Spermiogenesis in the red-ear turtle (Pseudemys scripta) and the domestic fowl (Gallus domesticus): a study of cytoplasmic events including cell volume changes and cytoplasmic elimination

Asymmetric Stem Cell Division and Germline Immortality

Germ cells are the only cell type that is capable of transmitting genetic information to the next generation, which has enabled the continuation of multicellular life for the last 1.5 billion years. Surprisingly little is known about the mechanisms supporting the germline's remarkable ability to continue in this eternal cycle, termed germline immortality. Even unicellular organisms age at a cellular level, demonstrating that cellular aging is inevitable. Extensive studies in yeast have established the framework of how asymmetric cell division and gametogenesis may contribute to the resetting of cellular age. This review examines the mechanisms of germline immortality-how germline cells reset the aging of cells-drawing a parallel between yeast and multicellular organisms.

Steps of spermiogenesis in the ostrich (Struthio camelus)

Few studies describe the sequence of morphological events that characterize spermiogenesis in birds. In this paper, the clearly observable steps of spermiogenesis are described and illustrated for the first time in a commercially important ratite, the ostrich, based on light microscopy of toluidine blue-stained plastic sections. Findings were supplemented and supported by ultrastructural observations, PNA labeling of acrosome development, and immunocytochemical labeling of isolated spermatogenic cells. Spermiogenesis in the ostrich followed the general pattern described in non-passerine birds. Eight steps were identified based on changes in nuclear shape and contents, positioning of the centriolar complex, and acrosome development. Only two steps could be recognized with certainty during development of the round spermatid which contributed to the fewer steps recorded for the ostrich compared to that described in some other bird species. The only lectin that displayed acrosome reactivity was PNA and only for the first three steps of spermiogenesis. This suggests that organizational and/or compositional changes may occur in the acrosome during development and merits further investigation. Immunological labeling provided additional evidence to support the finding of previous studies that the tip of the nucleus in the ostrich is shaped by the forming acrosome and not by the microtubular manchette. To our knowledge, this is the first complete description of spermiogenesis in ostrich and one of few in any avian species. In addition to comparative reproduction and animal science, this work has implications for evolutionary biology as the reported germ cell features provide a bridge between reptile and ratite-avian spermatogenesis.

Seasonal spermatogenesis in the red-eared slider (Trachemys scripta elegans): The roles of GnRH, actin cytoskeleton, and MAPK

Reproduction is the key to the ecological invasion of alien species. As an invasive species, the characteristic and regularity of red-eared slider (Trachemys scripta elegans) spermatogenesis is an index for evaluating reproduction and ecological adaptation. Here, we investigated the characteristics of spermatogenesis i.e., the gonadosomatic index (GSI), plasma reproductive hormone levels, and the histological structure of testes by HE and TUNEL staining, and then RNA-Seq in T. s. elegans. The histomorphological evidence confirmed that seasonal spermatogenesis in T. s. elegans has four successive phases: quiescence (December-May of the following year), early-stage (June-July), mid-stage (August-September), and late-stage (October-November). In contrast to 17β-estradiol, testosterone levels were higher during quiescence (breeding season) compared to mid-stage (non-breeding season). Based on RNA-seq transcriptional analysis, gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways were used to analyze the testis in the quiescent and mid-stage. Our study found that circannual spermatogenesis is regulated by interactive networks including gonadotropin-releasing hormone (GnRH) secretion, regulation of actin cytoskeleton, and MAPK signaling pathways. Moreover, the number of genes associated with proliferation and differentiation (srf, nr4a1), cell cycle (ppard, ccnb2), and apoptosis (xiap) were up-regulated in the mid-stage. With the maximum energy saving, this seasonal pattern of T. s. elegans determines optimal reproductive success and thus adapts better to the environment. These results provide the basis for the invasion mechanism of T. s. elegans and lay the foundation for deeper insight into the molecular mechanism of seasonal spermatogenesis in reptiles.

Avian Reproduction

There are about 10,400 living avian species belonging to the class Aves, characterized by feathers which no other animal classes possess and are warm-blooded vertebrates with four-chamber heart. They have excellent vision, and their forelimbs are modified into wings for flight or swimming, though not all can fly or swim. They lay hard-shelled eggs which are a secretory product of the reproductive system that vary greatly in colour, shape and size, and the bigger the bird, the bigger the egg. Since domestication, avian species have been basically reared for eggs, meat, pleasure and research. They reproduce sexually with the spermatozoa being homogametic and carry Z-bearing chromosomes, and the blastodisk carries either Z-bearing or W-bearing chromosomes, hence, the female is heterogametic, and thus, determines the sex of the offspring. The paired testes produce spermatozoa, sex hormones and the single ovary (with a few exceptions) produces yolk bearing the blastodisk and sex hormones. Both testis and ovary are the primary sex organs involved in sexual characteristics development in avian. In avian reproduction, there must be mating for fertile egg that must be incubated to produce the young ones. At hatch, hatchling sex is identified and reared to meet the aim of the farmer.

