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M A Koehl

Publications and source records attributed to M A Koehl.

16 recordsLinked to original sources
PubMed2001

Lobster sniffing: antennule design and hydrodynamic filtering of information in an odor plume.

The first step in processing olfactory information, before neural filtering, is the physical capture of odor molecules from the surrounding fluid. Many animals capture odors from turbulent water currents or wind using antennae that bear chemosensory hairs. We used planar laser-induced fluorescence to reveal how lobster olfactory antennules hydrodynamically alter the spatiotemporal patterns of concentration in turbulent odor plumes. As antennules flick, water penetrates their chemosensory hair array during the fast downstroke, carrying fine-scale patterns of concentration into the receptor area. This spatial pattern, blurred by flow along the antennule during the downstroke, is retained during the slower return stroke and is not shed until the next flick.

Animals↗
PubMed2001

Fluid dynamic design of lobster olfactory organs: high speed kinematic analysis of antennule flicking by Panulirus argus.

Many organisms use olfactory appendages bearing arrays of microscopic hairs to pick up chemical signals from the surrounding water or air. We report a morphometric and high speed kinematic analysis of the olfactory organs (lateral flagella of the antennules, which bear chemosensory aesthetasc hairs) of the spiny lobster, Panulirus argus. Panulirus argus sample specific locations by executing a rapid series of antennule flicks at one position, moving the antennule to a different spot and then performing another series of flicks. Odorant delivery to an aesthetasc depends on the water motion near it, which depends on its Reynolds number (Re, proportional to both the diameter and speed of the hair). High speed video enabled us to resolve that during a series of flicks, an antennule moves down rapidly (aesthetasc Re = 2) and up more slowly (Re = 0.5), pausing briefly ( approximately 0.54 s) before the next downstroke. The antennules of P. argus operate in a range of Re values and inter-aesthetasc spacings in which penetration of fluid between the hairs in an array is especially sensitive to changes in speed. Therefore, when antennules flick 'old' water is flushed out of the aesthetasc array during the leaky downstroke and is not picked up again during the less leaky upstroke, hence the antennules can take discrete samples. Thus, by operating in this critical Re range these antennules should be particularly effective at sniffing.

Animals↗
PubMed2000

How animals move: an integrative view.

Recent advances in integrative studies of locomotion have revealed several general principles. Energy storage and exchange mechanisms discovered in walking and running bipeds apply to multilegged locomotion and even to flying and swimming. Nonpropulsive lateral forces can be sizable, but they may benefit stability, maneuverability, or other criteria that become apparent in natural environments. Locomotor control systems combine rapid mechanical preflexes with multimodal sensory feedback and feedforward commands. Muscles have a surprising variety of functions in locomotion, serving as motors, brakes, springs, and struts. Integrative approaches reveal not only how each component within a locomotor system operates but how they function as a collective whole.

Animals↗
PubMed2000

Sniffing by a silkworm moth: wing fanning enhances air penetration through and pheromone interception by antennae.

Many organisms increase the air or water flow adjacent to olfactory surfaces when exposed to appropriate chemical stimuli; such 'sniffing' samples fluid from a specific region and can increase the rate of interception of odorant molecules. We used hot-wire anemometry, high-speed videography and flow visualization to study air flow near the feathery olfactory antennae of male silkworm moths (Bombyx mori L.). When exposed to conspecific female sex pheromone, male B. mori flap their wings through a stroke angle of 90-110 degrees at approximately 40 Hz without flying. This behavior generates an unsteady flow of air (mean speed 0.3-0.4 m s(-1)) towards the antennae from the front of the male. A pulse of peak air speed occurs at each wing upstroke. The Womersley number (characterizing the damping of pulsatile flow through the gaps between the sensory hairs on the antennae) is less than 1; hence, pulses of faster air (at 40 Hz) should move between sensory hairs. Calculation of flow through arrays of cylinders suggest that this wing fanning can increase the rate of interception of pheromone by the sensory hairs on the antennae by at least an order of magnitude beyond that in still air. Although wing fanning produces air flow relative to the antennae that is approximately 15 times faster than that generated by walking at top speed (0.023 m s(-1)), air flow through the gaps between the sensory hairs is approximately 560 times faster because a dramatic increase in the leakiness of the feathery antennae to air flow occurs at the air velocities produced by fanning.

Air↗
PubMed2000

Stomatopod antennule design: the asymmetry, sampling efficiency and ontogeny of olfactory flicking.

