The cytology of the testaceous rhizopod Lesquereusia spiralis (Ehrenberg) Penard. I. Ultrastructure and shell formation

Ultrastructure and shell formation in the testaceous ameba, Lesquereusia spiralis, were investigated with both scanning and transmission electron microscopy and X-ray microanalysis. The nucleus, surrounded by a fibrous lamina, contains multiple nucleoli. The cytoplasm, containing a well developed granular endoplasmic reticulum, also contains remnants of starch granules in stages of digestion. Spherical aggregates of ribosome-like particles may be seen. Golgi complexes seem to produce both a nonordered fibrous material and an electron dense vesicle. Only the latter appears to bleb off from the Golgi complex. X-ray microanalysis demonstration of silicon in Golgi vesicles and in some dense vesicles suggests that the fibrous component of the cisternae may take up and concentrate silica to form the electron-dense component of the vesicles. Membrane-bound siliceous crystals are often seen adjacent to the Golgi, suggesting either a Golgi origin or platelet formation in vesicles after release from the Golgi complex. Both electron-dense bodies and siliceous platelets are released from the cell by a process similar to apocrine secretion and may be seen outside the cell in route to the shell during shell morphogenesis. Shell development involves fusion of electron-dense bodies to form a matrix, positioning of siliceous platelets in this matrix parallel to the shell surface, and development of a system of matrix chambers. A particulate glycoconjugate is released to the shell surface upon rupture of the matrix chamber.     

Fine Structure of the Shell Wall in the Soil Testate Amoeba Cyclopyxis kahli (Rhizopoda)1

The common soil testate amoeba, Cyclopyxis kahli, is capable of building a complete organic shell when grown in culture in the absence of agglutinate material. Such shells are morphologically similar to agglutinate shells except in the structure and diameter of the aperture. Chemical analysis of organic shells suggests that the main inorganic elements present are calcium and iron. Comparisons are made with similar analyses of organic shells from other agglutinate species. This is the first report of a significant change in shell architecture due to agglutinate material.     

An ultrastructural study on spicule formation in the pennatulid colony Renilla reniformis

Examination of the coenenchyme tissues of Renilla reniformis revealed two regions of crystal formation: the endoderm containing small oval deposits of unknown composition and the mesoglea containing larger elongated spicules composed of calcite. Spicule formation takes place intracellularly in scleroblasts and may be explained by the following sequential processes: an organic matrix consisting of a homogeneous ground substance and fibers is formed in a large vacuole. Calcite needles 0.4 μ in diameter develop in close association with matrix fibers and vesicles, and grow to form the central core of the spicule. Large electron dense bodies dominate the scleroblast cytoplasm during these early stages of growth. In a later stage, smaller needles 0.2 μ in diameter develop surrounding the core to form the distal lobes of the spicule. ‘Lollipop’-shaped vesicles containing fibers appear in the scleroblast cytoplasm at the onset of this stage. This material is released at the calcification front and presumably is incorporated into the spicule as an organic matrix of crystals in the distal lobes.     

Studies on the acrosome. X. Differentiation of the starfish acrosome

The course of acrosomal differentiation observed during spermiogenesis in two starfishes shows that the central components of the mature acrosome are produced by Golgi activity. In the early spermatid, small Golgi-derived vesicles enter the hydrated acrosomal mass and appear to contribute their membrane constituents to the acrosomal-membrane precursor elements. A single lamella of smooth endoplasmic reticulum and fine-fibrillar material associated with it surround the membraneprecursor complex. In a drastic reorganization by which the spermatid acquires antero-posterior symmetry, the acrosome becomes embedded in the anterior part of the nucleus directly beneath the plasma membrane. All the other organelles congregate in the posterior cytoplasm; a thin layer of cytoplasm persists around the sides of the nucleus. During late spermiogenesis two additional acrosomal components become increasingly conspicuous. One is the layer of fine-fibrillar material associated with the smooth endoplasmic reticular vesicles surrounding the Golgi-derived elements. This material is finally pushed towards the center of the sperm head by a late accretion of fibrous product which appears to be synthesized throughout spermiogenesis by the ribosomes, and accumulates around the anterior part of the acrosome as the cytoplasmic matrix diminishes.     

