Electron microscope observations on chemo- and mechano-receptor cells of fishes

Summary This paper reports on the fine structure of chemo and mechano-receptor cells found in three species of fishes (Corydoras paleatus, Cnesterodon decemmaculatus, Fitzroyia lineata).Taste cells were studied in the food-finding barbels of adult species belonging to the Genus Corydoras. They are characterized by the presence of a great amount of vesicular material concentrated at the level of the apical and medial region. Most of these cells terminate at the barbel surface by means of a cylindrical or tapered extremity devoid of sensory hairs. It was possible to observe, in some cases, the existence of short and ill defined microvilli. The basal pole of each sensory cell contacts with several sensory nerve fibers. These fibers contain mitochondria and a few vesicles.The fine structure of the olfactory neurons was studied in full-developed embryos of Cnesterodon and Fitzroyia. The olfactory sensory hairs consist of long cilia which project into the lumen of the olfactory pit. Cilia arise from the olfactory knob which is merely an apical swelling of the dendrite. The dendrite of the olfactory neuron shows profiles of small tubules, aligned parallel to its length. Near the basement membrane of the epithelium groups of axons are seen encased in the surface of the sustentacular cells.The mechano-receptor cells studied were: 1.) The sensory cells existing in the neuromasts of the lateral line system of Cnesterodon and Fitzroyia, and 2.) the receptor cells of the ampullar crests of the same species.Neuromast receptor-cells have well developed sensory hairs which consist of cilia and microvilli. It is highly probable that each receptor cell, like those of the vestibular epithelium, bears only one cilium asymmetrically located in relation to the units of the sensory process. One of the most striking characteristics of this type of cell is the existence of a high amount of vesicular material accumulated in the cytoplasm of the basal region; it is at this level that the nerve fibers take contact with the receptor cell membrane.Three main types of neuroepithelial junction are described in the neuromasts (nerve fiber deeply recessed in the cytoplasm, calyx type and knob-like ending). In these junctions the vesicular material is almost exclusively concentrated in the cytoplasm of the receptor cell, while only few vesicles are seen within the neuroplasm of the sensory fibers.The receptor cells occuring in the ampullar crests of Cnesterodon and Fitzroyia show many structural characteristics similar to those present in neuromasts' receptor cells. Like these, they bear sensory hairs consisting of several microvilli and only one cilium which is always asymmetrically located within the group of hairs. The basal region of the cell is filled with a large amount of small vesicles. Nerve endings also show vesicles but they are less in number than inside the cytoplasm of the receptor cell.Comments are made on the apparent significance of the sensory hairs. These structures are considered (in chemo-receptor cells) as devices serving to enlarge the active surface of the cell and increasing by this way the effectiveness of the whole receptive system. In mechano-receptor cells cilia and microvilli may act as levers of different mechanical characteristics which convey stimuli to the receptor-cell cytoplasm.In this paper three main types of neuroepithelial junctions connecting receptor cells with the central nervous system are described.     

Electron microscope observations on the differentiation of protocorm cells of Dactylorhiza maculata

The fine structure of aseptically cultivated protocorm cells of Dactylorhiza maculata (L.) Sod was studied with special reference to differentiation of leaf cells. In the young protocorm the characteristic structures are compound amyloplasts and smooth ER. In the older, the green shoot pole contains many proplastids storing starch, some membrane–bound protein crystalloids and lipid droplets attached to the nuclear membrane and plasmalemma. In the leaves of the young seedling, two types of chloroplasts are found. One contains little or no starch but many osmiophilic plasto–globuli; the other has only a few plastoglobuli but a large starch grain.     

