Every cell contains DNA within the nucleus , containing the blueprint to build many different proteins in the cell. Different signals can cause embryonic cells to select specific parts of the DNA which can then be used to synthesize proteins, eventually building different cell types. Differentiation of cells in the embryo is brought about by both internal cellular factors as well as extracellular factors that act on the cell from the outside.
Much remains to be understood about the exact molecular interactions that govern cellular differentiation. It is understood, however, diversifying the ratio of and types of internal and external influences on certain cells, allows many divergent cell types to arise. There are two main types of cellular development that pertain to embryos: mosaic development or regulative development.
In mosaic development which is not characteristic of mammals, but of organisms such as annelids differentiation occurs in steps that are set in order and progression, without input occurring between neighboring cells. On the other hand, regulative development involves the interaction of adjacent cells, within what is known as embryonic fields.
This change affects the next generation of cells derived from that cell. In subsequent generations, it is the combination of different transcription factors that can ultimately determine cell type.
Although most of the tissues in adult organisms maintain a constant size, the cells that make up these tissues are constantly turning over. Therefore, in order for a particular tissue to stay the same size, its rates of cell death and cell division must remain in balance.
A variety of factors can trigger cell death in a tissue. For example, the process of apoptosis, or programmed cell death, selectively removes damaged cells — including those with DNA damage or defective mitochondria.
During apoptosis, cellular proteases and nucleases are activated, and cells self-destruct. Cells also monitor the survival factors and negative signals they receive from other cells before initiating programmed cell death. Once apoptosis begins, it proceeds quickly, leaving behind small fragments with recognizable bits of the nuclear material. Specialized cells then rapidly ingest and degrade these fragments, making evidence of apoptosis difficult to detect.
Figure 2 : Different cell types in the mammalian gut The gut contains a mixture of differentiated cells and stem cells. The a intestine, b esophagus, and c stomach are shown. Through asymmetric division, quiescent stem cells d probably give rise to more rapidly dividing active stem cells, which then produce progenitor cells while losing their multipotency and ability to proliferate.
All these progeny cells have defined positions in the different organs. To maintain its function and continue to produce new stem cells, a stem cell can also divide into and produce more stem cells at the same position symmetric division. Stem cells in gastroenterology and hepatology. All rights reserved. Figure Detail Tissue function depends on more than cell type and proper rates of death and division: It is also a function of cellular arrangement.
Both cell junctions and cytoskeletal networks help stabilize tissue architecture. For instance, the cells that make up human epithelial tissue attach to one another through several types of adhesive junctions. Characteristic transmembrane proteins provide the basis for each of the different types of junctions. At these junctions, transmembrane proteins on one cell interact with similar transmembrane proteins on adjacent cells.
Special adaptor proteins then connect the resulting assembly to the cytoskeleton of each cell. The many connections formed between junctions and cytoskeletal proteins effectively produces a network that extends over many cells, providing mechanical strength to the epithelium.
The gut endothelium — actually an epithelium that lines the inner surface of the digestive tract — is an excellent example of these structures at work. Here, tight junctions between cells form a seal that prevents even small molecules and ions from moving across the endothelium. As a result, the endothelial cells themselves are responsible for determining which molecules pass from the gut lumen into the surrounding tissues.
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Scientists are exploring how gene expression patterns and their timing regulate cell differentiation. Aa Aa Aa. Figure 1: Dolly the sheep. All rights reserved. Cell-Extrinsic Regulation of Gene Expression. Figure 2: Microarray data collected at different times during metamorphosis reveals the effects of the ecdysone pulse on many downstream genetic pathways.
A Changes in ecdysone levels affect the glycolytic pathway. Levels of a number of enzymes involved in this pathway are decreased as a result of the ecdysone pulse; these enzymes are listed in red next to the reactions they catalyze. B This array shows expression changes in various structural and regulatory genes involved in muscle formation myogenesis in response to the ecdysone pulse.
C This array shows how the ecdysone pulse alters expression of multiple genes involved in central nervous system restructuring, apoptosis, and cellular differentiation during metamorphosis. In both of the microarrays, red means that the gene was downregulated, while green means that the gene was upregulated. Expression levels were measured at various points before and after pupal formation PF.
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