Embedding in a collagen gel stabilizes the polarity of epithelial cells in thyroid follicles in suspension culture

Thyroid-on-a-Chip: An Organoid Platform for In Vitro Assessment of Endocrine Disruption

Thyroid is a glandular tissue in the human body in which the function can be severely affected by endocrine disrupting chemicals (EDCs). Current in vitro assays to test endocrine disruption by chemical compounds are largely based on 2D thyroid cell cultures, which often fail to precisely evaluate the safety of these compounds. New and more advanced 3D cell culture systems are urgently needed to better recapitulate the thyroid follicular architecture and functions and help to improve the predictive power of such assays. Herein, the development of a thyroid organoid-on-a-chip (OoC) device using polymeric membranous carriers is described. Mouse embryonic stem cell derived thyroid follicles are incorporated in a microfluidic chip for a 4 day experiment at a flow rate of 12 µL min^-1 . A reversible seal provides a leak-tight sealing while enabling quick and easy loading/unloading of thyroid follicles. The OoC model shows a high degree of functionality, where organoids retain expression of key thyroid genes and a typical follicular structure. Finally, transcriptional changes following benzo[k]fluoranthene exposure in the OoC device demonstrate activation of the xenobiotic aryl hydrocarbon receptor pathway. Altogether, this OoC system is a physiologically relevant thyroid model, which will represent a valuable tool to test potential EDCs.

Thyroid Gland Organoids: Current models and insights for application in tissue engineering

The incidence of treatment of thyroid disease and consequential hypothyroidism has been increasing over the past few years. To maintain adequate thyroid hormone levels, these patients require daily supplementation with levothyroxine (L-T4) for the rest of their lives. However, a large part of these patients experiences difficulties due to the medication, which causes a decrease in their quality of life. Regenerative medicine through tissue engineering could provide a potential therapy by establishing tissue engineering models, such as those employing thyroid-derived organoids. The development of such treatment options may replace the need for additional hormonal replacement therapy. This review aims to highlight the current knowledge on thyroid regenerative medicine using organoids for tissue engineering, and to discuss insights into potential methods to optimize thyroid engineering culture systems. Finally, we will describe several challenges faced when utilising these models.

Functional model of rat thyroid follicles cultured in Matrigel

BACKGROUND

Long-term maintenance of functional activity of thyroid cells is an essential requirement for basic in vitro studies on the physiology and pathology of the thyroid. An important prerequisite of thyrocytes' functional activity in vivo and in vitro is their follicle organization.

AIM

This study aimed at developing a method of cultivation of functionally active rat thyroid follicles in Matrigel under three-dimensional conditions.

METHODS

Undamaged rat thyroid follicles were isolated by enzymatic digestion with collagenase/dispase, then embedded into Matrigel, and cultivated for 2 weeks. Thyroglobulin, thyroxine and zonula occludens-1 (ZO-1) localization were revealed by immunofluorescence analysis. Iodide organification was tested by protein-bound 125I (PBI) measurement.

RESULTS

Integrity of the follicles was preserved during the whole period of cultivation and was confirmed by 3D reconstruction of ZO-1 localization. Thyroglobulin was detected in the thyrocyte cytoplasm, as well as in the intrafollicular lumen. Thyroxine was observed predominantly at the apical side of thyrocytes. Also, generated cultures were characterized by a high level of iodide organification: PB125I represented 39% of the total radioactivity in the Matrigel drop embedding the follicles; at the same time, methimazole almost totally inhibited this process (0.2% of total radioactivity).

CONCLUSION

The method of rat thyrocyte cultivation in Matrigel, as described here allows to maintain the structural integrity and the functional activity of thyroid follicles in vitro and could be used for wide ranges of basic and applied researches in thyroidology.

