Chemical dissection of mammalian spermatozoa

INSL-3 protein expression in normal and cryptorchid testes of Ziwuling black goats

Ziwuling black goats are typically found in loess plateaus regions and the Ziwuling Nature Reserve. Cryptorchidism is a common disease in this inbred goat, and its pathogenesis has been linked with the expression of insulin-like factor 3 (INSL-3). Therefore, this study aimed to investigate anatomical alterations caused by cryptorchism and the expression and distribution of INSL-3 in normal and cryptorchid testicular tissues. The testicular tissues of 6-month-old Ziwuling black goats were collected for microscopic analyses using histochemical, immunohistochemical, immunofluorescence and biometrical methods, as well as Western blotting to compare the expression and distribution of INSL-3. A lower expression of INSL-3 was observed in cryptorchid compared with normal testicular tissues (p < .01). Cryptorchidism caused a significant reduction in layers of spermatogenic epithelium and tubule areas in Ziwuling black goat (p < .01). The interstitial to seminiferous tubule area ratio was larger in cryptorchid than in normal group. Periodic Acid-Schiff (PAS) staining revealed pronounced positive bands in the interstitial tissue, while positive Alcian blue (AB) staining was not clear, and AB-PAS staining revealed a positive red band in the basement membrane of cryptorchid group. Immunofluorescence revealed a strong signal of INSL-3 expression in Sertoli and peritubular myoid cells, and moderate signal in Leydig and spermatogenic cells in the normal group. However, in cryptorchid testicular tissues, the signal of INSL-3 expression was strong in primary spermatocytes, occasional in Sertoli cells, limited in Leydig cells and absent in peritubular myoid cells. Furthermore, immunohistochemistry showed that INSL-3 expression was higher in normal testes compared with cryptorchid testicular tissues (p < .05), especially in primary spermatocytes and Sertoli cells. Collectively, our results indicate that cryptorchidism is closely related to the disorder of acid glycoprotein metabolism and the reduction in release of INSL-3 from Leydig cells. Moreover, Sertoli and peritubular myoid cells are crucial for INSL signalling and could underpin further research on the mechanism of cryptorchidism in animal.

The flagellar protein Enkurin is required for mouse sperm motility and for transport through the female reproductive tract

Enkurin was identified initially in mouse sperm where it was suggested to act as an intracellular adaptor protein linking membrane calcium influx to intracellular signaling pathways. In order to examine the function of this protein, a targeted mutation was introduced into the mouse Enkurin gene. Males that were homozygous for this mutated allele were subfertile. This was associated with lower rates of sperm transport in the female reproductive tract, including reduced entry into the oviduct and slower migration to the site of fertilization in the distal oviduct, and with poor progressive motility in vitro. Flagella from wild-type animals exhibited symmetrical bending and progressive motility in culture medium, and demembranated flagella exhibited the "curlicue" response to Ca2+ in vitro. In contrast, flagella of mice homozygous for the mutated allele displayed only asymmetric bending, nonprogressive motility, and a loss of Ca2+-responsiveness following demembrantion. We propose that Enkurin is part of a flagellar Ca2+-sensor that regulates bending and that the motility defects following mutation of the locus are the proximate cause of subfertility.

The IP3 R Binding Protein Released With Inositol 1,4,5-Trisphosphate Is Expressed in Rodent Reproductive Tissue and Spermatozoa

Besides its capacity to inhibit the 1,4,5-trisphosphate (IP3) receptor, the regulatory protein IRBIT (IP3 receptor binding protein released with IP3) is also able to control the activity of numerous ion channels and electrolyte transporters and thereby creates an optimal electrolyte composition of various biological fluids. Since a reliable execution of spermatogenesis and sperm maturation critically depends on the establishment of an adequate microenvironment, the expression of IRBIT in male reproductive tissue was examined using immunohistochemical approaches combined with biochemical fractionation methods. The present study documents that IRBIT is expressed in Leydig and Sertoli cells. In addition, pronounced IRBIT expression was detected in sperm precursors during early stages of spermatogenesis as well as in spermatozoa. Analyzing tissue sections of rodent epididymides, IRBIT was found to co-localize with the proton pumping V-ATPase and the cystic fibrosis transmembrane conductance regulator (CFTR) at the apical surface of narrow and clear cells. A similar co-localization of IRBIT with CFTR was also observed for Sertoli cells and developing germ cells. Remarkably, assaying caudal sperm in immunogold electron microscopy, IRBIT was found to localize to the acrosomal cap and the flagellum as well as to the sperm nucleus; moreover, a prominent oligomerization was observed for spermatozoa. The pronounced occurrence of IRBIT in the male reproductive system and mature spermatozoa indicates a potential role for IRBIT in establishing the essential luminal environment for a faithful execution of spermatogenesis and epididymal sperm maturation, and suggest a participation of IRBIT during maturation steps after ejaculation and/or the final fertilization process.