Ultrastructure evidence for vesicles and double-membrane structures involved in cytoplasmic elimination during spermiogenesis in large yellow croaker, Larimichthys crocea (Teleostei, Perciformes, Scienidae)

Spermatids eliminate excess cytoplasm to form streamlined sperm during spermiogenesis, which mechanism is insufficiently elucidated in fish. In this study, we investigated the cytoplasmic elimination procedure in spermatid during spermiogenesis in the large yellow croaker (Larimichthys crocea) using transmission electron microscopy. The early spermatid is subrotund with a centrally located nucleus. With further development, nucleus polarizes into one side of the cell while the cytoplasm with numerous vesicles near the membrane migrates to the caudal region. Furthermore, exocytosis-like structures were detected in middle spermatid. In late spermatid, the vesicles are reduced and rarely observed. These findings indicate that vesicles may be involved in cytoplasmic elimination possibly via exocytosis. In the later spermatid, a double-membrane, autophagosome-like structure envelopes the cytoplasm, which may develop into a single-membrane structure, and gets discarded from the cell as a residual body from the caudal region. This suggests its potential functions in the formation of residual body and cytoplasmic elimination. Overall, our results revealed that polarized development of spermatid causes polarized distribution of cytoplasm necessary for cytoplasmic elimination. Moreover, they provide ultrastructure evidence for vesicles and double-membrane structures involved in discarding spermatid cytoplasm in large yellow croaker, thus offering novel insights into cytoplasmic elimination during spermiogenesis in fish.

Process of cytoplasm elimination during spermiogenesis in Octopus tankahkeei: Polarized development of the spermatid and discarding of the residual body

The elimination of the spermatid cytoplasm during spermiogenesis enables the sperm to acquire a streamlined architecture, which allows for unhindered swimming. While this process has been well described in vertebrates, it has rarely been reported in invertebrates. In this study, we observed the process of cytoplasm elimination during spermiogenesis in Octopus tankahkeei (Mollusca, Cephalopoda) using light microscopy, transmission electron microscopy, and immunofluorescence. In the early spermatid, the cell is circular, and the nucleus is centrally located. With spermatid development, the cell becomes polarized. The nucleus gradually elongates and moves toward the end of the cell where the tail is forming. As a result, the cytoplasm moves past the nucleus at the anterior region of the future sperm head (the foreside of the acrosome). Following this, during the late stage of spermiogenesis, the cytoplasm condenses and collects on the foreside of the acrosome until finally the residual body is discarded from the top of the sperm head. This represents a distinct directionality for the development of cytoplasmic polarity and discarding of residual body compared with that reported for vertebrates (in which the cytoplasm of the elongating spermatids is polarized toward the caudal region). The fact that the cytoplasm also becomes concentrated suggests that water pumps may be involved in the elimination of water from the cytoplasm before the residual body is discarded. Furthermore, we found that microtubules, forming a manchette-like structure, are involved not only in reshaping of the nucleus but also in the transport of mitochondria and vesicles to the foreside of the acrosome, subsequently allowing them to be discarded with the residual body. This study broadens our understanding of the development of polarization and elimination of cytoplasm from spermatids in animals.

Characteristics of seasonal spermatogenesis in the soft-shelled turtle

Spermatogenesis in reptiles is a seasonally dependent physiological process that is not temporally associated with male mating behavior. Characteristics of seasonal spermatogenesis in reptiles, however, remain largely unknown. In this review, there is a coverage of the characteristics of soft-shelled turtle, Pelodiscus sinensis, during seasonal spermatogenesis that provides insights into spermatogenesis of testudines. The seminiferous epithelium of P. sinensis are undergoing spermatogenesis during the summer and fall, but are quiescent throughout the rest of the year; germ cells progress through spermatogenic stages in a temporal rather than a spatial pattern. While apoptotic germ cells mainly appear in the non-spermatogenic phase, these are seldom present during active spermatogenesis. It is inferred that apoptosis may be one of the reasons for germ cell loss during the resting phase of spermatogenesis. During the period when spermatogenesis is occurring, Sertoli cells become very narrow and are in contact with several round/elongated spermatids. Many residual spermatozoa can be internalized and degraded within Sertoli cells by entosis during the non-spermatogenic phase, which precedes the next reproductive cycle in P. sinensis. In the late spermatogenic phase, round-shaped mitochondria of spermatids become elongated and swollen, subsequently forming a crescent-like shape and develop into "onion-like" shaped mitochondria. As spermiogenesis progresses, the endoplasmic reticulum of spermatids is transferred into a specialized structure called the "Chrysanthemum flower center", which may be a source of autophagosomal membranes. The information provided in this review will help improve understanding of characteristics of seasonal spermatogenesis, which will hopefully promote interest in the study of reptilian species.