Ultrastructural localization of spicule matrix proteins in normal and metalloproteinase inhibitor-treated sea urchin primary mesenchyme cells

Studies of the sea urchin larval skeleton have contributed greatly to our understanding of the process of biomineralization. In this study we have undertaken an investigation of the morphology of skeleton formation and the localization of proteins involved in the process of spicule formation at the electron microscope level. Sea urchin primary mesenchyme cells undergo a number of morphological changes as they synthesize the larval skeleton. They form a large spicule compartment that surrounds the growing spicule and, as spicule formation comes to an end, the density of the cytoplasm decreases. Inhibition of spicule formation by specific matrix metalloproteinase inhibitors or serum deprivation has some subtle effects on the morphology of cells and causes the accumulation of specific classes of vesicles. We have localized proteins of the organic matrix of the spicule and found that one protein, SM30, is localized to the Golgi apparatus and transport vesicles in the cytoplasm as well as throughout the occluded protein matrix of the spicule itself. This localization suggests that SM30 is an important structural protein in the spicule. Another spicule matrix protein, SM50, has a similar cytoplasmic localization, but in the spicule much of it is localized at the periphery of the spicule compartment, and consequently it may play a role in the assembly of new material onto the growing spicule or in the maintenance of the integrity of the matrix surrounding the spicule.     

Microstructure of matrix and mineral components of eggshells from White Leghorn chickens (Gallus gallus)

The avian eggshell is a composite structure of organic matrix and mineral (calcium carbonate) that is rapidly and sequentially fabricated in the oviduct in <24 hr. The eggshell is an excellent vehicle for the study of biomineralization processes and the role of the organic matrix in the mineral-matrix composite. The organic matrix components of eggshells from White Leghorn chickens (Gallus gallus) were examined by transmission electron microscopy (TEM) and optical microscopy. The mineral phase was analyzed by TEM, scanning electron microscopy (SEM), X-ray compositional microanalysis, and electron diffraction. Ultrastructural examination of the matrices within the calcified eggshell reveals a complex architecture that differs within each of the major zones of the eggshell: the shell membranes, the mammillary zone, the palisade region, and the cuticle. The mammillary layer consists of the calcium reserve assembly (CRA) and crown region, each with a unique substructure. TEM images show that the matrix of the CRA consists of a dense, flocculent material partially embedded within the outer shell membrane (a mostly noncalcified region of the shell). The mantle of the collagen fibers of the shell membranes is rich in polyanions (cuprolinic blue-positive), as is the CRA matrix. The CRA is capped by a centrally located calcium reserve body sac (CRB sac) that contains numerous 300–400 nm, electron-dense, spherical vesicles. Directly above the CRB sac is a zone of matrix consisting of stacks of interconnected vesicles (similar in morphology to CRA vesicles) that are interspersed with a granular material. The palisade region, the largest of the mineralized zones, contains hollow vesicles ∼450 nm (s.d. = 75 nm) in diameter, with a crescent-shaped, electron-dense fringe. An interconnecting matrix material is also found between the vesicles in the palisades region. The cuticle is composed of two layers, a mineralized inner layer and an outer layer consisting of only organic matrix. The bulk of the mineral within the eggshell is calcite, with small amounts of needlelike hydroxyapatite in the inner cuticle and occasionally, vaterite micro crystals found at the base of the palisade (cone) region. The well-crystallized calcite crystals within the palisade are columnar, typically ∼20 μm wide by 100–200 μm long; aside from numerous entrapped vesicles and occasional dislocations, they are relatively defect-free. The bulk of the matrix found in the palisade and crown regions are thought to be residual components of the rapid mineralization process. The unique matrix structure within the CRB corresponds to the region of preferentially solubilized calcite used by the developing embryo and the hydroxyapatite found in the inner cuticle may play a role in the cessation of mineral growth. © 1996 Wiley-Liss, Inc.     

