Robert Edward Lee

Interactions between entorhinal axons and target hippocampal neurons: A role for glutamate in the development of hippocampal circuitry

A coculture system consisting of input axons from entorhinal cortex explants and target hippocampal pyramidal neurons was used to demonstrate that glutamate, released spontaneously from afferent axons, can influence both dendritic geometry of target neurons and formation of presumptive synaptic sites. Dendritic outgrowth was reduced in hippocampal neurons growing on entorhinal axons when compared with neurons growing off the axons. Presumptive presynaptic sites were observed in association with hippocampal neuron dendrites and somas. HPLC analysis showed that glutamate was released from the explants in an activity- and Ca2(+)-dependent manner. The general glutamate receptor antagonist D-glutamylglycine significantly increased dendritic outgrowth in pyramidal neurons associated with entorhinal axons and reduced presumptive presynaptic sites. Tetrodotoxin and reduction of extracellular Ca2+ also promoted dendritic outgrowth and reduced the formation of presumptive synaptic sites. The results suggest that the neurotransmitter glutamate may play important roles in the development of hippocampal circuitry.

AN ULTRASTRUCTURAL SURVEY OF CRYPTOMONAD PERIPLASTS USING QUICK-FREEZING FREEZE FRACTURE TECHNIQUES

A quick‐freezing technique for freeze fracturing was used to determine periplast plate types in 20 cryptomonads. With this technique cells are frozen so rapidly that major artifacts are eliminated. We propose that periplast plates are attached to the cell membrane by intramembrane particles (IMPs), consequently plate shapes are outlined by IMP distribution in fractured membranes. Round to oval, sometimes slightly angular, plates occur in Cryptomonas ovata, Cryptomonas tetrapyrenoidosa, Cryptomonas parapyrenoidifera, Cryptomonas obovata, Cryptomonas erosa and two unidentified species of Cryptomonas; large rectangular plates occur in Chroomonas pochmannii, Chroomonas coerulea and Hemiselmis sp.; small rectangular plates were found in Cryptomonas sp. (Strain SDB); square to slightly rounded plates occur in Cryptomonas chrysoidea and a single continuous plate or sheet, perforated by ejectisome pores, was observed in Cryptomonas caudata, Cryptomonas rostratiformis, Cryptomonas marssonii, Cryptomonas platyuris, Cryptomonas curvata, Cryptomonas ozolini, Chilomonas paramecium and Rhodomonas sp. Oval and square plates are described for the first time in Cryptomonas. Plate IMPs may be morphologically modified in size and shape, depending upon their location in relation to the plate, the plate ridges, and ejectisome chambers. Conformational changes in plate shapes, to form hexagons or polygons, may be induced when cells are subjected to fixation, desiccation, cryoprotectants or centrifugation.

Cell form and surface patterns in Chroomonas and Cryptomonas cells (Cryptophyta) as revealed by scanning electron microscopy

Fifteen freshwater cryptomonad species were freeze‐dried and examined with the scanning electron microscope. Surveys of cell surfaces revealed four general cell types. Chroomonas type cells lack a furrow but possess a shallow vestibular depression where the flagella are inserted. The presence of a gullet could not be detected. Cryptomonas spp. displayed three morphological types, all lacking gullets. The first type of Cryptomonas has a simple, shallow furrow with ridges that apparently can close to form a raphe but an oval opening or stoma remains at the posterior end and an opening from the vestibulum is formed at the anterior end. The second Cryptomonas type consists of a complex furrow with furrow ridges and folds that extend almost two‐thirds of the cell length. A sloma is present in the central region of the closed furrow. The folds apparently can separate thereby exposing the underlying furrow. The third type of Cryptomonas possesses a simple, non‐closing furrow. At the anterior end there is a vestibular ligule which extends from the dorsalleft side of the cell and covers the region of the vestibulum where the contractile vacuole discharges.

