Differentiation of primordial germ cells during embryogenesis of Allacma fusca (L.) (Collembola: Symphypleona)

The youngest primordial germ cells (PGCs) of Allacma fusca (L.) (Collembola: Sminthuridae) can be identified in embryos at the blastoderm stage as scattered in the yolk mass. They are arranged in pairs connected via intercellular bridges and dispersed among the yolk granules over a relatively small area but they never form multicellular clusters. With progressing development, the mesoderm of the germ band differentiates, the PGCs migrate to the abdominal part of the germ band and enter among mesoderm cells making two clusters of cells in the left and right parts of the abdomen. The mesoderm cells neighbouring the PGC cluster differentiate into a one-layered gonad envelope and produce a thin basal lamina separating the gonad from the rest of the mesoderm. The PGCs are still connected in pairs. At the end of the embryonic development, the gonads have regular spherical shapes and are enclosed within the envelope built up by a layer of flat somatic cells. Now, the PGCs do not occur only in pairs, but chains of cells connected with a sequence of intercellular bridges can also be seen.     

Cytological characterization of the germinal line during testicular differentiation in the lizard Liolaemus gravenhorsti (gray)

The gross morphology, histology, and ultrastructure of Liolaemus gravenhorsti gonads prior to and after differntiation are described. Special emphasis has been given to characterization and changes of the germ cell line throughout intrauterine development and 3 days postpartum. During the pregonadal stage, the primordial germ cell migrates toward gonadal rudiments by way of the mesenchyme. These cells can easily be identified by their great size, voluminous and lobulated nucleus, great quantities of yolk platelets, microtubules, and numerous lipid inclusions. In the undifferentiated gonad, the germ cells (type 1 gonocytes) have an ovoid or spherical shape and autodigestion of yolk platelets, great development of Golgi complex, and mitochondrial aggregation, though fewer liposomes, pseudopodes, and microtubules were noted. Concomitantly with the beginning of mitosis, a third type of germ cell appears, the type 2 gonocytes, which are smaller, with poorly defined membranous systems in various degrees of involution. The seminiferous cords are organized when somatic cells of the medullar portion of the gonad surround type 1 gonocytes. Germinal cells are centrally localized in the cords. Near birth many gonocytes migrate toward the basal lamina of cords and differentiate into spherical prespermatogonia, with few organoids. Sertoli cells eparate them from the basal lamina. In advanced pregnancy, Leyding cells become numerous with morphology typical of androgen-producing cells.     

Development of terminal filaments and ovariole envelopes in Thermobia domestica (Insecta, Zygentoma) larvae

The 3rd instar female larvae of Thermobia domestica have five pairs of gonad primordia, each enclosed within a basal lamina (tunica propria). At the end of the 3rd instar some somatic cells scattered on the outer surface of the lamina are seen. During the 4th larval instar the gonad primordia start to form the ovarioles. Each ovariole is elongated and polarized, having anterior and posterior ends. The anterior group of outer somatic cells proliferate to form the terminal filament. At the 6th larval stage the ovarioles are already formed. The terminal filament is separated from the germarium by a thick basal lamina (transverse septum). There are three types of cell building the terminal filament. 1/Basal cells with numerous fingerlike projections; 2/Cells with electron lucent cytoplasm and large nuclei, and 3/Cells with darker cytoplasm containing bundles of fibers and more compact nuclei. The outer surface of the filament is covered by a thick, fibrous basal lamina. The somatic cells that in the previous stages were scattered on the tunica propria as distinct cells, in the 6th larval stage form a cellular envelope (tunica externa). This envelope is formed by a layer of flat cells, and contains numerous tracheae.     

Development of the gonads and gonaducts in the polypterids (Pisces)]

No study of the development of the gonads and gonoducts in Polypterus has ever been made in a more or less complete way. The present study does not fill up this gap, but repairs several lacks of knowledge, by putting clear a few items that may contribute to a better understanding of this development. In larvae between 5,5 mm and 9,6 mm the primary gonocytes appear under the pronephric ducts in the anterior region of the opisthonephros. A genital crest is foreshadowed there; one of its characteristics is a cubial epithelium. The latter does not have any relation with peritoneal channels and it does not contribute to the formation of gonocytes. In the following stages the gonocytes get very numerous because of cellular multiplication. They cause a local transformation of the genital crest into a genital fold; when the number of gonocytes has increased very much, a part of the fold gets transformated into a gonad. At no stage is there any intrusion of medullary tissue into this gonad anlage.     

