The expression of differentiation by chick embryo thyroid in cell culture. II. Modification of phenotype in monolayer culture by different media.

Previous results with thyroid secretory cells in monolayer culture seem contradictory with respect to phenotypic stability of this cell type. On the one hand, in "minimal" medium the cells lose structural and functional specializations which can be returned only by three-dimensional growth in organ culture upon addition of fibroblasts derived from the thyroid capsule. On the other hand, in "rich" medium used for cloning, cytoarchitecture and function remain unaltered in either mass or clonal cultures. The apparent discrepancy has been resolved by plating cell suspensions in both media and changing to the alternate medium once the cells have become established. It has been shown that a number of characteristics, including hormone levels, are reversed each time such a change in medium is made. These modulations are discussed in terms of the normal variations in structure and function of the gland in vivo.

Interference with thyroid histogenesis by inhibitors of collagen synthesis.

Histogenesis of thyroid follicles in the chick embryo begins with a penetration by cells of the mesenchymal capsule into a solid epithelial primordium. Before penetration occurs, slits containing fibrillar material form between the epithelial cells. The fibrillar material is an epithelial cell product as shown by its formation within channels that form in cultures of isolated epithelial primordia. The drugs L-azetidine-2-carboxylic acid (LACA) and alpha, alpha'-dipyridyl, which interfere with collagen synthesis, prevent the formation of fibrils in cultured epithelial primordia and in cultures of whole thyroids. Furthermore, mesenchymal cells do not invade when whole thyroid primordia are cultured in the presence of either drug. The effects of alpha, alpha'-dipyridyl are reversed by washing out the drug; the effects of LACA are reversed by incubation with equimolar or greater amounts of L-proline added to the medium along with the drug. The results are interpreted to mean that the fibrillar material is collagen of epithelial origin, that the collagen in some way plays a role in mesenchymal penetration of the epithelial primordium, and that the epithelium is responsible for the pattern of lobulation within the developing gland.

Use of a touch sensitive screen and computer assisted image analysis for quantitation of developmental changes in pulmonary structure.

The extensive changes in pulmonary function occurring during early development may reflect variations in the anatomic structure of the respiratory apparatus during this period. Accurate definition of these alterations could yield important information concerning the structure-function correlations of the respiratory system. To facilitate the acquisition of morphometric data from histologic sections of pulmonary tissues, we propose the use of a computer assisted image analysis system with a touch sensitive screen as an interactive peripheral. This allows planimetric measurements and computation of the dimensions of areas of selected light intensities within an image. We present the description, design, and applications of such an image analysis system and report representative results regarding developmental changes in pulmonary structure. In addition, we correlate these results with previously published information regarding pulmonary mechanics during early development to help clarify the maturational changes in pulmonary structure-function relationships.

Follicle formation in the embryonic chick thyroid. III. Initiation of follicle formation.

It has been proposed that the follicular spaces in the thyroid form by either the coalescence of intracellular droplets or by separation of cell apices by secretion into the extracellular space. On the basis of examination of thyroid primordia in early chick embryos this study provides evidence that in the chick, at least, follicle formation conforms to the second model. The first indications of change in the chick thyroid is the appearance of interdigitations of the cell apices. These interdigitations form microvilli as the two surfaces become separated and the follicular space is established. Vesicles with two types of contents can be identified in proximity with the cell surface during follicle formation, but it is not clear if either the dense particulate or the more electron-lucid materials that they contain actually enter the follicular space. Neither removal of the pituitary gland by decapitation nor inhibition of collagen synthesis and a concomitant failure of the invasion of capsular mesenchyme prevents the initiation of normal follicle formation.

Calcium dependence and contraction in somite formation.

