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 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.

The acrosome reaction

acrosome reaction on a Sea Urchin cell

Sperm cells become even more excited when they approach an egg cell. They swim faster and their tail movements become more forceful and erratic. This behaviour is called "hyperactivity."

A recent discovery links hyperactivity to a sudden influx of calcium ion into the tails. The whip-like tail (flagellum) of the sperm is studded with ion channels formed by a protein called CatSper1. These channels are selective, allowing only calcium ion to pass. The opening of CatSper1 channels is responsible for the influx of calcium. The sudden rise in calcium levels causes the flagellum to spin faster, propelling the sperm more forcefully through the viscous environment. Sperm hyperactivity is necessary for breaking through two physical barriers that protect the egg from fertilization.

The first barrier to sperm is made up of so-called cumulus cells that encase the egg like chain mail. The cumulus cells develop with the egg and support it as it grows, and then provide a physical barrier to fertilization.

The second barrier is a membrane called the zona pellucida. One of the proteins that make up the zona pellucida binds to a partner molecule on the sperm. This lock-and-key type mechanism is species-specific and prevents the sperm and egg of different species from fusing. There is some evidence that this binding is what triggers the acrosome to release the enzymes that allow the sperm to fuse with the egg.

The female ovum is coated in a thick protective membrane. When a sperm cell reaches the egg the acrosome releases its enzymes. These enzymes break down the cell membrane, allowing the sperm cell passage into the egg where the sperm fuses with the egg cells membrane, and empties its genetic content into the egg.

Upon penetration the membrane of the egg cell undergoes a change and becomes impenetrable, preventing further fertilization of the ovum.

See also

  • Fertilization
  • Ovum

References
ISBN links support NWE through referral fees

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

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


External links

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