Nuclear shaping in the absence of microtubules in scorpion spermatids

Sperm bauplan and function and underlying processes of sperm formation and selection

The spermatozoon is a highly differentiated and polarized cell, with two main structures: the head, containing a haploid nucleus and the acrosomal exocytotic granule, and the flagellum, which generates energy and propels the cell; both structures are connected by the neck. The sperm's main aim is to participate in fertilization, thus activating development. Despite this common bauplan and function, there is an enormous diversity in structure and performance of sperm cells. For example, mammalian spermatozoa may exhibit several head patterns and overall sperm lengths ranging from ∼30 to 350 µm. Mechanisms of transport in the female tract, preparation for fertilization, and recognition of and interaction with the oocyte also show considerable variation. There has been much interest in understanding the origin of this diversity, both in evolutionary terms and in relation to mechanisms underlying sperm differentiation in the testis. Here, relationships between sperm bauplan and function are examined at two levels: first, by analyzing the selective forces that drive changes in sperm structure and physiology to understand the adaptive values of this variation and impact on male reproductive success and second, by examining cellular and molecular mechanisms of sperm formation in the testis that may explain how differentiation can give rise to such a wide array of sperm forms and functions.

Spermatozoa and sperm packages of the European troglophylous scorpion Belisarius xambeui Simon, 1879 (Troglotayosicidae, Scorpiones)

Studies on the sperm morphology in scorpions are rare, but the existing investigations already revealed a remarkable interfamiliar diversity. The present study reports for the first time on the spermatozoa and sperm packages of a representative of the family Troglotayosicidae, the troglophylous species Belisarius xambeui. The spermatozoa are characterized by (1) a thread-like nucleus, which is slightly bent anteriorly; (2) an asymmetrical cap-like acrosomal vacuole, which encloses the anterior tip of the nucleus; an acrosomal filament is absent; (3) an axoneme with a 9+0 microtubular pattern; (4) a midpiece consisting of elongated mitochondria coiling around the axoneme; the number can vary between 3 and 6 (mostly 4). At the end of spermiogenesis, the spermatozoa aggregate in order to form oval-shaped sperm packages in which all sperm cells show the same orientation. A single package consists of approximately 150 sperms. A secretion sheath is always absent. The present results might provide new characters for further systematic studies and their phylogenetic implications are briefly discussed.

Structural and functional characteristics of the centrosome in gametogenesis and early embryogenesis of animals

We present a description of the wide spectrum of centrosome behavior during gametogenesis, early development, and cell differentiation. During meiosis and terminal differentiation of gametes there occurs a process of centrosome maturation which includes alterations in characteristics such as the number of centriolar cylinders and their structure if the basal body is formed and ability to function as MTOC, reduplicate, split, and serve as a polar organizer. Such centrosome properties require modifications of the molecular composition. Maturation of the centrosome in gametes may be compared to transformation of centrosome characteristics during terminal differentiation of other cells. After fertilization different properties of maternal and paternal centrosomes are supposed to combine, adding to each other in the fused (hybrid) centrosome of a zygote. Restoration of centrosome features typical in diploid somatic cells takes place in cells of a developing embryo in the course of early cell cycles.

Giant, accordioned sperm acrosomes of the greater bulldog bat, Noctilio leporinus

Sperm of the greater bulldog bat Noctilio leporinus display an architecture that is totally unique among mammalian spermatozoa. The sperm head of Noctilio is extraordinarily large and flat and lies eccentrically with respect to the sperm tail. The major portion of the atypically large acrosome lies anterior to the nucleus and is shaped into a dozen accordionlike folds that run parallel to the long axis of the sperm. The ridge of each fold is shaped into approximately 60 minute, evenly spaced rises that extend along the entire length of the fold. We speculate that acrosome ridges may serve to strengthen the sperm head during transport.

