Uterine and eggshell structure and histochemistry in a lizard with prolonged uterine egg retention (Lacertilia,Scincidae, Saiphos)

The eggshell of lizards is a complex structure composed of organic and inorganic molecules secreted by the oviduct, which protects the embryo by providing a barrier to the external environment and also allows the exchange of respiratory gases and water for life support. Calcium deposited on the surface of the eggshell provides an important nutrient source for the embryo. Variation in physical conditions encountered by eggs results in a tradeoff among these functions and influences eggshell structure. Evolution of prolonged uterine egg retention results in a significant change in the incubation environment, notably reduction in efficiency of gas exchange, and selection should favor a concomitant reduction in eggshell thickness. This model is supported by studies that demonstrate an inverse correlation between eggshell thickness and length of uterine egg retention. One mechanism leading to thinning of the eggshell is reduction in size of uterine shell glands. Saiphos equalis is an Australian scincid lizard with an unusual pattern of geographic variation in reproductive mode. All populations retain eggs in the uterus beyond the embryonic stage at oviposition typical for lizards, and some are viviparous. We compared structure and histochemistry of the uterus and eggshell of two populations of S. equalis, prolonged egg retention, and viviparous to test the hypotheses: 1) eggshell thickness is inversely correlated with length of egg retention and 2) eggshell thickness is positively correlated with size of shell glands. We found support for the first hypothesis but also found that eggshells of both populations are surprisingly thick compared with other lizards. Our histochemical data support prior conclusions that uterine shell glands are the source of protein fiber matrix of the eggshell, but we did not find a correlation between size of shell glands and eggshell thickness. Eggshell thickness is likely determined by density of uterine shell glands in this species. J. Morphol., 2010. © 2010 Wiley‐Liss, Inc.     

Oviductal morphology and egg shelling in the oviparous lizards Crotaphytus collaris and Eumeces obsoletus

Morphological changes occurring in the oviduct and epithelial cells of the lizards Crotaphytus collaris and Eumeces obsoletus during the natural reproductive cycle were examined and quantified. Additionally, development of the eggshell at different stages of gravidity was described. The anterior uterus of each species has a distinct glandular type which differs between species: in E. obsoletus, the glands are tubular and in C. collaris, branched saccular. The branched saccular glands in the anterior uterus of C. collaris produce collagen-like material that forms the fibers of the shell membranes. However, fibers from the eggshell of E. obsoletus did not stain for collagen. The shell of both species is composed of a multilayered inner boundary covered externally by fibers of varying thickness. Initial layers are composed of thick fibers all lying along the same general axis. Outer layers of fibers are progressively thinner and an external surface layer composed of glycosaminoglycans (GAGs) is also present. In C. collaris, calcium, which is deposited in relatively small amounts on the shell surface, appears to be secreted by the epithelium of the anterior uterus. The nonciliated secretory epithelial cells covering the villi-like folds of the posterior infundibulum secrete GAGs. Epithelial cell height of the infundibular villi is greatest during early gravidity. A functional relationship may exist between luteal activity and oviductal secretory activity because the activity of the glandular epithelium varied as gravidity progressed.     

Anatomy and ultrastructure of the female reproductive system of Pleioplana atomata (Platyhelminthes: Polycladida)

The ultrastructure of the female reproductive system of the polyclad flatworm Pleioplana atomata is described. Numerous ovaries are scattered throughout the entire body but are mainly concentrated on the dorsal side. Within an ovary, a germinative zone with oogonia and prefolicular cells is located in the dorsal part of the ovary. The remaining part of the gonad is filled with previtellogenic and early vitellogenic oocytes enwrapped by follicular cells. During previtellogenesis, oocytes produce numerous eggshell globules, which are distributed into the cortical area of the cell in later stages. Eventually, these globules release their contents into the space between the eggshell cover and oolemma. Similar types of globules are also found in others flatworms, and may represent useful phylogenetic characters. Entolecital, vitellogenic oocytes pass to paired uteri, where vitellogenesis is completed. The remainder of the female reproductive system consists of paired thin uterine ducts that join a vagina. The distal part of the long, curved vagina forms a large Lang's vesicle, while the proximal part is connected to a female atrium leading to a female gonopore. We hypothesize that Lang's vesicle functions in the digestion of excess sperm received. Two kinds of different shell (cement) glands that release their secretion into the vagina are identified. Both are unicellular glands and each gland cell connects to the lumen of the vagina via an individual canal. Similar glands in other acotylean polyclads have been implicated in the formation of eggshell covers. J. Morphol. 2009. © 2008 Wiley‐Liss, Inc.     

