An ultrastructural study of the establishement of the testicular cysts during spermatogenesis in the sea anemone Actinia fragacea (Cnidaria: Anthozoa)

Spermatogenesis in the sea anemone Actinia fragacea takes place in numerous testicular cysts located in the mesoglea of the gonads. Prospermatogonia arise among the bases of the gonadal epithelial cells bordering the mesoglea, and later migrate into the mesoglea to establish the cysts. The prospermatogonia arise singly, but soon most are found as small groups within the endoderm. They are small cells, 6–7 μm in diameter, and have relatively large nuclei with a single nucleolus. Their cytoplasm is dense, and contains dense bodies and nuage material as well as Golgi, mitochondria, and individual cisternae of endoplasmic reticulum. Each prospermatogonium bears a flagellum, originating in a groove or channel in the cytoplasm. A small proportion of prospermatogonia enter the mesoglea singly, but most migrate as elongate groups or “slugs” of cells. As they enter, the groups often become constricted into hour-glass shapes, and they become covered by the endodermal basal lamina. During the later stages of entry, the last part of the group to enter retains contact with the bases of the epithelial cells, which are dragged into the mesoglea behind the germ cells. This contact between germ cells and endoderm persists throughout spermatogenesis and prevents closure of the mesoglea behind the group. The endodermal cells involved begin specialization to form the trophonema. Once entry is complete, the groups enlarge rapidly to form the testicular cysts. A small number of germ cells appear to remain behind in the endoderm after most have entered the mesoglea, and the possible significance of these cells is discussed.     

Entry of algal symbionts into oocytes of the coral Litophyton arboreum

On the Red Sea coral reefs Litophyton arboreum is a common octocoral whose endodermal cells are associated with endosymbiotic dinoflagellates (zooxanthellae). Colonies of this species are gonochoric and brood planulae which, upon release, are already associated with the algal symbionts. Algal cells within membrane-bound vacuoles are observed within the gastrovascular cavity of the polyps, adjacent to the oocytes and are gradually phagocytized by the follicular cells which surround the oocytes. During oogenesis, temporary gaps open in the mesoglea underlying the follicular cells. Symbionts within vacuoles, along with cytoplasm and various organelles derived from the follicular cells, are translocated through these gaps. Subsequently, groups of zooxanthellae accumulate at the perioocytic zone, flanked between the mesoglea and oocytic microvilli. At a later stage, prior to the commencement of the breeding season, symbionts pass through the oolemma and rest inside the periphery of the oocytes. It is proposed that early uptake of zooxanthellae by sexual progeny at the oocyte stage, indicates a highly specialized mode of interaction between this algal symbiont and its host.     

An ultrastructural investigation of the early stages of oocyte differentiation in Actinia fragacea (Cnidaria; Anthozoa)

Individuals from a population of the intertidal sea anemone Actinia fragacea (Tugwell) were collected at approximately monthly intervals over an 18 month period. Samples of gonad were removed from each anemone and examined by light and electron microscopy. During late spring and early summer, large numbers of small cells were seen in the endoderm of the female gonads, lying close to the mesoglea. For convenience, these cells were classified into three types. Type I cells are 6–9 μm in diameter, with relatively very large nuclei, which may contain synaptinemal complexes, and scant cytoplasm containing few organelles. Type II cells are larger, reaching 15 μ m in diameter, with more abundant cytoplasm containing more organelles and inclusions. The nucleus is more dense, but may also contain synaptinemal complexes. Type III cells are less common. They are similar in size to Type II cells, but their nuclei contain irregular dense chromatin masses, and the nuclear envelope is incomplete or absent. The possible significance of the various cell types is discussed. It is suggested that Type I cells are oocytes at a very early stage of differentiation and that Type II cells are rather later oocytes. The status of the Type III cells is uncertain.     

Ultrastructural features of the trophonema and oogenesis in the starlet sea anemone, Nematostella vectensis (Edwardsiidae)

