Chapter 22 Part 1 Stem Cells PDF

BIOS 367 Cell Biology Fall 2024 Chapter 22 Part 1 Stem Cells in Tissue Homeostasis and Regeneration Tissue homeostasis compared to a river In a self-renewing adult tissue, a constant flow of new cells produced by cell division “upstream” and...

BIOS 367 Cell Biology Fall 2024 Chapter 22 Part 1 Stem Cells in Tissue Homeostasis and Regeneration Tissue homeostasis compared to a river In a self-renewing adult tissue, a constant flow of new cells produced by cell division “upstream” and a constant loss of differentiated cells “downstream” maintain the tissue in a dynamic equilibrium Example – epidermis (made of epithelial tissue) https://encyclopedia.lubopitko-bg.com/structureofskin.html Dead cells sloughing off here, at the skin surface By the time the cells reach this layer, they are dead Epidermal cells maturing here, as they get pushed up from underneath Some mitosis here Frequent mitosis in this layer, creating new skin cells while maintaining epidermal Underlying connective tissue stem cells The defining characteristics of a stem cell Each daughter cell produced when a stem cell divides can either remain a stem cell, in the process of self-renewal, or commit to differentiation, usually after a number of cell divisions. The self-renewal process maintains the pool of stem cells in the tissue. A hierarchy of stem cells, progenitor cells, and differentiated cells In addition to self-renewal, tissue- specific stem cells generally produce progenitor (transit-amplifying) cells that divide a limited number of times before they terminally differentiate Stem cells and progenitor cells can be unipotent or multipotent, depending on whether they produce only one type or multiple types of differentiated cells. Renewal of the gut epithelial lining The stem cells in this instance express a specific G-protein coupled receptor called Lgr5 https://www.youtube.com/watch?v=qq5k1sWqLO0 Renewal of the gut epithelial lining The four main differentiated cell types found in the epithelial lining of the small intestine. Absorptive (brush-border) cells outnumber the other cell types in the epithelium by about 10:1 or more. The microvilli on their apical surface provide a 30-fold increase in surface area, not only for the import of nutrients but also for the anchorage of enzymes that perform the final stages of extracellular digestion, breaking down small peptides and disaccharides into monomers that can be transported across the cell membrane Goblet cells secrete mucus into the gut lumen; this mucus covers the epithelium with a protective coat Paneth cells secrete (along with some growth factors) cryptdins—proteins of the defensin family that kill bacteria. Enteroendocrine cells, of more than 15 different subtypes, secrete serotonin and peptide hormones that act on neurons and other cell types in the gut wall and regulate the growth, proliferation, and digestive activities of cells of the gut and other tissues. How to identify stem cells? Stem cells in adult tissues are usually rare and difficult to identify in conventional tissue sections, unless a stem cell– specific marker is available Recombinant DNA technology provides a general and powerful way to identify stem cells and their progeny in any renewing tissues using a technique called cell lineage tracing. The method uses transgenic animals to create a visible genetic mark in just a few random cells. Some stem cells will be marked randomly and will lead to a persistent clonal lineage that contains stem cells as well as differentiated cells Dividing progenitor cells will also be marked, but will eventually disappear. In this system, Cre recombinase is inactive, unless activated by tamoxifen. When tamoxifen is added, the recombinase is activated, and in some cells, will remove the blocking sequence by recombination, leading to expression of GFP. If the recombination If the event occurs in a stem recombination cell, a clonal lineage event occurs in a will be marked, and cell that is not a the labeling will stem cell, the persist over time as label will the marked stem cell disappear over self-renews and time as the produces marked cell differentiated cells. differentiates and is eventually lost. Lgr5-expressing stem cells and their progeny in the small intestine In this experiment, the Lgr5 promoter was used to drive expression of CreERT2, and treatment with a low dose of tamoxifen resulted in occasional stem cells expressing the marker protein LacZ These progeny can be shown to include all types of differentiated cells, as well as persistent Lgr5-expressing cells at the crypt base. This proves that Lgr5-expressing cells are multipotent stem cells The repair of skeletal muscle fibers by satellite cells Skeletal muscle cells are very long, multinucleate, and not capable of division. When skeletal muscle cells are damaged or stimulated to grow, quiescent satellite cells proliferate, and can fuse with