Oviductal morphology and eggshell formation in the lizard,Sceloporus woodi

Despite a great deal of work in recent years on the structure of reptilian eggshells, few studies have examined the structure and regulation of the female reproductive tract in the formation of eggshell components, and none have examined the entire process from ovulation to oviposition. In this study, we examined oviductal structure in the oviparous lizard, Sceloporus woodi, followed changes in oviductal structure during gravidity, and determined uterine function in the formation of eggshell components. The endometrial glands of the uterus produce the proteinaceous fibers of the eggshell membrane mainly during the first 24 hours following ovulation, and the fibers are secreted intact and subsequently wrapped around the in utero eggs. Eggshell fibers of different thicknesses are layered around each egg, ranging from an inner layer of thick fibers that gradually become thinner medially and finally forms an outer layer of densely packed particulate matter. These changes in the fibrous layer are reflected by the thickness and length of fibers released from the endometrial glands. Calcium deposition occurs from 3 days following ovulation through day 14 (oviposition) and is accompanied by cellular changes in the luminal epithelium suggestive of secretory activity. Deposition of the eggshell components within the uterus occurs on all eggs simultaneously, rather than sequentially. © 1993 Wiley-Liss, Inc.     

Fine structure and formation of eggshells in marine Gastrotricha

Summary The fine structure of the gastrotrich eggshell in the hermaphroditic species Turbanella ocellata (Hummon 1974) and the parthenogenetic species Aspidiophorus sp. is described using transmission electron microscopy. The presented evidence strongly suggests that the shell is produced by the egg itself prior to oviposition in both species. The layed egg in Aspidiophorus sp. is provided with a special attachment stalk that is also preformed in the mother animal. Freshly layed eggs of T. ocellata are adhesive all around their surface and lack any specialized structures for attachment. Formation of the spiny eggshell of Aspidiophorus sp. appears to begin with a sudden release of special vesicles containing the preformed spines of the outer eggshell covering. Additional material appears to be secreted by the egg in a more gradual process after the initial vesicle release. The formation of the two fibrous layers in the eggshell of T. ocellata is less well understood and deposition of eggshell material could be seen either as a continuous process or as two separate steps, similar to the events observed for Aspidiophorus sp. For T. ocellata, Tetranchyroderma sp. and Aspidiophorus sp. it is demonstrated that formation of the cuticle occurs as an independent process from that of eggshell formation. This is significantly different from the basic mode of cuticle formation in the annelid line of evolution. The paper argues further that the data support earlier claims of a pronounced difference between the Gastrotricha-Macrodasyida and the Gastrotricha-Paucitubulatina and agree well with the postulated ties of the Gastrotricha and Nematoda. The phylogenetic importance of the eggshell fine-structure is discussed in the framework of present theories on aschelminth phylogeny.Abbreviations cus cuticular spines - cut cuticle - cov coated vesicles - cv cup-shaped vesicles - dp dense particles - ep epidermis - emb embryo - erl lacunae of smooth ER - fgb fibrous and granular bodies - fl fibrous layer - ga Golgi apparatus - gc gut cell - gv Golgi vesicles - im intercellular matrix - isp intercellular space - isl inner shell layer - ld lipid droplet - mdb medium-dense bodies - mvb multivesicular bodies - oc oocyte - od oviduct - osl outer shell layer - o egg - sv spiny vesicles - sh eggshell - st egg-stalk - sl spiny layer - sub substrate - trm trilaminate membrane - yb yolk bodies - yg yolk granule - yoc young oocyte This work was supported by NSF Grant # GB-42211 to R.M. Rieger     

