how tissue specific stem cells produce specialised cells?

The short answer is that we don't fully know what kicks most stem cells into action. Many adult stem cells seem to divide quite infrequently, just ticking over to keep their numbers constant. Even in tissues that are being constantly renewed, like the skin, most of the cell proliferation that produces the new specialized cells takes place among the immediate progeny of the stem cell. Not surprisingly, injury is one stimulus that can start stem cells producing specialized tissue cells for replacement.

It usually takes several cell divisions for the progeny of a stem cell to become fully differentiated cells. Along this line will be cells that can still give rise to several different cell types but have lost the ability to maintain a supply of unspecialized stem cells. Such cells are called progenitor cells. Whether to classify a cell as a stem cell or a progenitor has spurred many a squabble among scientists, largely because it's hard to determine experimentally exactly what these cells can do.

In some cases, the frequency with which stem cells throw off specialized cells seems to be guided by an internal 'clock', which might perhaps be tracking the number of times the stem cell has divided. But stem cells also take cues from their environment, such as the temperature, and whether they are attached to something or floating free. Most important, stem cells communicate with the other cells that surround them — the stem-cell niche. The cells send signals back and forth, and in some way we don't yet fully understand, the stem cells go where they need to go and become what they need to become, whether in the developing embryo or the adult. Whether researchers can provoke or mimic these signals sufficiently precisely to use stem cells for repairing the damage to tissues that occurs in neurodegenerative, cardiovascular and other diseases still remains to be seen.

related:

major types of stem cells!

what are stem cells?

how a fertilized egg develop?

a rare surgery!

more!!!!!!!!

and here is your a/r solution 13

home!tests!notes!exams !colleges!magazine!orkut! forum!must read

major types of stem cells!

Embryonic stem cells (ES cells) are obtained by extracting cells from very early embryos — at the blastocyst (hollow ball) stage — and growing them in laboratory dishes. Human ES cells are generated mainly from blastocysts that are the result of in vitro fertilization for assisted reproduction, but are not needed for implantation into the mother. In some countries, such as Britain and the United States, the parents can donate these 'spare' blastocysts for medical research.

Adult stem cells are also called tissue-specific stem cells because each type of adult stem cell produces only a limited set of specialized cells characteristic of a particular tissue — epidermis, blood, and so on. In adults, tissue-specific stem cells are located throughout the body. The so-called hematopoietic stem cells in bone marrow and umbilical cord blood, which make all the different types of blood cells, are the easiest to isolate, and have been used in therapy for decades — as bone marrow transplants for diseases such as leukemia, where the normal development of blood cells has gone awry.

Other types of tissue-specific stem cells are usually found deep within tissues and are harder to get at and harder to study, especially in humans. Familiar examples are the epidermal stem cells which continually renew the outer layer of the skin as it gets worn away, and the epithelial stem cells in the gut, that are similarly continually replacing the gut lining. More recent discoveries are of bronchoalveolar stem cells from the lungs of adult humans, which are thought to renew the lining of parts of the lungs, for example. And adult stem cells have been found in the inner ear in mice that could be involved in renewing cells involved in balance sensing. But even when cells are discovered that appear to behave like stem cells when grown in the laboratory, it's hard to know whether they act like stem cells in the body, because their natural behavior is hard to observe.

more coming up.........

related:

what are stem cells?

how a fertilized egg develop?

a rare surgery!

more!!!!!!!!!

how tissue specific stem cells produce specialised cells?

and here is your a/r test 13

home!tests!notes!exams !colleges!magazine!orkut! forum!must read

how a fertilized egg develops?

The most versatile stem cells occur earliest in life. As a fertilized human egg divides, it first becomes a solid ball of cells, the morula. Next, about five days after fertilization, it becomes a hollow ball, the blastocyst. The cells of the outer layer of the blastocyst eventually form part of the placenta. Inside the ball is a small clump of cells, the inner cell mass, that will form all the tissues in the body. When isolated from blastocysts created by in vitro fertilization (IVF) and grown in culture, these are the cells known as embryonic stem cells (ES cells).

The floating blastocyst takes another day or so to attach to the wall of the uterus and begin to draw nutrients from it. The basic structure of the placenta forms in about three weeks.

Well before then the embryonic cells are already "too old" to make ES cells. They have already become committed to more restricted fates. The initial flat sheet of embryonic cells folds and twists and grows to form a recognizable embryo, with a rudimentary head and a central cavity surrounded by three "layers" of cells. The cavity lengthens to form the gut. Cells in the innermost layer, the endoderm, make the lining of the gut and associated organs like the pancreas and liver. Cells in the middle layer, the mesoderm, form pretty much everything else on our inside: muscle, bone, heart, and kidneys, and all the connective tissues in between. Those cells in the outermost layer, the ectoderm, become skin and brain.

