Media induced alterations of the membrane associated particles of the guinea pig sperm tail

Identification of the major tyrosine phosphorylated protein of capacitated hamster spermatozoa as a homologue of mammalian sperm a kinase anchoring protein

The molecular basis of mammalian sperm capacitation is unique in that, it is associated with a protein kinase A (PKA) dependent upregulation of protein tyrosine phosphorylation. Therefore, PKA activity during capacitation would be crucial for the downstream events of protein tyrosine phosphorylation, and mechanisms may exist to ensure that PKA phosphorylates its specific substrate. This could be achieved by bringing PKA close to its substrate, a function normally carried out by an A-kinase anchoring protein (AKAP). We showed previously that cauda epididymidal spermatozoa of hamster undergo a capacitation-dependent increase in protein tyrosine phosphorylation. In the present study, evidence is provided that two major tyrosine phosphorylated proteins of molecular weight 97 and 83 kDa are the hamster homologues of mouse pro-AKAP82 and AKAP82, and have been designated as hamster pro-AKAP83 and AKAP83 respectively. Hamster AKAP83 resembled the mouse AKAP82 with respect to its molecular weight, pI (pH 5-5.5) and cDNA and amino acid sequences. Sequence analysis indicated that the primary structure of pro-AKAP83 was highly conserved and exhibited 91% identity with mouse and rat AKAP82. Further, the functional domains, namely the region involved in binding the regulatory subunit of PKA and the proteolytic cleavage site between pro-AKAP83 and AKAP83, were identical with that observed in rat and mouse pro-AKAP82 and AKAP82. Immunoblot analysis using polyclonal hamster anti-AKAP83 antibodies indicated that AKAP83 was present both in caput and cauda epididymidal spermatozoa. The antibody also identified the pro-AKAP82 and AKAP82 in mouse caput and cauda epididymidal spermatozoa. Immunofluorescence studies indicated that AKAP83 in hamster spermatozoa was localized along the length of principal piece of the tail. It was also demonstrated that hamster pro-AKAP83/AKAP83 gene expression was testis specific and was not expressed in other organs in either sex. This is the first report implicating AKAP in capacitation in rodents.

Acrosome reaction: methods for detection and clinical significance

The present article reviews the methods for detection and the clinical significance of the acrosome reaction. The best method for the detection of the acrosome reaction is electron microscopy, but it is expensive and labour-intensive and therefore cannot be used routinely. The most widely used methods utilize optical microscopy where spermatozoa are stained for the visualization of their acrosomal status. Different dyes are used for this purpose as well as lectins and antibodies labelled with fluorescence. The acrosome reaction following ionophore challenge (ARIC) can separate spermatozoa that undergo spontaneous acrosome reaction from those that are induced, making the result of the inducible acrosome reaction more meaningful. Many different stimuli have been used for the induction of the acrosome reaction with different results. The ARIC test can provide information on the fertilizing capability of a sample. The ARIC test was also used to evaluate patients undergoing in vitro fertilization since a low percentage of induced acrosome reaction was found to be associated with lower rates of fertilization. The cut-off value that could be used to identify infertile patients is under debate. Therapeutic decisions can also be made on the basis of the value of the ARIC test.

Cholesterol efflux-mediated signal transduction in mammalian sperm: cholesterol release signals an increase in protein tyrosine phosphorylation during mouse sperm capacitation