Localization of osteopontin in oviduct tissue and eggshell during different stages of the avian egg laying cycle

The avian eggshell is an acellular bioceramic containing organic and inorganic phases that are sequentially assembled during the time the egg moves along the oviduct. As it has been demonstrated in other mineralized tissues, mineralization of the eggshell is regulated by extracellular matrix proteins especially the anionic side chains of proteoglycans. Among them, osteopontin has been found in the avian eggshell and oviduct. However, its precise localization in the eggshell or in different oviduct regions during eggshell formation, nor its function have been established. By using anti-osteopontin antibody (OPN 1), we studied its immunolocalization in the isthmus, red isthmus and shell gland of the oviduct, and in the eggshell during formation. In the eggshell, osteopontin was localized in the core of the non-mineralized shell membrane fibers, in the base of the mammillae and in the outermost part of the palisade. In the oviduct, OPN 1 was localized in the ciliated epithelial but not in the tubular gland cells of the isthmus, in the ciliated epithelial cells of the red isthmus, and in the non-ciliated epithelial cells of the shell gland. The occurrence of osteopontin in each of the oviduct regions, coincided with the concomitant presence of the egg in such region. Considering the reported inhibitory function of osteopontin in other mineralized systems, together with its main occurrence in the non-mineralized parts of the eggshell and at the outermost part of the shell, suggests that this molecule could be part of the mechanism regulating the eggshell calcification.     

Parataxonomic review of the Upper Cretaceous dinosaur eggshells belonging to the oofamily Megaloolithidae from India and Argentina

The eggshell oospecies from India and Argentina are compared and reviewed in detail. These eggshells resemble each other in having a nodular outer surface ornamentation and clearly arched growth lines of the shell units. Microstructurally, the eggshell oospecies belonging to the oofamily Megaloolithidae shows fan-like shell units, which are sharply separated from each other throughout the thickness of the eggshell and can be traced up to the surface of the eggshell. Comparisons between four oospecies from India and Argentina reveal three groupings, which show similarities between megaloolithids of both countries: (1) Megaloolithus jabalpurensis, M. matleyi and M. patagonicus; (2) M. cylindricus, M. rahioliensis and Tipo 1d; and (3) M.megadermus and Tipo 1e. The other two types of eggshell oospecies from India and Argentina show partially fused external nodes and shell units. As a result, growth lines enter into the adjacent shell units with a marked concavity. A new oogenus Fusioolithus have been erected due to fusion between shell units and tubospherulitic morphotype, which include two new oospecies F. baghensis and F. berthei. Till date, morphostructurally, a total of 15 eggshell oospecies belonging to different oofamilies have been recorded from India and seven oospecies from Argentina.     

Progesterone stimulates prostaglandin synthesis in eggshell gland mucosa of estrogen-primed chickens.

1. Prostaglandins may be involved in calcium translocation in the avian shell gland, since indomethacin, administered at the beginning of shell formation, reduces eggshell thickness as well as 45Ca-uptake and prostaglandin synthesis by a homogenate of eggshell gland mucosa. 2. The stimulus for calcium transport in the shell gland during shell formation remains unknown. 3. The present study was undertaken to investigate the effects of progesterone on prostaglandin formation by the eggshell gland mucosa of the domestic fowl. 4. Progesterone significantly stimulated synthesis of PGF2 alpha, PGE2 and TXB2 by eggshell gland mucosa homogenate. 5. Progesterone treatment also induced the synthesis of the biotin-binding protein, avidin. 6. A microsomal fraction prepared from the eggshell gland mucosa had a high affinity for binding PGE2. 7. Progesterone treatment reduced the KD value of this binding without affecting the maximal number of binding sites. 8. Progesterone did not change the total calcium content of shell gland mucosa. 9. The role progesterone plays in prostaglandin formation and calcium transport in the eggshell gland mucosa is discussed.     