Primordial germ cells and early stages of oogenesis in Ophryotrocha puerilis (Polychaeta,Dorvillediae)

Summary Each setigerous segment of the protandric polychaete Ophryotrocha puerilis contains two primordial germ cells. A ventral furrow in the gut wall together with the peritoneal lining of the gut forms a genital blood vessel. The gonocytes are located within the peritoneum of this genital blood vessel. At sexual maturity the gonocytes undergo a proliferation cycle, the first division of which gives rise to a cell which is extruded into a forming outpocketing of the coelomic lining. The stem cell remains within the peritoneum. Inside the forming gonad the detached cell goes through a series of four mitotic divisions. The resulting 16 cells are interconnected by cytoplasmic bridges. These bridges are arranged in a very regular pattern which allows the mitotic cycles to be followed. While remaining still within the gonad the 16 cells begin to synthesize yolk and to take up exogenous yolk precursors. At this stage a differentiation into oocytes and nurse cells becomes visible. The oocytes deposit yolk platelets of the definitive size whereas the polyploid nurse cells produce only small yolk bodies that are passed to the adjacent oocytes. In a later stage the cell bridges between adjacent nurse cells are cut and pairs of one oocyte and one nurse cell are released to the coelomic cavity during breakdown of the gonadal sac. Oocyte-nurse cell-complexes then freely float in the coelomic fluid. The proliferation of gonadal cells is well synchronized within one segment. In anterior segments, however, gonadal proliferation usually begins earlier than in posterior segments but smaller oocytes in posterior segments catch up within a few days. Finally a batch of oocytes is produced in which all the oocytes are of the same size (120 m). The origin of the primordial germ cells remains unknown.     

Postembryonic development of the ovary in the penicillate diplopod,Eudigraphis nigricans (Miyosi) (Diplopoda,Penicillata)

Postembryonic development of the ovary through the larval stages was studied in a penicillate diplopod, Eudigraphis nigricans. In the first instar larva a single young cell cluster, consisting of about 20 spherical gonial cells and some smaller interstitial cells, exists beneath the alimentary canal in the third body segment. The gonadal epithelium encompasses the upper surface of this young cell cluster by the end of the first instar. The epithelium then extends forward and backward to form a single long sac-like gonad, leaving the young cell cluster on the center of the gonadal floor as a mound-shaped germarium. In an early second-instar larva, very early previtellogenic oocytes accompanied by some interstitial cells appear in the front and rear surfaces of the ovarian germarium. During the period from the third through the seventh (the last) larval instar, some cell clusters containing several previtellogenic oocytes and interstitial cells successively separate forward and backward from the germarium to form a series of paired patch-shaped vitellarial areas on the extending ventral ovarian epithelium. In each vitellarial area, some of the interstitial cells surround the oocytes to form the follicles. In the seventh instar, the ovarian lumen is extremely expanded, and the late previtellogenic oocytes in the vitellarial areas encroach upward into the ovarian lumen. These oocytes floating in the ovarian lumen are still connected with their own vitellarial areas by partial extensions of their follicles. Some phylogenetic implications of the basic characteristics in structure and postembryonic development of the ovary are discussed. © 1995 Wiley-Liss, Inc.     

The sys-1 gene and sexual dimorphism during gonadogenesis in Caenorhabditis elegans

In wild-type Caenorhabditis elegans, the hermaphrodite gonad is a symmetrical structure, whereas the male gonad is asymmetric. Two cellular processes are critical for the generation of these sexually dimorphic gonadal shapes during early larval development. First, regulatory "leader" cells that control tube extension and gonadal shape are generated. Second, the somatic gonadal precursor cells migrate and become rearranged to establish the adult pattern. In this paper, we introduce sys-1, a gene required for early organization of the hermaphrodite, but not the male, gonad. The sys-1(q544) allele behaves genetically as a strong loss-of-function mutant and putative null. All hermaphrodites that are homozygous for sys-1(q544) possess a grossly malformed gonad and are sterile; in contrast, sys-1(q544) males exhibit much later and only partially penetrant gonadal defects. The sys-1(q544) hermaphrodites exhibit two striking early gonadal defects. First, the cell lineages of Z1 and Z4, the somatic gonadal progenitor cells, produce extra cells during L2, but the regulatory cells that control gonadal shape are not generated. Second, somatic gonadal precursor cells do not cluster centrally during late L2, and the somatic gonadal primordium typical of hermaphrodites is not established. In contrast, the early male gonadal lineage is asymmetric as normal, the somatic gonadal primordium typical of males is established correctly, and the male adult gonadal structures can be normal. We conclude that the primary role of sys-1 is to establish the shape and polarity of the hermaphrodite gonad.     

