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group_5_presentation_1_-_regeneration_stem_cells [2019/01/30 19:50] kingr6 [5. Regeneration in Humans] |
group_5_presentation_1_-_regeneration_stem_cells [2019/02/01 17:37] (current) smithl12 |
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| + | ========Regeneration and Stem Cells======== | ||
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| + | ======1. Introduction to Regeneration====== | ||
| + | Regeneration is a fascinating concept among us humans; the thought of amputated limbs growing back into fully functioning body parts has long been popularized in cartoons and fictional characters. For instance, Piccolo, a character from Dragon Ball Z is popular for its remarkable regenerative capability. | ||
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| + | {{ :dragonball.gif?nolink&300 |}} | ||
| + | Figure 1. Cartoon character, Piccolo, displaying regeneration. | ||
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| + | Regeneration is the process by which an organism is able to replace or restore lost tissue, organs, limbs, or other body structures (King & Newmark, 2012). Interestingly, the phenomena of repair and regeneration are universal, but the capacity for regeneration is a characteristic that varies remarkably among organisms. Certain invertebrates, such as hydra, possess the ability to regenerate into two genetically identical individuals when cut in half (Bosch, 2007). Amphibians such as salamanders can readily replace whole body parts (Morrison et al., 2006). Mammals however, are fairly limited in their ability to regenerate. Although the function of a damaged organ may be recovered, most mammals are unable to restore missing body structures (King & Newmark, 2012). The ability to replace lost or damaged organs with new body structures that are genetically and functionally identical to the original is known as complete regeneration. Incomplete regeneration describes the process by which organ or tissue function may be recovered, but missing or damaged structures cannot be restored (King & Newmark, 2012). | ||
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| + | ======2. Defining Key Terms====== | ||
| + | The healing and regeneration of a complex body structure involves diverse cellular processes such as wound healing, programmed cell death, stem cell proliferation, and dedifferentiation (King & Newmark, 2012). | ||
| + | In order to gain a better understanding of the cellular processes involved in regeneration, it is important to begin by defining some key terms. Stem cells can be defined as undifferentiated progenitor cells that possess the ability to develop into specialized cells, while also maintaining self-renewal (Singh et al., Chandra, 2016). Cellular differentiation describes the process by which a cell changes from one cell type to another, typically to a more specialized type (Slack, 2007). Dedifferentiation is another important biological phenomenon whereby specialized cells regress back to a simpler state reminiscent of stem cells (Cai et al., 2007). | ||
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| + | Stem cells can be classified as totipotent, pluripotent, multipotent or unipotent, depending on their potency (Singh et al., 2016). Totipotent cells have the potential to develop into all the cell types in an organism, plus extra-embryonic cell types. Pluripotent cells can give rise to all the cell types that make up the body, except extra-embryonic cell lines. Multipotent stem cells are lineage specific with limited differentiation potential, and tend to develop only within specific tissue or cell lines. The developmental potential of unipotent cells is further reduced as they are able to give rise to only one cell type. It is important to note that both the self-renewal capacity and the differentiation potential of stem cells is reduced down the stem cell hierarchy from totipotent to unipotent cell states (Singh et al., 2016) | ||
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| + | The blastema is a general term used to describe a regeneration bud containing a mass of undifferentiated progenitor cells capable of growth and renewal. Several organisms such as planarian flatworms, zebrafish and salamanders utilize a regenerative blastema for healing and regrowth during adult stages (Godwin, 2014). Injury or amputation among such organisms can trigger the formation of a blastema from adult tissue; the blastema then functions to reform the diverse tissues of the missing body structure (Kragl et al., 2009). The identity and potency of the type of cells that make up the blastema is not completely known. It is believed that cells near the site of injury lose their specialized properties and revert back to a primordial state via de-differentiation; these cells then multiply rapidly and redifferentiation to form the new tissue needed to rebuild the missing body structure (Bosch, 2007). Recent research also suggests that in some organisms, cells within the blastema may contribute to regrowth via memory retention of the tissue of origin (Kragl et al., 2009). | ||