Nuclear morphogenesis and the role of the manchette during spermiogenesis in the ostrich (Struthio camelus)

Nuclear condensation during spermiogenesis in the ostrich follows the basic pattern established in other vertebrates. The fine granular nuclear substance of early spermatids is gradually replaced by numbers of coarse dense granules which appear to arise by aggregation of smaller dispersed elements of the chromatin. The granules increase in size and eventually coalesce to form the compact homogenous mass of chromatin typical of the mature sperm. In ostrich spermatids, however, the aggregation of the nuclear material produces large numbers of longitudinally oriented rod-shaped structures in addition to some granular material. Although fibrillar chromatin has been observed during spermiogenesis in a number of vertebrate species, the hollow nature of the rod-shaped chromatin granules in ostrich spermatids is a unique phenomenon. The spiralisation of the chromatin material observed in ostrich spermatids and in some other nonpasserine birds is possibly related to the reduction in nuclear length demonstrated during spermiogenesis in these species. In common with other nonpasserine birds, spermiogenesis in the ostrich is characterised by the appearance both of a circular and a longitudinal manchette. The circular manchette consists of a single row of microtubules reinforced by additional peripherally arranged microtubules. Links between adjacent microtubules, and between the nucleolemma and some of the microtubules, are evident. The longitudinal manchette consists of arrays of interconnected microtubules arranged in approximately 4-6 staggered, ill defined rows. This structure seems to originate as a result of the rearrangement of the microtubules of the circular manchette and is only formed once the process of chromatin condensation is well advanced. Based on the sequence of morphological events observed during spermiogenesis in the ostrich, it is concluded that the circular manchette is responsible for the initial transformation in shape of the spermatid nucleus. Thereafter, the chromatin condenses independently within the confines of the nucleolemma with the circular manchette merely acting to maintain the shape of the nucleus while this process is underway, to compress the nuclear membrane, and possibly to orientate the subunits of the condensing chromatin. The longitudinal manchette appears to assist in the translocation of material during spermatid elongation. There are indications that the developing acrosome is instrumental in effecting nuclear shaping of the apical (subacrosomal) head region of the ostrich spermatid.

A case of an infertile man with short-tailed spermatozoa

Ejaculated spermatozoa from an infertile patient were examined by scanning and transmission electron microscopy. All spermatozoa had abnormalities in the tail region. The abnormalities were divided into three types: (a) spermatozoa with a spherical tail. The tail was larger than the head in volume. This type of abnormality accounted for about 60% of the population; (b) spermatozoa with a blunt tail. The tails were about 2 microns in diameter and about 7 microns in length. This type of abnormality accounted for about 30% of the population of an ejaculate; (c) spermatozoa without a tail. Some of these had only a rudimentary tail. About 10% of the population belonged to this type. In types A and B, all components of the tail, except for dynein arms, were observed, but they were severely disarranged. Biopsy specimens of the testes and of the nasal mucosa of this patient were also investigated. The testicular biopsy showed defects in manchette formation. Normal development of the manchette could not be observed. The nasal epithelium showed absence of the inner dynein arms in the cilia.