The existence of a calcium-dependent contractile process in the formation of somites from segmental plate mesoderm was investigated using a Ca2+ agonist and Ca2+ and calmodulin antagonists. The contribution of cell movement and apical constriction in the segmentation process were assessed using SEM of normal and drug-treated somite and segmental plate tissue. Explants that contained segmental plates of stage 14-15 chick embryos were cultured on vitelline membranes in calcium- and magnesium-free (CMF) Hands' solution, liquid culture medium, and medium containing drugs. Ca2+ ionophore A23187 promoted the rapid completion of one new somite pair. CMF halted segmentation. The Ca2+ antagonists verapamil and papaverine reversibly inhibited segmentation. Theophylline did not inhibit segmentation, suggesting that the effects of the Ca2+ antagonists are not due to inhibition of phosphodiesterase activity. These results suggest that somitogenesis is Ca2+-dependent. Two drugs that inhibit the binding of calmodulin, chlorpromazine and trifluoperazine (TFP), halted segmentation. The inhibitory effect of TFP was reversible. The effects of TFP on somites were compared with those of cytochalasin D. The contribution of microtubules to cell shape and movement in somitogenesis was examined by incubation with nocodazole, a reversible inhibitor of tubulin polymerization. Cell elongation and somitogenesis were inhibited.

Mesenchymal control of branching pattern in the fetal mouse lung.

The effect of mesenchyme on specialization of respiratory epithelium in the fetal mouse was tested in organ cultures. Heterologous combinations were made between respiratory and non-respiratory lung epithelia and the corresponding mesenchymes. Isolated terminal respiratory buds of fetal mouse lungs were recombined with mesenchyme from chick lung parabronchi, mouse trachea or from the avascular, non-respiratory air sacs of chick lungs. Isolated non-branching chick air sacs were combined with mouse terminal bud mesenchyme or mesenchyme from the respiratory branches of chick lungs. Air sac epithelia branched in a pattern characteristic of the chick lung when combined with chick respiratory mesenchyme and in a pattern characteristic of mouse lung when combined with mouse terminal bud mesenchyme. Mouse terminal bud epithelia did not branch with either mouse tracheal mesenchyme or chick air sac mesenchyme but branched in a chick pattern with chick parabronchial mesenchyme. Electron microscopic examination of the cultures showed that all chick air sac epithelial cultures failed to produce surfactant (lamellar bodies) even when they branched. Control cultures of mouse terminal buds contained large numbers of lamellar bodies; mesenchyme which suppressed branching reduced the number of lamellar bodies to only a few in a small proportion of the cells. Culture medium supplemented with growth factors and hormones increased the number of lamellar bodies in heterologous mouse combinations but did not bring the number to control levels. Supplemented medium had no effect on lamellar body production by chick air sac epithelium. The results indicate that branching pattern is determined by the mesenchyme surrounding the epithelial primordium. However, the capacity to synthesize surfactant is determined by the source of the epithelium; mesenchyme may control the degree of expression but not the absolute presence or absence of the differentiated condition.

Morphogenesis of the lung: control of embryonic and fetal branching.

Lung development differs in the embryo and fetus with regard to branching pattern and organization of the epithelial cells. The surrounding mesodermal component, the capsule, has long been known to play a role in branching. As a result of recent analyses of distribution of components of the extracellular matrix coupled with interference with their expression, we are beginning to understand how branching is controlled. Insoluble macromolecules of the basal lamina and deeper extracellular matrix may act as physical barriers or traps to sequester soluble components. The soluble growth factors activate genes regulating cell proliferation.

Development of the eye of the chick embryo.

Vertebrate eye development begins with the formation of the optic vesicles as outgrowths of the forebrain. These initial pouches grow laterally and can be subdivided into optic stalk and optic vesicle. The axis of growth then shifts to produce optic vesicles that enlarge dorsally to lie alongside the expanding diencephalon. Concomitant invagination of the optic vesicles and the overlying ectoderm produces the optic cup and lens. During later stages, the lens detaches from the surfaces ectoderm and the optic cup forms the neural retina and the pigmented epithelium. Experimental analysis of eye development has revealed an intimate relationship between invagination of the lens and optic cup. The primordia of the lens and neural retina become adherent, as a result of changes in the extracellular matrix, before invagination commences. Interference with matrix synthesis causes abnormal development of the optic cup, and subsequent abnormalities of the lens. The forces that control invagination are under investigation. Lens formation may result from internal contractile forces as well as from forces exerted by surrounding cells. Characteristic changes in cell shape and cytoplasmic organization occur during invagination of the neural retinal primordium. These and the effects of inhibitory drugs suggest the involvement at least in part of a contractile mechanism during optic cup formation.

