Pronephros and mesonephros characterization during the embryonic development of the giant South American river turtle, Podocnemis expansa (Podocnemididae: Testudines).

This study aimed to describe pronephros and mesonephros morphology during the embryonic development of Podocnemis expansa. Eggs were collected on an artificial beach at Balbina, Amazonas, Brazil, during the entire incubation period (mean of 59 days). The kidney-gonad complex was processed using light microscopy and the mesonephros using transmission electron microscopy. The pronephros was present for the first time on stage 4, composed of external glomeruli devoid of a capsule, protruding into the coelomic cavity, and internally composed of a capillary network. The pronephros degenerated after development stage 15. The first sign of the appearance of the mesonephros occurred around stage 8, indicated by the early formation of renal corpuscles. The mesonephros comprised an renal corpuscles, neck segment, proximal tubule, intermediate segment, distal tubule, collector tubule, and collector duct. Ultrastructural analysis of the mesonephros brush border was done in the proximal tubule, and the presence of cells with structural characters indicative of secretory activity was detected in the juxtatubular region. Renal corpuscles and proximal tubules were the main components that underwent morphological alterations during mesonephros degeneration. The pronephros is a transient kidney, and the mesonephros became the functional embryonic kidney in P. expansa. Mesonephros degeneration occurs in the cranial-caudal direction, and histologically, the degeneration is identified by changes in the morphology of the renal corpuscle and proximal tubule. However, the mesonephros is still present after hatching.

Live imaging of the developing mouse mesonephros.

Embryonic development is a highly dynamic process involving complex tissue interactions and movements. Recent progress in cell labeling, image acquisition, and image processing technologies has brought the study of embryo morphogenesis to another level. It is now possible to visualize in real time the dynamic morphogenetic changes occurring in vivo and to reconstitute and quantify them in 4D rendering. However, extended live embryo imaging remains challenging in terms of embryo survival and minimization of phototoxicity. Here, we describe a procedure to image the developing mesonephros for up to 16 h in intact mouse embryos. This method can easily be adapted to the imaging of other structures at similar developmental stages.

Nonrigid registration of CLSM images of physical sections with discontinuous deformations.

When biological specimens are cut into physical sections for three-dimensional (3D) imaging by confocal laser scanning microscopy, the slices may get distorted or ruptured. For subsequent 3D reconstruction, images from different physical sections need to be spatially aligned by optimization of a function composed of a data fidelity term evaluating similarity between the reference and target images, and a regularization term enforcing transformation smoothness. A regularization term evaluating the total variation (TV), which enables the registration algorithm to account for discontinuities in slice deformation (ruptures), while enforcing smoothness on continuously deformed regions, was proposed previously. The function with TV regularization was optimized using a graph-cut (GC) based iterative solution. However, GC may generate visible registration artifacts, which impair the 3D reconstruction. We present an alternative, multilabel TV optimization algorithm, which in the examined samples prevents the artifacts produced by GC. The algorithm is slower than GC but can be sped up several times when implemented in a multiprocessor computing environment. For image pairs with uneven brightness distribution, we introduce a reformulation of the TV-based registration, in which intensity-based data terms are replaced by comparison of salient features in the reference and target images quantified by local image entropies.

Morphology and ultrastructure of the pronephros of Testudo hermanni Gmelin, 1789 (chelonian reptiles).

The aim of this study was to investigate the morphology and ultrastructure of the pronephros of Testudo hermanni as observed in the earlier part of its development. This paired structure appears during renal ontogenesis and originates from the first somites localised in the cephalic part of the embryo. The first pronephric evidence is noted at stage 12. The kidney is composed of large glomeruli that are devoid of a capsule and protrude into the coelomic cavity. A ciliated nephrostome provides access to short renal tubules, lined with a well-developed brush border. Two nephric ducts on the lateral sides of the embryo are connected to the tubules. The cytological characteristics of the pronephric structure suggest that it might be functional, at least in the first part of development. The pronephros of Testudo hermanni regresses after a short time around stage 18, while the mesonephros is already well differentiated. Its plesiomorphic characteristics, similar to those observed in amphibians, might be related to the phyletic position of chelonians within the reptiles.

Role of the mesonephros as a transient haematopoietic organ in the bovine embryo.