Self-Recognition: A Constraint on the Formation of Electrical Coupling in Neurons

Electrical coupling between specific neurons is important for proper function of many neuronal circuits. Identified cultured neurons from the snail Helisoma show a strong correlation between electrical coupling and presence of gap junction plaques in freeze-fracture replicas. Gap junction plaques, however, were never seen between overlapping neurites from a single neuron, even though those same neurites formed gap junctions with neurites from another essentially identical identified neuron. This observation suggests that a form of self-recognition inhibits reflexive gap junction formation between sibling neurites. When one or both of those growth cones had been physically isolated from the neuronal cell body, both electrical coupling and gap junction plaques, between growth cones from the same neuron, were observed to form rapidly (within 30 min). Thus, inhibition of electrical coupling between sibling neurites apparently depends on cytoplasmic continuity between neurites, and not the molecular composition of neurite membrane. The formation of gap junctions is not likely due to the isolation process; rather, the physical isolation appears to release an inhibition of reflexive gap junction formation. These data demonstrate the existence of a previously unknown constraint on the formation of electrical synapses.

KATABLEPHARIS OVALIS, A COLORLESS FLAGELLATE WITH INTERESTING CYTOLOGICAL CHARACTERISTICS1

Katablepharis ovalis Skuja, isolated from an impoundment in Colorado, has a cell covering composed of two layers over the cell body and flagella. The outer component of the cell covering contains 25‐nm‐diameter hexagonal scales arranged in rows. The inner component of the cell covering is composed of a layer of interwoven microfibrils. The inner component of the cell covering is joined to the plasma membrane by one or more attachment strips that always occur outside, and along, one of the microtubular groups of the outer array. The attachment strips resemble hemidesmosomes and are composed of rows of electron‐dense material, 12 nm apart, that protrude through the plasma membrane into the extracellular space, to attach to the inner wall. The two flagella are inserted subapically into a raised area of the cell. The flagella do not have any fibrillar or tubular hairs and are covered only by the two‐layered cell covering. The cell has an inner and outer array of microtubules, both of which are spindle‐shaped, arising at the anterior end of the cell and continuing into the posterior end of the cell. A single large Golgi apparatus occurs in the anterior cytoplasm. The nucleus is in the center of the cell. Two rows of large ejectisomes occur posterior to the area of flagellar attachment. Smaller ejectisomes occur under the plasma membrane in the posterior and medial areas of the cell. Each ejectisome is composed of a single body containing a spirally wound, tapered ribbon. On discharge, the ejectisome ribbon rolls inward, creating a tubular structure. The possible relationship between Katablepharis, the green algae, and the cryptophytes is discussed.

Ancient atmospheric CO2 and the timing of evolution of secondary endosymbioses

Abstract We postulate that the evolutionary success of those diverse algal groups characterized by secondary endosymbiotic origin of plastids was directly connected to changing atmospheric CO2 in the late Palaeozoic. Atmospheric CO2 levels were reduced to historic lows, probably lower than todays atmosphere, during the Pennsylvanian and Permian periods. This most likely resulted in dissolved inorganic carbon (DIC) becoming the limiting factor in photosynthesis in these ancient oceans. Fossil and molecular data suggest that a number of algal groups with secondary endosymbioses evolved between 260 and 285 Ma during the minimum in atmospheric CO2. It has been hypothesized that these algae were able to more efficiently utilize DIC because their chloroplasts were contained within an acidic compartment where DIC was largely in the form of CO2. In this paper we postulate that secondary endosymbioses arose continuously from the time of evolution of the chloroplast (about 2000 Ma). However, these secondary endosymbioses were quickly eliminated because they possessed no selective advantage over existing phytoplankton in waters high in DIC. It was not until the ancient atmospheric CO2 minimum that secondary endosymbioses were selected for, because these algae were able to utilize the low DIC more efficiently and outcompete existing algae. Under these favourable conditions, a number of secondary endosymbioses evolved and survived, and it is their ancestors that constitute most of the eukaryotic phytoplankton in todays oceans.

