Cytomembranes in first cleavage xenopus embryos

Summary The ultrastructure and interrelationships of the Golgi body, endoplasmic reticulum and lipid droplets have been studied in the first cleavage Xenopus embryos. Lipid droplets, usually spherical or sometimes multilobed, did not have a discernible limiting membrane, although some had an incomplete electron dense partition. The Golgi bodies and endoplasmic reticulum were seen continuous with lipid droplets and the profiles indicated a probable formation of these membranes from lipid droplet material. Rough endoplasmic reticulum (ER) mainly consisted of paired tubular cisternae and vesicles containing filamentous material that gave a fringed appearance. The relationships of paired cisternae with the Golgi body suggested a transformation of ER membranes into the Golgi body membranes. In addition, paired ER cisternae showed a close apposition with the limiting membrane of the yolk platelet. Lone ER cisternae that contained moderately electron dense material instead of filaments were also present and showed numerous associated vesicles near the Golgi body. The Golgi body showed several morphological forms including a single fenestrated cisterna, two to four flat or cup-shaped cisternae, or up to seven cisternae, some of which were dilated and similar to fringed ER in appearance. These forms could be different developmental stages of the organelle. Coated vesicles were seen continuous with the cisternae of the Golgi body. A probable route for the assembly of the cell surface material has been proposed.This work was supported by a grant from the Medical Research Council of Canada to one of us (E.J.S.).     

The action of cytochalasin B during egg cleavage in Xenopus laevis: dependence on cell membrane permeability

By exposing Xenopus eggs during the first cleavage to cytochalasin B (CCB) for successive periods of 4 min, it has been shown that CCB sensitivity becomes manifest approximately 7 min after the onset of furrow formation. However, even before this time furrow regression can be induced by the injection of CCB under the membrane in the furrow. This shows that during the first 7 min of cleavage the operative contractile system is CCB sensitive. Using microelectrode techniques, electrical membrane characteristics (membrane potential and resistance) were measured continuously in normally cleaving eggs and in cleaving eggs injected with CCB. It was found that the onset of sensitivity to externally applied CCB coincides with a rapid alteration of the membrane potential and resistance. We have concluded that externally applied CCB can only enter the egg when the membrane permeability increases. No evidence has been found that CCB alters the ionic permeability of preexisting cell membrane.     

Development of spherical organelles from the endoplasmic reticulum in the nucellus of some Euphorbia species

Summary The development of characteristic cytoplasmic inclusions from endoplasmic-reticulum (ER) cisternae in nucellar cells of some species of Euphorbia has been studied by electron microscopy. The formation of these organelles is preceded by the appearance of rough ER cisternae filled with an electron-dense material and forming complicated networks. Vesicular structures are formed which grow rapidly to give electron-dense, spherical dilations. On the outer surface of their limiting membrane numerous ribosomes and often polysomes are present. This membrane can be seen to remain continuous with the membranes of one or more cisternae of the rough ER up to when the dilations have a maximum diameter of 2.5–3 . At this time, continuity between the ER cisternae and the spherical dilations ceases. After this the new cell organelles remain unchanged in size, shape, and electron-density until the cell is disintegrated by the growing embryo-sac. The fate of the contents of these organelles is discussed.This work was supported by a grant of the Italian National Research Council (C.N.R.). We acknowledge with appreciation the excellent technical assistance of Mr. Sergio Casini.     

Polysomes and intracisternal accumulations in enucleate sieve elements of rice (Oryza sativa L.)

Polysomes in sieve elements of rice (Oryza sativa L.) were studied with the electron microscope. The polysomes were found on the rough endoplasmic reticulum (ER) present in immature sieve elements and also on the cisternae of aggregated ER in the parietal layer of mature, enucleate sieve elements. In the immature sieve elements the ER cisternae existed as narrow profiles while in the mature sieve elements the ER cisternae were considerably dilated and contained a fibrillar material and, occasionally, electron-opaque inclusions. In addition to the aggregated ER, single profiles of ER were found applied to the lateral walls and also the sieve plates. These cisternae also bore ribosomes and were separated from the plasmalemma by a narrow, dense space. In the mature sieve elements much of the surface of the ER membranes was covered with polysomes. The dimensions of the polysomes are described and the possibility that they contribute to the formation of the fibrillar material in the intracisternal space is discussed.Abbreviations ER endoplasmic reticulum     

NEW MEMBRANE FORMATION DURING CYTOKINESIS IN NORMAL AND CYTOCHALASIN B-TREATED EGGS OF XENOPUS LAEVIS : II. Electrophysiological Observations