During vertebrate embryogenesis, haematopoietic stem cells (HSC) arise in the aorta-gonads-mesonephros (AGM) region. In the present study, we examined serial sections of 22-34 days post-insemination (dpi) bovine embryos, a species with a well-developed and functional mesonephros. We describe the temporo-spatial distribution of presumptive c-kit positive HSC and the occurrence of haematopoietic foci in the mesonephros and fetal liver using specific antibodies directed against haematopoietic cell markers and conventional electron microscopy. In the mesonephros, presumptive HSC were found at 23-24 dpi in the blood stream, in the endothelial lining of the filtering capillaries and in the septal stroma of the cranial part of the mesonephros, the mesonephric giant corpuscle (MGC), suggesting a colonalization via the blood stream from the haematopoietic clusters of the dorsal aorta. From 25 to 30 dpi, presumptive HSC predominate in the septal stroma of the MGC, were they first expand but then decline and disappear following 32 dpi. In parallel, we found ongoing erythropoiesis and myelopoiesis starting in the MGC at 24 dpi and extending during the complete observation period. In the embryonic liver, colonization with presumptive c-kit positive HSC occurs slightly later, at 25 dpi. Active formation of blood cells in the liver increases following 30 dpi. In conclusion, the mesonephros of bovine embryos, in particular its MGC, functions as a primitive haematopoietic organ, temporarily intercalated between extraembryonic erythropoiesis and haematopoiesis within in the fetal liver, thus recapitulating for a short period a phylogenetically old site of blood formation.

Live imaging of emerging hematopoietic stem cells and early thymus colonization.

We recently demonstrated in zebrafish the developmental migration of emerging hematopoietic stem cells (HSCs) that is thought to occur in mammalian embryos, from the aorta-gonad-mesonephros (AGM) area to the successive hematopoietic organs. CD41 is the earliest known molecular marker of nascent HSCs in mammalian development. In this study, we show that in CD41-green fluorescent protein (GFP) transgenic zebrafish embryos, the transgene is expressed by emerging HSCs in the AGM, allowing us for the first time to image their behavior and trace them in real time. We find that the zebrafish AGM contains no intra-aortic cell clusters, so far considered a hallmark of HSC emergence. CD41GFP(low) HSCs emerge in the subaortic mesenchyme and enter the circulation not through the dorsal aorta but through the axial vein, the peculiar structure of which facilitates their intravasation. The rise in CD41-gfp expression among c-myb(+) HSC precursors is asynchronous and marks their competence to leave the AGM and immediately seed the caudal hematopoietic tissue (which has a hematopoietic function analogous to that of the mammalian fetal liver). Imaging the later migration of CD41-GFP(+) precursors to the nascent thymus reveals that although some reach the thymus by extravasating from the nearest vein, most travel for hours through the mesenchyme from surprisingly diverse and remote sites of extravasation.

The quail mesonephros: a new model for renal senescence?

BACKGROUND/AIMS: Renal senescence during normal aging is associated with specific vascular alterations and tissue degeneration. Although the degenerative program executed during embryonic kidney development is known to include vascular alterations, studies yet have to examine whether it involves replicative senescence. In this study, we assessed the potential of the quail mesonephros, a transitory embryonic kidney, as a model of human renal senescence. METHODS: Quail embryos with developing or degenerating mesonephros were studied on day 6 or day 11 of incubation, respectively. Senescence-associated beta-galactosidase activity, a marker of replicative senescence, was examined on whole mounts and sections. Senescent vascular characterization was performed by the scanning electron-microscopic analysis of vascular corrosion casts. RESULTS: Senescence-associated beta-galactosidase activity was found only in old mesonephros. Moreover, at 11 days of incubation glomerular capillaries showed discontinuities and were thinner and more tortuous than those observed at 6 days, characteristics also reported for the aging human kidney. CONCLUSION: The degenerating quail mesonephros is a potential model of renal senescence, showing biochemical and morphological characteristics of the aging human kidney.

Fish mesonephric model of polycystic kidney disease in medaka (Oryzias latipes) pc mutant.

