海金沙精细胞中生毛体发育及多层结构起源的超微结构研究

蕨类植物海金沙（LygodiumJaponicum（Thunb．）Sw．）的游动精子发育过程中,生毛体在精母细胞的细胞质中出现,它是直径为0．5－0．6μm的椭球体,其结构紧密,由辐射排列的具轮辐结构的管状亚单位和无定形基质组成。大量微管从生毛体伸向细胞质。随着精细胞的发育,生毛体结构变得松散,亚单位分化形成的中心粒彼此分开扩散到外围,中心为无定形物质。伴随着中心粒的分化,多层结构出现,一端与无定形基质相连。多层结构由外侧的微管带及内侧的片层组成,形成后与一线粒体相连,移向靠近核的位置,并正对着核上出现凹点。研究发现在精原细胞后期出现一团絮状结构,为无定形基质,其中有深染色的小管状结构分布,同时可见微管从絮状结构边缘伸出,这一絮状结构可能与生毛体的产生有一定的关系。     

Ultrastructure of Blepharoplast and Other Organelles of the Spermatid in Ginkgo biloba

Both blepbaroplast and osmiophilic globule were characteristic structures to the spermatid of Ginkgo biloba. The blepharoplast of Ginkgo biloba ranged from 3 ～ 4 μm in diameter and consisted of a number of basal centrioles radiating from an electron dense core that contained electron-lucent areas with microtubule structure. Microtubules extended radially from the blepharoplast into the cytoplasm. A large round osmiopbilie globule with a diameter of about 10～20/μm, was located between the blepharoplast and the nucleus, while a filbrillogranular body in the cytoplasm was opposite to the osmiophilic globule. There were numerous mitochondria, plastids, endoplasmic reticulia and dictiosomes in the cytoplasm, particularly around the blepharoplast and the osmiophilic globule of sperm cells. The nucleus of spermatid in Ginkgo biloba was large and roundly elliptical in shape. The large spheroidal nucleolus was the most obvious structure in the nucleus, There were two regions in the nucleolus distinguished by TEM: A ring-shaped granular component, which contained maturing ribosomal precursor particles; and a centrally placed fibrillar component. The nuclear pore complexes in the nuclear envelope were plentiful but not evenly distributed.     

CENTRIOLE REPLICATION : II. Sperm Formation in the Fern, Marsilea, and the Cycad, Zamia

Sperm formation was studied in the fern, Marsilea, and the cycad, Zamia, with particular emphasis on the centrioles. In Marsilea, the mature sperm possesses over 100 flagella, the basal bodies of which have the typical cylindrical structure of centrioles. Earlier observations by light microscopy suggested that these centrioles arise by fragmentation of a body known as the blepharoplast. In the youngest spermatids the blepharoplast is a hollow sphere approximately 0.8 µ in diameter. Its wall consists of closely packed immature centrioles, or procentrioles. The procentrioles are short cylinders which progressively lengthen during differentiation of the spermatid. At the same time they migrate to the surface of the cell, where each of them puts out a flagellum. A blepharoplast is found at each pole of the spindle during the last antheridial mitosis, and two blepharoplasts are found in the cytoplasm before this mitosis. Blepharoplasts are also found in the preceding cell generation, but their ultimate origin is obscure. Before the last mitosis the blepharoplasts are solid, consisting of a cluster of radially arranged tubules which bear some structural similarity to centrioles. In Zamia, similar stages are found during sperm formation, although here the number of flagella on each sperm is close to 20,000 and the blepharoplast measures about 10 µ in diameter. These observations are discussed in relation to theories of centriole replication.     