FEEDING APPARATUS OF THE COLORLESS FLAGELLATE KATABLEPHARIS (CRYPTOPHYCEAE)1

The feeding apparatus of Kalablepharis ovalis (isolated from a freshwater impoundment in Colorado) and Katablepharis clone G‐2 (isolated from the littoral of the Black Sea near Yalta in the Crimea) consists of inner and outer oval‐shaped arrays of microtubules that begin at the anterior end of the cell and pass into the posterior of the cell. Each array of microtubules contains groups of microtubules with two to eight microtubules per group depending on the position of the array in the cell. A specialized area of the plasma membrane, the mouth, occurs at the anterior end of the cell. The mouth is oval with the long axis oriented dorsoventrally and consists of a raised ridge surrounding a central depression. The anterior end of the microtubules of the inner and outer arrays supports the raised ridge of the mouth. In freeze‐fracture replicas, the protoplasmic face of the plasma membrane contains intramembrane particles on the raised ridge of the mouth. Three small membrane‐cisternae occur on the protoplasmic side of the plasma membrane in the area of the mouth. Katablepharis clone G‐2 also has five or six additional large membrane‐cisternae associated with the inner microtubular array in the anterior portion of the cell. These larger membrane‐cisternae do not occur in K. ovalis. Vesicles with electron‐dense contents occur in association with the microtubular arrays. Katablepharis ovalis has a second type of vesicle containing a single‐membrane profile associated with the microtubule arrays. The structure of the microtubular arrays in Katablepharis is compared with similar structures in suctorian ciliates and dinoflagellates.

ULTRASTRUCTURAL VARIATIONS IN CRYPTOMONAD FLAGELLA1

The arrangement of flagellar appendages in 19 cryptomonad species was examined and four new flagellar types are described. The first new type has a single row of mastigonemes on both flagella and hairs on the side opposite the mastigonemes. The second type, which is common, has unilateral rows of mastigonemes on both flagella, but no hairs. A third type has an acronematic short flagellum and a single row of mastigonemes on the long flagellum. A fourth type lacks mastigonemes but has a unilateral row of curved “spikes” on the short flagellum and hairs on both flagella. These additional flagellar variations may contribute to a more natural system of classification for cryptomonads.

Ultrastructure of fertilization in a cryptomonad

By means of various electron microscopic techniques, the ultrastructure of fusing gametes in a cryptomonad is described for the first time. The isolate used in this study is bisexual, and vegetative cells may act as isogametes. Plasmogamy usually is initiated at the posterior end of one gamete and the mild‐ventral region of the other gamete. A posterior, pointed protuberance may be a specialized mating structure which initiates the fusion process. Fusion proceeds toward the anterior end, forming a quadriflagellate cell which becomes spherical and settles to the bottom of the culture flask. The quadriflagellate, spherical cell contains two nuclear‐nucleomorph‐chloroplast complexes which remain intact throughout karyogamy. During karyogamy the nuclei are positioned close to each other and become lobed on the sides where fusion takes place. At the points where the lobes touch, the nuclear membranes break down and direct karyogamy is initiated. Nuclear fusion continues and eventually a single zygotic nucleus is formed. The zygote nucleus and the two nucleomorphs and chloroplasts become enclosed in a common periplastidial compartment. The nucleomorphs, however, remain apart and do not fuse. Meiosis presumably is zygotic, but the stages of post‐karyogamy remain to be elucidated.

ULTRASTRUCTURE AND SYSTEMATICS OF TWO NEW FRESHWATER RED CRYPTOMONADS, STOREATULA RHINOSA, SP. NOV. AND PYRENOMONAS OVALIS, SP. NOV.

The ultrastructure and systematics of two red colored freshwater cryptomonads, Storeatula rhinosa, sp. nov. and Pyrenomonas ovalis, sp. nov., are described for the first time. Storeatula, which had been described from marine waters only, has a single inner periplast sheet and a fibrous surface periplast component. Cells lack a furrow but possess a gullet, a bilobed chloroplast connected by a pyrenoid and a nucleomorph located in an indentation of the pyrenoid. This freshwater Storeatula possesses the same general features as the marine species, but it has a contractile vacuole and lacks the lobed chloroplast of S. major. P. ovalis has the generic characteristics described for marine species of Rhodomonas. These characteristics include a short furrow, a deep gullet, square inner periplast plates with beveled corners, a slightly fibrillar surface periplast component, a single chloroplast with two lobes connected by a pyrenoidal bridge and a nucleomorph located in an indentation of the pyrenoid.