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| + | ======3. Types of Regeneration====== | ||
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| + | Regeneration can occur in three different ways: epimorphosis, morphallaxis and compensatory regeneration. | ||
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| + | **Epimorphosis**: This type of regeneration is commonly found within Salamanders, usually results in a new limb or appendage. In this type of regeneration, the remaining part of the limb attached to the body is able to form a ‘blastema’ at the site of the cut. The cells on the wound are able to help construct the missing structures in an ‘add-on’ type of manner. | ||
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| + | **Morphallaxis**: This form of regeneration is commonly found in Hydras, there is no ‘blastema’ formed and the remaining cells are used to regenerate the rest of the body. | ||
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| + | **Compensatory**: Regeneration of this form occurs with the human liver, cells divide but they maintain their differentiated functions. Meaning that the cells produce other cells of their own kind and there is no undifferentiated tissue needed. | ||
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| + | ======4. Regeneration in Animals====== | ||
| + | ==A. Salamander Description== | ||
| + | Salamanders are of exceptional biological interest because of their unmatched capacity for healing and renewal. These amphibians are typically characterized by a lizard-like appearance with a basal tetrapod body, four short limbs, and a long tail (Morrison et al., 2006). Salamanders can regenerate complex body structures such as entire limbs, tails and jaws through an epimorphosis regeneration process; the result is complete and functional restoration of tissue architecture (Morrison et al., 2006). | ||
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| + | ==B. Mechanism - Epimorphosis== | ||
| + | Salamanders have the ability to regenerate most of their body parts including the spinal cord and tail. However, the most fascinating is its ability for limb regeneration. It follows the epimorphosis method of regeneration which depends on blastema formation. This is also known as monodirectional regeneration, where the new limb grows distally from the wound. | ||
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| + | Regeneration can be broken down into two stages; blastema formation and blastema growth & patterning. Each have their own sub-stages consisting of; early bud, medium bud, late bud, palette and early differentiation (Figure 1). | ||
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| + | {{:buds.png?nolink&200|}} | ||
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| + | Figure 1. The stages of regeneration of a Newt following amputation (Iten & Bryant, 1973) | ||
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| + | ==I. Apical Epidermis Cap & Blastema Formation== | ||
| + | The process starts when one of its limbs are removed. Initially, the amphibians blood vessels tighten to minimize the amount of blood loss, and epidermal cells proliferate and cover the amputated area, similar to the idea of how a cut on a person may form a scab. An essential component called the apical epithelial cap forms from the initial epidermal cells, this occurs from the thickening of the epidermis (Brito, 2018). The apical epithelial cap functions as a signaling center which enhances blastema proliferation (Stocum, 2017). This is an important stage of the regeneration process because if this structure were to be damaged or missing then regeneration would seize to occur (Goss, 1969). | ||
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| + | ==II. Limb Blastema Growth & Patterning== | ||
| + | There are two mechanisms that explain how the limb blastema is formed since the origin is still up for debate. One source suggests that it accumulates through the activation of nearby stem cells (Simon & Tanaka, 2013). On the other hand, some say that the blastema is formed through the dedifferentiation of mature cells (Stocum, 2017). There are many different studies that explore which specific tissues contribute to the formation of the blastema, however, we will only focus on one of those in this section. A study reports that dermal fibroblast-derived cells contribute towards roughly 50% of the blastema makeup (Muneoka, Fox & Bryant, 1986). | ||
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| + | Fibroblasts, which are a type of connective tissue, relocate to the middle of the injury and multiply until they become a blastema (Muneoka, Han & Garinder, 2008). This is caused by a protein called nAG (Kumar et al., 2007). Hox genes contained in the fibroblast cells determine which specific limb needs to be replaced based on its location in the body (Muneoka, Han & Garinder, 2008). The blastema proliferates creating numerous undifferentiated cells, which then become new blood vessels, bone, and muscle tissues (Kumar et al., 2007). The undifferentiated cells differentiate due to the Hox gene in the fibroblasts and become a new limb (Conger, 2008). | ||
