From the very earliest stage of pregnancy, after the sperm fertilizes the egg, zygote is form to blastomere to blastocytes in an embryo form. Around 3—5 days after a sperm fertilizes an egg, the embryo takes the form of a blastocyst or ball of cells.
The blastocyst contains stem cells and will later implant in the womb. Embryonic stem cells come from a blastocyst that is 4—5 days old. It has been demonstrated that hESCs can be generated from single blastomeres Klimanskaya et al. No death of embryo in harvesting cells …..
Single blastomeres are removed from the embryos by using a technique similar to preimplantation genetic diagnosis PGD. The biopsied embryos were shown to grown to the blastocyst stage and frozen. The blastomeres were cultured by using a modified approach aimed at recreating the ICM niche, which substantially improved the efficiency of the hESC derivation to rates comparable to whole embryo derivations.
When scientists take stem cells from embryos, these are usually extra embryos that result from in vitro fertilization IVF.
In IVF clinics, the doctors fertilize several eggs in a test tube, to ensure that at least one survives. They will then implant a limited number of eggs to start a pregnancy.
Scientists have used MSCs to create new body tissues, such as bone, cartilage, and fat cells. They may one day play a role in solving a wide range of health problems. Scientists create these in a lab, using skin cells and other tissue-specific cells. These cells behave in a similar way to embryonic stem cells, so they could be useful for developing a range of therapies. However, more research and development is necessary.
To grow stem cells, scientists first extract samples from adult tissue or an embryo. They then place these cells in a controlled culture where they will divide and reproduce but not specialize further. Stem cells that are dividing and reproducing in a controlled culture are called a stem-cell line. Researchers manage and share stem-cell lines for different purposes. They can stimulate the stem cells to specialize in a particular way.
This process is known as directed differentiation. Until now, it has been easier to grow large numbers of embryonic stem cells than adult stem cells.
However, scientists are making progress with both cell types. Types of stem cells Researchers categorize stem cells, according to their potential to differentiate into other types of cells. Embryonic stem cells are the most potent, as their job is to become every type of cell in the body. The full classification includes: Totipotent: These stem cells can differentiate into all possible cell types.
The first few cells that appear as the zygote starts to divide are totipotent. Pluripotent: These cells can turn into almost any cell. Cells from the early embryo are pluripotent. Multipotent: These cells can differentiate into a closely related family of cells. Adult hematopoietic stem cells, for example, can become red and white blood cells or platelets.
Oligopotent: These can differentiate into a few different cell types. Adult lymphoid or myeloid stem cells can do this. Unipotent: These can only produce cells of one kind, which is their own type.
A stem cell which is capable of giving rise to any kind of differentiated cells in a particular organism is considered as totipotent. That means these cells contain the highest potential of differentiation. Zygote and spore are two examples of totipotent cells.
But some differentiated cells are also capable of returning to totipotency state. In humans, the zygote is formed after the fertilization of the ovum by the sperm. Zygote is divided by mitosis , generating identical cells which later become totipotent. Zygote forms the morula, which is further divided to form the blastocyte. After the implantation of blastocyte in the endometrium, the differentiation process begins.
This stage is referred to as the embryonic stage, and it has separated two cell masses called outer trophoblast and inner cell mass. Hence, the trophoblast and the inner cell mass are differentiated from the totipotent cells in the morula.
Then, inner cell mass becomes pluripotent by differentiating into three germ layers: endoderm, mesoderm, or ectoderm.
These three germ layers give rise to different types of specialized cells in the body by becoming multipotent. Therefore, the totipotent stem cells in humans are capable of differentiating into any type of a body cell; there are more than distinct types of human body cells.
Figure 1: Differentiation of totipotent embryonic stem cells. A stem cell that is capable of differentiating into any of the three germ layers is considered as pluripotent. The three germ layers are endoderm, ectoderm, and mesoderm.
Each of these three germ layers is then differentiated into different organs and tissues by becoming multipotent.
Multipotent cells are capable of differentiating into several types of cells which are functionally related to each other. Endoderm gives rise to the interior stomach lining, gastrointestinal tract, and lungs. Ectoderm gives rise to epidermal tissues and the nervous system. Mesoderm gives rise to bones, muscles, and blood. However, some cells like embryonic cells and induced pluripotent stem cells iPS are completely pluripotent. Stem cells are formed and differentiate during early embryonic development, and are also a natural part of the tissue renewal processes throughout life.
Stem cells roles in early development and tissue renewal make them particularly interesting from a therapeutic standpoint, potentially useful for healing otherwise incurable conditions and injuries. Stem cells are categorized based on their potency , or the diversity of cell types they can become as they differentiate. In most vertebrates, totipotent and pluripotent stem cells are very rare, occurring in abundance only during embryonic development.
Totipotent and pluripotent stem cells are a critical part of embryonic development and show how the single cell of a zygote can develop into every distinct tissue and cell type found in an organism. Many healing processes instead rely on multipotent stem cells, or basic cell division. For example, when a nerve or heart-muscle cell dies, the body often has no way to replace these highly specialized cells.
These cells do not divide in the same way as skin cells and other highly replaceable cells. There is also no supply of pluripotent cells ready to take on this role. Therfore, injuries to some organs or tissues are permanent. Even in tissues that can heal, the healing process often creates scar tissue that impedes long-term function.
Similarly, some diseases such as type 2 diabetes and AIDS, target specific cells types, limiting their ability to replenish themselves by cell division. Stem cells provide the possibility of replenishing cells that have been damaged or lost, thereby restoring functions that the body cannot restore on its own. This is in part how some vertebrates manage limb regeneration. Bone marrow is home to multipotent stem cells which produce blood cells, and a bone marrow transplant gives the recipient an entirely new selection of blood cells over time.
This is lifesaving for patients whose original blood cells were destroyed, such as in the aftermath of leukemia treatment. Future stem cell therapies make similar promises for other tissues.
This concept hinges on the fact that stem and most other cells have the same genetic code.
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