Development of terminal buds in the fetal mouse lung.

Development of the lungs in mammals begins with the formation of pharyngeal buds that undergo repeated branching to establish the bronchial tree. Late in fetal life the most superficial buds begin to form side branches that will develop into the respiratory surfaces of the lung; the alveolar ducts, alveolar sacs, and alveoli. The bronchial ciliated cells and Clara cells first appear on the seventeenth day of gestation in the mouse. In the respiratory region, a few lamellar bodies are found by 16 days but identifiable type II pneumocytes and extracellular surfactant are not found until 18 days. Flattened type I pneumocytes do not form until after birth, on day 19 or 20. The epithelial branches of the lung are surrounded by mesenchyme and covered by a capsule. At 16 days, the branches are separated by a compact mesenchyme that is highly cellular. By 18 days, the mesenchymal space begins to be reduced. By 19 days, the air passages expand and the mesenchyme forms a thin layer between branches. Little experimental work has been done on the role of the mesenchyme in differentiation of the respiratory surfaces. Since branching of the embryonic lung is influenced by the kind of mesenchyme surrounding the epithelium at early stages, it is likely that mesenchymal control is exerted at later stages in the fetus.

Calcium regulation of neural fold formation: visualization of the actin cytoskeleton in living chick embryos.

The involvement of calcium ions during chick neurulation was studied by treating neural plate stage embryos with agonists and antagonists of calcium transport and with inhibitors of calmodulin activity. Organotypic shape changes were examined by light and scanning electron microscopy. Changes in size of cell apices were quantitated by computer image analysis of actin filaments labeled with fluorescent phallicidin in time lapse recordings of living embryos. Both ionomycin and A23187 caused precocious fold elevation around the median hinge point and convergence by bending at the lateral furrows only when Ca2+ was in the external medium. As judged by decreased perimeters of 100 fluorescent apical polygons, cell apices constricted medial to the lateral furrows but did not change significantly within the median hinge point. Pretreatment with dihydrocytochalasin B or cytochalasin D prevented precocious folding and apical constriction. Papaverine and verapamil prevented folding but could be reversed by subsequent ionophore treatment. Calmidazolium and trifluoperazine irreversibly blocked folding. The demonstration in living embryos of constriction of lateral cell apices in a calcium-dependent manner is consistent with a contractile process operating during the formation of neural folds.

Immunolocalization of basal lamina components during development of chick otic and optic primordia.

Immunolocalization of laminin, fibronectin, and type IV collagen was examined during early morphogenetic shape changes of the avian inner ear and eye. The ear was studied from formation of the otic placode to invagination of the otic pit and the eye from the optic vesicle stage to formation of an optic cup. Distribution and intensity of immunoreactivity were compared in the two organ primordia and in adjacent epithelial layers. Laminin formed a continuous layer at the basal surface of the otic ectoderm and adjacent neural tube at all stages. The basal surfaces of the optic and lens epithelia also were continuously covered with laminin throughout development. The otic placode became attached to the neural ectoderm through a single layer of fibronectin and collagen IV between the layers of laminin. The ring-like attachment between the edges of the optic cup and lens primordium had the same structure. In addition, the central regions of the optic and lens primordia were attached by fibrils containing type IV collagen, whereas finer strands containing fibronectin and laminin also connected the otic epithelium and neural tube. The results are discussed in terms of models of invagination for the two primordia.

Computer simulation of organogenesis: an approach to the analysis of shape changes in epithelial organs.

