Fine structure observations in frozen-etched bovine spermatozoa

Bacterial protein HU dictates the morphology of DNA condensates produced by crowding agents and polyamines

Controlling the size and shape of DNA condensates is important in vivo and for the improvement of nonviral gene delivery. Here, we demonstrate that the morphology of DNA condensates, formed under a variety of conditions, is shifted completely from toroids to rods if the bacterial protein HU is present during condensation. HU is a non-sequence-specific DNA binding protein that sharply bends DNA, but alone does not condense DNA into densely packed particles. Less than one HU dimer per 225 bp of DNA is sufficient to completely control condensate morphology when DNA is condensed by spermidine. We propose that rods are favored in the presence of HU because rods contain sharply bent DNA, whereas toroids contain only smoothly bent DNA. The results presented illustrate the utility of naturally derived proteins for controlling the shape of DNA condensates formed in vitro. HU is a highly conserved protein in bacteria that is implicated in the compaction and shaping of nucleoid structure. However, the exact role of HU in chromosome compaction is not well understood. Our demonstration that HU governs DNA condensation in vitro also suggests a mechanism by which HU could act as an architectural protein for bacterial chromosome compaction and organization in vivo.

Fine surface structure of bovine acrosome-intact and reacted spermatozoa observed by atomic force microscopy

The atomic force microscope (AFM) provides nanometer resolution, topographic data of the natural surface structure of materials. We studied the topology of the surface structure of bovine sperm heads during the acrosome reaction by AFM. In addition, we numerically analyzed the areas of the median sagittal plane of the sperm heads. Bovine frozen-thawed spermatozoa were washed, capacitated by heparin, and incubated with lysophosphatidylcholine (LPC) to induce the acrosome reaction, smeared on a cover glass, air-dried, and observed with AFM using the dynamic force (tapping) mode. AFM analysis of spermatozoa showed the clear surface structure of acrosomes, equatorial segments, postacrosomal regions and necks. Although AFM images of spermatozoa capacitated by heparin had complete acrosomes, most spermatozoa treated with LPC had no acrosomal caps as shown by AFM. These observations coincided with those obtained by light microscopy after staining with naphthol yellow S and erythrosin B. Furthermore, numerical analysis of AFM images indicated that areas of the median sagittal plane of the anterior portions of acrosome-reacted sperm heads (2679 +/- 616 pixels) were approximately 40% less than those of intact heads (4535 +/- 174 pixels, P<0.05). These results indicate that AFM can usefully observe and numerically analyze the fine surface structures of bovine spermatozoa.

Formation of native-like mammalian sperm cell chromatin with folded bull protamine

The DNA of most vertebrate sperm cells is packaged by protamines. The primary structure of mammalian protamine I can be divided into three domains, a central DNA binding domain that is arginine-rich and amino- and carboxyl-terminal domains that are rich in cysteine residues. In native bull sperm chromatin, intramolecular disulfide bonds hold the terminal domains of bull protamine folded back onto the central DNA binding domain, whereas intermolecular disulfide bonds between DNA-bound protamines help stabilize the chromatin of mature mammalian sperm cells. Folded bull protamine was used to condense DNA in vitro under various solution conditions. Using transmission electron microscopy and light scattering, we show that bull protamine forms particles with DNA that are morphologically similar to the subunits of native bull sperm chromatin. In addition, the stability provided by intermolecular disulfide bonds formed between bull protamine molecules within in vitro DNA condensates is comparable with that observed for native bull sperm chromatin. The importance of the bull protamine terminal domains in controlling the bull sperm chromatin morphology is indicated by our observation that DNA condensates formed under identical conditions with a fish protamine, which lacks cysteine-rich terminal domains, do not produce as uniform structures as bull protamine. A model is also presented for the bull protamine.DNA complex in native sperm cell chromatin that provides an explanation for the positions of the cysteine residues in bull protamine that form intermolecular disulfide bonds.

DNA in human and stallion spermatozoa forms local hexagonal packing with twist and many defects

In human and other mammal sperm nuclei, DNA is packed in a highly condensed state, the structure of which remains unsolved. Cryoelectron microscopy of vitrified sections provides a first direct view of the local arrangement of the nucleoprotamine filament. DNA aligns in parallel in layers and its orientation rotates along a single-twist direction as in a cholesteric liquid crystal. The structure contains numerous defects, which introduce locally double-twist configurations. Destruction of the SS bonds with dithiotrehitol relaxes the twist and favors the extension of the hexagonal close packing of the filaments, though keeping constant their interfilament distance.

