Cell culture has become an indispensable tool to help uncover fundamental biophysical and biomolecular mechanisms by which cells assemble into tissues and organs, how these tissues function, and how that function becomes disrupted in disease. Cell culture is now widely used in biomedical research, tissue engineering, regenerative medicine, and industrial practices. Although flat, two-dimensional (2D) cell culture has predominated, recent research has shifted toward culture using three-dimensional (3D) structures, and more realistic biochemical and biomechanical microenvironments. Nevertheless, in 3D cell culture, many challenges remain, including the tissue-tissue interface, the mechanical microenvironment, and the spatiotemporal distributions of oxygen, nutrients, and metabolic wastes. Here, we review 2D and 3D cell culture methods, discuss advantages and limitations of these techniques in modeling physiologically and pathologically relevant processes, and suggest directions for future research.

Tracheal pressure, central airflow, and alveolar capsule pressures in cardiac lobes were measured in open-chest dogs during 0.1- to 20-Hz pseudorandom forced oscillations applied at the airway opening. In the interval 0.1-4.15 Hz, the input impedance data were fitted by four-parameter models including frequency-independent airway resistance and inertance and tissue parts featuring a marked negative frequency dependence of resistance and a slight elevation of elastance with frequency. The models gave good fits both in the control state and during histamine infusion. At the same time, the regional transfer impedances (alveolar pressure-to-central airflow ratios) showed intralobar and interlobar variabilities of similar degrees, which increased with frequency and were exaggerated during histamine infusion. Results of simulation studies based on a lung model consisting of a central airway and a number of peripheral units with airway and tissue parameters that were given independent wide distributions were in agreement with the experimental findings and showed that even an extremely inhomogeneous lung structure can produce virtually homogeneous mechanical behavior at the input.

This paper deals with a unifying hypothesis addressed at lung tissue resistance and its responses to neurohumoral and biophysical stimuli. The hypothesis holds that dissipative and elastic processes within lung tissue are coupled at the level of the stress-bearing element. Such a description leads naturally to consideration of a readily measured attribute of organ-level dissipative behavior called lung tissue hysteresivity, eta. On preliminary analysis this attribute is found to be nearly frequency independent and numerically conserved across species. To the degree that the numerical value of eta might be conserved during an intervention in which tissue dynamic elastance changes, such behavior would be consistent with the notion that elastic energy storage and dissipative energy loss reside within the very same stress-bearing element and, moreover, that those processes within the stress-bearing element bear an approximately fixed relationship. Tissue hysteresivity is closely related to the parameter K used by Bachofen and Hildebrandt (J. Appl. Physiol. 30: 493-497, 1971) to describe energy dissipation per cycle, and both lend themselves directly to interpretation based on processes ongoing at the levels of microstructure and molecule. Intraparenchymal connective tissues, surface film, and contractile elements appear to submit individually to this description and, in doing so, yield respective hysteresivities that are relatively well matched; this suggests that such hysteretic matching may be a necessary condition for synchronous expansion of the alveolar duct. The overriding simplicity with which this description organizes diverse observations implies that it may capture some unifying attribute of underlying mechanism.

The interrupter method for measuring respiratory system resistance involves rapidly interrupting flow at the mouth while measuring the pressure just distal to the point of interruption. The pressure signal observed invariably exhibits two distinct phases. The first phase is a very rapid jump, designated delta Pinit, which occurs immediately on interruption of flow. The second phase is designated delta Pdif and is a further pressure change in the same direction as delta Pinit but evolving over several seconds. The physiological interpretations of delta Pinit and delta Pdif have been somewhat unclear. Delta Pinit has been taken to equal the pressure drop across the pulmonary airways, possibly with a contribution from the tissues of the respiratory system. Delta Pdif can arise, in principle, from two sources: gas redistribution throughout the lung after interruption of flow and stress recovery within the tissues. To resolve these issues we performed interruption experiments on anesthetized paralyzed, tracheotomized, open-chest normal dogs during passive expiration while measuring alveolar pressures at three sites with alveolar capsules. We found that, in the absence of the chest wall, delta Pinit reflects only the resistance of the airways and that delta Pdif can be ascribed almost entirely to the stress recovery properties of lung tissues.

In six excised canine lungs, regional alveolar pressures (PA) were measured during small-amplitude high-frequency oscillations applied at the airway opening. Both the regional distribution of PA's and their relationship to pressure excursions at the airway opening (Pao) were assessed in terms of amplitude and phase. PA was sampled in several capsules glued to the pleural surface and communicating with alveolar gas via pleural punctures. Pao and PA were measured over the frequency (f) range 1-60 Hz, at transpulmonary pressures (PL) of 5, 10, and 25 cmH2O. The amplitude of PA excursions substantially exceeded Pao excursions at frequencies near the resonant frequency. At resonance the ratio [PA/Pao] was 1.9, 2.9, and 4.8 at PL's of 5, 10, and 25 cmH2O, respectively. Both spatial homogeneity and temporal synchrony of PA's between sampled lung regions decreased with f and increased with PL. Interregional variability of airway impedance [(Pao - PA)/Vao] and tissue impedance (PA/Vao) tended to be larger than differences due to changing PL but not as large as between-dog variability. These data define the baseline nonhomogeneity of the normal canine lung and also suggest that there may be some advantage in applying high-frequency ventilation at frequencies at least as high as lung resonant frequency.