Somites. Drat! You have mislabeled tubes that contain embryos from three differe

Question

Somites. Drat! You have mislabeled tubes that contain embryos from three different species: chick, mouse, and goose. You can't tell them apart by just looking at them. However, you know that these organisms differ in the number of cervical vertebrae, with the goose having 17, the chick having 14, and the mouse has 7. Your boss gets mad at you and tells you perform in situ hybridization for the Hoxc6 mRNA to figure out which embryos are the goose embryos. Describe how the pattern of Hoxc6 expression in the somites can help you sort this mess out.

Explanation / Answer

Hoxc6 mRNA expression differs in all the three embryos- chick, mouse and goose as the expression is localized in diffferent somites. Differentially localized expression of Hoxc-6 in different somites (thoracic) helps in distinguishing the embryos of different organisms from one another due to their distinct Hoxc-6 expression pattern.

In case of a chick embryo, the insitu hybridization Hoxc6 expression labeling begins at somite level 19-20, aligned with the middle of the forelimb bud.

In the mouse embryo, Hoxc-6 expression begins at somite level 12-13, in axial alignment with the middle of the forelimb bud. Hoxc-6 specifically marks a key morphological boundary, the cervical-thoracic transition in the chick and the mouse even though this transition is transposed by seven vertebrae.

Finally, in the goose embryos, which has 17 cervical vertebrae, and thus the forelimb is transposed caudally by 3 segments relative to the chick.The anterior limit of expression falls at somite level 22-23, the first thoracic vertebra, and thus is also transposed by three segments relative to the chick, in concert with the position of the forelimb.

Therefore, studying the insitu hybridization to localize Hoxc-6 mRNA in all the three embryos helps in distinguishing them from one another as the expression border of Hoxc-6 is located at the somites which  map to the first thoracic vertebra (that carries the last spinal nerve to the brachical plexus) in the chick, the goose and the mouse.

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