Biomedical subjects
K R Porter
Publications and source records attributed to K R Porter.
Microtubules in intracellular locomotion.
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The ultrastructural morphology and possible functional significance of mesothelial microvilli.
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A scanning electron microscope study of surface features of viral and spontaneous transformants of mouse Balb-3T3 cells.
Cells of the mouse line Balb/3T3 as well as three virus-induced transformants and two spontaneous transformants grown in vitro have been studied for their topography by scanning electron microscopy. The parent cell in confluent culture closely resembles an endothelial cell in its form and in the structure of its association with adjacent cells. The tumorigenic transformants produced by SV40, murine sarcoma virus, or polyoma viruses are fusiform to pleomorphic and distinctly different from the cell of origin. They show relatively smooth surfaces except for blebs and marginal microvilli. Perhaps most surprising is the similarity they bear to one another. This is made the more singular by the very different form shown by the tumorigenic transformants of spontaneous origin. One of these, S2-4, possesses a thickened rather than the lamellar form of the parent A31 cell and is covered by long microvilli and many spherical blebs. The other, TuT(3), more closely resembles the cell of origin but shows extensive ruffling at its margins. All transformants grow without evidence of contact inhibition. The significance of the surface morphologies and the factors influencing cell form are discussed.
The response of ventral horn neurons to axonal transection.
The morphological changes induced in the frog ventral horn neurons by axonal transection have been studied with the electron microscope. During the first 2 wk after axotomy the neuronal nucleus becomes more translucent and the nucleolus becomes enlarged and less compact. The cisternae of the granular endoplasmic reticulum vesiculate and ribosomes dissociate from membranes. Free ribosomes and polysomes are dispersed in the cytoplasmic matrix. Neurofilaments and neurotubules are increased in number. These structures appear to be important in the regeneration of the axon. It is proposed that neurotubules, neurofilaments, and axoplasmic matrix are synthesized by the free polyribosomes in the chromatolytic neuron. By the fourth postoperative week, the neurons show evidence of recovery. The cytoplasm is filled with profiles of granular endoplasmic reticulum and many intercisternal polysomes. The substances being manufactured by the newly formed granular endoplasmic reticulum are not clearly defined, but probably include elements essential to electrical and chemical conduction of impulses. The significance of these observations in respect to recent studies of axoplasmic flow is discussed.
Fine structural observations relating to the production of color by the iridophores of a lizard. Anolis carolinensis.
This paper presents the results of light and electron microscopy done on iridophores in the dorsal skin of the lizard Anolis carolinensis. New fine-structural details are revealed, and their importance is discussed. Of some interest is the complex of filaments between crystalline sheets in the cell. It is proposed that this complex is involved in the arrangement of crystals into crystalline sheets, and that the crystal arrangement and spacing are critical for the production of the cells' blue-green color. Tyndall scattering and thin-film interference are discussed as possible explanations for iridophore color production in relation to the fine-structural data obtained.
Observations on the structure of the skin of the teleost Fundulus heteroclitus (L).
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An electron microscope study of differentiation of the molting muscles of Rhodnius prolixus.
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Centriole replication during ciliogenesis in the chick tracheal epithelium.
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A reinterpretation of the structure and development of the basement lamella: an ordered array of collagen in fish skin.
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Fine structure of the visual system of the honeybee (Apis mellifera). I. The retina.
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Microtubules in the formation and development of the primary mesenchyme in Arbacia punctulata. I. The distribution of microtubules.
Prior to gastrulation, the microtubules in the presumptive primary mesenchyme cells appear to diverge from points (satellites) in close association with the basal body of the cilium; from here most of the microtubules extend basally down the lateral margins of the cell. As these cells begin their migration into the blastocoel, they lose their cilia and adopt a spherical form. At the center of these newly formed mesenchyme cells is a centriole on which the microtubules directly converge and from which they radiate in all directions. Later these same cells develop slender pseudopodia containing large numbers of microtubules; the pseudopodia come into contact and fuse to form a "cable" of cytoplasm. Microtubules are now distributed parallel to the long axis of the cable and parallel to the stalks which connect the cell bodies of the mesenchyme cells to the cable. Microtubules are no longer connected to the centrioles in the cell bodies. On the basis of these observations we suggest that microtubules are a morphological expression of a framework which opeartes to shape cells. Since at each stage in the developmental sequence microtubules appear to originate (or insert) on different sites in the cytoplasm, the possibility is discussed that these sites may ultimately control the distribution of the microtubules and thus the developmental sequence of form changes.
Fine structure of cell division in Chlamydomonas reinhardi. Basal bodies and microtubules.
