Reverse Dissection and DiceCT Reveal Otherwise Hidden Data in the Evolution of the Primate Face

Summary

Facial expressions are a mode of visual communication produced by mimetic muscles. Here, we present protocols for the novel techniques of reverse dissection and DiceCT to fully visualize and assess mimetic muscles. These combined techniques can examine both morphological and physiological aspects of mimetic musculature to determine functional aspects.

Abstract

Facial expressions, or facial displays, of social or emotional intent are produced by many mammalian taxa as a means of visually communicating with conspecifics at a close range. These displays are achieved by contraction of the mimetic muscles, which are skeletal muscle attached to the dermis of the face. Reverse dissection, removing the full facial mask from the skull and approaching mimetic muscles in reverse, is an effective but destructive way of revealing the morphology of mimetic muscles but it is destructive. DiceCT is a novel mechanism for visualizing skeletal muscles, including mimetic muscles, and isolating individual muscle fascicles for quantitative measurement. Additionally, DiceCT provides a non-destructive mechanism for visualizing muscles. The combined techniques of reverse dissection and DiceCT can be used to assess the evolutionary morphology of mimetic musculature as well as potential contraction strength and velocity in these muscles. This study further demonstrates that DiceCT can be used to accurately and reliably visualize mimetic muscles as well as reverse dissection and provide a non-destructive method for sampling mimetic muscles.

Introduction

Mimetic musculature, or facial expression musculature, is skeletal muscle and is found throughout Mammalia¹. While most mammalian skeletal muscle attaches to discrete bony landmarks, mimetic musculature is unique in its attachments primarily into the skin of the face, scalp, and the ventral aspect of the neck^1,2,3,4. Mimetic musculature contraction deforms the "facial mask" into expressions or facial displays of social and emotional intent, changes the size and shape of the sphincters of the eye, nasal cavity,....

Protocol

Because these procedures use animals that died from natural causes at zoos or were sacrificed in research labs where they were part of unrelated studies, these protocols do not require IACUC approvals.

1. Reverse Dissection

Note: The protocol for reverse dissection is effective for very small mammals, such as laboratory mice, all the way to large land mammals, such as the domestic horse. The mimetic muscles are often best preserved and best visualized when left with the overlying dermis instead of leaving them behind on the skull. See Figure 1-3.<....

Results

This section presents examples of results on facial musculature form that can be achieved by using "reverse dissection" in concert with DiceCT scanning. By using "reverse dissection" to create a facial mask, a fuller representation of mimetic (facial) muscle can sometimes be seen than in traditional dissection methodology. This method works on a range of body sizes from the tiny, small-bodied primates, for example the common marmoset Callithrix jacchus (Figur.......

Discussion

Following the steps for the "reverse dissection" protocol typically produces a facial mask that can be slowly and methodically dissected to reveal mimetic musculature, regardless of the size of the head. It is especially important to move slowly and continuously assess whether the muscles have been completely cut through inadvertently, especially in smaller bodied species.

In order to determine where the musculature is located, it is especially critical to allow the developing mask to .......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Nikon XTH 225 ST	Nikon		no catalog numbers
10% buffered formalin	Fisher Scientific	SF98-4
Iodine, ACS Grade	Lab Chem, Inc.	LC155901
Sodium thiosulfate	Acros Organics	AC450620010
Potassium Iodide	Alfa Aesar	A1270430