傅氏凤尾蕨精子发生超微结构的研究

超微结构研究显示傅氏凤尾蕨（Pteris fauriei Hieron）精子发生过程包括生毛体、多层结构和鞭毛等运动细胞器重新发生,环状线粒体形成,核塑形等过程,最后形成一个螺旋形的游动精子,这与其他真蕨类精子发生过程相似。本研究观察到的一些新现象包括：精细胞在分化早期呈极性,细胞核位于精细胞的近极端,生毛体、线粒体和质体等细胞器主要分布远极端;在生毛体分化早期,可见大量微管从其发出,其周围线粒体丰富;基体分化经历了前中心粒、中心粒和基体3个阶段,它们的内部结构不同;研究表明生毛体内的不定形物质是微管组织者,多层结构、附属微管带及鞭毛等细胞器均由不定形物质分化形成;精细胞在分化过程中产生了丰富的膜结构,它们可能为精核塑形提供原料。本研究报道了傅氏凤尾蕨精细胞分化的一些细节,这有助于进一步揭示蕨类植物精子发生的细胞学机制。     

Identification and function of the centrosome centromatrix

Centrosomes direct the organization of microtubules in animal cells. However, in the absence of centrosomes, cytoplasm has the potential to organize microtubules and assemble complex structures such as anastral spindles. During cell replication or following fertilization, centrioles that are incapable of organizing microtubules into astral arrays are introduced into this complex cytoplasmic environment. These centrioles become associated with pericentriolar material responsible for centrosome-dependent microtubule nucleation, and thus the centrosome matures to ultimately become a dominant microtubule organizing center that serves as a central organizer of cell cytoplasm. We describe the identification of a novel structure within the pericentriolar material of centrosomes called the centromatrix. The centromatrix is a salt-insoluble filamentous scaffold to which subunit structures that are necessary for microtubule nucleation and abundant in the cytoplasm bind. We propose that the centromatrix serves to concentrate and focus these subunits to form the microtubule organizing center. Since binding of these subunits to the centromatrix does not require nucleotides, we propose a model for centrosome assembly which predicts that the assembly of the centromatrix is a rate-limiting step in centrosome assembly and maturation.     

Structural and immunocytochemical characterization of the Ginkgo biloba L. sperm motility apparatus

Summary. Ginkgo biloba and the cycads are the only extant seed plants with motile sperm cells. However, there has been no immunocytochemical characterization of these gametes to determine if they share characteristics with the flagellated sperm found in bryophytes and pteridophytes or might give clues as to the relationships to nonflagellated sperm in all other seed plants. To determine characteristics of proteins associated with the motility apparatus in these motile sperm, we probed thin sections of developing spermatogenous cells of Ginkgo biloba with antibodies to acetylated and tyrosinated tubulin and monoclonal antibodies that recognize mammalian centrosomes and centrin. The blepharoplast that occurs as a precursor to the motility apparatus consists of an amorphous core, pitted with cavities containing microtubules and a surface studded with probasal bodies. The probasal bodies and microtubules within the blepharoplast cavities are labeled with antibodies specific to acetylated tubulin. Positive but weak reactions of the blepharoplast core occur with the centrosomereactive antibodies MPM-2 and C-9. Reactions to centrin antibodies are negative at this developmental stage. From this pre-motility apparatus structure, an assemblage of about 1000 flagella and associated structures arises as the precursor to the motility apparatus for the sperm. The flagellar apparatus consists of a three-layered multilayered structure that subtends a layer of spline microtubules, a zone of amorphous material similar to that in the blepharoplast, and the flagellar band. Centrin antibodies react strongly with the multilayered structure, the transition zone of the flagella, and fibrillar material near the flagellar base at the surface of the amorphous material. Both the spline microtubules and all of the tubules in the flagella react strongly with the antibodies to acetylated tubulin. These localizations are consistent with the localizations of these components in pteridophyte and bryophyte spermatogenous cells, although the blepharoplast material surrounding and connecting flagellar bases does not occur in the seedless (nonseed) land plants. These data indicate that despite the large size of ginkgo gametes and the taxonomic separation between pteridophytes and Ginkgo biloba, similar proteins in gametes of both groups perform similar functions and are therefore homologous among these plants. Moreover, the presence of acetylated tubulin in bands of microtubules may be a characteristic shared with more derived non-flagellated sperm of other conifers and angiosperms. Correspondence and reprints: Southern Weed Science Research Unit, USDA Agricultural Research Service, P.O. Box 350, Stoneville, MS 38776, U.S.A.     