Purification and characterization of a novel calcium-binding protein from the extrapallial fluid of the mollusc, Mytilus edulis

In the bivalve mollusc Mytilus edulis shell thickening occurs from the extrapallial (EP) fluid wherein secreted shell matrix macromolecules are thought to self-assemble into a framework that regulates the growth of CaCO(3) crystals, which eventually constitute approximately 95% of the mature shell. Herein is the initial report on the purification and characterization of a novel EP fluid glycoprotein, which is likely a building block of the shell-soluble organic matrix. This primary EP fluid protein comprises 56% of the total protein in the fluid and is shown to be a dimer of 28,340 Da monomers estimated to be 14.3% by weight carbohydrate. The protein is acidic (pI = 4.43) and rich in histidine content (11.14%) as well as in Asx and Glx residues (25.15% total). The N terminus exhibits an unusual repeat sequence of histidine and aspartate residues that occur in pairs: NPVDDHHDDHHDAPIVEHHD approximately. Ultracentrifugation and polyacrylamide gel electrophoresis demonstrate that the protein binds calcium and in so doing assembles into a series of higher order protomers, which appear to have extended structures. Circular dichroism shows that the protein-calcium binding/protomer formation is coupled to a significant rearrangement in the protein's secondary structure in which there is a major reduction in beta-sheet with an associated increase in alpha-helical content of the protein. A model for shell organic matrix self-assembly is proposed.     

Initiation and ontogeny of vesicles in cultured Frankia sp. strain HFPArI3.

Removal of combined nitrogen from the medium of Frankia sp. strain HFPArI3 induced the formation of specialized structures, called vesicles, which are the proposed site of nitrogen fixation. After 5 to 6 h of culture on N-free medium, newly formed vesicles, termed provesicles, arose from the tips of some hyphae. These structures were spherical, phase dark, ca. 1.5 to 2.0 micron in diameter, and were not associated with acetylene reduction (nitrogenase) activity. Provesicles reached their greatest frequency after ca. 24 h of N-free culture. Provesicles increased in size to become mature vesicles which first appeared after 18 to 20 h of N-free culture. They were ca. 2.5 micron in diameter, phase bright, and reached their greatest frequency after 5 to 6 days, at which time nitrogenase activity peaked. Some vesicles eventually became damaged structurally and took on the appearance of ghosts. Transmission electron micrographs revealed an increase in size from provesicle to mature vesicle. Also evident with the micrographs were the presence of a septum between the young provesicle and parental hypha, the presence of glycogen in some young vesicles, the development of internal septations as vesicles matured, and the degradation of cytoplasm and internal septae in ghost vesicles. The extent to which the formation of vesicles is reversible by the addition of NH4+ was investigated. Commitment times of 3.2 and 6.5 h were obtained for provesicles and vesicles, respectively. A concentration-dependent inhibition of nitrogenase by NH4+ was demonstrated. The structure of preexisting vesicles was also affected by addition of NH4+ to the culture medium.     

Ultrastructural studies of a vesicle system associated with endoplasmic reticulum in exo-erythrocytic forms of Plasmodium berghei

Fine structural studies of a specialized vesicle system associated with the endoplasmic reticulum (ER) of exo-erythrocytic Plasmodium berghei suggest that this system may be the equivalent of a Golgi apparatus. Patches of ER, randomly distributed in the cytoplasm of developing parasites, are formed of smooth and ribosome-studded cisternae intermingled with each other. The vesicle systems are located between as well as at the edges of ER aggregates and appear to be in different stages of budding from the cisternae. Prolonged osmication reveals distinct staining of the nuclear envelope and ER of the parasites as well as part of the Golgi apparatus of the hepatocytes. However, the small vesicles associated with the parasite's ER are unstained, as are the coated vesicles in the Golgi region of the liver cell. These sites in the parasite cytoplasm seem comparable to the concave surface of the Golgi apparatus in liver cells. The pinched-off vesicles fuse with others to form the prominent peripheral vacuolization characteristic of the nearly mature exoerythrocytic form. The formation of these peripheral vacuoles and their subsequent fusion with the parasite membrane may be an exocytosis mechanism supplying the rapidly expanding parasite with new plasma membrane material.     

