Functional differentiation of the chick endocrine pancreas: insulin storage and secretion

Ultrastructure of endocrine pancreatic granules during pancreatic differentiation in the grass snake, Natrix natrix L. (Lepidosauria, Serpentes)

We used transmission electron microscopy to study the pancreatic main endocrine cell types in the embryos of the grass snake Natrix natrix L. with focus on the morphology of their secretory granules. The embryonic endocrine part of the pancreas in the grass snake contains four main types of cells (A, B, D, and PP), which is similar to other vertebrates. The B granules contained a moderately electron-dense crystalline-like core that was polygonal in shape and an electron-dense outer zone. The A granules had a spherical electron-dense eccentrically located core and a moderately electron-dense outer zone. The D granules were filled with a moderately electron-dense non-homogeneous content. The PP granules had a spherical electron-dense core with an electron translucent outer zone. Within the main types of granules (A, B, D, PP), different morphological subtypes were recognized that indicated their maturity, which may be related to the different content of these granules during the process of maturation. The sequence of pancreatic endocrine cell differentiation in grass snake embryos differs from that in many vertebrates. In the grass snake embryos, the B and D cells differentiated earlier than A and PP cells. The different sequence of endocrine cell differentiation in snakes and other vertebrates has been related to phylogenetic position and nutrition during early developmental stages.

Hormonal effects on amino acids and related compounds in plasma, amniotic fluid, and allantoic fluid of the chicken embryo

So far, more than 40 free amino acids and related compounds have been identified in plasma, amniotic fluid, and/or allantoic fluid of the 13-day chicken embryo. Concentration differences, and greatly varying behavior of these compounds under experimental conditions, revealed the presence of specific barriers among the three fluids. We tested the hypotheses that (1) the absence of an innervation of amnion and allantois indicates a hormonal control of their barriers, and (2) changes in the concentrations of certain amino compounds in the three fluids indicate anabolic or catabolic actions of hormones. Insulin, prolactin, and stress caused complex changes of the concentrations of amino compounds in all three fluids within 30 min. Some of these changes indicated breakdown of embryonic tissues, while others must have been due to transfer of amino compounds among the three fluid compartments. However, there was no significant effect on the glucose concentration in any of the three compartments under any of the experimental conditions. This is the first demonstration of hormonal effects on the amino compounds in the extraembryonic fluids of nonmammalian amniotes.

Morphogenesis and differentiation of the avian endocrine pancreas, with particular reference to experimental studies on the chick embryo

The avian pancreas has three or four lobes and develops from a dorsal and two ventral buds. The cells that will contribute to formation of the dorsal bud are at first located in the mid-dorsal endoderm, those of the ventral buds in the floor of the foregut. The determination of endoderm to form dorsal and ventral bud derivatives occurs before formation of the buds. The highest concentration of endocrine tissue is in the splenic lobe. The lobes contain A and B islets in which glucagon and insulin cells, respectively, predominate. Islets contain somatostatin and pancreatic polypeptide (PP) cells, both of which also occur scattered in the exocrine parenchyma. Pancreatic endocrine cells arise from endoderm: glucagon, insulin, and somatostatin cells differentiate early, PP cells later. To establish culture conditions suitable for avian insulin cells, the epithelial component of dorsal buds of 5-day chick embryos was cultured under various conditions. At the end of 7 days the proportion of insulin cells was determined. In raising the proportion of insulin cells, Matrigel was superior to collagen gel and a serum-free medium (incorporating insulin, transferrin, and selenium) was superior to a serum-containing medium. Modifications to the serum-free medium were tested. Raising the level of glucose or of glucose and essential amino acids increased the proportion of insulin cells. This proportion was also increased by replacement of insulin by insulin-like growth factor-I, whereas addition of transforming growth factor beta1 reduced the proportion. Transfer of explants from poor to favourable culture conditions showed that the improved conditions stimulated quiescent insulin progenitor cells to develop.