银杏精细胞结构及受精作用的细胞学研究

采用常规石蜡制片技术和环氧树脂半薄切片技术,对银杏的精细胞结构及受精过程进行了研究。结果表明,临近受精前,精原细胞分裂形成两个半球形的精细胞,每个精细胞内含液泡状结构、生毛体和纤维性颗粒体各一个。两精细胞的液泡状结构其位置可同时或分别位于近极面和远极面,这种位置的变动可能是鞭毛摆动导致精细胞的旋转所引起,这说明银杏的精细胞在花粉管内也许即可以旋转运动。在银杏受精前后,珠孔端颈卵器室附近的珠心组织表现出向上隆起、出现受精滴、皱褶等规律性变化,这些现象是判断银杏受精时期的良好形态特征。在银杏受精时带有鞭毛的完整精细胞进入颈卵器,随后鞭毛带及精细胞质遗留于颈卵器口下方、卵细胞上面,仅精核进入卵细胞。进入卵细胞的精核直径约30μm,小于成熟精细胞的精核直径（约40μm）。这些对探讨银杏的系统地位及裸子植物的生殖演化具有一定意义。     

Spermiogenesis and sperm structure in the crabUca tangeri (Crustacea,Brachyura), with special reference to the acrosome differentiation

Summary Early spermatids of the crabUca tangeri consists of the nucleus of granular chromatin and the cytoplasm, which contains a proacrosomal vesicle in close association with membrane lamellae. In the mid spermatids an invagination of the acrosomal vesicle membrane gives rise to the formation of the perforatorium, a spindle-shaped tubule which encloses tubular membranous structures. The pair of centrioles located at the base of the acrosome is not directly involved in perforatorial differentiation. The acrosomal vesicle shows a heterogeneous content composed of the operculum, the thickened ring, and three layers of different materials concentrically arranged around the perforatorium. During the late spermatid stage the nuclear profile differentiates numerous slender arms and the chromatin arranges into fibers. Membranous tubules from the cytoplasm become incorporated into the tubular structures of the perforatorium. The mature spermatozoon has the typical structure of the branchyuran sperm, with a complex acrosome, cupped by the nucleus, and a thin cytoplasmic band intervening between the former main elements. The centrioles are degenerate. The nuclear arms are unusually numerous (more than 20) and lack microtubules or microtubular derivatives.     

Electron and immunofluorescence microscopy on the fertilization ofFucus distichus (Fucales,Phaeophyceae)

Summary Processes of fertilization and zygote development inFucus distichus were studied by indirect immunofluorescence microscopy using anti- tubulin antibody and electron microscopy. Just after plasmogamy, sperm aster formation occurs during migration of a sperm nucleus toward an egg nucleus at the center of cytoplasm. Only sparse microtubules (MTs) exist around the egg nucleus. The sperm aster can be observed till karyogamy, but afterwards vanishes. Accompanying sperm aster formation, cortical MTs which are reticulately arranged develop further in the zygotes. In 4 h-old zygotes, characteristic structures which are composed of fine granular masses and consist of intermixed dense and lighter staining areas appear around the nucleus. These structures cannot be detected with anti- tubulin immunofluorescence microscopy. The two centrioles derived from the sperm separate and migrate to both poles. In 4 h-and 8 h-old zygotes, there are no defined MT foci around the zygote nucleus and MTs radiate from the circumference of it. In 12 h-old zygotes, each centriole has migrated to the poles and derivative centrioles are generated. The fine granular masses also migrate to both poles and finally disappear accompanying the appearance of numerous MTs radiating from the poles. Therefore, two distinct MT foci appear from 12 h onwards. Progressive stages of nuclear division were also examined with electron and immunofluorescence microscopy in 16 h-old zygotes. The sperm chloroplast with an eyespot and the sperm mitochondria with an intercristal tubular structure, which are distinctive from those of egg, can be detected after plasmogamy and karyogamy. The sperm chloroplast is still present in 16 h-old zygotes.     

