Middleton Lab bio photo

Middleton Lab

University of Missouri - Integrative Anatomy

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Bone Structure and Biomechanics

We have a long-term collaborative project with Dr. Theodore Garland, Jr. in the Department of Biology at the University of California, Riverside, studying the effects of exercise and artificial selection on skeletal morphology and bone mechanics in house mice.

The Garland Lab has been artificially selecting mice for high levels of voluntary wheel running for more than 60 generations. These “high-runner” mice can run more than 25 km in one night, which is taxing to most physiological systems. In the lab, we are particularly interested in the effects (both separate and combined) of exercise and artificial selection on skeletal form, growth, physiology, and function.

Inbred High Activity Mini-Muscle Mice

A major project in the lab is the establishment of an inbred strain of high-activity mice that exhibit the mini-muscle phenotype. This project was begun in 2007. As of Fall 2014, we are over 20 generations into inbreeding and can now officially say that the strain is inbred.

The strain of mice that results from the project, which will both exhibit the mini-muscle allele and have genes enabling them to run great distances are a potential model organism for human musculoskeletal diseases and disorders. For example, mice can be crossed with other strains of mice known to have high or low bone mass to dissect the complex interaction between muscle function and normal bone growth.

Left: When standardized to body mass, mice with the mini-muscle phenotype (left) have significantly longer and thinner femora than randomly bred controls (right). Right: Comparison of mini-muscle (left) and wild-type (right) phenotype triceps surae muscle. The mini-muscle is about 50% less massive. (Photo courtesy of Mark Chappell)

Future experiments may include physiological measurements such as maximal oxygen consumption during strenuous exercise. High-runner mice generally have higher oxygen consumption, but this varies depending of the presence of the mini-muscle allele. Additionally, inbreeding may affect the physiological capacity of mice, so comparisons between the mice described in this experiment and “regular” high-runner mice will provide insights into the complex regulation of physiology. Research in the lab also concerns differences in bone shape and materials between mini-muscle mice and both high-runner mice and “standard” strains of lab mice.

Comparisons of shape and breaking strength with other types of mice will help us understand how genetics control the formation and, ultimately, function of the skeleton. In humans, for instance, it has been shown that certain bone shapes are more prone to fracture, and that these shape differences are genetically determined. By studying the same relationship in a model organism, in which we can have much greater experimental control, we will be able to better understand human biology.

Dinosaur Cranial Function and Evolution

In collaboration with Casey Holliday (University of Missouri), Larry Witmer (Ohio University), and Julian Davis (University of Southern Indiana), we are beginning an NSF-funded project to study the evolution of cranial kinesis across the non-avian theropod dinosaur to bird transition.

Cranial kinesis (movement among the bones of the skull, excepting the jaw) required a significant reorganization of skeletal tissues and was likely a major evolutionary factor in the highly diverse feeding apparatus and ultimately the diversification of modern birds. The goal of this project is to understand the bioechanical environment of the feeding system.

We will generate accurate 3D musculoskeletal models of heads across the dinosaur to bird transition and use these models to estimate muscle forces and joint loads.

Feather Biomechanics

Transformations associated with a return to a primarily aquatic ecology have been intensely studied in organisms ranging from insects to cetaceans. However, comparatively little attention has been paid to similar transitions in birds. Co-option of the aerial flight stroke for underwater propulsion has evolved multiple times in diverse lineages within crown clade Aves and has been associated with extremes of body size, growth rate, skeletal modification, and integumentary specialization. While the transition from aerial to aquatic “flight” (wing-propelled diving) has been recognized to involve a profound reorganization of the musculoskeletal system, bone microstructure, and integument, proposed patterns of character acquisition have remained largely hypothetical.

This project, in collaboration with Dr. Julia Clarke (University of Texas at Austin) and Dr. Daniel Ksepka (Bruce Museum; March of the Fossil Penguins blog), encompasses phylogenetic, histologic, functional, and sensory evolution. Work in the lab addresses two major questions.

How does feather structure change with the loss of aerial flight?

In water, feathers must produce forces in a medium that is more than 800 times denser than air. We are study both the microstructure of feathers and their bending properties under loads. We have evidence for the first known fossilized penguin feathers, which will provide valuable comparative data on the early evolution of these unusual structures.

Tarrin Casey (MU School of Medicine Summer Research Fellow from Xavier University, New Orleans) adjusts the apparatus that we use to load feathers in three-point bending.
Our lab-built apparatus for loading feather rachises in cantilever bending. Instron doesn't make off-the-shelve fixtures for our needs, so we have to build them outselves.

Character Evolution

What is the sequence of character evolution across multiple aerial to aquatic transitions? Most hypotheses concerning the evolution of wing-propelled diving center on musculoskeletal changes within the pectoral skeleton to enable propulsive force in an aqueous medium. By identifying osteological correlates of wing-propelled diving and quantifying their three-dimensional morphology, we are addressing relative rates of evolution among forelimbs and hind limbs using techniques that we developed.

Other Interests

Theropod Evolution

During the last 20 years, our understanding of the fossil history of birds has grown tremendously. My interest is in taking what we can learn about living birds and applying that to fossils to understand their functional anatomy. I have been working in collaboration with Dr. Julia Clarke at the University of Texas, Austin to understand skeletal evolution in Mesozoic birds. We have developed new techniques for quantitatively studying morphological evolution using Bayesian phylogenetic methods.

With the evolutionary transition that converted the forelimbs of their theropod dinosaur ancestor into wings, birds lost their grasping hands. Some time early in avian evolution, the opposed or “reversed” hallux (digit I - homologous with your big toe) evolved. Perching provides birds with the ability to grasp, much in the same way that an opposable thumb allows humans to grip.

Left: The foot of Tyrannosaurus rex which is restored with the hallux (erroneously) in a perching orientation. Right: The earliest bird, Archaeopteryx lithographica, occupies a central position in debates over whether the earliest birds were perching or not.

We have studied foot structure in living and fossil birds to understand the evolution of this complex behavior. We have also studied foot function in different kinds of living birds to investigate how the evolution of perching might have impacted walking and running on the ground.

Effects of Altered Oxygen Environments on Alligator Bone Structure

In collaboration with Dr. James Hicks (UC, Irvine), Dr. Tomasz Owerkowicz (CSU, San Bernardino), and Dr. Bryan Rourke (CSU, Long Beach), we are studying the effects of different oxygen environments on the growth microstructure of alligator bones. During the last ca. 550 million years, atmospheric oxygen levels have varied when compared to the present atmospheric oxygen level of 21% (normoxia).

During the Permian, atmospheric oxygen is estimated to have been as high as 30-35% (hyperoxia) and in the Late Triassic as low as 12% (hypoxia). Vertebrate have been living and evolving in these variable conditions. Recent interest in the bone structure of fossil vertebrate has led to a great expansion comparative paleohistology. However, comparisons are restricted to animals that live in present-day, normoxic environments.

Estimates of atmospheric oxygen levels during the last ~550 million years before the present. Estimates have ranged from extremely hypoxic to extremely hyperoxic relative to present day (~21% O2). Data from Berner (2006 and 2009).

By raising alligators in different oxygen environments, first in eggs and then post-hatching, we will mimic a range of atmospheric conditions, from hypoxia to hyperoxia. These experiments will allow us to assess the effects of the prevailing environment on growth rates and microstructure of bone. We will also be able to provide a comparative context for future paleohistological studies.

The images above show representative cross-sections of alligator femora that have been injected with fluorochrome labels (red and yellow rings). The large dark areas are vascular canals, and the small dark dots are osteocytes.