Transformation of the endostyle of the anadromous sea lamprey,Petromyzon marinus L., during metamorphosis. II. Electron microscopy

Electron microscopy was used to follow the transformation of the endostyle to a thyroid gland in the anadromous sea lamprey, Petromyzon marinus L., throughout metamorphosis (stages 1–7). Transformation of the larval (ammocoete) endostyle begins at the first signs of external change (stages 1–2), and the adult form of the gland is reached by stage 5. Only slight modifications of the gland accompany further development to the end of metamorphosis. Development of the thyroid gland involves degeneration, proliferation, and reorganization of the cells in the endostyle, and changes in their fine structure. Ultrastructural changes during early stages are most obvious in the type 1 cells that make up the shrinking glandular tracts, and involves the accumulation of cytoplasmic microfilaments and a variety of cytoplasmic inclusions. The glandular tracts and their cells gradually disappear through autolysis and, apparently, through phagocytosis by neighboring epithelial cells and macrophages. Although the fine structure of the type 2, 3, 4, and 5 cells is not altered in the early stages, by stage 3, many of these cells become either vacuolated, undergo autolysis, or are extruded. Phagocytosis of some of each of these cell types likely occurs. Thyroid follicles are first observed during stage 4. Some of their lumina seem to arise from the accumulation of material in intercellular spaces and from vacuoles among cell clusters. Other lumina may represent a portion of the original lumen of the endostyle. Many follicles appear to be comprised of cells with cytological characteristics similar to those of larval cell types 3 and 2c. Some of the other larval cell types, such as type 5, may also be involved. In young adult lampreys follicles are composed of cuboidal to columnar cells that lack the dilated cisternae of rough endoplasmic reticulum seen in follicular cells of higher vertebrates. Dense collagenous connective tissue surrounding the follicles contains relatively few blood vessels. The transformation process described may have some relevance to our understanding of the development and evolution of the vertebrate thyroid gland.     

Structural and ultrastructural differentiation of the thyroid gland during embryogenesis in the grass snake Natrix natrix L. (Lepidosauria, Serpentes)

The differentiation of the thyroid primordium of reptilian species is poorly understood. The present study reports on structural and ultrastructural studies of the developing thyroid gland in embryos of the grass snake Natrix natrix L. At the time of oviposition, the thyroid primordium occupied its final position in the embryos. Throughout developmental stages I-IV, the undifferentiated thyroid primordium contained cellular cords, and the plasma membranes of adjacent cells formed junctional complexes. Subsequently, the first follicular lumens started to form. The follicular lumens were of intracellular origin, as in other vertebrate species, but the mechanism of their formation is as yet unclear. At developmental stages V-VI, the thyroid anlage was composed of small follicles with lumens and cellular cords. Cells of the thyroid primordium divided, and follicles were filled with a granular substance. At developmental stage VI, the cells surrounding the follicular lumen were polarized, the apical cytoplasm contained dark granules and the Golgi complex and the rough endoplasmic reticulum (RER) developed gradually. Resorption of the colloid began at developmental stage VIII. At the end of this stage, the embryonic thyroid gland was surrounded by a definitive capsule. During developmental stages IX-X, the follicular cells contained granules and vesicles of different sizes and electron densities and a well-developed Golgi apparatus and RER. At developmental stage XI, most follicles were outlined by squamous epithelial cells and presented wide lumens filled with a light colloid. The Golgi complex and RER showed changes in their morphology indicating a decrease in the activity of the thyroid gland. At developmental stage XII, the activity of the embryonic thyroid gradually increased, and at the time of hatching, it exhibited the features of a fully active gland.     

Embryonic induction and development of the ultimobranchial body in the embryogenesis of amphibia

At early embryonal and larval stages of development 7 species of amphibia have been studied. The ultimobranchial anlage and processes resulted in formation of the secretory follicle are investigated. Dynamics on changes of amount of the gland cells and the first appearance of capillaries are analyzed. In Anura and Urodela the anlage of the ultimobranchial gland develops from the epithelial lining of the pharynx behind the last branchial pocket rudiment. The gland is asymmetric and can be laid either in the right or in the left side of the body. Death of calcitonin-secreting cells is compensated at the expense of repeated anlage of follicles from the pharyngeal epithelium. The newly formed follicles can either incorporate into the existing gland, or form independent follicles. For amphibia formation of the capillary network around the gland after beginning of the follicular secretion is specific. Owing to these data, it is possible to conclude that the stimulus for the gland to become overgrown with capillaries is the beginning of calcitonin secretion.     