We previously demonstrated that mouse sperm capacitation is accompanied by a time-dependent increase in protein tyrosine phosphorylation that is dependent on the presence of BSA, Ca2+, and NaHCO(3), all three of which are also required for this maturational event. We also demonstrated that activation of protein kinase A (PK-A) is upstream of this capacitation-associated increase in protein tyrosine phosphorylation. BSA is hypothesized to modulate capacitation through the removal of cholesterol from the sperm plasma membrane. In this report, we demonstrate that incubation of mouse sperm medium containing BSA results in a release of cholesterol from the sperm plasma membrane to the medium; release of this sterol does not occur in medium devoid of BSA. We next determined whether cholesterol release leads to changes in protein tyrosine phosphorylation. Blocking the action of BSA by adding exogenous cholesterol-SO-(4) to the BSA-containing medium inhibits the increase in protein tyrosine phosphorylation as well as capacitation. This inhibitory effect is overcome by (1) the addition of increasing concentrations of BSA at a given concentration of cholesterol-SO-(4) and (2) the addition of dibutyryl cAMP plus IBMX. High-density lipoprotein (HDL), another cholesterol binding protein, also supports the capacitation-associated increase in protein tyrosine phosphorylation through a cAMP-dependent pathway, whereas proteins that do not interact with cholesterol have no effect. HDL also supports sperm capacitation, as assessed by fertilization in vitro. Finally, we previously demonstrated that HCO-(3) is necessary for the capacitation-associated increase in protein tyrosine phosphorylation and demonstrate here, by examining the effectiveness of HCO-(3) or BSA addition to sperm on protein tyrosine phosphorylation, that the HCO-(3) effect is downstream of the site of BSA action. Taken together, these data demonstrate that cholesterol release is associated with the activation of a transmembrane signal transduction pathway involving PK-A and protein tyrosine phosphorylation, leading to functional maturation of the sperm.

Capacitation of mouse spermatozoa. I. Correlation between the capacitation state and protein tyrosine phosphorylation

The molecular basis of mammalian sperm capacitation, defined functionally as those processes that confer on the sperm the acquisition of fertilization-competence either in vivo in the female reproductive tract or in vitro, is poorly understood. We demonstrate here that capacitation of caudal epididymal mouse sperm in vitro is accompanied by a time-dependent increase in the protein tyrosine phosphorylation of a subset of proteins of M(r) 40,000-120,000. Incubation of sperm in media devoid of bovine serum albumin, CaCl2 or NaHCO3, components which individually are required for capacitation, prevent the sperm from undergoing capacitation as assessed by the ability of the cells to acquire the pattern B chlortetracycline fluorescence, to undergo the zona pellucida-induced acrosome reaction and, in some cases, to fertilize metaphase II-arrested eggs in vitro. In each of these cases the protein tyrosine phosphorylation of the subset of capacitation-associated proteins does not occur. Protein tyrosine phosphorylation of these particular proteins, as well as sperm capacitation, can be recovered in media devoid of each of these three constituents (bovine serum albumin, CaCl2 or NaHCO3) by adding back the appropriate component in a concentration-dependent manner. The requirement of NaHCO3 for these phosphorylations is not due to an alkalinization of intracellular sperm pH or to an increase in media pH. Caput epididymal sperm, which lack the ability to undergo capacitation in vitro, do not display this capacitation-dependent subset of tyrosine phosphorylated proteins in complete media even after extended incubation periods, and do not fertilize metaphase II-arrested eggs in vitro.(ABSTRACT TRUNCATED AT 250 WORDS)

Actin polymerization in boar spermatozoa: fertilization is reduced with use of cytochalasin D

The aggregational state of actin in boar spermatozoa after capacitation and the acrosome reaction has been examined by several methods. In vitro fertilization (IVF) experiments were conducted in the presence and absence of cytochalasin D (CD) to evaluate the role of actin polymerization in the events of fertilization. The fertilizing capacity was very high in controls, but, when CD (an inhibitor of the polymerization of actin) was added to the capacitation medium, there was a marked decrease in the fertilizing capacity of the boar spermatozoa. There was a further decrease when CD was present during both capacitation and fertilization processes. In addition to the IVF tests, biochemical and immunoelectron microscopic methods were used to analyze the state of aggregation of actin in boar spermatozoa after capacitation, and the acrosome reaction. By immunoelectron microscopy with a phalloidin probe, there were no gold particles, indicating the presence of F-actin on boar sperm heads capacitated and acrosome-reacted in media containing CD. By sodium dodecyl sulfate-polyacrylamide gel electrophoresis there were differences in NP-40 solubility, reflecting actin polymerization, between CD-treated and untreated sperm. These results suggest that actin polymerizes during capacitation and the acrosome reaction and that this polymerization is essential to the fertilization process.

