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How Cells In The Body Reproduce

Source

Cell Reproduction

Most human cells are frequently reproduced and replaced during the life of an individual.  However, the process varies with the kind of cell.  Somatic , or body cells, such as those that make up skin, hair, and muscle, are duplicated by mitosis .  The sex cells, sperm and ova, are produced by meiosis in special tissues of male testes and female ovaries .  Since the vast majority of our cells are somatic, mitosis is the most common form of replication.

Most human cells are frequently reproduced and replaced during the life of an individual.  However, the process varies with the kind of cell.  Somatic, or body cells, such as those that make up skin, hair, and muscle, are duplicated by mitosis.  The sex cells, sperm and ova, are produced by meiosis in special tissues of male testes and female ovaries.  Since the vast majority of our cells are somatic, mitosis is the most common form of replication.

 

Mitosis

The cell division process that produces new cells for growth, repair, and the general replacement of older cells is called mitosis.  In this process, a somatic cell divides into two complete new cells that are identical to the original one.  Human somatic cells go through the 6 phases of mitosis in 1/2 to 1 1/2 hours, depending on the kind of tissue being duplicated.

Phases of mitosis

Some human somatic cells are frequently replaced by new ones and other cells are rarely duplicated.  Hair, skin, and finger nails are replaced constantly and at a rapid rate throughout our lives.   In contrast, brain and nerve cells in the central nervous system are rarely produced after we are a few months old.  Subsequently, if they are destroyed later, the loss is usually permanent, as in the case of paraplegics.  Liver cells usually do not reproduce after an individual has finished growing and are not replaced except when there is an injury.  Red blood cells are also somewhat of an exception.  While they are being constantly produced in our bone marrow, the specialized cells from which they come do not have nuclei nor do the red blood cells themselves.

 

mitosis close up

 

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Meiosis

Meiosis is a somewhat similar but more complex process than mitosis.  This is especially true in females.  While mitosis produces 2 daughter cells from each parent cell, meiosis results in 4 sex cells, or gametes in males and 1 in females.  Unlike the cells created by mitosis, gametes are not identical to the parent cells.  In males, meiosis is referred to as spermatogenesis because sperm cells are produced.  In females, it is called oögenesis because ova, or eggs, are the main ultimate product.   The illustration below shows the 8 phases of spermatogenesis.

Phases of spermatogenesis (i.e., meiosis in males)

 

meiosis close up

 

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Sperm carries the father's chromosomes to the mother's ovum where they combine with her chromosomes at the time of conception.  Sperm cells are microscopic, but ova may be large enough in some species to be barely visible with the naked eye.

Human sex cells (not to the same scale)

The two sequential division processes of meiosis culminate in the production of gametes with only 1/2 the number of chromosomes of somatic cells.  As a result, human sperm and ova each have only 23 single-stranded chromosomes.

Summary of reduction division in meiosis

Human somatic cells, with their full set of 46, have a diploid number of chromosomes.  Gametes have a haploid number (23).  When conception occurs, a human sperm and ovum combine their chromosomes to make a zygote (fertilized egg) with 46 chromosomes.  This is the same number that the parents each had in their somatic cells.  In doing this, nature is acting conservatively.   Each generation inherits the same number of chromosomes.  Without the halving of them in meiosis first, each new generation would have double the number of chromosomes in their cells as the previous one.  Within only 15 generations, humans would have over 1½ million chromosomes per cell and would be a radically different kind of animal.  In fact, when a zygote has an extra set of chromosomes, it usually is spontaneously aborted by the mother.

