Specific paracrystalline structures of rough endoplasmic reticulum in the follicular (stellate) cells of the dog adenohypophysis

Chapter 6: cubic membranes the missing dimension of cell membrane organization

Biological membranes are among the most fascinating assemblies of biomolecules: a bilayer less than 10 nm thick, composed of rather small lipid molecules that are held together simply by noncovalent forces, defines the cell and discriminates between “inside” and “outside”, survival, and death. Intracellular compartmentalization—governed by biomembranes as well—is a characteristic feature of eukaryotic cells, which allows them to fulfill multiple and highly specialized anabolic and catabolic functions in strictly controlled environments. Although cellular membranes are generally visualized as flat sheets or closely folded isolated objects, multiple observations also demonstrate that membranes may fold into “unusual”, highly organized structures with 2D or 3D periodicity. The obvious correlation of highly convoluted membrane organizations with pathological cellular states, for example, as a consequence of viral infection, deserves close consideration. However, knowledge about formation and function of these highly organized 3D periodic membrane structures is scarce, primarily due to the lack of appropriate techniques for their analysis in vivo. Currently, the only direct way to characterize cellular membrane architecture is by transmission electron microscopy (TEM). However, deciphering the spatial architecture solely based on two-dimensionally projected TEM images is a challenging task and prone to artifacts. In this review, we will provide an update on the current progress in identifying and analyzing 3D membrane architectures in biological systems, with a special focus on membranes with cubic symmetry, and their potential role in physiological and pathophysiological conditions. Proteomics and lipidomics approaches in defined experimental cell systems may prove instrumental to understand formation and function of 3D membrane morphologies.

Unusual configurations of endoplasmic reticulum in pinealocytes of the Djungarian hamster (Phodopus sungorus)

Ultrastructural observations of the pineal gland of Phodopus sungorus revealed different morphological forms of endoplasmic reticulum. These included crystalloids composed of 200A-wide tubules with an intervening space of 200A and connected at right angles to form a rectangular lattice; complexes of undulating 500A-wide tubules in an electron-dense cytoplasmic matrix; and stacks of flattened cisternae covered with granular electron-dense material. All these structures are continuous with the surrounding endoplasmic reticulum. Frequently one structure is directly connected with another. The exact function of these structures derived from the endoplasmic reticulum is still unclear, but they can be assumed to be morphological features of augmented secretory activity.

Phasic secretion by follicular cells of the bat adenohypophysis during the prearousal period of their annual life cycle

The fine structure of the follicular (nongranulated) cells of the adenohypophysis of the bat was examined at different periods of the annual life cycle. Follicular cells were found in all regions of the pars distalis of both active and hibernating bats. Throughout the active phase of the life cycle and during most of the hibernating period, the structure of follicular cells is unchanged. Toward the end of hibernation, however, striking ultrastructural changes occur. The most obvious of these alterations is the appearance in follicles of dense rod-shaped bodies within the follicular luminal space. Just prior to arousal, the rodlike bodies become much wider and appear more as paracrystalline aggregates of dense material. Images indicate that the dense rod-shaped bodies are packaged in vesicles in the cytoplasm of the follicular cells. The fusion of the membrane of filled vesicles with the plasma membrane of the follicular cell results in the release of their dense rod-shaped bodies into the follicular lumen by exocytosis. These cyclic ultrastructural changes indicate that follicular cells have a phasic secretory activity.