Testicular Structure and Spermatogenesis in the Naked Mole-Rat Is Unique (Degenerate) and Atypical Compared to Other Mammals

The naked mole-rat (NMR) queen controls reproduction in her eusocial colony by usually selecting one male for reproduction and suppressing gametogenesis in all other males and females. Simplified, polymorphic and slow-swimming spermatozoa in the NMR seem to have been shaped by a low risk of sperm competition. We hypothesize that this unique mammalian social organization has had a dramatic influence on testicular features and spermatogenesis in the NMR. The testicular structure as well as spermatogenic cell types and its organization in breeding, subordinate and disperser males were studied using light microscopy and transmission electron microscopy. Even though the basic testicular design in NMRs is similar to most Afrotheria as well as some rodents with intra-abdominal testes, the Sertoli and spermatogenic cells have many atypical mammalian features. Seminiferous tubules are distended and contain large volumes of fluid while interstitial tissue cover about 50% of the testicular surface area and is mainly composed of Leydig cells. The Sertoli cell cytoplasm contains an extensive network of membranes and a variety of fluid-containing vesicles. Furthermore, Sertoli cells form numerous phagosomes that often appear as extensive accumulations of myelin. Another unusual feature of mature NMR Sertoli cells is mitotic division. While the main types of spermatogonia and spermatocytes are clearly identifiable, these cells are poorly organized and many spermatids without typical intercellular bridges are present. Spermatid heads appear to be malformed with disorganized chromatin, nuclear fragmentation and an ill-defined acrosome formed from star-like Golgi bodies. Rudimentary manchette development corresponds with the occurrence of abnormal sperm morphology. We also hypothesize that NMR testicular organization and spermiation are modified to produce spermatozoa on demand in a short period of time and subsequently use a Sertoli cell "pump" to flush the spermatozoa into short tubuli recti and simplified rete testis. Despite the difficulty in finding cellular associations during spermatogenesis, six spermatogenic stages could be described in the NMR. These numerous atypical and often simplified features of the NMR further supports the notion of degenerative orthogenesis that was selected for due to the absence of sperm competition.

Ultrastructural differentiation of spermiogenesis in Scincus scincus (Scincidae, Reptilia)

BACKGROUND

Knowledge of spermiogenesis in reptiles, especially in lizards, is very limited. Lizards found in Arabian deserts have not been considered for detailed studies due to many reasons and the paucity of these animals. Therefore, we designed a study on the differentiation and morphogenesis of spermiogenesis, at an ultrastructural level, in a rare lizard species, Scincus scincus.

RESULTS

The spermiogenesis process includes the development of an acrosomal vesicle, aggregation of acrosomal granules, formation of subacrosomal nuclear space, and nuclear elongation. A role for manchette microtubules was described in nuclear shaping and organelle movement. During head differentiation, the fine granular chromatin of the early spermatid is gradually replaced by highly condensed contents in a process called chromatin condensation. Furthermore, ultrastructural features of sperm tail differentiation in S. scincus were described in detail. The commencement was with caudal migration toward centrioles, insertion of the proximal centriole in the nuclear fossa, and extension of the distal centrioles to form the microtubular axoneme. Subsequently, tail differentiation consists of the enlargement of neck portion, middle piece, the main and end pieces.

CONCLUSIONS

This study aids in the understanding of different aspects of spermiogenesis in the lizard family and unfurls evolutionary links within and outside reptiles.

Ultrastructure of spermatid development within the testis of the Yellow-Bellied Sea Snake, Pelamis platurus (Squamata: Elapidae)

Little is known about spermatid development during spermiogenesis in snakes, as there is only one complete study in ophidians, which details the spermatid ultrastructure within the viperid, Agkistrodon piscivorus. Thus, the following study will add to our understanding of the ontogenic steps of spermiogenesis in snakes by examining spermatid maturation in the elapid, Pelamis platurus, which were collected in Costa Rica in 2009. The spermatids of P. platurus share many similar ultrastructural characteristics to that described for other squamates during spermiogenesis. Three notable differences between the spermatids of P. platurus and those of other snakes is a round and shorter epinuclear lucent zone, enlarged caudal nuclear shoulders, and more prominent 3 and 8 peripheral fibers in the principal and endpieces. Also, the midpiece is much longer in P. platurus and is similar to that reported for all snakes studied to date. Other features of chromatin condensation and morphology of the acrosome complex are similar to what has been observed in A. piscivorus and other squamates. Though the spermatids in P. platurus appear to be quite similar to other snakes and lizards studied to date, some differences in subcellular details are still observed. Analysis of developing spermatids in P. platurus and other snakes could reveals morphologically conserved traits between different species along with subtle changes that could help determine phylogenetic relationships once a suitable number of species have been examined for ophidians and other squamates.

