Polarity reversal of inside‐out thyroid follicles cultured within collagen gel: an ultrastructural study

Abstract

Inside-out porcine thyroid follicles in culture undergo polarity reversal after being embedded in collagen gel. The newly-formed follicles reexpress some specific thyroid functions lost in inside-out follicles (Chambard et al., 1984. We present here an ultrastructural study of the inversion of polarity in this model system. This process takes place within 24 to 48 hr, without any opening of the original tight junctions, as shown by fixation in the presence of ruthenium red. A general shrinkage of cellular aggregates was noted soon after embedding. At the apical pole, three different modifications were observed: structural changes appeared in the kinocilium, microvilli and underlying cytoskeleton as early as 10 min after embedding, mainly when the apical pole of the cells was in close contact with the collagen fibers; large cytoplasmic lamellipod- or pseudopod-like extensions, covering the adjacent apical domain, protruded from outer apical regions; some other apical areas invaginated and formed channels inside the aggregates. The last two processes prevented close contact between apical cell surfaces and collagen fibers and allowed a persistence of the initial polarity in some of the cells. Newly-formed lumens were closed 24 hr after embedding in gel and the outer surface of the cellular aggregates in close contact with collagen fibers looked like a basal membrane. These mechanisms proceeded at different rates and involved different numbers of cells, but they all appeared to be related to the transformation of inside-out follicles into follicular structures.

Polarity reversal of inside-out thyroid follicles cultured within collagen gel: structure of the junctions assessed by freeze-fracture and lanthanum permeability

“…2a-e) following the invagination processes. As indicated in our previous work [1], pseudopods appeared early after collagen embedding. They were not invelved in phagocytotic processes and did not internalize collagen.…”

“…As previously shown, early events of the inversion process principally occurred in the first 6 hr following embedding within collagen gel [1]. At this time, TJs seemed to persist, whereas apical poles were still oriented towards the collagen.…”

“…These newly formed follicles reexpress specific thyroid functions, lost in the inside-out structures [5]. In a previous study [1], we described by electron microscopy the cellular organization and the structure of inside-out follicles during their polarity reversal induced by embedding within hydrated collagen gel. Mainly, we showed that two mechanisms seem to be involved in the reversal process: a rapid transformation of the apical pole into a basal " Correspondence and reprints: one, along with a preservation of the initial cell polarity by invagination processes or pseudopod extensions.…”

“…3a-b). Most of them probably derived from invagination processes [1]. Microvilli lining the apical surfaces, in inner cavities, remained numerous.…”

Tight junctions and apical/basolateral polarity

“…4e). However, exposure of this surface to collagen provokes the formation of a lumen at the core of the follicle, reversion of polarity, and migration of the TJ in the plane of the lateral membrane until it occupies the innermost end of the intercellular space [4,5,26,27,104]. HCO3-secreting cells of the cortical collecting duct of the kidney possess CI-/HCO3 exchangers in the apical domain and proton pumps in the basolateral membrane.…”