The electrical membrane potential (E_m) and electrical membrane resistance (R_m) were measured continuously during the first cleavage of Xenopus eggs, using intracellular microelectrodes. A sharp hyperpolarization of E_m and decrease in R_m can be observed from 6 to 8 min after the onset of cleavage. This moment coincides with the onset of the insertion of new membrane (Bluemink and de Laat, 1973) leading to the formation of the interblastomeric membrane during normal cleavage. Removal of the vitelline membrane or exposure to cytochalasin B (CCB) leads to exposure of the entire surface area of the membrane newly formed during cleavage. These conditions allow for a direct measurement of the permeability properties of the new membrane. It was found that under these conditions E_m reaches values about 3 times more negative and R_m reaches values about 1.5–3 times smaller than during normal cleavage. The extent of reduction of R_m can be correlated with the surface area of the newly formed membrane. We conclude that the new membrane has different ionic permeability properties than the pre-existing membrane (most probably a relatively high permeability for K⁺ ions). Its mean specific resistance is 1–2 kΩ·cm², as against 74 kΩ·cm² for the pre-existing membrane. No influence of CCB on the permeability properties of the pre-existing or new membrane could be detected.     

The ultrastructural differentiation of the endoplasmic reticulum in choroidal epithelial cells of the chick embryo

During the development of the choroidal epithelium in the chick embryo, a substantial concentration of granular endoplasmic reticulum differentiates in the subnuclear cytoplasm of the epithelial cells. The formation of the membranous components of this organelle is preceded by the appearance of a dense, localized population of small, free polyribosomes. Subsequently, numerous membrane-bound vesicles appear in the perinuclear cytoplasm. These primordial ER vesicles measure from 0.1 μ to 0.5 μ or more and they originate from evaginations of the outer nuclear membrane. These vesicles commonly occur in successive rows situated around the margin of the nucleus, and they expand and/or interconnect to form incipient ER tubules. Most vesicles and early tubules are smooth to nearly smooth in appearance. With continued development nuclear evaginations cease, and ER tubules expand in Situ to form an elaborate, laminated system of 7–12 ‘bag-like’ cisternae. Throughout this period of expansive growth, small polyribosomes attach to the developing ER cisternae. As the ER cisternae progressively attain their granular appearance, the number of small, free polyribosomes diminishes. During later stages of development larger polyribosomes appear in association with the subnuclear concentration of ER, and the first accumulations of electron-dense material develop within cisternal spaces.     

The effect of monensin on gametogenesis and zoosporogenesis in the aquatic fungus,Allomyces macrogynus

Summary Cytoplasmic cleavage in the gametangia and zoosporangia ofA. macrogynus was studied using monensin, an ionophore known to disrupt several endomembrane functions in plant and animal cells. Monensin interfered with normal gamete and zoospore formation in a dose dependent manner such that at a 20 M concentration very abnormal cells were released from the reproductive structures. It was evident that monensin's effect was most pronounced during the first 25 minutes of gametogenesis and parallels in time the onset and continuation of the cytoplasmic cleavage events. Observations using fluorescence and differential interference contrast microscopy indicated that the ionophore inhibited normal cytoplasmic cleavage resulting in the production of multinucleate cells, many of which had either no flagella or multiple flagella. Transmission electron microscopy showed that the monensin-treated gametangia had many large vacuoles which contained amorphous electron-opaque material. X-ray microprobe analysis demonstrated that the elemental composition of the large vacuoles was identical to that of the dense globular inclusions seen in untreated gametangia, and morphological analysis confirmed the relationship between these endomembrane structures. Thus this swollen endomembrane component probably is not involved in the cleavage process. Single endomembrane cisternae which were very common in untreated gametangia were seldom seen in monensin-treated preparations. Instead, many smaller electron-transparent vacuoles were observed. These swollen cisternae may both represent monensin-modified Golgi apparatus equivalents and/or play a critical role during the process of gametogenesis and zoosporogenesis inA. macrogynus.     