Capacitated and acrosome reacted spermatozoa of goat (Capra indicus): a fluorescence and electron microscopic study

Plasma membrane alterations accompanying in vitro capacitation and acrosome reaction of goat spermatozoa were studied using lectin labelling, scanning electron microscopy, and freeze-fracture methods. Fluorescein isothiocyanate linked lectins namely; Canavalia ensiformis (ConA), Maclura pomifera (MPA), Arachis hypogaea (PNA), Glycine max (SBA) and Triticum vulgaris (WGA) agglutinin were used to examine the distribution of surface carbohydrates during these two events. The head and the sperm tail reveal altered lectin labelling features after capacitation and acrosome reaction. After capacitation the surface coat components for MPA, SBA, and WGA are shed from the spermatozoon head. ConA receptors on the head are retained after capacitation but are partially shed in the acrosome reacted spermatozoa. SBA receptor sites appear on the sperm tail of the capacitated spermatozoa. Unusual morphological changes attending capacitation involve the sperm tail-end which develops a novel entity, which we have termed 'spatula'. The 'spatula' shows strong binding with ConA and WGA only. In the acrosome reacted spermatozoa the spatulated tail-end unwinds with a concomitant loss of lectin labelling. Highly ordered membrane particles, 'ladders' of the middle piece of the epididymal sperm tail, disappear and IMP clearings appear on the middle piece and in the spatulated ends of the capacitated spermatozoa. But in the acrosome reacted sperm IMPs reappear and are randomly disposed on the middle-piece and are clustered in small patches on the principal-piece. IMP free areas appear on the plasma membrane covering the acrosome and the outer acrosomal membrane (OAM) of the capacitated spermatozoa. The plasma membrane and OAM fuse at multiple foci and appear as acrosomal 'ghosts' which remain associated with the sperm head even after acrosome reaction.

Flow cytometric analysis of mouse sperm using monoclonal anti-sperm antibody OBF13

The variable reactivity of OBF13 monoclonal antibody to mouse sperm was studied using a fluorescein activated cell sorter. Sperm from cauda epididymis were incubated with ionophore A23187, subjected to indirect immunostaining and analyzed by a cell sorter. Three peaks showing different fluorescence intensities were observed. These peaks contained (i) not stained (N), (ii) acrosomal cap or anterior region stained (A) and (iii) head stained (H) sperm, respectively. H type sperm showed more intense integral fluorescence than A type sperm. It was also noted that the H type were observed when cauda epididymal sperm were incubated with A23187, but not among non-incubated, or A23187 treated caput epididymal sperm. When sperm were pre-categorized as "dead" or "alive" by propidium iodide staining, no A type were observed in the "live" population. These results suggest that the sperm exhibiting the OBF13 antigen in the acrosome region lost their viability before they accomplished a "true" acrosome reaction.

Carbohydrate determinants involved in mammalian fertilization

During fertilization in mammals, the male and female gametes undergo a form of highly complex cell-cell recognition whereby a nonspecific initial binding is followed by a species-specific penetration of the zona pellucida. Recent data from many species have demonstrated the involvement of surface carbohydrates in regulating fertilization at both these stages. The potential benefits as well as drawbacks of three major techniques used so far are discussed, and the need for a cautious interpretation of the data is emphasized. During capacitation, the carbohydrate components of the entire surface of spermatozoa undergo striking changes which may be linked to the concurrent metabolic events within motile spermatozoa, leading to the appearance of egg-specific glycoconjugates in a time-dependent manner. A multiple set of glycoproteins on the sperm surface, possessing oligosaccharides synthesized by the lipid-linked pathway, are probably required during different stages of fertilization, including sperm-oocyte fusion. The oviductal glycosaminoglycans may also be involved in regulating the timing and species specificity of mammalian fertilization by masking the sperm receptor sites on the zona and triggering the physiological acrosome reaction. Future biochemical and high-resolution localization studies involving specific probes for surface glycoconjugates, glycosyltransferases, and hydrolytic enzymes should greatly aid our understanding not only of the role of the individual surface macromolecules but also of the surface domains to which they are localized.

