoön. Diagrammatic. A. Surface view. B.Profile view. In C the head, neck, and connecting piece are more highly magnified.
Gray's subject #258 1243
MeSH Spermatozoa

A spermatozoon, or spermatozoan, (pl. spermatozoa), from the ancient Greek σπερμα (seed) and ζων (alive), is more commonly known as a sperm cell. It is the haploid male gamete cell, meaning it is the reproductive cell containing a single set of chromosomes. It fertilizes an ovum, which serves as the female haploid gamete. Together, the sperm cell and ovum form a zygote, or fertilized egg, which can then grow and develop into a new organism.

Because sperm cells are haploid, they contribute half of the genetic information to the diploid offspring, which contain two sets of chromosomes. In mammals, the sex, or gender, of the offspring is determined by the sperm cell since the ovum always provides an X chromosome. A spermatozoon bearing a Y chromosome will lead to a male (XY) offspring, while a spermatozoon bearing an X chromosome will lead to a female (XX) offspring.

Sperm cells were first observed by a student of Anton van Leeuwenhoek in 1677.[1]

Sperm Cell Structure

In humans, a sperm cell consists of a head, which is 5 µm by 3 µm, and a 50 µm long tail, or flagellum. The dense nucleus is covered by a vesicle called the acrosome, which contains enzymes that are crucial for fertilization. The sperm cell contains a minimum amount of cytoplasm. The midpiece, which is located between the head and tail of the sperm cell, contains centrioles, microtubules, and a mitochondrial spiral. These structures are used to aid in movement and fertiliaztion.

During fertilization, the sperm's mitochondria are destroyed by the egg cell. Only the mother is able to provide the offspring's mitochondrial DNA. This plays an important fact in tracing maternal ancestry. However, it has been recently discovered that mitochondrial DNA may be recombinant, or a combination of genes not found together in either parents.

Spermatazoan stream lines are straight and parallel. The tail flagellates and propels the human sperm cell at about 1 to 3 mm per minute by rotating like a propeller. The Reynolds number associated with spermatazoa is in the order of 1E-2. The Reynolds number is a ratio of inertial to viscous forces and is used to determine whether flow will be laminar or turbulent. A small number indicates viscous forces are dominant and therefore laminar flow is present, meaning sperm cells exhibit smooth and constant fluid motion.

In marine invertebrates, the sperm cell is a flagellate cell consisting of a flagella, acrosome, and perforatorium. Such organisms practice external fertilization (Baccetti 1986).

The largest spermatozoa belongs to the fruit fly.

Sperm Cell Production

Once a male reaches puberty, the period when the gonad,(reproductive organs) mature in the early teen years, sperm cells are produced continuously throughout the rest of the male's lifetime (gonads become inactive after birth until puberty). Sperm production does dimish with age but never completely ceases. Women, on the other hand, are born with all the eggs they will ever have. After approximately 40 years, their reproductive cycle ends during a period known as menopause.

Spermatozoa, or sperm cells, are all derived from germ cells, or the embryonic gonadal cells that produce gametes. Gametes are the reproductive cells that unite to form a new individual. In the testes of a newborn boy, immature spermatogonia, which are germ cells, are present. Some of these spermatogonia continually duplicate themselves through the process of mitosis. Other spermatogonia undergo meiosis and eventually develop into sperm.

During the process of spermatogenesis, germ cells mature to become sperm cells. The first part of the process occurs in the seminiferous tubules of the male testes, which are the male gonads. It takes approximately 64 days. Final maturation of sperm cells occurs in the epididymus (hollow duct) over a 12 day period. In the seminiferous tubules, spermatogonia initially undergo meiosis to become primary spermatocytes. Then, in the first meiotic division, each primary spermatocyte divides into two secondary spermatocytes. Each of the two secondary spermatocytes divide into two spermatids during the second meiotic division. Spermatids are haploid cells and contain 23 single chromosomes. The spermatids then mature into sperm as they lose most of their cytoplasm and develop a flagellated tail. Also, the nucleus' chromatin condenses into a dense structure as a vesicle called the acrosome covers most of the surface of the nucleus. Although sperm cells have been formed at this point in spermatogenesis, the sperm cells are not yet mature or able to swim. The final maturation process takes place once the sperm cells are moved into the epididymis, where they mature over 12 days or so.

Once the process of spermatogenesis is complete for one primary spermatocyte, the end result is the creation of four sperm cells. The average life span of sperm is between 4 to 6 days.

The entire process, from spermatogonium to mobile and functional sperm, takes approximately 76 days. However, at any one time different cells may be in a different stage of the development process. This staggering of developmental stages allows sperm cell production to stay steady at about 200 million sperm per day (Silverthorn 2004). Although this number may seem excessively large, it is about the number of sperm released in a single ejaculation.