Abstract. The starlet sea anemone, Nematostella vectensis Stephenson 1935, is a burrowing, estuarine species that has become a model organism for fundamental studies of cnidarian and metazoan development. During early oogenesis, oocytes appear in the basal region of the gastrodermis in the reproductive mesenteries and gradually bulge into the adjacent connective tissue space (mesoglea) where the majority of oocyte growth and vitellogenesis occurs. However, oocytes do not physically contact the cellular and amorphous matrix of the mesogleal compartment due to a thin, intervening basal lamina. Oocytes retain limited contact with the basal gastrodermal epithelium via groups of ultrastructurally modified gastrodermal cells called trophocytes. Trophocytes are monociliated accessory cells of somatic origin that collectively form a structure called the trophonema, a unique accessory cell/oocyte association not observed outside the Cnidaria. The trophonema consists of 50–60 trophocytes that maintain contact with <1% of the oocyte surface and forms a circular, bowel‐shaped depression on the luminal surface of the gastrodermis as they sink into the mesoglea with the oocyte. The oocyte remains highly polarized throughout oogenesis with the germinal vesicle positioned near the trophonema and presumably representing the future animal pole of the embryo. Contact between the trophonema and the oocyte is restricted to cell junctions connecting peripheral trophocytes and narrow extensions from the oocyte. Previous studies suggest that the trophonema plays a role in transport of extracellular digestive products from the gastrovascular cavity to the oocyte, and the ultrastructural features described in this study are consistent with that view. Vitellogenesis is described for the first time in a sea anemone. Yolk synthesis involves both autosynthetic and heterosynthetic processes including the biosynthetic activity of the Golgi complex and the uptake of extraoocytic yolk precursors via endocytosis, respectively.     

Preliminary ultrastructural and autoradiographic evidence that the trophonema of the sea anemone Actinia fragacea has a nutritive function

The developing oocyte of the sea anemone Actinia fragacea is associated with a distinct group of gonad epithelial cells which constitute the trophonema. Electron microscopy has shown that the cells of the trophonema extend through a pore in the mesoglea which surrounds the oocyte, and make intimate contact with the oocyte surface. The ooplasm beneath this region of contact differs from the rest of the oocyte in containing numerous small vesicles, but few yolk granules or other organelles. Light microscope autoradiography has shown that oocytes within gonads can take up and incorporate tritiated glucose and leucine from solution. The cells of the trophonema appear more active in precursor incorporation than other gonad epithelial cells. The evidence therefore suggests that these cells have a nutritive function during oogenesis.     

From primordial germ cell to primordial follicle: mammalian female germ cell development

In mammals, the final number of oocytes available for reproduction of the next generation is defined at birth. Establishment of this oocyte pool is essential for fertility. Mammalian primordial germ cells form and migrate to the gonad during embryonic development. After arriving at the gonad, the germ cells are called oogonia and develop in clusters of cells called germ line cysts or oocyte nests. Subsequently, the oogonia enter meiosis and become oocytes. The oocyte nests break apart into individual cells and become packaged into primordial follicles. During this time, only a subset of oocytes ultimately survive and the remaining immature eggs die by programmed cell death. This phase of oocyte differentiation is poorly understood but molecules and mechanisms that regulate oocyte development are beginning to be identified. This review focuses on these early stages of female germ cell development.     

The cytoskeleton organizes germ nuclei with divergent fates and asynchronous cycles in a common cytoplasm during oogenesis in the chordate Oikopleura

Germline cysts are conserved structures in which cells initiating meiosis are interconnected by ring canals. In many species, the cyst phase is of limited duration, but the chordate, Oikopleura, maintains it throughout prophase I as a unique cell, the coenocyst. We show that despite sharing one common cytoplasm with meiotic and nurse nuclei evenly distributed in a 1:1 ratio, both entry into meiosis and subsequent endocycles of nurse nuclei were asynchronous. Coenocyst cytoskeletal elements played central roles as oogenesis progressed from a syncytial state of indistinguishable germ nuclei, to a final arrangement where the common cytoplasm had been equally partitioned into resolved, mature oocytes. During chromosomal bouquet formation in zygotene, nuclear pore complexes clustered and anchored meiotic nuclei to the coenocyst F-actin network opposite ring canals, polarizing oocytes early in prophase I. F-actin synthesis was required for oocyte growth but movement of cytoplasmic organelles into oocytes did not require cargo transport along colchicine-sensitive microtubules. Instead, microtubules maintained nurse nuclei on the F-actin scaffold and prevented their entry into growing oocytes. Finally, it was possible to both decouple meiotic progression from cellular mechanisms governing oocyte growth, and to advance the timing of oocyte growth in response to external cues.     