By about eight weeks after fertilization, all the major structures and tissues of a human body, including the heart and even the eyelids, are in place, although they've still got a lot of developing to do. The result, less than 4 centimetres (1.5 inches) long, is now called a fetus.

As pregnancy continues, stem cells become more and more specialized. Fetuses and adults have many types of stem cells, but each type generally makes fewer different kinds of cells than stem cells from earlier stages in development. The so-called bronchoalveolar stem cells found in the lungs in adults, for example, make only cells found in particular parts of the lung.

this is second post of stem cell series! to know the basics- what are stem cells?

discussing more on stem cells tomorrow........

major types of stem cells!

how tissue specific stem cells produce specialised cells?

get more interesting articles here

here is your a/r solution 12

home!tests!notes!exams !colleges!magazine!orkut! forum!must read

what are stem cells?

Without stem cells, wounds would never heal, your skin and blood could not continually renew themselves, fertilized eggs would not grow into babies, and babies would not grow into adults. Stem cells are quite unlike the specialized, or differentiated, cells in your body — such as the nerve cells, muscle cells and blood cells that enable you to function. In contrast, they are the body's silent reserves. At any given moment, many of the stem cells in your body won't be doing very much. They will only spring into action when you need either to produce more stem cells or make more of other, specialized types of cells. And they're not just found in people. All multicellular organisms, from plants to humans, need stem cells.

Usually, when a stem cell divides into two, one daughter cell goes on to make a more specialized type of cell, or even gives rise to several different cell types. The other daughter cell remains a stem cell, ready to produce more stem cells when they are needed. Only stem cells have this versatility, although some fully specialized cells, such as liver cells, can divide to give more cells exactly like themselves.

A fertilized egg is the ultimate stem cell, as it is the source of every type of cell in the body, from oxygen-carrying red blood cells to electricity-conducting nerve cells and throbbing heart muscle cells. But of course this doesn't happen all at once. As the fertilized egg divides to make an embryo, cells become specialized gradually.

Within three to six days after a human egg is fertilized, it has grown into a ball of a few hundred cells called a blastocyst. Within this ball lie a small number of cells that will go on to develop into the embryo. Scientists have learned to extract these stem cells from a thickening in the blastocyst called the inner cell mass and to grow them in the laboratory. These are known as embryonic stem cells or ES cells, and they have the potential to produce all the cell types in the human body. When a blastocyst implants in a woman's uterus, the cells of the inner cell mass will keep on dividing and differentiating into the earliest types of embryonic cells. Human ES cells in culture are not created from eggs that have been fertilized inside a woman's body. They come from the inner cell masses of 'spare' blastocysts that have been created in the laboratory as part of in vitro fertilization (IVF) programmes.

Less than three weeks after a human egg has been fertilized, these most flexible of stem cells have disappeared and embryonic cells become gradually more restricted in their potential. Instead of dividing to make one more specialized daughter cell and a back-up all-purpose stem cell, later embryonic cells are more likely to make two types of more differentiated cells when they divide. At this stage, the embryo's cells have 'committed' to become one of three general types of tissue that each has distinct types of stem cells. Many of these persist into adult life. As embryonic development continues, cells become even more specialized, forming recognizable tissues such as heart, muscle and blood.

In adults, dozens of stem-cell types have been described; more remain to be discovered. These stem cells are called tissue-specific as they will normally only replace one particular tissue. They are also sometimes called adult stem cells. The best understood are the stem cells that grow new blood cells, those that renew the skin, those that renew the gut lining, and those that can grow new skeletal muscles. Stem cells in the bone marrow make blood cells and have been used therapeutically for years. These are the cells that make it possible for a bone marrow transplant to renew a person's complete blood system.

Scientists across the world are trying to figure out exactly what these stem cells are capable of becoming in the lab and in the body; right now, however, tissue-specific stem cells appear to be specialists, quite good at making a few types of cells. Stem cells found in bone marrow naturally make new red blood cells and new white blood cells, for instance, but not new brain cells, at least, not robustly. Stem cells occurring in the brain make new neurons plus the cells that support them, but they don't seem to make muscle cells.

discussing more on stem cells tomorrow.......

how a fertilized egg develop?

major type of stem cells!

get more interesting posts here

and now:

here is your a/r solution 11

here is your a/r test 12

home!tests!notes!exams !colleges!magazine!orkut! forum!must read