![]() ![]() During the first few days of its development (exact number of days varies depending on the species), the early mammalian embryo travels down the fallopian tubes into the uterus. However, even in placental mammals, not all development occurs in the uterus. Mammals, particularly placental mammals, are an exception. In most animals, development occurs outside the body and the embryo is not physically connected to the mother. Preimplantation Development and Embryonic Stem (ES) Cells The only known indisputably totipotent cell is the zygote. Finally, totipotent stem cells can make all three embryonic germ layers and the extra-embryonic tissue. In contrast, pluripotent stem cells can make multiple lineages from all three embryonic germ layers but not from extra-embryonic tissue. To be classified as “multipotent,” stem cells must make at least two different lineages, usually from the same embryonic germ layer. Extra-embryonic tissues are “outside the embryo,” referring primarily to the cells of the amniotic sac and the placenta: organs that are essential for embryonic development but are discarded after birth. These three lineages are referred to as the embryonic germ layers.įor mammals, even before gastrulation occurs, the tissues of the embryo are classified into two other broad categories: extra-embryonic and embryonic. Gastrulation subdivides the cells in the group into three broad layers: endoderm, which gives rise to the cells of many internal organs, mesoderm, which gives rise to the muscles and the blood, and ectoderm, which gives rise to the nervous system and epithelial layers. In basic embryology, blood originates from the mesoderm, the middle layer of an embryo, which forms as the embryo undergoes a process called “gastrulation” shortly after fertilization. The hematopoetic stem cells mentioned earlier are a multipotent cell type: they are able to give rise to many kinds of cells, but only of the blood lineage. Broadly speaking, they are characterized as totipotent, pluripotent and multipotent. Not all stem cells have the same “potency,” the capacity to give rise to similar cell types. Differentiation refers to its ability to make other cell types, performing other biological functions.įor instance, hematopoietic stem cells are found in the bone marrow, where they generate progenitor cells that give rise to the cells of the immune system and red blood cells. Self-renewal refers to a cell’s capacity to divide and make other cells with the same properties. Despite these challenges, the promise of producing human neuroendocrine cell types in vitro gives opportunities for unique insights and is therefore worthwhile.Ī stem cell is defined as any cell type with two fundamental capacities (1) self-renewal and (2) differentiation. An additional challenge involves providing these cells with the appropriate environment to induce their normal behavior outside the body. Progress in this direction continues to be made, on multiple fronts, and it involves using small molecules and proteins to mimic developmentally important signals, as well as building on advances in “reprogramming” to directly convert one cell type into another by forced expression of sets of transcription factors. While we are able to isolate pluripotent stem cells from patients in a minimally invasive manner, we do not yet fully understand how to direct these cells to many of the medically important neuroendocrine fates. This work has enabled us to produce neurons that are genetically matched to individual patients. In contrast, pluripotent stem cells are easy to obtain, due to the paradigm-shifting work on direct reprogramming of human skin fibroblasts into induced pluripotent stem cells. As adult neural stem cells reside in the brain, their isolation would require considerably invasive and dangerous procedures. The challenge in utilizing adult stem cells for disease research is obtaining cells that are genetically matched to people with disease phenotypes, and being able to differentiate them into the appropriate cell types of interest. Multipotent adult stem cells are found throughout the body, and they include neural stem cells. ![]() Stem cells are usually categorized as multipotent (able to give rise to multiple cells within a lineage), pluripotent (able to give rise to all cell types in an adult) and totipotent (able to give rise to all embryonic and adult lineages). Stem cell technology can allow us to produce human neuronal cell types outside the body, but what exactly are stem cells, and what challenges are associated with their use? Stem cells are a kind of cell that has the capacity to self-renew to produce additional stem cells by mitosis, and also to differentiate into other-more mature-cell types. ![]()
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