Elucidation of the signaling pathways that underpin capacitation-associated surface phosphotyrosine expression in mouse spermatozoa

Recent studies from within our laboratory have demonstrated a causal relationship between capacitation-associated surface phosphotyrosine expression and the ability of mouse spermatozoa to recognize the oocyte and engage in sperm-zona pellucida interaction. In the studies described herein we have sought to investigate the signaling pathways that underpin the tyrosine phosphorylation of sperm surface protein targets and validate the physiological significance of these pathways in relation to sperm-zona pellucida adhesion. Through selective pharmacological inhibition we have demonstrated that surface phosphotyrosine expression is unlikely to be mediated by the canonical cAMP-dependent protein kinase A (PKA) signaling cascade that has been most widely studied in relation to sperm capacitation. Rather, it appears to be primarily driven by the extracellular signal-regulated kinase (ERK) module of the mitogen-activated protein kinase (MAPK) pathway. Consistent with this notion, the main components of the ERK module (RAS, RAF1, MEK, and ERK1/2) were localized to the periacrosomal region of the head of mature mouse spermatozoa and their phosphorylation status within this region of the cell was positively modulated by capacitation. Furthermore, inhibition of several elements of this pathway suppressed sperm surface phosphotyrosine expression and induced a concomitant reduction sperm-zona pellucida interaction. Collectively, these data highlight a previously unappreciated role of the ERK module in the modification of the sperm surface during capacitation to render these cells functionally competent to engage in the process of fertilization.

Lactate and adenosine triphosphate in the extender enhance the cryosurvival of rat epididymal sperm

We evaluated the cryosurvival of rat epididymal sperm preserved in raffinose-modified Krebs-Ringer bicarbonate-egg yolk extender supplemented with various energy-yielding substrates (glucose, pyruvate, lactate, and ATP) and assessed the effect on sperm oxygen consumption. The incubation of sperm at 37 degrees C for 10 min in lactate-free extender decreased sperm motility and oxygen consumption before and after thawing compared with those of sperm in glucose- and pyruvate-free mediums. We then focused on the effect of supplementing the extender with lactate (0, 10.79, 21.58, 32.37, and 43.16 mM) and found that sperm frozen and thawed in extender supplemented with 32.37 mM lactate exhibited the highest motility. When we supplemented extender containing 32.37 mM lactate with ATP (0, 0.92, 1.85, 3.70, and 5.55 mM), sperm frozen and thawed in the extender supplemented with 1.85 mM ATP exhibited considerably higher motility and viability than those of sperm frozen and thawed in ATP-free extender. These results provide the first evidence that supplementation of the raffinose-modified Krebs-Ringer bicarbonate-egg yolk extender with 32.37 mM lactate and 1.85 mM ATP increases of number of motile sperm before freezing and enhances the cryosurvival of rat sperm. These supplements to the extender may enhance sperm cryosurvival by improving the metabolic capacity of sperm before freezing.

Cadherins expression during gamete maturation and fertilization in the rat

A role for adhesion molecules in gamete fusion, preceding fertilization, has been previously suggested. We investigated the presence of cadherins, Ca(2+) dependent cell-cell adhesion molecules, in rat oocytes and spermatozoa using an anti-pan-cadherin antibody and specific antibodies against the 3 classical cadherins: E- (epithelial), P- (placental), and N- (neural) cadherins. Electrophoretic separation was performed on samples of lysed oocytes of different stages: germinal vesicle oocytes, metaphase II eggs, newly fertilized and 2-cell embryos, as well as spermatozoa from testes, caput and cauda epididymis and ejaculate. Localization of cadherins was determined on intact, gametes by immunocytochemistry, using confocal microscopy. Immunoblotting with the pan-cadherin antibody revealed a major band of approximately 120 kD in all oocyte and sperm extracts. Oocytes presented E-cadherin at appropriate molecular weight but N-cadherin only as a specific 40 kD band. In sperm lysate, at all stages, both E- and N-cadherin were demonstrated as major protein bands but a series of lower molecular weight proteins (that may represent protein degradation) were also detected. Immunohistochemical evaluation showed that E- and N-cadherins are already present on the plasma membrane of immature unfertilized oocytes, although their concentration increases after fertilization in early cleavage stage embryos. Cadherin localization on spermatozoa changed during maturation from a dispersed pattern over the entire head plasma membrane of testicular spermatozoa to a restricted equatorial and post-acrosomal plasma membrane staining in ejaculated spermatozoa. These findings suggest a specific cadherin organization at the fusogenic domains of both gametes.