Derivation of Thyroid Follicular Cells From Pluripotent Stem Cells: Insights From Development and Implications for Regenerative Medicine

Stem cell-based therapies to reconstitute in vivo organ function hold great promise for future clinical applications to a variety of diseases. Hypothyroidism resulting from congenital lack of functional thyrocytes, surgical tissue removal, or gland ablation, represents a particularly attractive endocrine disease target that may be conceivably cured by transplantation of long-lived functional thyroid progenitors or mature follicular epithelial cells, provided a source of autologous cells can be generated and a variety of technical and biological challenges can be surmounted. Here we review the emerging literature indicating that thyroid follicular epithelial cells can now be engineered in vitro from the pluripotent stem cells (PSCs) of mice, normal humans, or patients with congenital hypothyroidism. We review the in vivo embryonic development of the thyroid gland and explain how emerging discoveries in developmental biology have been utilized as a roadmap for driving PSCs, which resemble cells of the early embryo, into mature functional thyroid follicles in vitro. Finally, we discuss the bioengineering, biological, and clinical hurdles that now need to be addressed if the goals of life-long cure of hypothyroidism through cell- and/or gene-based therapies are to be attained.

Development of an In Vitro Human Thyroid Microtissue Model for Chemical Screening

Thyroid hormones (TH) are essential for regulating a number of diverse physiological processes required for normal growth, development, and metabolism. The US EPA Endocrine Disruptor Screening Program (EDSP) has identified several molecular thyroid targets relevant to hormone synthesis dynamics that have been adapted to high-throughput screening (HTS) assays to rapidly evaluate the ToxCast/Tox21 chemical inventories for potential thyroid disrupting chemicals (TDCs). The uncertainty surrounding the specificity of active chemicals identified in these screens and the relevance to phenotypic effects on in vivo human TH synthesis are notable data gaps for hazard identification of TDCs. The objective of this study was to develop a medium-throughput organotypic screening assay comprised of reconstructed human thyroid microtissues to quantitatively evaluate the disruptive effects of chemicals on TH production and secretion. Primary human thyroid cells procured from qualified euthyroid donors were analyzed for retention of NK2 homeobox 1 (NKX2-1), Keratin 7 (KRT7), and Thyroglobulin (TG) protein expression by high-content image analysis to verify enrichment of follicular epithelial cells. A direct comparison of 2-dimensional (2D) and 3-dimensional (3D) 96-well culture formats was employed to characterize the morphology, differential gene expression, TG production, and TH synthesis over the course of 20 days. The results indicate that modeling human thyroid cells in the 3D format was sufficient to restore TH synthesis not observed in the 2D culture format. Inhibition of TH synthesis in an optimized 3D culture format was demonstrated with reference chemicals for key molecular targets within the thyroid gland. Implementation of the assay may prove useful for interpreting phenotypic effects of candidate TDCs identified by HTS efforts currently underway in the EDSP.

Switching from MAPK-dependent to MAPK-independent repression of the sodium-iodide symporter in 2D and 3D cultured normal thyroid cells

Loss of sodium-iodide symporter (NIS) expression in thyroid tumour cells primarily caused by constitutive MAPK pathway activation is often refractory to small molecule MAPK inhibitors. Suggested mechanisms are rebound MAPK signalling and activation of alternative signalling pathways. Here we provide evidence that failure to recover down-regulated NIS by MEK inhibition is not specific to tumour cells. NIS mRNA levels remained repressed in TSH-stimulated primary thyroid cells co-treated with epidermal growth factor (EGF) and pan-MEK inhibitor U0126 in the presence of 5% fetal bovine serum or, independently of serum, in 3D cultured thyroid follicles. This led to inhibited iodide transport and iodination. In contrast, U0126 restituted thyroglobulin synthesis in EGF-treated follicular cells. Serum potentiated TSH-stimulated NIS expression in 2D culture. U0126 blocked down-regulation of NIS only in serum-starved cells with a diminished TSH response. Together, this suggests that morphogenetic signals modify the expression of NIS and recovery response to MEK inhibition.

The follicular thyroid cell line PCCL3 responds differently to laminin and to polylaminin, a polymer of laminin assembled in acidic pH

