Origin of the avian glycogen body. II. Observations in support of a glial nature in the chick embryo.

Stem cells of the glycogen body in chick embryos at 7-8 days of incubation (stages 31-34) were studied by light microscopy autoradiography, immunocytochemistry, and following induced spina bifida to discern their nature and origin. Glycogen cells displayed the presence of glial fibrillar acid protein and vimentin at the time of their appearance at 7-8 days of incubation (stages 31-34). Displayed also were indications of DNA synthesis by thymidine incorporation during that period of development. Observations suggest that glycogen body cells are glial in nature, possibly radial glia which arise from neuroepithelium that comprises the ependyma and roof plate of the avian lumbosacral spinal cord.

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.

Origin of the avian glycogen body: I. Effects of tail bud removal in the chick embryo.

The tail bud was removed from chick embryos at stage 16-17 in a first study directed to learn of the origin of the glycogen body in the lumbosacral spinal cord of birds. Results of tail bud removal and chorio-allantoic grafting of caudal portions of the embryo containing the tail bud or the neural tube suggest that the glycogen body does not arise from the tail bud, but from the preexisting neural tube craniad or anterior to the tail bud. The stem cells of the glycogen body are most likely among those components of the anterior portion of the lumbosacral neural tube derived from primary neurulation.

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.

The effects of lead nitrate on the central nervous system of the chick embryo. II. Electron microscopy and histochemistry: peroxisomes.

The deleterious effects of lead on the developing central nervous system were observed in chick embryos that were exposed to lead nitrate at 10 days of incubation. At 16 days, cervical, thoracic, and lumbar regions of the spinal cord were fixed in gluteraldehyde and processed for electron microscopy, either directly or following incubation in media containing 3,3-diaminobenzidine and hydrogen peroxide for the histochemical detection of catalase-reactive peroxisomes. The results indicate that peroxisomes are not destroyed as are other components of the neuropil after lead exposure. Catalase-reactive peroxisomes appear to increase in number in the spinal cords of lead-treated embryos suggesting that these organelles may play a part in the response of glial cells to metabolic alterations induced by lead. Further experimentation using the chick embryo as an animal model for studies of lead-induced neuropathy is encouraged.

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 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.

Growth and differentiation of chicken embryos in simplified shell-less cultures under ordinary conditions of incubation.

The growth and differentiation of chicken embryos maintained in evaporating dishes as shell-less cultures were compared to that of control embryos in ovo under the same conditions of incubation. The development of cultured embryos appears to be indistinguishable from that of embryos in ovo up to 10 days of total incubation. Thereafter, the growth of shell-less embryos is significantly retarded as evidenced by morphological criteria of the Hamburger-Hamilton stages of development, body weight, tibia length, and total protein content. Cultured embryos are smaller in size than controls, but show normal morphological features including a fully-differentiated skeleton. Skeletal defects and other anomalies were not detected in any of the specimens studied. Limitations as to the use of the simplified method of culture are evaluated and discussed.

The effects of lead nitrate on the central nervous system of the chick embryo I. Observations of light and electron microscopy.

The effects of lead on the developing central nervous system were studied for the first time by light and electron microscopy in the chick embryo after injecting lead nitrate into the air space of the egg at 10 days of incubation. Lead-treated embryos showed curled toes as a sign of neurological injury, weighed less and were smaller in size than controls of the same incubation age. Light microscope studies of brain and spinal cord revealed the presence of numerous accumulations of blood cells which appeared to accompany the extensive injury found in those tissues. Ultrastructure of spinal cord suggests that neuroglial astrocytes in the vicinity of blood vessels are altered in leaded embryos. Prominent morphological changes were extensive vacuolation and disorganization of the endoplasmic reticulum of the cell. The evidence indicates that the spinal cord like the developing brain is subject to the biological effects of lead. The results encourage further experimentation using the chick embryo for studies of lead neuropathies.

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.

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.