Spermatogenesis in nonmammalian vertebrates

Spermatogenesis appears to be a fairly conserved process throughout the vertebrate series. Thus, spermatogonia develop into spermatocytes that undergo meiosis to produce spermatids which enter spermiogenesis where they undergo a morphological transformation into spermatozoa. There is, however, variation amongst the vertebrates in how germ cell development and maturation is accomplished. This difference can be broadly divided into two distinct patterns, one present in anamniotes (fish, amphibia) and the other in amniotes (reptiles, birds, mammals). For anamniotes, spermatogenesis occurs in spermatocysts (cysts) which for most species develop within seminiferous lobules. Cysts are produced when a Sertoli cell becomes associated with a primary spermatogonium. Mitotic divisions of the primary spermatogonium produce a cohort of secondary spermatogonia that are enclosed by the Sertoli cell which forms the wall of the cyst. With spermatogenic progression a clone of isogeneic spermatozoa is produced which are released, by rupture of the cyst, into the lumen of the seminiferous lobule. Following spermiation, the Sertoli cell degenerates. For anamniotes, therefore, there is no permanent germinal epithelium since spermatocysts have to be replaced during successive breeding seasons. By contrast, spermatogenesis in amniotes does not occur in cysts but in seminiferous tubules that possess a permanent population of Sertoli cells and spermatogonia which act as a germ cell reservoir for succeeding bouts of spermatogenic activity. There is, in general, a greater variation in the organization of the testis and pattern of spermatogenesis in the anamniotes compared to amniotes. This is primarily due to the fact there is more reproductive diversity in anamniotes ranging from a relatively unspecialized condition where gametes are simply released into the aqueous environment to highly specialized strategies involving internal fertilization. These differences are obviously reflected in the mode of spermatogenesis and this is particularly true of the stage of spermiogenesis where the morphology of the species-specific spermatozoon is determined. Moreover, unlike amniotes, many anamniotes display a spermatogenic wave manifest, depending upon the species, either at the level of the cyst or seminiferous lobule. This variation in the organization of the testis makes certain anamniotes perfect models for investigating germ cell development and maturation. For instance, the presence of a spermatogenic wave provides an opportunity to manually isolate discrete germ cell stages for analysis of specific Sertoli/germ cell interactions. Furthermore, for many anamniotes, germ cells mature in association with a morphologically poorly developed Sertoli cell. This seeming independence of Sertoli cell regulation allows the in vitro culture of isolated germ cells of some species of anamniotes through several developmental stages. Thus, due either to the anatomical organization of the testis, or structural simplicity of the germinal units, nonmammalian vertebrates can provide excellent experimental animal models for investigating many basic problems of male reproduction.

Linkage of manchette microtubules to the nuclear envelope and observations of the role of the manchette in nuclear shaping during spermiogenesis in rodents

Structural features of the mouse and rat manchette and the role of the manchette in shaping the spermatid nucleus were investigated. Rod-like elements about 10 nm in diameter and 40-70 nm in length were seen linking the innermost microtubules of the manchette and the outer leaflet of the nuclear envelope in step 8 through step 11 rat and mouse spermatids that either had been routinely fixed for electron microscopy or had been isolated and detergent extracted. Rod-like linkers were also seen joining the nuclear ring to the plasma membrane and nuclear envelope. These linkers may ensure that under normal conditions the manchette remains in a defined position relative to these membranous components. A variety of compounds (taxol, cytoxan, and 5-fluorouracil) were found to perturb the manchette and to affect nuclear shaping. In addition, sys and azh mutant mice were used to determine the consequences of defective manchette formation. These genetic conditions and chemical treatments either produced manchettes that were not in their normal position (azh, sys, and taxol) and/or caused the manchette to appear abnormal (azh, sys, cytoxan, 5-fluorouracil, and taxol), and all resulted in a deformation of the step 9-11 spermatid nucleus. In all instances where the manchette was present, either in normal or ectopic locations, the sectioned nuclear envelope was parallel to the long axis of the microtubules of the manchette. In general, areas of the nuclear envelope where the manchette was not present, or where it was expected to be present but was not, were rounded (normal animals, sys, cytoxan). In addition, there are indications using certain compounds (cytoxan and 5-fluorouracil) as well as in the azh and sys mouse that the manchette may exert pressure to deform the nucleus. It is suggested that the rod-like linkages of the manchette ensure that the nuclear envelope remains at a constant distance from the manchette microtubules and that this is a major factor acting to impart nuclear shape changes on a region of the head caudal to the acrosome during the early elongation phase of spermiogenesis. The manchette microtubules, which are also known to be linked together, may act as a scaffold to deform this part of the nucleus from its spherical shape, perhaps in concert with forces initiated by other structural elements. Evidence from sys animals indicates that structural elements, such as the acrosomal complex over the anterior head (acrosome-actin-nuclear envelope), may affect nuclear shaping over the acrosome-covered portion of the spermatid head.(ABSTRACT TRUNCATED AT 400 WORDS)

Spermiogenesis in Xenopus laevis: from late spermatids to spermatozoa

Spermatogenesis is a complex morphogenetic process in which microfilaments and microtubules have been shown to play an important role. The last steps of Xenopus spermatogenesis, i.e., the corkscrew shaping of the sperm head, have been followed to study actin and microtubule distribution by conventional and immunoelectron microscopy. During sperm head morphogenesis, actin is absent in the elongating spermatids, but it is present in the Sertoli cells where results localized at the periphery of their cytoplasm that surrounds the developing germ cells. Sertoli cell actin and microtubules may assist the elongation and the shaping of the spermatids and function in maintaining the Sertoli-spermatid association.