EVIDENCE FOR RECYCLING OF SYNAPTIC VESICLE MEMBRANE DURING TRANSMITTER RELEASE AT THE FROG NEUROMUSCULAR JUNCTION

When the nerves of isolated frog sartorius muscles were stimulated at 10 Hz, synaptic vesicles in the motor nerve terminals became transiently depleted. This depletion apparently resulted from a redistribution rather than disappearance of synaptic vesicle membrane, since the total amount of membrane comprising these nerve terminals remained constant during stimulation. At 1 min of stimulation, the 30% depletion in synaptic vesicle membrane was nearly balanced by an increase in plasma membrane, suggesting that vesicle membrane rapidly moved to the surface as it might if vesicles released their content of transmitter by exocytosis. After 15 min of stimulation, the 60% depletion of synaptic vesicle membrane was largely balanced by the appearance of numerous irregular membrane-walled cisternae inside the terminals, suggesting that vesicle membrane was retrieved from the surface as cisternae. When muscles were rested after 15 min of stimulation, cisternae disappeared and synaptic vesicles reappeared, suggesting that cisternae divided to form new synaptic vesicles so that the original vesicle membrane was now recycled into new synaptic vesicles. When muscles were soaked in horseradish peroxidase (HRP), this tracerfirst entered the cisternae which formed during stimulation and then entered a large proportion of the synaptic vesicles which reappeared during rest, strengthening the idea that synaptic vesicle membrane added to the surface was retrieved as cisternae which subsequently divided to form new vesicles. When muscles containing HRP in synaptic vesicles were washed to remove extracellular HRP and restimulated, HRP disappeared from vesicles without appearing in the new cisternae formed during the second stimulation, confirming that a one-way recycling of synaptic membrane, from the surface through cisternae to new vesicles, was occurring. Coated vesicles apparently represented the actual mechanism for retrieval of synaptic vesicle membrane from the plasma membrane, because during nerve stimulation they proliferated at regions of the nerve terminals covered by Schwann processes, took up peroxidase, and appeared in various stages of coalescence with cisternae. In contrast, synaptic vesicles did not appear to return directly from the surface to form cisternae, and cisternae themselves never appeared directly connected to the surface. Thus, during stimulation the intracellular compartments of this synapse change shape and take up extracellular protein in a manner which indicates that synaptic vesicle membrane added to the surface during exocytosis is retrieved by coated vesicles and recycled into new synaptic vesicles by way of intermediate cisternae.     

THE FINE STRUCTURE OF THE GALL BLADDER EPITHELIUM OF THE MOUSE

Sections of mouse gall bladder epithelium fixed by perfusion with buffered osmium tetroxide have been studied in the electron microscope as an example of simple columnar epithelium. The free surface presents many microvilli, each presenting a dense tip, the capitulum, and displaying a radiating corona of delicate filaments, the antennulae microvillares. Very small pit-like depressions, representing caveolae intracellulares, are encountered along the cell membrane of the microvilli. The free cell surface between microvilli shows larger cave-like depressions, likewise representing caveolae intracellulares, containing a dense material. The lateral cell borders are extensively folded into pleats, which do not interdigitate extensively with corresponding folds of the adjacent cell membrane. The terminal bars are shown to consist of thickened densities of the cell membrane itself in the region of insertion of the lateral cell wall with the free cell surface. This thickening is associated with an accumulation of dense cytoplasmic material in the immediate vicinity. The terminal bar is thus largely a cytoplasmic and cell membrane structure, rather than being primarily intercellular in nature. The basal cell membrane is relatively straight except for a conical eminence near the center of the cell, projecting slightly into the underlying tunica propria. The basal cell membrane itself is overlain by a delicate limiting membrane, which does not follow the lateral contours of the cell. Unmyelinated intercellular nerve terminals with synaptic vesicles have been encountered between the lateral walls of epithelial cells. A division of the gall bladder epithelial cell into five zones according to Ferner has been found to be convenient for this study. The following cytoplasmic components have been noted, and their distribution and appearance described: dense absorption granules, mitochondria, Golgi or agranular membranes, endoplasmic reticulum or ergastoplasm, ring figures, and irregular dense bodies, perhaps lipoid in nature. The nucleus of these cells is also described.     

Reorganization of the Endoplasmic Reticulum during Meiotic Maturation of the Mouse Oocyte