Classification of the follicle population based on oocyte diameter and number of granulosa cells in the ovary of large-eared hedgehog, Hemiechinus auritus gmelin

The present study includes the classification of ovarian follicle population in H. auritus on the basis of oocyte diameter and granulosa cell layers. Our observations revealed that there was direct relationship in the oocyte diameter and the follicle size under a certain limit. Diameter of oocytes increased from 10 microns to 84 microns with consequent increase in the follicle size from 20 microns to 320 microns. However, the diameter of oocyte did not show further increase, but the follicle size was enlarged gradually up to 500 microns. Based on the diameter of follicle and the number of granulosa cell layers, the follicle population was divisible into 5 categories: 1. primordial follicle which ranged 20 to 80 microns in diameter with few cells to one complete layer of granulosa cells in the largest cross section; 2. small follicle which was 81 to 200 microns in diameter having 2 to 3 complete layers of granulosa cells; 3. secondary or medium-sized follicle which ranged 201 to 320 microns in diameter and consisted of 4 to 6 complete layers of granulosa cells; 4. transitory follicle having diameter 321 to 360 microns and 7 complete layers of granulosa cells; 5. large-sized follicle which ranged 361 to 500 microns in diameter and had more than 7 layers of granulosa cells.     

Origin of germ cells and formation of new primary follicles in adult human ovaries

Recent reports indicate that functional mouse oocytes and sperm can be derived in vitro from somatic cell lines. We hypothesize that in adult human ovaries, mesenchymal cells in the tunica albuginea (TA) are bipotent progenitors with a commitment for both primitive granulosa and germ cells. We investigated ovaries of twelve adult women (mean age 32.8 ± 4.1 SD, range 27–38 years) by single, double, and triple color immunohistochemistry. We show that cytokeratin (CK)+ mesenchymal cells in ovarian TA differentiate into surface epithelium (SE) cells by a mesenchymal-epithelial transition. Segments of SE directly associated with ovarian cortex are overgrown by TA, forming solid epithelial cords, which fragment into small (20 micron) epithelial nests descending into the lower ovarian cortex, before assembling with zona pellucida (ZP)+ oocytes. Germ cells can originate from SE cells which cover the TA. Small (10 micron) germ-like cells showing PS1 meiotically expressed oocyte carbohydrate protein are derived from SE cells via asymmetric division. They show nuclear MAPK immunoexpression, subsequently divide symmetrically, and enter adjacent cortical vessels. During vascular transport, the putative germ cells increase to oocyte size, and are picked-up by epithelial nests associated with the vessels. During follicle formation, extensions of granulosa cells enter the oocyte cytoplasm, forming a single paranuclear CK+ Balbiani body supplying all the mitochondria of the oocyte. In the ovarian medulla, occasional vessels show an accumulation of ZP+ oocytes (25–30 microns) or their remnants, suggesting that some oocytes degenerate. In contrast to males, adult human female gonads do not preserve germline type stem cells. This study expands our previous observations on the formation of germ cells in adult human ovaries. Differentiation of primitive granulosa and germ cells from the bipotent mesenchymal cell precursors of TA in adult human ovaries represents a most sophisticated adaptive mechanism created during the evolution of female reproduction. Our data indicate that the pool of primary follicles in adult human ovaries does not represent a static but a dynamic population of differentiating and regressing structures. An essential mission of such follicular turnover might be elimination of spontaneous or environmentally induced genetic alterations of oocytes in resting primary follicles.     

A polypeptide domain that specifies migration of nucleoplasmin into the nucleus

Nucleoplasmin is the most abundant protein of the nucleus of Xenopus laevis oocytes. It rapidly enters the nucleus after being injected into oocyte cytoplasm. Partial proteolysis of the nucleoplasmin pentamer reveals two structural domains within each subunit: a relatively exposed "tail" and a protected "core." When all the "tails" have been removed from the pentamer the residual "core" remains pentameric. This pentameric core fails to enter the nucleus, remaining stably in the cytoplasm. A single tail region attached to the pentamer is sufficient to transport it into the nucleus. The rate of accumulation in the nucleus, but not its final extent, depends on the number of tails per pentamer. The detached (monomeric) tails rapidly accumulate in the oocyte nucleus, indicating that the tail structure is sufficient for selective accumulation. Pentameric cores diffuse throughout the nucleus but are retained when microinjected into the nucleus, indicating that the tail is necessary for entry but not for retention within the nucleus. An improved method for purification of nucleoplasmin is also described.     