Localized surface antigens of guinea pig sperm migrate to new regions prior to fertilization

We have previously defined distinct localizations of antigens on the surface of the guinea pig sperm using monoclonal antibodies. In the present study we have demonstrated that these antigen localizations are dynamic and can be altered during changes in the functional state of the sperm. Before the sperm is capable of fertilizing the egg, it must undergo capacitation and an exocytic event, the acrosome reaction. Prior to capacitation, the antigen recognized by the monoclonal antibody, PT-1, was restricted to the posterior tail region (principle piece and end piece). After incubation in capacitating media at 37 degrees C for 1 h, 100% of the sperm population showed migration of the PT-1 antigen onto the anterior tail. This redistribution of surface antigen resulted from a migration of the surface molecules originally present on the posterior tail. It did not occur in the presence of metabolic poisons or when tail-beating was prevented. It was temperature-dependent, and did not require exogenous Ca2+. Since the PT-1 antigen is freely diffusing on the posterior tail before migration, the mechanism of redistribution could involve the alteration of a presumptive membrane barrier. In addition, we observed the redistribution of a second surface antigen after the acrosome reaction. The antigen recognized by the monoclonal antibody, PH-20, was localized exclusively in the posterior head region of acrosome-intact sperm. Within 7-10 min of induction of the acrosome reaction with Ca2+ and A23187, 90-100% of the acrosome-reacted sperm population no longer demonstrated binding of the PH-20 antibody on the posterior head, but showed binding instead on the inner acrosomal membrane. This redistribution of the PH-20 antigen also resulted from the migration of pre-existing surface molecules, but did not appear to require energy. The migration of PH-20 antigen was a selective process; other antigens localized to the posterior head region did not leave the posterior head after the acrosome reaction. These rearrangements of cell surface molecules may act to regulate cell surface function during fertilization.

Effect of seminal plasma and calcium on the stability of the surface protein composition of ejaculated bull spermatozoa

Ejaculated bull spermatozoa were incubated in different media to elucidate the effect of seminal plasma and calcium on the stability of sperm surface compositions. The spermatozoa were radioiodinated prior to or after the incubation and the labelled proteins from the incubation media and spermatozoa were analysed by SDS-polyacrylamide gel electrophoresis. Seminal plasma detached efficiently sperm surface components. Even with concentration of 2% considerable amounts of the 44K and 17K proteins were released into the medium. The 17K proteins of seminal plasma were also adsorbed by sperm surfaces during incubation. The release of the 27K protein seemed to be dependent on calcium.

Changes in plasma membrane glycoproteins of rat spermatozoa during maturation in the epididymis

Glycoproteins on the plasma membrane of testicular and cauda epididymidal spermatozoa have been labeled with galactose oxidase/NaB [3H]4 and sodium metaperiodate/NaB[3H]4, followed by analysis on SDS polyacrylamide gels. The major glycoprotein labeling on testicular spermatozoa has a molecular weight 110,000 whereas on cauda epididymidal spermatozoa greater than 90% of the radio-label is incorporated into proteins of molecular weight 32,000. These 32,000-mol wt X proteins are homologous with proteins of similar molecular weight purified from the epididymal secretion and which have been shown previously to be synthesized in the caput epididymidis under hormonal control. Immunofluorescence revealed that the 32,000-mol wt proteins are present on the flagellum of mature but not immature spermatozoa and that they have a patchy distribution suggesting that they are mobile within the plane of the membrane. The membrane-bound 32,000-mol wt proteins possess hydrophobic domains as revealed by charge-shift electrophoresis and they also label with a lipophilic photoaffinity probe suggesting that they are in contact with the lipid bilayer. The evidence indicates that there is a considerable reorganization of the molecular structure of the plasma membrane of spermatozoa during maturation in the epididymis and that some of the changes are brought about by a direct interaction with epididymal secretory proteins.

Detection of modifications in the tail of capacitated guinea pig sperm using lectins