Effect of cryopreservation on the fine structure of spermatozoa of rainbow trout (Oncorhynchus mykiss) and brown trout (Salmo trutta F. fario)

Freeze-fracture electron microscopy has revealed that changes are induced in the organization of the plasma membranes of spermatozoa of rainbow and brown trout when they are cryopreserved. Electron-micrographic images of spermatozoa that had not been exposed to cryopreservation showed a protoplasmic surface with particles homogeneously distributed. The concentration of the particles was low. In the median portion of the tail the authors observed longitudinal strips, consisting of particles located along the bands. In the neck of the spermatozoon the particles were aggregated in a chaotic manner. Electron micrographs of trout spermatozoa that had been cryopreserved showed particles grouped in rounded clusters on the protoplasmic surfaces of both the head and the tail. In some spermatozoa, folding of the protoplasmic membrane, with the particle-free sites, were found. The dimensions of the clusters and the spaces that were free of particles suggest that, after thawing, the spermatozoa of the brown trout is likely to experience greater difficulty in restoring their physiological integrity than those of the rainbow trout. These results suggest that the membrane proteins of spermatozoa of the species of trout that were studied possess high motility and diffuse, with the formation of clusters, in very short periods of time (about 30 s). The changes in membrane structure of the trout spermatozoa following cryopreservation appear to indicate high lability. Copyright 1998 Academic Press.

Targeting of the domain-specific integral membrane protein PM52 to the periacrosomal plasma membrane during guinea pig spermiogenesis

The sperm plasma membrane is segregated into functionally, biochemically, and structurally distinct domains yet the protein sorting pathways and assembly mechanisms that assemble these domains during spermiogenesis are incompletely understood. We previously characterized two structurally related size-variant, integral membrane proteins of 52 kDa (PM52) and 35 kDa localized to the periacrosomal plasma membrane of guinea pig cauda epididymal spermatozoa (Westbrook-Case et al., 1994). In this study we used light and electron microscopic immunocytochemistry to define the expression pattern and sorting pathway that establishes the domain-specific distribution of PM52 during spermiogenesis. The PM52 is first expressed in acrosome-phase spermatids and it localizes exclusively to the cytoplasmic lobe. Immunoelectron microscopy revealed that both cytoplasmic vesicles and the plasma membrane of the cytoplasmic lobe labeled with anti-PM52. During early stages of expression, PM52 appeared to be absent from the head region, but significant PM52 accumulation over the spermatid head was noted in late acrosomal phase spermatids. Throughout spermiogenesis PM52 extended posteriorly to the annulus, which represents a barrier preventing PM52 diffusion into the posterior tail. Following the migration of the annulus to the midpiece-principal piece junction, PM52 began to disappear from the flagellar region and at the completion of spermiogenesis most of the PM52 was restricted to the acrosomal segment. Spermatids and epididymal sperm PM52 exhibited identical sizes by SDS-PAGE and immunoblotting, indicating that they are not proteolytically modified during epididymal maturation. The PM52 antibodies were also used to screen a guinea pig testis cDNA library, and sequence determination of full-length PM52 clones demonstrated identity of a sperm membrane protein recently termed "sperad" (Quill and Garbers, 1996). Membrane barriers and potential mechanisms establishing the domain-specific residence of PM52 are discussed.

AFM analysis of DNA-protamine complexes bound to mica

A novel method for reconstituting sperm chromatin was used to investigate how protamine 1 condenses DNA. Complexes formed in vitro using linearized plasmid DNA were imaged and measured by atomic force microscopy (AFM). The structures formed were found to be highly dependent on the sample preparation method used for reconstitution. Interstrand, side-by-side fasiculation of DNA and toroidal-like structures only 1-2 DNA diameters thick were observed for complexes formed in solution following direct mixing of the DNA and protamine. Large chromatin aggregates were also observed on the mica. However, if the DNA was first allowed to attach to the mica prior to addition of the protamine, well-defined toroidal complexes were formed without any observed DNA fasiculation or aggregate formation. The diameter of the toroids measured 30.6-50.2 nm (mean 39.4 nm). The dimensions of these structures indicate that the condensed DNA is stacked vertically by four to five turns, with each coil containing as little as 360-370 bp of 'B'-form DNA. This approach for preparing and imaging DNA-protamine complexes permits the analysis of intermediate structures 'trapped' on the mica as partially formed toruses of nucleoprotamine.