Ultrastructure of Oogenesis in Dryopteris crassirhizoma Nakai

The ultrastructure ofoogenesis in Dryopteris crassirhizoma Nakai has been investigated using transmission electron microscopy. The nucleus in the young egg is rounded with an uneven outline. As it develops, it becomes amoeboid and extends nuclear protrusions that are not only sac-like nuclear evaginations like those often seen in the oogenesis of other ferns, but also mushroom-like and finger-like, with an opening at their end allowing the nucleolus material to flow out from the openings. This has not been observed previously. The nuclear protrusions differ from Dryopterisfilix-mas (L.) Schott. in the absence of sheets of nuclear membrane in the form of a closed ring. As the egg matures, the nucleus transforms into a tuber-like structure with a smooth surface, lying transversely in the egg cell. In the immature egg, vesicles almost encircle the nucleus twice and are most remarkable. In the maturing egg, the vesicles are distributed at the periphery, except for at the top of the egg, and affect the formation of the separation cavity and extra egg membrane. Simultaneously, vesicles from the venter canal cell move to the egg and take part in the formation of separation cavity and extra egg membrane. In the mature egg, a large number of small vesicles containing fragments of lamellae or osmiophilic material emerge from the cytoplasm. The origin of these vesicles is obscure. Irregular plastids containing a cylindrical starch grain dedifferentiated progressively.Mitochondria seem to have been undeveloped during the process, but return to normal at later stages of oogenesis. There is a high frequency of ribosomes in the mature egg. Microtubules, rarely seen in the eggs of D.filix-mas (L.) Schott. and Pteridium aquilinum (L.) Kuhn, have been observed inside the plasmalemma of the maturing egg in D. crassirhizoma.     

Ultrastructure of Oogenesis in Dryopteris crassirhizoma Nakai

The ultrastructure of oogenesis in Dryopteris crassirhizoma Nakai has been investigated using transmission electron microscopy. The nucleus in the young egg is rounded with an uneven outline. As it develops, it becomes amoeboid and extends nuclear protrusions that are not only sac-like nuclear evaginations like those often seen in the oogenesis of other ferns, but also mushroom-like and finger-like, with an opening at their end allowing the nucleolus material to flow out from the openings. This has not been observed previously. The nuclear protrusions differ from Dryopteris filix-mas (L.) Schott. in the absence of sheets of nuclear membrane in the form of a closed ring. As the egg matures, the nucleus transforms into a tuber-like structure with a smooth surface, lying transversely in the egg cell. In the immature egg, vesicles almost encircle the nucleus twice and are most remarkable. In the maturing egg, the vesicles are distributed at the periphery, except for at the top of the egg, and affect the formation of the separation cavity and extra egg membrane. Simultaneously, vesicles from the venter canal cell move to the egg and take part in the formation of separation cavity and extra egg membrane. In the mature egg, a large number of small vesicles containing fragments of lamellae or osmiophilic material emerge from the cytoplasm. The origin of these vesicles is obscure. Irregular plastids containing a cylindrical starch grain dedifferentiated progressively.Mitochondria seem to have been undeveloped during the process, but return to normal at later stages of oogenesis. There is a high frequency of ribosomes in the mature egg. Microtubules, rarely seen in the eggs ofD. filix-mas (L.) Schott. and Pteridium aquilinum (L.) Kuhn, have been observed inside the plasmalemma of the maturing egg in D. crassirhizoma.     