The surfaces of uncapacitated, capacitated, and trypsinized guinea pig sperm have been compared using 10 lectins. A previously described assay was used to assess the degree and pattern of agglutination induced by each lectin. Our method of evaluating the agglutination pattern was significantly improved by observing assay plates with an inverted phase contrast microscope at 200 X 320 X magnification rather than with a dissecting microscope. Following in vitro capacitation or trypsinization, the agglutinability of sperm by SBA, DBA, and WGA increased, and the agglutination pattern induced by SBA, DBA, WGA, and RCA120 changed to a predominantly reticular form. These changes did not occur when sperm were incubated in Ham's F-10, a control medium which does not support capacitation. Since binding of these lectins is inhibited by N-acetyl-D-galactosamine (SBA, DBA), D-galactose (SBA, RCA120, DBA), or N-acetyl-D-glucosamine (WGA), we conclude that surface receptors containing these residues are affected by capacitation. By examining assay plates with an inverted phase contrast microscope, we were able to show that the change in the agglutination pattern for all four lectins was due to an increase in agglutinability of the principal piece of the flagellum. It is suggested that the modification of the principal piece detected by this lectin assay may be important in the development of activated motility. Results further show that capacitation and trypsin treatment affect the sperm surface in a similar manner, and are thus consistent with the idea that a trypsin-like enzyme may modify the sperm during capacitation.

Lectins as probes of the spermatozoon surface

Studies involving the use of lectins to probe the spermatozoon surface are reviewed, with particular emphasis on mammalian spermatozoa and their various transformations during maturation, capacitation, and the acrosome reaction. Although the numbers of lectin bindings sites on spermatozoa are generally comparable to those of many somatic cells, the distribution of most lectins thus far studied is highly restricted to certain domains of the cell surface. These domains are often related to specific underlying morphological entities such as the acrosome, postacrosomal region, mitochondria associated with the middle piece, and so on. Changes in the patterns of lectin binding have been noted during sperm maturation in the epididymis, during capacitation, and after the acrosome reaction, and, in some instances, these studies contribute to the understanding of mechanism that may mediate these transformations. This is particularly true of situations where lectin labeling or binding studies have been used in conjunction with biochemical or other correlative methods to synthesize a more detailed picture of the sperm surface.

Orderly particle arrays on the mitochondrial outer membrane in rapidly-frozen sperm

By deep-etching and rotary replication of unfixed, non-cryoprotected tissue frozen on a helium-cooled copper block, previously undemonstrable organellar surfaces and intramembranous structures can be examined. Among the more remarkable features of mammalian spermatozoa thus prepared are the highly ordered particulate arrays on the surface of the mitochondrial outer membrane. In the midpiece of the sperm, mitochondria curl around dense fibers and the axoneme. The surface of the mitochondrion that faces the plasmalemma carries closely packed rods in haphazard dispersement, composed of two to four 70-to 80-A particles, less than 20 A apart, while the concave aspect of the organelle contains rods in stepladder pattern. These ladders are parallel, with their particles in neighboring rungs apparently in register at a 40--45 degree angle relative to the mitochondrial axis. This organizational disparity between the convex and concave surfaces of the organelle not only affords evidence of a new mitochondrial substructure, but represents a type of topographical heterogeneity rarely found except within specialized areas of the plasma membrane. Other novel findings in the sperm cell include the observation of "lipid" tracts flanking intramembranous particle-strands in the plasmalemma of the cytoplasmic droplet, and a gridiron design on the cytoplasmic faces of the droplet's microcisternae, as well as both within and atop its plasma membrane--a motif consistent with the presence of exocytotic or endocytotic activity in this portion of the cell. Additional recent observations are the differing internal and external periodicities of axonemal microtubules and the subunit structure of rectangles on the tail surface overlying the intramembranous particles of the zipper.

Surface domains of the guinea pig sperm defined with monoclonal antibodies

Four monoclonal antibodies have been produced that recognize surface molecules of the guinea pig sperm. Each antibody binds to a unique, localized region of the sperm surface. The four antibody-binding patterns are restricted to the anterior head, of the sperm, the posterior head, the whole head and the posterior tail. Based on absorption experiments, the surface molecules detected by the antibodies are differentiation antigens. Three of the antibodies immunoprecipitate 125I-labeled protein antigens. The molecular weights of the polypeptides observed after SDS-PAGE are 52,000 (anterior head), 42,000 (whole head), and 60,000 (posterior head). The observed monoclonal antibody-binding patterns suggest that the guinea pig sperm surface is divided into a minimum of four domains.

