Fibroblasts can condense a hydrated collagen lattice to a tissue-like structure 1/28th the area of the starting gel in 24 hr. The rate of the process can be regulated by varying the protein content of the lattice, the cell number, or the con- centration of an inhibitor such as Colcemid. Fibroblasts of high population doubling level propagated in vitro, which have left the cell cycle, can carry out the contraction at least as efficiently as cycling cells. The potential uses of the system as an immu- nologically tolerated "tissue" for wound hea ing and as a model for studying fibroblast function are discussed.

Production of a tissue-like structure by contraction of collagen lattices by human fibroblasts of different proliferative

Similarity in morphological form may arise from common ancestry (failure to evolve), from parallel evolution, from convergence, or from reversal to an apparently ancestral condition. Homoplasy from convergence, parallelism, and reversal is common, and its ubiquity creates difficulties in phylogenetic analysis. Convergent evolution often is considered one of the most powerful lines of evidence for adaptive evolution. But an alternative explanation for convergence and other evolved similarities is that limited developmental and structural options exist. Identical forms can be obtained when particular developmental phenomena are triggered by very different kinds of stimuli or when constraints exist that shape external form or limit morphological expression to a few options. Examples from plethodontid salamanders are used to illustrate an approach combining internalist and externalist analytical methods. In order to explain how morphologies evolve in lineages, both functionalist and structuralist approaches a...

Homoplasy: the result of natural selection, or evidence of design limitations?

This article investigates the neural mechanisms underlying salamander locomotion, and develops a biologically plausible connectionist model of a central pattern generator capable of producing the typical aquatic and terrestrial gaits of the salamander. It investigates, in particular, what type of neural circuitry can produce and modulate the two locomotor programs identified within the salamander's spinal cord; namely, a traveling wave of neural activity for swimming and a standing wave for trotting. A two-dimensional biomechanical simulation of the salamander's body is developed whose muscle contraction is determined by the locomotion controller simulated as a leaky-integrator neural network. While the connectivity of the neural circuitry underlying locomotion in the salamander has not been decoded for the moment, this article presents the design of a neural circuit that has a general organization corresponding to that hypothesized by neurobiologists. In particular, the locomotion controller is based on a body central pattern generator (CPG) corresponding to a lamprey-like swimming controller, and is extended with a limb CPG for controlling the salamander's limbs. The complete controller is developed in three stages: first the development of segmental oscillators, second the development of intersegmental coupling for the making of a lamprey-like swimming CPG, and finally the development of the limb CPG and its coupling to the body CPG. A genetic algorithm is used to determine the parameters of the neural circuit for the different stages, given a high-level description of the desired state space trajectories of the different subnetworks. A controller is thus developed that can produce neural activities and locomotion gaits very similar to those observed in the real salamander. By varying the tonic (i.e. non-oscillating) excitation applied to the network, the speed, direction and type of gait can be varied.

A connectionist central pattern generator for the aquatic and terrestrial gaits of a simulated salamander.

The evolution of avian flight can be interpreted by analyzing the sequence of modifications of the primitive tetrapod locomotor system through time. Herein, we introduce the term "locomotor module" to identify anatomical subregions of the musculoskeletal system that are highly integrated and act as functional units during locomotion. The first tetrapods, which employed lateral undulations of the entire body and appendages, had one large locomotor module. Basal dinosaurs and theropods were bipedal and possessed a smaller locomotor module consisting of the hind limb and tail. Bird flight evolved as the superimposition of a second (aerial) locomotor capability onto the ancestral (terrestrial) theropod body plan. Although the origin of the wing module was the primary innovation, alterations in the terrestrial system were also significant. We propose that the primitive theropod locomotor module was functionally and anatomically subdivided into separate pelvic and caudal locomotor modules. This decoupling freed the tail to attain a new and intimate affiliation with the forelimb during flight, a configuration unique to birds. Thus, the evolution of flight can be viewed as the origin and novel association of locomotor modules. Differential elaboration of these modules in various lineages has produced the diverse locomotor abilities of modern birds.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1558-5646.1996.tb04496.x

Locomotor modules and the evolution of avian flight.

We quantified midline kinematics with synchronized electromyograms (emgs) from the red and white muscles on both sides of bluegill sunfish (Lepomis macrochirus) during escape behaviors which were elicited from fish both at a standstill and during steady speed swimming. Analyses of variance determined whether or not kinematic and emg variables differed significantly between muscle fiber types, among longitudinal positions, and between swimming versus standstill trials.

