Susan Fisher

Characterization of the rabbit sperm membrane autoantigen, RSA, as a lectin-like zona binding protein☆

Adhesion between spermatozoa and the eggs extracellular coat, the zona pellucida, involves the sperms zona binding proteins (ZBP) and their interaction with the carbohydrate residues of the zona. To investigate this interaction in more detail, a purified nonenzymatic ZBP, the rabbit sperm membrane autoantigen, RSA, was used. RSA-zona binding was demonstrated on nitrocellulose blots and by using the Denny-Jaffe crosslinking reagent which identified an 87,000 molecular weight zona component as the ligand for RSA. The RSA-zona binding was of high affinity with a dissociation constant of 5.6 X 10(-13) M. Furthermore, the binding of capacitated spermatozoa to intact zona was inhibited in the presence of RSA. Characterization of the RSA-zona interaction with a variety of simple and complex carbohydrates indicated that the sulfated, complex carbohydrates fucoidin, dextran sulfate, chondroitin sulfate B, and heparin strongly inhibited RSA-zona binding while chondroitin sulfates A and C, cholesterol-3-sulfate, and monosaccharides such as galactose inhibited RSA-zona binding only weakly. It is concluded that RSA functions as a sperm lectin-like molecule to bind the spermatozoon to the zona pellucida.

Modeling hair using level-of-detail representations

We present a novel approach for modeling hair using level-of-detail representations. The set of representations include individual strands, hair clusters, and hair strips. They are represented using subdivision curves or surfaces, and have the same underlying base skeleton to maintain consistent high-level physical behavior when a transition between different levels-of detail occurs. This framework supports automatic simplification of dynamic simulation, collision detection, and graphical rendering of animated hair. It also offers flexibility to balance between the overall performance and visual quality, and can be used to model and render different hairstyles. We have used these level-of-detail representations to animate various hairstyles and obtained noticeable performance improvement, with little loss in visual quality.

Localization of zona pellucida binding sites on rabbit spermatozoa and induction of the acrosome reaction by solubilized zonae

The binding of mammalian spermatozoa to the eggs extracellular coat, the zona pellucida, is a complex process which culminates in species-specific penetration of the sperm to the egg plasma membrane. To investigate where on the spermatozoons surface the zona binding sites are located, whole rabbit zonae were labeled with FITC, heat solubilized and used to observe the surface binding patterns on live spermatozoa. Before the acrosome reaction the zona binding sites are located either over the entire head as well as the middle piece or alternatively in patches along the apical ridge of the head. After the acrosome reaction there is a 29% loss of fluorescence and the zona binding sites are present in the posterior aspect of the acrosomal region, the anterior postacrosomal region and the middle piece. These results demonstrate the presence of zona binding sites after the acrosome reaction which would account for the sperms ability to remain bound to the zona after the acrosome reaction. Further, we report for the first time that solubilized rabbit zonae pellucidae will induce the acrosome reaction in in vitro capacitated rabbit sperm whereas solubilized pig zonae pellucidae will not. Since rabbit sperm bind pig zonae, the induction and specificity of the physiological acrosome reaction must reside in the affinity of the binding rather than the binding itself.

An implicit finite element method for elastic solids in contact

Focuses on the simulation of mechanical contact between nonlinearly elastic objects such as the components of the human body. The computation of the reaction forces that act on the contact surfaces (contact forces) is the key for designing a reliable contact handling algorithm. In traditional methods, contact forces are often defined as discontinuous functions of deformation, which leads to poor convergence characteristics. This problem becomes especially serious in areas with complicated self contact such as skin folds. We introduce a novel penalty finite element formulation based on the concept of material depth, the distance between a particle inside an object and the objects boundary. By linearly interpolating pre-computed material depths at node points, contact forces can be analytically integrated over contact surfaces without increasing the computational cost. The continuity achieved by this formulation supports an efficient and reliable solution of the nonlinear system. This algorithm is implemented as part of our implicit finite element program for static, quasistatic and dynamic analysis of nonlinear viscoelastic solids. We demonstrate its effectiveness on an animation showing realistic effects such as folding skin and sliding contacts of the tissues involved in knee flexion. The finite element model of the leg and its internal structures was derived from the Visible Human data set.

Fast penetration depth estimation for elastic bodies using deformed distance fields

We present a fast penetration depth estimation algorithm between deformable polyhedral objects. We assume the continuum of non-rigid models are discretized using standard techniques, such as finite element or finite difference methods. As the objects deform, the pre-computed distance fields are deformed accordingly to estimate the penetration depth, allowing an enforcement of non-penetration constraints between two colliding elastic bodies. This approach can automatically handle self-penetration and inter-penetration in a uniform manner. We demonstrate its effectiveness on moderately complex simulation scenes.

Deformed distance fields for simulation of non-penetrating flexible bodies

We present a novel penetration depth estimation algorithm based on the use of deformed distance fields for simulation of non-penetrating flexible bodies. We assume that the continuum of non-rigid models are discretized using standard techniques, such as finite element methods. As the objects deform, the distance fields are deformed accordingly to estimate penetration depth, allowing enforcement of non-penetration constraints between two colliding elastic bodies. Our approach can automatically handle self-penetration and inter-penetration in a uniform manner. We demonstrate its effectiveness on moderately complex animated scenes.

An improved finite element contact model for anatomical simulations

This work focuses on the simulation of mechanical contact between nonlinearly elastic objects, such as the components of the human body. In traditional methods, contact forces are often defined as discontinuous functions of deformations, which leads to poor convergence characteristics and high-frequency noises. We introduce a novel penalty method for finite-element simulation based on the concept of material depth, which is the distance between a particle inside an object and the object’s boundary. By linearly interpolating precomputed material depths at node points, contact forces can be analytically integrated over contact surfaces without raising the computational cost. The continuity achieved by this formulation reduces oscillation and artificial acceleration, resulting in a more reliable simulation algorithm.

Sperm Membrane and Zona Pellucida Interactions during Fertilization

In most species, fertilization is a necessary and essential step for the passage of one generation to the next. The genome is passed via spermatozoon and ovum into the zygote, and development begins anew. The spermatozoon’s passage through the female reproductive tract and its capacitation, which is necessary for successful egg penetration, encompass the fertilization process. Successful fertilization and therefore the survival of the species depends upon a series of steps which has been described as a hierarchy of specificities (O’Rand, 1985): from geographic isolation to the egg plasma membrane. The meeting of the sperm’s plasma membrane and the egg’s zona pellucida is almost the last step in this hierarchy and is the subject of the present discourse.

Automatic simplification of particle system dynamics

We present a novel framework for automatically simplifying the dynamics computation of particle systems to improve simulation speeds. Our approach is based on a physically-based subdivision scheme to generate a hierarchy of approximated motion models or simulation levels of detail (SLOD). At each time step, the SLODs are updated on-the-fly, and the appropriate SLOD is chosen adaptively to reduce computational costs. We have tested a prototype implementation on the simulation of a water fountain and a galaxy system. The preliminary results show a significant performance gain on these scenarios with little loss in the visual appearance of the simulation, indicating the potential to generalize this approach to other dynamical systems.