Many crustaceans detect odors from distant sources using chemosensory sensilla (aesthetascs) on their antennules. The greater the flow of water through arrays of aesthetascs, the faster the access of odorant to receptors inside the aesthetascs. Stomatopods facilitate odorant access by flicking their antennules, thus increasing the relative velocity of the water reaching their aesthetascs. We used dynamically scaled physical models to investigate how aesthetasc size and spacing and antennule flicking velocity affect flow penetration into the simple aesthetasc arrays of the stomatopod Gonodactylaceus mutatus. Particle image velocimetry of flow fields near models of juvenile and adult antennules revealed that velocity gradients around the aesthetascs are steeper during the outward part of the flick than during the return stroke and that the velocity gradients are steeper at the aesthetasc tips than at their bases. More fluid per unit time flows between aesthetasc rows during the outward stroke than during the return stroke, ensuring that odor sampling is pulsatile. During flicking, velocity gradients are steeper near adult aesthetascs than near juvenile aesthetascs, and adults process more fluid per unit time than juveniles. The resulting differences in odorant access can be related to size- and age-dependent changes in stomatopod ecology.

Aging↗
PubMed1999

Measurements of mechanical properties of the blastula wall reveal which hypothesized mechanisms of primary invagination are physically plausible in the sea urchin Strongylocentrotus purpuratus.

Computer simulations showed that the elastic modulus of the cell layer relative to the elastic modulus of the extracellular layers predicted the effectiveness of different force-generating mechanisms for sea urchin primary invagination [L. A. Davidson, M. A. R. Koehl, R. Keller, and G. F. Oster (1995) Development 121, 2005-2018]. Here, we measured the composite elastic modulus of the cellular and extracellular matrix layers in the blastula wall of Strongylocentrotus purpuratus embryos at the mesenchyme blastula stage. Combined, these two layers exhibit a viscoelastic response with an initial stiffness ranging from 600 to 2300 Pa. To identify the cellular structures responsible for this stiffness we disrupted these structures and correlated the resulting lesions to changes in the elastic modulus. We treated embryos with cytochalasin D to disrupt the actin-based cytoskeleton, nocodazole to disrupt the microtubule-based cytoskeleton, and a gentle glycine extraction to disrupt the apical extracellular matrix (ECM). Embryos treated less than 60 min in cytochalasin D showed no change in their time-dependent elastic modulus even though F-actin was severely disrupted. Similarly, nocodazole had no effect on the elastic modulus even as the microtubules were severely disrupted. However, glycine extraction resulted in a 40 to 50% decrease in the elastic modulus along with a dramatic reduction in the hyalin protein at the apical ECM, thus implicating the apical ECM as a major mechanical component of the blastula wall. This finding bears on the mechanical plausibility of several models for primary invagination.

Actins↗
PubMed1999

Ecological biomechanics of benthic organisms: life history, mechanical design and temporal patterns of mechanical stress.

We can gain biomechanical insights if we couple knowledge of the environments, ecological roles and life history strategies of organisms with our laboratory analyses of their mechanical function or fluid dynamics, as illustrated by studies of the mechanical design of bottom-dwelling marine organisms. Obviously, measurements of the spatial and temporal distribution of loads on an organism in nature reveal the magnitudes and rates at which biomechanical tests should be performed in the laboratory. Furthermore, knowledge of the population biology and ecological interactions of the organisms being studied is crucial to determine when during the life of an individual particular aspects of mechanical performance should be measured; not only can the size, shape and material properties of an individual change during ontogeny, but so can its habitat, activities and ecological role. Such ecological information is also necessary to determine whether the aspects of mechanical performance being studied are biologically important, i.e. whether they affect the survivorship or fitness of the organisms. My point in raising these examples is to illustrate how ecological studies can enhance or change our understanding of biomechanical function.

Animals↗
PubMed1998

Adhesion and reattachment of compound ascidians to various substrata: weak glue can prevent tissue damage.

Sessile, soft-bodied, compound ascidians are successful competitors for substrata in crowded benthic and epibiotic marine communities and can be effective colonists of new sites, through adult rafting and reattachment. Adhesion to the substratum is essential for these ecologically important functions; we therefore studied the material properties of colony attachment to various substrata in the rafting ascidians Botrylloides sp. We found that, compared with the strength of the colony tissues, the glue attaching Botrylloides sp. to the substratum is very weak. This relative weakness may protect the soft-bodied colonies from damage if they are ripped from their host. For sessile animals, such a weak-glue 'strategy' is only effective if the animals can later reattach to a substratum. By detaching Botrylloides sp. colonies from host eelgrass blades and allowing them to reattach, before measuring peel strengths, we learned that the initial reattachment of a colony depends upon rapid new growth of the colony rather than on fresh secretion of glue beneath old zooids. We also found that the propagation peel force necessary to remove Botrylloides sp. from different substrata (e.g. mussel shells, barnacle basal plates or eelgrass blades) depends upon the surface texture of the host. Thus, the overall tenacity of a colony is affected by the types of substrata that it overgrows.