Gonads in Histiostoma mites (Acariformes: Astigmata): Structure and development

The development of male and female gonads in arrhenotokous and thelytokous species of Histiostoma was studied using transmission electron microscopy (TEM). All instars were examined: larvae, protonymphs, facultative heteromorphic deutonymphs (=hypopi), tritonymphs, and adults. In testis primordium, spermatogonia surrounding a testicular central cell (TCC) with a gradually enlarging, branched nucleus are present already at the larval stage. Spermatogonia and the TCC are connected via narrow, tubular intercellular bridges revealing that the TCC is a germline cell. Spermatocytes appear at the protonymphal stage. At the heteromorphic deutonymph stage, the testis primordium is similar to that of the protonymph, but in the tritonymph it is much larger and composed as in the adult: spermatids as well as sperm cells are present. The latter are congregated ventrally in the testis at the entrance of the deferent duct.In the larval ovary, an eccentrically located ovarian nutritive cell (ONC) is surrounded by oogonia which are connected with the ONC via tubular intercellular bridges. In later stages, the ovary grows and oocytes appear in the protonymph. Meiotic synaptonemal complexes in oocytes occur from the tritonymph stage. At about the time of the final molting, tubular intercellular bridges transform into peculiar diaphragm-crossed bridges known only in Histiostoma mites. In the adult female, growing oocytes at the end of previtellogenesis lose intercellular bridges and move ventro-laterally to the ovarian periphery towards the oviduct entrance. Vitellogenesis occurs in oviducts.Germinal cells in both the testis and ovary are embedded in a few somatic stroma cells which may be well discernible already in the larval ovary; in the testis, somatic stroma cells are evident not earlier than the end of the tritonymphal stage. The ovary has a thin wall of flat somatic cells, whereas the testis is covered by a basal lamina only.The obtained results suggest that gonads in Histiostoma and other Astigmata originate from two primordial cells only.     

黄胫小车蝗受精囊的亚显微结构

利用组织学方法,观察了黄胫小车蝗Oedaleus infernalis 受精囊的显微与亚显微结构。结果表明,黄胫小车蝗受精囊为单个,由高度卷曲的受精囊管和蚕豆状的端囊构成。受精囊壁主要由表皮层、上皮层、基膜和肌肉层构成;上皮层包含上皮细胞、导管细胞和腺细胞。上皮细胞在靠表皮层的边缘有大量的微绒毛,两相邻上皮细胞的细胞膜相互嵌入,并有细微的突起延伸在导管细胞及腺细胞之间,直到基膜,达基膜处的上皮细胞膜折叠,与腺细胞膜的折叠,一起形成迷宫样的指状突起,附着在基膜上。导管细胞有一个较大的核和分泌导管,连接于腺细胞的细胞腔和受精囊腔,将腺细胞中分泌物运输到受精囊腔中。腺细胞具有典型的分泌细胞特征： 含发达内质网、高尔基复合体及不同大小的囊泡。肌肉层位于受精囊最外层,附在基膜上。在受精囊不同部位的结构有差异。在交配前和交配后,受精囊腺细胞的亚显微结构也有差异。     

Mammary ductal elongation: differentiation of myoepithelium and basal lamina during branching morphogenesis