The endoplasmic reticulum (ER) of live metaphase II mouse eggs and prophase I-arrested oocytes was compared using the fluorescent, lipophilic dicarbocyanine dye, DiI. DiI, dissolved in soybean oil, was microinjected into oocytes and eggs; the dye diffused throughout the cytoplasm to label the ER, which was imaged by confocal microscopy. The mature egg had a fine reticular network of ER throughout the cell and numerous dense accumulations of membrane in the cortex. These ER accumulations, 1-2 μm in diameter, were generally absent deeper in the cytoplasm. A similar staining pattern was observed when the eggs were fixed within 1 min of injection, providing evidence that the cortical accumulations of membrane are part of a continuous ER membrane system, since membrane trafficking could not occur in a fixed egg. Cortical ER accumulations were localized to the same region of the egg as the cortical granules and were not observed in the cortical granule-free region adjacent to the meiotic spindle. In contrast, ER accumulations were rarely found in the cortex of the immature, prophase I-arrested oocyte, but larger and less well-defined membrane clusters were found throughout the deeper cytoplasm of the oocyte. The appearance of ER clusters in the egg cortex following oocyte maturation correlates with an increased ability of the mature egg to release calcium at fertilization. Since the ER is a calcium store, structural reorganization of the ER may be necessary to permit the large release of calcium and resulting cortical granule exocytosis at fertilization.     

Endocytosis in nonciliated epithelial cells of the ductuli efferentes in the rat

The nonciliated cells lining the ductuli efferentes presented three distinct cytoplasmic regions. The apical region contained, in addition to cisternae of endoplasmic reticulum and mitochondria, two distinct membranous elements. The tubulovesicular system consisted of dilated tubules connected to the apical plasma membrane and subjacent distended vesicular profiles. The apical tubules, not connected to the cell surface, consisted of numerous densely stained tubules of small size which contain a compact, finely granulated material. The supranuclear region, in addition to a Golgi apparatus and ER cisternae, contained dilated vacuoles, pale and dense multivesicular bodies, as well as numerous dense granules identified cytochemically as lysosomes. The basal region contained the nucleus and many lipid droplets. The endocytic activity of these cells was investigated using cationic ferritin (CF) and concanavalin-A-ferritin (Con-A-ferritin) as markers of adsorptive endocytosis; and native ferritin (NF), concanavalin-A-ferritin in the presence of alpha-methyl mannoside, and horseradish peroxidase or albumin bound to colloidal gold for demonstrating fluid-phase endocytosis. These tracers were injected separately into the rete testis, and animals were sacrificed at various time intervals after injection. At 1 min, CF or Con-A-ferritin were seen bound to the apical plasma membrane, to the membrane of microvilli, and to the membrane delimiting elements of the tubulovesicular system. Between 2 and 5 min, these tracers accumulated in the densely stained apical tubules and at 15 min in the dilated vacuoles. Between 30 min and 1 hr, the tracers appeared in multivesicular bodies of progressively increasing density, whereas at 2 hr and later time intervals, many dense lysosomal elements became labeled. The tracers for fluid-phase endocytosis showed a distribution similar to that for CF or Con-A-ferritin except that they did not bind to the apical plasma membrane, microvilli, or membrane delimiting the tubulovesicular system. At no time interval were any of the tracers observed in the abluminal spaces. Thus, the nonciliated epithelial cells of the ductuli efferentes are actively involved in fluid-phase and adsorptive endocytosis, both of which result in the sequestration of endocytosed material within the lysosomal apparatus of the cell.     

ELECTRON MICROSCOPY OF ORAL CELLS IN VITRO : II. Subsurface and Intracytoplasmic Confronting Cisternae in Strain KB Cells

Two special areas involving membranous components in strain KB cells were studied by electron microscopy. The first area described is that of the subsurface regions of two apposing cells in which flattened cisternae (one cisternae in each subsurface region) with membranes spaced 110–230 A apart were found in a confrontation alignment. The long dimension of the profiles of these cisternae ranges from 0.5 to 2 µ. At these intercellular contact areas, each cisterna is closely applied to the adjacent plasma membrane; the intervening space is 60–100 A. We have named the cisternae in these roughly symmetrical areas of cell contact the subsurface confronting cisternae. Communications between these cisternae and those of the rough-surfaced endoplasmic reticulum also were observed. The second area described is that of the intracytoplasmic confronting cisternae. These cisternae were observed as oval or round images about 0.3–1.4 µ in diameter, each image being composed of a pair of concentrically arranged confronting cisternae with membranes spaced 200–400 A apart. The apposing membranes of the two confronting cisternae are electron opaque, smooth, and free of ribosomes, whereas the unapposed membranes are less dense, scalloped, and associated with ribosomes. The spacing between the two intracytoplasmic confronting cisternae is 70–110 A.     