Adhesiveness↗
PubMed1997

Cellular mechanism underlying neural convergent extension in Xenopus laevis embryos.

Convergent extension, the simultaneous narrowing and lengthening of a tissue, plays a major role in shaping and patterning the neural ectoderm in vertebrate embryos. In this paper, we characterize the cellular mechanism underlying convergent extension of the neural ectoderm in the Xenopus laevis late gastrula and neurula embryo. Neural ectoderm in X. laevis consists of two components, a superficial layer of epithelial cells overlying deep mesenchymal cells. To investigate the force contribution of the deep cells to convergent extension, we explanted single layers of neural deep cells from late gastrula stage embryos. These "neural deep cell explants" undergo active convergent extension autonomously, implying that these cells contribute force for neural convergent extension in vivo. Using time-lapse videorecording of these explants, we observed the neural deep cell behaviors (previously hidden behind an opaque epithelium) underlying convergent extension. We show that neural deep cells mediolaterally intercalate to form a longer, narrower tissue and that cell shape change and cell division contribute little to their convergent extension. Moreover, we characterize the neural deep cell motility driving mediolateral intercalation, also using time-lapse videorecordings. Analyses of these videos revealed that, on average, neural deep cells exhibit mediolaterally biased protrusive activity which is expressed in an episodic fashion. We propose that neural deep cells accomplish mediolateral intercalation by applying their protrusions upon one another, exerting traction, and pulling themselves between one another. This mechanism is similar to that previously described for convergent extension of the mesodermal cells. However, because the neural deep cells do not mediolaterally elongate during their convergent extension as the mesodermal cells do, we predict that a given intercalation will result in more extension for neural deep cells than for the mesodermal cells. Intercalation of neural cells also likely occurs in a more episodic manner than that of the mesodermal cells because the neural cells' mediolateral protrusive activity is episodic, whereas the protrusive activity of mesodermal cells is more continuous. These differences in protrusive activity and cell shape changes between the neural and mesodermal regions may reflect specializations of the same basic mechanism of mediolateral intercalation, tailored to accommodate other aspects of patterning and development of each tissue. These descriptions of the active cell motility underlying neural convergent extension in X. laevis are the first high-resolution video documentation of protrusive activity during neural convergent extension in any system. Our findings provide an important step in the investigation of neural convergent extension in X. laevis and further our understanding of convergent extension in general.

Animals↗
PubMed1995

The dorsal involuting marginal zone stiffens anisotropically during its convergent extension in the gastrula of Xenopus laevis.

Physically, the course of morphogenesis is determined by the distribution and timing of force production in the embryo and by the mechanical properties of the tissues on which these forces act. We have miniaturized a standard materials-testing procedure (the stress-relaxation test) to measure the viscoelastic properties of the dorsal involuting marginal zone, prechordal mesoderm, and vegetal endoderm of Xenopus laevis embryos during gastrulation. We focused on the involuting marginal zone, because it undergoes convergent extension (an important and wide-spread morphogenetic process) and drives involution, blastopore closure and elongation of the embryonic axis. We show that the involuting marginal zone stiffens during gastrulation, stiffening is a special property of this region rather than a general property of the whole embryo, stiffening is greater along the anteroposterior axis than the mediolateral axis and changes in the cytoskeleton or extracellular matrix are necessary for stiffening, although changes in cell-cell adhesions or cell-matrix adhesions are not ruled out. These findings provide a baseline of data on which future experiments can be designed and make specific, testable predictions about the roles of the cytoskeleton, extracellular matrix and intercellular adhesion in convergent extension, as well as predictions about the morphogenetic role of convergent extension in early development.

Animals↗
PubMed1995

How do sea urchins invaginate? Using biomechanics to distinguish between mechanisms of primary invagination.

The forces that drive sea urchin primary invagination remain mysterious. To solve this mystery we have developed a set of finite element simulations that test five hypothesized mechanisms. Our models show that each of these mechanisms can generate an invagination; however, the mechanical properties of an epithelial sheet required for proper invagination are different for each mechanism. For example, we find that the gel swelling hypothesis of Lane et al. (Lane, M. C., Koehl, M. A. R., Wilt, F. and Keller, R. (1993) Development 117, 1049-1060) requires the embryo to possess a mechanically stiff apical extracellular matrix and highly deformable cells, whereas a hypothesis based on apical constriction of the epithelial cells requires a more compliant extracellular matrix. For each mechanism, we have mapped out a range of embryo designs that work. Additionally, the simulations predict specific cell shape changes accompanying each mechanism. This allows us to design experiments that can distinguish between different mechanisms, all of which can, in principle, drive primary invagination.