The extracellular-matrix protein laminin forms polymers both in vivo and in vitro. Acidification of pH leads to the formation of an artificial polymer with biomimetic properties, named polylaminin (polyLM). Follicle cells in the thyroid are in close contact with laminin, but their response to this important extracellular signal is still poorly understood. PCCL3 thyroid follicular cells cultured on glass, on regular laminin (LM) or on laminin previously polymerized in acidic pH (polyLM) showed different cell morphologies and propensities to proliferate, as well as differences in the organization of their actin cytoskeleton. On polyLM, cells displayed a typical epithelial morphology and radially organized actin fibers; whereas on LM, they spread irregularly on the substrate, lost cell contacts, and developed thick actin fibers extending through the entire cytoplasm. Iodide uptake decreased similarly in response to both laminin substrates, in comparison to glass. On both the LM and polyLM substrates, the expression of the sodium iodide symporter (NIS) decreased slightly but not significantly. NIS showed dotted immunostaining at the plasma membrane in the cells cultured on glass; on polyLM, NIS was observed mainly in the perinuclear region, and more diffusely throughout the cytoplasm on the LM substrate. Additionally, polyLM specifically favored the maintenance of cell polarity in culture. These findings indicate that PCCL3 cells can discriminate between LM and polyLM and that they respond to the latter by better preserving the phenotype observed in the thyroid tissue.

EFFECTS OF SUCRALFATE, CIMETIDINE AND RABEPRAZOLE ON MUCOSAL HYDROXYPROLINE CONTENT IN HEALING OF ETHANOL-HCL-INDUCED GASTRIC LESIONS

1. No general consensus has been reached on the treatment of acute gastric lesions. The aims of the present study were to clarify the effects of sucralfate, cimetidine and rabeprazole monotherapies and combination therapies on acute gastric lesions from the viewpoint of connective tissue regeneration. 2. Gastric lesions were experimentally created by the oral administration of 50% ethanol-0.15 mol/L HCl to rats. After 30 min, the anti-ulcer agents sucralfate (100 mg/kg), cimetidine (20 mg/kg) and rabeprazole (2 mg/kg) were administered separately or in combination and the stomach was excised at different times to measure the level of hydroxyproline in the gastric mucosa and determine lesion index. Immunostaining against prolylhydroxylase was performed on some specimens. 3. In the control group, lesion index decreased linearly from 30 min after ethanol-HCl administration and the level of mucosal hydroxyproline peaked between 2 and 4 h later. Although sucralfate significantly promoted lesion healing, it had no effect on mucosal hydroxyproline level. Cimetidine suppressed increases in mucosal hydroxyproline and prolonged lesion healing, but these findings were reversed by combining cimetidine and sucralfate. Rabeprazole had no significant effect on lesion healing, but promoted lesion healing in combination with sucralfate. Immunohistochemical analysis showed that prolylhydroxylase was expressed in spindle cells that lined the glandular cells in a boundary area between normal and injured tissues. 4. Under conditions in which the effects of intragastric pH are minimal, sucralfate is superior to antisecretory agents in promoting the healing of acute gastric lesions.

Thyrocyte integration, and thyroid folliculogenesis and tissue regeneration: perspective for thyroid tissue engineering

The thyroid gland is composed of many ball-like structures called thyroid follicles, which are supported by the interfollicular extracellular matrix (ECM) and a capillary network. The component thyrocytes are highly integrated in their specific structural and functional polarization. In conventional monolayer and floating culture systems, thyrocytes cannot organize themselves into follicles with normal polarity. In contrast, in 3-D collagen gel culture, thyrocytes easily form stable follicles with physiological polarity. Integration of thyrocyte growth and differentiation results ultimately in thyroid folliculogenesis. This culture method and subacute thyroiditis are two promising models for addressing mechanisms of folliculogenesis, because thyroid-follicle formation actively occurs both in the culture system and at the regenerative phase of the disorder. The understanding of the mechanistic basis of folliculogenesis is prerequisite for generation of artificial thyroid tissue, which would enable a more physiological strategy to the treatment of hypothyroidism caused by various diseases and surgical processes than conventional hormone replacement therapy. We review here thyrocyte integration, and thyroid folliculogenesis and tissue regeneration. We also briefly discuss a perspective for thyroid tissue regeneration and engineering.