Spermatogenesis and sperm structure of Acerella muscorum, (Ionescu, 1930) (Hexapoda, Protura)

The general organization of the male genital system, the spermatogenesis and the sperm structure of the proturan Acerella muscorum have been described. At the apex of testis apical huge cells are present; their cytoplasm contains a conventional centriole, a large amount of dense material and several less electron-dense masses surrounded by mitochondria. Spermatocytes have normal centrioles and are interconnected by cytoplasmic bridges. Such bridges seem to be absent between spermatid cells and justify the lack of synchronization of cell maturation. Spermatids are almost globular cells with a spheroidal nucleus and a large mass of dense material corresponding to the centriole adjunct. Within this mass a centriole is preserved. Mitochondria of normal structure are located between the nucleus and the plasma membrane. The spermatids are surrounded by a thick membrane. No flagellar structure is formed. Sperm have a compact spheroidal nucleus, a large cap of centriole adjunct material within which a centriole is still visible. A layer of mitochondria is located over the nucleus. The cytoplasm is reduced in comparison to spermatids; many dense bodies are interspersed with sperm in the testicular lumen. The sperm are small, immotile cells of about 2.5-3 μm in diameter.     

DEVELOPMENTAL FEATURES OF THE SPERMATOGENOUS CELL IN GINKGO BILOBA

A “double-blepharoplast” originates de novo in the spermatogenous cell of Ginkgo biloba L. Initially, the double-blepharoplast consists of two opposing hemispherical bodies comprised of densely staining material. The two blepharoplasts seemingly are pushed apart by the formation of densely packed fibrils which are oriented perpendicular to the distal, rounded edges of the two future blepharoplasts. As the latter move apart, each one develops lightly staining channels which are often organized in a hub and spoke configuration (procentrioles). Microtubules extend from the blepharoplasts as the latter move to their final position in the cell, and centrioles (probasal bodies) become organized at the periphery of each blepharoplast. Two large “osmiophilic globules,” conspicuous entities close to the nucleus of the mature spermatogenous cell, arise de novo. A fibrillogranular body in the cytoplasm, always closely associated with the nucleus, also arises de novo.     

蕨类植物分株紫萁精子发生过程中生毛体和多层结构的超微结构

应用电镜技术对蕨类植物分株紫萁(Osmunda cinnamomea L. var.asiatica Fernald)精子发育过程中的生毛体和多层结构的超微结构进行了研究.生毛体在幼精子细胞中出现,正在分化的生毛体略呈球状,球状体的中央由一团染色深的颗粒状物质构成,外围分化出若干柱状体.已分化的生毛体由柱状体分散或辐射状排列构成,呈球状,球体中心不含染色深的物质.多层结构位于精子细胞内的基体和巨大线粒体之间,刚形成时仅由片层构成,片层相互平行排列形成片层带.多层结构在分化中期由微管带、片层带和蚀斑三层构成.多层结构在分化末期又形成附属微管带、嗜锇冠和嗜锇层.微管带从多层结构长出,沿细胞核的表面伸展,并与核膜之间形成复合结构.基体由柱状体转变而成,它向两端生长,在远端产生鞭毛的轴丝,在近轴端形成楔状结构.本文首次详细阐明了原始薄囊蕨分株紫萁生毛体和多层结构发育的超微结构特点,并与其他蕨类进行了比较,发现其片层带出现在微管带形成之前.     