Development of embryonic chick insulin cells in culture: beneficial effects of serum-free medium, raised nutrients, and biomatrix

A previous finding that insulin cells do not survive or differentiate in explants of embryonic avian pancreas cultured in collagen gel with a serum-containing medium has provided a model system for identification of conditions favorable for development of these cells. To this end, we here modify the substrate and the medium. The epithelial component of dorsal pancreatic buds of 5-d chick embryos was cultured for 7 d on Matrigel in serum-containing and in serum-free medium, the latter incorporating insulin, transferrin, and selenium. Endocrine cell types were distinguished by immunocytochemistry; insulin cell counts were expressed as a proportion of insulin plus glucagon cells. With serum-containing medium, Matrigel stimulated a significant increase in this proportion as compared with collagen gel--3.1% as against 0.2%; the serum-free medium further increased this proportion to 17.3%. Raising the level of essential amino acids approximately fivefold increased the latter figure somewhat (to 18.9%), but it was more than doubled (to 37.4%) by raising the glucose concentration from 10 mM to 20 mM. Raising the levels of amino acids and glucose simultaneously yielded a lesser increase (to 31.8%). Some cultures grown in collagen gel and serum-containing medium for 7 d were transferred to Matrigel and serum-free medium for a further 7 d. Insulin cell development recovered, indicating that progenitor cells had survived and were stimulated to develop by the improved conditions. This study indicates that components of the biomatrix and the medium (in particular, a raised glucose concentration) are important for the survival and differentiation of embryonic insulin cells.

Glycoconjugate saccharidic moieties of the exocrine and endocrine pancreas in the chick embryo, newborn and adult

A battery of horseradish peroxidase-conjugated lectins (PNA, SBA, DBA, WGA, ConA, LTA and UEAI) was used to study the distribution of glycoconjugate sugar residues in the exocrine and endocrine pancreas of chick embryos, 1-day-old chicks and adult animals. During the period of incubation considered here, the time of appearance and changes in the oligosaccharides were noted in the acinar cells, in the endocrine cells of the "light islets" and in the capillary endothelial cells. In the developing exocrine pancreas, the basolateral surface of the acinar cells reacted with PNA, WGA, LTA and ConA for the entire period of incubation while SBA, WGA, LTA and ConA reactivity was detected in the apical plasma membrane. Zymogen granule membrane reacted with PNA, WGA and LTA from the 12th day onward, while ConA reactivity was detectable from the 7th day onward. Owing to its early appearance, alpha-D-mannose, revealed by ConA lectin, might in some way contribute to the maturation of the zymogen granule. In the 1-day-old chick and in the adult the cellular surface reacted with WGA and ConA. The zymogen granule membrane reacted with PNA, WGA and ConA in the 1-day-old chick. In the adult, WGA and ConA reactivity was observed in the zymogen granule membrane, whereas at this site PNA reactivity was revealed only after neuraminidase digestion. The hypothesis formulated by others that granule membrane lectin reactivity is equal to the apical membrane reactivity, owing to exocytotic processes, is not consistent with our results in the chick embryos, the 1-day-old chick and the adults. The surface and the granules of the islet beta cells in different periods of incubation reacted with WGA, SBA and, after neuraminidase treatment, with PNA. The same reactivity was seen also in 1-day-old chick while in the adult the granules of the beta cells also reacted with ConA. These findings show the achievement of an almost adult-like state of glycosylation of the glycoconjugates in the beta cells at an early stages of development.

Insulin improves survival but does not maintain function of cultured chick wing bud apical ectodermal ridge

Previously we demonstrated that high levels of insulin (5 micrograms/ml) permit the survival of isolated chick apical ectodermal ridge in culture (Boutin and Fallon, Dev. Biol., 104:111-116, 1984). Here we address whether lower levels of insulin or insulin-like growth factors (IGFs) can also improve the survival of cultured apical ectodermal ridge and whether ridge function is maintained along with ridge survival. Neither IGF I nor IGF II (100 ng/ml) decreased ridge cell death; however, cell death was significantly decreased with 50 ng/ml insulin. No further improvement was obtained in the presence of both IGF I and insulin. These data suggest that insulin improved the survival of the isolated apical ectodermal ridge by binding its own receptor. To test for the maintenance of function, stage 20 ridges were cultured for 0, 6, 12, 18, or 24 hr with or without insulin (5 micrograms/ml or 5 ng/ml) and used to make recombinant limbs. Isolated ridges cultured for 12 hr or more produced fewer outgrowths and these were rarely distally complete. The medium in which the ridges had been cultured did not influence ridge activity, despite the major differences in cell survival. Recombinants made with ridges cultured with limb mesoderm for 18 hr did not yield outgrowths as often as those with freshly isolated ridges, but most of the limbs that did form were distally complete. These results suggest that the decline in function of cultured, isolated apical ectodermal ridge was not due merely to ridge cell death but rather, at least in part, to its separation from limb mesoderm.