Formation of three-dimensional thyroid follicle-like structures by polarized FRT cells made communication competent by transfection and stable expression of the connexin-32 gene

Pig thyrocytes, either in the intact gland or cultured under conditions leading to thyroid follicle reconstitution, coexpress two gap junction proteins, connexin-32 (Cx32) and connexin-43 (Cx43). As thyrocytes cultured in the form of a monolayer only express Cx43, we hypothesized that Cx32 could play a role in thyroid folliculogenesis. In the present work, we analyzed the ability of polarized FRT cells (that are gap junction deficient) to form follicle-like structures after stable transfection with either Cx32 or Cx43 genes. Wild-type and transfected FRT cells, while growing, showed the capacity to form three-dimensional structures corresponding to domes that result from the accumulation of fluid underneath limited areas of the cell layer. The number of domes formed by FRT cells expressing Cx32 (FRT-Cx32) was 2- to 3-fold higher than that obtained with either wild-type or Cx43-transfected FRT cells (FRT-Cx43). Domes generated by FRT-Cx32 cells were stable (beyond 3 weeks of culture), whereas those formed from wild-type or FRT-Cx43 cells were transient, disappearing when cells reached confluence. Inspection of the cell organization within domes formed from FRT-Cx32 cells by phase contrast and confocal microscopy revealed a progressive transition from domes toward closed structures with a lumen. The tightness of the lumen was demonstrated by the retention of a fluorescent probe, lucifer yellow, introduced by microinjection. Electron microscope examinations showed that the neoformed follicle-like structures had an inside-out polarity. Analyses of cell motion and division with time, by fluorescence video microscopy, indicated that the transformation of domes into inside-out follicles brings into play the migration of cells and, to a lesser extent, cell multiplication underneath the domes. In conclusion, FRT cells forced to express Cx32 give rise to domes that transform into closed inside-out follicles. This gain of function appears Cx specific, as FRT-Cx43 cells did not form similar structures. Our data suggest that the formation and/or functioning of Cx32 gap junctions might represent a key event in thyroid epithelium morphogenesis, i.e. formation of a lumen from a tight epithelial cell layer.

Beta1B integrin interferes with matrix assembly but not with confluent monolayer polarity, and alters some morphogenetic properties of FRT epithelial cells

Beta1B is a beta1 integrin splice variant that differs from the ubiquitous beta1A in the terminal portion of the cytosolic tail. The expression of this variant in CHO cells results in reduced fibroblast adhesion and motility (Balzac, E et al., J. Cell Biol. 127, 557-565 (1994)). We have evaluated the phenotypic changes induced by the expression of beta1B in the FRT epithelial cell line. Stable transfectants of FRT cells expressing beta1B or beta1A human integrins were obtained. The transfected integrins associated with the endogenous alpha subunits and were delivered to the plasma membrane. Beta1B expressing cells attached less efficiently and spread less on fibronectin, laminin or type IV collagen coated dishes. A great reduction of fibronectin fibrils associated to the basal membrane of non-confluent beta1B transfected cells was observed. This was paralleled by the disappearance of microfilament bundles and loss of basally located focal adhesions. On the contrary, upon beta1A transfection, a higher amount of fibronectin fibrils, together with microfilament bundles and focal adhesions, was observed. Expression of beta1B did not significantly modify the ability to manifest the polarized phenotype when cells were grown to confluence on filters in two-chamber-systems. Beta1B-transfected cells showed reduced motile properties when embedded as aggregates in type I collagen gels. Moreover, formation of polarized cysts in suspension culture was impaired. The results show that beta1B, by interfering with focal adhesion organization, microfilament and fibronectin assembly, cell spreading and migration, affects some morphogenetic properties of FRT epithelial cells.

Transforming growth factor-beta1 induces a mesenchyme-like cell shape without epithelial polarization in thyrocytes and inhibits thyroid folliculogenesis in collagen gel culture

Transforming growth factor-beta1 (TGFbeta1) induces a mesenchyme-like cell shape in some epithelial cell types. To clarify the role of TGFbeta1 in the morphological regulation of thyrocytes, we performed collagen gel culture of porcine thyrocytes with serum-free medium. TGFbeta1-nontreated cells organized follicles. In contrast, the cells treated with 10 ng/ml TGFbeta1 became spindle shaped, i.e. they resembled mesenchymal fibroblasts, and did not form follicles. To characterize the spindle-shaped cells, we examined the fine structures and expression of thyroglobulin (Tg) and cytoskeletal proteins using electron microscopy, immunohistochemistry, and immunoblotting. TGFbeta1-nontreated cells had microvilli at the apical side facing follicle lumen and had basal lamina at the basal side in contact with collagen gel. TGFbeta1-treated cells showed both microvilli and basal lamina at the basal side. TGFbeta1-nontreated cells expressed Tg, whereas TGFbeta1-treated cells showed no expression. TGFbeta1-nontreated cells barely expressed vimentin, but they expressed enough cytokeratin. TGFbeta1-treated cells extensively displayed vimentin along with the change in shape to become spindle-like and retained a decreased expression of cytokeratin. TSH (10 mU/ml) did not essentially influence any TGFbeta1 effects on the cells. These results indicate that TGFbeta1 induces a mesenchyme-like cell shape accompanied by cytoskeletal molecular change and the loss of both epithelial polarization and a function in thyrocytes, and that it results in inhibiting thyroid folliculogenesis with or without TSH.