Elongation of mammary ducts in the immature mouse takes place as a result of rapid growth in end buds. These structures proliferate at the apex of elongating ducts and are responsible for penetration of the surrounding adipose stroma; by turning and branching, end buds give rise to the characteristic open pattern of the mammary ductal tree. We have used a variety of techniques to determine the cellular and structural basis for certain of these end bud activities, and now report the following. (1) The end bud tip is covered with a monolayer of epithelium, the "cap cells," which are characterized by a relative lack of intercellular junctions and other specialized features. (2) The cap cell layer extends along the end bud flank and neck regions where it is continuous with the myoepithelium which surrounds the subtending mature duct. A linear sequence of differentiative changes occur in the cap cells in this region as they progressively alter in shape and accumulate the cytological features of mature myoepithelium. Cap cells may therefore be defined as a stem cell population providing new myoepithelial cells for ductal morphogenesis and elongation. (3) Differentiation of cap cells into myoepithelium is associated with conspicuous changes in the basal lamina. At the tip, cap cells form a 104-nm lamina similar to that described in expanding mammary alveoli and in embryonic tissues. Along the end bud flanks the basal lamina is raised from the cell surface and extensively folded, resulting in a greatly thickened lamina, measuring as much as 1.4 microns. At the surface of the subtending ducts the lamina becomes structurally simplified and resembles that at the tip, but has a significantly greater thickness, averaging 130 nm. (4) The codifferentiation of myoepithelium and its basement membrane is associated with changes in the surrounding stroma. Undifferentiated mesenchymal-like cells attach to the surface of the basal lamina in the midportion of the end buds and become increasingly numerous in the neck region, forming a monolayer over the myoepithelial basal lamina. These stromal cells progressively differentiated into fibrocytes which participate in collagen fibrillogenesis and give rise to the fibrous components of the stroma surrounding the mature duct.     

Inhibition of primordial germ cell proliferation by the medaka male determining gene Dmrt1bY

Background  

Dmrt1 is a highly conserved gene involved in the determination and early differentiation phase of the primordial gonad in vertebrates. In the fish medaka dmrt1bY, a functional duplicate of the autosomal dmrt1a gene on the Y-chromosome, has been shown to be the master regulator of male gonadal development, comparable to Sry in mammals. In males mRNA and protein expression was observed before morphological sex differentiation in the somatic cells surrounding primordial germ cells (PGCs) of the gonadal anlage and later on exclusively in Sertoli cells. This suggested a role for dmrt1bY during male gonad and germ cell development.     

Xenogenesis in teleost fish through generation of germ-line chimeras by single primordial germ cell transplantation

Primordial germ cells (PGCs) are the only cells in developing embryos with the potential to transmit genetic information to the next generation. PGCs therefore have the potential to be of value for gene banking and cryopreservation, particularly via the production of donor gametes with germ-line chimeras. Currently, it is not clear how many PGCs are required for germ-line differentiation and formation of gonadal structures. In the present study, we achieved complete germ-line replacement between two related teleost species, the pearl danio (Danio albolineatus) and the zebrafish (Danio rerio), with transplantation of a single PGC into each host embryo. We isolated and transplanted a single PGC into each blastula-stage, zebrafish embryo. Development of host germ-line cells was prevented by an antisense dead end morpholino oligonucleotide. In many host embryos, the transplanted donor PGC successfully migrated toward the gonadal anlage without undergoing cell division. At the gonadal anlage, the PGC differentiated to form one normally sized gonad rather than the pair of gonads usually present. Offspring were obtained from natural spawning of these chimeras. Analyses of morphology and DNA showed that the offspring were of donor origin. We extended our study to confirm that transplanted single PGCs of goldfish (Carassius auratus) and loach (Misgurnus anguillicaudatus) can similarly differentiate into sperm in zebrafish host embryos. Our results show that xenogenesis is realistic and practical across species, genus, and family barriers and can be achieved by the transplantation of a single PGC from a donor species.     

The gon-1 gene is required for gonadal morphogenesis in Caenorhabditis elegans

In wild-type Caenorhabditis elegans, the gonad is a complex epithelial tube that consists of long arms composed predominantly of germline tissue as well as somatic structures specialized for particular reproductive functions. In gon-1 mutants, the adult gonad is severely disorganized with essentially no arm extension and no recognizable somatic structure. The developmental defects in gon-1 mutants are limited to the gonad; other cells, tissues, and organs appear to develop normally. Previous work defined the regulatory "leader" cells as crucial for extension of the gonadal arms (J. E. Kimble and J. G. White, 1981, Dev. Biol. 81, 208-219). In gon-1 mutants, the leader cells are specified correctly, but they fail to migrate and gonadal arms are not generated. In addition, gon-1 is required for morphogenesis of the gonadal somatic structures. This second role appears to be independent of that required for leader migration. Parallel studies have shown that gon-1 encodes a secreted metalloprotease (R. Blelloch and J. Kimble, 1999, Nature 399, 586-590). We discuss how a metalloprotease may control two aspects of gonadal morphogenesis.     

