No abstract

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.

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.

The ultrastructure of the accessory lobes of Lachi (Hofmann's nuclei) and that of the glycogen body were studied by transmission and scanning electron microscopy in 1-day-old chicks. Glycogen cells of the accessory lobes appeared like those of the glycogen body with nuclei of similar form which were flanked by organelles and by glycogen. The predominant cell type in each structure possessed glycogen-filled processes which were in contact with those of neighboring cells and with blood vessels throughout the tissue. The primary difference between the tissues is that the glycogen cells of the accessory lobes are associated with an extensive meshwork of nerve cells of the marginal nerve nuclei. Glycogen processes of accessory lobe cells appeared to intertwine with each other in such fashion as to completely envelop the multipolar nerve cells that are embedded in those structures. Axo-somatic and axo-dendritic synapses, as well as lamellar whorls were also noted in the vicinity of the perikarya of these nerve cells. The glycogen body and accessory lobes contained extensive extracellular spaces which when viewed by scanning electron microscopy appeared as sinusoid-like networks of channels between the glycogen cells and their processes. The close proximity of these glycogen-rich structures in the lumbosacral nerve cord and the morphological similarities shared by their cell types suggest that they have some specialized functional relationship within the avian nervous system. '

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.