The structure of the spiny mouse (Acomys cahirinus) ovary during development

The study presents the structure of the ovaries of the spiny mouse (Acomys cahirinus) during the first months of life. The ovaries in neonate females exhibit a large number of primordial and primary follicles, sometimes clustered in nests. The growing follicles were also observed within the ovary at that period. The first, early antral follicles appeared in the ovary during the second week of life. In the group of 60-day old females, the structure of the ovaries was characterized by a significant increase in the connective tissue elements. Moreover, ovarian follicles at various stages of development were observed, except for the antral ones with cumulus oophorus. The first mature follicles were identified in 3-month old females. In the ovarian follicles, apoptosis occurs at all stages of follicle development, especially in the early antral follicles. In the atretic follicles, apoptotic cells were identified in the layer of granulosa cells.     

Sex differences in the development of the thyroid gland in human prenatal ontogeny

A comparative study of the thyroid gland development in the human male and female embryos and prefoetuses was carried out. The development of incretory part of the testicles was shown to occur earlier than the differentiation of follicles in the thyroid gland and the appearance of colloid in it. Sexual differences were noted in the appearance of follicles, connective tissue, nerve fibres in the thyroid gland (earlier in the male foetuses). The processes of organ differentiation in the male foetuses were more active than in the female ones during the whole prenatal period of development. A suggestion is put forward that the sexual hormones of embryonic testicles influence the thyroid gland differentiation.     

Expression of an Otx gene in the adult rudiment and the developing central nervous system in the vestibula larva of the sea urchin Holopneustes purpurescens

Expression of the Otx gene, HprOtx, from the sea urchin Holopneustes purpurescens, is described during the development of the adult echinoid rudiment in the vestibula larva of this species. The adult rudiment forms directly after gastrulation in the vestibula larva since, unlike the pluteus larva of most other sea urchin species, it is not a feeding larva. The expression is described during the period from hatching to a late vestibula larva. At hatching, HprOtx is expressed throughout the ectoderm of the gastrula. A short time later, expression is absent from the ectoderm on the oral side of the gastrula where the vestibule will form. In an early vestibula larva, HprOtx is not expressed in the ectodermal floor of the vestibule but is expressed in an asymmetric pattern in the aboral ectoderm. As the vestibule invaginates, HprOtx is newly expressed in the ectodermal floor of the vestibule as it develops into the neuroectoderm that is the anlage of the circum-oral central nervous system. The expression is at first in the central part of the floor, then it extends outwards to the ectoderm around the five primary podia and to the epineural folds between the podia. The epineural folds later close to form the radial nerves and the circum-oral nerve ring. In a late vestibula larva, HprOtx is expressed in the radial nerves and the nerve ring. The expression of an Otx gene in the developing echinoid central nervous system is interpreted as an instance of conserved gene expression in echinoderm development.     

Biosynthesis of thyroglobulin in the endostyle of larva (ammocoetes) of a fresh water lamprey, Lampetra planeri B1]

The biosynthesis of thyroglobulin (Tg) in larva of a fresh-water lamprey, Lampetra planeri B1. has been established. This glycoprotein presents the same characters as in thyroid follicles of adult lampreys, as shown by its 18-19 S sedimentation coefficient and by the incorporation (in vivo and in vitro experiments of 4, 12, 72 h) of 125I, 3H-leucine and 3H-galactose. 3-8 S fractions and a 12 S monomer are the precursors of the 18-19 S protein. Total I % of Tg is very low (0.002 %) ; about 5 % of 125I are present in thyroid hormones (T3 and T4) in the 125I-labeled protein. The biosynthesis of 18-19 S Tg proceeds in larvs before the morphological differentiation of thyroid cells and follicles after metamorphosis. However, the biosynthesis of this protein is much slower in the endostyle of larvs, in which a primitive mechanism of storage is poorly efficient, compared to the accumulation of Tg in the colloid of the follicles of adults.     