The complete meiosis process in human males takes about 74 hours.  Spermatogenesis usually begins at 12-13 years of age and continues throughout life. Several hundred million sperm cells are produced daily by healthy young adult males.  Between 200 and 600 million sperm cells are normally released in each ejaculation.  Since only one sperm cell is required for conception, this huge number would seem to be an extreme overkill.  However, as many as 20% of sperm cells are likely to be defective and the female reproductive tract is hostile even to healthy ones--it is acidic and contains antibodies that seek out and destroy the sperm cells.  Ejaculating large numbers of sperm cells at the same time is nature's way of overcoming these difficulties and increasing the likelihood that conception will take place. The number of sperm cells produced can be significantly diminished by psychological and physiological stress.  In addition, sperm count progressively declines with age after reaching a peak, usually in the early 20's.

Human female reproductive system

Meiosis in human females is more complex.  Beginning in the 5th to 7th month after conception, several million immature sex cells begin to develop in the fetal ovaries but stop at an early stage of meiosis (after prophase I).  They remain in this primary oöcyte phase until puberty when hormones cause a resumption of meiosis for one to several cells each month.  They proceed to the 1st and 2nd reduction divisions and once again stop developing.  At this stage they are secondary oöcytes.  When a secondary oöcyte is finally released from the ovaries into the fallopian tube (during ovulation ), the egg still has not completed the last stage of meiosis.  That happens only at conception as a result of chemical changes that occur when the main part of a sperm cell enters the ovum.

Virtually all (99.9%) sex cells in a woman's ovaries never develop beyond the primary oöcyte stage and eventually are reabsorbed by her body.  By puberty, each of the two ovaries has lost all but about 200,000 of them.  Normally, women have on average of 11-14 ovulations per year for 33-36 years.  This means that less than 500 secondary oöcytes usually are produced out of the store of hundreds of thousands of primary oöcytes.  The actual number of ovulations is highly variable and often much lower, however, since the process is governed by hormones and ultimately other factors including psychological stress, nutrition, physical activity, and pathological conditions.  The fact that women rarely have more than a few children is evidence that only a small fraction of the successfully ovulated eggs are fertilized and become viable zygotes.

 

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This page was last updated on May 20, 2008 6:24 AM.
Copyright © 1997, 2000 by Dennis O'Neil. All rights reserved.
Illustration credits.

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1: Biol Signals Recept 2001 Jan-Apr;10(1-2):26-56

Structural changes of mitochondria related to apoptosis.

Wakabayashi T, Karbowski M.

Department of Cell Biology and Molecular Pathology, Nagoya University School of Medicine, Nagoya, Japan. twakaba@tsuru.med.nagoya-u.ac.jp

The original concept of apoptosis stressed the morphological changes of the nucleus, condensation with the aggregation of chromatin, and the intactness of intracellular organelles including mitochondria. However, the application of molecular biology and flow-cytometric techniques to the research field of apoptosis has led to the proposal of the apoptotic processes which emphasizes the 'swelling of mitochondria' due to the opening of the 'permeability transition pores' of the mitochondrial membranes followed by a series of events including the collapse of the transmembrane potential of mitochondria and release of cytochrome c from mitochondria into the cytosol. Enlargement of mitochondria induced by various pathological conditions are classified into two categories: the swelling and the formation of megamitochondria (MG). Recently, we have found that free radical-induced formation of MG is succeeded by apoptotic changes of the cell. If the MG formation is actually related to apoptosis, this will be a new aspect of the structural changes of mitochondria involved in apoptosis besides the simple swelling of mitochondria. First, we will discuss the 'swelling of mitochondria' which characterizes the currently accepted hypothesis on the apoptotic processes of the cell, as described above, in the light of the literature. Second, the mechanisms controlling the size and distribution of mitochondria in the cell are dealt with paying special attention to the genetic regulation and cytoskeletons. Third, we have tried to characterize the MG formation to correlate apoptotic changes of the cell. Finally, we will discuss several problems to be solved in the future which involve mitochondria in apoptotic processes of the cell. Copyright 2001 S. Karger AG, Basel

Publication Types:

• Review

• Review, Academic

PMID: 11223639 [PubMed - indexed for MEDLINE]

 

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