Surface modification of guinea pig sperm during in vitro capacitation: an assessment using lectin-induced agglutination of living sperm

Plant lectins have been used to advantage to study carbohydrate-containing cell surface receptors in numerous systems. In this study, a simple, reliable assay was developed to quantitate lectin-induced agglutinability of sperm. This assay was used successfully to compare some of the surface properties of uncapacitated and capacitated guinea pig sperm. Capacitation was induced by incubating sperm in minimum capacitation medium (MCM) or modified Tyrodes solution (T-PL). Control incubations were done in Ham's F-10 or Hank's balanced salt solution which do not support capacitation. At timed intervals during incubation, sperm samples were assessed for pattern and degree of lectin-induced agglutination. Results establish that: (1) soybean agglutinin (SBA) and to a lesser extent concanavalin A (Con A) induced agglutinability of guinea pig sperm increase during in vitro capacitation in MCM; (2) a similar increase in SBA induced agglutinability occurs during capacitation in T-PL, but not in the non-capacitating media; and (3) for sperm incubated in MCM or T-PL, there is a significant increase in tail to tail agglutination after capacitation. The results with SBA demonstrate that D-galactose and/or N-acetyl-D-galactosamine containing receptor sites or the guinea pig sperm surface are affected by capacitation, and this effect occurs, at least in part, in the sperm tail. Possible explanations for the observed increase in agglutinability are discussed. The agglutination assay may prove useful as a direct test for the occurrence of capacitation and may be especially valuable for species having a small acrosome or limited number of eggs.

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.

Gradionation: hypothesis for positioning and patterning

The interaction of contiguous proteins is explored in microtubules, rosettes, and membranes based on the well established molecular phenomena of cooperativity and allosterism. It is proposed that conformational gradients in protein arrays cause the formation of gradions by nearest-neighbor interactions. Gradions are repeating functional molecular sequences that contain several conformational forms of one or more proteins, with the result that different reactive sites can exist in the same molecular architecture at any one time. Gradionators are small controlling molecules that may be microscopically visible as layers of linkages, but could alse be smaller. Some of the presently available supporting evidence and its functional implications are discussed, including the possibility that the raison d'etre for membrane-particle arrays is to enhance the regulation and amplification capabilities of cell systems.

Morphological changes in the midpiece of wooly opossum spermatozoa during epididymal transit

Several ultrastructural changes were found to occur in the midpiece region of wooly opossum spermatozoa during epididymal maturation. The changes include alterations in mitochondrial morphology, development of structural specializations of the plasma membrane, and acquisition of a prominent extracellular coating. The lamellar membrane network which is wound about the periphery of the mitochondria becomes more densely packed during sperm development and the reticular network of membranes noted in the center of the mitochondria of immature sperm disappears leaving a homogeneous electron dense central zone. During epididymal transit the plasma membrane over the sperm midpiece region shows extensive structural modification. In cross sections of paired spermatozoa the plasma membrane of the midpiece regions shows a very regular, repetitive scalloping. In longitudinal sections the scalloping is observed as continuous parallel ridges which extend slightly obliquely to the flagellar long axis. Each ridge appears to be greater in density than the interridge areas. In the epididymis a prominent extracellular coating of dense material is deposited over the midpiece surface; this material is similar in appearance to dense material seen in restricted areas of the epididymal lumen. At the proximal and distal ends of the midpiece the plasma membrane comes into intimate contact with underlying structural specializations and it is suggested that these zones of fusion may serve to preserve regional differences in membrane composition.

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.

Morphological variations in gap junctions of ovarian granulosa cells

Granulosa cells in ovarian follicles of rat, mouse, rabbit and hamster were studied by lanthanum tracer and freeze-fracture techniques. Abundant gap junctions exhibited striking intraspecific variation in size and pattern of particle aggregation. The smaller gap junctions showed close packing of the intramembranous A face particles. In large gap junctions, ranging up to 6 mu in diameter, particles were packed in rectilinear arrays separated by a labyrinthine network of particle-free 'aisles'. Small clusters of particles in a particle-poor circumferential zone suggested enlargement of junctions by peripheral accretion. Linear intramembranous structures, resembling those of occluding junctions, occasionally bounded large gap junctions. Spherical intracytoplasmic structures limited by gap junctional membranes were shown by tracer studies to arise by invagination of the cell surface. These were intrepreted as a means of disposal of junctions by interiorization.