What is a cell?
Ever so often, we need to repeat the basic questions before we can go on to the more difficult ones. So, what's a cell?
The cell is the smallest complete unit of a living organism. Bacteria are one-cell organisms; humans have many millions of cells. For multicellular organisms, the cell (zygote) created by the fertilization of a egg by a sperm can multiply, differentiate into many different tissue types (e.g., muscle, nervous, cardiac), form different organ systems (e.g., cardiovascular, skin, digestive tract), and become an entire human being. In humans, as with all mammals, development into an independent living organism is only possible if the zygote implants itself in a female's uterus within a limited number of cell disivions after fertilization.
During differentiation, most cells follow a one-way path towards one of the many different tissue types needed by the body. Although nearly all cells of a human being contain the same genetic information (genotype), cells of one tissue type (e.g., muscle) are different in appearance and function (phenotype) from all cells of other tissue types (e.g., nerve and skin).
Because the body often needs different numbers of certain cells, the body's remarkably efficient construction system maintains a supply of cells that have not completed their differentiation. In particular, stem cells in bone marrow can produce lymphocytes, leukocytes, and other blood cells. Except for these "undifferentiated" cells, one tissue type cannot produce another tissue type; e.g., muscle cells cannot produce nerve cells.
Two cell types of the human body are "special". The sex cells -- oocytes and spermatocytes -- produce specialized reproductive cells -- eggs (ova) and sperm -- which have only half the genetic material of other body cells. Human females produce all their egg cells before birth; males produce their sperm cells over many years. An egg or sperm must combine with its matching type (forming a zygote) before it can multiply. The other special type of cell is red blood cells (erythrocytes), which have differentiated so far they no longer contain any genetic material and can no longer produce new cells.
Marie Godfrey, PhD
lack of nuclei in red blood cells
I think the lack of DNA (and mitochondria and their DNA, by the way) may be an energy and space-saving issue. The red blood cell is packed with hemoglobin molecules so that it can carry the greatest amount of CO2 to the lungs and O2 back from the lungs. The more space there is, the more efficient the red blood is. However, red blood cells are somewhat fragile, since their membranes are quite thin. When they wear out and disintegrate or are "eaten" by the white blood cells, there's no great loss to the body, since new red blood cells can be made.
I have no factual basis for this suggestion, however--so, I'll have to see what I can find out from reputable sources and get back to you.
Marie Godfrey, PhD
other sources on anucleate red blood cells
Dear Sabrina,
I lucked out quickly in a Google search and found a response written to a similar question. Here's the response I found:
Re: Why do birds have a nucleus in their red blood cells and we don't?
Date: Thu Apr 6 23:27:07 2000
Posted By: Robin Geller, , corporate regulatory and clincal affairs, Baxter Healthcare Corp
Area of science: Cell Biology
ID: 953481677.CbMessage:
Actually, this question is best answered by turning it around and asking why human red blood cells don't have a nucleus. The reason that human (acutally all mammalian)mature red blood cells lack a nucleus appears to be so that the red blood cell has room for more hemoglobin and therefore can carry more oxygen per cell.Remember the human red blood cells doen't always lack a nucleus. Up until the final steps of red blood cell maturation, they do have one.
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Admin note:
Monique Stins adds the following:Hi Dianna,
This is a very interesting question and when I was looking for an answer I found the same question posted on different sites, including an earlier one on the Mad Scientist Network. I consulted some websites (Sue Thornquist of Clinical pathology CVM, www.vet.orst.edu/clinpath/learning/vm736/avianhem.htm) and colleagues. We came up with the following explanation (facts and speculations):
Red blood cells (RBC) or erythrocytes are continually formed in the bone marrow. RBC originate from nucleated stem cells, which mature into nucleated erythroblasts, then differentiate into a-nuclear reticulocytes and finally into RBC. RBC are terminally differentiated cells (they cannot divide anymore) and are shed from the bone marrow into the blood circulation. They live approximately 120-180 days.
In contrast to mammals, RBCs in birds, reptiles and other "lower" vertebrates have a nucleus. The a-nucleated erythrocyte, as it is seen in mammals, is considered more evolutionarily "advanced". (see www.ultranet.com/~jkimball/BiologyPages/V/Vertebrates.html). The lower vertebrates (e.g., birds) are considered earlier on the evolution ladder and have a different circulatory system (see www.sciencenet.org, www.historyoftheuniverse.com/blood.html, http://library.thinkquest.org/3564/lessons/lesson3/lesson3.html ). In addition to the differences in the circulatory system, mammals have smaller end-bloodvessels (capillaries of about 3 micron in diameter) than birds. In order to squeeze through these small blood capillaries, RBC which are about 10 micron in diameter, must be very flexible. The presence of a nucleus would prevent big nucleated RBC to squeeze through these small capillaries. Therefore, during the evolutionary development, nature has found that it was better to get rid of the nucleus and also other cell organelles (e.g., endoplasmic reticulum for protein synthesis) which were not needed for their actual function as oxygen carrier.
Sue Thornquist ( www.vet.orst.edu/clinpath/learning/vm736/avianhem.htm) also thinks that the absence of the nucleus in birds is based on evolutionary differences but she’s not sure whether this theory has been proved. As homeotherms evolved, they had increased oxygen demands due to different metabolic requirements. Birds appear to have adapted to increased oxygen demands by developing a "flow-through" respiratory system (interconnecting tubes for continuous flow, rather than blind-ended alveoli) that's more efficient than mammals'. Mammals may have diverged here and developed anucleated RBC's with increased oxygen carrying capacity to adapt to the increased oxygen demands.
We hope that this will answer your question.
Monique
I didn't check the credentials of the comments' authors, but I can do that if you're interested.
Marie Godfrey, PhD
Evolution of a cell...?
OK, I went back to the beginning today just out of curiosity.
The last sentence of this post caught my attention. "The other special type of cell is red blood cells (erythrocytes), which have differentiated so far they no longer contain any genetic material and can no longer produce new cells." Why is that do you suppose? Is there an evolutionary adaptation going on here? Is it possible that viruses or bacteria were incorporating the dna of red blood cells into their makeup and using this as a way into the body and that those without dna in these cells were protected from invasion.
OK, it's stretching things quite a lot. But... ?
Or, is there some other much more logical explination?
Sabrina