Abstract
This paper is part of the emerging field of Evolutionary Developmental Pathology, dedicated to study the links between normal and abnormal development, evolution and human pathologies. We analyzed the head musculoskeletal system of several ‘natural mutant’ newborn lambs displaying various degrees of abnormality, from mild defects to cebocephaly and to cyclopia, and compared them with humans. Interestingly, muscle defects are less marked than osteological ones, and contrarily to the latter they tend to display left-right assymetries. In individuals with cebocephalic and even cyclopic skulls almost all head muscles are normal. The very few exceptions are some extraocular muscles and facial muscles that normally attach to osteological structures that are missing in the abnormal heads: such muscles are instead attached to the ‘nearest topological neighbor’ of the missing osteological structure, a pattern also found in cyclopic humans. These observations support Alberch’s ill-named “logic of monsters” - as a byproduct of strong developmental/topological constraints anatomical patterns tend to repeat themselves, even severe malformations displayed by distantly related taxa. They also support the idea that mammalian facial muscles reverted to an ancestral ‘nearest-neighbor’ muscle-bone type of attachment seen in non-vertebrate animals and in vertebrate limbs, but not in other vertebrate head muscles.
Highlights
The links between normal and abnormal development, evolution, and pathologies begun to be intensively studied centuries ago by authors such as St
We provide just a short summary of the results obtained from our dissections and morphometric analyses, as detailed descriptions of each head muscle dissected by us in each specimen included in the present work are given in SI1 Table 1 and the results of our morphometric analyses are given in SI2
The typical configuration of the head muscles of the osteologically less defective newborn lambs dissected by us is basically similar to the normal newborn myological lamb phenotype (Fig. 2)
Summary
The links between normal and abnormal development, evolution, and pathologies begun to be intensively studied centuries ago by authors such as St. We dissected 27 sides (left and right: see SI1 Table 1) of heads of abnormal newborn lambs that were collected during various years by Howard Evans and colleagues to illustrate various degrees of skull (osteological) abnormality, from mild defects (stage 2 sensu the present work, stage 1 being a normal phenotype) to cebocephalic (stage 3 sensu the present work) and cyclopic (stage 4 sensu the present work) configurations (see[5] and Fig. 1). It has been demonstrated that cyclopamine was the primary alkaloid from this plant exerting such teratological effects on developing embryos by selectively blocking Sonic hedgehog signal transduction; since cyclopamine was used as a probe to understand the biological development of a variety of mammalian, including human, organs, and as a potential therapeutic agent for the treatment of tumors arising from the disruption of the Hedgehog pathway[12]
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