The embryonic development of Schistosoma mansoni eggs: proposal for a new staging system

Schistosomiasis is a water-borne parasitic illness caused by neoophoran trematodes of the genus Schistosoma. Using classical histological techniques and whole-mount preparations, the present work describes the embryonic development of Schistosoma mansoni eggs in the murine host and compares it with eggs maintained under in vitro conditions. Two pre-embryonic stages occur inside the female worm: the prezygotic stage is characterized by the release of mature oocytes from the female ovary until its fertilization. The zygotic stage encompasses the migration of the zygote through the ootype, where the eggshell is formed, to the uterus. Fully formed eggs are laid still undeveloped, without having suffered any cleavage. In the outside environment, eight embryonic stages can be defined: stage 1 refers to early cleavages and the beginning of yolk fusion. Stage 2 represents late cleavage, with the formation of a stereoblastula and the onset of outer envelope differentiation. Stage 3 is defined by the elongation of the embryonic primordium and the onset of inner envelope formation. At stage 4, the first organ primordia arise. During stages 5 to 7, tissue and organ differentiation occurs (neural mass, epidermis, terebratorium, musculature, and miracidial glands). Stage 7 is characterized by the nuclear condensation of neurons of the central neural mass. Stage 8 refers to the fully formed larva, presenting muscular contraction, cilia, and flame-cell beating. This staging system was compared to a previous classification and could underlie further studies on egg histoproteomics (morphological localizome). The differentiation of embryonic structures and their probable roles in granulomatogenesis are discussed herein. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Histology and functional morphology of the female reproductive tract of the tortoise Gopherus polyphemus

The oviducts of 25 tortoises (Gopherus polyphemus) were examined by using histology and scanning electron microscopy to determine oviductal functional morphology. Oviductal formation of albumen and eggshell was of particular interest. The oviduct is composed of 5 morphologically distinct regions; infundibulum, uterine tube, isthmus, uterus, and vagina. The epithelium consists of ciliated cells and microvillous secretory cells throughout the oviduct, whereas bleb secretory cells are unique to the infundibulum. The epithelium and endometrial glands of the uterine tube histologically resemble those of the avian magnum which produce egg albumen and may be functionally homologous. The isthmus is a short, nonglandular region of the oviduct and appears to contribute little to either albumen or eggshell formation. The uterus retains the eggs until oviposition and may form both the fibrous and calcareous eggshell. The endometrial glands are histologically similar to the endometrial glands of the isthmus of birds, which are known to secrete the fibers of the eggshell. These glands hypertrophy during vitellogenesis but become depleted during gravidity. The uterine epithelium may supply "plumping water" to the egg albumen as well as transport calcium ions for eggshell formation. The vagina is extremely muscular and serves as a sphincter to retain the eggs until oviposition. Sperm are found within the oviductal lumen and endometrial glands from the posterior tube to the anterior uterus throughout the reproductive cycle. This indicates sperm storage within the female tract, although the viability and reproductive significance of these sperm are unknown.     

Structure of shells from eggs of kinosternid turtles

Shells from eggs of five species of kinosternid turtle (Sternotherus minor, Kinosternon flavescens, K. baurii, K. Hirtipes, and K. alamosae) were examined with light and scanning electron microscopy. Except for possible differences among species in thickness of eggshells, structure of shells from all eggs was similiar. In general, kinosternid turtles lay eggs having a rigid calcareous layer composed of calcium carbonate in the form of aragonite. The calcareous layer is organized into individual shell units with needlelike crystallites radiating from a common center. Most of the thickness of the eggshell is attributable to the calcareous layer, with the fibrous shell membrane comprising only a small fraction of shell thickness. Pores are found in the calcareous layer, but they are not numereous. The outer surface of the eggshells is sculptured and may have a thick, organic layer in places. The outer surface of the shell membrane of decalcified eggshells is studded with spherical cores which presumably nucleate growth of shell units during shell formation. The shell membrane detaches from eggs incubated to hatching, carrying with it remnants of the calcareous layer. Such changes in shell structure presumably reflect withdrawal of calcium from the eggshell by developing embryos.     