MMM1 encodes a mitochondrial outer membrane protein essential for establishing and maintaining the structure of yeast mitochondria

In the yeast Saccharomyces cerevisiae, mitochondria are elongated organelles which form a reticulum around the cell periphery. To determine the mechanism by which mitochondrial shape is established and maintained, we screened yeast mutants for those defective in mitochondrial morphology. One of these mutants, mmm1, is temperature-sensitive for the external shape of its mitochondria. At the restrictive temperature, elongated mitochondria appear to quickly collapse into large, spherical organelles. Upon return to the permissive temperature, wild-type mitochondrial structure is restored. The morphology of other cellular organelles is not affected in mmm1 mutants, and mmm1 does not disrupt normal actin or tubulin organization. Cells disrupted in the MMM1 gene are inviable when grown on nonfermentable carbon sources and show abnormal mitochondrial morphology at all temperatures. The lethality of mmm1 mutants appears to result from the inability to segregate the aberrant-shaped mitochondria into daughter cells. Mitochondrial structure is therefore important for normal cell function. Mmm1p is located in the mitochondrial outer membrane, with a large carboxyl-terminal domain facing the cytosol. We propose that Mmm1p maintains mitochondria in an elongated shape by attaching the mitochondrion to an external framework, such as the cytoskeleton.

Atomic force microscopy of mammalian sperm chromatin

We have used the atomic force microscope (AFM) to image the surfaces of intact bull, mouse and rat sperm chromatin and partially decondensed mouse sperm chromatin attached to coverglass. High resolution AFM imaging was performed in air and saline using uncoated, unfixed and unstained chromatin. Images of the surfaces of intact chromatin from all three species and of an AFM-dissected bull sperm nucleus have revealed that the DNA is organized into large nodular subunits, which vary in diameter between 50 and 100 nm. Other images of partially decondensed mouse sperm chromatin show that the nodules are arranged along thick fibers that loop out away from the nucleus upon decondensation. These fibers appear to stretch or unravel, generating narrow smooth fibers with thicknesses equivalent to a single DNA-protamine complex. High resolution AFM images of the nodular subunits suggest that they are discrete, ellipsoid-shaped DNA packaging units possibly only one level of packaging above the protamine-DNA complex.

Molecular organization of the plasma membrane in the post-acrosomal region of some farm animals

According to the distribution of IMP, three different regions can be recognized on PF of the post-acrosomal plasma membrane of bull, ram, and boar spermatozoa. They are: (1) a region with linear aggregation of IMP, (2) a region with fewer and scattered IMP, and (3) a region with more numerous IMP. In the last two regions IMPs are randomly distributed or a clustering of certain particles is visible. In stallion spermatozoa the last two areas are undistinguishable. There are evident interspecies differences in the arrangement of linear aggregations of IMP which are characteristic for each species.

Plasma membrane alterations of maturing goat (Capra indicus) spermatozoa: lectin-binding and freeze-fracture study

A qualitative and quantitative analysis of lectin-binding sites has been undertaken on spermatozoa recovered from different regions of the epididymis of the goat (Capra indicus) using fluorescein isothiocyanate-linked lectins (Bauhinia purpurea BPA, Concanavalin A Con A, Dolichos biflorus DBA, Maclura pomifera MPA, Arachis hypogaea or peanut agglutinin PNA, Glycine max or soyabean agglutinin SBA, Ulex europaeus UEA, and Triticum vulgaris or wheat-germ agglutinin WGA), in conjunction with scanning and transmission electron microscopy, and freeze-fracture techniques. Flow cytometric analysis has also been used to quantitize binding affinity. Spermatozoa from caput to cauda epididymidis show no significant variation in lectin-binding ability, but the samples removed from the corpus epididymidis contain a greater number of binding sites. The passage of spermatozoa through the epididymidis is accompanied by a redistribution of the plasma membrane lectin-receptors covering the sperm head and tail. Receptors for BPA, DBA, PNA and SBA are specifically restricted to the anterior region of the acrosome in caudal spermatozoa. Freeze-fracture replicas, examined to study changes in organisation of intramembranous particles of the plasma membrane during sperm maturation, reveal distinct changes in their distribution in the acrosome, post-acrosome and spermatozoon tail, especially in the corpus and cauda epididymidis.