Several hormones are required to initiate and maintain gametogenesis, which is the formation of gametes. Without gametogenesis, sperm cells could never form. Follicle stimulating hormone, or FSH, from the anterior pituitary, along with sex hormones, is required for the gametogensis process. The hypothalamus, a part of the brain, controls the release of FSH through gonadotropin releasing hormone, or GnRH.

While in the seminiferous tubules, various cells aid sperm development. Sertoli cells, also called sustentacular cells, are one such type of cell. They function to regulate sperm development by providing nourishment for the spermatogonia. They manufacture various proteins that span from hormones to growth factors to enzymes.

Fertilization and the Acrosomal Reaction

acrosome reaction on a Sea Urchin cell

The main function of sperm is to fertilize an ovum, or egg, to form a zygote. In order to do so, a sperm cell must locate the egg, penetrate its protective layerings, and then finally fuse its genetic material with the egg. The process through which a sperm cell breaks through the barriers of the egg is called the acrosomal process.

Once sperm has entered the female's vagina, usually through copulation (sexual intercourse), the sperm begins its task of locating the egg. The sperm mostly likely locates the egg through possible chemical attractants produced by the egg. It essentially smells its way from the vagina to the to the location of the egg in the distal parts of the female's Fallopian tubes. Once the sperm meets the egg, fertilization can occur.

For successful fertilization to take place, a sperm must first penetrate various layers surrounding the egg. The outer layer of the egg is the loosely connected granulosa cells. These cells make up what is known as the corona radiata and develop with the egg to support its growth and then serve to provide a physical barrier to fertilization. Once past this outer layer, a sperm has to surpass the protective glycoprotein coat of the zona pellucida. In order to get past these two major barriers, however, a sperm must release its powerful enzymes contained in the acrosome of the sperm head. The release of these enzymes begins the acrosomal process.

Once a sperm nears an ovum, capacitation and hyperactivity occur. The sperm begins to swim more rapidly and forcefully. A recent discovery links hyperactivity to a sudden influx of calcium ions into the tail of the sperm. The flagellum contains ion channels formed by a protein called CatSper1. These ion channels are selective and allow only calcium ions to flow in. Hence, the opening of CatSper1 channels leads to the influx of calcium. The sudden rise in calcium levels in the tail causes increased activity in the flagellum, propelling the sperm more forcefully through the viscous environment of the female uterus. Sperm hyperactivity is necessary for breaking through the physical barriers that protect the egg from fertilization. Once a sperm is capacitated and reaches the egg, enzymes are released from the acrosome in order to dissolve cell junctions and the zona pellucida coat (Carlson 2003).

After the sperm has wiggled its way toward the egg, one of the proteins that makes up the zona pellucida binds to a partner molecule receptor on the sperm. The zona pellucida consists of three or four glycoproteins, one of them being ZP3, or zona pellucida glycoprotein 3. It is the sperm receptor on the egg surface and functions in the initial binding and induction of the sperm acrosomal reaction. This lock-and-key type mechanism is species-specific and prevents the sperm and egg of different species from fusing. The fused section of the membranes opens and the nucleus of the sperm is transferred to the egg cytoplasm. Changes in the egg cell follow and help to prevent polyspermy, or the fertilization of the egg by more than one sperm.

Pathologies

Problems with sperm production, motility, or count may lead to infertility. In immotile cilia syndrome, which is an autosomal recessive defect, immobile or poor motility of the cilia of the airways and sperm result. Consequently, an egg cannot be fertilized and male infertility results.

Azoospermia can lead to infertility as well. In males afflicted with azoospermia, a non-measurable amount of sperm is present in the semen. Azoospermia has two forms: obstructive azoospermia, where sperm are created but cannot be mixed with the rest of the ejaculatory fluid due to a physical obstruction, and non-obstructive azoospermia, where there is a problem with spermatogenesis. It can be caused by cystic fibrosis, obstruction, chemotherapy, and Klinefelter syndrome.

A third pathology, impairment of sperm transport, may lead to infertility. It can be caused by a variety of factors such as obstruction of the epididymis or vas deferens and cystic fibrosis (Wilson 1991).

See also

  • Fertilization
  • Ovum
  • Testes

References
ISBN links support NWE through referral fees

Baccetti, B. 1986. Evolutionary Trends in Sperm Structure. PubMed PMID: 2876819. (See external link below).

Carlson, Anne, et al. 2003. CatSper1 required for invoked Ca2+ entry and control of flagellar function in sperm. Proceedings of the National Academy of Sciences. vol. 100 no. 25.

Silverthorn, D. 2004. Human Physiology, An Integrated Approach (3rd Edition). San Francisco: Benjamin Cummings. ISBN 013102153

Wilson, Jean D., et al. 1991. Harrison's Principles of Internal Medicine (12th Edition). New York: McGraw- Hill, Inc. ISBN 0070708908

External links

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