Aspects of shell formation in eggs of the tuatara,Sphenodon punctatus

The flexible shell from eggs of the tuatara (Sphenodon punctatus) is comprised of both calcareous and fibrous components. The calcareous material is organized into columns that extend deep into the fibrous shell membrane. Many of the fibers of the membrane are enclosed within the crystalline matrix of the columns. Columns widen and flatten slightly at the outer surface of the eggshell to form cap-like structures composed of a compact crystalline matrix containing no fibers. The outer surface of eggs laid prior to completion of shell formation consists of a series of nodes obscured by a densely fibrous matrix. Similar nodes also are found at the inner surface of partially shelled eggs. The nodes represent the outer and inner aspects of columns that had not completed formation prior to oviposition. Our interpretation is that a layer (or layers) of the shell membrane forms first, with nucleation of columns occurring shortly thereafter. Columns grow into the membrane a short distance and enclose fibers of the membrane, but the primary direction of column growth is toward what will become the outer aspect of the shell. Calcareous columns and the shell membrane form more or less in concert until crystal growth outstrips that of the membrane and a cap-like apex of compact crystalline material is formed. The end result is an eggshell in which the shell membrane and calcareous material form a single unit for much of the thickness of the shell.     

Relationship between eggshell strength and keratan sulfate of eggshell membranes

Eggshell strength is an important factor in an effort to minimize eggshell breakage, which is a significant problem in the egg production industry. In the current study, we isolated and quantified the specific glycosaminoglycans (GAGs) from the calcified eggshell and shell membranes, which are related to eggshell strength. Our data suggest that GAGs exist in calcified eggshell may influence morphology of shell but do not affect on increase of shell amount while GAGs of shell membranes are maybe highly associated with shell strength with an increase of shell weight. Shell strength showed a strong correlation with the content of GAGs (r=0.942, p<0.0005) and a weak relationship with uronic acid content (r=0.564, p=0.056) in shell membranes. Monosaccharides in shell membranes were determined by Bio-LC analysis for the identification of any specific GAGs related with shell strength. It indicates that the galactose content as a component of keratan sulfate (KS) has a significant correlation with eggshell strength (r=0.985, p<0.0005). These results suggest that eggshell strength is proportional to the KS content of eggshell membranes with an increase of eggshell weight.     

Cloning of ovocalyxin-36, a novel chicken eggshell protein related to lipopolysaccharide-binding proteins, bactericidal permeability-increasing proteins, and plunc family proteins

The avian eggshell is a composite biomaterial composed of noncalcifying eggshell membranes and the overlying calcified shell matrix. The shell is deposited in a uterine fluid where the concentration of different protein species varies at different stages of its formation. The role of avian eggshell proteins during shell formation remains poorly understood, and we have sought to identify and characterize the individual components in order to gain insight into their function during elaboration of the eggshell. In this study, we have used direct sequencing, immunochemistry, expression screening, and EST data base mining to clone and characterize a 1995-bp full-length cDNA sequence corresponding to a novel chicken eggshell protein that we have named Ovocalyxin-36 (OCX-36). Ovocalyxin-36 protein was only detected in the regions of the oviduct where egg-shell formation takes place; uterine OCX-36 message was strongly up-regulated during eggshell calcification. OCX-36 localized to the calcified eggshell predominantly in the inner part of the shell, and to the shell membranes. BlastN data base searching indicates that there is no mammalian version of OCX-36; however, the protein sequence is 20-25% homologous to proteins associated with the innate immune response as follows: lipopolysaccharide-binding proteins, bactericidal permeability-increasing proteins, and Plunc family proteins. Moreover, the genomic organization of these proteins and OCX-36 appears to be highly conserved. These observations suggest that OCX-36 is a novel and specific chicken eggshell protein related to the superfamily of lipopolysaccharide-binding proteins/bactericidal permeability-increasing proteins and Plunc proteins. OCX-36 may therefore participate in natural defense mechanisms that keep the egg free of pathogens.     