Insulin gene expression in chicken ontogeny: pancreatic, extrapancreatic, and prepancreatic

Insulin has metabolic, growth, and differentiation effects in chicken embryos in vivo and it is required for normal development. Whether the pancreas is the sole source of insulin in embryogenesis is controversial. In the present study we investigated (1) the developmental pattern of expression of the chicken insulin gene in the pancreas; (2) the expression of the insulin gene in three nonpancreatic tissues, liver, brain, and lower limb, during chicken development; and (3) the expression of the insulin gene at prepancreatic stages and during chicken embryo organogenesis. Hybridization of synthetic species-specific insulin oligonucleotides to pancreatic frozen section in situ and to Northern blots revealed a major increase in insulin messenger RNA (mRNA) levels during the third week of embryonic development. The hybridization histochemistry showed both an increase in the levels of insulin mRNA per pancreatic islet and, in addition, an increase in the number of insulin mRNA containing islets with development. By Northern analysis there was a major polyadenylated transcript of 0.6 kb, which increased in abundance approximately 30-fold during this interval. Under the same stringency conditions used for pancreatic RNA an insulin transcript was detected in liver RNA blots. The abundance of this hepatic insulin mRNA was about 100-fold less than the pancreatic insulin mRNA and, in contrast to the latter, did not increase in late development. Primer extension experiments demonstrated that the insulin transcripts of pancreas and liver had similar 5' ends. No insulin mRNA was detected by Northern analysis or primer extension either in whole brain or lower limb total RNA from several developmental stages. A very low abundance insulin mRNA was detected in whole embryo at Day 8 and body regions at Day 4 and Day 5 when organogenesis of the pancreas takes place. Interestingly, a polyadenylated insulin transcript was detected, as well, in whole Day 2 and Day 3 embryos (stages 10 to 20, with 20 to 40 somites) before differentiation of beta cells occurs. Thus, there is differential developmental regulation of the insulin gene in several chicken embryo tissues and the expression of insulin precedes pancreatic maturation. These findings support the proposed role of insulin in differentiation and development in vivo and suggest a paracrine type of action of the hormone in early embryos before blood circulation begins.

Immunohistochemical localization of insulin in the chick embryo during development

Since an extrapancreatic source of insulin has been postulated in the chick embryo, insulin was sought by immunohistochemistry in the embryo before and after development of the pancreas and other tissues. Using for the first time an antiserum specific for chicken insulin, localization of insulin was investigated in whole embryos (Days 1-8) and in brain, kidney, liver, and pancreas (Days 9-21 and 30). Results indicate the presence of immunoreactive insulin in pancreatic islets from Day 5 onward with an increase in staining intensity during development. Insulin was not detected in other tissues of the early developing embryo or in brain, liver, or kidney in later development. These data suggest that although extrapancreatic insulin has been reported, concentrations are not high enough in tissues to be detected with a specific, homologous antiserum and sensitive immunohistochemical techniques.

Interaction between endocrine and paracrine peptides in prenatal growth control

The evidence reviewed here shows that the endocrinology of fetal growth is very different from that operating postnatally. Pituitary hormones play little part in stimulating growth of the lean body mass or skeleton although growth hormone (GH) may be involved, in some as yet ill defined way in the ontogeny of the fetal pancreatic islet and insulin secretion. Insulin is important because it stimulates fetal cellular anabolism but acts in a permissive manner: with too little insulin growth is inhibited, with too much growth proceeds at a genetically predetermined rate. Placental lactogen (PL), or other peptides within the GH/PL family, may act as a true growth-promoting hormone in the fetus; it stimulates both cellular metabolism and mitosis. The part played by endocrine control mechanisms in the fetus is set in context by an appreciation of the importance of locally acting tissue growth factors, and in particular the somatomedins. Their part in fetal growth control is intimately bound up with the plane of nutrition experienced by the fetus. It is concluded that the simplest analysis that makes biological sense involves a consideration of hormones, tissue growth factors and nutrition, not hierarchically but as mutually interacting variables.

Insulin inhibition of tyrosine aminotransferase induction by dexamethasone is species-related

Dexamethasone induces TAT in the embryonic chick liver, but this effect is not blocked by prior administration of insulin at a dose sufficient to increase the hepatic protein/DNA ratio. Similarly, insulin does not block dexamethasone induction of TAT in chick embryo hepatocytes in vitro. In contrast, insulin reduces TAT induction by dexamethasone in fetal rat hepatocytes by 35-40%. No significant differences in insulin binding were noted between chick and rat hepatocytes.