Red and white muscle activity and kinematics of the escape response of the bluegill sunfish during swimming

Summary Aquatic neotenic and terrestrial metamorphosed salamanders {Ambystoma tigrinum) were videotaped simultaneously with electromyographic (EMG) recording from five epaxial myotomes along the animal's trunk during swimming in a flow tank and trotting on a treadmill to investigate axial function during aquatic and terrestrial locomotion. Neotenic and metamorphosed individuals swim using very similar axial wave patterns, despite significant differences in axial morphology. During swimming, both forms exhibit traveling waves of axial flexion and muscle activity, with an increasing EMG-mechanical delay as these waves travel down the trunk. In contrast to swimming, during trotting metamorphosed individuals exhibit a standing wave of axial flexion produced by synchronous activation of ipsilateral epaxial myotomes along the trunk. Thus, metamorphosed individuals employ two distinct axial motor programs - one used during swimming and one used during trotting. The transition from a traveling axial wave during swimming to a standing axial wave during trotting in A. tigrinum may be an appropriate analogy for similar transitions in axial locomotor function during the original evolution of terrestriality in early tetrapods.

Kinematic and electromyographic analysis of the functional role of the body axis during terrestrial and aquatic locomotion in the salamander ambystoma tigrinum

The orientation of the fibers in the dermis of the tiger salamander, Ambystoma tigrinum, undergoes a dramatic repatterning at metamorphosis. The pre-metamorphic, larval dermis is a tight layer composed of crossed fibers that wind helically around the trunk. This condition is retained by neotenic adults which do not undergo metamorphosis. In contrast, the neotenic adults which do not undergo metamorphosis. In contrast, the metamorphosed adult dermis consists of a superficial, loose network of fibers invested with large multicellular glands--the stratum spongiosum--and a deeper tight layer of fibers--the stratum densum. However, unlike the crossed fibers of the pre-metamorphic dermis, there is no preferred orientation to the fibers in either layer of the post-metamorphic dermis. In order to evaluate whether these two distinctly different fiber patterns are constructed from biochemically similar fibers, the collagen types present in the pre- and post-metamorphic dermis were determined using SDS-polyacrylamide gel electrophoresis. Type I collagen is the predominant collagen of the dermis and the same major collagen types are present for all individuals, whether pre- or post-metamorphic. Thus, the major types of collagen that compose the dermal fibers do not change during metamorphic repatterning of the dermis.

Collagen type conservation during metamorphic repatterning of the dermal fibers in salamanders.

The plethodontid salamander species Ensatina eschscholtzii comprises seven subspecies that are geographically distributed in a ring surrounding California's Central Valley. The subspecies are differentiated primarily on the basis of colour pattern, although there is also significant electrophoretic protein variation both within and among subspecies. The subspecies intergrade around the north end of the valley, whereas at the southern end, two morphologically distinct subspecies meet but do not interbreed. Thus, the species is believed to be at or near speciation. The present study examines osteological variation in Ensatina eschscholtzii using x-radiographs of 436 individuals from 19 localities around the ring-shaped range of the species. In marked contrast to the distinct geographic differentiation in colour pattern variation, skeletal conservatism predominates and the small amounts of variation seen in the limbs and vertebral column show no correlation with geography. Thus, skeletal variation does not further elucidate the status of speciation in E. eschscholtii.



Rare skeletal variants, especially in the limb, follow patterns of variation that implicate developmental processes in constraining skeletal variability. In the digits, variation in the number of phalanges in E. eschscholtzii is congruent with variation produced by mitotic inhibition experiments in the developing axolotl (Ambystoma mexicanum) limb. In the mesopodium, the sequence of mineralization (in rare large individuals where mineralization occurs) matches the sequence of prechondrogenic condensation in the developing Ambystoma limb. Although constraints on morphological variability by internal developmental mechanisms can account for the patterns of variation seen among these rare limb variants, the question remains open as to whether developmental processes are responsible for the overall skeletal conservatism seen in E. eschscholtzii.

Larry M. Frolich

Papers

Kinematic and electromyographic analysis of the functional role of the body axis during terrestrial and aquatic locomotion in the salamander ambystoma tigrinum

Collagen type conservation during metamorphic repatterning of the dermal fibers in salamanders.

Osteological conservatism and developmental constraint in the polymorphic ‘ring species' Ensatina eschscholtzii (Amphibia: Plethodontidae)