Fine structure of the eggshell of Ommatissus binotatus Fieber (Homoptera,Auchenorrhyncha, Tropiduchidae)

The external morphology and fine structure of the eggshell of Ommatissus binotatus Fieber (Homoptera : Tropiduchidae) was investigated by light, scanning and transmission electron microscopy. The egg surface has 2 main regions: a specialized area and an unspecialized egg capsule. The specialized area is characterized by a large respiratory plate containing the operculum and a short respiratory horn. The latter consists of an external hollow tube and an internal coneshaped projection hosting a micropylar canal. The eggshell has 4 layers: the vitelline envelope, a wax layer, the chorion and an outer mucous layer. The chorion has inner, intermediate and outer parts. The functions of the different parts of the eggshell are discussed. Characters useful to define the eggs and the oviposition habit in the family Tropiduchidae were provided. The size and morphology of the egg, plate, respiratory horn and operculum are suggested as useful characters for ootaxonomic analysis.     

Ultrastructure of avian eggshell during resorption following egg fertilization

For skeletal mineralization, the avian embryo mobilizes calcium from its calcitic eggshell. This occurs through dissolution of specific interior regions of the shell in a process that also weakens the shell to allow hatching. Here, we have examined eggshell ultrastructure during dissolution occurring between laying of a fertilized egg (with incubation) and hatching of the chick (Gallus gallus). We have focused on changes in shell mammillae where the majority of dissolution takes place. Using scanning electron microscopy, we describe differences in matrix–mineral structure and relationships not observed in unfertilized eggs (unresorbed eggshell). We document changes in the calcium reserve body – an essential sub-compartment of mammillae – consistent with it being an early, primary source of calcium essential for embryonic skeletal growth. Dissolution events occurring in the calcium reserve sac and in the base plate of the calcium reserve body, and similar changes in surrounding bulk mammillae structure, all correlate with advancing skeletal embryonic calcification. The changes in mammillae sub-structures can generally be characterized as mineral dissolutions revealing fine surface topographies on remaining mineral surfaces and the exposure of an extensive, intracrystalline (occluded) organic matrix network. We propose that this mineral-occluded network regulates how shell mineral is dissolved by providing dissolution channels facilitating calcium release for the embryonic skeleton.     

The proteome of the insoluble Schistosoma mansoni eggshell skeleton

In schistosomiasis, the majority of symptoms of the disease is caused by the eggs that are trapped in the liver. These eggs elicit an immune reaction that leads to the formation of granulomas. The eggshell, which is a rigid insoluble structure built from cross-linked proteins, is the site of direct interaction between the egg and the immune system. However, the exact protein composition of the insoluble eggshell was previously unknown. To identify the proteins of the eggshell of Schistosoma mansoni we performed LC-MS/MS analysis, immunostaining and amino acid analysis on eggshell fragments. For this, eggshell protein skeleton was prepared by thoroughly cleaning eggshells in a four-step stripping procedure of increasing strength including urea and SDS to remove all material that is not covalently linked to the eggshell itself, but is part of the inside of the egg, such as Reynold’s layer, von Lichtenberg’s envelope and the miracidium. We identified 45 proteins of which the majority are non-structural proteins and non-specific for eggs, but are house-keeping proteins that are present in large quantities in worms and miracidia. Some of these proteins are known to be immunogenic, such as HSP70, GST and enolase. In addition, a number of schistosome-specific proteins with unknown function and no homology to any known annotated protein were found to be incorporated in the eggshell. Schistosome-specific glycoconjugates were also shown to be present on the eggshell protein skeleton. This study also confirmed that the putative eggshell protein p14 contributes largely to the eggshell. Together, these results give new insights into eggshell composition as well as eggshell formation. Those proteins that are present at the site and time of eggshell formation are incorporated in the cross-linked eggshell and this cross-linking does no longer occur when the miracidium starts secreting proteins.     