Direct effect of insulin on the synthesis of specific plasma proteins: biphasic response of hepatocytes cultured in serum- and hormone-free medium

Monolayers of chicken embryo hepatocytes. cultured in chemically defined medium, retain the ability to synthesize a wide spectrum of plasma proteins for several days in the absence of added hormones. Addition of insulin to the medium elicited a biphasic stimulation of plasma protein synthesis: a rapid response of the synthesis of a limited number of plasma proteins (e.g., albumin and alpha 1-globulin "M"), then, after prolonged exposure to the hormone, the involvement of additional plasma proteins (e.g., fibrinogen and lipoproteins). Synthesis of transferrin and a few other plasma proteins was not affected by the presence of insulin. The degree of stimulation for the most response plasma proteins ranged between 2- to 4-fold during the early phase and 10- and even 30-fold during the late phase of the cells' response t insulin. Stimulated synthesis in the early phase was detected within 1 hr and was rapidly reversible. Plasma protein synthesis in culture was sensitive to concentrations of insulin below 0.35 nM, well within the physiological range. The delayed response was elicited only at higher hormone levels. Parallels between the control of synthesis of plasma proteins in this system and that observed in diabetic animals suggest that the embryonic chicken hepatocytes may be a useful model for studying liver function in diabetes as well as insulin action in general.

Ouabain-resistant hyperpolarization induced by insulin in aggregates of embryonic heart cells

Spheroidal aggregates formed from trypsin-dissociated 14-day embryonic chicken hearts after 48 hr of rotation on a gyratory shaker. Intracellularly recorded resting membrane potentials of aggregates bathed in 1.3 mM K+ balanced salt solution had a mean (+/- SD) of 64 +/- 4 mV. After a stable potential was achieved, addition of 1-100 nM sodium bovine insulin caused a slow hyperpolarization of up to 19 mV after 4-5 min, followed, in some cases, by a further, more rapid, shift to a potential near EK. Equivalent hyperpolarizations were observed when insulin was added in the presence of 10 mM ouabain, indicating that enhanced Na+,K+ pump activity was not responsible for the change in membrane potential. The concentration of insulin that produced half-maximal hyperpolarization (2 nM) corresponded to the association constant of a high-affinity insulin receptor, suggesting that binding to this class of receptors led to the change in membrane potential. Steady-state current-voltage curves from current clamp experiments suggested that insulin produced an increase in slope conductance at potentials near rest by inducing an outward current with an apparent potential negative to -90 mV.

Muscle trophic factor: III. Effect of hormones and tissue extracts on muscle trophic-factor activity

The ability of classical hormones or extract from adult chicken tissues to replace or influence the activity of chicken skeletal muscle cell trophic factor was investigated. Pituitary gland extract did not replace the trophic factor in a range equivalent to the serum concentrations of somatotropin, although high concentrations of such extract showed a significant ability to mimic trophic-factor activity. Insulin, triiodothyronine, testosterone, dihydrotestosterone (Stanolone), and estradiol did not show any ability to mimic trophic-factor activity. Both pituitary extract and insulin showed a potentiating effect on the trophic factor when added to the medium; however, the concentrations necessary for this effect were too high to be considered physiologic. Triiodothyronine, testosterone, Stanolone, and estradiol did not potentiate the trophic factor; indeed, at higher concentrations, these hormones actually suppressed the activity of the trophic factor. Every tissue extract examined showed, to a greater or lesser extent, the ability to potentiate the trophic factor.

Functional differentiation of the chick endocrine pancreas. II. The alpha cells and glucagon

A radioimmunoassay for glucagon, together with electron microscopic observations of early embryonic alpha cells were utilized to examine developmental aspects of glucagon accumulation and release in the chick embryo. Immunoreactive glucagon was detected in both the pancreas and blood plasma from the fifth embryonic day onwards. In addition, emiocytotic events were observed in alpha cells as early as the fifth embryonic day. The early appearance of glucagon and its subsequent developmental profile correlate well with major events in carbohydrate metabolism occurring in the embryonic chick, and are discussed in relation to a functionally responding system, the developing liver. The present data show that glucagon is secreted at earlier embryonic stages than hitherto demonstrated, and suggest a developmental role for glucagon in hepatic glycogen metabolism.

Radioimmunological evidence for early functional activity in chick embryonic alpha cells

A radioimmunoassay for glucagon was utilized to investigate early developmental aspects of glucagon synthesis and release in the chick embryo. Immunoreactive glucagon was detected in both the pancreas and blood plasma from the fifth embryonic day onwards. The present data show that the chick embryonic alpha cell has the potential for secretory activity very early in development, and suggest a developmental role for glucagon in hepatic glycogen metabolism.