[Relations between the contractile system and the phenomenon of endocytosis. Bibliographic review and data on the thyroid cell]

Evidence for a role of the cytoskeleton in the in vitro folliculogenesis of the thyroid gland of the fetal rat

Thyrotropic hormone (TSH) or cAMP accelerate the formation of follicular cavities in the explanted thyroid gland of the 15-day-old rat fetus. Cytochalasin B or vinblastine and nocodazole or colchicine, which disorganize microfilamental and microtubular structures respectively, inhibit or completely block in vitro-induced folliculogenesis. Exposure of the thyroid tissue to lumicolchicine, a structural isomer of colchicine deprived of antimicrotubular activity, does not inhibit the activation of folliculogenesis induced by TSH. These results are strong evidence for the supposition that microfilaments and microtubules are involved in the TSH-stimulated mechanisms resulting in thyroid folliculogenesis. Folliculogenesis requires the integrity of both microfilaments and microtubules.

Polarization of thyroid cells in culture: evidence for the basolateral localization of the iodide "pump" and of the thyroid-stimulating hormone receptor-adenyl cyclase complex

When cultured in collagen gel-coated dishes, thyroid cells organized into polarized monolayers. The basal poles of the cells were in contact with the collagen gel, whereas the apical surfaces were facing the culture medium. Under these culture conditions, thyroid cells do not concentrate iodide nor respond to acute stimulation by thyroid-stimulating hormone (TSH). To allow the free access of medium components to the basal poles, the gel was detached from the plastic dish and allowed to float in the culture medium. After release of the gel, the iodide concentration and acute response to TSH stimulation were restored. Increased cAMP levels, iodide efflux, and formation of apical pseudopods were observed. When the thyroid cells are cultured on collagen-coated Millipore filters glued to glass rings, the cell layer separates the medium in contact with the apical domain of the plasma membrane (inside the ring) from that bathing the basolateral domain (outside the ring). Iodide present in the basal medium was concentrated in the cells, whereas no transport was observed when iodide was added to the luminal side. Similarly, an acute effect of TSH was observed only when the hormone was added to the basal medium. These results show that the iodide concentration mechanism and the TSH receptor-adenylate cyclase complex are present only on the basolateral domain of thyroid cell plasma membranes.

A new experimental method for the dynamic study of the antibody transfer mechanism from mother to fetus in the rat

The yolk-sac is known to be a route for the transport of passive immunity from mother to fetus in the rat. The main purpose of the present paper is to describe an experimental system for ultrastructurally studying the kinetics of the uptake and transport of immunoglobulin by rat yolk-sac. This system has the advantage of enabling the membrane to be externalized and then exposed to protein under controlled environmental conditions whilst at the same time maintaining the conceptus in connection with the in situ placenta. Preliminary investigations have utilized homologous anti-horseradish peroxidase (HRP) IgG (detected as antibody by application of HRP) or HRP alone. Comparison has been made with the localization of endogenous IgG transmitted in vivo after immunization of the female rat with HRP. The results show the rapid binding of IgG to membrane since only 30 sec were sufficient for this attachment to occur. Moreover, the endocytic process also appears to be very fast as localization of IgG in clusters or patches, caveolae or pits and even rare microvesicles is observed within 8 min. On the other hand, no binding of HRP to microvilli was observed and, unlike IgG, HRP became located in the apico-tubulocanalicular system.