The biflagellate alga Chlamydomonas reinhardi was studied with the light and electron microscopes to determine the behavior of flagella in the living cell and the structure of the basal apparatus of the flagella. During normal forward swimming the flagella beat synchronously in the same plane, as in the human swimmer's breast stroke. The form of beat is like that of cilia. Occasionally cells swim backward with the flagella undulating and trailing the cell. Thus the same flagellar apparatus produces two types of motion. The central pair of fibers of both flagella appear to lie in the same plane, which coincides with the plane of beat. The two basal bodies lie in a V configuration and are joined at the top by a striated fiber and at the bottom by two smaller fibers. From the area between the basal bodies four bands of microtubules, each containing four tubules, radiate in an X-shaped pattern, diverge, and pass under the cell membrane. Details of the complex arrangement of tubules near the basal bodies are described. It seems probable that the connecting fibers and the microtubules play structural roles and thereby maintain the alignment of the flagellar apparatus. The relation of striated fibers and microtubules to cilia and flagella is reviewed, particularly in phytoflagellates and protozoa. Structures observed in the transitional region between the basal body and flagellar shaft are described and their occurrence is reviewed. Details of structure of the flagellar shaft and flagellar tip are described, and the latter is reviewed in detail.

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Flagellar motion and fine structure of the flagellar apparatus in chlamydomonas

Axon hillocks and initial segments have been recognized and studied in electron micrographs of a wide variety of neurons. In all multipolar neurons the fine structure of the initial segment has the same pattern, whether or not the axon is ensheathed in myelin. The internal structure of the initial segment is characterized by three special features: (a) a dense layer of finely granular material undercoating the plasma membrane, (b) scattered clusters of ribosomes, and (c) fascicles of microtubules. A similar undercoating occurs beneath the plasma membrane of myelinated axons at nodes of Ranvier. The ribosomes are not organized into Nissl bodies and are too sparsely distributed to produce basophilia. They vanish at the end of the initial segment. Fascicles of microtubules occur only in the axon hillock and initial segment and nowhere else in the neuron. Therefore, they are the principal identifying mark. Some speculations are presented on the relation between these special structural features and the special function of the initial segment.

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The axon hillock and the initial segment

Publisher Summary This chapter discusses lysosomes and related particles. Lysosomes have a fairly narrow size range, but there are exceptions such as the large droplets in kidney cells and phagocytes. Some lysosomes, as in liver parenchyma and in reticuloendothelial cells, contain ferritin-like grains; in the cells of the proximal convolution, they contain a material of variable, but generally moderate, electron opacity. Golgi apparatus as a site for the segregation of lysosomal hydrolases and insoluble materials like the ferritin-like material observed within the granules of these para-erythroblasts. There is also evidence suggesting the presence of other cytoplasmic particles closely related to but not identical with lysosomes. These are particles in which intravenously injected peroxidase is concentrated. These peroxidase-containing particles when isolated shortly after the injection of the protein behave very much like lysosomes. The procedures that cause the release of hydrolases from lysosomes also free the peroxidase from their particles.

