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

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.

Mechanisms of thyroid development and dysgenesis: an analysis based on developmental stages and concurrent embryonic anatomy

Thyroid dysgenesis is the most common cause of congenital hypothyroidism that affects 1 in 3000 newborns. Although a number of pathogenetic mutations in thyroid developmental genes have been identified, the molecular mechanism of disease is unknown in most cases. This chapter summarizes the current knowledge of normal thyroid development and puts the different developmental stages in perspective, from the time of foregut endoderm patterning to the final shaping of pharyngeal anatomy, for understanding how specific malformations may arise. At the cellular level, we will also discuss fate determination of follicular and C-cell progenitors and their subsequent embryonic growth, migration, and differentiation as the different thyroid primordia evolve and merge to establish the final size and shape of the gland.

Implantation: a cell biological paradox

Implantation initiation in eutherian mammals requires, at least in invasive types of implantation, that 1) the trophoblast or subpopulations of it have reached a state of "invasiveness" and, synchronously, 2) the endometrium has reached a state of "receptivity" ("implantation window"). The cell biological basis of "receptivity" as well as of "invasiveness" has long remained unknown, but recently it appears that the application of modern concepts of cell and developmental biology is opening new views of it, concentrating on cell adhesion and cell polarity phenomena. Implantation initiation requires that the trophoblast attaches via its apical plasma membrane to the apical plasma membrane of the uterine epithelium. Since apical plasma membranes of epithelia are normally non-adhesive, this represents a cell biological paradox. In development, cells can express two major phenotypes and switch between these: 1) the mesenchymal/fibroblastoid phenotype that is compatible with cells moving individually; 2) the epithelioid phenotype which is characterized by cells expressing apico-basal polarity and a strong association with neighbouring cells via various junctions, so that they can migrate as sheets but not as individual cells. Application of this concept to embryo implantation would seem to lead to the hypothesis that the trophoblast of blastocysts has to give up part of its typical epithelial organization when becoming invasive; it must express cell-cell adhesion molecules or matrix receptors non-typically at its apical plasma membrane, change the spectrum of expressed adhesion molecules, and alter its motility apparatus. This applies in a somewhat similar way to the uterine epithelium at "receptivity". Interestingly, recent data show that a substantial number of differentiation parameters of these cells change in addition. It appears that part of the epithelial differentiation program is down-regulated at this phase. This also suggests interesting new aspects of the basis of steroid hormone action at the endometrium, as well as of trophoblast invasiveness, possibly involving switches in the activity of regulatory "master" genes as are also involved in decision making during development.

Terminally differentiating epithelial tissues in primary explant culture: a model of growth and development

Many epithelial tissues are characterized by the presence of basal cells which serve the dual roles of self-renewal and of progression through terminal differentiation, to a functional state. Such tissues, when grown in vitro as primary explants, exhibit a characteristic pattern of outgrowth and development which includes both renewal and efforts toward normal differentiation. The degree of differentiation achieved depends upon conditions of culture and may be modulated in a variety of ways. The human prostate constitutes such a system and offers numerous possibilities for investigating basic control mechanisms in growth and development. Information on a variety of epithelial tissues is reviewed and experimental results using human prostate tissue are presented.

Co-inoculation of tumorigenic rat prostate mesenchymal cells with non-tumorigenic epithelial cells results in the development of carcinosarcoma in syngeneic and athymic animals

Co-inoculation of NbF-I and NbE-I s.c. into either adult male syngeneic rats or athymic nude mice induced the development of tumors that resembled carcinosarcoma on histopathologic evaluation. These tumors were composed of a mixture of adenocarcinoma and fibrosarcoma and were induced by the mixtures of NbF-I and NbE-I cells at a ratio ranging from 0.001 to 3.2; inoculation of NbF-I alone resulted in the development of fibrosarcoma. Flow-cytometric analysis showed that the epithelial cells subcloned from the carcinosarcoma had a DNA profile like that of their parental cell line and remained non-tumorigenic. When co-inoculated with the tumorigenic fibroblasts in syngeneic hosts, however, the subcloned epithelial cells again formed carcinosarcomas. Our results indicate that cell fusion between epithelial cells and fibroblasts is an unlikely explanation for tumorigenicity. We propose that prostatic fibroblasts exert a directive influence on their adjacent epithelial cells through a paracrine mechanism that determines epithelial growth and tumorigenicity in vivo.

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.

The differentiation behaviour of MDCK cells grown on matrix components and in collagen gels

MDCK cells are grown on various substrates (Thermanox pure, extracellular matrix (ECM), dried or wet collagen type I or type III), on floating collagen and enclosed in collagen gels, and their differentiation behaviour is investigated electron microscopically. The cells grown on ECM or dried collagen (type I and type III) do not show any changes as compared with the controls (Thermanox). Differentiation processes can only be observed when the cells are grown on wet collagen (type I and type III), especially on floating collagen and enclosed in collagen gels. These differentiation processes comprise changes in the cell shape, an increase in the number of microvilli, an increase in the length of the lateral contact zone with the formation of gap junctions and desmosomes, and an increase in the number and size of the cell organelles. A basement membrane only develops in the form of short segments. Moreover, on floating collagen and in collagen gels three-dimensional, organoid structures develop: cell aggregates with central lumina and tubuli. They are formed by cuboid cells that also exhibit indications of differentiation. Basement membrane fragments occur more often and are longer. It can be concluded from these findings that the chemical structure of the substrate does not play the primary role in the described process. It is rather the physical properties, probably the plasticity, that are of significance. Due to this property the cells change their shape and the contact areas increase in size. The establishment of contacts might be the triggering factor for differentiation. Organoid structures with lumina develop when the apical surface comes into contact with other cells or collagen gels. The pronounced tendency towards polarization necessitates a re-arrangement of three-dimensionally growing cells to structures with lumina. The formation of the basement membrane is the result and not the cause of differentiation.