Targeted disruption of the transition protein 2 gene affects sperm chromatin structure and reduces fertility in mice

During mammalian spermiogenesis, major restructuring of chromatin takes place. In the mouse, the histones are replaced by the transition proteins, TP1 and TP2, which are in turn replaced by the protamines, P1 and P2. To investigate the role of TP2, we generated mice with a targeted deletion of its gene, Tnp2. Spermatogenesis in Tnp2 null mice was almost normal, with testis weights and epididymal sperm counts being unaffected. The only abnormality in testicular histology was a slight increase of sperm retention in stage IX to XI tubules. Epididymal sperm from Tnp2-null mice showed an increase in abnormal tail, but not head, morphology. The mice were fertile but produced small litters. In step 12 to 16 spermatid nuclei from Tnp2-null mice, there was normal displacement of histones, a compensatory translationally regulated increase in TP1 levels, and elevated levels of precursor and partially processed forms of P2. Electron microscopy revealed abnormal focal condensations of chromatin in step 11 to 13 spermatids and progressive chromatin condensation in later spermatids, but condensation was still incomplete in epididymal sperm. Compared to that of the wild type, the sperm chromatin of these mutants was more accessible to intercalating dyes and more susceptible to acid denaturation, which is believed to indicate DNA strand breaks. We conclude that TP2 is not a critical factor for shaping of the sperm nucleus, histone displacement, initiation of chromatin condensation, binding of protamines to DNA, or fertility but that it is necessary for maintaining the normal processing of P2 and, consequently, the completion of chromatin condensation.

Electron microscopic observations of mouse sperm whole mounts after extraction for nuclear matrix and intermediate filaments

Nuclear matrix and intermediate filaments (NM-IF) can be isolated by sequential treatment with non-ionic detergent, high salt. and nuclease. Extracted cells are easily observed by unembedded whole-mount transmission electron microscopy. Different somatic cell types have been subjected to this procedure and retained their essential architecture. To our knowledge, this work describes the first application of NM-IF extraction to sperm. After chemical dissection the general appearance of mouse sperm cells was preserved, except for head-from-neck separation in some cases. The cell membrane, acrosome and mitochondria were not present. The nucleus showed no apparent changes and revealed no details excepting pore complexes in the posterior part. Tissue-specific cytoskeletal elements (perforatorium, postacrosomal sheath, capitulum, segmented columns, outer dense fibers, submitochondrial reticulum, annulus, and fibrous sheath) were retained, which permitted a parallel between them and intermediate filaments of somatic cells. Tail microtubules were also relatively well preserved, showing high intrinsic stability. Cell structures could be observed well, with some details in the tail even better visible than in ultrathin sections. Observation of mouse sperm whole mounts after NM-IF extraction not only revealed intermediate filament-like properties of their cytoskeletal elements but also offered an additional viewpoint to sperm ultrastructure.

Ultrastructure of the domestic tom cat () and tiger () spermatozoa

The ultrastructure of spermatozoa from the domestic tom cat and the Siberian tiger was studied. Semen was collected from anesthetized tom cats and Siberian tigers by electroejaculation. Spermatozoa were fixed and processed for examination by transmission electron microscopy. The principle differences between the spermatozoa from the two species were the head shape, mitochondrial organization in the neck area and structure of the fibrous sheath. Tom cat spermatozoa had an elongated oval-shaped head, while tiger spermatozoa had a more rounded head shape. Circularly oriented mitochondria in the neck area, near the proximal centriole, were frequently observed in tiger cells but rarely observed in tom cat cells. The semicircular ribs of tom cat spermatozoa were larger than the ribs of tiger spermatozoa. Also, the dense fibers (Numbers 3 and 8) of the corresponding microtubule doublets were fused or connected to the longitudinal columns in tiger spermatozoa but showed only occasional attachment in tom cat spermatozoa. These differences could influence results when the tom cat is used as a model for studying tiger semen.