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| + | Macrophages are also a key component in limb regeneration. Macrophages are critical to the immune system and generate pro and anti-inflammatory signals. Studies have shown that reduced amounts of these cells in salamanders result in closure of the wound, but not limb regeneration. In a study, adding the macrophages back to the salamander resulted in regular limb regeneration (Godwin, Pinto & Rosenthal, 2013). | ||
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| + | ==C. Hydra Description== | ||
| + | Hydra, a bilayered freshwater polyp was subject to some of the first scientific investigations concerning regeneration in animals. Hydra belong to the Phylum Cnidaria, a phylum that includes corals, sea anemones, and jellyfish (Solomon, Berg, & Martin, 2002). Hydra has a tubular, radially symmetric body and exhibits distinct polarity along the oral/aboral axis (Zamaraev, 1956). The animal body is made up of two cell layers: the ectoderm and the endoderm - separated by an extracellular matrix; neurons and interstitial cells can be found between these two layers (King & Newmark, 2012). The cells of the Hydra belong to either the ectodermal, endodermal or interstitial cell lineage (Bosch, 2007). Scientists are especially interested in Hydra because of their remarkable regenerative abilities; tiny fragments of the animal tissue possess the ability to regenerate into complete organisms. Even dissociated Hydra tissue cells can reaggregate, reestablish polarity, and grow into whole new organisms (King & Newmark, 2012). | ||
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| + | ==D. Mechanism - Morphallaxis== | ||
| + | Regeneration in Hydra is accomplished through epithelial cells (Bosch, 2007). Additionally, a mesoglea which separates the two cell layers is required for the progression of regeneration. When a Hydra is cut in half, the head portion will regenerate a basal disc and vice versa. A Hydra can be spliced into many different pieces and both the head and basal disc components will regenerate accordingly. This is accomplished through polarity coordination via morphogenetic gradients. | ||
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| + | In essence, there are signaling peptides that play an important role in the regeneration of the Hydra. Experiments have shown that there are two different types of signaling molecules; activators and inhibitors. These include head and foot activator peptides which enhance apical and basal regeneration respectively. There are different molecular pathways that can explain the biochemical mechanisms that describe Hydra regeneration, however, we will focus only on the MAPK pathway. | ||
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| + | When the Hydras head is removed, the MAPK pathway becomes activated followed by the Wnt-beta-catenin pathway. This is all organized by the gastric tissue located in the regenerating tip of the Hydra. The MAPK pathway causes cell death in interstitial cells. This then causes Wnt genes to become activated in the progenitors of interstitial cells and carries over to the endodermal cells as well (Galliot, 2013). Resulting in organizing the cells involved with the head regeneration. Head regeneration in Hydra has shown to be flexible as decreases in cellular proliferation during Wnt activation did not have a significant effect on regeneration. Interstitial cycling and proliferation is important for this process. The cells located near the regenerating area increase proliferation and the cells farther away move closer. However, due to the flexibility of regeneration, the epithelial cells have been seen to regenerate the head without the interstitial cells, but this process takes longer (Galliot, 2013). It has been found that especially the HyWnt3 gene is important for head regeneration. Experiments that have silenced this gene have resulted in diminished head regeneration in the Hydra (Galliot, 2010). | ||
| + | Investigators have taken an interest in these animal models as their remarkable regeneration capacities have inspired new ideas about how their regeneration can be applied to humans. | ||
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| + | ==E. Human Application== | ||
| + | {{ :salamander.png?nolink |}} (Heinrich, 2016) | ||
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| + | The regeneration in model organisms is applicable to humans as scientists can copy or use these ideas as a framework for future research. For example, using this idea of dedifferentiating cells, scientists are able to create induced multipotent stem cells out of fat cells. Through aspirating fat cells with two different solutions, the fat cells can convert back to a less specific state which can be used for wound healing. However, this cannot be used to regrow limbs, but this is a great step in the right direction for regenerative medicine. (Heinrich, 2016) | ||
| ======5. Regeneration in Humans====== | ======5. Regeneration in Humans====== | ||