Germ cells are not the primary factor for sexual fate determination in goldfish

The presence of germ cells in the early gonad is important for sexual fate determination and gonadal development in vertebrates. Recent studies in zebrafish and medaka have shown that a lack of germ cells in the early gonad induces sex reversal in favor of a male phenotype. However, it is uncertain whether the gonadal somatic cells or the germ cells are predominant in determining gonadal fate in other vertebrate. Here, we investigated the role of germ cells in gonadal differentiation in goldfish, a gonochoristic species that possesses an XX-XY genetic sex determination system. The primordial germ cells (PGCs) of the fish were eliminated during embryogenesis by injection of a morpholino oligonucleotide against the dead end gene. Fish without germ cells showed two types of gonadal morphology: one with an ovarian cavity; the other with seminiferous tubules. Next, we tested whether function could be restored to these empty gonads by transplantation of a single PGC into each embryo, and also determined the gonadal sex of the resulting germline chimeras. Transplantation of a single GFP-labeled PGC successfully produced a germline chimera in 42.7% of the embryos. Some of the adult germline chimeras had a developed gonad on one side that contained donor derived germ cells, while the contralateral gonad lacked any early germ cell stages. Female germline chimeras possessed a normal ovary and a germ-cell free ovary-like structure on the contralateral side; this structure was similar to those seen in female morphants. Male germline chimeras possessed a testis and a contralateral empty testis that contained some sperm in the tubular lumens. Analysis of aromatase, foxl2 and amh expression in gonads of morphants and germline chimeras suggested that somatic transdifferentiation did not occur. The offspring of fertile germline chimeras all had the donor-derived phenotype, indicating that germline replacement had occurred and that the transplanted PGC had rescued both female and male gonadal function. These findings suggest that the absence of germ cells did not affect the pathway for ovary or testis development and that phenotypic sex in goldfish is determined by somatic cells under genetic sex control rather than an interaction between the germ cells and somatic cells.     

Ultrastructural events in horse gonadal morphogenesis.

The establishment and sexual differentiation of the gonads of horse embryos were studied using high-resolution techniques. The most dramatic observation is the early cytodifferentiation of the somatic cells into steroidogenic cells which takes place before sexual differentiation of the gonads. A unique morphogenetic pattern is established during this process: the seminiferous cords of the testis are completely segregated from the steroidogenic tissue by a basal lamina, while in the medulla of the ovary, steroidogenic cells differentiate inside the epithelial cords which contain germ cells. This early difference in the topographical distribution of steroidogenic cells favours the hypothesis that the interactions between somatic and germ cells vary with the genetic sex. The possibility of finding qualitative differences in steroidogenesis before and during sexual differentiation of the gonad suggests the horse gonad as a good model for the study of the role of the steroid hormones in the sexual differentiation of the mammalian gonad.     

Structure Organization of Urinary System in the Yellow Spotted Mountain Newts （Salamandridae： Neurergus microspilotus）

This study deals with the histomorphology of the mesonephros in male and female Neurergus microspilotus. The slender and narrow kidneys are positioned in the retro peritoneal position up against the ventral aspect of vertebral column and may extend the length from the esophagus-stomach junction to cloaca. The kidney in both sexes is composed of sexual（anterior） and pelvic（posterior） parts. The duct of sexual kidney is a narrow duct which is lying alongside its lateral edge. In the female, it is connected to the ureters and then the duct of defi nitive kidney. Before entering the cloaca, two ureters are joined together and open to the apex of the cloaca. In the male, after entering the sexual kidney, the sperm leave the testis through efferent ducts, then these ducts join together and eventually form Bidder＇s duct. The Bidder＇s duct joins the Bowman＇s capsule of the nephrons in the sexual kidney and the nephrons make collecting ducts which are fi lled with both sperm and urine. After leaving the kidney, all the collecting ducts are connected to the Wolffi an duct. Wolffi an duct joins the ureters（merge from defi nitive kidney） just before entering the cloaca. Based on serial paraffi n sections, nephrons consist of a fi ltration unit, the Malpighian corpuscle, and a renal tubule, which can be divided into 4 morphologically distinct segments： proximal tubule（first and second segment）, distal tubule, and collecting tubule. Collecting tubules merge and form a branch system that opens into collecting ducts.     

The mammalian gonocytes fate: oogenesis or spermatogenesis

Germ line cells become gonocytes after the completion of migration and colonization of gonadal anlages. After the contact with the somatic cells of genital ridges the reprogramming of gonocytes takes place. Upon entry the embryonic gonads germ cells undergo the most complete demethylation during germ line development, their chromatin tends to have an open conformation for a short period. This event promotes the susceptibility to meiosis-inducing factors signaling. The choice of the further path of gonocytes development just after the gonadal sex differentiation, mitotic arrest and meiotic entry are discussed. Analisis of the action mechanisms of meiosis-inducing and meiosis-preventing factors, especially retinoic acid and enzymes of its degradation and synthesis, was performed.     