Nuage material and the origin of dense-core vesicles in oocytes of Nassarius reticulatus (Mollusca,Gastropoda)

Summary

In late previtellogenic oocytes nuage material accumulates in the vicinity of the nucleus and is often seen to be intimately associated with cisternae of endoplasmic reticulum (ER). A similar association observed in Ilyanassa has given rise to the proposition that nuage granules become completely enclosed in an envelope of ER and that this structure is transformed into so-called double-membrane vesicles, organelles which have only been found in eggs of Ilyanassa and Nassarius. This study provides evidence that, in Nassarius, the association of nuage material with ER has a temporary character. At later stages the nuage granules dissociate from the ER, move away from the nucleus, and become surrounded by mitochondria. Eventually they disintegrate. Evidence is presented that double-membrane vesicles originate from cisternae of ER by the accumulation and transformation of material within the lumen of the cisternae. Since only a single membrane is present in these vesicles, and a dense core is consistently found if the appropriate fixation is employed, I suggest that these vesicles be called dense-core vesicles instead of double-membrane vesicles.     

Quantitative cellular analysis of growth and reproduction in freshwater planarians (Turbellaria; Tricladida). I. A cellular description of the intact organism

Summary

Changes in egg shape and surface morphology during maturation may be related to the localization of cytoplasmic determinants in the embryos of organisms with spiral cleavage. The eggs of the polyclad turbellarian Hoploplana inquilino undergo pronounced shape changes during the meiotic divisions which have been examined with the scanning electron microscope. Unfertilized eggs have a smooth surface that becomes covered with microvilli and microblebs within 10 min of fertilization. First polar body extrusion is accompanied by the asymmetric appearance of large blebs (Blebbing Cycle I) primarily in the animal hemisphere with one quadrant characteristically smoother than the others and bearing fewer blebs. Blebbing Cycle II, which is less pronounced than the first but is still characterized by a relatively un blebbed quadrant of the zygote, coincides with second polar body formation. These asymmetric shape changes in the animal hemisphere during meiosis may possibly correlate with a primitive form of morphogenetic segregation and beginning quadrant specialization in polyclads, the most primitive spiralians with mosaic development.     

FERTILIZATION IN OEDOGONIUM. III. KARYOGAMY

Karyogamy is described in Oedogonium cardiacum from ultrastructural studies. Close proximity of the two gamete nuclei in the fusion cell is established by plasmogamy, whereas karyogamy appears to be initiated by multiple contacts formed between the outer membranes of the adjoining nuclear envelopes. Blebs of endoplasmic reticulum (ER) originate from the outer membrane of each nuclear envelope; these ER blebs presumably contact and fuse with the outer membrane of the nuclear envelope of the opposing nucleus. This is followed by the fusion of the inner membranes of the opposing nuclear envelopes, thereby resulting in a series of small connective bridges between the two gamete nuclei. It is estimated that in this manner 30–50 bridges are formed, perhaps many more. Several of these bridges enlarge relative to the others; one presumably becomes the major connection between the fusing nuclei. As it continues to enlarge, any organelles positioned between the fusing nuclei are pushed aside. There is also evidence, particularly in later stages of karyogamy, that the smaller connective bridges fuse to form larger ones. Temporary cytoplasmic channels often result at the juncture of fusion. In other instances, isolated inclusions of cytoplasm may be delimited by remnants of nuclear envelope deep within the developing zygote nucleus; these inclusions disappear with subsequent development. Throughout karyogamy the contribution of the male gamete nucleus is readily recognized by the characteristic appearance of its highly condensed chromatin. Ultimately, however, this distinction is lost and the content of the mature zygote nucleus assumes a more uniform appearance very similar to that of an egg nucleus. The complete process of fertilization in Oedogonium may occur within 15 min of mixing the spermatozoids with eggs.     

ONTOGENY,HISTOCHEMISTRY AND FINE STRUCTURE OF CELLULAR INCLUSIONS IN VEGETATIVE CELLS OF ANTITHAMNION DEFECTUM (CERAMIACEAE,RHODOPHYTA)1

The ultrastructure and histochemistry of developing and mature cell inclusions in vegetative cells of Antithamnion defectum Kylin were examined. Those studied were chloroplast inclusions, cytoplasmic crystals and spherical bodies within the vacuole. Chloroplasts of mature vegetative cells contain an interthylakoidal, apparently noncrystalline deposit of undetermined chemical identity. The bodies are parallel to the long axis of the plastid, are square (0.13 μm) in cross-section, and up to 3 μm long. Spherical vacuolar bodies (0.5–1.5 μum diam) are formed during early stages of vacuole formation by accumulation of protein deposits in swelling endoplasmic reticulum (ER) cisternae. Swelling of smooth ER contiguous to the ER containing the deposits results in the vacuole enclosing the spherical bodies. In mature cells, vesicles appear to be secreted into the preformed vacuole. Cytoplasmic proteinaceous crystalloids develop without a bounding membrane and may serve as protein reserves.     