Preferential differentiation of the bile ducts along the portal vein in the development of mouse liver

The development of bile ducts in the mouse liver was studied histochemically, with special reference to their preferential differentiation around the portal vein. Both portal vein and hepatic vein shared a common origin, the omphalomesenteric vein. In the early development of the liver, haematopoietic cells were predominant around both veins. With the progressive development of intrahepatic bile ducts, the following three steps were observed: cluster formation of type I hepatocytes around the portal vein, formation of primitive bile duct structures and basal lamina, then formation of ducts surrounded by connective tissue structures composed of type I and type III collagens and lectin-binding sites, which were predominant around the portal vein compared to the hepatic vein. These results suggest that the deposition of abundant connective tissue structures around the portal vein is a prerequisite for the cell differentiation and basal lamina formation in the bile duct precursors. A possible mechanism of the aggregation of type I hepatocytes around the portein vein is also discussed.

Autonomous growth and function of cultured thyroid follicles from cats with spontaneous hyperthyroidism

Spontaneous feline hyperthyroidism is a unique experimental model of toxic nodular goiter. To determine whether feline toxic goiter is caused by extrathyroidal stimulating factors or by the intrinsic autonomy of follicular cells, primary cultures of enzymatically dissociated follicles from 15 hyperthyroid cat goiters and from 3 normal cat thyroid glands were embedded in collagen gels. Growth and function in chemically defined media were assessed by autoradiography after double labeling with 3H-thymidine and 131I-Na. Iodine organification in follicles from normal glands was TSH dependent, but intense radioiodine organification occurred in follicles from hyperfunctioning goiters even in the absence of TSH. Similarly, twice as many follicular cells of hyperfunctioning thyroid tissue, maintained without TSH in the medium, were labeled after exposure to 3H-thymidine than in follicles from normal glands. The results strongly suggest that intrinsic alterations of cell function lead to autonomy of follicular growth and function and subsequently to the development of hyperplastic nodules, causing thyrotoxicosis. The reason for the focal nature of the disease remains an unresolved challenge. Further investigation using this model may further understanding of the growth of autonomous endocrine tumors.

Cell sorting out: the self-assembly of tissues in vitro

The question posed by the science of analytical histology is how the properties and interactions of the components of the tissues determine their organization in the organs. The relevant components of the tissues are the cells and the extracellular matrix. The ability of cohering populations of cells to self-assemble structured tissues by cell sorting out offers an important opportunity for the experimental study of the mechanisms by which the cells and extracellular matrix interact to determine structure. The investigator can manipulate the initial organization and the cellular composition of the system and, in favorable situations, the composition of the extracellular matrix and the activities of candidate adhesive molecules. It can reasonably be expected that the recent progress in the characterization of the molecular species involved in cell-cell and cell-extracellular matrix interaction will allow the analysis of the molecular basis of tissue organization, with study of the self-assembly of tissue structure during sorting out playing an important role in this analysis. The importance of the differential adhesion hypothesis is its success in describing the rules by which macroscopic tissue structure is governed by the adhesive interactions of cell with cell and cell with extracellular matrix. The DAH describes how the physical forces of cell-cell and cell-matrix adhesion determine structure. Elucidation of the particular adhesive molecules involved in these interactions (e.g., the CAMs, junctional proteins, and matrix adhesion molecules) will yield an explanation at the biochemical level. A complete understanding of structure requires both levels of explanation.

