Differentiation of the thyroid in the hypophysectomized chick embryo

The influence of hypophyseal hormones on the normal development of the thyroid gland was investigated by decapitating chick embryos at a time before the hypophysis has begun to form. Thyroids from decapitated embryos were compared with thyroids of normal embryos from shortly after decapitation to the time when the embryo should hatch. The thyroids were examined for ultrastructural differences and for accumulation of thyroglobulin. Thyroxine levels in the blood were also examined. It was concluded that the thyroids in decapitated embryos develop normally up to about 12 days of development, by which time morphogenesis and differentiation of the gland is essentially complete. The rates of thyroid growth and accumulation of thyroglobulin are diminished after 12 days of development. The amount of thyroxine in the blood is decreased by about one-half. Thyroxine was found in the yolk of unincubated eggs at levels sufficient to sustain blood thyroxine levels throughout embryonic development. We conclude that hormones that are synthesized by the embryonic hypophysis affect thyroid development only through their influence on generalized growth and metabolic activity.     

The mature mesonephric nephron of the rabbit embryo

Summary In freeze-fracture replicas of the entire cross-fractured mesonephros of 18 day rabbit embryos the basolateral and luminal cell faces of the different nephron segments were studied and compared with their metanephric counterparts. In the proximal tubule, the shallow zonula occludens exhibited only 1–2 strands and resembled the corresponding metanephric zonula, a very leaky type, which was found with a considerable paracellular flow component in sites of isotonic reabsorption. Gap junctions were restricted to the proximal tubule and were seen more frequently in its terminal segment. The distal tubule harboured two types of tight junctions. The most common type, a band of 5–8 closely parallel strands, matched the zonula occludens of the metanephric straight distal tubule. The observed particle density of the basolateral membrane (2,500±328/m²) was less than that of the proximal tubule (2,642±306). In addition, the collecting tubule exhibited a zonula occludens of the tight variety similar to that which occurred in the metanephric collecting duct. Rod-shaped particles of the luminal membrane were mainly concentrated in some of the intercalated cells but also had developed on principal cells, and occasionally, in the distal tubule. The Wolffian duct, with a deep tight zonula occludens, had an obviously rather inactive epithelium with no conspicuous transport-linked membrane specializations.     

Development of cell-to-cell relationships in the thyroid gland of the chick embryo

Summary Thyroid glands of 7 to 21 day-old chick embryos were examined by electron microscopy using freeze-fracture and thin-section preparations. The primitive follicle lumen first appears between two adjacent epithelial cells in 8 day-old embryos, and is formed in the region of a focal tight junction (macula occludens). The focal tight junction develops into the zonula occludens when the primitive follicle lumen first forms.The zonula occludens is, at first, composed of 4.6 ± 2.45 strands, but with increasing embryonic age the number of strands increases to 5.9 ± 1.41 in 13 day-old, and 8.0 ± 1.75 in 19 day-old embryos.Thyroglobulin stored within the embryonic gland lumina is isolated from the mesenchymal tissue even at the first appearance of these follicle cavities.Well developed gap junctions already occur in the thyroid gland of the 7 day-old embryo, so that an intimate relationship and communication between these cells already exists at the time of their functional differentiation.This study was supported by a grant from the Japan Ministry of Education     

The mature mesonephric nephron of the rabbit embryo

Summary Transmission electron micrographs of the mesonephric nephron in 18 day rabbit embryos reveal major cytological structures reappearing in the nephron of the definitive rabbit kidney. The initial segment of the proximal tubule resembles (despite quite different cell proportions) the cell picture of the metanephric S₂-segment. The changes occurring at the end of the terminal proximal segment, the decrease in cell size, flattening of the nuclei, shortening of the brush border and reduction of Golgi profiles and endocytotic organelles largely parallel those between S₂ and S₃. The type of increased basolateral cell face of the proximal and distal tubule cells shows only quantitative differences to their metanephric counterparts. The distal tubule, which cannot be further subdivided (except the macula densa-region) exhibits varying degrees of cell interdigitations with vertically arranged and partially arching lateral ridges. This tubule matches closely the metanephric medullary straight part of the distal tubule, so that the sequence of the first mesonephric nephron segments is similar to the metanephric ones with the exception that the thin limb of Henle is absent. The large macula densa-region is characterized by its cell height and distended infranuclear spaces. The principal cells of the collecting tubule, with a few basal infoldings and intense short lateral interlockings resemble metanephric cells of the outer medullary collecting duct. The mitochondria-rich intercalated cells occur in dark and light contrasting forms and are more frequent than was evident from our SEM-study. The homogeneous cell population of the Wolffian duct is characterized by large glycogen deposits and comparatively smooth cell faces.     