EXOCYTOSIS OF LATEX BEADS DURING THE ENCYSTMENT OF ACANTHAMOEBA

Cells of Acanthamoeba castellanii (Neff) are known to form mature cysts characterized by a cellulose-containing cell wall when transferred to a nonnutrient medium. Amebas which engulfed latex beads before encystment formed mature cysts essentially devoid of bead material. The encystment of bead-containing cells appeared to be similar to that of control cells since no important differences between the two were observed with respect to cellular levels of glycogen or protein, cellulose synthetase activity, the amount of cyst wall polysaccharide formed, or the percentage of cysts formed. Actinomycin D and cycloheximide inhibited encystment as well as bead expulsion. Ultrastructural analysis revealed that the beads, which initially were contained in phagocytic vesicles, were released from the cell by fusion of vesicular membranes with the plasma membrane. Exocytosis was observed in cells after 3 hr of encystment, with most of the beads being lost before cyst wall formation. Each bead-containing vesicle involved in expulsion was conspicuously demarcated by an area of concentrated cytoplasm, which was more homogeneously granular than the surrounding cytoplasm. Beads were not observed in the cytoplasm of mature cysts but were occasionally found in the cyst wall.     

Ultrastructural Studies of a Vesicle System Associated with Endoplasmic Reticulum in Exo-Erythrocytic Forms of Plasmodium berghei1

ABSTRACT. Fine structural studies of a specialized vesicle system associated with the endoplasmic reticulum (ER) of exo-erythrocytic Plasmodium berghei suggest that this system may be the equivalent of a Golgi apparatus. Patches of ER, randomly distributed in the cytoplasm of developing parasites, are formed of smooth and ribosome-studded cisternae intermingled with each other. The vesicle systems are located between as well as at the edges of ER aggregates and appear to be in different stages of budding from the cisternae. Prolonged osmication reveals distinct staining of the nuclear envelope and ER of the parasites as well as part of the Golgi apparatus of the hepatocytes. However, the small vesicles associated with the parasite's ER are unstained, as are the coated vesicles in the Golgi region of the liver cell. These sites in the parasite cytoplasm seem comparable to the concave surface of the Golgi apparatus in liver cells. The pinched-off vesicles fuse with others to form the prominent peripheral vacuolization characteristic of the nearly mature exo-erythrocytic form. The formation of these peripheral vacuoles and their subsequent fusion with the parasite membrane may be an exocytosis mechanism supplying the rapidly expanding parasite with new plasma membrane material.     

Developmental Features of Cell Wall Formation in Sieve Elements of the Grass Aegilops comosa var. thessalica

In differentiating sieve elements of Aegilops comosa var. thessalicadictyosomes are abundant and they produce numerous smooth vesicles.Coated vesicles seem to bud from smooth ones. Since both kindsof vesicles appear both in the cytoplasm and in associationwith the plasmalemma, it is proposed that they move to and fusewith the plasmalemma transferring products for cell wall synthesis.During differentiation sub-plasmalemmal microtubules are initiallyscarce and randomly oriented but soon afterwards they becomenumerous and transversely oriented to the long axis. Cellulosemicrofibrils in the cell wall appear to run parallel to themicrotubules and the latter may regulate microfibril orientation. Root protophloem sieve elements develop wave-like wall thickenings,which are, during development, overlaid by microtubules perpendicularto the long axis. Just after maturation these thickenings progressivelybecome smooth and finally the walls appear uniform in thickness.The wave-like wall thickenings may function as stored wall material,utilized in later stages of development when wall material willbe needed and its synthesis will be impossible because of theabsence of a synthesizing mechanism in the highly degraded protoplastsof mature sieve elements. It is suggested that in this way thethickenings may enable root protophloem sieve elements to growand keep pace with the active clongation of the surroundingcells. Aegilops comosa var. thessalica, sieve elements. cell wall, microtubules, dictyosomes, coated vesicles, wave-like thickenings     

