Development of the glycogen body of the Japanese quail, Coturnix japonica: II. Observations of electron microscopy.

Transmission electron microscopic observations of the relationships of the cells of the glycogen body and those of nervous tissue in the lumbosacral spinal cord show that one day after hatching, glycogen cells at the lateral margins of the glycogen body lie in close association with elements of the neuropil in the adjacent spinal cord. Glycogen cells and their processes appear to extend into the neuropil, where they contact other glycogen cells, blood vessels, neurons, and neuroglia. Junctional complexes and synapses occur among glycogen cells, astrocytes, and oligodendrocytes. Other indications of specialized activities were surmised by the presence of annulate lamellae in continuity with extensive arrays of smooth endoplasmic reticulum in several glycogen cells. These observations enhance our earlier views that cells of the avian glycogen body are metabolically active in the synthesis and degradation of glycogen for neuronal support and myelination in the central nervous system.

Development of the glycogen body of the Japanese quail, Coturnix japonica. I. Light microscopy of early development.

The glycogen body is a functionally enigmatic structure located in lumbosacral region of the spinal cord in birds. This tissue is unique to birds, and, although it is believed to be present in all species, studies on the glycogen body to date have been confined largely to the domestic chicken. The present study is the first to describe the glycogen body of the Japanese quail (Coturnix japonica) during incubation and at hatching. Light microscopy and histochemistry were used to identify the glycogen body in the spinal cord of the developing quail beginning at 7 days of incubation and to ascertain the presence of nerve fibers in that tissue at hatching.

Effects of oxytetracycline treatment on enzymes of hepatic glycogen metabolism in genetically diabetic (db/db) mice.

The effects of daily oxytetracycline treatment on the activities of hepatic glycogen synthase, glycogen phosphorylase, plasma glucose, and insulin, and on liver glycogen, free fatty acid, and triglyceride levels were examined in 8- to 15-week-old genetically diabetic and lean mice. Oxytetracycline administration resulted in substantial reductions in the plasma glucose and immunoreactive-insulin levels in both diabetic and lean mice. The drug had no significant effect on the liver glycogen content in either phenotype, regardless of age, but it increased hepatic lipids and depressed body weights in lean animals. The most prominent effect of the drug was in markedly altering the activities of both glycogen synthase and phosphorylase in the liver of older diabetic mice. Oxytetracycline treatment produced a three-fold increase in the percentage of glycogen synthase I activity and reduced by one-third the percentage of glycogen phosphorylase a activity in 15-week-old diabetic mice. In age-matched lean mice treated with oxytetracycline, the percentage of glycogen synthase I activity increased significantly, but the percentage of phosphorylase a activity was unchanged. These data suggest that the drug may alter an aspect of hepatic glycogen metabolism which might lead to an inhibition of glycogenolysis and subsequent diminution of blood sugar levels in the diabetic. The present results show that, while oxytetracycline may be effective in reducing the severity of some of the diabetic symptoms associated with carbohydrate metabolism in this animal model of maturity-onset diabetes, the drug may have adverse effects on aspects of protein and lipid metabolism in these animals.

Minireview. The hypothalamus and blood glucose regulation.

Complex neural circuits exist in the hypothalamus for the control of metabolic hormones, enzyme activities, and substrate flux that appear to be involved with the normal adaptation to feeding and subsequent blood sugar regulation. Abnormalities within this system have been found in experimental obesity and in human diabetes, and such maladaptive changes are thought to contribute to the pathophysiology of both disorders. The present report is an attempt to present a coherent picture of the manifold factors which may be involved in the homeostatic regulation of blood glucose levels by the hypothalamus. A wide variety of evidence is touched upon here to show that this part of the ancient vertebrate forebrain mediates a neural glucoregulatory mechanism involving the endocrine pancreas, the liver, and the adrenal medulla.

A longitudinal and comparative study of some structural and hormonal alterations in the endocrine pancreas of spontaneously diabetic and streptozotocin-induced diabetic mice.