Changes in the distribution of intramembranous particles and filipin-reactive membrane sterols during in vitro capacitation of golden hamster spermatozoa

Membrane alterations accompanying in vitro capacitation of hamster spermatozoa were examined using the freeze-fracture technique with or without use of filipin, a sterol-binding probe. In the spermatozoa prior to or at 10 min after start of incubation in capacitating medium, large (about 11 nm) and small (8-9 nm) intramembranous particles (IMPs) were present in the periacrosomal region of the sperm plasma membrane (PAPM). Filipin sterol complexes (FSCs) were densely (about 500/micron 2) distributed in the PAPM prior to incubation. The density of FSCs in the PAPM was reduced by 70-80% of the original density by 2 hr of incubation. At the same time, small patches of IMP-free areas appeared in the plasma membrane above the equatorial and middle segments of the acrosome. By the end of 3 hr of incubation, the majority of small IMPs had disappeared from the PAPM. Remaining large and small IMPs tended to aggregate in the PAPM. During incubation in capacitation medium, "cords," or linear arrangements of closely packed IMPs, appeared near the posterior ring of the sperm head. These observations strongly suggest that the acrosome reaction of the hamster spermatozoa is preceded by the removal (deletion) of filipin-reactive sterols (FRSs) and the disappearance of small IMPs from the lipid bilayer of PAPM.

Nucleus of the boar spermatozoon: structure and modifications in frozen, frozen-thawed, or sodium dodecyl sulfate-treated cells

After cryosubstitution and Epon embedding, or after Nanoplast embedding and very thin sectioning, the chromatin of ejaculated or diluted boar spermatozoa appears to be formed of DNA fibers embedded in a quite homogeneous matrix. After sodium dodecyl sulfate (SDS) treatment, and to a lesser extent after freeze-thawing, the DNA fibers are present mostly between cords, probably proteinaceous in nature. The quantity of free sulfhydryl (SH) groups, as calculated from staining by DACM and flow fluorometry, is increased in thawed or SDS-treated cells. The quantity of NH2 groups, calculated from electron microscopy image analysis of alcoholic phosphotungstic acid-stained cells, is decreased in thawed nuclei. The DNA is more accessible to the fluorochrome ethidium bromide after freeze-thawing, and its sensitivity to HCl hydrolysis is modified, during the Feulgen-like staining procedure using acriflavine. The X-ray energy dispersive analysis of cryosections of nuclei indicates that the slight separation of DNA and nucleoproteins in freeze-thawed spermatozoa could result from a dramatic modification of the nuclear ionic environment during thawing.

A birefringence, electron microscopy, and histochemical survey of chromosomal structure in the sperm nuclei of an echinothurid sea urchin, Araeosoma owstoni

The mature sperm head of Araeosoma owstoni, an echinothurid sea urchin, showed positive birefringence reflecting that the overall orientation of DNA molecules was semiperpendicular toward the nuclear axis of the sperm head. Transmission electron microscopical observation of sperm in this species revealed a highly electron-dense cylindrical coil with an empty central core extending along the major axis of the sperm head. This coil had seven to eight turns along its entire length of 3.5 micron. The maximum width was 0.35 micron near the distal end of the nucleus, and the minimum width was 0.17 micron near the apical end. Lamellar substructures were also present in the sperm nucleus, appearing at the periphery of the electron-dense cylinder in a radial manner. Staining with Feulgen's reaction and acid-orcein indicated that the coil was probably composed of sperm chromosomes.

Characterization of the postacrosomal sheath of bovine spermatozoa

A purified head fraction was prepared from bovine epididymal spermatozoa and was utilized to identify the solubility characteristics and major polypeptide components of the postacrosomal sheath. Sperm heads extracted in nonionic-detergent-containing or high-salt-containing solutions retained an intact postacrosomal sheath, but it was readily solubilized by high pH extraction solutions. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed a major polypeptide of 58,000 daltons (58-kD) in the high pH extract solution. Antibodies to the 58-kD polypeptide specifically reacted with the postacrosomal segment by immunofluorescence and by electron microscopic immunohistochemistry were shown to bind the postacrosomal sheath. We conclude that this 58-kD polypeptide is a constituent of the postacrosomal sheath and that its distribution is restricted to the postacrosomal segment.