The mature mesonephric nephron of the rabbit embryo

Summary The luminal surface ultrastructure of the mature mesonephric nephron in 18 day rabbit embryos was studied in order to classify the nephron segments and to compare them with their metanephric counterparts. The proximal tubule has two slightly different segments. Its brush-bordered cells, with lateral ridges and basal microvilli (revealed in disjoined cells) exhibit structural principles similar to those of metanephric cells. The short distal tubule, starting with an abrupt border, cannot be subdivided. Its surface differs from one specimen to the next; the various cellular patterns are regarded as different functional states rather than evidence of a true cellular heterogeneity. Cells with leaf-like meandering borders correspond to similar metanephric cells favoring a paracellular transport mechanism. The collecting tubule shares common features with the metanephric collecting duct in spite of its different origin. Among principal cells, clearly demarcated by marginal microvillous rows and studded with sparse apical microvilli, non-ciliated and strongly bulging intercalated cells occur in small numbers. The latter have exaggerated, sometimes branched microvilli, and occasional microplicae. In the Wolffian duct, which has no metanephric counterpart, the single cilia dominate the picture of a homogeneous cell population. Apical globular protrusions of the tubular epithelia, which have been depicted in almost every paper on the mesonephros, are all fixation artefacts that can only be avoided by properly perfusing the living embryo.     

Development of an embryonic thyroid grafted into a chick embryo

A morphological and physiological study of an embryonic thyroid grafted in a chick embryo showed that it developed according to the endocrine status of the host. Its relative age appreciated at various stages of embryonic life is different from that of a gland developing normally during similar lengths of time.     

Role of protein kinase C in chick embryo skeletal myoblast fusion

The involvement of Ca2+ and PGE1 in myoblast fusion has been well documented. Extracellular Ca2+ is essential for myoblast adhesion, alignment, and fusion. There is an obligatory increase in Ca2+ influx immediately preceding fusion and the Ca2+ ionophore A23187 promotes precocious fusion. PGE1 receptors appear just prior to fusion, and an antagonist of PGE1 binding to cell surface receptors blocks fusion when added prior to Ca2+ influx. Finally, exogenous PGE1 induces precocious fusion. The present study was an initial test of the hypothesis that membrane protein phosphorylation by protein kinase C (PKC) links PGE1 receptor occupancy and the increase in Ca2+ influx. Our conclusion that PKC is an essential component in the regulation of myoblast fusion is based in part on the following evidence: (1) an activator of PKC, the tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA), at low concentration and for a brief exposure period, induces precocious fusion and stimulates Ca2+ influx; (2) 4 alpha-phorbol-12,13-didecanoate, an inactive analog of TPA, has no discernible effect on fusion or Ca2+ influx; (3) 1-oleoyl-2-acetyl glycerol, an analog of endogenous diacylglycerol (DAG) which activates PKC, promotes precocious fusion, as does the DAG kinase inhibitor R59022 (6-[2-[4-[(4-fluorophenyl)phenylmethylene]-1-piperidinyl]ethyl]-7- methyl-5H-thiazole-[3,2 alpha]-pyrimidin-5-one) which raises the level of endogenous DAG by inhibiting its catabolism; (4) 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H-7), a highly potent PKC inhibitor, reversibly blocks myogenesis at a point between alignment and fusion; and (5) H-7 also blocks the normal increase in Ca2+ influx preceding fusion.     

Protein absorption by tubular mesonephric and metanephric structures in the human embryo

Summary The presence of different serum proteins in the cells of the proximal tubule of both meso- and metanephric nephrons in human embryos (7th–12th week of intrauterine life) was investigated using immunohistochemistry. Endogenous lysozyme, alpha-1-antitrpysin and ferritin were detected in mesonephric proximal tubules and, starting from the 8th week, also in metanephric proximal tubules. Our observations provide information concerning the appearance and distribution of tubular protein reabsorption during the early stages of development.     

Ultrastructural changes in thyroid epithelium during involution of the hyperplastic thyroid gland

The ultrastructure of the thyroid epithelial cell was examined at various time intervals after induction of involution of the hyperplastic thyroid gland. Thyroids were made hyperplastic by the feeding of thiouracil in a Remington low-iodine diet to male Fischer rats for 3 weeks. Involution was induced by replacing the thiouracil-containing diet with Purina Laboratory Chow, a high-iodine diet. During involution, organelles that play a role in the synthesis and secretion of thyroglobulin, such as the rough endoplasmic reticulum, Golgi apparatus, and apical vesicles, were well preserved and prominent features of the epithelial cell. The apical plasma membrane of many cells was highly irregular for approximately 2 weeks with signs suggesting rapid discharge of apical-vesical contents into the lumen of the follicle. Pseudopods and colloid droplets were present but were not very prominent features of the cell. No signs of extensive autophagy or obvious increased incidence of lysosomes were present, although there was an elevation in the incidence of small dense bodies starting about day 8, and prominent by 15 days. Some of these may be phagosomes formed from luminal debris. The observations indicate that involution of the hyperplastic thyroid in which there is maintenance of the protein synthetic apparatus and little sign of autophagy or death of the epithelial cells is remarkably different from phenomena occurring during involution of prostate or mammary glands.     

Erythropoiesis in the developing chick embryo

The types of erythroid cells of chick embryos developing in ovo have been correlated with the hemoglobins of the embryos. Prior to 5 days, when primitive cells constitute the only erythroid cells, two hemoglobins can be resolved by polyacrylamide gel electrophoresis. The two adult hemoglobins and a minor hemoglobin found only in embryos and young chicks first appear simultaneously with initiation of definitive erythropoiesis.