Immunohistochemical localization of NPY, VIP and 5-HT in the thyroid gland of the lizard, Podarcis sicula

The thyroid gland of the lizard Podarcis sicula was immunohistochemically studied in adult male specimens using specific antibodies against NPY, VIP and 5-HT and the avidin-biotin peroxidase complex (ABC) procedure to localize the three peptides. Fine beaded VIP-immunoreactive nerve fibers ran between the follicles, and VIP-immunoreactivity was evenly distributed in the apical cytoplasm of follicular cells. NPY-immunoreactive fibers were found around the follicles, and, in the cells, immunoreactivity was localizated only in the cellular apices. Immunoreactivity to 5-HT was observed in the colloid, with a concentration in the follicular lumen exceeding that in the follicular cells. In fact, most follicles showed immunoreactivity in the cytoplasmic bridges formed between the apical portion of the follicular cells and the colloid.     

Alterations within the rat thyroid gland during vitamin A deficiency

Thyroid glands from female rats kept vitamin A deficient for one, two, and three months were examined by electron microscopy. After one month on the diet, no consistent alterations were noted. After two months, the colloid in some follicles displayed a peripheral zone of decreased density. In addition, ultimobranchial follicles within the gland had become keratinized. After two to three months on the diet, cells were seen entering the colloid. Many of these cells were identified as follicular cells since they often occurred in groups and occasionally exhibited remnants of desmosomes. Often the cells within the colloid appeared vacuolated, and by light microscopy were thought to contain lipid. However, electron microscopy revealed that these cells contained many digestive vacuoles rather than lipid droplets. Quantitative and autoradiographic studies indicated that thyroids of vitamin A deficient rats took up less radioiodide than thyroids of control rats. The keratinization of ultimorbranchial follicles in vitamin-A deficiency has been suggested as preliminary in the histogenesis of squamous cell carcinoma. However, an effect of vitamin A deficiency on thyroid follicular cells has not heretofore been reported. It's possible that the presence of follicular cells in the colloid reflects an accelerated turnover of these cells and could indicate an early pathological sign.     

Immunocytochemical studies on differentiation of the thyroid gland in rabbit fetuses and chick embryos

Summary The morphogenesis of the thyroid gland in rabbit fetuses and chick embryos was investigated using the PAS stain and an immunoperoxidase method with anti-19S-thyroglobulin antiserum. In rabbit fetuses, the reaction for precursor components was firstly detected in the apical portions of follicular cells, arranged in clusters but not yet forming follicles, at 16 days of gestation. Although the first primordial follicles storing colloid droplets were observed on day 18, a drastic increase of follicle formation, the true onset of thyroid function, did not occur until day 22. The colloid in primordial follicles revealed very strong immunoreactivity for 19S-thyroglobulin. The follicles gradually increased in size with age. At 25 days of gestation the cytoplasm of follicular cells was stained densely by slightly diluted 19S-thyroglobulin antiserum, whereas the colloid was stained with highly diluted antiserum; these immunoreactions of follicular cells and colloid were comparable to those of postnatal animals. In chick embryos, significant numbers of primordial follicles were observed throughout the whole thyroid parenchyma at 9 days of incubation. On day 12, the follicles stored more PAS-positive and immunoreactive colloid. At 14 days of incubation follicles with enlarged follicular lumina, having an immunoreactivity similar to mature rollicles, became increasingly common.     

Postembryonic development of the ovary in the penicillate diplopod,Eudigraphis nigricans (Miyosi) (Diplopoda,Penicillata)

Postembryonic development of the ovary through the larval stages was studied in a penicillate diplopod, Eudigraphis nigricans. In the first instar larva a single young cell cluster, consisting of about 20 spherical gonial cells and some smaller interstitial cells, exists beneath the alimentary canal in the third body segment. The gonadal epithelium encompasses the upper surface of this young cell cluster by the end of the first instar. The epithelium then extends forward and backward to form a single long sac-like gonad, leaving the young cell cluster on the center of the gonadal floor as a mound-shaped germarium. In an early second-instar larva, very early previtellogenic oocytes accompanied by some interstitial cells appear in the front and rear surfaces of the ovarian germarium. During the period from the third through the seventh (the last) larval instar, some cell clusters containing several previtellogenic oocytes and interstitial cells successively separate forward and backward from the germarium to form a series of paired patch-shaped vitellarial areas on the extending ventral ovarian epithelium. In each vitellarial area, some of the interstitial cells surround the oocytes to form the follicles. In the seventh instar, the ovarian lumen is extremely expanded, and the late previtellogenic oocytes in the vitellarial areas encroach upward into the ovarian lumen. These oocytes floating in the ovarian lumen are still connected with their own vitellarial areas by partial extensions of their follicles. Some phylogenetic implications of the basic characteristics in structure and postembryonic development of the ovary are discussed. © 1995 Wiley-Liss, Inc.     