BACKGROUND: Polycystic kidney disease (PKD) is a common hereditary disease. A number of murine and zebrafish mutants have been generated and used for the study of PKD as metanephric and pronephric models, respectively. Here, we report a medaka (Oryzias latipes) mutant that develops numerous cysts in the kidney in adulthood fish in an autosomal-recessive manner as a mesonephric model of PKD. METHODS: The phenotypes of the medaka pc mutant were described in terms of morphologic, histologic, and ultrastructural features. The pc see-through stock was produced by crossing a pc mutant and a fish from the see-through stock and used for observing the kidney through the transparent body wall of a live fish. RESULTS: The mutant developed bilateral massive enlargement of the kidney in adulthood. They sexually matured normally within 2 months of age and died within 6 months of age. The affected kidney was occupied by numerous, fluid-filled cysts, which were lined by attenuated squamous epithelial cells. Developmentally, cystic formation began in the pronephros in 10-day-old fry and in the mesonephros in 20-day-old fry at the microscopic level. The pc see-through stock was useful in observing disease progression in live fish. CONCLUSION: The kidney disorder that develops in the medaka pc mutant is a mesonephric counterpart of PKD, particularly an autosomal-dominant PKD, based on its morphologic, histologic, and ultrastructural features, and slow progression.

[The blood vessels of the mesonephros of domestic cattle (Bos taurus), a corrosion cast study]. / Die Blutgefässe der Urniere (Mesonephros) des Hausrindes (Bos taurus), eine korrosionsanatomische Untersuchung.

The blood vessels of the bovine mesonephros - a corrosion cast study Using scanning electron microscopy plastoid corrosion cast specimens of the mesonephros of 30-45-day-old bovine foetuses were investigated. The studies showed that the blood vessels of the mesonephros are organized in two separate networks similar to these of the teleosts and amphibians. The glomerula mesonephrica originate from the aorta implying that they belong to the high pressure system. The tubuli mesonephrici are surrounded by a filamentous independent capillary network which is shown for the first time in these studies. This capillary network originates from the caudal cardinal veins and represents a mesonephric portal system with a relatively low blood pressure. The blood from the two capillary networks diverts into terminal veins and into the subcardinal veins. The questions of haemodynamics in the mesonephros are discussed.

Expression of natriuretic peptides, nitric oxide synthase, and guanylate cyclase activity in frog mesonephros during the annual cycle.

Natriuretic peptides (NPs), a family of structurally related hormones and nitric oxide (NO), generated by nitric oxide synthase (NOS), are believed to be involved in the regulation of fluid balance and sodium homeostasis. Differential expression and regulation of these factors depend on both physiological and pathological conditions. Both NPs and NO act in target organs through the activation of guanylate cyclase (GC) and the generation of guanosine 3',5'-cyclic monophosphate (cGMP), which is considered a common messenger for the action of these factors. The present study was designed to investigate--by histochemical methods--the expression of some NPs (proANP and ANP) and isoforms of NOS (neuronal NOS, nNOS, and inducible NOS, iNOS) in the mesonephros of Rana esculenta in different periods of the year including hibernation, to evaluate possible seasonal changes in their expression. We also studied the enzyme activity of NOS-related nicotinamide adenine dinucleotide phosphate diaphorase (NADPHd) and of GC. The experiments were performed on pieces of kidney of R. esculenta collected in their natural environment during active and hibernating life. The study was carried out using immunohistochemical techniques to demonstrate proANP, ANP, and some NOS isoforms. Antigen capture by enzyme linked immunosorbent assay (ELISA) was also performed to determine the presence of NPs in the frog kidney extract. Enzyme histochemistry was used to demonstrate the NOS-related NADPHd activity at light microscopy; GC activity was visualized at the electron microscope, using cerium as capture agent. The application of the immunohistochemical techniques demonstrated that frog mesonephros tubules express different patterns of distribution and/or expression of ANP and NOS during the annual cycle. Comparing the results obtained on active and hibernating frogs has provided interesting data; the NOS/NADPHd and GC activities showed some variations as well. Furthermore, the presence of NPs in the frog kidney extract was evidenced by dose-dependent response in the ELISA. The data suggest that both ANP and NO are intra-renal paracrine and/or autocrine factors which may modulate the adaptations of frog renal functions to seasonal changes through the action of the cGMP generated from GC activity.

Origin and development of the pronephros in the chick embryo.

The process by which the pronephros develops was morphologically examined in chick embryos from Hamburger-Hamilton stage (ST) 8+ to ST34. The intermediate mesoderm, from which the pronephros arises, was first seen as a faint ridge of undifferentiated mesoderm between the segmental plate and lateral plate at ST8+. It formed a cell cord at the level of the 6th to the presumptive 13th somites at ST9 to ST10. This cell cord then separated into dorsal and ventral parts, the former becoming the nephric duct and the latter the tubules by ST14. The primordia of the external glomeruli (PEGs) appeared at ST15 through some epithelial cells protruding in the nephrostome (the opening of the nephric tubule into the body cavity). PEGs formed gradually in the caudal direction until ST18, while the pronephric tubules and PEGs in cranial locations disappeared. At this stage, only a few PEGs remained at the level of the 13th and 14th somites and these developed from ST23 to ST29 to become ultrastructurally similar to the glomeruli of the functional kidney. From these observations in the avian pronephros, we infer that the pronephric duct and tubules both form from a cell cord in the intermediate mesoderm and at the same time, but later develop differently.