Melanophore differentiation in leaf frogs

In melanosomes of Pachymedusa (Agalychnis) dacnicolor and other leaf frogs, the pteridine dimer, pterorhodin, is found in fibers concentric to a kernel of eumelanin. The kernel is a remnant mature larval melanosome that is renovated at metamorphic climax and on which pterorhodin is deposited at the completion of metamorphosis. When pterorhodin is initially detected by chemical means in the skin of stage 25 individuals, flocculations of fibers are first seen in melanosomes. At stage 25+ a more intense chemical demonstration is accompanied by larger flocculations. These larval melanosomes are smaller than those of other vertebrates, but are formed from classical premelanosomes. At metamorphosis, the melanosome's limiting membrane is elevated from the surface of the eumelanin, and small spheroids are seen in the space and on the pigment surface. The Golgi complex is extremely active, numerous small vesicles are seen in the cytoplasm, and blebbing of the outer membrane of the nuclear envelope occurs. At stage 25 small thick-walled vesicles appear in the cytoplasm in contact with or within the melanosome; they may represent the transport of pterorhodin or elements necessary for its formation.     

White Vesicles in the Skin of Aplysia californicaCooper: A Proposed Excretory Function

White patches of skin that appear early in the development ofthe shell-less mollusc, Aplysia californica, are composed ofaggregations of vase-shaped vesicles, each a single, large cellwith an enlarged nucleus. Two layers of collagen, at 90°to each other, surround the vesicle membrane, together witha non-contiguous external layer of muscle. Energy dispersivespectroscopy and electron diffraction of the contents with scanningelectron microscopy (SEM) and transmission electron microscopy(TEM) failed to find a signal for halogens (involved with unpalatableskin and, therefore, defence against predation), but did recordprominent signals for calcium. Immature vesicles are filledwith an homogenous material that appeared to mature progressively,likely by nucleation, into numerous, small spherules, visibleby both SEM and TEM. Ultra-thin uranyl acetate stained sectionsof mature vesicles showed that the spherules had an intracrystallinematrix of radially arranged electron-dense material. The vesiclecell had none of the modifications characteristic of calciumor rhogocyte cell-types, which serve as calcium reservoirs forthe movement of this element to sites of shell formation andregeneration. These vesicles are likely involved with the excretionof excess calcium and not, as originally hypothesized, witheither defence against predators or calcium flux. In addition,the spherules that fill the vesicles are possibly composed ofthe less common polymorphs of calcium carbonate, vaterite andamorphous calcium carbonate. Excretion of the vesicle contentsappeared to be passive, since vesicles lacked an abundant contiguouslayer of muscle or a muscular release valve in the vesicle neck,but did have organized layers of collagen just outside the vesiclemembrane. We propose that release of vesicle contents probablyoccurs when the neck of the vesicle is broken. (Received 5 December 2005; accepted 28 April 2006)     

ULTRASTRUCTURE OF DEVELOPING SIEVE ELEMENTS IN THLASPI ARVENSE L. II. MATURATION

Developing sieve elements of pennycress (Thlaspi arvense L.) were studied with the electron microscope. The maturation of sieve elements involved loss of ribosomes from cytoplasm; degeneration of nulcei; modification of endoplasmic reticulum (ER); loss of tonoplast; and disappearance of dictyosomes and dictyosomes vesicles, coated vesicles, microtubules, and microbodies. Such changes produce a mature, presumably conducting cell that contains no nucleus or central vacuole but which retains a thin layer of peripheral cytoplasm with plastids, mitochondria, and smooth ER. Some similar changes have been described in a variety of developing sieve elements of angiosperms, but coated vesicles and microbodies previously have not been followed through sieve-element maturation. Likewise, few developmental studies have been made of plant sieve elements that exhibit two types of P-protein, the tubular type and the granular P-protein body.     