Control of lateral migration and germ cell elimination by the Drosophila melanogaster lipid phosphate phosphatases Wunen and Wunen 2

In most organisms, primordial germ cells (PGCs) arise far from the region where somatic gonadal precursors (SGPs) are specified. Although PGCs in general originate as a single cluster of cells, the somatic parts of the gonad form on each site of the embryo. Thus, to reach the gonad, PGCs not only migrate from their site of origin but also split into two groups. Taking advantage of high-resolution real-time imaging, we show that in Drosophila melanogaster PGCs are polarized and migrate directionally toward the SGPs, avoiding the midline. Unexpectedly, neither PGC attractants synthesized in the SGPs nor known midline repellents for axon guidance were required to sort PGCs bilaterally. Repellent activity provided by wunen (wun) and wunen-2 (wun-2) expressed in the central nervous system, however, is essential in this migration process and controls PGC survival. Our results suggest that expression of wun/wun-2 repellents along the migratory paths provides faithful control over the sorting of PGCs into two gonads and eliminates PGCs left in the middle of the embryo.     

Structure of the body cavity of the sea spider <Emphasis Type="Italic">Nymphon brevirostre</Emphasis> Hodge, 1863 (Arthropoda: Pycnogonida)

The microscopic anatomy and ultrastructure of the body cavity and adjacent organs in the sea spider Nymphon brevirostre Hodge, 1863 (Pycnogonida, Nymphonidae) were examined by transmission electron microscopy. The longitudinal septa subdividing the body cavity are described: (1) Dohrn’s horizontal septum, (2) lateral heart walls, and (3) paired ventral septa consisting of separate cellular bands. The body cavity is a hemocoel, it has no epithelial lining and is only bordered by a basal lamina. The epidermis, heart, and Dohrn’s septum are not separated from each other by basal laminae and may have a common origin. The cellular bands forming the longitudinal ventral septa are not covered with the basal lamina and presumably derive from cells belonging to the hemocoel. The roles of the morphological structures studied for the circulation of hemolymph are discussed. The gonad lies inside Dohrn’s septum, it is covered with its own basal lamina and surrounded by numerous lacunae of the hemocoel entering the septum. The gonad wall is formed with a single layer of epithelium. The same epithelial cells form the gonad stroma. The gonad cavity is not lined with the basal lamina; muscle cells are present in the gonad wall epithelium, thus rendering the lumen similar to a coelomic cavity. Freely circulating cells of two types are found in the hemocoel: small amebocytes containing electronic-dense granules that are similar to granulocytes of other arthropods, as well as hemocytes with large vacuoles of varying structure that are comparable with plasmatocytes; however some of these may be activated granulocytes.     

Ultrastructure of the protonephridia of larval Rugiloricus cf. cauliculus, male Armorloricus elegans, and female Nanaloricus mysticus (Loricifera)

The protonephridial system of several Loricifera was studied by transmission electron microscopy. A larval specimen of Rugiloricus cf. cauliculus possesses two protonephridia, which are "capped" frontally by a compact mass of still undifferentiated gonadal cells. Each protonephridium consists of four monociliary terminal cells and four canal cells with a diplosome but no cilia. Because of incomplete series of sections and unsatisfactory fixation, the outleading cell(s) could not be detected. In a male specimen of Armorloricus elegans, each gonad contains two protonephridia that open into the gonadal lumen. Each protonephridium consists of two monociliary terminal cells, each forming a filter, two nonciliated canal cells, and two nephroporus cells. The protonephridial lumina of the latter cells fuse to one common lumen, which unites with the gonadal lumen. Preliminary observations on the protonephridia of a female Nanaloricus mysticus reveal a more complicated arrangement of interdigitating terminal and canal cells. One or two terminal cells form their own individual filter or four cells form a common compound filter. The cilium of the terminal cells of all species investigated are surrounded by a palisade of nine microvilli that support the filter barrier made of an extracellular matrix. An additional filter diaphragm could be traced between the pores in the cell wall of each terminal cell of A. elegans. The urogenital system of the Loricifera differs from that of the Priapulida in that the protonephridia of the former are completely integrated into the gonad, whereas the excretory organs of the latter open into the urogenital duct caudally of the gonads.