Some observations on the fine structure of the mantis shrimp heart

Summary The electron microscopic structure of the myocardium of the mantis shrimp is descriped. Particular attention is paid to the organization of the nerve terminal and the sarcotubular system. The general appearance of this myocardium is characterized by deep invagination of the plasma membrane at the level of Z-band and large irregular shaped mitochondria. It possesses a very well developed sarcotubular system, consisting of the longitudinal system and two transverse elements making two sets of contact to each other. One forms dyad and the other forms triad at the level of M- and Z-band, respectively. The organization of the myoneural junction in this muscle is very simple and undifferentiated. In general, a special structural differentiation is invariably observed at both sides of the contact area. In spite of the fact that synaptic vesicles and a differentiated membrane are found at the naked process of this cardiac nerve, specialization such as subsynaptic fold formation has not been observed at the muscle side which is in contact with the nerve process. Observations made on the sarcotubular system and the nerve termination have been discussed with reference to their physiological significance. This investigation was supported by the Public Health Service Research Grants HE 06968-04 and NB 03348-04 of the National Institutes of Health, Bethesda, and the U. S. Department of the Army through its Far East Research Office.     

The fine structure of isolated Acetabularia nuclei

Summary Acetabularia nuclei were isolated in solutions of sucrose and fixed, dehydrated and embedded for electron microscopy using a number of techniques. The dimensions of the nucleic changed during all the procedures used, but the addition of calcium to the fixative prevented swelling. The structure observed in isolated nuclei was in good agreement with that seen in nuclei in intact cells and amputated rhizoids. A layer of cytoplasm about 0.1 thick was found on the outside of even the cleanest nuclei — this layer probably gives the nucleus protection when it is isolated.     

Some observations on the fine structure of the corpus striatum of the rat brain

Summary The rat corpus striatum was perfused vitally with glutaraldehyde, immersed in OsO₄ and then observed under an electron microscope.Numerous small cells in the neostriatum show a simple cytoplasmic structure, while the large cells possess a complicated fine structure. These are differentiated under the elctron microscope into two kinds, which seem to have functional differences. The large pallidal cells containing much pale cytoplasm are covered with many varied axonal boutons from the cell body to the dendritic terminal making numerous axo-somatic or axo-dendritic trunk synapses. Numerous axo-dendritic, or spine synapses are recognized in the neostriatal neuropil.These numerous axon terminals, which belong to striatal nerve cells or other nuclei of the brain, are classified morphologically into several types. At least five types of synaptic vesicles are distinguished by their size or by the presence of fine dense granules on their membranes, and seem to be specific to the neostriatum.Many myelin interruptions and several kinds of glial cells in the corpus striatum are observed. Moreover, the ventricular wall of the caudate nucleus, namely, the ependyma, and two kinds of subependymal cells are described and discussed with reference to the subependymal layer.     

Isolation of gametes and zygotes from Setaria viridis

Setaria viridis, the wild ancestor of foxtail millet (Setaria italica), is an effective model plant for larger C₄ crops because S. viridis has several desirable traits, such as short generation time, prolific seed production and a small genome size. These advantages are well suited for investigating molecular mechanisms in angiosperms, especially C₄ crop species. Here, we report a procedure for isolating gametes and zygotes from S. viridis flowers. To isolate egg cells, ovaries were harvested from unpollinated mature flowers and cut transversely, which allowed direct access to the embryo sac. Thereafter, an egg cell was released from the cut end of the basal portion of the dissected ovary. To isolate sperm cells, pollen grains released from anthers were immersed in a mannitol solution, resulting in pollen-grain bursting, which released sperm cells. Additionally, S. viridis zygotes were successfully isolated from freshly pollinated flowers. Isolated zygotes cultured in a liquid medium developed into globular-like embryos and cell masses. Thus, isolated S. viridis gametes, zygotes and embryos are attainable for detailed observations and investigations of fertilization and developmental events in angiosperms.

