In Vitro Models for Biomechanical Stud- ies of Neural Tissues

In vitro models are invaluable tools for studying cell behavior in a highly controlled setting. Cell and tissue culture models of the nervous system can be utilized to elucidate neurobiological phenomena that are difficult to observe, manipulate, or measure in vivo. In the context of biomechanics, culture models that accurately mimic specific brain features can be used to determine tissue properties and tolerances to mechanical loading. There are several criteria that culture models must meet in order to complement in vivo and macroscopic biomechanical studies. In addition to providing an environment that is conducive to cell survival, cell type and source are critical to the interpretation of results. In this review, we present design criteria for ideal cultures, the current state of the art in neural cell and tissue culturing methods, and the advantages and limitations to using culture mimics. We will further present what insights in vitro models can provide to complement in vivo and macroscopic biomechanics in terms of mesoto microscale material properties and tissue-level tolerance criteria. The discussion will focus primarily on central nervous system (CNS) tissue, which is inherently complex in cytoarchitecture and organization. In addition, the CNS is not typically exposed to mechanical loading beyond physiological motion; therefore, it is expected that cell death and functional failure may be particularly prominent at large deformations and high loading rates. These and other B. Morrison III (&) Biomedical Engineering, Columbia University, 351 Engineering Terrace, MC 8904, 1210 Amsterdam Avenue, New York, NY 10027, USA e-mail: [email protected] D. K. Cullen Neurosurgery, University of Pennsylvania, 105 Hayden Hall, 3320 Smith Walk, Philadelphia, PA 19104, USA M. LaPlaca Biomedical Engineering, Georgia Institute of Technology, 313 Ferst Drive, Atlanta, GA 30332-0535, USA Stud Mechanobiol Tissue Eng Biomater (2011) 3: 247–285 247 DOI: 10.1007/8415_2011_79 Springer-Verlag Berlin Heidelberg 2011 Published Online: 30 April 2011 factors must be considered when attempting to extract and culture CNS tissue or its components for studying neurobiological or neuromechanical phenomena. 1 Neural Tissue Structure and Composition 1.1 Heterogeneity of Brain Tissue Brain is a heterogeneous tissue that is divided into distinct anatomical and functional regions. The gray and white matter comprise the cellular constituents, and the blood supply, cerebrospinal fluid/ventricular system, interstitial fluid, and extracellular matrix (ECM) make up the extracellular components. The intracellular space (ICS) and the extracellular space (ECS) can be considered as one scale of heterogeneity. The ICS is a composite with neurons and their extensions, glia cells, vascular cells, and other support cells. The orientation is quite organized in some areas and apparently random in others. Most neuronal communication in the brain exists on a local level (i.e. shorter interneurons, which comprise most neurons in the brain), with 1,000s of synapses on some neurons, creating an extremely complex network [1]. The brain has a very high cellularity compared to other organs and a very diverse cell population. Cell sizes range from\10 to[100 lm in cell soma diameter with axons ranging from microns to 100s of microns long. Dendrites are highly branched, permitting diversification and maximization of interactions between communicating neurons [1]. Brain tissue can be divided further by anatomical regions. Just rostral to the spinal cord is the brain stem, a deep, well-protected portion of the brain comprised of the medulla oblongata, pons, and midbrain, which houses the controls to many homeostatic functions such as respiratory and cardiovascular regulation. The cerebellum, dorsal to the brainstem, helps control balance, posture, motor coordination, sensory and motor relaying. The diencephalon is the deep region of the brain that contains the thalamus (primary relay for sensory input and motor output) and the hypothalamus (neuroendocrine structure critical for maintaining homeostasis). Traveling toward the brain surface are the white matter tracts, the cerebral nuclei (basal ganglia, hippocampus, amygdala) and the cerebral cortex, comprised of six distinct, organized, interconnected layers. The isolation of any brain region (for primary culture) should consider the connections with other brain areas not represented in the culture and the possible effects (e.g., activation of compensatory responses) harvest and dissection may have. Interneuronal connections and interregional junctions are disrupted during tissue dissociation as are projection axons for most culture techniques, therefore the culturing procedures must consider these microinjuries and provide conditions conducive for repair and healthy culture maturation and reconnection. Some regions, such as the hippocampus are particularly vulnerable to excitotoxicity and the pH of the medium and constituents should allow for normal receptor function and neurotransmitter reuptake. Yet, this region is ideal for studying phenomena related to excitotoxicity, such as ischemia and trauma. 248 B. Morrison III et al.