Cellular and subcellular localization of the small G protein RhoA in the human and rat embryonic and adult kidney.

Rho proteins, a subgroup of the Ras GTPase superfamily, control many cellular processes and morphogenetic events by acting as signaling molecules in the transduction pathways of various receptors. Among the "Rho-dependent" receptors are the extracellular matrix- and growth factor-binding sites; these are particularly involved in the modulation of renal development since they control the epithelial-mesenchymal interactions that drive kidney organogenesis. The present study has addressed the immunohistochemical localization of RhoA in developing and adult kidneys of rats and humans because: a) Rho proteins are known to have a morphogenetic role, b) data in the literature on expression of Rho GTPases during mammalian histogenesis and organogenesis are scarce, and c) their involvement in the transduction pathways of receptors is implicated in kidney development. In particular, RhoA peptide was found to be localized in the mesonephric duct and vesicles in both rats and humans; metanephric anlagen were mainly stained in ampullar-derived cells. Periglomerular tubules of fetal and adult kidneys as well as collecting ducts of adult kidneys showed intense staining. Therefore, the present study provides new information on the distribution patterns of RhoA during early stages of mammalian kidney development suggesting that this signaling molecule may take part in epithelial-mesenchymal induction processes that control kidney organogenesis. RhoA expression in adult structures may be linked with renewal of renal epithelial cells and the maintenance of their morphology and polarity.

The genus Acipenser as a model system for vertebrate urogenital development: nephrostomial tubules and their significance for the origin of the gonad.

Early gonadal development was studied in the sterlet, Acipenser ruthenus, by means of histological and semithin serial sections and scanning electron microscopy. Special attention was given to the role of opisthonephric nephrostomial tubules and their coelomic funnels (nephrostomes, coelomostomes) in the origin of the gonad. Specimens of about 1 mm in length (about 7 days post hatching) have a continuous kidney complex (holonephros) that extends from the branchial region to the level of the cloaca and may be divided into a cranial pronephros and a caudally following opisthonephros, with no overlapping of either portion. In specimens of 10 to 25 mm in length the regression of pronephros and cranial opisthonephros can already been seen; as a consequence, these parts of the kidney complex are not involved in gonadogenesis. The initial gonadal anlage is seen in specimens of 30-40 mm in length. The somatic cells of the gonadal primordium develop from the medial lips of segmentally arranged opisthonephric nephrostomes situated in a line that extends from the level of the stomach through that of the spiral valve. The nephrostomes involved in this process belong to the first-order set of nephrons, since nephrons of higher order that arise continuously from blastema cells during further growth of the animals never send nephrostomial tubules to the coelomic surface. The cells of the medial nephrostomial lips proliferate by many mitoses. They grow over and surround the large germ cells that have accumulated on the medial side of the nephrostomes. The proliferating nephrostomial cells are elongated in shape, and their long axes are oriented in the cranio-caudal direction. By their size, shape and arrangement they replace not only the flat, polygonal, mesothelial cells of lateral plate origin on the medial side of the nephrostomes, but also those in the interstices in between. The result is the formation of a continuous gonadal crest situated medially from the nephrostomial line. At 100-130 mm in length, the gonadal crest has reached the stage of a gonadal fold that is attached to the dorsal body wall by a thin mesogonadium. Stroma cells and blood vessels start to invade the gonadal fold. At 240-290 mm in length, parts of the gonadal fold are converted into a fat body, a structure that is typically present in adult sturgeons of either sex. Nephrostomial tubules and their nephrostomes are visible only for a short period in Acipenser ruthenus. They are completely developed in specimens of 25-40 mm in length and start continuous retrogression immediately afterwards. Therefore, they cannot play an important role in excretion. Their only purpose apparently is to function as precursor tissue for other organs, such as the gonad, in establishing a route by which cells of the intermediate mesoderm can gain access to and spread out over the coelomic surface. In conclusion, the observations made in Acipenser prove that this species is a most suitable model to explain the origin of the gonad from opistho/mesonephric nephrostomial tubules in vertebrates with a less-developed or early regressing pronephros. Most mammals, including man, belong to this category.