Formation of the digestive system in zebrafish. II. Pancreas morphogenesis

Recent studies have suggested that the zebrafish pancreas develops from a single pancreatic anlage, located on the dorsal aspect of the developing gut. However, using a transgenic zebrafish line that expresses GFP throughout the endoderm, we report that, in fact, two pancreatic anlagen join to form the pancreas. One anlage is located on the dorsal aspect of the developing gut and is present by 24 h postfertilization (hpf), the second anlage is located on the ventral aspect of the developing gut in a position anterior to the dorsal anlage and is present by 40 hpf. These two buds merge by 52 hpf to form the pancreas. Using heart and soul mutant embryos, in which the pancreatic anlagen most often do not fuse, we show that the posterior bud generates only endocrine tissue, while the anterior bud gives rise to the pancreatic duct and exocrine cells. Interestingly, at later stages, the anterior bud also gives rise to a small number of endocrine cells usually present near the pancreatic duct. Altogether, these studies show that in zebrafish, as in the other model systems analyzed to date, the pancreas arises from multiple buds. To analyze whether other features of pancreas development are conserved and investigate the influence of surrounding tissues on pancreas development, we examined the role of the vasculature in this process. Contrary to reports in other model systems, we find that, although vascular endothelium is in contact with the posterior bud throughout pancreas development, its absence in cloche mutant embryos does not appear to affect the early morphogenesis or differentiation of the pancreas.     

Difference between the thyroid gland of normal and hypomyelinated jimpy mice--a light microscopic study

A light microscopic quantitative analysis was performed on normal and jimpy male mice for studying the difference between the structures of the thyroid glands of the two animals. The results of this analysis showed that the thyroid gland of the normal mice consisted of numerous homogenous round follicles with cuboidal follicular cells, separated by thin interlobular and interfollicular connective tissue and a few adipose tissue. The thyroid gland of jimpy mice consisted of a few, small follicles surrounded by columnar follicular cells and intraepithelial capillaries, separated by thick connective tissue and abundant adipose tissue. The number of thyroid follicles are significantly less in the jimpy mice than in the normal mice. Another striking difference is that almost every follicular cell surrounding the follicular lumen of jimpy mice is accompanied by an intraepithelial capillary. In addition, the ratio of the number of intraepithelial capillaries to the number of the thyroid follicular cells are significantly higher in the jimpy mice than in the normal mice. The S-follicles or ultimobranchial cysts of the thyroid gland are well developed in the jimpy mice. The parafollicular cells are normal in appearance. Morphological evidence suggested that the thyroid follicular cells of the jimpy mice are very active in the transport, synthesis and release of thyroglobulin, and secretion of thyroid hormones. But owing to the significantly decreased number of thyroid follicles, the inadequate secretion of the thyroid hormones result in the hypothyroidism and the hypomyelination of the jimpy mice.     

Development and cytodifferentiation of C cell complexes in dog fetal thyroids

Summary The development of C-cell complexes was investigated in dog fetuses by an immunoperoxidase method with three specific antisera: anti-calcitonin, anti-C-thyroglobulin (C-Tg), and anti-19S thyroglobulin. Ultimobranchial bodies joined with the thyroid anlage and then dispersed into the parenchyma to form large C cell groups. Sparse reaction products of C-Tg initially appeared in C cells with small amounts of cytoplasm. Later at about day 39 of gestation, when the immunoreactivity of calcitonin and 19S thyroglobulin appeared weakly in C cells and follicular cells, C-cell complexes were identified as large cell masses containing numerous undifferentiated cells without no immunoreactivity for any of the antisera. As development proceeded, the undifferentiated cells developed progressively the morphology of C cells. In addition, the undifferentiated cells developed 19S thyroglobulin immunoreactivity, that is, within some of the complexes small clusters of cells filled with material immunoreactive for 19S thyroglobulin. They were not organized into follicles during the fetal period, and were very slow in development. Depending on the degree of development of the undifferentiated cells, several features of the complexes were noted. The present study indicates that not only C cells but also follicular thyroid cells appear to be derived from the ultimobranchial bodies.     