Chicken sperm transcriptome profiling by microarray analysis

It has been confirmed that mammalian sperm contain thousands of functional RNAs, and some of them have vital roles in fertilization and early embryonic development. Therefore, we attempted to characterize transcriptome of the sperm of fertile chickens using microarray analysis. Spermatozoal RNA was pooled from 10 fertile males and used for RNA preparation. Prior to performing the microarray, RNA quality was assessed using a bioanalyzer, and gDNA and somatic cell RNA contamination was assessed by CD4 and PTPRC gene amplification. The chicken sperm transcriptome was cross-examined by analysing sperm and testes RNA on a 4 × 44K chicken array, and results were verified by RT-PCR. Microarray analysis identified 21,639 predominantly nuclear-encoded transcripts in chicken sperm. The majority (66.55%) of the sperm transcripts were shared with the testes, while surprisingly, 33.45% transcripts were detected (raw signal intensity greater than 50) only in the sperm and not in the testes. The greatest proportion of up-regulated transcripts were responsible for signal transduction (63.20%) followed by embryonic development (56.76%) and cell structure (56.25%). Of the 20 most abundant transcripts, 18 remain uncharacterized, whereas the least abundant genes were mostly associated with the ribosome. These findings lay a foundation for more detailed investigations on sperm RNAs in chickens to identify sperm-based biomarkers for fertility.

Spermiogenesis in birds

Current knowledge on avian spermiogenesis, including strengths and weaknesses, has been reviewed. Information on avian spermiogenesis considerably lags behind that in mammals because of the paucity of reports in birds. Spermiogenesis in passerine birds has received even much less attention than in non-passerine birds. Mechanisms underlying morphogenesis of the acrosome and nucleus, and roles of microtubular assemblies are poorly understood. The proximal centriole found in non-passerine birds, but hitherto considered to be absent in passerine birds, has recently been described in spermatids and mature spermatozoa of 2 passeridan species, including the Masked weaver for which new and detailed spermiogenetic information is provided in this review. A great deal more studies on spermiogenesis, and spermatogenesis generally, in various avian species are required to considerably enhance knowledge of this phenomenon, contribute to comparative spermatology, provide a basis for appropriate applied studies, and contribute to understanding of phylogeny in this vast order of vertebrates.

A novel transient structure with phylogenetic implications found in ratite spermatids

BACKGROUND

A novel transient structure was observed in the spermatids of three ratite species using transmission electron microscopy.

RESULTS

The structure first appeared at the circular manchette stage of sperm development, was most prominent during the longitudinal manchette phase and disappeared abruptly prior to spermiation. It was composed of regularly-spaced finger-like projections which were closely associated with the outer nuclear membrane, giving the nucleus a cogwheel-like appearance. The projections were approximately 30 nm long and 14 nm wide. Although a similar structure has been described in certain lizard and crocodile species, this is the first report of a similar structure in the developing spermatids of birds.

CONCLUSIONS

The potential value of non-traditional characters, such as spermiogenesis and sperm ultrastructure, as phylogenetic markers has recently been advocated. The morphologically unique structure found in ratite spermatids provides additional evidence of a possible phylogenetic link between the reptiles and birds. It also endorses the basal positioning of the ratites as a monophyletic group within the avian phylogenetic tree.

Spermiogenesis in the Australian cockatiel Nymphicus hollandicus

Information on the ultrastructure of parrot spermatids and spermatozoa is limited to only four species with no comprehensive study of spermiogenesis conducted within the order Psittaciformes. The present study was undertaken to describe the development of the cockatiel spermatid using electron microscopy. Four phases of spermatid maturation were documented on the basis of nuclear morphology, development of the acrosome, perforatorium, and axial filament. These phases included 1) round nuclei, 2) irregular nuclei, 3) elongated nuclei with granular chromatin, and 4) elongated nuclei with homogenous chromatin. While development of the cockatiel spermatid was comparable to that of other domestic avian species, we have noted the hollow nature of some chromatin granules, an abnormal formation of the axoneme, the absence of the fibrous sheath around the axoneme of the principal piece, and the absence of an annulus.