Development of spermatozoa in the rhea

We have examined the ultrastructural changes that take place during spermiogenesis in the rhea. Spermatozoa are characterized by a curved head and a midpiece. A thin rod extends from the anterior tip of the spermatozoon through the center of the nucleus. A 3-mu-long distal centriole occupies the entire midpiece. The principal piece is characterized by a small fibrous sheath and tiny dense fibers that are only observed in the region of the principal piece, which is immediately behind the annulus. During development a circular manchette surrounds the nucleus of young spermatids. Later the microtubules of the circular manchette become reorganized into a longitudinal manchette. A long distal and short proximal centriole are observed in early round spermatids. The distal centriole becomes associated with the plasma membrane. Later the proximal centriole is observed in association with the nucleus. The area around the centriole pair then accumulates dense material, which is associated with either the centrioles or the circular manchette. The longitudinal manchette forms and then disappears and mitochondria subsequently associate with the distal centriole. The long centriole of the rhea enables this species to develop a midpiece similar to the midpiece of mammalian sperm without the complex intercellular movements that characterize mammalian spermiogenesis.

Nuclear shaping in spermatids of the Thai leaf frog Megophrys montana

Transmission electron microscopy of Thai leaf frog testis revealed a unique pattern of spermatid nuclear morphogenesis. Chromatin condenses into a continuous cylindrical coil within a roughly spherical nucleus. Later the nuclear membrane conforms to the contours of the uncoiling nuclear contents. In the mature sperm, the long, tapering nucleus is helically shaped. This developmental sequence occurs in the absence of a microtubular manchette, raising questions about the role of this structure in nuclear shaping in spermatozoa of other species.

Spermiogenesis in Platichthys flesus

Early spermatids of Platichthys flesus (flounder) have a central spherical nucleus and cytoplasm with numerous dispersed mitochondria and a pair of peripheral centrioles. One of the centrioles acts as a kinetosome for axoneme formation. After the start of chromatin condensation the centrioles and developing axoneme migrate to adopt a tangential orientation adjacent to a flattened, lateral margin of the nucleus. During migration, pericentriolar material becomes reorganised. The proximal centriole becomes surrounded by nine fibres, and the distal centriole by a complex collar. As the nucleus condenses it undergoes a rotation which shifts it to the anterior of the centrioles and deepens the articular fossa which houses, and is connected to, the two centrioles and their associated structures. This rotation also results in the mitochondria becoming relocated to form a ring around the proximal region of the axoneme to give the cell its mature ultrastructure.

Microtubules in the spermatids of stick insects

Spermatids from two phasmid species were seen to possess an unusually large amount of microtubules along the nucleus and tail. Some of the microtubules have a loosely fitting sleeve for half a micron or more. During late stages in spermiogenesis the microtubules aggregate and form one or several "microtubular crystals" consisting of electron-lucid tubular elements with a diameter of about 360 A. The tail flagellum contains five kinds of microtubular structures, which all have a substructure of longitudinal protofilaments that is clearly visible after fixation in the presence of tannic acid. The so-called accessory tubules have 17 protofilaments that have the same appearance as that in ordinary, 13-unit microtubules, but are somewhat thicker than those. It is evident that the protofilaments in both the 17-unit and the 13-unit microtubules run parallel or nearly parallel to the long axis of the microtubules. It is of interest that both types of microtubules possess a prime number of protofilaments which may give the fagellum certain functional advantages.