Evidence of the occurrence of calcitonin cells in the ultimobranchial follicle of the rat postnatal thyroid.

A study on thyroid glands of Wistar rats of ages ranging from 1 to 120 days was carried out. The glands were serially sectioned and stained for calcitonin using the peroxidase antiperoxidase method. All the thyroids contained ultimobranchial follicles (UBF) located partially embedded among the usual follicles but in a 5-day-old rat this structure showed an unusual position in the interstitium of connective tissue between the cartilage of the trachea and the thyroid gland. We have observed in the wall of that UBF the presence not only of resting C cells but also mitotic figures of C cells. Furthermore, on the opposite side of the same UBF an active area of formation of thyroid follicles was found. These observations provided the first evidence of the contribution of the UBF in the formation of C cells during the postnatal life of the rat. Furthermore, it is suggested that some C cells may share a common origin with ultimobranchially derived follicular cells.     

Programmed cell death and heterolysis of larval epithelial cells by macrophage-like cells in the anuran small intestine in vivo and in vitro.

The degenerative processes in the larval small intestine of Xenopus laevis tadpoles during spontaneous metamorphosis and during thyroid hormone-induced metamorphosis in vitro were examined by electron microscopy. Around the beginning of spontaneous metamorphic climax (stages 59-61), both apoptotic bodies derived from larval epithelial cells and intraepithelial macrophage-like cells suddenly increase in number. The macrophage-like cells become rounded and enlarged because of numerous vacuoles containing the apoptotic bodies. Mitotic profiles of the macrophage-like cells, however, are localized in the connective tissue where different developmental stages of macrophage-like cells are present. After stage 62, the intraepithelial macrophage-like cells decrease in number, while large macrophage-like cells which include the apoptotic bodies and retain intact cell membranes and nuclei appear in the lumen. Degenerative changes similar to those during spontaneous metamorphosis described above could be reproduced in vitro. In tissue fragments isolated from the small intestine of stage 57 tadpoles and cultured in the presence of thyroid hormone, the number of intraepithelial macrophage-like cells reaches its maximum around the 3rd day of cultivation when the larval epithelial cells most rapidly decrease in number. These results suggest that the rapid degeneration of larval epithelial cells occurs not only because of apoptosis of the epithelial cells themselves but also from heterolysis by macrophages. The macrophages probably originate in the connective tissue, actively proliferate, migrate into the larval epithelium around the beginning of metamorphic climax, and are finally extruded into the lumen.     

The insertion of mesenchyme cells into the ectoderm during differentiation in Sea urchin embryos

Summary During the course of sea urchin development, from early blastula to pluteus larva, there are two major visible processes toward which all activities seem to be focused. They are the differentiation of the larval skeleton by the primary mesenchyme cells and the differentiation of the primitive gut by the secondary mesenchyme cells. These activities take place within the shell-like layer of epithelial cells, or ectodermal wall. The interactive role of the ectodermal wall with the mesenchyme cells is not yet clearly understood. A number of earlier studies have proposed that the ectoderm may have an inductive influence on the mesenchyme cells and that its inner surface forms a molecular template for guiding the mesenchyme cells. In this report, we suggest an additional role for the ectodermal wall. We show that some primary mesenchyme cells and secondary mesenchyme cells insert between the cells of the ectodermal wall in order to firmly anchor the anlage of the larval skeleton and primitive gut during differentiation. This mechanism may provide a physical basis for maintaining the stable positional relationship of the anlage during development.     

Experimental heteroxenous cycle of Lagochilascaris minor Leiper, 1909 (Nematoda: Ascarididae) in white mice and in cats.

Reports of natural infections of sylvatic carnivores by adult worms of species similar to Lagochilascaris minor in the Neotropical region led to attempts to establish experimental cycles in laboratory mice and in cats. Also, larval development was seen in the skeletal muscle of an agouti (Dasyprocta leporina) infected per os with incubated eggs of the parasite obtained from a human case. In cats, adult worms develop and fertile eggs are expelled in the feces; in mice, larval stages of the parasite develop, and are encapsulate in the skeletal muscle, and in the adipose and subcutaneous connective tissue. From our observations, we conclude that the larva infective for the mouse is the early 3rd stage, while for the final host the infective form is the later 3rd stage. A single moult was seen in the mouse, giving rise to a small population of 4th stage larvae, long after the initial infection.