existing muscle cells. Satellite cells or some subset of them are thus the stem cells of adult skeletal muscle, normally held in reserve in a quiescent state but available when needed as a self-renewing source of terminally differentiated myoblasts Blood Cells Rescue of an irradiated mouse by a transfusion of bone marrow cells A single blood stem cell injected into the mouse is sufficient to reconstitute its entire hematopoetic system. Therefore, a blood stem cell is multipotent, able to give rise to all blood cell types, as well as more stem cells A simplified scheme of mouse and human hematopoiesis Some tissues do not require stem cells for their maintenance Some types of cells can divide even though fully differentiated, allowing for renewal and regeneration without the use of stem cells. Examples: -Pancreatic Beta cells, which produce insulin -Hepatocytes in the liver Other tissues lack stem cells, and are incapable of regeneration Examples: Auditory epithelium in the ear Retinal photoreceptive epithelium in the eye  How do stem cells maintain their identity, and how do their daughter cells choose between self-renewal and a commitment to differentiation? Stem cells undergo self-renewal only in a specialized microenvironment where they are exposed to the necessary signal molecules. These signal molecules can be provided by niche- supporting cells or by a specialized extracellular matrix that serves to concentrate them. The niche environment can be very small. In some cases, a stem cell directly adjacent to the niche is capable of self-renewal, while a cell just one cell-diameter away from the niche is not Genesis of a minigut from a single Lgr5-expressing cell cultured in a cell-free matrix At random, one or more of the cells in this vesicle differentiates as a Paneth cell (blue). The Paneth cells secrete Wnt proteins that stimulate stem-cell self-renewal and maintain Lgr5 expression (yellow) in their immediate neighbors Stem-cell niche of the C. elegans gonad. Fluorescence micrograph showing the nuclei (blue) in a portion of the C. elegans gonad. The nucleus of the distal tip cell is shown in red The same tissue is stained to show the cytoplasm and processes of the distal tip cell in red and the differentiated germ cells in green. The distal tip cell processes extend across multiple stem- cell diameters, maintaining stem-cell identity through Notch signaling, which is cell contact dependent. Those cells moved out of reach of the distal tip cell processes initiate meiosis and begin the differentiation program that will produce either eggs or sperm Stem-cell niche of the C. elegans gonad. The same tissue is stained to show the cytoplasm and processes of the distal tip cell in red and the differentiated germ cells in green. The distal tip cell processes extend across multiple stem-cell diameters, maintaining stem-cell identity through cell contact dependent Notch signaling. Those cells moved out of reach of the distal tip cell processes initiate meiosis and begin the differentiation program that will produce either eggs or sperm Asymmetric and symmetric stem-cell divisions In the asymmetric stem-cell division schematized here, a cell-fate determinant (red) that maintains stem-cell identity is localized to the cell cortex at one side of the cell. When the cell divides, only one daughter cell inherits the determinant and remains a stem cell, while the other daughter commits to differentiation In the symmetric stem-cell division shown, both daughter cells inherit such determinants and remain stem cells. How the plane of division in a dividing, germ-line stem cell in the Drosophila testis determines which daughter cell maintains contact with the niche How the plane of division in a dividing, germ-line stem cell in the Drosophila testis determines which daughter cell maintains contact with the niche. When a stem cell divides, its mitotic spindle is oriented so that only one daughter cell maintains contact with the niche and remains a stem cell; the other daughter loses contact with the niche cells and is therefore deprived of self-renewal signals and commits to differentiation The independent-choice mechanism for how the two daughters of a stem-cell division choose their fates The choice to differentiate or remain a stem cell might be determined by the local environment a daughter finds itself in In other cases, the choice to differentiate or remain a stem cell is made stochastically (randomly) With a choice made randomly by each daughter there is, for example, a 25% chance at the first division that both daughters will commit to differentiation, so that the clone will eventually disappear. Or, at this division or later, a preponderance of daughters might choose to remain stem cells, creating a clone that persists and increases in size. If this choice is not actually random, but influenced by a changing environment, this strategy can adjust to changing conditions