     

New observations on the fine structure of graptoblasts

Graptoblasts are redefined as widenings of the sealed terminal portions of graptolite stolothecae resulting in ovoid bodies composed of an outer fusellar layer, named here as blastotheca, and an inner secondary layer of electron dense material, named the blastocrypt. In their proximal portion a number of graptoblasts display a scar with fuselli stripped and the crassal material of the blastotheca exposed and displaying a rough and dull surface. The boundary of this area, termed the talus, is edged by a distinct escarpment, probably the margin of the preserved part of the blastotheca. The outer wall of the talus is usually abruptly terminated. Only exceptionally is the outer wall conical and gradually widening. Thus the transverse partition between the parental stolotheca and the graptoblast was produced by the blastocrypt without fusellar covering. It seems, therefore, that graptoblasts were most probably lacking any internal portion of their blastothecae which could possibly be compared with the internal parts of a dendroid auto-theca. It is concluded that graptoblasts housed a zooid that never functioned as an autozooid, but suffered arrested development and encystation. The biological role of graptoblasts as dormant bodies is discussed.     

Some observations on the fine structure of the synaptic area in the ciliary ganglion of the chick

Summary The fine structure of the synaptic area in the ciliary ganglion from 10-days chick embryo to the adult was studied by electron microscopy.The ciliary ganglion cell is unipolar and a considerable surface of which is covered by the calyx terminal. The peripheral part of the calyx divides into several terminal knobs and form a basket terminal.Four types of contact configurations were observed in the calyciform ending: 1) contact area without membrane specialization which occupies the most part of the contact, 2) desmo-some-like structure which is observed in various places of the contact surface, 3) synaptic complex and 4) close apposition of apposing plasma membranes.The presence of the synaptic complex and the close appositon of apposed plasma membranes seems to correspond to the dual natures of the transmission obtained by Martin and Pilar (1963a, b).In addition, some considerations were made on the subsurface cistern and on the possible functional significance of the myelin sheath surrounding the ganglion cell and the calyx.This work was supported in part by Grant NB-03348-03 from the National Institutes of Health, United States Public Health Service.Dr. Takahashi wishes to express his sincere thanks to Professor S. Watanabe, Department of Anatomy, Sapporo Medical College, who offered an opportunity for doing work at the Department of Anatomy, Hiroshima University.     

Some observations on the fine structure of the giant nerve fibers of the earthworm,Eisenia foetida

Sectioned dorsal giant fibers of the earthworm Eisenia foetida have been studied with the electron microscope. The giant axon is surrounded by a Schwannian sheath in which the lamellae are arranged spirally. They can be traced from the outer surface of the Schwann cell to the axon-Schwann membranes. Irregularities in the spiral arrangement are frequently observed. Desmosome-like attachment areas occur on the giant fiber nerve sheath. These structures appear to be arranged bilaterally in columns which are oriented slightly obliquely to the long axis of the giant fiber and aligned linearly from the axon to the periphery of the sheath. At these sites they bind together apposing portions of Schwann cell membrane comprising the sheath. Longitudinal or oblique sections of the nerve sheath attachment areas are reminiscent of the Schmidt-Lantermann clefts of vertebrate peripheral nerve. Septa of the giant fibers have been examined. They are symmetrical or non-polarized and consist of the two plasma membranes of adjacent nerve units. Characteristic vesicular and tubular structures are associated with both cytoplasmic surfaces of these septa.     

Some observations on the fine structure of the myotendinous junction in myotomal muscles of the tadpole tail

Summary Myotendinous junctions in the myotomal tail muscles of the tadpole of Rana rugosa were examined by electron microscopy. At the site of the myotendinous junction, the sarcolemma is covered on its sarcoplasmic aspect by the connecting filament layer and the attachment layer, and on the extracellular aspect by the intermediary layer and the external lamina, with associated collagen fibrils. The intermediary layer consists of filamentous structures which closely resemble microfibrils (Hanak and Böck, 1971), spine-like or thread-like profiles (Korneliussen, 1973) and intermediary layer (Nakao, 1975a, b) in the myotendinous junctions of other vertebrate skeletal muscles.Particularly interesting is the fact that all the coverings and linings of the sarcolemma, including the external lamina, are completely absent in the terminal segment of the finger-like sarcolemmal invagination characteristic of the myotendinous junction. Furthermore, special types of coupling between a sac of sarcoplasmic reticulum and a part of the sarcolemmal invagination are frequently observed. These couplings always occur along the region of the sarcolemma where the external lamina is absent. The couplings show features similar to those of the triad, such as SR feet , scalloped SR membranes and granular content of the SR sac, suggesting that they are analogous and functionally similar to the triad and other equivalent structures.     

