The present study deals with the morphology and ultrastruclure of the nephron in the mesonephros of the toad, Bufo bufo (Linnaeus, 1758). Based on serial sections in paraffin, Araldite and Epon, the ...position of the different segments of the nephron within the kidney tissue was determined, and a nephron subsequently reconstructed. The nephron consists of the following parts: Malpighian corpuscle, neck segment, proximal tubule, intermediate segment, early distal tubule, late distal tubule and collecting tubule. The late distal tubule was subdivided into three morphologically different sections. The total number of nephrons in the toad mesonephros was estimated at 6000 units. The length of the segments in the reconstructed nephron was calculated. The cytology of the epithelial cells constituting the segments was described using transmission and scanning electron microscopy. Heterocellularity was found in the late distal tubule section I and III and in the collecting tubule. The proportional distribution and number of intercalated (mitochondria‐rich) cells in the late distal tubule and collecting tubule was calculated. Only one morphological type of intercalated cell could be distinguished. Late distal tubules were removed from fresh Bufo kidneys for preliminary studies of the intercalated cells with Nomarski optics.
Reproduction and/or sperm structure was studied in 18 species belonging to five genera of Galeommatidae from Phuket Island, Andaman Sea, Thailand, and Hong Kong. Incubation of the ova occurs in the ...inner and outer demibranchs of both gills, and sperm is most probably transferred to the ctenidial brood chamber as masses of agglutinated and non-encapsulated spermatozoa. The smallest specimens are males, but change into the female sex and there is a strong indication that some of the species are alternate hermaphrodites. Dwarf males occur in one species (Galeomma layardi). The sperm of all 18 species studied are of the ent-aquasperm type with a more or less hemispherical acrosome that is tilted with respect to the long axis of the moderately elongated nucleus. It is suggested that this type of sperm affords the best autapomorphy for the family Galeommatidae.
Mysella gregaria new species (Bivalvia: Galeommatoidea: Montacutidae) is described from Wrightsville Beach, North Carolina, USA. Several individuals were collected from the body column of an ...intertidal burrowing unidentified actinian. The morphology of the shell and soft parts is described and compared with other species of the genus from the W. Atlantic Ocean and with other bivalves associated with solitary antho-zoans. Mysella gregaria is a ctenidial brooder and specimens are either males or females; no true hermaphrodites were found. Contrary to many less social commensal bivalves, reproduction in M. gregaria does not involve sperm storage. We suggest that this may be correlated with the species' gregari-ousness.
The mitochondria-rich (mr) cell of amphibian skin epithelium is differentiated as a highly specialised pathway for passive transepithelial transport of chloride. The apical membrane of mr cells ...expresses several types of Cl
− channels, of which the function of only two types has been studied in detail. (i) One type of channel is gated by voltage and external chloride concentration. This intriguing type of regulation leads to opening of channels only if Cl
−
o is in the millimolar range and if the electrical potential is of a polarity that secures an inwardly directed net flux of this ion. Reversible voltage activations of the conductance proceed with long time constants, which depend on
V in such a way that the rate of conductance activation increases when
V is clamped at more negative values (serosal bath grounded). The gating seems to involve processes that are dependent on F-actin localised in the submembrane domain in the neck region of the flask-shaped mr cell. (ii) The other identified Cl
− pathway of mr cells is mediated by small-conductance apical CFTR chloride channels as concluded from its activation via β-adrenergic receptors, ion selectivity, genistein stimulation and inhibition by glibenclamide.
bbCFTR has been cloned, and immunostaining has shown that the gene product is selectively expressed in mr cells. There is cross-talk between the two pathways in the sense that activation of the conductance of the mr cell by voltage clamping excludes activation via receptor occupation, and vice versa. The mechanism of this cross-talk is unknown.
Interna development within the intertidal tropical crab, Leptodius exaratus, requires up to 4.5 months; the roots become distributed from the CNS along the major nerves and, when the crab moults, ...results in emergence of numerous externae located symmetrically on the abdomen and bases of the thoracic appendages. As many externae drop off, their number is continuously reduced; the survivors reach sexual maturity and ovulate after 2.5 months and complete embryonic development after 2 more months. At the next moult a new and more numerous generation of externae appears, and this is repeated until the crab dies. Presence of externae significantly prolongs the host's normal intermoult period and minimizes its growth. The parasite has little effect on the host's sexual characters although females are normally sterilized. The cuticle of the externa is moulted once, leaving a distinct annulus around the stalk. Spermatogonia are presumably injected by male cyprids into the female externa and develop into sperm within one or more vesicular spermatogenic bodies in its mantle. The eggs are laid through a single oviduct into the mantle cavity where they become fertilized and develop into cyprids that are liberated at the disintegration of the externa.
Three species of galeommatoidean bivalves live commensally with
Brissus latecarinatus (Echinoidea, Spatangoidea) near Phuket Marine Biological Center, Thailand.
Scintillona brissae Morton and Scott ...(family Galeommatidae) and
Montacutella echinophila gen. et sp. n. (family Montacutidae), live on the periproct, have very similar shells, but differ with regard to the number of demibranchs, occurrence of seminal receptacles, and sperm structure.
Brachiomya stigmatica (Pilsbry) gen. n. (family Montacutidae), occupies the host's ambulacra. The external morphology and internal anatomy are described. Reproduction involves formation of dimorphic sperm enclosed in syncytial sperm packages produced in the testis and transferred to the gills of a sexual partner. On account of the structure of the animal, the shell hinge, and the sperm,
B. stigmatica is transferred from the Galeommatidae to the Montacutidae.
Jespersen, Å. and Lützen, J. 2001. Ultrastructure of the seminal receptacle and the dimorphic sperm in the commensal bivalve Mysella bidentata (Veneroida: Galeommatoidea: Montacutidae). —Acta ...Zoologica (Stockholm) 82: 107–115
The seminal receptacle and the euspermatozoa and paraspermatozoa of Mysella bidentata were examined at an ultrastructural level and the results were compared with earlier findings of the same and other species of the Montacutidae. The euspermatozoon has a slender 13 µm long nucleus and a 1.1 µm long bullet‐shaped acrosome. The acrosome of the paraspermatozoon is almost identical in ultrastructure to that of the euspermatozoa but is longer (1.9 µm) and more slender and is bent at an angle to the diminutive nucleus (1.1 µm long). The unpaired seminal receptacle is lined by a heavily ciliated epithelium and a non‐ciliated epithelium with short and broad microvilli. Euspermatozoa only are stored in the receptacle. They are densely packed and orientated with their heads towards the non‐ciliated epithelium. In this position they develop numerous extremely fine microvilli from the acrosome which apparently serve to attach them to the epithelial microvillar surface. Stored sperm may presumably remain functional for at least six months. A possible function of paraspermatozoa could be to clump sperm into sperm bags to keep them in suspension.
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