Cell division in log-phase cultures of the unicellular, biflagellate alga, Chlamydomonas reinhardi, has been studied with the electron microscope. The two basal bodies of the cell replicate prior to cytokinesis; stages in basal body formation are presented. At the time of cell division, the original basal bodies detach from the flagella, and the four basal bodies appear to be involved in the orientation of the plane of the cleavage furrow. Four sets of microtubules participate in cell division. Spindle microtubules are involved in a mitosis that is marked by the presence of an intact nuclear envelope. A band of microtubules arcs over the mitotic nucleus, indicating the future cleavage plane. A third set of microtubules appears between the daughter nuclei at telophase, and microtubules comprising the "cleavage apparatus" radiate from the basal bodies and extend along both sides of the cleavage furrow during cytokinesis. Features of cell division in C. reinhardi are discussed and related to cell division in other organisms. It is proposed that microtubules participate in the formation of the cleavage furrow in C. reinhardi.
Cytoplasmic fibrils in living cultured cells. A light and electron microscope study.
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Intestinal triglyceride absorption in the rat. An electron microscopical study.
This report provides information on the morphology of fat absorption in rat intestinal epithelial cells. Three types of experiments were performed: (a) intubation of corn oil into fasted rats, (b) injection of physiological fatty-chyme prepared from fat-fed donor rats into ligated segments of jejunum of fasted animals, and (c) administration of electron-opaque particles in corn oil and markers given concurrently with the fat. These results support the hypothesis that fat is absorbed by selective diffusion of monoglycerides and fatty acids from micelles rather than by pinocytosis of unhydrolized triglycerides. Evidence is presented that the pits between the microvilli, previously believed to function in the transport of fat, are not involved in this process. Instead they appear to contribute their contents to lysosomes in the apical cytoplasm. Arguments are offered that the monoglycerides and fatty acids diffuse from the micelle while the latter is associated with the microvillous membrane of the absorptive cell. These micellar components penetrate the plasma membrane and diffuse into the cytoplasmic matrix where they encounter the SER. Triglyceride synthesis occurs in the SER and results in the deposition of fat droplets within its lumina. The synthesis of triglycerides and their sequestration into the SER establishes an inward diffusion gradient of monoglycerides and fatty acids.
Studies on the microtubules in heliozoa. II. The effect of low temperature on these structures in the formation and maintenance of the axopodia.
When specimens of Actinosphaerium nucleofilum are placed at 4 degrees C, the axopodia retract and the birefringent core (axoneme) of each axopodium disappears. In fixed specimens, it has been shown that this structure consists of a highly patterned bundle of microtubules, each 220 A in diameter; during cold treatment these microtubules disappear and do not reform until the organisms are removed to room temperature. Within a few minutes after returning the specimens to room temperature, the axonemes reappear and the axopodia begin to reform reaching normal length 30-45 min later. In thin sections of cells fixed during the early stages of this recovery period, microtubules, organized in the pattern of the untreated specimens, are found in each reforming axopodium. Reforming axopodia without birefringent axonemes (and thus without microtubules) are never encountered. From these observations we conclude that the microtubules may be instrumental not only in the maintenance of the axopodia but also in their growth. Thus, if the microtubules are destroyed, the axopodia should retract and not reform until these tubular units are reassembled. During the cold treatment short segments of a 340-A tubule appeared; when the organisms were removed from the cold, these tubular segments disappeared. It seems probable that they are one of the disintegration products of the microtubules. A model is presented of our interpretation of how a 220-A microtubule transforms into a 340-A tubule and what this means in terms of the substructure of the untreated microtubules.
Microtubules in the spermatids of the domestic fowl.
Spermiogenesis in chicken has been examined in order to see whether the radical changes observed in cell shape can be related to the presence of cytoplasmic microtubules. A highly ordered array of tubules has been found which surrounds the nucleus as it elongates from a sphere to a slender cylinder. The structure of the array has been determined by following the tubules through 12-14 adjacent serial sections, and it is a left-handed double helix. Faint cross-bridges connect consecutive turns of the two helices. After the change in nuclear shape is complete, the helical system of microtubules disappears and is replaced by a set of almost straight tubules which run parallel to the long axis of the nucleus. These tubules remain while the spermatid nucleus condenses isotropically to its final size. We suggest that the helix is the agent which effects nuclear elongation and that the subsequent system of paraxial tubules determines the curvature of the final sperm head. Evidence for these suggestions is found in the form of spermatids which have failed to develop properly. In an appendix we consider the kinematics of single and multiple helix systems and discuss the revelance of these models to the morphogenesis of chicken spermatids.
Specializations of the unit membrane.
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