Establishment of the urogenital junction in the male bovine embryo: an ultrastructural study.

The ultrastructure of the developing extratesticular rete testis, the efferent ductules and the establishment of the urogenital junction were studied in bovine embryos and fetuses of 41 through 95 days post conceptionem. The efferent ductules originate as a new set of secondary mesonephric tubules from the dorsal aspect of the nephric giant corpuscle and grow in the direction of the Wolffian duct. Cytological differentiation of the efferent ductules proceeds in a proximo--distal direction. At about 50-60 days, the simple columnar epithelium of the proximal portions of the efferent ductules already consists of the two typical cell types, i.e. reabsorptive principal cells with an endocytotic apparatus and a brush-border and ciliated cells. The lumen of the proximal portion is temporarily filled with intraductular blood vessels and perivascular tissue which may represent vestigial rudiments of glomeruli associated with the efferent ductules. At 50 to 60 days, the extratesticular rete still has a blastema--like appearance and consists of irregular cells with abundant glycogen. Extensions of the extratesticular rete come into contact with the efferent ductules and create the first end-to-side anastomoses with the latter. Somewhat later, the separating basal laminas vanish and invading rete cells intermingle with the epithelium of the efferent ductules, thus establishing the urogenital junction.

The differential fate of mesonephric tubular-derived efferent ductules in estrogen receptor-alpha knockout versus wild-type female mice.

We investigated mesonephric tubular-derived efferent ductules in female wild-type (WT) and estrogen receptor-alpha knockout (ERalphaKO) mice from late fetal to adult life. On gestational day 17, efferent ductules in both fetal WT and ERalphaKO females were well developed and morphologically similar, although one third the size of the male counterpart. Unexpectedly, efferent ductules with a ciliated epithelium were still present on postnatal day 10 in WT and ERalphaKO females. By day 23, however, marked phenotypic differences occurred in efferent ductules of WT and ERbetaKO vs. ERalphaKO female mice. In the latter, efferent ductules became hypertrophied and dilated, whereas only small tubules remained in WT and ERbetaKO adult mice. The serum testosterone concentrations were similar in 21- to 25-day-old ERalphaKO, heterozygous, and WT female mice, suggesting that increased testosterone was not inducing enlargement of efferent ductules in ERalphaKO females. In conclusion, remnants of efferent ductules persisted in normal adult female mice, although these structures were greatly reduced in size compared with efferent ductules in ERalphaKO female mice. The underlying mechanism inducing hypertrophy and dilation of efferent ductules in ERalphaKO females is not clear, but secretory and/or reabsorptive function of female efferent ductules may involve ERalpha.

Peroxisomes in permanent and provisional kidneys. Phylogenic and ontogenic considerations.

Peroxisomes in three forms of vertebrate kidney (pronephros, mesonephros, and metanephros), as permanent or provisional kidney, are summarized concerning their ultrastructure and developmental changes. Because the peroxisome is known to be diverse in mammalian metanephros, and species difference is its distinctive feature among cell organelles, information should be obtained on each kidney of each species. The ultrastructural and biochemical features of peroxisomes have at least been partly delineated in the metanephros and mesonephros, but nothing is known about the pronephros. Ultrastructural studies of the metanephric peroxisomes are present in mammals, birds, and reptiles, but information on their development is restricted to mammals and birds. As for the mesonephric peroxisomes, both ultrastructural and developmental data have been accumulating on mammals and amphibians, and ultrastructural information is present on fishes, but not on birds and reptiles. At present, studies on peroxisomes of provisional kidney have been restricted to mammalian mesonephros. The common features of renal peroxisomes previously examined are that they are spherical cell organelles with a single limiting membrane in ultrastructure, and are positive for catalase. Information on the ultrastructure and enzymes is not sufficient at present for comparing the ontogenesis of renal peroxisomes with their phylogenesis.

Endogenous galectins and effect of galectin hapten inhibitors on the differentiation of the chick mesonephros.