The ultrastructural characteristics of the spermatozoa stored in the cauda epididymidis in Chinese soft-shelled turtle Pelodiscus sinensis during the breeding season

The ultrastructure of spermatozoa in cauda epididymidis of soft-shelled turtle, P. sinensis during breeding season was investigated by light microscopy (LM) and electron microscopy (TEM and SEM). The mature spermatozoa appeared elongated and filiform. In general, the turtle spermatozoon contains a characteristic head, midpiece and tail, similar in morphology to that of birds, amphibians and other reptiles. However, several features are unique. These include (1) three intranuclear tubules containing dense core extend from the subacrosomal cone through the rostral nucleus and deep into the nuclear body; (2) the midpiece is composed of 40 mitochondria which present a staggered rings-and-columns arrangement (8 parallel rings and 5 columns); (3) unusual spherical mitochondria with a dense core are surrounded by 8-10 concentric layers of cristae. Surprisingly, about 21.4±3.6 percent immature spermatozoa with normal morphology are also observed in this season. Different from the mature spermatozoa, a variable amount of cytoplasm droplets are attached to the immature spermatozoa under SEM. Some spermatozoa still show the tail coiled tightly around the cytoplasm. These spermatozoa in transverse sections under TEM, showed a large amount of cytoplasm wrapped by plasma membrane; even some free mitochondria and higher electron density material still seen in the cytoplasm. Among the immature spermatozoa, most of them possess a cytoplasmic droplet which is located eccentrically on the midpiece, and contains a lot of lipid droplets in addition to hollow vesicles. Lipid droplets are closely associated with mitochondrial membranes and may function in the formation or degradation of mitochondria. These immature spermatozoa may be the dormant cells, but whether or not they can fertilize the ovum or not is unknown. Thus, in the present study we hypothesized that the cauda epididymidis might be involved in the sperm maturation in this species.

Reptilian spermatogenesis: A histological and ultrastructural perspective

Until recently, the histology and ultrastructural events of spermatogenesis in reptiles were relatively unknown. Most of the available morphological information focuses on specific stages of spermatogenesis, spermiogenesis, and/or of the mature spermatozoa. No study to date has provided complete ultrastructural information on the early events of spermatogenesis, proliferation and meiosis in class Reptilia. Furthermore, no comprehensive data set exists that describes the ultrastructure of the entire ontogenic progression of germ cells through the phases of reptilian spermatogenesis (mitosis, meiosis and spermiogenesis). The purpose of this review is to provide an ultrastructural and histological atlas of spermatogenesis in reptiles. The morphological details provided here are the first of their kind and can hopefully provide histological information on spermatogenesis that can be compared to that already known for anamniotes (fish and amphibians), birds and mammals. The data supplied in this review will provide a basic model that can be utilized for the study of sperm development in other reptiles. The use of such an atlas will hopefully stimulate more interest in collecting histological and ultrastructural data sets on spermatogenesis that may play important roles in future nontraditional phylogenetic analyses and histopathological studies in reptiles.

Ultrastructure of spermiogenesis in the Cottonmouth, Agkistrodon piscivorus (Squamata: Viperidae: Crotalinae)

To date multiple studies exist that examine the morphology of spermatozoa. However, there are limited numbers of data detailing the ontogenic characters of spermiogenesis within squamates. Testicular tissues were collected from Cottonmouths (Agkistrodon piscivorus) and tissues from spermiogenically active months were analyzed ultrastructurally to detail the cellular changes that occur during spermiogenesis. The major events of spermiogenesis (acrosome formation, nuclear elongation/DNA condensation, and flagellar development) resemble that of other squamates; however, specific ultrastructural differences can be observed between Cottonmouths and other squamates studied to date. During acrosome formation vesicles from the Golgi apparatus fuse at the apical surface of the nuclear membrane prior to making nuclear contact. At this stage, the acrosome granule can be observed in a centralized location within the vesicle. As elongation commences the acrosome complex becomes highly compartmentalized and migrates laterally along the nucleus. Parallel and circum-cylindrical microtubules (components of the manchette) are observed with parallel microtubules outnumbering the circum-cylindrical microtubules. Flagella, displaying the conserved 9 + 2 microtubule arrangement, sit in nuclear fossae that have electron lucent shoulders juxtaposed on either side of the spermatids basal plates. This study aims to provide developmental characters for squamates in the subfamily Crotalinae, family Viperidae, which may be useful for histopathological studies on spermatogenesis in semi-aquatic species exposed to pesticides. Furthermore, these data in the near future may provide morphological characters for spermiogenesis that can be added to morphological data matrices that may be used in phylogenetic analyses.

Sertoli cell proliferation during the post hatching period in domestic fowl

There has been no study aimed at directly determining of the periods of Sertoli cell proliferation in birds even domestic fowl. The aims of this study were to observe the cessation of post-hatching mitotic proliferation of Sertoli cells in domestic fowl, and to determine the volume density of Sertoli and germ cells during this period. A total of 50 Leghorn chicks were used in this study. The testes sections of the animals were immunostained with BrdU to observe the proliferation of cells from one to 10 weeks of age. The volume density of the Sertoli and germ cells were determined using the standard point counting method. The volume density of the germ cell nuclei was initially less than that of the Sertoli cells but the volume density converged by week 6, and remained relatively constant until the commencement of meiosis. Clear labeling of Sertoli and germ cells was observed from week 1 to week 7. The only those cells still labeled after 8 weeks were germ cells, indicating that Sertoli cell proliferation had ceased. Therefore, it is recommended that any research into the testes of domestic fowl should consider the cessation of Sertoli cell proliferation by approximately 8 weeks.