Embryonic development of epithelial organ primordia often involves changes in several parameters, such as cell height, cell width, cell volume, amount of extracellular space, and cell number. Since these changes often occur simultaneously, it becomes difficult to "separate out" the role that each plays in the developmental process. A computer program has been written that allows the shape of epithelial organs to be reproduced based upon measurements of the primordium. A developmental sequence can be simulated by changing the dimensions of the primordium based upon either measurements of the developmental stages or theoretical projections of changes. The primordium is divided into blocks representing groups of cells, based upon characteristics of the different cell groups. The program allows differences in cell height and circular and spiral curvatures of the primordium to be simulated. Analysis of the optic primordium using this method has allowed recognition of several regional changes during optic cup formation. These are sequential constriction of cell apices at the margin of the optic cup, expansion of the apical surface toward the center of the retinal disc, and spreading of the future pigmented layer. Simulation of other organs permits regions of morphogenetic activity to be identified.

Embryologic origin of the various epithelial cell types in the second kind of thyroid follicle in the C3H mouse.

The thyroid gland of the C3H mouse is composed largely of the usual follicle but it also contains a second kind of follicle. To ascertain the embryologic origin of the cell types in each of these follicles, ventral pharyngeal outpocketings and ultimobranchial outpocketings were isolated (before they fused to form the thyroid gland) from the 12-day-old fetus of the C3H mouse. The outpocketings were implanted into different kidney capsules of adult C3H mice and were allowed to grow for several months. Transplants were then excised and examined by electron microscopy. The ventral contribution formed large aggregates of follicles and was recognized as a distinct bump on the surface of the kidney. It formed only typical thyroid ultimobranchial contribution usually formed only a small number of follicles, generally of small size. It was readily located because the follicles occurred next to a white plaque of bone or cartilage at the site of implantation. The ultimobranchial contribution formed follicles containing four cell types: a ciliated cell, a cell with abundant agranular reticulum, a cell with many free ribosomes and fiber and occasional hemidesmosomes, and the C cell which was the most frequent cell type. No typical thyroid epithelium was observed in the ultimobranchial transplant. These observations suggest that the C cell in the usual follicle is derived from the ultimobranchial contribution, and that the second kind of follicle is largely an ultimobranchial contribution but the typical thyroid epithelium in it is largely or entirely a ventral contribution.

Accumulation of CPC-precipitable material at apical cell surfaces during formation of the optic cup.

Accumulation of extracellular material at the apical surfaces of cells in the optic vesicle was studied by precipitation with cetylpyridinium chloride (CPC) and scanning electron microscopy. Treatment at low salt concentration to preserve all precipitable material indicated an initial appearance of surface material at the tme that the retinal primordium first formed. The amount of precipitate increased as the optic cup formed, particularly at the margins of the cup. Stability of the precipitate during subsequent washing at higher salt concentrations suggested that the apical cell surface material contained highly acidic glycosaminoglycans. The greatest resistance to extraction occurred during the period in which invagination was most pronounced.

The development of pharyngeal endocrine organs in mouse and chick embryos.

Some interesting differences exist between development of the pharyngeal endocrine organs in mouse and chicken. In both, the thyroid forms as an evagination of the pharyngeal floor in the midline at the level of pharyngeal arch II and moves caudally to the base of the neck. In the chicken, the thyroid divides to form paired organs whereas in the mouse it forms a moustache-shaped bar that later fuses with the parathyroid and ultimobranchial bodies. The thymi, parathyroids and ultimobranchial bodies form as lateral evaginations of the pharyngeal pouches. The chick forms two pairs of thymi and parathyroids from the third and fourth pouches whereas the mouse forms a single pair of each primordium from the third pouch. Ultimobranchial evaginations form from the caudal wall of the sixth pouch in the chick and from the posterior pharynx in the mouse. In the chick all of these evaginations detach from the pharynx and come to lie along the carotid arteries as separate organs that surround the thyroids. In the mouse the parathyroids and ultimobranchial bodies fuse with the lateral thyroid lobes. The thymi move to the ventral midline and fuse to form a single organ. Positioning of these organs appears to be related to three developmental events. Outgrowth of the original evaginations and their ventral movements conform to the curved shape of the pharyngeal arches. Ventral growth of the primordia should result in their convergence near the origin of the ventral aortic roots. The thyroid appears to be attached to the common carotid arteries. Lateral growth of the carotids is consistent with the change in thyroid shape. In the mouse the thymi lie ventral to the other organs. Their movement appears to be related to ventral closure of ventral ectoderm in the neck region.