Influence of centriole behavior on the first spindle formation in zygotes of the brown alga Fucus distichus (Fucales, Phaeophyceae)

The influence of centrioles, derived from the sperm flagellar basal bodies, and the centrosomal material (MTOCs) on spindle formation in the brown alga Fucus distichus (oogamous) was studied by immunofluorescence microscopy using anti-centrin and anti-beta-tubulin antibodies. In contrast to a bipolar spindle, which is formed after normal fertilization, a multipolar spindle was formed in polyspermic zygote. The number of mitotic poles in polyspermic zygotes was double the number of sperm involved in fertilization. As an anti-centrin staining spot (centrioles) was located at these poles, the multipolar spindles in polyspermic zygotes were produced by the supplementary centrioles. When anucleate egg fragments were fertilized, chromosome condensation and mitosis did not occur in the sperm nucleus. Two anti-centrin staining spots could be detected, microtubules (MTs) radiated from nearby, but the mitotic spindle was never produced. When a single sperm fertilized multinucleate eggs (polygyny), abnormal spindles were also observed. In addition to two mitotic poles containing anti-centrin staining spots, extra mitotic poles without anti-centrin staining spots were also formed, and as a result multipolar spindles were formed. When karyogamy was blocked with colchicine, it became clear that the egg nucleus proceeded independently into mitosis accompanying chromosome condensation. A monoastral spindle could be frequently observed, and in rare cases a barrel-shaped spindle was formed. However, when a sperm nucleus was located near an egg nucleus, the two anti-centrin staining spots shifted to the egg nucleus from the sperm nucleus. In this case, a normal spindle was formed, the egg chromosomes arranged at the equator, and the associated MTs elongated from one pole of the egg spindle toward the sperm chromosomes which were scattered. From these results, it became clear that paternal centrioles derived from the sperm have a crucial role in spindle formation in the brown algae, such as they do during animal fertilization. However, paternal centrioles were not adequate for the functional centrosome during spindle formation. We speculated that centrosomal materials from the egg cytoplasm aggregate around the sperm centrioles and are needed for centrosomal activation.     

STUDIES OF SPERMIOGENESIS IN A NEMATODE, NIPPOSTRONGYLUS BRASILIENSIS

The fine structure of the developing spermatids and the mature sperm of Nippostrongylus brasiliensis was investigated. Immature spermatids are found at one end of the tubelike testis, and the mature sperm at the other. The spermatid has a prominent nucleus, with the chromatin clumped at the margin. It also contains a pair of centrioles, located near the nucleus. The cytoplasm is filled with ribosomal clusters, but it lacks an organized Golgi area or endoplasmic reticulum. Besides the normal mitochondria, the spermatid has specialized mitochondrionlike inclusions with dense matrix, few broad cristae, and a crystalloid structure always facing the nucleus. As spermiogenesis proceeds, the nucleus elongates, comes to lie at one end, and later evaginates to form a separate head structure, leaving the mitochondria and other cytoplasmic organelles in a broad cytoplasmic region. The nuclear material becomes filamentous and spiral, and the centrioles come to lie at one end near the junction of the head and the cytoplasmic portion of the sperm. Microtubules are found in the cytoplasmic region extending from the tubelike nucleus. The specialized mitochondria are about eighteen in number, and are arranged in rows in staggered groups of three around the microtubules in the cytoplasmic region. The mature sperm is aflagellate and lacks an acrosome. No movement of the sperm was ever observed.     