Spermiogenesis in soft-shelled turtle, Pelodiscus sinensis

Spermiogenesis in the soft-shelled turtle, Pelodiscus sinensis, was examined by transmission electron microscopy. The process includes nuclear elongation, chromatin condensation, acrosomal and flagellar development, and elimination of excess cytoplasm. In stage I, the proacrosomal vesicle occurs next to a shallow fossa of the nucleus, and a dense acrosomal granule forms beneath it. A smaller subacrosomal granule in the middle of the fibrous layer is related to the development of intranuclear tubules. The nucleus begins to move eccentrically. In stage II, the round proacrosomal vesicle is flattened by protrusion of the nuclear fossa, and the dense acrosomal granule diffuses into the vesicle, as the fibrous layer forms the subacrosomal cone. Circular manchettes develop around the nucleus, and the chromatin coagulates into small granules. The movement of the nucleus causes rearrangement of the cytoplasm. In stage III, the front of the elongating nucleus protrudes out of the spermatid and is covered by the flat acrosome; coarse granules replace the small ones within the nucleus. At the posterior pole of the head, mitochondria move backward. Numerous microtubules begin to assemble the axoneme of flagellum. In stage IV, the chromatin concentrates to dense homogeneous phase. The circular manchette is reorganized longitudinally. The Sertoli process covers the acrosome and the residues of the cytoplasmic lobes are eliminated. In stage V, the sperm head matures. After dissolution of the longitudinal manchette, the mitochondria arrange themselves around the proximal and distal centrioles. Caudal to the mitochondrial mass, a fibrous sheath surrounds the proximal portion of the flagellum.

The Effect of Dietary l-Carnitine on Semen Traits of White Leghorns

A previous study conducted in our laboratory showed that feeding 500 ppm of dietary L-carnitine to young and aging White Leghorns for 5 wk improved sperm concentration and reduced sperm lipid peroxidation during the last half of supplementation. The current study examined the effect of feeding dosimetric as well as lower levels of L-carnitine for longer durations on semen traits of White Leghorns. In experiments 1 and 2, White Leghorns consumed diets supplemented with 0, 125, 250, or 500 mg of L-carnitine/kg of feed. For experiment 1, an 8-wk trial was conducted with 48 White Leghorns from 46 to 54 wk of age. For experiment 2, a 17-wk trial was conducted with 96 White Leghorn roosters from 46 to 63 wk of age. For experiment 3, 84 roosters were provided for ad libitum consumption a diet formulated to contain 0 or 125 ppm of L-carnitine beginning at hatch until 37 wk of age. Long-term consumption of 125 ppm of L-carnitine beginning at hatch was the only dietary treatment that sustained a persistent increase in sperm concentration. These results suggest that L-carnitine's antioxidant influence on sperm production begins before the onset of sexual maturity.

Ultrastructural examination of spermiogenesis within the testis of the ground skink,Scincella laterale (Squamata, Sauria, Scincidae)

Although the events of spermiogenesis are commonly studied in amniotes, the amount of research available for lizards (Sauria) is lacking. Many studies have described the morphological characteristics of mature spermatozoa in lizards, but few detail the ultrastructural changes that occur during spermiogenesis. The purpose of this study was to gain a better understanding of the subcellular events of spermiogenesis within the temperate ground skink (Scincella laterale). The morphological data presented here represent the first complete ultrastructural study of spermiogenesis within the Scincidae clade. Samples of testes from 20 specimens were prepared using standard techniques for transmission electron microscopy. Many of the ultrastructural changes occurring during spermiogenesis within the ground skink are similar to that of other saurians. However, there were a few unique characteristics that to date have not been described during spermiogenesis in other lizards. For example, during early round spermatid development within the ground skink testis, proacrosomal granules begin to form within the acrosomal vesicle before making contact with the apex of the nucleus. Also, a prominent microtubular manchette develops during spermiogenesis; however, the circular component of the manchete is absent in this species of skink. This developmental difference in manchette formation may lead to the more robust and straight mature spermatozoa that are common within the Scincidae family. These anatomical character differences may be valuable nontraditional sources that along with more traditional sources (i.e., mitochondrial DNA) may help elucidate phylogenetic relationships, which are historically considered controversial at best, among species within Scincidae and Sauria.