A comparative study of the endocrine pancreas was carried out in genetically diabetic (db) mice and in mice with streptozotocin-induced (Sz) diabetes over a 12-week period of pronounced diabetes. Mice were examined at 9, 12 and 21 weeks of age. Plasma and pancreatic levels of immunoreactive insulin and immunoreactive glucagon were measured in both experimental animal models, and the biochemical data obtained were correlated with ultrastructural observations on the endocrine pancreas. Both pancreatic and plasma immunoreactive insulin levels were severely depressed in all Sz mice. Although pancreatic immunoreactive insulin concentrations in db mice were consistently lower than control values, these animals displayed a hyperinsulinemia which gradually dropped to control levels by 21 weeks. Pancreatic immunoreactive glucagon levels in 12- and 21-week-old db mice were markedly lower than those found in either control or in Sz mice. However, both db and Sz mice in all age groups exhibited a marked and persistent hyperglucagonemia. Pancreatic islet tissue was examined concurrently in control and experimental animals. The ultrastructural changes occurring in the endocrine cells are reported and discussed with regard to the pancreatic and plasma levels of the hormones presently monitored and in light of other recent studies on these animal models.

An hypothesis of function for the avian glycogen body: a novel role for glycogen in the central nervous system.

Our own and other recent data have led us to hypothesize that the glycogen body, heretofore generally considered to be metabolically inert, may be functionally geared to support the process of myelin formation in the avian central nervous system (CNS). We envision that the abundant glycogen stores in this tissue, unlike those in the liver or in skeletal muscle, can serve as a recyclable substrate for the ultimate production of reducing equivalents that would be available for the synthesis of myelin lipid cholesterol. In addition, it is suggested that such glycogen also may serve as a source of organic acids which might provide alternate substrates to the CNS under conditions of metabolic stress.

Glycogen synthase and phosphorylase activities in skeletal muscle from genetically diabetic (db/db) mice.

Glycogen synthase and glycogen phosphorylase activities were determined in skeletal muscle from lean and genetically diabetic (db/db) mice. The activities of both forms of glycogen synthase and of glycogen phosphorylase in diabetic mice were found to be comparable to those activities observed in muscle tissue from lean animals. Enzyme activities in muscle from diabetic animals remained essentially unaltered throughout the 12-week period studied while glycogen content was consistently depressed, despite the presence of hyperglycemia, hyperglucagonemia and transient hyperinsulinemia. These findings suggest that an impairment in muscle glycogen synthase activation may exist in db/db mice in vivo and that skeletal muscle glycogen synthesis may be resistant to the action of insulin.

Glycogen metabolism in the developing accessory lobes of Lachi in the nerve cord of the chick: metabolic correlations with the avian glycogen body.

Glycogen synthase, glycogen phosphorylase, glucose-6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase, and glucose-6-phosphatase were determined for the first time in the necessary lobes of Lachi from late embryonic chicks. The activities of these enzymes were compared with those found in other glycogen-metabolizing tissues, specifically the glycogen body, liver, and skeletal muscle, obtained from the same embryos. The data show that, as in the glycogen body, the accessory lobes of Lachi lack glucose-6-phosphatase, but contain relatively high activity levels of glycogen synthase I, total and active glycogen phosphorylase, and the dehydrogenases of glucose-6-phosphate and 6-phosphogluconate. The percent of glycogen synthase I activity in the Lachi lobes is from two- to 20-fold greater than observed in the glycogen body, liver, or muscle, whereas the percent of glycogen phosphorylase a activity is comparable to that of the liver, but greater than that in the glycogen body or muscle. The activity of each dehydrogenase of the pentose phosphate cycle in the Lachi lobes is similar to that noted in the glycogen body, but is over two- or fivefold greater than that activity found in muscle or liver. Our data, together with other recent evidence, suggest that the role of glycogen in these functionally enigmatic tissues may be to support the precocious process of myelin synthesis in the developing bird, as well as possibly to provide alternate sources of energy for the avian central nervous system.