Changes in the distribution of intramembranous particles and filipin-sterol complexes during epididymal maturation of golden hamster spermatozoa

Golden hamster spermatozoa in various segments of the excurrent duct system were studied by freeze-fracture with and without filipin treatment. Two types of regular IMP (intramembranous particle) patterns temporarily appear on the plasma membrane covering the sperm head. One is a hexagonal arrangement seen in the acrosomal region, and the other is a linear arrangement near the posterior ring. Both patterns are seen in the spermatozoa from the corpus epididymidis. The FSC (filipin-sterol complex) density in the plasma membrane covering the acrosome increases from about 400 to 500 FSC/microns2 during epididymal passage. In this region, the majority of the membrane sterols appears to reside on the outer leaflet of the lipid bilayer. When the spermatozoa reach the cauda epididymidis, FSCs in the outer acrosomal membrane virtually disappear from the apical segment, while they increase in the middle segment (250 FSC/microns2). These observations are discussed in relation to epididymal maturation.

Membrane changes in Chinese hamster spermatozoa during epididymal maturation

Chinese hamster spermatozoa during epididymal maturation were examined by thin sectioning, freeze-fracture, and surface replica. Membrane-limited vesicles and tubules (MVTs) attach to the plasma membrane over the acrosome of the spermatozoa in the distal caput through proximal cauda epididymidis. The origin of these MVTs is likely to be spermatozoa degenerating in the epididymal lumen. The attachment of MVTs to the plasma membrane seems to be mediated by a paste-like substance covering the plasma membrane. A parallel striation pattern of intramembranous particles (IMPs) is seen in the plasma membrane of almost the entire postacrosomal region of the epididymal spermatozoa. The patterned domain begins to appear in the proximal caput epididymidis. The number and density of IMPs in the plasma membrane of the postacrosomal region increases with the development of striated pattern of IMPs in this region. In the cauda epididymidis, the redundant nuclear envelope elongates to form a shirt-like membrane covering the mitochondrial sheath. The elongated portion of the nuclear envelope is devoid of nuclear pores and has few IMPs.

Distribution of intramembranous particles and filipin-sterol complexes in mouse sperm membranes: polyene antibiotic filipin treatment

The distribution of intramembranous particles (IMPs) and membrane filipin-sterol complexes (FSC) was examined ultrastructurally in mouse spermatozoa from the male reproductive tract and ejaculates. IMPs were qualitatively analyzed on freeze-fracture replicas of glutaraldehyde-fixed tissue, while membrane FSC were quantitatively analyzed on replicas of filipin-treated cells. The distribution pattern of IMPs of mouse spermatozoa was fundamentally similar to that of other mammalian spermatozoa. 1) In the head, the plasma membrane had a heterogeneous population density, e.g., few IMPs on the acrosomal region, particularly few on the marginal segment, and somewhat regularly arranged IMPs on the postacrosomal region. The acrosomal membrane had many IMPs in hexagonal arrays. The nuclear membrane had many IMPs on the P-face, few IMPs on the variegated E-face, and an intense population density on the P-face of the basal plate. 2) In the neck, the plasma membrane had many IMPs with square arrangements of small IMPs in some areas on the P-face; the redundant nuclear membrane had a few IMPs on both P- and E-faces. 3) In the tail, the plasma membrane had diagonal rows of IMPs in some areas amongst larger IMPs on the middle piece, while it had "zippers" composed of IMPs running parallel to the axis on the principal piece. The distribution of sperm membrane FSC may be summarized as follows: 1) In the head, the acrosomal plasma membrane, which was heavily labeled with filipin, had much more FSC in the equatorial segment than in the marginal segment throughout the study. The postacrosomal plasma membrane generally had no FSC, but some sperm in ejaculates were slightly positive to filipin. The acrosomal membranes (both outer and inner) had no FSC. The nuclear membrane in the main part of the head had less FSC in vas deferens and ejaculated sperm than in the epididymal sperm. The nuclear membrane on the basal plate had no FSC. 2) In the neck, the plasma membrane had little FSC. The redundant nuclear envelope had scattered FSC with a higher incidence in the epididymal sperm than in those from the vas deferens and ejaculates. The membrane scroll, which was elongated from the extreme caudal end of the redundant nuclear envelope, had abundant FSC in the vas deferens and ejaculated sperm. 3) The tail plasma membrane (both middle and principal piece), which was weakly labeled with filipin, had less FSC in sperm from the vas deferens and ejaculates than in those from the epididymis. The limiting membrane covering the mitochondria had no FSC.