Cytological evaluation of spermatogenesis and organization of the germinal epithelium in the male slider turtle, Trachemys scripta

The germ cell development in the slider turtle (Trachemys scripta) testis was investigated by viewing the histology of the seminiferous epithelium in plastic sections with a light microscope. Germ cell morphologies in the slider turtle testis were similar to the morphologies of other vertebrate germ cell types. However, the slider turtle seminiferous epithelium contained germ cells that progress through spermatogenesis in a temporal rather than a spatial pattern, resulting in a single spermatogenic event that climaxed with one massive sperm release in November. Mature sperm then are stored within the epididymis until breeding commences in the following spring. The germ cell development strategy in the slider turtle is different from that of other amniotes and is more reminiscent of the developmental strategy found in the anamniotic testis. This temporal progression of germ cells through spermatogenesis within a tubular testis represents a transitional model that may be evolutionarily significant.

Cytological steps during spermiogenesis in the house sparrow (Passer domesticus, Linnaeus)

Spermiogenesis of the domestic sparrow was investigated with the light and electron microscopes and a step by step classification is proposed. Three cell populations corresponding to early, mid and late spermatids were easily divided according to their positions in the seminiferous epithelium. In addition to this initial separation, six steps were recognized, based on nuclear morphology and the degree of chromatin condensation, in association to their acrosomal and flagellar development. Early spermiogenesis is the period previous to chromatin condensation. The first step can be recognized by the extending flagellum and the second by the pro-acrosome development in contact with the nucleus. During the third or intermediate step, chromatin condenses and the cell becomes polarized with the pro-acrosomic vesicle and the tail occupying opposite sides of the nucleus. Late spermiogenesis, including steps IV-VI, is marked by complete chromatin condensation. The final cellular modifications lead to the formation of a spiraled spermatozoon. This shape is due to the twisting of the acrosome and nucleus, as well as the helical arrangement of mitochondria around the axoneme along most of the flagellum, making an exceptionally long middle piece.

Spermiogenesis and spermiation in a monotreme mammal, the platypus, Ornithorhynchus anatinus

Spermatogenesis in the platypus (Ornithorhynchus anatinus) is of considerable biological interest as the structure of its gametes more closely resemble that of reptiles and birds than marsupial or eutherian mammals. The ultrastructure of 16 steps of spermatid development is described and provides a basis for determining the kinetics of spermatogenesis. Steps 1-3 correspond to the Golgi phase of spermatid development, steps 4-8 correspond to the cap phase, steps 9-12 are the acrosomal phase, and steps 13-16 are the maturation phase. Acrosomal development follows the reptilian model and no acrosomal granule is formed. Most other features of spermiogenesis are similar to processes in reptiles and birds. However, some are unique to mammals. For example, a thin, lateral margin of the acrosome of platypus sperm expands over the nucleus as in other mammals, and more than in reptiles and birds. Also, a tubulobulbar complex develops around the spermatid head, a feature which appears to be unique to mammals. Further, during spermiation the residual body is released from the caudal end of the nucleus of platypus sperm leaving a cytoplasmic droplet located at the proximal end of the middle piece as in marsupial and eutherian mammals. Other features of spermiogenesis in platypus appear to be unique to monotremes. For example, nuclear condensation involves the formation of a layer of chromatin granules under the nucleolemma, and development of the fibrous sheath of the principal piece starts much later in the platypus than in birds or eutherian mammals.

The ultrastructure of spermatogenesis and epididymal spermatozoa of the tuatara Sphenodon punctatus (Sphenodontida, Amniota)

By using transmission electron microscopy (TEM) the events of spermatogenesis are described for the first time in the tuatara Sphenodon punctatus punctatus (Gray), a representative of the ‘reptilian’ order Sphenodontida. Secondary spermatocytes contain two greatly elongate (8.0 μm), rod-shaped centrioles which lie parallel to one another and are each associated with a small deposit of dense material and a short centriole. Spermatids contain only one rod-shaped centriole (associated with a short centriole) which gives rise to the flagellar axoneme thereby becoming the distal centriole. Four stages of spermatid development can be distinguished: (i) the early stage (nucleus round; nuclear contents granular with a thin, condensed periphery; mitochondria scattered; acrosomal vesicle spheroidal, slightly depressed onto nuclear surface); (ii) the middle stage (nucleus pyriform with two endonuclear canals formed; nuclear contents fibro-granular with thick periphery; mitochondria chiefly posterior; acrosomal vesicle flattened; centriolar complex attached to nucleus); (iii) the advanced stage (nucleus elongate and rod shaped; nuclear contents coarsely granular; mitochondria (containing linear cristae) clustered around the distal centriole; acrosomal vesicle conical; centriolar complex attached to posterior fossa of nucleus); (iv) the late stage (nucleus very elongate and associated with a longitudinally arranged microtubular sheath; nuclear contents very condensed; midpiece fully formed and featuring mitochondria with concentric cristae and a dense intramitochondrial body; centrioles associated with a dense, lateral body). Testicular sperm have a conical acrosomal vesicle (length 4 μm) and subacrosomal cone, an elongate (length 54- 56 μm) helical nucleus, a midpiece (length 8 μm, featuring spheroidal mitochondria containing concentric cristae and a dense body), an annulus, an elongate principal piece (length 74-78 μm, featuring a dense, fibrous sheath) and a short end piece (length 2-4 μm). Epididymal sperm differ from those in the testis by having a more developed lateral body in the midpiece and a sheath of flocculent material surrounding the fibrous sheath in the principal piece. The relatively large number of epididymal sperm still associated with a cytoplasmic droplet suggests that sperm spend a significant period maturing within the epididymis. The features of spermatogenesis and mature sperm suggest that the Sphenodontida are primitive amniotes, with only chelonians having fewer spermatozoal apomorphies while the crocodilians are little more advanced.