Early preantral mouse follicle in vitro maturation: oocyte growth, meiotic maturation and granulosa-cell proliferation

Mechanically isolated early preantral mouse follicles were cultured singly for 16 d and fully grown oocytes were obtained from these follicles. We then compared in vitro and in vivo follicle growth by trypsinising the follicles and counting their cell numbers in a Neubauer-counting chamber and recording the diameter and meiotic status of oocytes under an inverted microscope. As long as the granulosa cells were within the basal membrane, proliferation was slow. From Day 6, when granulosa cells had broken through the basal membrane, the proliferation rate progressed up to Day 10 and decreased thereafter to approximately 12,000 cells per culture droplet. Incorporation of BrdU revealed that proliferating cells were evenly distributed throughout the follicle until antrum formation. As granulosa cell differentiation progressed, proliferation of mural-granulosa cells ceased, while cells around the oocytes continued dividing. Oocyte diameter increased discontinuously in relation to follicle remodelling. During the first growth phase, diameters increased from 56.5 (+/- 4.4 microns) to 67 (+/- 4.1 microns) until the onset of antral-like cavity formation. The last growth phase started after Day 10, and by Day 14 oocyte diameters were not significantly different from those of 26-d-old in vivo control oocytes. The potential to resume meiosis after mechanical removal of granulosa cells was first reached on Day 8; thereafter, removal of the corona showed that all oocytes cultured with FSH remained arrested at the GV stage up to Day 16. After Day 8, approximately 70% of all oocytes underwent GVBD as a result of granulosa-cell removal, but only 23% of these reached MII after 24 h. The in vivo controls reached a comparable GVBD rate (66%) when the granulosa was removed, but most of the oocytes (82%) underwent first polar body extrusion 24 h later. These results suggest that although oocyte diameters after IVM are not different from those of the controls, culture conditions are not yet adequate to support complete meiotic maturation.     

Assembly of ovarian follicles in the caecilians Ichthyophis tricolor and Gegeneophis ramaswamii: light and transmission electron microscopic study

Though much is known about various aspects of reproductive biology of amphibia, there is little information on the cellular and mechanistic basis of assembly of ovarian follicles in this group. This is especially true of the caecilians. Therefore, taking advantage of the abundant distribution of caecilians in the Western Ghats of India, two species of caecilians, Ichthyophis tricolor and Gegeneophis ramaswamii, were subjected to light and transmission electron microscopic analysis to trace the sequential changes during the assembly of ovarian follicles. The paired ovaries of these caecilians are elongated sac-like structures each including numerous vitellogenic follicles. The follicles are connected by a connective tissue stroma. This stroma contains nests of oogonia, primary oocytes and pregranulosa cells as spatially separated nests. During assembly of follicles the oocytes increase in size and enter the meiotic prophase when the number of nucleoli in the nucleus increases. The mitochondrial cloud or Balbiani vitelline body, initially localized at one pole of the nucleus, disperses through out the cytoplasm subsequently. Synaptonemal complexes are prominent in the pachytene stage oocytes. The pregranulosa cells migrate through the connective tissue fibrils of the stroma and arrive at the vicinity of the meiotic prophase oocytes. On contacting the oocyte, the pregranulosa cells become cuboidal in shape, wrap the diplotene stage oocyte as a discontinuous layer and increase the content of cytoplasmic organelles and inclusions. The oocytes increase in size and are arrested in diplotene when the granulosa cells become flat and form a continuous layer. Soon a perivitelline space appears between the oolemma and granulosa cells, completing the process of assembly of follicles. Thus, the events in the establishment of follicles in the caecilian ovary are described.     

Ultrastructure and transovarial transmission of endosymbiotic microorganisms in Palaeococcus fuscipennis (Burmeister) (Insecta, Hemiptera, Coccinea: Monophlebidae)

Ovaries ofPalaeococcus fuscipennis (Burmeister) are accompanied by large organs termed bacteriomes which are composed of large cells termed bacteriocytes. Each bacteriocyte is surrounded with small epithelial cells. The bacteriocyte cytoplasm is tightly packed with pleomorphic bacteria, whereas in epithelial cells small coccoid microorganisms are present. The number of coccoid bacteria is significantly lower than pleomorphic bacteria. The ovarioles containing choriogenic oocytes are invaded both by pleomorphic as well by coccoid bacteria. Microorganisms traverse the follicular epithelium and enter the perivitelline space. During advanced choriogenesis, endosymbionts are accumulated in the deep depression of the oocyte. Bacteria do not enter the ooplasm until the end of oocyte growth.     

