Lecture 1 (Introduction To Stem Cells) Flashcards
Define ‘germ cell’.
A germ cell describes any cell, in sexually reproducing organs, that can potentially contribute to the formation of offspring including gametes and their immature precursors.
Give a brief summary of how multicellular organisms are formed from stem cells.
Stem cells differentiate into specialised cells, which aggregate and condense into tissues. These tissues are then organised into the organs which comprise the organism. This process carries a high physiological cost, however it is associated with a significant evolutionary advantage.
Define ‘somatic cell’.
Somatic cell is the term used to describe any plant or animal cell that is not germ-line and therefore cannot produce a new organism under standard conditions.
Define ‘cell lineage’.
Cell lineage describes the development of tissues and organisms through patterns of mitotic cell division. A cell line culminates in the formation of a terminally differentiated cell.
Define ‘differentiation’.
Differentiation is the process by which a cell becomes specialised in order to perform a specific function as one of the 250+ cell types within the human body. Once a cell has undergone differentiation, it will have a restricted development potential.
Define ‘stem cell’.
A stem cell is a self-renewing cell that can divide symmetrically to give rise to two daughter cells whose development potential is identical to that of the parental stem cell or asymmetrically to generate daughter cells with different developmental potential.
Summarise the stages of embryogenesis.
The fertilised ovum undergoes cell division to form a two-cell embryo, followed by a four-cell embryo. Subsequent divisions occur until 5-6 days following fertilisation, when then multi-cell embryo hollows to form a blastocyst. The blastocyst is the stage of the mammalian embryo that is composed of approximately 64 cells that have separated into two cell types - the trophectoderm, which will form extra-embryonic tissues, and the inner cell mass, which gives rise to the embryo proper. The blastocyst implants in the uterine wall and the tissues of the embryo begin to form after days 11-14, with cells becoming multipotent.
Define ‘totipotency’.
A cell exhibiting totipotency is one of the most versatile cell types, as it has the ability to give rise to any and all cells within the human body. The earliest stages of the embryo are composed of totipotent cells, until the formation of the blastocyst instigates specialisation into pluripotent stem cells.
Define ‘pluripotent’.
A pluripotent cell is similar to a totipotent cell in that they can give rise to all tissue types, however, a pluripotent cell cannot form an entire organism without the trophectoderm outer layer.
Define ‘multipotency’.
Multipotent cells are more differentiated then totipotent and pluripotent cells, but can still give rise to any cell within a specific tissue type. For example, haematopoietic adult stem cells, derived from the mesoderm, can differentiate into any one of the blood cells.
Define ‘unipotent’.
A unipotent cell has restricted potential and is only able to produce one specific type of cell within a tissue e.g. adult intestinal epithelial stem cells.
Describe the typical hierarchy of differentiation, from the tissue stem cells to specialised cells.
Stem cells give rise to committed progenitors; these are not fully differentiated cells but have different properties from stem cells - they are an intermediate cell type. Committed progenitors, also called transit amplifying cells, are multipotent, divide rapidly, and do not self-renew in the same fashion as stem cells. The committed progenitors produce fully differentiated and functional cells following a series of divisions.
Describe the formation of blood cells from haematopoietic stem cells in the bone marrow.
Multipotent long-term repopulating haematopoietic stem cells may divide symmetrically to self-renew or divide asymmetrically to form one daughter cell that is multipotent, like the parent cell, and another daughter with limited self-renewal capacity. Ultimately, this daughter generates myeloid and lymphoid stem cells; although these multipotent cells are capable of limited self-renewal, they are committed to one of the two haematopoietic lineages. Depending on the types and amounts of cytokines present, the myeloid and lymphoid stem cells undergo rapid rounds of cell division and generate different types of progenitor cells. These progenitors are termed multipotent or unipotent in that they can give rise to several or only a single type of differentiated blood cell, respectively; they respond to one or a few specific cytokines. These progenitors are detected by their ability to form colonies containing the differentiated cell types, measured as colony-forming cells (CFCs). The colonies are detected by plating the progenitor cells in a viscous medium, formed by a polymer such as methylcellulose, such that all of the daughter differentiated cells remain localised and thus form a colony.
Describe a progenitor cell.
Progenitor cells, also called transiently amplified cells, intermediate cells or precursor cells, are produced by the asymmetrical division of stem cells. Progenitor cells only have a limited capacity for population and therefore cannot maintain a population of cells indefinitely, often dying within a few weeks. Progenitor cells often divide to produce another progenitor cell and a terminally-differentiated cell.
Describe the structure of the intestinal crypt.
The wall of the intestine contains villi and crypts. Villi are structures which protrude from the intestinal wall in order to maximise the surface area available for nutrient adoption, however crypts are embedded into the wall itself. At the base of the crypt lie Paneth cells and stem cells, which divide asymmetrically and symmetrically to give rise to intestinal epithelial cells whilst also maintaining their own population. The newly-produced intestinal epithelial cells migrate up the sides of the crypt, from the proliferative region through to the crypt-villus junction and the differentiated region.