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.

Spermatozoal ultrastructure in three species of parrots (aves, Psittaciformes) and its phylogenetic implications

BACKGROUND

DNA-DNA hybridization studies suggest that Psittaciformes are highly, but not the most, derived nonpasserines. Multilocus protein electrophoresis indicates that cockatoos (Cacatuinae) form a monophyletic lineage distant from the other Australo-Papuan psittacids (Psittacinae).

METHODS

Transmission electron microscope procedures are applied to the spermatozoa of three parrots, in the Cacatuninae and Psittacinae, to investigate these relationships.

RESULTS

Psittaciform sperm have the following characteristics: (1) conical acrosome vesicle; rodlike perforatorium; cylindrical, highly condensed nucleus; proximal and distal centriole embedded in dense material; elongate periaxonemal mitochondrial midpiece, (2) nine dense peripheral axonemal fibers (coarse fibers), (3) no fibrous sheath around the axoneme, (4) mitochondria with linear cristae, lacking intra- (or inter-) mitochondrial dense bodies, (5) restriction of the endonuclear perforatorial canal to the anterior region of the nucleus, (6) a short distal centriole, and (7) nucleus abutting on but not penetrating the acrosome.

CONCLUSIONS

(1) These features are tetrapod symplesiomorphies, (2) is an amniote synapomorphy; the fibers differ from those of reptiles in being uniform in size, (3) loss of the fibrous sheath is an apomorphy known elsewhere only in columbiforms, (4) are apomorphies relative to basal aminiotes (Chelonia, Sphenodon, and Crocodilia), (5) is an apomorphic condition shared with other nonpasserines (galliforms and the white-naped crane) and crocodilians, (6) the latter taxa differ from parrots in a plesiomorphic elongation of the distal centriole, and (7) is a unique apomorphy of parrot sperm relative to other nonpasserines and reptiles. The short midpiece of N. hollandicus distinguishes this cacatuine from the two psittacines.

The comparative cell biology of accessory somatic (or Sertoli) cells in the animal testis

A comparative account is given of recent advances in the cell biology of testicular accessory somatic (or Sertoli) cells in mammals, nonmammalian vertebrates, and invertebrates by comparing and contrasting their structure and function. Their structure is discussed in relation to the nucleus, cytoplasmic organelles, and inclusions (lipids, the cytoskeleton, junctional complexes, and blood-testis barrier, which show great diversity and a variable testicular architecture), and mode of spermatogenesis. A very limited somatic cell-germinal association or its complete absence is observed in some groups of invertebrates. Wherever the somatic accessory cells are present, their comparative functions are discussed in relation to (1) mechanical support and nutrition; (2) translocation of germ cells; (3) paracrine regulation and a combination of male germ cell proliferation and differentiation by secretion of regulatory proteins, including peptide growth factors and hormones; (4) phagocytosis; (5) steroid hormone synthesis and metabolism; and (6) spermiation. Comparative cellular and molecular aspects of Sertoli cell-germ cell and peritubular cell interactions and the regulatory (hormonal) mechanisms involved as well as gaps in our knowledge about the molecular aspects of these interactions are emphasized for a better understanding of diversity in the patterns and regulation of spermatogenesis in animals.

Process of nuclear envelope reduction in spermiogenesis of a mosquito, Culex tigripes

When the Culex tigripes spermatid begins to elongate, the nucleus exhibits on its surface invaginations of the nuclear envelope. These invaginations have a uniform diameter of 0.3 microns. They separate from the envelope of the nucleus and form spherical intranuclear vesicles. In the old spermatids these vesicles are imprisoned in the condensed chromatin. The spermatozoon also possesses these vesicles which are then ovoid in shape. This process of vesiculation permits the diminution of the surface of the nucleus when it decreases in volume during spermiogenesis.