"The evolution of mammalian keratinized structures"
Title of Source:
Zoological Society of London. Symposia
Date of Publication:
Bibliog.: p. 79-81
Summary. 1. Hair follicles appear to be new structures which have evolved in the epidermis between reptilian-type scales. 2. A granular layer was first formed in the pilo-sebaceous canals of the hair follicles. 3. Evolutionary changes resulted in an extension of granular layer formation into the epidermis adjacent to the follicles. This was associated with a change in the horny layer produced from the rigid reptilian scale to a flexible type in which the horn cells instead of being uniformly keratinized are mainly karatinized at the cell periphery. 4. This flexible type of horny layer was necessary because of the increased muscular movements and greater agility of mammals as compared with reptiles. 5. Primary follicles producing protective overhairs were first formed in the epidermis adjacent to the scales as seen in rodent tails. Later secondary follicles producing wool evolved as an adaption to conserve heat loss, and this could not have been before physiological heat regulation had been evolved. Increase in numbers of secondary follicles would increase the extension of the granular layer and tend towards loss of reptilian type scales. 6. Parakeratosis in mammals is a reversion to a more primitive form of keratinization.
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