Galectins are galactoside-binding lectins. In the mesonephros of the chick embryo, the 16-kDa galectin is abundant in the glomerular and tubular basement membranes where it colocalizes with fibronectin and laminin. To test whether galectin-glycoprotein interactions could play a role in mesonephric development, the effects of the galectin hapten inhibitors thiodigalactoside (TDG) and lactose on the differentiation of the cultured mesonephros were investigated. When compared to control saccharide-free or maltose-treated cultures, mesonephroi cultured in the presence of TDG and lactose exhibited defects in tissue organization. These included a distorted tubule shape, pseudo-stratification of the tubular epithelium, and detachment of glomerular podocytes from the basement membrane. The presence of molecular differentiation markers in the developing mesonephros was investigated. In vivo, expression of the epithelial-specific cell adhesion molecule E-cadherin is restricted to differentiated tubular epithelial cells, whereas the intermediate filament protein vimentin is present in mesonephrogenic mesenchyme and is undetectable in tubular epithelial cells. In mesonephroi cultured in the absence of sugars or in the presence of maltose, the expression pattern of these two marker molecules resembles that found in the mesonephros in vivo. In contrast, in the mesonephroi cultured in the presence of TDG and lactose, the epithelial tubular cells expressing E-cadherin also express vimentin. Re-expression of vimentin in the tubular epithelial cells could indicate a partial reversal to a mesenchymal phenotype. Results suggest that galectin-glycoprotein interactions in the basement membrane are important in the maintenance of the renal epithelial phenotype. Dev Dyn 1999;215:248-263.

Functional specialization of the epithelium in the mesonephric tubules.

An electron microscopy study was aimed to correlate structural differentiation of the epithelium in mesonephric proximal tubules (PT) with the expression of membrane activities of alkaline phosphatase (AP) and 5'-nucleotidase (AMP). Tissue samples of mesonephros were taken from 5 to 16 days old chick embryos. Both enzymes were detected with cerium technique, Mayahara modification of lead capture method was used also for localization of AP. Control incubation was performed with levamisole. The formation of absorptive apparatus was characterized by the differentiation of PT epithelium. Activities of AP and AMP appeared to increase rapidly with the differentiation of epithelium. Reaction products of AP and AMP were detected on brush border as well as on membranes of tubular invaginations, transport tubules and endocytotic vacuoles. The basolateral cell surfaces of epithelium were projected in short interdigitating microvilli and the expression of AP and AMP activities on their membranes suggested the transport role of this structural specialization.

Male adnexal tumour of probable Wolffian origin occurring in a seminal vesicle.

AIMS: Adnexal tumours of probable Wolffian origin are rare low-grade malignant neoplasms that have been previously described in the broad ligament, ovaries and retroperitoneum of females. All are characterized by small, bland epithelial cells growing in a diffuse, trabecular, or tubular pattern. The majority of the cases reported have pursued a benign clinical course. However, recurrences and distant metastases have been described. We present a case of a male adnexal tumour of probable Wolffian origin occurring in the left seminal vesicle of a 29-year-old man with 23 years of follow-up. RESULTS: The diagnosis is supported by immunohistochemical and electron microscopic findings: The tumour cells were immunoreactive for cytokeratin and vimentin while smooth muscle antigen and S100 protein were uniformly negative. By electron microscopy cells were arranged in an acinar pattern and surrounded flocculent, electron-dense material. Individual cells contained numerous dense bodies and free ribrosomes. The patient had recurrences at 14 and 23 years after initial diagnosis. CONCLUSION: This is the first report of this entity in a male. The literature on this unusual tumour is reviewed and the clinicopathological, immunohistochemical and ultrastructural features are described. The differential diagnosis of this seemingly indolent tumour is discussed with genitourinary tumours having a more aggressive clinical course.

Development of the genital peritoneal folds in sheep.

Development of the genital peritoneum in the sheep was studied in embryos/fetuses that were 27-69 days old. The plica mesonephrica was the first visible fold of the urogenital organs of the embryos and was the origin of the proximal mesorchium or mesovarium. The plica suspensoria gonadalis fixed the gonad to the mesonephros and held the cranial genital ligament and the cranial part of the gubernaculum. The distal and the cranial part of the gernaculum. The distal mesorchium or mesovarium evolved from this fold. The inguinal fold fixed the caudal part of the gubernaculum. The suspensory fold of the gonad and plica inguinalis formed a system of serosal folds called the Plica gonadoinguinalis. The fold of the genital ducts had two parts, divided by the crossing with the gubernaculum. The mesepididymis or mesosalpinx evolved from the cranial part of the fold, while the caudal part was the origin of the mesoductus deferens or mesometrium. The caudal part of the right and left folds contributed to defining another system of peritoneal folds, the Plica genitalis. A rearrangement of some accepted terms in the nomenclature is proposed, as are some new terms. An analysis of the meaning of some terms, especially those applied to systems of peritoneal folds, is given to aid in their interpretation.