Expression and topographical localization of cell surface fucosyltransferase activity during epididymal sperm maturation in the mouse

We have demonstrated previously that spermatogenic cells in the mouse testis have high levels of fucosyltransferase activity. Furthermore, a significant portion of this activity has been localized to the cell surface (Millette et al.: Cell Biology of the Testis and Epididymis, 1987). Differential expression of fucosyltransferases and their function as ecto-enzymes may be important in the processes of sperm maturation and fertilization in mammals. Accordingly, here we report the activity levels of fucosyltransferase (FT) in spermatozoa isolated from the mouse caput and cauda epididymides. Calculated on a per cell basis, spermatozoa from the caput epididymis have significantly more FT activity than do spermatozoa from the cauda epididymis (18.07 +/- 2.2 pmol/million cells compared with 2.8 +/- 0.09 pmol/million cells). Furthermore, caput sperm exhibit a more significant increase in FT activity when assayed in the presence of Nonidet P-40. Calculated on the basis of cell surface area, however, FT activity remains constant on the head portion of spermatozoa isolated from all portions of the male reproductive tract and from capacitated spermatozoa. Measurements of FT activity in extracts of isolated sperm tails from cells at different stages of maturation indicate a greatly diminished activity in tails from sperm in the cauda epididymis. The total sperm surface area is composed predominantly of the plasma membrane surrounding the flagellar apparatus. Therefore, our data demonstrate that FT activity is retained selectively on the different topological regions of sperm, with losses during sperm maturation in the epididymis being restricted to the tail segment. Maintenance of high levels of FT activity of the plasma membranes of the mouse sperm head raise the possibility that FT is indeed involved in some aspects of sperm-egg recognition.

Biochemical studies of metalloendoprotease activity in the spermatozoa of three mammalian species

Ejaculated porcine and human spermatozoa, hamster spermatozoa from the cauda epididymidis, isolated hamster sperm heads and hamster cytoplasmic droplets contained activity that hydrolyzed the metalloendoprotease substrate ABZ-Ala-Gly-Leu-Ala-NBA (AAGLAN). Hamster sperm heads were isolated by treating spermatozoa with proteinase K and removing sperm tails with Dowex-50W beads. Hamster sperm activity was characterized using spermatozoa from which cytoplasmic droplets were removed by sonication and centrifugation. Porcine sperm preparations were essentially free of cytoplasmic droplets, while human sperm preparations retained somewhat more droplet material. Activity from all of these sources was inhibited by the metalloendoprotease inhibitors phosphoramidon, 1,10-phenanthroline, CBZ-D-Phe and CBZ-L-Phe but was not competitively inhibited by the metalloendoprotease substrate CBZ-Ser-Leu-amide. The AAGLAN hydrolyzing activity found in intact spermatozoa of all three species had a pH optimum of 6.2, while the optimum of the hamster sperm cytoplasmic droplet activity was 7.0. In addition, hamster sperm preparations were inhibited by ZnCl2 and dithiothreitol, but were not affected by toluene, benzamidine or chymostatin. The AAGLAN hydrolyzing activity of hamster sperm preparations was reduced, but not eliminated, by dialysis. It is concluded that spermatozoa from all three species, hamster sperm heads and hamster cytoplasmic droplets contain metalloendoprotease activity. Furthermore, metalloendoprotease activity found in hamster cytoplasmic droplets is different from that found in spermatozoa.