Lysosomes and Related Particles

surface between the AB corner and the GH corner and are approximately related by the pseudo-dyad axis of symmetry named dyad 1 by Cullis et al. 4• However, the xenon atom in the oc-chain lies nearer the GH corner and thPt in the f3-chain closer to the AB corner. At first it might appear surprising that the xenon positions in the two chains are different from one another, and not the same as in myoglobin. The AB corners of the etand f3-chains differ in both the sequence and tho number of residues they contain, giving rise to structurally different environments. The amino-acid sequences in the GH corners are also different, though here the two chains are of equal length. It should also be noted that the amino-acid sequences of myoglobin• and haemoglobin are quite different. Any change in atomic distribution near a cavity could easily change the electronic interaction with xenon towards an energetically unfavourable state. The exact analysis of the xenon sites will have to await determination of the haemoglobin structure at high resolution. On the basis of Perutz's tentative atomic model of haemoglobin', the nearest neighbours of both xenon atoms are valine, leucine and pheny lalanine. This complex is presumably stabilized, as in myoglobin, by dipoleand quadrupole-induced dipole and quadrupole moments and London interactions. A theoretical investigation by Kittel and Shore8 of xenon polarizability has shown that the quadrupolar (as well as the dipolar) polarizability is particularly high, thus favouring binding in situations like this where one might not otherwise expect it. An analysis of the change in protein-bound water between haemoglobin and the haemoglobin-xenon complex by a microwave t echnique showed an increase of protein-bound water, due to the presence of xenon. Any attempts to demonstrate this directly by X-ray methods must also await the fina l analysis of haemoglobin at high resolution, but changes in the charge distribution caused by xenon atoms located close to the surface of the molecule could account for tho increase in bound water. I thank Dr. M. F. Porutz for his advice and use of his structural data of haemoglobin. This work was supported in part by the U.S. Public Health Service grant NB 03625 to R. M. Featherstone, chairman, Department of Pharmacology, University;,.of California Medical Center, San Francisco. 1 Schoenborn, B. P., Watson, H . C., and Kendrew, ;r, C., Nature, 207, 28 (1965). ' Cullen, S., and Cross, E., Scie11C<l,l13, 580 (1951). • Muehlbacher, C., De Bon, F. L., and Featherstone, R.l\\L,lntern. Artesthesiol. Clinics , 1, 937 (1963). 'Cullis, A. F. , llfuirhead, H .• Perutz, M. F ., Rossmann, l\\f. G., and North, A. C. T .. Proc. Roy. Soc., A,265, 161 (1962). 'Matsuda, G., Gehring-Mfiller, R., and Braunilzer, G., Biochemistry, 2, a38, 669 (1963). • Edmundson. A. B., Nature, 205, 883 (19B5). 'Perutz, M. F .• J. Mol. Biol.,l3 (in the press). 8 Kitto! . C., and Shore, H. (private communieatiou). 'Schoenborn, B. P., Featherstone, R. M., Vogelhut, P. 0., and Sil3skin<l, C., Nature, 202, 695 (1964).

Polymorphism and mitochondrial activity in sleeping sickness trypanosomes

Iron metabolism in the bone marrow as seen by electron microscopy: a critical review.

SYNOPSIS. An electron microscope study of Leishmania donovani revealed major differences between the intracellular leishmanial and the extracellular leptomonad stages. Parasites were also removed from infected hamster spleen, inoculated into cultures, and examined at intervals during incubation in order to follow changes during the transformation from one stage to the other. Prominent changes are evident primarily in the kinetoplast and the nucleoli after several hours of cultivation when the parasite is in an intermediate stage of transition. The kinetoplast, a composite organelk containing DNA and cristae mitochondriales, is a regular, sausage-like structure in the leishmanial stage, but becomes irregular and enlarges extensively after a few hours in vitro, developing branches that are mitochondrial in nature and which, very likely, become new mitochondria. In the nucleus, significant nucleolar changes occur. The two homogeneous nucleoli characteristic of the leishmanial stage merge during transformation into one which is larger, less compact, and composed of small centrally located granules about 100 A in diameter and larger particles over 200 A in diameter at the periphery. These transformations of the kinetoplast and nucleolus are undoubtedly expressions of adaptive changes in the metabolic pattern of an intracellular parasite transferred to a markedly different extracellular environment of much lower temperature.



It was also found that the leishmanial stage of the parasite has two unit membranes which extend over the flagellum and the flagellar pocket; the intermediate stage and the leptomonad have only a single membrane. Furthermore, some leishmanias in spleen cells were found to be covered by a capsule-like structure formed by the deposition of fine granular material between the two unit membranes. Since two to four organisms were occasionally found within such capsules, it is possible that encapsulation is a preparatory step for reproduction.

The Fine Structure of Leishmania donovani and the Role of the Kinetoplast in the Leishmania-Leptomonad Transformation*

Blood collected from rats infected with Plasmodium berghei was centrifuged and the pellet was fixed for 1 hour in 1 per cent buffered OsO 4 with 4.9 per cent sucrose. The material was embedded in n -butyl methacrylate and the resulting blocks sectioned for electron microscopy. The parasites were found to contain, in almost all sections, oval bodies of the same density and structure as the host cytoplasm. Continuity between these bodies and the host cytoplasm was found in a number of electron micrographs, showing that the bodies are formed by invagination of the double plasma membrane of the parasite. In this way the host cell is incorporated by phagotrophy into food vacuoles within the parasite. Hematin, the residue of hemoglobin digestion, was never observed inside the food vacuole but in small vesicles lying around it and sometimes connected with it. The vesicles are pinched off from the food vacuole proper and are the site of hemoglobin digestion. The active double limiting membrane is responsible not only for the formation of food vacuoles but also for the presence of two new structures. One is composed of two to six concentric double wavy membranes originating from the plasma membrane. Since no typical mitochondria were found in P. berghei , it is assumed that the concentric structure performs mitochondrial functions. The other structure appears as a sausage-shaped vacuole surrounded by two membranes of the same thickness, density, and spacing as the limiting membrane of the body. The cytoplasm of the parasite is rich in vesicles of endoplasmic reticulum and Palade9s small particles. Its nucleus is of low density and encased in a double membrane. The host cells (reticulocytes) have mitochondria with numerous cristae mitochondriales. In many infected and intact reticulocytes ferritin was found in vacuoles, mitochondria, canaliculi, or scattered in the cytoplasm.