Interactions of nuclear proteins with DNA, during sperm differentiation in the ram

Ram spermatid nuclei and caput epididymal sperm nuclei were prepared and treated with DTT under conditions avoiding proteolysis. Whole-mount preparations for the electron microscope were made in the presence or absence of the detergent Joy. The chromatin of the less mature, non-round spermatid nuclei displayed a nucleosomal organization that gradually disappeared at the time the histones leave the nuclei (elongating spermatids). Digestion with micrococcal nuclease suggests that polynucleosome arrays are scarcer and more accessible to nuclease in the elongating than in the round nuclei, with increasing amounts of DNA becoming devoid of nucleosomes. In the protamine-containing nuclei (elongated spermatids), only smooth filaments were observed, which formed thick fibers by parallel aggregation. The change from a nucleosomal organization to bundles of smooth filaments appeared to result from a complex process involving the transitory presence of conspicuous "knobby fibers" that suggest a periodicity in the organization of the spermatidal proteins along the DNA molecules. X-ray diffraction patterns obtained with protamine-containing spermatid nuclei and with sperm nuclei confirm that the DNA is arranged in smoothly bent bundles of parallel molecules. No higher-order reflections that might correspond to nucleosome structures were detected in the 30-200 A region.

Morphogenesis and cytochemistry of the postacrosomal dense lamina during mouse spermiogenesis

During the elongation phase of spermiogenesis in the mouse, a layer of electron-dense material appears just below the posterior portion of the acrosomal zonule. Subsequently this material accumulates on the outer side of the nuclear envelope immediately subjacent to the caudal tip of the acrosomal zonule--the anlage of the future postnuclear band--as well as on the inner side of the plasma membrane vis-à-vis to this region--the anlage of the future postacrosomal dense lamina (PADL). Corresponding to further development the postacrosomal region of the nucleus becomes directly enveloped by the plasma membrane, and the PADL, situated on its inner side, grows adequately. The postnuclear band, however, staying the same size as in the preceding elongation phase, gets shifted to the caudal end of the PADL, where it closes the perinuclear space. Since the anlagen and the mature PADL and postnuclear band show the same cytochemical reactions as the dense basal plaque of the acrosomal zonule and the thin layer on the nuclear envelope vis-à-vis to it, a relationship between these structures can be assumed. Furthermore, the demonstration of ribonucleoproteins in all these structures is discussed in connection with a possible nucleolar genesis.

Partially irreversible cold-induced lipid phase transitions in mammalian sperm plasma membrane domains: freeze-fracture study

In an effort to investigate the nature of the cellular injury caused when mammalian spermatozoa are cooled prior to cryopreservation, the occurrence of thermal phase transitions amongs the lipid components of the sperm plasma membrane was investigated by the use of freeze-fracture electron microscopy. The mechanisms by which glycerol and egg yolk exert protective effects during semen cooling and freezing were also examined. Ram and blackbuck spermatozoa, maintained at 30 degrees C prior to fixation at this temperature, exhibited randomly distributed intramembranous particles over the acrosomal, postacrosomal, and flagellar regions of the plasma membrane. In contrast, spermatozoa fixed at 5 degrees C after slow cooling to this temperature exhibited particle clustering over the postacrosomal region of the head as well as over the tail. These effects were not influenced by the presence of egg yolk or glycerol during the cooling procedure, although these substances protected the spermatozoa against loss of motility. Particle clustering over the sperm tail, induced by the slow cooling process, was found to be only partially reversible. The extensive areas of particle-free lipid, noted to result from the cooling procedure, were absent if the spermatozoa were rewarmed to 30 degrees C; however, the original distribution of particles was not restored and numerous small particle-free domains persisted. It is proposed that this type of irreversible change within the sperm plasma membrane may contribute to the loss of motility and fertility suffered by spermatozoa after cooling and freezing. Furthermore, it is suggested that protective substances such as egg yolk may exert their effects by countering these deleterious changes, rather than by preventing their occurrence.