Electron microscope visualization of giant polysomes in sea urchin embryos

Chromatin spreading techniques have been applied to the electron microscopic visualization of polysomes in sea urchin (Strongylocentrotus purpuratus) eggs and embryos. Polysomes of giant size are commonly found after the 8-cell stage. The largest seen, from an early gastrula, was 13.6 m in length, carried 277 ribosomes, with a message calculated to contain 6.49×10⁴ nucleotides and potentially to encoded 2.38×10⁶ daltons of peptide. Polysomes are rare and very large ones absent from lysates of unfertilized eggs. Giant polysomes appear in 4- to 8-cell stages and are common in 16-cell stages and thereafter. They are of two forms: a compact form with no spacing between ribosomes characteristic of stages through early mesenchyme blastulae, and an extended form found only after late mesenchyme blastulae. Both have potential for massive informational content. Some of each type have ribosome-free tails at one end, as long as 733 Å in the compact forms, and 7,890 Å in the extended ones. Occasionally they have a single array of fibrous material increasing from one end of a polysome to the other, interpreted to be nascent peptide chains. Polysomes are not found after brief, mild exposure of lysates to RNase A, or from embryos treated with puromycin. Very large polysomes are present in lysates of blastulae exposed since fertilization to actinomycin D, cycloheximide, or cordycepin. They appear in parthenogenetically activated or fertilized enucleate merogones, but are absent from unactivated merogones, demonstrating that egg masked messages can generate them. A potential embryological significance of giant, potentially polycistronic polysomes is suggested.     

Electron microscope observations of the melanocyte of the human epidermis

Proteins and colloidal materials, administered orally to suckling rats and mice, were ingested by columnar absorptive cells of the jejunum and ileum, but not of the duodenum. Bovine gamma globulin and ovalbumin were identified in the apical cytoplasm by staining with fluorescent antibody; trypan blue, Evans blue, saccharated iron oxide, and colloidal gold were detected intracellularly by their color, specific staining, and appearance in the electron microscope. Each substance was segregated in membrane-enclosed vacuoles, apparently part of a system of potentially interconnecting vacuoles and tubules in the apical cytoplasm which is continuous in places with the apical cell membrane. We postulate that ingestion of foreign materials was accomplished by pinocytosis, that is, by invagination of the apical cell membrane to form vacuoles containing material from the intestinal lumen. Approximately 18 days after birth columnar absorptive cells lost the ability to ingest proteins and colloids, and no longer contained large vacuoles and numerous tubules. At this age rats and mice lose the ability to absorb antibodies from the intestine in an immunologically intact form, and we conclude that cellular ingestion is part of the mechanism of absorption of intact proteins in suckling animals. Particulate fat apparently is absorbed in both newborn and adult animals by micropinocytosis. Thus adult animals may not have lost the capacity for pinocytosis, but rather have become selective as to what substances provoke it. Cortisone acetate, administered subcutaneously to rats 8 to 10 days old alters the columnar absorptive cells within 72 hours so that they resemble the cells in adult animals and no longer ingest proteins.     

Electron microscope observations on the early development of the rat

Summary The early development of the rat, from the mature oocyte through fertilization until the 8-cell stage, has been studied with the electron microscope. The fine structure is described and discussed, with particular reference to the following topics. The middle piece of the spermatozoon is found in every stage studied, within the ovum cytoplasm; it is not significantly altered by this situation. The nucleoli are numerous during the 1-cell stage and often present in positions that suggest their extrusion into the cytoplasm; in subsequent stages a branching structure develops around them. The dividing cell presents the mitotic apparatus with its centrioles, hollow looking fibers, chromosomes, but without centromeres; in the cytoplasm the small granules align in rows. Mitochondria are evenly distributed during the 1-cell stage and can be found in the 8-cell stage constricted as if dividing. The multivesicular bodies constitute an abundant population of cytoplasmic elements that may be related to the endoplasmic reticulum or the Golgi complex, neither of which is clearly recognizable. In the 8-cell stage the cytoplasm segregates into two zones, one of which contains all the multivesicular bodies, while the mitochondria are found in both of them; this distinction provides some basis to adscribe to the multivesicular bodies the properties of the so called metachromatic particles.The support of the Gildemeister Foundation is gratefully acknowledged.