The involvement of nurse cells in oogenesis and embryonic development in the marine sponge,Haliclona ecbasis

Oogenesis and embryonic development in the marine sponge, Haliclona ecbasis, were studied using standard histological procedures. When the oocytes reach a diameter of about 30 μ, nurse cells begin to aggregate around them. Then when the oocytes are about 36 μ in diameter, they begin to engulf the associated nurse cells. Whole nurse cells are engulfed; and although the nucleus of the nurse cells disappears either as or soon after the cells are engulfed, the cytoplasm remains essentially unchanged. The accumulation of these cells within the oocytes most of the cytoplasm is nurse cell cytoplasm. During cleavage of the egg, the engulfed nurse cells are gradually fragmented, but otherwise appear unchanged. At the same time the cytoplasm of the nurse cells is progressively incorporated into that of the blastomeres by what appears to be fusion process. When the latter process is complete, the embryo develops into a typical parenchymula larva.     

An unusual lysosome compartment involved in vitellogenin endocytosis by Xenopus oocytes

We have investigated the lysosomal compartment of Xenopus oocytes to determine the possible role of this organelle in the endocytic pathway of the yolk protein precursor, vitellogenin. Oocytes have lysosome-like organelles of unusual enzymatic composition at all stages of their development, and the amount of hydrolase activity increases steadily throughout oogenesis. These unusual lysosomes appear to be located primarily in a peripheral zone of oocyte cytoplasm. At least two distinct populations of lysosomal organelles can be identified after sucrose density gradient fractionation of vitellogenic oocytes. Most enzyme activity resides in a compartment of large size and high density that appears to be a subpopulation of yolk platelets that are less dense than most platelets within the cell. The appearance of this high density peak of lysosomal enzyme activity coincides with the time of onset of vitellogenin endocytosis during oocyte development. The data suggest that endocytic vesicles that contain vitellogenin fuse with modified lysosomes shortly after their internalization by the oocyte. Pulse-chase experiments with radiolabeled vitellogenin suggest that the ligand passes through the low density platelet compartment en route to the heavy platelets. The accumulation of yolk proteins apparently results from a failure of these molecules to undergo complete digestion after their entry into an unusual lysosomal compartment. The yolk platelets that these proteins finally enter for prolonged storage appear to be a postlysosomal organelle.     

The C.elegans MAPK phosphatase LIP-1 is required for the G(2)/M meiotic arrest of developing oocytes

In the Caenorhabditis elegans hermaphrodite germline, spatially restricted mitogen-activated protein kinase (MAPK) signalling controls the meiotic cell cycle. First, the MAPK signal is necessary for the germ cells to progress through pachytene of meiotic prophase I. As the germ cells exit pachytene and enter diplotene/diakinesis, MAPK is inactivated and the developing oocytes arrest in diakinesis (G(2)/M arrest). During oocyte maturation, a signal from the sperm reactivates MAPK to promote M phase entry. Here, we show that the MAPK phosphatase LIP-1 dephosphorylates MAPK as germ cells exit pachytene in order to maintain MAPK in an inactive state during oocyte development. Germ cells lacking LIP-1 fail to arrest the cell cycle at the G(2)/M boundary, and they enter a mitotic cell cycle without fertilization. LIP-1 thus coordinates oocyte cell cycle progression and maturation with ovulation and fertilization.     

The critical role of the MAP kinase pathway in meiosis II in Xenopus oocytes is mediated by p90(Rsk)

BACKGROUND: During oocyte maturation in Xenopus, progesterone induces entry into meiosis I, and the M phases of meiosis I and II occur consecutively without an intervening S phase. The mitogen-activated protein (MAP) kinase is activated during meiotic entry, and it has been suggested that the linkage of M phases reflects activation of the MAP kinase pathway and the failure to fully degrade cyclin B during anaphase I. To analyze the function of the MAP kinase pathway in oocyte maturation, we used U0126, a potent inhibitor of MAP kinase kinase, and a constitutively active mutant of the protein kinase p90(Rsk), a MAP kinase target. RESULTS: Even with complete inhibition of the MAP kinase pathway by U0126, up to 90% of oocytes were able to enter meiosis I after progesterone treatment, most likely through activation of the phosphatase Cdc25C by the polo-like kinase Plx1. Subsequently, however, U0126-treated oocytes failed to form metaphase I spindles, failed to reaccumulate cyclin B to a high level and failed to hyperphosphorylate Cdc27, a component of the anaphase-promoting complex (APC) that controls cyclin B degradation. Such oocytes entered S phase rather than meiosis II. U0126-treated oocytes expressing a constitutively active form of p90(Rsk) were able to reaccumulate cyclin B, hyperphosphorylate Cdc27 and form metaphase spindles in the absence of detectable MAP kinase activity. CONCLUSIONS: The MAP kinase pathway is not essential for entry into meiosis I in Xenopus but is required during the onset of meiosis II to suppress entry into S phase, to regulate the APC so as to support cyclin B accumulation, and to support spindle formation. Moreover, one substrate of MAP kinase, p90(Rsk), is sufficient to mediate these effects during oocyte maturation.     

