Modified Outside In Hypothesis Essay

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The modified outside-in hypothesis contains parts of the classical outside-in and the inside-out hypothesis. For clarity, I will refer to the original outside-in hypothesis already discussed in the paper as the classical outside-in hypothesis and I will refer to the hypothesis I will now be discussing as the modified outside-in hypothesis. The conodonts have been removed from this discussion as there is no consensus about the structural homology of their denticles with any of the vertebrate odontodes (2). In agreement with the classical outside-in hypothesis, the modified outside-in hypothesis states that teeth are indeed derived from odontodes which are ectodermal in origin (2). The pharyngeal denticles seen when discussing the inside-out …show more content…
Huysseune and colleagues suggest that teeth may have evolved before the origin of jaws due to the invasion of competent, odontode-forming ectoderm into the oropharyngeal cavity through the gill slits. Their hypothesis is based upon the assumption that endoderm alone, cannot form teeth (2). Their hypothesis is also based on the observation that pharyngeal teeth are present only in species that are known to possess gill slits. Lastly, their hypothesis is based on the assumption that the dental lamina is not required for tooth development (2). This modified hypothesis suggested by these researchers is not the end all be all of the evolutionary origins of vertebrate dentition. Scientists have not arrived at a consensus yet as to which hypothesis is the most plausible. It is important to note the limitations of the modified outside-in hypothesis. The modified outside-in hypothesis can explain the complete loss of teeth from the post-mandibular branchial arches, but it cannot explain how in certain arches, there is a spatially restricted loss of teeth (2). This occurs in various teleosts as well as frogs. This hypothesis also cannot explain the ontogenetic loss of teeth in specific taxa such as certain marine
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