A congenital anomaly characterized by the presence of a single or partially divided eye has been described in many vertebrate species, including humans, and is sometimes associated with the presence of a proboscis. A new study analyses the embryonic origin of this malformation in a chicken embryo.
Chick embryo analysed in the study. / Paul Palmquist-Gomes et al.
Polyphemus is surely the most famous Cyclops in Greek mythology. In Homer's Odyssey, this one-eyed giant, son of Poseidon and the nymph Thoosa, locked himself in his cave and devoured some of the men who accompanied Odysseus (or Ulysses), the Trojan War hero. The latter managed to flee cunningly, but not before injuring the bad-tempered Cyclops in the eye.
Outside literature, cyclopia, which has been known for thousands of years, is a less frequent but real malformation.
This congenital anomaly, which has been described in many vertebrate species, including humans, is associated with other serious facial malformations, such as the absence of a nose or, on rare occasions, the presence of a proboscis located above the ocular structures, and which until now had been believed to derive from the remains of the embryonic nose.
“Despite the low incidence of cyclopia in the population, human foetuses with this malformation are not viable and die during development or shortly after birth,” Paul Palmquist-Gomes, researcher at the University of Malaga and co-author with José María Pérez-Pomares and Juan Antonio Guadix of a study published in the Journal of Experimental Zoology Part B: Molecular and Developmental Evolution, has explained to Sinc.
Different forms of this malformation have been described so far. These include the appearance of a single eye in a central orbit or an incomplete fusion of the two eyes into a single central orbit. On other occasions, a structure with the shape of a proboscis may appear on the single eye.
Finally, this proboscis can emerge between the two eyes as if it were an incomplete formation of the nose and, in some extreme cases, two proboscises may appear where the eyes should be.
Different types of cyclopia. / Paul Palmquist-Gomes
In order to understand the origin of the appearance of this cyclopia-associated proboscis, the new work has analysed a spontaneous congenital case of cyclopia in a chicken embryo. The results support those of many previous analyses that indicate that this anomaly results from an alteration of the normal programme of midline embryonic development of organisms.
The most important conclusion of the research in this case is that the cellular identity of some of the facial tissues during the embryonic development of the malformation is complex.
“Our analysis suggests that, on certain occasions, the proboscis base of this type of Cyclops is more related to the ocular tissues than to the nasal tissues,” the researchers from the University of Malaga reveal to Sinc.
Moreover, the study also underlines that, during embryonic development, the Cyclops eye is associated with olfactory tissues rather than with the progenitors of the retina.
An in-depth analysis of the different structures found in this Cyclops chicken embryo can reveal why, in some individual cases of Cyclopes, the central structure that is usually considered an eye lacks the tissues that characterise this organ, as is the case with the retina.
“These discoveries shed light on the origin of this complex facial malformation,” they conclude.
Paul Palmquist-Gomes et al. “Cellular identities in an unusual presentation of cyclopia in a chick embryo” Journal of Experimental Zoology Part B: Molecular and Developmental Evolution 2019, 332:179-186.