Cholesteric organization of DNA in the stallion sperm head

The fine structure of chromatin in sperm heads was investigated by different microscopic techniques: in vivo examinations in the polarizing microscope, thin sections and freeze-fracture replicas observed by transmission electron microscopy. The freeze-fractured chromatin appears to be formed of superimposed lamellae, each one 330 A thick. These lamellae are parallel to the flattening plane of the sperm head. This situation was already described in other mammal spermatozoa and in particular in the bull and the rabbit. This work presents a new interpretation of this lamellated aspect. The chromatin structure of these spermatozoa is that of a cholesteric liquid crystal. This structure resembles that of a plywood, made of superimposed layers of parallel filaments, but instead of having a right angle between two successive layers, there is a progressive rotation and similar orientation occurs at each 180 degrees rotation. The apparent lamellae result from cleavages due to freeze-fracture between levels of parallel filament orientation. The thickness of lamellae corresponds therefore to the half helicoidal pitch of the cholesteric liquid crystal. This model is consistent with our observations by polarizing microscopy. The lamellation is not visible in thin sections of stallion spermatozoa. There are however biochemical methods to decondense chromatin and we are able to observe this lamellation in sections normal to the flattening plane of sperm heads. The methods used classically to decondense the sperm chromatin lead to extremely varied aspects which are discussed, some of them being closely related to the structure of cholesteric liquid crystals.

Substructure of the postacrosomal sheath of bovine spermatozoa

The substructure of the postacrosomal sheath and its relationship to the plasma membrane and nuclear membrane complex were examined in thin-section, negative-stain, surface-replica, and freeze-fracture preparations. The matrix of the postacrosomal sheath contains a single layer of closely associated 10- to 12-nm filamentous elements aligned parallel to the long axis of the sperm. A precise lateral interaction of the filaments is suggested from negative-stain images which reveal a second set of parallel striations extending over the surface of the sheath at 60 degrees relative to the filament long axis. Several structural differences between the posterior and anterior segments and the outer and inner surface of the postacrosomal sheath were identified. Data on structural specializations of the plasma membrane and nuclear membrane complex which relate to the asymmetric structure are presented and their potential significance in fertilization events discussed.

Initial experiments with cyproterone acetate and 1-amino-3-chloro-2-propanol hydrochloride in the male Virginia opossum (Didelphis virginiana)

Cyproterone acetate (CA) was toxic for Didelphis virginiana at a dose range of 22-28 mg/kg IM. A loss of spermatozoal pairing int the cauda epididymidis did not occur until 2 wk after testicular degeneration caused by CA. The isomers of 1-amino-3-chloro-2-propanol hydrochloride (CL 88,236 and CL 88,237) were well tolerated by the opossum at doses up to 30 mg/kg IM. After 8 days of CL 88,236 and CL 88,237 treatment there was no effect on spermatozoal pairing in the cauda epididymidis or on the seminiferous epithelium.

A model for the structure of chromatin in mammalian sperm

DNA in mammalian, and most vertebrate sperm, is packaged by protamines into a highly condensed, biochemically inert form of chromatin. A model is proposed for the structure of this DNA-protamine complex which describes the site and mode of protamine binding to DNA and postulates, for the first time, specific inter- and intraprotamine interactions essential for the organization of this highly specialized chromatin. In this model, the central polyarginine segment of protamine binds in the minor groove of DNA, crosslinking and neutralizing the phosphodiester backbone of DNA while the COOH- and NH2-terminal ends of protamine participate in the formation of inter- and intraprotamine hydrogen, hydrophobic, and disulfide bonds. Each protamine segment is of sufficient length to fill one turn of DNA, and adjacent protamines are locked in place around DNA by multiple disulfide bridges. Such an arrangement generates a neutral, insoluble chromatin complex, uses all protamine sulfhydryl groups for cross linking, conserves volume, and effectively renders the chromatin invulnerable to most external influences.

