Genetics Lecture 19 Stem Cells. Stem Cells 4/10/2012

Transcription

1 Genetics Lecture 19 Stem Cells Stem Cells Much of the excitement about stem cells in the scientific and medical communities comes from their largely untapped and unproven potential for treating human conditions) through regenerative medicine creating cells, tissues, and organs for tissue or organ repair and replacement. For some people the isolation and use of these cells engender excitement, fear, anger, and a range of other emotions

2 4 What Are Stem Cells? Developmental biologists have known about Stem cells for decades, yet only relatively recently have stem cells generated tremendous attention from the scientific community worldwide. So what are stem cells and what makes them such attractive ti candidates for repairing ii failing tissues and organs? As you will soon learn, there are many different types of stem cells, but in general they all share two basic characteristics that make them distinctive from other cell types: self renewal and differentiation into specialized cell types: 5 1. Self renewal: Stem cells grow and divide (proliferate) indefinitely by mitosis to create populations of identical stem cells

3 7 Differentiation is a complex process involving many genes that must be activated and silenced in carefully coordinated temporal patterns of expression, and differentiating cells rely on chemical signals such as growth factors and hormones from other cells to help them change. Some stem cells possess greater differentiation ability than others, and scientists refer to this ability as the potency of the cell type. A totipotent cell, such as the zygote, can form not only all adult body cell types, but also the specialized tissues needed for development of the embryo, such as the placenta. Many types of stem cells are called pluripotent because they have the potential to eventually differentiate into a variety of different cell types to form all of the 220 cell types in the human body. 8 Sources and Types of Stem Cells Where do stem cells come from? At one time, it was commonly thought that stem cells were only present in an embryo. VV e now know that there are several sources and different types of stem cells. To understand what stem cells are, we need to briefly consider the development of a human embryo). 9 3

4 The zygote divides rapidly and after three to five days first forms a compact ball of about 12 cells called a morula, meaning little mulberry." Around five to seven days after fertilization, the dividing cells create an embryo consisting of a small hollow cluster of approximately 100 cells called a blastocyst. The blastocyst is approximately one seventh of a millimeter in diameter and contains an outer row of single cells called the trophoblast; this layer develops to form the fetal portion of the placenta that nourishes the developing embryo Within the blastocyst is a small cluster of around 30 cells that form a structure known as the inner cell mass. The inner cell mass is the source of human embryonic stem cells (hescs). During embryonic development, cellsof the inner cell mass develop to form the embryo itself, and hescs can differentiate to form all cell types in the body. hescs are pluripotent, as are other types of. 12 4

5 Successful isolation and culturing of the first hescs from a human blastocyst was reported in 1998 by Iames Thomson of the University of Wisconsin at Madison who had cultured ESCs from rhesus monkeys two years earlier. When hescs are isolated, scientists use a holding pipette that applies a brief suction to hold" the blastocyst in place, and a glass micropipette is then inserted into the blastocyst to gently remove cells from the inner cell mass, which are then cultured in dishes and flasks in the lab Initially, the main source of hescs was leftover embryos produced by assisted reproductive technologies such as in vitro fertilization (IVF). In IVF, multiple eggs are removed from a woman and fertilized in vitro. Resulting embryos are then implanted into a woman s uterus, but typically only a few of the embryos produced this way are implanted. Excess embryos are typically y frozen at ultra low temperatures for use by the couple in the future if desired. Alternatively, these embryos may be destroyed or, with the couple s consent, they may be donated for research. 15 5

6 Human embryonic stem cells avoid senescence (cell aging) in part because they express high levels of telomerase. Several groups have maintained stem cells for over threeyears andover 600 rounds of division without apparent problems. Cultured cells such as these that can be maintained and grown successively are called cell lines. Stem cells also grow rapidly and can be frozen for long periods of time and still retain their properties. 16 Stimulating ESCs to Differentiate Under the right conditions, when stimulated with different factors, ESC lines and other types of cells can be coaxed to differentiate into different types of cells in vitro. This directed differentiation of stem cells into specific differentiated cells of interest is key for creating tissues for regenerative medicine applications. A major focus of stem cell research is to determine what controls the pluripotency of stem cells and to identify if the factors that stimulate differentiation of stem cells into discrete cell types. These signals include substances called growth factors, hormones, and small proteins (peptides) that stimulate differentiation in tissue specific ways. 17 Adult Derived Stem Cells (ASCs) Research on hescs is controversial largely because of the source of ESCs a human embryo. It was long thought that the early embryo was the primary and perhaps the only major source of stem cells, but adult derived stem cells (ASCS) do reside in differentiated t d tissues of the body. ASCs appear in small numbers, but they can be isolated; this has been done from the brain, intestine, hair, skin, pancreas, bone marrow, fat, mammary glands, teeth, muscle, and blood and almost every adult tissue. 18 6