Animals↗
PubMed1995

Fluid flow through hair-bearing appendages: feeding, smelling and swimming at low and intermediate Reynolds numbers.

Many different types of animals use appendages bearing rows of hairs to capture food or molecules from the surrounding fluid, to locomote or to move fluids past themselves. The performance of these appendages, whose hairs operate at Reynolds numbers (Re) of 10(-5) to 10, depends on how much of the fluid that they encounter flows through the gaps between the hairs rather than around the perimeter of the whole array. We have employed mathematical modeling, microcinematography of hairy appendages on small aquatic animals and flow visualizations around dynamically scaled physical models to elucidate the factors that determine the leakiness of arrays of hairs. We found that rows of hairs operating at very low Re function as paddles, whereas those at Re near 1 operate like leaky sieves. The Re range through which the transition in leakiness occurs depends on the geometry of the appendage. We have discovered that different aspects of morphology and behavior are important in determining the leakiness of a hair-bearing appendage at different Re. Our study has revealed conditions under which morphological diversity of hairy appendages has little consequence for performance and other conditions under which simple changes in speed, size or mesh coarseness can lead to novel physical mechanisms of operation.

Animals↗
PubMed1993

A role for regulated secretion of apical extracellular matrix during epithelial invagination in the sea urchin.

Epithelial invagination, a basic morphogenetic process reiterated throughout embryonic development, generates tubular structures such as the neural tube, or pit-like structures such as the optic cup. The 'purse-string' hypothesis, which proposes that circumferential bands of actin microfilaments at the apical end of epithelial cells constrict to yield a curved epithelial sheet, has been widely invoked to explain invaginations during embryogenesis. We have reevaluated this hypothesis in two species of sea urchin by examining both natural invagination of the vegetal plate at the beginning of gastrulation and invagination induced precociously by Ca2+ ionophore. Neither type of invagination is prevented by cytochalasin D. In one species, treatment with A23187 three hours before the initiation of invagination resulted in the deposition of apical extracellular matrix at the vegetal plate, rather than invagination. This apical matrix contains chondroitin sulfate, as does the lumen of the archenteron in normal gastrulae. When the expansion of this secreted matrix was resisted by an agarose gel, the vegetal plate buckled inward, creating an archenteron that appeared 3-4 hours prematurely. Pretreatment with monensin, which blocks secretion, inhibits both Ca2+ ionophore-stimulated folding and natural invagination, demonstrating that secretion is probably required for this morphogenetic event. These results indicate that alternatives to the purse-string hypothesis must be considered, and that the directed deposition of extracellular matrix may be a key Ca(2+)-regulated event in some embryonic invaginations. A bending bilayer model for matrix-driven epithelial invagination is proposed in which the deposition of hygroscopic material into a complex, stratified extra-cellular matrix results in the folding of an epithelial sheet in a manner analagous to thermal bending in a bimetallic strip.

Animals↗
PubMed1990

The mechanics of notochord elongation, straightening and stiffening in the embryo of Xenopus laevis.

We have examined the biomechanical development of the notochord of Xenopus early tail-bud embryos by: (1) quantifying morphological and mechanical changes in the embryo during stages 20-28, and (2) conducting manipulative experiments to elucidate mechanical roles of various components of the notochord. The notochord, which is composed of a stack of flat cells surrounded by a connective tissue sheath, elongates dramatically and begins straightening between stages 21 and 25. At this time the fiber density in the notochord sheath goes up, the osmotic activity of the notochord cells increases, vacuoles within these cells swell, the internal pressure of the notochord increases 2- to 3-fold, and the flexural stiffness of the notochord rises by an order of magnitude. We suggest that the tendency of the notochord cells to osmotically swell is resisted by the sheath, thereby permitting the internal pressure to rise. This pressure increase results in the greater stiffness that permits the notochord to elongate and straighten without being buckled by the surrounding tissues.

Animals↗
PubMed1984

Technique for revealing the three-dimensional architecture of whole preserved spiculated invertebrates.

Procedures for revealing the three-dimensional arrangement of calcareous sclerites, spicules, or ossicles embedded within connective tissue in formalin fixed invertebrates are described. Spicules are stained with alizarin red S following maceration of preserved animals or colonies with either trypsin or KOH solutions. Connective tissue is stained with alcian blue in different samples prior to maceration. Stained animals or colonies are cleared in glycerin. This method for revealing spicular structure and arrangement and the gross morphology of connective tissues offers several advantages over either scanning electron microscopy or reconstruction from serial sections.

Alcian Blue↗