Necrosis and its formation in a spheroid
The term necrosis refers to cell or tissue death and is typically caused by factors such as injury, infection, or inadequate blood supply. Necrosis can also form within a spheroid, which is a 3D culture of cells.
Here are a few reasons why necrosis can form in a spheroid (1,2,3):
1. Nutrient and oxygen deprivation: As a spheroid grows, the cells located in the core of the structure may become deprived of essential nutrients and oxygen. The limited diffusion capacity within the spheroid may restrict the availability of these vital resources, leading to cellular stress, dysfunction, and eventually necrosis.
2. Accumulation of metabolic waste: spheroids consist of densely packed clusters of cells and the accumulation of metabolic waste products such as lactic acid and carbon dioxide can occur within these structures. The inefficient removal of these waste products due to limited diffusion may result in acidic and toxic microenvironment, causing cellular damage and necrosis.
3. Physical compression: As the spheroid grows, the outer layer of cells may exert physical pressure on the inner cells. This compression can restrict blood flow, impair nutrient and oxygen delivery, and induce necrosis in the compressed region.
4. Apoptosis and necrosis cascade: The presence of cells undergoing apoptosis within the spheroid can also contribute to necrosis. Apoptotic cells release various factors and signals that can trigger an inflammatory response and subsequent necrosis in the surrounding cells.
The occurrence of necrosis in a spheroid can vary depending on the cell type, culture conditions, and experimental setup. Seeding density and duration of cultivation affect the size and thus the diameter of spheroids which in turn determine the viability of the cells in the core. As spheroids grow larger in size, the penetration of nutrients and oxygen into the core and removal of metabolites becomes restricted, resulting in oxygen gradients and hypoxic zones within the inner layers.
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