7 Opponents of hesc research have often claimed that ASCs are a more acceptable alternative than hescs because isolating ASCs does not require the destruction of an embryo. ASCs can be harvested from people by fine needle biopsy, where a thin diameter needle is inserted into muscle or bone tissue. It may even be possible to isolate ASCs from cadavers. We also know that ASCs are present in fat (adipose) tissue, which could potentially be an outstanding source of stem cells, especially if you consider that over 500,000 liters of fat tissue collected by liposuction and other cosmetic surgery techniques are discarded in the United States each year. For instance, an ASC isolated from muscle tissue could be used to develop into a blood cell. But other studies have demonstrated that ASCs may not be as pluripotent as hescs. 19 Amniotic Fluid Derived Stem Cells Stem cells can be isolated from human amniotic fluid, the protective fluid that surrounds a developing fetus. In the lab, these amniotic fluid derived stem cells have been coaxed to become neurons, muscle cells, adipocytes, bone, blood vessels, and liver cells. It is not entirely clear if these cells are truly different from hescs or ASCS, but if they are, they may produce a key breakthrough in stem cell technologies. 20 Cancer Stem Cells Cancer stem cells (CSCs), also called tumor initiating cells, have been identified and implicated in the development of cancers, tumor progression, tumor metastasis, and the recurrence of cancers. Similar to normal stem cells, CSCs can self renew and can differentiate to form tissues from which they were derived. Certain CSCs grow slowly l in clusters or niches ih within ihi a tissue. It is not clear what properties CSCs may have besides the ability to form a tumor. Researchers are also not sure if CSCs are derived from normal cells that have undergone mutations or if they are involved in cancer tumor resistance to chemotherapies, but these cells are a focus of intense research and potential therapeutic treatments for the treatment of cancers. 21 7

8 Tests for Pluripotency This is important for determining whether cells are able to form other cell types, and this property is indicative of a stem cell s potential to do so in vivo or in vitro. It turns out that embryonic cells are pluripotent p for a relatively brief period of time during development. When properly isolated and cultured, ESCs can be maintained in a pluripotent state in vitro indefinitely. Frequently, heterogeneic populations of ESCs are often produced in which cells have different capacities to differentiate in viva or in vitro. The concept of pluripotency is not difficult, but actually determining the potency of a stem cell line is not so easy. 22 One key test for pluripotency is to determine whether stem cells can form the three primary germ layers: ectoderm, mesoderm, and endoderm. During embryological development, the inner cell mass forms three specialized layers of tissue the primary germ layers, which then form the specific tissues of the body. The outer layer, ectoderm, gives rise to skin, brain, and nervous tissue. The middle layer, or mesoderm, gives rise to blood cells, the heart, bone, kidneys, muscles, and cartilage. The innermost layer, the endoderm, develops into lungs, the liver, and the digestive system. In vitro it has been demonstrated that ESCs can differentiate into cell types representing cells that would originate from the primary germ layers. In vitro tests are also often used to determine whether stem cells can aggregate into a cluster of cells called an embryoid body. 23 Stem cell biologists use in vivo tests to demonstrate pluripotency as a way to validate the viability of stem cells, especially ESCs. In these assays, ESCs are injected into mice to see if the cells respond to signals in viva to stimulate tissue development. lmmunodeficient mice those lacking an immune system are used so that the host mice do not reject the injected stem cells. When stem cells are placed dinto these mice, tumors form including teratomas. Within these tumors differentiated tissues typically form, and these tumors can be analyzed to determine which tissue types have formed. 24 8

9 25 Nuclear Reprogramming Research on stem cell biology is an extremely active field. One primary area of focus continues to be alternative approaches for producing pluripotent stem cells without destroying an embryo. One of the most promising new approaches for creating and isolating stem cells without an embryo involves nuclear reprogramming of somatic cells. It was previously thought h that once cells differentiated i d to become a specific, specialized cell type, for example, a skin cell, their differentiation fate was irreversible. But we now know this is not the case. The basic idea is to take a differentiated, adult cell and to alter its patterns of gene expression in order to reprogram the cell to an early stage in its differentiation pathway that is, to push the cells back to an undifferentiated, pluripotent state