Read on to know ..........WHY?
As we know that all the information about an individual living organism is the information contained in its genes. Gene expression is a process of converting this genetic information into proteins that make body's structures and carry out its vital processes.
Although teeth usually do not become visible until after birth, their formation starts early in development. Teeth develop from the epithelium and mesenchyme, two key tissue layers within the mammalian embryo. The first sign of tooth development in mammals is the thickening of the epithelium along the jaw line to form a band of cells called the dental lamina. Researchers confirmed that it was indeed the mesenchyme that carried tooth initiation signals later in development, but how those signals were restricted to the area beneath the tooth row was unknown. Past studies had shown bone morphogenic protein 4 (BMP4) to be an important factor for the initiation of teeth, and that a protein called Msx1 amplifies the BMP4 tooth-generating signal.
A recent study published in Journal Science provides the first solid proof that the precise space where mammals can develop teeth (called the "tooth morphogenetic field") is shaped and restricted by the effect of Oddskipped related-2 (Osr2) gene on the expression of the Bmp4 gene within the mesenchymal cell layer. Osr2 gene is present in high amounts towards the tongue side of the jaw and decreases towards cheek. Since the presence of Osr2 restricts the teeth forming effect of BMP4 gene, so teeth will be formed at a place with low concentration of Osr2 (high concentration of BMP4). So Osr2 restricts Bmp4 expression to the tooth mesenchyme under the dental lamina only, due to which teeth grow only in the region of dental lamina.
These results suggest that diversity in the number of tooth rows across species may be due to evolutionary changes in the control of the BMP4/Msx1 pathway. In mammals, Osr2 suppresses this pathway to restrict teeth within a single row.
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