Functional properties of human thyroid follicles cultured within collagen gel

Cultures of human thyroid follicles embedded in collagen gel were performed to investigate certain functional properties under bovine thyrotropin (TSH) stimulation. Follicles obtained from normal glands responded to increasing concentrations of TSH administered on day 4 in culture and for 3 days by increased amounts of cyclic AMP (cAMP), thyroglobulin (Tg) and triiodothyronine (T3) and by decreased levels of thyroxine (T4). Effect was maximal at 2000 microU/ml TSH (cAMP) or 200 microU/ml (Tg, T3, T4). When methimazole or propylthiouracil (PTU) were added, the T3 levels decreased. Follicle lumens contained a periodic acid-Schiff substance which was identified by immunoreaction as Tg. Thyroid follicles obtained from Graves' disease glands gave modified results with an earlier and intensified T3 response and no increase in Tg. These data show that (1) Tg and T3 are secretory products of functional follicles giving a cAMP-mediated response to TSH. (2) The detected T3 also derives from T4 5'-deiodination inhibited by PTU. (3) Intensified T3 response in Graves' follicles is probably due to enhanced conversion of T4 to T3.

In vitro cytodifferentiation of perinatal rat islet cells within a tridimensional matrix of collagen

Cell suspensions prepared by collagenase digestion of pancreases obtained from rat fetuses (21.5 d old) and newborns (2.5 d old) were mixed with a collagen solution and inoculated on a collagen base layer. At the onset of the culture, most acinar cells became necrotic, whereas other epithelial cells proliferated. Most of the cell clusters arranged themselves into simple polarized structures composed of epithelial cells forming hollow spheres, and from these budded neoformed endocrine islets. Scarce fibroblasts were located close to these structures. Immunocytochemical localization of insulin and glucagon, as well as ultrastructural characteristics of the cell types revealed an intrainsular distribution similar to the in vivo localization. Tridimensional matrix of collagen offers, to perinatal pancreatic cells in culture, an environment close to the in vivo conditions: cells reorganize themselves in tissuelike structures and cell interactions concerned in the cytodifferentiation of pancreatic islets occur. This system allows for the study of undifferentiated epithelial cells--the presumed stem cells--differentiating and differentiated endocrine cells in the same preparation.

Expression of type I and III collagen during morphogenesis of fetal rat testis and ovary

The distribution of interstitial collagen type I and III was studied immunocytochemically and ultrastructurally in the fetal rat testis and ovary from the undifferentiated stage (day 12) until birth. The results suggest that there is a correlation between the differentiation, organization, and abundance of the mesenchyme and the differentiation of the testicular vs. ovarian cords. Type III collagen was already present in the undifferentiated gonadal mesenchyme, and it appeared at an early stage around the organizing gonadal cords. Type I collagen appeared later in a similar mesenchymal distribution as type III collagen. Fragmentation of the subepithelial basement membrane in the gonads starting morphogenesis was considered to indicate that the surface epithelium participates in the gonadal cord formation. The expression of type III collagen at first on the surface of the developing testicular cords and later around the ovarian cords suggests that the mesenchymal premyoid cells are actively involved in the male cord formation. Focal discontinuities were found in the basement membrane of the ovarian cords, which in part were separated from each other by a ramified and relatively sparse mesenchyme. A complex linkage between the cytoskeleton and the extracellular matrix is illustrated both in the cord forming Sertoli and granulosa cells, and in the adjacent mesenchymal cells. Depletion of the mesenchyme and the basement membrane around the germ cell-granulosa cell associations of the wide ovarian medullary cords may be causal for their subsequent degeneration.

Long-term maintenance of monoxygenase activities in cultured fetal rat hepatocytes

Fetal hepatocytes cultured in the presence of dexamethasone even in low concentration were maintained alive for several weeks. The expression of monoxygenase in these cells is switched from fetal to adult type. Their aldrin epoxidase and ethoxycoumarin-o-de-ethylase activities were maintained at a high level. Cytochrome P-450 concentration remains stable in these cells throughout the culture period. Cell-cell and cell-biomatrix interactions seem to play an important role in the control of growth, maturation and enzymatic activity expression of the cells in culture. This model may constitute an interesting approach for the study of drug metabolism and drug toxicity in vitro.