Optic cup formation: a calcium-regulated process.

Invagination of the optic vesicle to form the optic cup is an important event in the formation of the eye in the early embryo. To obtain support for earlier conclusions that a contractile process is involved, calcium dependency of optic cup formation was tested. Heads were excised from chicken embryos at the optic vesicle stage of development (stage 13) and incubated in nutrient medium containing antagonists or agonists of calcium transport. Invagination was reversibly inhibited by the Ca2+ antagonists verapamil and papaverine. It was initiated in a precocious fashion by the Ca2+ ionophore A23187 but only in the presence of external Ca2+. Neither caffeine, theophylline, nor A23187 (in the absence of external Ca2+) were able to initiate precocious optic cup formation. Trifluoperazine and chlorpromazine reversibly inhibited optic cup formation while chlorpromazine sulfoxide had no effect at the concentrations used. The binding of [3H]trifluoperazine to isolated stage 13 heads revealed a class of Ca2+-dependent binding sites having a Kd similar to that of calmodulin. These results indicate a Ca2+-dependence for optic cup formation and that the source of the Ca2+ may be extracellular. This Ca2+ dependence probably is conferred to the system by calmodulin.

Embryologic origin of various epithelial cell types in the thyroid gland of the rat.

Ventral pharyngeal outpocketings and ultimobranchial outpocketings from the 14-day-old fetus of the Fischer rat were isolated before they fused to form thyroid glands. The outpocketings were implanted into different kidney capsules of adult male Fischer rats, and were allowed to grow for several months. Transplants were then excised and examined by electron microscopy. The ultimobranchial outpocketing gave rise to two types of follicles. One contained ciliated cells, cells with an abundant agranular reticulum, U cells with basal hemidesmosomes, and two types of cells with secretory vesicles. The other contained C cells separated from the follicular lumen by a single flat fiber-containing cell. The ventral outpocketing formed typical thyroid epithelium making up the usual thyroid follicles differing from follicles in the thyroid in situ by the absence of C cells. These follicles were functional as determined by autoradiographic studies with 125I but differed from thyroid follicles in situ with respect to size distribution. The results suggest that (1) in the adult thyroid gland the C cell in the usual follicle is an ultimobranchial contribution and (2) the so-called ultimobranchial follicle is ultimobranchial in origin but that the typical thyroid epithelium in mixed follicles of U cells and typical thyroid epithelium is a ventral contribution. The reason for the absence in the thyroid gland in situ of the variety of other cell types observed in ultimobranchial transplants is unknown.

Cellular dynamics during evagination of the thyroid primordium in the chick embryo.

The thyroid forms as an outpouching of the ventral pharynx. Evidence supports the conclusion that formation of the thyroid pit is mediated by changes in the cytoskeleton that cause constriction of cell apices. However, it seems unlikely that a relatively flat epithelial sheet can be converted into a pit without either distortions of the surface or considerable rearrangement of cells to reduce surface area. Possible cellular rearrangements were investigated by tracing the movements of individual cells by using time-lapse video microscopy. Changes in shape of the primordium were investigated by marking with carbon and DiI and by scanning electron microscopy. Cell movements occurred only over short distances, mostly shifts relative to a neighbor, especially at the edge of the pit. Instead, cells rearranged into clusters that piled up at the edge of the pit and then tilted inside. Adjacent rings of pharyngeal cells were annexed by the growing thyroid, undergoing rearrangement into clusters, piling up at the edge, and moving inside the pit. The consequence was the formation of a series of shelf-like extensions within the cavity, representing successive generations of cell rings moving inside. These results have implications for the formation of other organs by evagination.