Fine structure of hippocampal mossy fiber synapses following rapid high-pressure freezing

Synapses of hippocampal neurons play important roles in learning and memory processes and are involved in aberrant hippocampal function in temporal lobe epilepsy. Major neuronal types in the hippocampus as well as their input and output synapses are well known, but it has remained an open question to what extent conventional electron microscopy (EM) has provided us with the real appearance of synaptic fine structure under in vivo conditions. There is reason to assume that conventional aldehyde fixation and dehydration lead to protein denaturation and tissue shrinkage, likely associated with the occurrence of artifacts. However, realistic fine-structural data of synapses are required for our understanding of the transmission process and for its simulation. Here, we used high-pressure freezing and cryosubstitution of hippocampal tissue that was not subjected to aldehyde fixation and dehydration in ethanol to monitor the fine structure of an identified synapse in the hippocampal CA3 region, that is, the synapse between granule cell axons, the mossy fibers, and the proximal dendrites of CA3 pyramidal neurons. Our results showed that high-pressure freezing nicely preserved ultrastructural detail of this particular synapse and allowed us to study rapid structural changes associated with synaptic plasticity.

Lung Structure and the Intrinsic Challenges of Gas Exchange

Structural and functional complexities of the mammalian lung evolved to meet a unique set of challenges, namely, the provision of efficient delivery of inspired air to all lung units within a confined thoracic space, to build a large gas exchange surface associated with minimal barrier thickness and a microvascular network to accommodate the entire right ventricular cardiac output while withstanding cyclic mechanical stresses that increase several folds from rest to exercise. Intricate regulatory mechanisms at every level ensure that the dynamic capacities of ventilation, perfusion, diffusion, and chemical binding to hemoglobin are commensurate with usual metabolic demands and periodic extreme needs for activity and survival. This article reviews the structural design of mammalian and human lung, its functional challenges, limitations, and potential for adaptation. We discuss (i) the evolutionary origin of alveolar lungs and its advantages and compromises, (ii) structural determinants of alveolar gas exchange, including architecture of conducting bronchovascular trees that converge in gas exchange units, (iii) the challenges of matching ventilation, perfusion, and diffusion and tissue-erythrocyte and thoracopulmonary interactions. The notion of erythrocytes as an integral component of the gas exchanger is emphasized. We further discuss the signals, sources, and limits of structural plasticity of the lung in alveolar hypoxia and following a loss of lung units, and the promise and caveats of interventions aimed at augmenting endogenous adaptive responses. Our objective is to understand how individual components are matched at multiple levels to optimize organ function in the face of physiological demands or pathological constraints. © 2016 American Physiological Society. Compr Physiol 6:827-895, 2016.