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Phagotrophy and two new structures in the malaria parasite Plasmodium berghei.

The feeding apparatus of Suctoria consists of long, thin, stiff tubes called tentacles. When a swimming prey attaches to the tip of the tentacle a number of events follow in rapid succession. The tentacle broadens, a stream of tiny granules starts to move upward at its periphery to the tip, the prey becomes immobilized and shortly thereafter the cytoplasm of the still living prey begins to flow through the center of the tentacle to the body of the predator. An electron microscope study of the tentacle in Tokophrya infusionum, a protozoan of the subclass Suctoria, has disclosed a number of structural details which help to clarify some of the mechanisms involved in this unusual way of feeding. Each tentacle is composed of two concentric tubes. The lumen of the inner tube is surrounded by 49 tubular fibrils most probably of contractile nature. In the inner tube the cytoplasm of the prey is present during feeding, and in the outer tube are small dense bodies. It was found that the dense bodies originate in the cytoplasm of Tokophrya. They have an elongate, missile-like appearance, pointed at one end, rounded at the other, and are composed of several distinct segments. At the tip of the tentacle they penetrate the plasma membrane, with their pointed ends sticking out. It is assumed that the missile-like bodies play a major role in the feeding process. Their composite structure suggests that they might contain a number of enzymes which most probably are responsible for the various events preceding the actual food intake.

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The fine structure and function of the tentacle in tokophrya infusionum

SYNOPSIS. Leishmania tarentolae requires at least 10 mμg riboflavin per ml for optimal growth on successive subculture in a denned medium. At this level many organisms are aflagellate or with a short flagellum; higher concentrations of riboflavin did not increase the yield of leptomonads but did increase the proportion with a flagellum at least as long as the body. At limiting levels of riboflavin, acriflavin markedly inhibited growth without producing akinetoplastic forms. With riboflavin at 20 mμ/ml or higher, acriflavin at around 500 mμg/ml permitted an approximately 10-fold multiplication. Seventy % of the leptomonads formed were akinetoplastic and they could not give rise to subcultures.



In electron micrographs of acriflavin-treated organisms the nucleus was unaffected but the cytoplasmic organelles, notably the kinetoplast and mitochondria, were altered. The mitochondria appeared swollen with their cristae replaced by circular profiles. In the kinetoplast the dense fibrillar material (probably DNA) lost its structure, became diffuse, and eventually disappeared. The observed changes again indicate close relationship between the kinetoplast and the mitochondria of hemoflagellates.

The riboflavin requirement and the effects of acriflavin on the fine structure of the kinetoplast of leishmania tarentolae.

SYNOPSIS. An electron microscope study of thin sections of Babesia rodhaini parasitizing mouse erythrocytes showed that this intracellular parasite feeds on its host cell in the same way as do malarial parasites. Large portions of erythrocyte cytoplasm are engulfed by invaginations of the plasma membrane leading to the formation of food vacuoles. The digestion of hemoglobin seems to be complete in Babesia since no pigment (hemozoin) could be detected either in the food vacuoles or in the cytoplasm.

The fine structure of Babesia rodhaini is very similar to that of Plasmodium berghei. In both, typical mitochondria are lacking. Instead a structure was found composed of concentric membranes which it is assumed might perform mitochondrial functions. As in Plasmodium berghei a double-membraned vacuole with a matrix of low density is present in almost all sections. The endoplasmic reticulum is represented by small vesicles; the ground-substance is filled with Palade's small particles. The nucleus is large and surrounded by two membranes.

Babesia reproduces by budding and binary fission. Reproduction is not easy to identify since the parasite forms large pseudopods resembling stages in division.

Maria A. Rudzinska

Papers

The Fine Structure of Leishmania donovani and the Role of the Kinetoplast in the Leishmania-Leptomonad Transformation*

Phagotrophy and two new structures in the malaria parasite Plasmodium berghei.

The fine structure and function of the tentacle in tokophrya infusionum

The riboflavin requirement and the effects of acriflavin on the fine structure of the kinetoplast of leishmania tarentolae.

Intracellular phagotrophy in Babesia rodhaini as revealed by electron microscopy.