蕨类植物分株紫萁精子发生过程中生毛体和多层结构的超微结构

应用电镜技术对蕨类植物分株紫萁(Osmunda cinnamomea L. var．asiatica Fernald)精子发育过程中的生毛体和多层结构的超微结构进行了研究。牛毛体在幼精了细胞中出现,正在分化的生毛体略呈球状,球状体的中央由一团染色深的颗粒状物质构成,外围分化出若干柱状体。已分化的生毛体由柱状体分散或辐射状排列构成,呈球状,球体中心不含染色深的物质。多层结构位于精子细胞内的基体和巨大线粒体之间,刚形成时仅由片层构成,片层相互平行排列形成片层带。多层结构在分化中期由微管带、片层带和蚀斑三层构成。多层结构在分化末期又形成附属微管带、嗜锇冠和嗜锇层。微管带从多层结构长出,沿细胞核的表面伸展,并与核膜之间形成复合结构。基体由柱状体转变而成,它向两端生长,在远端产生鞭毛的轴丝,在近轴端形成楔状结构。本文首次详细阐明了原始薄囊蕨分株紫其生毛体和多层结构发育的超微结构特点,并与其他蕨类进行了比较,发现其片层带出现在微管带形成之前。     

LIGHT MICROSCOPE AND ULTRASTRUCTURAL STUDIES OF THE MALE GAMETOPHYTE IN GINKGO BILOBA: THE SPERMATOGENOUS CELL

A light microscope and ultrastructural study was made of the pollen tube of Ginkgo biloba, with special emphasis given to the spermatogenous cell that gives rise to two motile sperms. Just prior to the mitotic division that results in the formation of two sperms, the spermatogenous cell consists of a large nucleus, two blepharoplasts, two large osmiophilic globules, and a conspicuous lipo-protein body. Other organelles in the cytoplasm include numerous electron-dense proplastids (with some lamellar development), mitochondria, small vacuoles, and lipid bodies. Ribosomes are present in abundance, but endoplasmic reticulum and dictyosomes are sparse. The nucleus, prior to mitosis, is relatively Feulgen-negative, due undoubtedly to the diffuse distribution of DNA. Each blepharoplast, the main organelle of interest, is nearly spherical, measures 3.5–4.5 μm in diam, and supports about 1,000 probasal bodies. The interior of a blepharoplast consists of an electron-dense matrix and of less dense regions which appear to be infiltrated by a network of microtubules. Each probasal body is composed of a cylinder of nine separate tubules (singlets) at the basal or proximal end. The cylinder becomes elaborated distally into nine pairs of subtubules (doublets) and then into nine sets of subtubules (triplets). A central tubule is present the entire length of the probasal body. Some of the subtubules, as well as microtubules from the interior of the blepharoplast, extend into the cytoplasm and probably constitute the “astral rays” as seen with the light microscope. Comparisons are made with other published accounts of the organization of blepharoplasts in plants and of centrosomes and centrioles in animals.     

Ultrastructural Study of Asters Induced by Microinjection with Sperm Centriolar Fraction in Sea Urchin Eggs

Multiple asters can be artificially induced in sea urchin fertilized eggs by the microinjection of the centriolar fraction of sperm homogenate. Investigation was continued by the electron microscopy to determine whether the multi-aster formation was due to the centrioles or the contaminants in the injected sperm fraction. Thirty three asters in 3 operated eggs were thoroughly examined, and we confirmed that the presence of centrioles in the central region of 26 asters. We considered that the rest of them might contained the centrioles in the sections lost during the preparation procedures. Fragmented axoneme, the plug of electron dense material, and the centriolar fossa, which were usually accompanied with the isolated centrioles, disappeared from the centrioles in these multiple asters. However, electron dense, amorphous materials were formed associating with the triplet blades and distributed around the centrioles. Many astral microtubules were terminated in these pericentriolar materials. Results obtained suggest that, although the pericentriolar material is acting as the microtubule organizing center, all multiple asters, except those derived from fertilization (2 asters per egg), are most likely induced by the injected centrioles and not by the contaminants.     

