Wednesday, 26 December 2012

stem cells, a brief description


Stem cells are biological cells found in all multicellular organisms, that can divide (through mitosis) and differentiate into diverse specialized cell types and can self-renew to produce more stem cells. In mammals, there are two broad types of stem cells: embryonic stem cells, which are isolated from the inner cell mass of blastocysts, and adult stem cells, which are found in various tissues. In adult organisms, stem cells and progenitor cells act as a repair system for the body, replenishing adult tissues. In a developing embryo, stem cells can differentiate into all the specialized cells (these are called pluripotent cells), but also maintain the normal turnover of regenerative organs, such as blood, skin, or intestinal tissues.
  • Bone marrow, which requires extraction by harvesting, that is, drilling into bone (typically the femur or iliac crest),
  • Adipose tissue (lipid cells), which requires extraction by liposuction,
  • Blood, which requires extraction through pheresis, wherein blood is drawn from the donor (similar to a blood donation), passed through a machine that extracts the stem cells and returns other portions of the blood to the donor
Stem cells can also be taken from umbilical cord blood just after birth. Of all stem cell types, autologous harvesting involves the least risk. By definition, autologous cells are obtained from one's own body, just as one may bank his or her own blood for elective surgical procedures.
Highly plastic adult stem cells are routinely used in medical therapies, for example in bone marrow transplantation. Stem cells can now be artificially grown and transformed (differentiated) into specialized cell types with characteristics consistent with cells of various tissues such as muscles or nerves through cell culture. Embryonic cell lines and autologous embryonic stem cells generated through therapeutic cloning have also been proposed as promising candidates for future therapies. Research into stem cells grew out of findings by Ernest A. McCulloch and James E. Till at the University of Toronto in the 1960s.
Recently medical advances have obtain stem cells from a unusual source which compared with the discomfort of biopsy, is less painful. Yuanyuan Zhang and his colleagues at the Wake Forest University School of Medicine in Winston-Salem, North Carolina, have made urethra-like tissue by growing stem cells extracted from the urine of four healthy volunteers on scaffolds made from pig gut tissue.
To do this, the team first converted stem cells extracted from urine into urothelial cells and smooth muscle cells - vital cell lines for making ureters, which empty fluid from the kidneys into the bladder, and urethras, which conduct it from the bladder out of the body. Zhang then chemically stripped all pig cells from layers of pig gut tissue, leaving just the underlying inert collagen scaffold. He coated this scaffold with the two types of cell. Two weeks later, the deposited cells had formed layers on the scaffolds resembling urethras and ureters. In another experiment, the same structures developed after the seeded scaffolds had been implanted in mice lacking an immune system, proving that the cells can survive and grow in live animals.
 Zhang plans further experiments in larger animals and eventually in humans. He and his colleagues hope to emulate the clinical success seen two years ago when researchers replaced a woman's damaged windpipe by growing her stem cells on a section of donated windpipe that had been stripped of the donor's cells. There seem to be ample stem cells in urine to make these structures. A single colony of converted cells can coat a scaffold up to 10 cubic centimetres in volume, and just 200 millilitres of urine contains enough stem cells to form 15 colonies, say the team. While harvesting cells may sound unique, an alternative macabre method maybe put into practice.
Dead bodies can provide organs for transplants, now they might become a source of stem cells too. Huge numbers of stem cells can still be mined from bone marrow five days after death to be potentially used in a variety of life-saving treatments.
Human bone marrow contains mesenchymal stem cells, which can develop into bone, cartilage, fat and other cell types. MSCs can be transplanted and the type of cell they form depends on where they are injected. Cells injected into the heart, for example, can form healthy new tissue, a useful therapy for people with chronic heart conditions. Unlike other tissue transplants, MSCs taken from one person tend not to be rejected by another's immune system. In fact, MSCs appear to pacify immune cells. It is this feature which has made MSC treatments invaluable for children with graft-versus-host disease, in which transplants aimed at treating diseases such as leukaemia attack the child instead. Stem cell therapies require a huge numbers of cells though, and it can be difficult to obtain a sufficient amount from a living donor.
Paolo Macchiarini, who researches regenerative medicine at the Karolinska Institute in Stockholm, Sweden, describes the work as an excellent advance but says that the cells may not be as healthy as they seem. Their DNA may be affected by the death of surrounding tissue and exposure to cold temperatures. "We need to make sure the cells are safe," he says.
Corneal stem cells taken from the eyes of fresh cadavers have already been used to treat blindness in people with eye conditions that result from injury and scarring, but Chris Mason at University College London sees a potential hurdle in using such MSCs in therapy. "The work is novel and intriguing... but it would be better to use a living donor," he says. That's partly because medical regulators oppose treating individuals with stem cells from more than one source. "You can always go back and get more stem cells from a living donor if you need them, but if you use a cadaver, you'll eventually run out."
Stem cells have a multitude of possible cures, as each cell is considered as a building block of a bio organism, the theory allows the possibility to program the blocks into specific cells for replacement or repair.

In 2006, Shinya Yamanaka made a groundbreaking discovery that would win him the Nobel Prize in Physiology or Medicine just six years later: he found a new way to ‘reprogramme’ adult, specialized cells to turn them into stem cells. These laboratory-grown stem cells are pluripotent – they can make any type of cell in the body - and are called induced pluripotent stem cells, or iPS cells. Only embryonic stem cells are naturally pluripotent. Yamanaka’s discovery means that theoretically any dividing cell of the body can now be turned into a pluripotent stem cell. Stem cells is still a relatively new science and while small breakthroughs in the news are helping to turn peoples mind on the ethical issues. As alternatives to using embryonic stem cells are making great advances it is uncertain how we might harvest cells for repair...


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