Binding of fluorescein isothiocyanate conjugated lectins to rat spermatogenic cells in tissue sections. Enhancement of lectin fluorescence obtained by fixation in Bouin's fluid

The testes from three months old Sprague-Dawley rats were fixed in Bouin's fluid or neutral buffered 10% formalin, embedded in paraffin, sectioned and after deparaffination stained with the following fluorescein isothiocyanate coupled lectins: PNA, WGA, Con A, RCA, SBA, DBA and UEA. The results show that there are considerable differences in the staining pattern of various spermatogenic cells between different lectins. The fixation in Bouin's fluid enhanced the staining of all the lectins compared to formalin fixation in which only a weak staining could be seen in the acrosomes of spermatids after WGA or PNA staining. PNA and WGA stained specifically the acrosome of the developing spermatids, which was seen from the beginning of the acrosome formation and lasted up to late spermiogenesis. However, the staining with PNA decreased in the late spermatids whereas the intensity of the staining remained unchanged with WGA. Con A did not stain the acrosome but stained unspecifically the cytoplasm of all spermatogenic cells. RCA stained faintly the acrosome throughout the spermatid differentiation. DBA and UEA stained specifically the chromosomes of B spermatogonia. DBA also faintly stained the cell membranes of early spermatids. SBA did not show any specific staining of the spermatogenic cells. Based on this it is suggested that the carbohydrates and glycoproteins which are known to be present in the acrosome are formed already in the beginning of the acrosome formation. The decrease in the PNA staining in late spermatids possibly reflects the fact that the receptor molecules are not synthesized in late spermatids but are formed in earlier developmental stages and are thereafter preserved in the acrosome.(ABSTRACT TRUNCATED AT 250 WORDS)

Calmodulin, a calmodulin acceptor protein, and calcimedins: unique antibody localizations in hamster sperm

A calmodulin acceptor protein has been identified in isolated hamster caudal sperm by immunofluorescence and Western transfer techniques. The protein shows a localization in sperm heads identical to calmodulin. Fluorescence of both calmodulin and the acceptor protein are lost by treatment with MgCl2, conditions which release the acrosome. These results are consistent with the proposed function of calmodulin in a sperm function.

Importance of zinc for human sperm head-tail connection

Head-tail detachment of ejaculated human spermatozoa was studied with phase contrast microscopy. The frequency of head-tail detachment was assessed after sperm exposure to the anionic detergent sodium dodecyl sulfate (SDS) for 60 min at 22 degrees C. Decapitation was enhanced by EDTA. Zinc reversibly inhibited native as well as EDTA induced head-tail disconnection. Still a certain proportion of spermatozoa were resistant to EDTA-treatment and this proportion significantly increased upon 24 h of saline storage. The development of EDTA resistance was enhanced by EDTA treatment before storage. A physiological role for zinc as a preserver of an inherent mechanism for head-tail detachment is suggested.

Flow cytometry of human spermatozoa

Methods are given for the preparation and staining of human spermatozoa for flow cytometric DNA measurements. Using agents for the reductive cleavage of disulfide crosslinks and suitable proteolytic enzymes an effective decondensation of the sperm chromatin and a DNA-proportional uptake of fluorochromes is achieved. Thus reliable and precise measurements of the relative DNA content of human spermatozoa are possible and the two subpopulations of haploid spermatozoa can be distinguished according to the difference in their DNA content.

Functional anatomy of haploid and diploid rabbit spermatozoa

Ultrastructural studies of surface replicas of haploid and diploid rabbit spermatozoa after treatment with various chemical media have shown that the various structural components of the diploid sperm head possess the same stabilities and labilities as those of the haploid. Therefore, at least in terms of functional anatomy, diploid rabbit spermatozoa should be capable of penetrating the egg investments and undergoing syngamy.

RNA polymerase activity in bovine spermatozoa

Washed mature spermatozoa from bulls incorporate ribonucleoside triphosphates into RNA using an endogenous template. Maximum incorporation was observed at 31 degrees C in the presence of MgCl2, all four ribonucleoside triphosphates, beta-mercaptoethanol, and glycine sodium hydroxide buffer at pH 9.0. The amount of synthesis was linearly dependent upon the concentration of spermatozoa and continued for at least 4 h. Digestion studies revealed the RNA to be present in a protected (intracellular?) location in the spermatozoa. The RNA synthesis was inhibited by ethidium bromide, rifampicin, acriflavine, actinomycin D, and caffeine, but not by alpha-amanitine or rifamycin SV. Fractionation of the spermatozoa by sonication and separation of the heads and tails by centrifugation through a discontinuous gradient revealed that more than half of the total RNA polymerase activity was associated with the tail fraction.