Male gametophyte development inPlumbago zeylanica: cytoplasm localization and cell determination in the early generative cell

Summary The behavior of the generative cell during male gametophyte development inPlumbago zeylanica was examined by epifluorescence microscopy and electron microscopy with organelle nucleoid as a cytoplasm marker. When the thin sections stained with 4,6-diamidino-2-phenylindoIe (DAPI) were observed under an epifluorescence microscope, two types of fluorescence spots were detected in the cytoplasm of the pollen cells before the second mitosis. The spots emitting stronger fluorescence were confirmed as plastid nucleoids and those emitting dimmer fluorescence were mitochondrial nucleoids. Before the first mitosis, both plastid and mitochondrial nucleoids distributed randomly in the cytoplasm of the microspore. A small lenticular generative cell formed with attachment to the interior of the intine after the mitosis. Small vacuoles were found in the lenticular cell. In the cytoplasm of the lenticular cell, both plastid nucleoids and the small vacuoles were distributed randomly at the very beginning but began to migrate in opposite directions immediately. Plastid nucleoids aggregated to the side of the cell that faces the pollen center and the small vacuoles aggregated to the side of the cell that attaches to the inline. As the result, the lenticular generative cell appeared highly polarized in cytoplasm location soon after the first mitosis. In accordance with the definition of the cytoplasm polarization, the primary wall between the generative and the vegetative cells began to flex and the lenticular generative cell started to protrude towards the pollen center. When the generative cell peeled away from the inline, it was spherical in shape with the pole that aggregated plastids towards the vegetative nucleus. But the cell direction appeared to be transformed immediately. The pole that aggregated small vacuoles turned to the position towards the vegetative nucleus and the pole that aggregated plastid nucleoids turned to the position countering to the vegetative nucleus. A cellular protuberance formed at the edge of the pole that aggregated small vacuoles and elongated into a tapered end that got into contact with the vegetative nucleus. The polarization of the cytoplasm kept constant throughout the second mitosis. The small vacuoles that apportioned to the sperm cell which attached the vegetative nucleus (the leading sperm cell) disappeared during sperm cell maturation. Plastid nucleoids were apportioned to the other sperm cell (the trailing sperm cell) completely. Mitochondrial nucleoids became undetectable after the second mitosis.     

The ultrastructural organization of the mature spermatozoon of Luidia clathrata (Say) (Echinodermata: Asteroidea)

The sperm of Luidia clathrata are morphologically typical of asteroid sperm. The head is spherical and contains the nucleus and acrosomal complex. The nucleus has an anterior indentation in which rests the acrosomal complex. There is no evidence of a centriolar fossa along the posterior border of the nucleus. The acrosome is a cup-shaped structure containing a less electron dense central region. The periacrosomal material is homogeneous in nature, and the subacrosomal specialization of the periacrosomal materials appear as bands of varying electron density. The middle piece is an annular band of mitochondria which surrounds the proximal and distal centrioles. The centrioles exhibit the typical nine triplet arrangement. Both the centrioles and the axoneme project to one side of the middle piece region. Associated with the distal centriole is an elaborate pericentriolar process.     

Behavior of the cell center during epithelial cell spreading

In the epithelial cells of mouse embryo renal channels, centrioles are located near the plasma membrane of the apical part of the cell. In most of the cells an active centriole carries a cilium, which comes out into the channel lumen. In the epithelial cells, suspended after trypsinisation and in single cells adhering to the substrate, the centrioles are located near the nucleus, and the outcoming cilia are not observed. In the spread cells of epithelial islets, the centrioles are also found near the nucleus, and in most cases an active centriole carries a cilium, which comes out of the cytoplasm at the upper side of the cell. In the peripheral cells of the islet, centrioles are positioned between the nucleus and the active edge of the cell. In the epithelial cells in situ, a relatively small number of microtubules radiate from the active centrioles. In the suspended cells, the activation of microtubule formation is observed in the cell center. In the spread cells of the epithelial islets there occurs a further increase in the number of microtubules radiating from the active centrioles. In the peripheral cells which cause translocation of the epithelial islet in the culture, the number of microtubules, radiating from the centrioles does not differ significantly from that of the inner cells of the islet. The cell center of the epithelial cells does not seem to be actively involved in the locomotion of the epithelial cells in the culture.