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The trophotaenial absorptive cells (TACs) in the goodeid teleosts, Xenotoca eiseni and Xenoophorus captivus facilitate macromolecular transport from mother to foetus. Various endocytic pathways operate in these cells as indicated by a highly compartmentalized vacuolar apparatus. A time-sequence analysis of the endocytotic activity involved in horseradish peroxidase (HRP) ingestion revealed that the solute marker was, for the most part, channelled to lysosome-like vacuoles. Alternatively, the electrostatic ligand cationized ferritin (CF) was also either transcytosed or regurgitated. In the TACs of Xenotoca eiseni, dose-dependent responses to ammonium ion (5 mM, 10mM, 20 mM), chloroquine (50 μM, 150 μM, 300 μM), and monensin (5 μM, 10 μM, 20 μM) were registered after incubation of embryos in HRP-saline supplemented with the respective reagents. Each drug produced qualitatively distinct structural alterations in the cell's vacuolar apparatus. Treatment with raising concentrations of NH4Cl caused progressive vacuolation. In a dose-related way, the chloroquine effect was reflected in the formation of a uniformly labelled labyrinthic membrane system, apparently a consequence of indiscriminate fusion events. Monensin-treated cells always had some densely labelled, lysosome-like vacuoles, but typical endosomes were for the most part missing. At higher concentrations of the ionophore, dense apical tubules progressively disintegrated. In the TACs of Xenoophorus captivus, CF-trafficking was traced in the presence of NH4Cl (10 mM), chloroquine (150 μM), or monensin (10 μM). Ammonia caused endosomal swelling and thus seemingly affected the lysosomal pathway, but recycling and transcytosis were qualitatively unaffected albeit at distinctly lower rates. After treatment with chloroquine, the ligand was uniformly distributed within a tubulo-lamellar membrane complex. Both recycling pathways and ligand processing pathways were probably blocked due to an indiscriminate fusion of vacuolar compartments. Monensin did not apparently inhibit lysosomal sequestration, but transcytotic vesicles were only rarely observed. A perturbation of the recycling mechanisms was indicated by the structural disintegration of many dense apical tubules.

The trophotaenial absorptive cells (TACs) in goodeid embryos facilitate nutrient absorption during prolonged periods of intraovarian gestation. In a study of membrane differentiations associated with solute and ligand transfer in the trophotaeniae of Xenotoca eiseni, embryos were incubated in vivo with cationized ferritin (CF) prior to freeze-cleaving. This exposure to high concentrations of an adsorptive ligand was meant to induce swelling of the endosomal compartment. Macromolecular trafficking in TACs occurs via an apical endocytic complex consisting of plasma membrane invaginations, a large population of small vesicles, uniformly thick apical tubules, and endosomes. Freeze-fracture replicas showed that the microvillar plasma membrane P-face of TACs was studded with intramembrane particles (IMPs) at a fairly high density, whereas that of the cell surface proper contained a distinctly lower density and the tubulovesicular endocytic pits contained almost no IMPs. The majority of small vesicles and apical tubules in a near surface position displayed P-fracture faces with only a few odd IMPs, indicating that membrane, shuttling between the apical plasma membrane and intracellular sorting organelles, obviously does not carry along many large-sized integral membrane proteins. The distended endosomal compartment had many P-face-associated particles primarily clustered into patches. Specializations of the lateral plasma membrane included 4–8 tight junctional strands, relatively large complements of gap junction proteins, and numerous plaques of desmosomal membrane particles. A system of lamellar cisternae underlay the lateral cell surface that was in continuity with the intraepithelial space by numerous tubular canals, giving rise to an intracellular amplification of the basolateral plasma membrane. Their outward openings appeared as tiny pits on the cytoplasmic faces of freeze-cleaved cell membrane. The density of IMPs on the P-faces of the surface plasma membrane was apparently lower than that on its invaginated lamellar complex. Hence, it is concluded that the mobility of integral membrane proteins in the plane of the membrane may be hampered in movement across the surface pores.

Le trophotaenia et l'epiderme abdominal des embryons de Xenoophorus captivus ont ete etudies par microscopies optique et electronique et par des techniques histochimiques. Les specialisations morphologiques associees a la respiration cutanee ont ainsi ete examinees

The absorptive epithelium of the trophotaeniae of goodeid embryos is involved in the micropinocytotic uptake of protein macromolecules from the ovarian embryotrophe. Incubations of viable Xenoophorus captivus embryos in vitro with horseradish peroxidase (HRP) and/or cationized ferritin (CF) allows the tracing of the fluid-phase and receptor-mediated pathways, respectively. Effects of lowered temperature on both these endocytotic mechanisms have been investigated. At 10° C, trophotaenial absorptive cells (TACs) have a strong capacity to ingest marker proteins from double tracer media. Surface-bound ligands (CF) and solutes (HRP), taken up in primary pinocytic vesicles, are rapidly channelled to the endosomal compartment. Part of the ingested CF is segregated into dense apical tubules and small vesicles indicating that membrane recycling and transcytosis continue at 10° C. Adsorptive endocytosis of CF at 5° C proceeds at a decreased rate. After incubation periods of 30 min and 1 h, tracer molecules can be found in vesicular, tubular and vacuolar compartments of the apical endocytic zone. At 0° C, no uptake of ligand worth mentioning could be ascertained. Fluid-phase endocytosis, on the other hand, is observable at this temperature. Enzyme reaction product accumulates in flattened vacuoles rather than typical voluminous endosomes. After prolonged exposure to HRP, the epithelial junctional complex becomes leaky and the marker protein penetrates the intercellular space and the lateral lamellar membrane invaginations of TACs.

Des echanges nutritionnels entre la mere et l'embryon de Xenoophorus captivus et les macromolecules proteiques du liquide nutritionnel ovarien ont ete etudies par focalisation isoelectrique du serum sanguin de l'embryon et de la mere. Le tissu ovarien a ete observe par microscopie electronique

The endodermal trophotaenial epithelium in goodeid embryos acts as a placental exchange site. Fine structural and cytochemical data indicate that the trophotaenial absorptive cells are endocytotically highly active. To test their micropinocytotic capacity and characterize the cellular mechanisms involved in membrane, solute and ligand movements, living embryos of Xenoophorus captivus were incubated in saline media containing horseradish peroxidase (HRP) and/or cationized ferritin (CF) in vitro, and the uptake of these tracer proteins examined by both time sequence analysis and pulse-chase procedures. In some embryos, the effects of prolonged exposure to CF injected into the ovarian cavity, was also investigated. Labelling of the free cell surface was detectable with CF only, but interiorization of both probes was quick from all incubation media. Adsorptive pinocytosis of CF and fluid-phase uptake of HRP sequentially labelled pinocytic vesicles, endosomes, and lysosome-like bodies. In addition, CF-molecules were sequestered within apical tubules and small vesicles. HRP was largely excluded from both organelles and ended up in the lysosomal compartment. For CF, two alternative pathways were indicated by the pulse-chase experiments; transcellular passage and regurgitation of tracer molecules to the apical cell surface. The latter procedure involves membrane and receptor recycling, in which apical tubules are thought to mediate. In double-tracer experiments, using an 8:1 excess of HRP, external labelling with CF was light or lacking after 1-3 min, and the initial uptake-phase produced pinocytic vesicles and endosomes that mainly contained HRP-reaction product. Prolonged incubation, however, resulted in densely CF-labelled plasmalemmal invaginations and pinocytic vesicles that predominantly carried ferritin granules. After 60 min, the vacuoles of the endosomal compartment contained either high concentrations of HRP-reaction product, both tracers side by side, or virtually exclusively CF.