Nuclear elongation of dissociated newt spermatids in vitro and their nuclear shortening by antimicrotubule agents

Dissociated newt spermatids with an initial cell length of 20-35 microns increased in length at an average rate of 35-46 microns during 5 days of culture at 22 degrees C. 10(-5) M vinblastine sulfate shortened the length of nearly all the spermatids of various initial lengths to that of round spermatids within 24 h at 22 degrees C. Application of vinblastine to the spermatids immediately following initiation of nuclear elongation caused the nuclei to become completely round within 1 h. 3 X 10(-6) M colcemid, 10(-4) M colchicine, 2 X 10(-5) M nocodazole and 10(-4) M griseofulvin also shortened the spermatid length. The effects of these five antimicrotubule agents were irreversible. Neither 10(-4) M beta-, gamma-lumicolchicine nor 1.0 micrograms/ml cytochalasin B (CB) had any effect on spermatid elongation. Spermatids incubated at 4 degrees C for 6 days shortened by 20-50%, but after transfer to 22 degrees C they started to elongate. An ultrastructural study showed that during nuclear elongation the number of microtubules increased in proportion to the elongation, and that the microtubules surrounded the whole nucleus from its apical to caudal end. After addition of vinblastine many microtubular crystals appeared in the cytoplasm of the spermatids. It was concluded that microtubules are a prerequisite for nuclear elongation of newt spermatids, and it is speculated that microtubules act directly in the initiation and continuation of the nuclear elongation of newt spermatids.

Determinants of sperm nuclear shaping in the genus Xenopus

The morphogenesis of sperm nuclei was investigated in six different species or subspecies of the genus Xenopus (Pipidae, Anura). The sequence of nuclear morphogenesis was similar in each species used in this study. Electrophoretic comparison of the basic chromatin proteins from late spermatids and sperm of each species demonstrated that the complements of histones and spermatid-sperm-specific basic proteins were extremely diverse suggesting that shape was not determined by specific basic proteins or mechanisms of histone removal. This conclusion was reinforced by the observation that Xenopus sperm DNA decondensed by 2.0 M NaCl remained contained in residual structures which resembled intact sperm nuclei. These observations suggested that morphogenesis of sperm nuclei is directed by proteins or RNA molecules which are not directly responsible for chromatin condensation.

The early development of the tail and the transformation of the shape of the nucleus of the spermatid of the domestic fowl, Gallus gallus

The differentiation of the spermatid, especially in reference to the formation of the flagellum, and transformation of the shape of the nucleus was investigated in the domestic fowl. In the early stage of the spermatid, a prominent Golgi apparatus appears around the centrioles. The Golgi vesicles then surround the axial-filament complex which develops from the distal centriole. These vesicles fuse to form continuous membrane at the earliest stage of flagellar formation, and in the succeeding stage Golgi lamellae are attached to the plasma membrane of the developing flagellum. From these observations, it is assumed that Golgi apparatus may be a source of the membrane system of the flagellum. The microtubules distributed around the nucleus form the circular manchette. The anterior region of the nucleus with the manchette is cylindrical in shape and the posterior region without it remains irregular in shape. When the circular manchette has been completed, the whole nucleus acquires a slender cylindrical shape. The circular manchette then changes into the longitudinal manchette. The nuclei of spermatids without a longitudinal manchette are abnormal in shape. In view of these observations it is assumed that the nuclear shaping of the spermatid may be accomplished by circular manchette and the maintenance of shape of the elongated nucleus by longitudinal manchette.

Spermiogenesis in the teleost Gambusia affinis with particular reference to the role played by microtubules

During nuclear elongation in spermatids of Gambusia affinis, a deep fossa is formed at the base of the nucleus in which the centriolar complex and proximal portion of the flagellum reside. To stabilize the positional relationship between the nucleus and centriolar complex, while nuclear morphogenesis is taking place, a series of microtubules develop which emanate from the centriolar complex and extend to the nuclear envelope lining the fossa. Buttressing microtubules also develop within the nuclear fossa which both originate and insert along the nuclear envelope. These appear to stabilize nuclear shape prior to the time when chromatin condensation has proceeded to the stage where it could lend structural stability to nuclear form. Microtubules develop only after specific nuclear morphogenic events have taken place. It is therefore concluded that the spermatid nucleus is capable of "self-assembly" involving microtubules in a supportive role in addition to stabilizing the nuclear-flagellar relationship in G. affinis. The pattern of nuclear fossa-associated microtubules in G. affinis is significantly different from that observed in other poeciliid teleosts indicating a degree of species specificity with regard to both the timing of appearance and total number of microtubules.