Nucleocytoplasmic distribution of snRNPs and stockpiled snRNA-binding proteins during oogenesis and early development in Xenopus laevis

The distribution of small nuclear ribonucleoprotein particles containing U snRNAs (U snRNPs) during oogenesis and early development in Xenopus was analyzed with a lupus antibody (anti-Sm) that reacts with snRNA-binding proteins. Fully grown oocytes and embryos prior to gastrulation were found to be relatively depleted of U snRNPs in their nuclei and to contain an excess of snRNA-binding proteins stored in the cytoplasm. During late blastula-early gastrula, or after microinjection of U snRNAs into the cytoplasm of a mature oocyte, the proteins migrate into the nucleus. Dot hybridization analysis showed that small previtellogenic oocytes already contain a maximal amount of U1 (and U2) snRNAs, which then decreases to about 20% of that value in fully mature oocytes, even though the cell's volume has increased enormously. Thus fully grown oocytes and eggs accumulate snRNA-binding proteins for use during early development, but this is not coupled with the accumulation of U snRNA.     

Membrane depolarization facilitates sperm entry, large fertilization cone formation, and prolonged current responses in sea urchin oocytes

Depolarization of the sea urchin egg's membrane is required for two processes during fertilization: the entry of the fertilizing sperm and the block to polyspermy which prevents the entry of supernumerary sperm. In an immature sea urchin oocyte, the depolarization is very small in response to the attachment of a sperm. The purpose of this study was to determine whether the depolarization evoked by sperm attaching to an oocyte can facilitate sperm entry or induce the block to polyspermy. Individual oocytes of the sea urchin with diameters which ranged from 86 to 102% that of the average diameter for mature eggs from the same female were examined. The oocytes have a membrane potential of -73 +/- 6 mV (SD, n = 80) and a very low input resistance compared to that of mature eggs. Single sperm, following attachment to an oocyte, elicit a brief, small depolarization with a maximum amplitude of 8 +/- 1.4 mV (SE, n = 15), frequently followed by the formation of tiny filament-like fertilization cones, but the sperm fail to enter. If oocytes are voltage-clamped at membrane potentials more negative than -20 mV, following attachment of the sperm small transient inward currents occur, similar filament-like cones form, and the sperm do not enter. When many sperm attach to an oocyte which is not voltage clamped, the depolarizations sum to create a large depolarization with an amplitude of 60 to 80 mV, which shifts the oocyte's membrane potential to a value between -10 and +5 mV; more positive values are not attained. At such membrane potentials, whether the potential is maintained by the summed depolarizations of many attached sperm or by voltage clamp, large fertilization cones form, the sperm enter, and the oocytes can become highly polyspermic. In oocytes voltage clamped at +20 mV, however, both sperm entry and fertilization cone formation are suppressed. Therefore, both types of voltage-dependence for sperm entry are present in oocytes, although the depolarization caused by a single sperm is not large enough to permit its entry, nor is the depolarization caused by many sperm sufficient to prevent the entry of supernumerary sperm.     

Scanning electron microscopy of the muscle system of Hydra magnipapillata

The fine structure of the ectodermal and endodermal muscle layers of Hydra magnipapillata has been analyzed by scanning electron microscopy after hydrolytic removal of the mesoglea with NaOH and subsequent exposure of the basal and lateral aspects of the layers by mechanical dissection. The ectodermal muscle layer consists of fibrous processes of epithelial cells extending longitudinally to the body axis, whereas the endodermal muscle layer comprises cells with hexagonal bases and several strands of myonemes oriented circularly. In each layer, the muscular elements tightly interdigitate, extending a continuous muscle sheet along the mesoglea. The ectodermal and endodermal muscle sheets communicate with each other via foliate microprojections penetrating the mesoglea. On the lateral aspect of the ectodermal epithelium, spiny nerve fibers run along the upper surface of the muscle processes. The spines are often attached to muscle processes, suggesting that the former monitor muscle contraction. Nerve fibers occasionally come into contact with the mesoglea through narrow gaps between the muscle processes. In the hypostomal ectoderm, a small spindle-shaped cell, probably sensory in nature, extends an apical cilium and a long basal process.