Global diversity and adaptations of avian eggshell thickness indices

The amniote eggshell is a fundamental aspect of the embryo life-support system, protecting it from UV light, microbes and mechanical damage, while regulating gas exchange and providing calcium for growth. The thickness of eggshells is highly diverse across modern birds and influences multiple eggshell functions, yet the selective pressures driving eggshell thickness have not been clearly identified. Here, we use a global dataset of avian eggshell thickness indices for 4260 (> 41%) avian species to assess trends in eggshell thickness across the phylogeny, as these indices are strongly correlated with direct measures of eggshell thickness and are non-destructive to the sample. We analysed the dataset within a phylogenetic framework to assess the relative importance of climatic and ecological explanations for variation in eggshell thickness indices. The distribution of avian eggshell thickness indices across species was found to be primarily driven by phylogenetic relatedness, in addition to evolutionary processes that do not match a Brownian model of evolution. Across modern birds, thicker eggshells were more prevalent in species (1) with precocial young, (2) which exhibit a scavenger-based diet, (3) which primarily feed on vertebrates or plants (excluding nectivores, seed and fruit specialists) and (4) which breed in open habitats. Thicker eggshells found in species with precocial young probably enable higher rates of calcium removal for the more advanced embryo development. Excessive light transmission through the shell damages developing embryos, while too little light transmission can impede development. Eggs in shaded habitats experience low light exposure, and thus thinner shells are more prevalent in these environments potentially to increase light transmission through the shell. Overall, variation in eggshell thickness indices appears to be driven largely by phylogeny, with certain life-history traits linked to embryo growth rate, calcium content of their food, and the need to mitigate excessive light transmission through the shell.     

Development of the uterine shell glands during the preovulatory and early gestation periods in oviparous and viviparous Lacerta vivipara

The evolutionary process leading to the emergence of viviparity in Squamata consists of lengthening the period of egg retention in utero coupled with marked reduction in the thickness of the eggshell. We used light microscopy and scanning electron microscopy to study uterine structure during the reproductive cycle of oviparous and viviparous females of the reproductively bimodal Lacerta vivipara. We compared the structure of the uterine shell glands, which secrete components of the eggshell, during preovulatory and early gestation phases of the reproductive cycle and also compared histochemistry of the eggshells. The uterine glands of both reproductive forms undergo considerable growth within a period of a few weeks during folliculogenesis and vitellogenesis preceding ovulation. The majority of the proteinaceous fibers of the shell membrane are secreted early in embryonic development and the uterine glands regress shortly thereafter. This supports previous observations indicating that, in Squamata, secretion of the shell membrane occurs very rapidly after ovulation. The most striking differences between reproductive modes were larger uterine glands at late vitellogenesis in oviparous females, 101 microm compared to 60 microm in viviparous females, and greater thickness of the shell membrane during early gestation in oviparous females (52-73 microm) compared to viviparous females (4-8 microm). Our intraspecific comparison supports the conclusions of previous studies that, prior to ovulation, the uterine glandular layer is less developed in viviparous than in oviparous species, and that this is the main factor accounting for differences in the thickness of the shell membrane of the two reproductive forms of squamates.     

Ultrastructural studies of the formation of the egg capsule in the hermaphroditic species, Isohypsibius granulifer granulifer Thulin, 1928 (Eutardigrada: Hypsibiidae)

The egg capsule of Isohypsibius granulifer granulifer Thulin 1928 (Eutardigrada: Hypsibiidae) is composed of two shells: the thin vitelline envelope and the multilayered chorion. The process of the formation of the egg shell begins in middle vitellogenesis. The I. g. granulifer vitelline envelope is of the primary type (secreted by the oocyte), but the chorion should be regarded as a mixed type: primary (secreted by the oocyte), and secondary (produced by the cells of gonad wall). During early choriogenesis, the parts of the chorion are produced and then connected into a permanent layer. The completely developed chorion consists of three layers: (1) the inner, medium electron dense layer; (2) the middle labyrinthine layer; (3) the outer, medium electron dense layer. After the formation of the chorion, a vitelline envelope is secreted by the oocyte.