Evolution of the endoplasmic reticulum during spermiogenesis of the rooster: an electron microscopic study

The endoplasmic reticulum (ER) of rooster's spermatids was analyzed during spermiogenesis, which was subdivided into eight distinct steps on the basis of changes observed with the electron microscope in the nucleus, acrosome-perforatorium system, manchette, and flagellum. In steps 1 and 2, spermatids' ER cisternae presented the following specializations: A loose network of tubular cisternae was distributed throughout the cytoplasm. Six to eight tight networks of anastomosed tubular cisternae parallel to each other were closely stacked to form a discoid body (1.5-2.5 microns in diameter and 0.5-0.8-micron thick) in which spheroidal vesicles (0.4 micron in diameter) were inserted. Close to and connected with this body, called the alveolar body, there was a stack of annulate lamellae. Large, flattened ER cisternae were seen singly or in piles of two or three running parallel to the nuclear surface. A collection of tubular ER cisternae faced plaques of thickened plasma membranes. These elements of the ER system appear continuous with each other. During steps 3-5 of spermiogenesis, no modification of the alveolar body-annulate lamellae complex was noted; the large flattened ER cisternae disappeared, however, and the broad network of tubular cisternae developed markedly. During steps 6 and 7, the latter network of tubular cisternae fragmented into vesicles that swelled to give a vacuolated appearance to the cytoplasm. The alveolar body-annulate lamellae complex remained visible until late step 7, when it disintegrated just before spermiation. Thus the system of ER cisternae underwent marked structural modifications during spermiogenesis.     

CHANGES IN THE RIBOSOME CONTENT, PRINCIPAL MICROSOMAL PROTEIN COMPOSITION, AND SECRETORY CHARACTER OF MAMMARY EPITHELIAL ROUGH ENDOPLASMIC RETICULUM DURING DIFFERENTIATION : Evidence that Messenger RNAs Specific for Milk Proteins are Incorporated into Rough Endoplasmic Reticulum Formed De Novo after Parturition

The equilibrium density distribution, protein composition, and secretory character of mouse mammary epithelial rough microsomes have been determined during differentiation. The density range exhibited by the rough microsomes broadens during mammary development; rough microsomes within the 1.25–1.29 g/ml density range appear soon after conception and then within the 1.30–1.34 range after the onset of lactation. The appearance of these denser microsomes represents the progressive increase of the average ribosome content of the rough endoplasmic reticulum (ER) during gestation and lactation. Fractionation of rough microsomal proteins by sodium dodecyl sulfate-polyacrylamide gel electrophoresis reveals that two proteins, having molecular weights of 57,000 and 76,000, occur to a significant extent only during lactation and are then most prominent in the very dense rough microsomes of the 1.30–1.34 range. Nascent polypeptide chains discharged (by incubation with puromycin) from 17-days lactation rough microsomes in either the 1.21–1.29 or 1.30–1.34 density range are distributed equally between the intra- and extravesicular compartments. Whereas 36% of the chains are discharged intravesicularly from 1-day lactation rough microsomes in the 1.30–1.34 range, only 25% are so discharged from those in the 1.21–1.29 range. The results indicate (a) that there is no correlation between the relative levels in lactation rough microsomes of the two microsomal proteins which become prominent during lactation and the extent of secretory activity and (b) that for a short period after parturition the rough ER elements bearing high surface densities of ribosomes have a greater proportion of ribosomes synthesizing milk proteins than the rough ER elements with moderate ribosome densities.     

CLEAVAGE FURROW FORMATION IN A TELOLECITHAL EGG (LOLIGO PEALII) : I. Filaments in Early Furrow Formation

Formation of the first cleavage furrow in the telolecithal egg of Loligo was studied with the electron microscope. Before the actual furrow forms, a dense filamentous band develops below the plasma membrane from membrane-bounded dense bodies which appear to be Golgi-derived. The egg surface is thrown into a number of longitudinal folds which parallel the furrow and eventually become incorporated into it. These longitudinal folds contain a network of tubules and vesicles. Frequently, multivesiculate bodies are associated with the furrow and possibly give rise to the network of tubules and vesicles. Apparently part of the membrane between the two new blastomeres is derived from the surface of the longitudinal folds. The theory of furrow formation by contraction is discussed in light of the filamentous band.