Change of inverted thyroid follicle into a spheroid after embedding in a collagen gel

When inverted thyroid follicles in suspension culture are embedded in a collagen gel, there is extensive reorganization of the follicle. To identify intermediate stages in the reorganization, a suspension of inverted follicles was mixed with a cold solution of collagen (0.1 mg/ml) in culture medium and the resultant was warmed and allowed to gel. Prior to embedding, the epithelial cells bounding the lumens formed a monolayer of attenuated cells with their microvilli-bearing surface in contact with the medium. The first change noted was a shrinkage of the lumen in many follicles by 18 h. The cells became cuboidal to columnar. Some of the cells had long sheet-like processes extending into the lumen in contact with those of other cells. In late stages of the reorganization, 48 h, the cells were arranged in a compact spheroid. The spheroids contained two different kinds of colloid-filled lumens, possibly of different origins, one a spherical microlumen, the other very long and narrow in section. The peripheral cells of the spheroid had a smooth plasma membrane (without microvilli) in contact with collagen. Although most of the cells in a section had a microvilli-bearing surface forming part of the boundary of a lumen, it is not certain that all cells were in contact with a lumen.

Developmental changes in interstitial collagens of fetal rat genital ducts

The distribution of interstitial collagen types I and III was studied by immunocytochemistry in the mesenchyme of progressing and regressing mesonephric and paramesonephric ducts of male and female rat fetuses from the age of 15 days until birth. Immunocytochemistry revealed a collagen-poor mesenchymal area around the genital ducts and in continuation with the coelomic epithelium on the lateral edge of the mesonephric ridge of 15-day-old fetuses. Ultrastructurally, collagen fibrils were accumulated along the continuous lamina densa of the mesonephric ducts, whereas they were absent on the medial side of the male and female paramesonephric ducts. In males, the amount of collagen fibrils increased with the histological maturation of the mesenchyme around the mesonephric duct, whereas around the regressing paramesonephric duct collagens disappeared from the basement membrane region and the surrounding mesenchyme of the 16-day-old male duct. After the completion of the paramesonephric regression, the mesenchyme acquired a uniformly collagen containing interstitial matrix. In females, the collagens increased in the mesenchyme around the progressing paramesonephric duct, and the original site of the regressing mesonephric duct became occupied with a collagen-containing mesenchyme by the age of 19 days. The results suggest a close structural linkage between the mesonephric duct and the established early paramesonephric duct. The differences in the developmental maturation of the periductal mesenchyme and the observed changes in the composition of the interstitial matrix probably reflect the functional differences in the regulatory factors acting on the progression and regression of the male and female genital ducts.

Functional properties of normal and inverted rat thyroid follicles in suspension culture

Rat thyroid follicles in both the normal and the inverted configuration have been cultured in suspension. It has been found that normal follicles (i.e., those having a structural organization similar to the follicles in vivo) express most functional properties of the thyroid gland. They trap iodide, synthesize, secrete, and iodinate thyroglobulin, and secrete thyroid hormone into the culture medium. On the contrary, in inverted follicles (i.e., those having a cell polarity reversed with respect to normal) we were unable to detect expression of some of these functions. We conclude that both the follicular organization and the proper cell polarity are needed for the full expression of thyroid functional properties.

LLC-PK1 cysts: a model for the study of epithelial polarity

In the present work, we have taken advantage of the properties of two recently isolated clonal subpopulations of the pig kidney-derived LLC-PK1 cell line to study aspects of the establishment of epithelial polarity. When grown in suspension, LLC-PK1/D + Sc cells reaggregated within a few hours and, during the following days of culture, formed free-floating, hollow spheres or cysts, lined by a monolayer of polarized cells. In contrast, LLC-PK1/D- cells were unable to develop such polarized structures even upon prolonged culture in suspension. The polarity of the LLC-PK1/D + Sc cells lining the cysts was inverted compared to that in intact renal tubules, the microvilli-rich "apical" pole being oriented toward the external medium. However, upon embedding these preformed cysts in collagen gels, a reversal of polarity was observed within hours, the microvilli-rich pole now facing the cyst cavity. Thus, in the same clonally derived cell population, cell-to-cell contact and interaction with the extracellular matrix differentially affect the orientation of cellular polarity. The LLC-PK1/D + Sc cysts provide a suitable in vitro model system for further study of the sequential events by which extracellular matrix components induce an appropriately oriented polarization. In addition, the comparison between LLC-PK1/D + Sc and D- cells, which differ in their ability to polarize in response to cell-to-cell contact, should help define some of the cellular determinants involved in epithelial organization.