Changes in intramembranous particle distribution epididymal spermatozoa of the boar

Membranes of boar spermatozoa from different regions of the epididymis and after ejaculation were studied by the freeze-fracture replica technique. The ordered pattern of the intramembranous particles of spermatozoan plasma membranes was different in the five arbitrary zones of the epididymis and in the semen. A distinctive ordered pattern was absent in zone 1, which is the proximal segment of the epididymis. In zone 2, paired parallel rows of the particles were present in the plasma membrane over the acrosomal region. This parallel arrangement was not present in zone 3 spermatozoa. Anterior to the posterior ring, cords formed by packed particles were apparent in zone 2 spermatozoa and reached their maximum prominence in zone 3, and persisted in zones 4 and 5 and in the semen. The plasma membrane over the marginal ridge of the acrosome had a hexagonal array of particles only in zones 4 and 5 spermatozoa. A similar pattern appeared on the post-acrosomal region of spermatozoa in zone 5 and in the semen. The plasma membrane of the middle piece had a rectilinear arrangement of the particles in zone 2 spermatozoa in which the migration of the cytoplasmic droplet was complete. Rudiments of the rectilinear arrangement persisted in spermatozoa in zones 4 and 5 and in the semen. These changes are discussed in relation to sperm maturation in the epididymis. The acrosomal membrane had a hexagonal arrangement of particles in the equatorial segment. The marginal ridge of the outer acrosomal membrane had parallel rows of intramembranous particles. The organization of the acrosomal membrane particles did not change during the epididymal passage of boar spermatozoa.

Fine structure of spermatozoa of the Asiatic musk shrew, Suncus murinus

The fine structure of spermatozoa of the Asian musk shrew, Suncus murinus, has been investigated using air-dried and critical-point-dried replicas in addition to freeze-fractured specimens. Asymmetry in the structure of the head, notable the acrosome, can be recognized in critical-point-dried replicas as a slight dorsoventral convexity not apparent in air-dried specimens. Freeze-etching reveals a dense accumulation of intramembranous particles just anterior to the posterior ring, but relatively few other characteristic membrane conformations associated with the head. Gelatin smears visualized with the light microscope show a vigorous lytic response, but preliminary efforts to remove the acrosome by physiological or enzymatic incubation have thus far not been successful.

Nuclear pores in the spermatozoon of the rat

The paper describes a hexagonal array of nuclear pores in a non-redundant region of the nuclear envelope underlying the basal surface of the rat spermatozoon head. It is concluded that intranuclear material protruding through these pores is the cause of the characteristic rows of circular 'bumps' found in surface replicas of this region.

Fine structural localization of Concanavalin A binding sites on hamster spermatozoa

The plasma membrane of epididymal spermatozoa of the golden hamster (Mesocricetus auratus) exhibits morphological differences over various parts of the head and tail as detected by air-dried replicas and freeze-etching techniques. In an attempt to ascertain whether any topographical differences exist in the number or distribution of carbohydrate moieties associated with the cell surface, cells were labeled with Concanavalin A and marked with hemocyanin. It was found that while the plasma membrane over the acrosomal region differed from that of the postacrosomal region in membrane components revealed by freeze fracturing, there was no apparent difference in the distribution or density of Con A binding sites detectable by hemocyanin localization. The tail regions exhibited differences in both fracture face appearance and the distribution of detectable carbohydrate moieties. It was also found that binding sites for Concanavalin A exist on the inner and outer acrosomal membranes in addition to those on the plasma membrane.

Acrosomal disruption in sperm. Freeze-fracture of altered membranes

"Capacitation" is a physiological event which alters sperm to permit rapid penetration through oocyte investments and fusion between gametes. Acrosomal "reaction," the physiological release of acrosomal contents, occurs after this facilitating process. In this study, acrosomal "disruption" of guinea pig and rat sperm was achieved in vitro by incubating sperm together with the follicular contents of superovulated mice. The samples contained both "reacted" and "disrupted" sperm. Thin sections of affected sperm revealed rupture and vesiculation of the plasma membrane overlying the acrosome, as well as loss of both the outer acrosomal membrane and the acrosomal content. Freeze-fracture revealed disintegration of the characteristic geometric patterns in regions of the acrosomal and plasma membranes thus disrupted and major modifications in particle distribution in the sperm tail. In the guinea pig, strands of 6-8-nm particles, usually confined to the plasma membrane of the midpiece, which overlies mitochondria, also appeared in the principal piece. Likewise, in rat sperm, bands of similarly small particles formed acute angles throughout the membrane of the principal piece. Compared with the membranes of control preparations, these membrane alterations are apparently a direct consequence of incubation with ovarian follicular contents.