FINE STRUCTURE AND HISTOCHEMISTRY OF VESICLE CELLS OF THE RED ALGA ANTITHAMNION DEFECTUM (CERAMIACEAE)1

The ultrastructure and histochemistry of the refractile, vesiculate cells (“blasenzellen,”“cellules secretrices,”“gland cells”) of Antithamnion defectum Kylin were examined. The refringent vacuolar contents disclosed two components of differing density: an electron opaque, proteinaceous matrix material surrounding cores of irregularly shaped, less opaque material. The cores contain less protein and more unknown material than the matrix. Part or all of the vacuolar material is synthesized by abundant rough endoplasmic reticulum (ER) and deposited in smooth surfaced cisternae that swell to form vesicles. Mitochondria are usually associated with stacks of the swelling cisternae. The vesicles enlarge by continued deposition of synthesized material and coalescence with other vesicles. All vesicles eventually coalesce to form the mature vacuole. A crystalline array of fibrils develops in the cytoplasm during later stages of vacuole enlargement. The crystal contains a sulfated, acidic polysaccharidic material. The chloroplasts, if present, and nucleus degenerate at vacuole maturity. Active release of the vacuolar material does not occur, and organelles for extracellular secretion are not present. Structural evidence suggests a storage, rather than secretory, function for the cells.     

银耳（Tremella fuciformis）原基分化前期双核菌丝细胞和分生孢子的边缘体与质膜体

本文报道银耳（Ｔｒｅｍｅｌｌａｆｕｃｉｆｏｒｍｉｓ）原基分化前期．在双核菌丝的幼细胞、成熟细胞和分生孢子中,与质膜相关联的两类膜结构──边缘体和质膜体的形成与功能。根据相似结构的存在．支持小泡或多泡体排出质膜之外附在细胞壁上成为边缘体和参于细胞壁合成的假定。银耳原基分化前期．双核菌丝迅速分裂的幼细胞．其质膜内陷产生泡状质膜体,内含数个小泡,或产生膜状质膜体;在成熟细胞中．质膜内陷通常形成回旋的膜结构──膜状质膜体．内含１—２个电子致密小泡．当这两类质膜体脱离质膜进入细胞质后,有的膜层和小泡局部被消化．因此,推断质膜体具有内吞和输送养料的作用。另外．在桶孔隔膜闭塞一侧电子致密度高的细胞质中．还观察到一种罕见的只有单个膜层的质膜体．其内充满３个电子致密小泡．估计它的形成与功能同膜状质膜体相似。作者认为．桶孔闭塞和质膜体的出现是与银耳原基细胞分化有关联的两个重要特征。最后,在成熟细胞中,尚可以观察到质膜体的膜层能够散开形成内质网．因此．内质网也可以来源于质膜体。     

Evolution of the shell coat and yolk in amniotes: a marsupial perspective

Two characters distinguish oogenesis and early development in marsupials and monotremes: (1) the shell coat that persists from the zygote to somite stages in marsupials or until hatching in monotremes; and (2) the numerous, apparently almost empty vesicles that appear in primary oocytes, increase during oogenesis in marsupials and monotremes before being shed into the cleavage cavity and are preferentially distributed to the trophoblast lineage in marsupials, but comprise the latebra in monotremes. Analysis of these unusual characters used Southern analysis of genomic DNA dot blots and histology and electron microscopy. The evidence suggests that the marsupial shell coat protein, CP4, was probably characteristic of the egg of the mammalian ancestor. Further, the vesicles, present in marsupials during oogensis and cleavage and in eutherian mammals during blastocyst formation are the residual elements of white yolk present in the larger yolky eggs of monotemes and sauropsids. By comparison with the function of the vesicle components in marsupials, it is suggested that one role for the white yolk in monotremes and the sauropsids is to provide extracellular matrix (ECM), especially hyaluronan containing stabilizing proteins, for epithelial construction. Thus, as oviparity was replaced by viviparity, egg size was reduced, the germinal cytoplasm was retained, and yellow yolk was markedly reduced or lost in marsupials and eutherians. The white yolk was retained in monotremes and marsupials where blastocyst epithelial construction requires ECM support, and its appearance is heterochronously shifted to after compaction, when blastocyst formation and expansion occurs, in eutherian mammals.