Some observations on the fine structure of the sinus gland of a land crab,Gecarcinus lateralis

The dilated axon endings of the sinus glands of the brachyuran crab, Gecarcinus lateralis, are filled with homogeneously dense granules, each granule being bounded by a delicate membrane. The granules are of two orders of magnitude: 0.05 to 0.1 micro and 0.15 to 0.2 micro in diameter. Each axon ending contains granules of a nearly uniform size. Endings with granules of the larger size range predominate. Non-nervous cells endogenous to the sinus gland are scattered among the nerve endings. The cell contours are irregular, and cytoplasmic processes ramify between endings. The axons are unmyelinated, having only thin limiting membranes, and they possess many neurofibrils. Granules in preterminal portions of the axons tend to lie at the periphery of the fiber, and in some cases in chains at the core of the fiber. The granules appear to be storage and release centers for neurosecretory substances or their precursors.     

Further observations on the fine structure of the human spermatozoon

Summary The limiting membrane of the equatorial segment of the acrosome has a five layered appearance due to a thin dense layer apposed to the inner aspect of the unit membrane. Shorter localized areas with the same appearance are seen in the inner membrane of the anterior segment of the acrosome, but most of this membrane and all of the outer membrane of this segment show the tri-laminar unit membrane structure.In longitudinal sections the postacrosomal sheath is seen to consist of two different portions separated by a circumferential groove in the cell membrane. The anterior part is composed of a homogeneous dense layer from which regularly arranged projections extend toward the cell membrane. The posterior part is composed of a dense granular or homogeneous material apposed to the inner aspect of the cell membrane. This layer is limited posteriorly by a close union of the cell membrane with the nuclear envelope via a dense homogeneous material.In the implantation fossa the two layers of the nuclear envelope are interconnected by regularly spaced linear structures. The centriole found in this region has a constant orientation with respect to the long axis of the flagellum, and a system of numbering the triplets is proposed.The coarse fibers of the flagellum are finely cross striated with a period of 40 A. Near the abaxial surface of the fibers every forth line is somewhat thicker than the others, thus demarcating a major period of 160 A with three intraperiod lines of which the middle one is slightly thicker than the others.—A somewhat regular mode of termination of the tubular elements of the flagellum in the end piece is described.Supported by a Public Health Service International Postdoctoral Research Fellowship (number 1 F05 TW 1681-01), by Contract 69-2107 from the Center for Population Studies, National Institute of Child Health and Human Development, and by Statens Laegevidenskabelige Forskningsraad and P. Carl Petersens Fond, Denmark.     

Some observations on the fine structure of light and dark cells in the gall bladder epithelium of the mouse

Summary Electron microscopic observations have been made of the two epithelial cell types, light barrel-shaped and dark rod-shaped cells in the gall bladder of the mouse.The light cells have a voluminous cytoplasm of low electron opacity in which cell organelles such as mitochondria, elements of granular endoplasmic reticulum, and free ribosomes undergo more or less degenerative changes. However, there are a relatively abundant Golgi apparatus and numerous lysosomal dense bodies. The ultrastructural features of the light cells suggest that they are an aged, degenerative cell type with declining functional activity and a high degree of hydration.The dark cells are characterized by a high concentration of mitochondria and free ribosomes, more or less distinctive elements of granular endoplasmic reticulum, and well developed components of the Golgi apparatus. Such ultrastructural characteristics indicate that the dark cell type has a high synthetic activity.What has been observed in the present study can well be correlated with the results of previous studies on the same cells by methods of light microscopic histochemistry.