The splitting of sperm heads from tails in eight mammalian species and the measurement of their sizes

Ejaculated spermatozoa from man, the Euopean wild boar and the bull, and spermatozoa from the cauda epididymes of the rabbit, rat, mouse, hamster and Guinea pig were treated with a sonic bath, a sonic probe, trypsin with and without prior treatment with a sulphhydryl reagent, pronase and alkalis. The fragments produced were counted and sized in an accurately calibrated Coulter Counter, Model ZB Industrial, before and after Zaponin treatment to lyse accompanying debris and the peripheral cytoplasm. Head and tail fractions were separated on sucrose gradients. Each species required different conditions for cleavage or fragmentation. Rabbit and bull spermatozoa were cleaved by the ultrasonic bath exactly into heads and tails, producing twice the number of particles with two peaks in the size distribution curves butith some 60% loss of total sperm volume which became the soluble fraction. The ultrasonic probe, and for the bull, pronase, produced the same cleavage but these more drastic treatments dissolved a considerable portion of the tail fraction. Rodent spermatozoa, especially the rat, were cleaved perfectly into heads and tails by mild trypsin treatment. All the nonrodent spermatozoa were resistent to trypsin cleavage, although prior treatment with a sulphhydryl reagent caused swelling and subsequent trypsin action caused digestion into miscellaneous pieces. Spermatozoa from the boar and from man could not be cleaved by any of the procedures. The sonic probe produced fragmentation with progressive dissolution of the tail fragments and a single peak in the size distribution curve corresponding to small stripped heads. The soluble fraction always constituted a large proportion of the original whole spermatozoa.

Studies on sperm antigenicity, 5. In vivo and in vitro cellular reactivity in guinea pigs sensitized with fractions of human spermatozoa

In vitro delayed type hypersensitivity was demonstrated with peritoneal exudate cells from guinea pigs immunized with different preparations of human and guinea pig seminal components emulsified with complete Freund's adjuvant-H37Ra. The seminal components were intact human spermatozoa (HuSp); intact guinea pig spermatozoa (GPSp); human seminal plasma (HuSePlFr); and fractions of human spermatozoa obtained by centrifugation of the homogenate at 5,000 X g (5S30 and 5p30), at 20,000 X g (20S30 and 20p30), and at 144,000 X g (144p120). Cellular sensitivity was demonstrated in vivo by skin testing and in vitro by the macrophage inhibition technique. Peritoneal exudate cells from guinea pigs sensitized with fractions 5p30 and 20p30 elicited a delayed hypersensitivity reaction which could be detected only with intact human spermatozoa. Other human spermatozoal fractions (5S30, 20S30, and 144p120) were weak immunogens. Sensitization of guinea pigs with fractions of human spermatozoa, in addition to causing delayed hypersensitivity reactions, elicited low titers of spermatoxic antibodies. Antibodies to human spermatozoal fractions 5S30, 5p30, 20S30, and 20p30 cross-reacted with intact human spermatozoa and intact guinea pig spermatozoa. It is postulated that the existence of "spermatozoa-specific" coating antigen(s) derived from other components of the reproductive tract might be responsible for human spermatozoal antigenicity.

Membrane differentiations in freeze-fractured mammalian sperm

A correlated thin-sectioning and freeze-fracturing study has been made of guinea pig and rat spermatozoa. In sections, the cell membrane over the acrosome has a concanavalin A and ruthenium red reactive glycocalyx which exhibits an ordered pattern related to the lattice of crystalline domains within the plane of the membrane revealed by freeze-fracturing. The cleaved acrosomal membrane also shows a finer linear periodicity in some areas. The membrane over the equatorial segment of the guinea pig acrosome is marked by a palisade of oblique ridges not observed in the rat. The plasmalemma of the postacrosomal region is rich in membrane intercalated particles, many randomly dispersed, others clustered in rectilinear arrays. A particle-poor zone is found just anterior to the posterior ring. The fold of redundant nuclear envelope posterior to the ring has many nuclear pores in close hexagonal array. The nuclear envelope lining the implantation fossa is devoid of pores. When cleaved it has a particle-free central area surrounded by a broad zone of large, closely packed, hollow particles. The membrane of the mid-piece in the guinea pig (but not the rat) contains linear strands of 6-8-nm particles oriented circumferentially. The membrane investing the principal piece exhibits the usual randomly distributed particles but in addition, a double row of larger (9 nm) particles runs longitudinally within the membrane over outer dense fiber 1. In the corresponding position in thin sections a local thickening of the membrane is discernible. These observations form a basis for further studies on the functional correlates of these regional specializations of the sperm membrane.