Glucagon and insulin relationships in genetically diabetic (db/db) and in streptozotocin-induced diabetic mice.

An inappropriate molar ratio of circulating insulin to glucagon is frequently associated with the metabolic alterations accompanying diabetes mellitus. Plasma immunoreactive insulin (IRI) and immunoreactive glucagon (IRG) levels were determined and the IRG:IRI ratio calculated at various intervals in overt diabetes in genetically diabetic (db/db) and in streptozotocin-treated mice. Plasma IRI levels in genetic mutants are elevated at nine weeks of age, but are comparable to values found in lean littermates by 21 weeks. The presence of a prevailing hyperglucagonemia is established for the first time in the intact db/db mice. Streptozotocin diabetics are found to have characteristically low plasma IRI and high plasma IRG values. The hormonal imbalance present in these two experimental animal models is accentuated when the data are expressed as the IRG:IRI ratio, which is seen to increase with the progression of diabetes.

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.

Ultrastructural characterization of the accessory lobes of Lachi (Hofmann's nuclei) in the nerve cord of the chick. I. Axoglial synapses.

Light microscopic observations of the accessory lobes of Lachi of one-day-old chicks show that this tissue contains abundant amounts of glycogen and consists of cells which are similar in appearance to those of the glycogen body. Ultrastructural studies reported here for the first time confirm the presence of glycogen-rich cells in the accessory lobes and reveal that these cells are intimately associated with nerve axons. The finding of synaptic complexes and other junctional specializations between nerves and accessory lobe cells suggests that they may have a functional relationship with the nervous system. It is felt that the accessory lobe cells may be neuroglia, possibly of the astrocytic type, which have an innate capacity for glycogen storage. While the functional significance of such glycogen remains obscure, the close morphological association between neurons and the accessory lobe cells enhances the hypothesis put forth by us regarding the glycogen body, namely that neural glycogen is involved in myelin synthesis in the avian nervous system.

Morphogenesis of the carpal elements in the regenerating forelimb of adult Notophthalmus viridescens viridescens.

This study is concerned with the morphogenesis of the carpal elements in the regenerating forelimb of the adult newt. Blastema cells surrounding the remnant bony stumps begin to differentiate into cartilage on the twentieth post-amputation day. Subsequently, masses of carilage build up from the radial and ulnar stumps. The radial mass is larger and differentiates more rapidly than the ulnar mass. By the fifty-fifth post-amputation day, the eight basic carpal elements are formed, with fusion of two of the units, intermedium with ulmare, occurring by the seventieth day. The completed regenerate possesses the seven carpal elements characteristic of the normal adult limb. The present results show that during limb regeneration in the adult newt the carpal elements are restored to their original number and position and that the pattern of such carpal differentiation proceeds in a proximodistal direction influenced by the stump remnants of the radius and ulna.

Glycogen metabloism in the developing chick glycogen body: functional significance of the direct oxidative pathway.

Glucose-6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase, and glucose-6-phosphatase were quantitatively determined for the first time in glycogen body tissue from late embryonic and neonatal chicks. For comparative purposes, the activities of these enzymes were examined also in liver and skeletal muscle from pre- and post-hatched chicks. The present data show that both the embryonic and neonatal glycogen body lack glucose-6-phosphatase, but contain relatively high levels of glucose-6-phosphate dehydrogenase. The activity of each dehydrogenase in either embryonic or neonatal glycogen body tissue is two- to five-fold greater than that found in muscle or liver from pre- or post-hatched chicks. The relatively high activities observed for both dehydrogenases in the glycogen body, together with the absence of glucose-6-phosphatase activity in that tissue, suggest that the direct oxidative pathway (pentose phosphate cycle) of glucose metabolism is a functionally significant route for glycogen utilization in the glycogen body. It is hypothesized that the glycogen body is metabolically linked to lipid synthesis and myelin formation in the central nervous system of the avian embryo.

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.