Comparisons of capillary maturation in control and hypothyroid rat spinal cord: an ultrastructural study

Differential effect of thyroid hormone deficiency on the growth of calretinin-expressing neurons in rat spinal cord and dorsal root ganglia

The development of spinal cord or dorsal root ganglia neurons expressing calretinin (CR) was studied in thyroid hormone-deficient rats. Immunocytochemical and morphometric analyses showed that the hypothyroidism induced a significant decrease in the number and size of immunoreactive neurons in the spinal cord, as well as stunted growth and arborization of the axons and dendrites. These alterations were observed at different embryonic ages and persisted during the whole postnatal life. In adult hypothyroid rats, the mean number of CR-positive neurons per spinal cord section (31.2 +/- 2.3 in laminae I and II and 30.5 +/- 5.5 in laminae III-X) was significantly decreased (P < 0.001 and P = 0.024, respectively) compared with adult normal rats (68.7 +/- 8.9 and 50.0 +/- 11.0, respectively). In the peripheral nervous system, hypothyroidism altered the growth of sensory neurons expressing CR protein mainly during embryonic life. In comparison with normal rats, hypothyroid embryonic animals showed not only reduced cell size but also a significantly decreased percentage of CR-positive neurons (6.6 +/- 0. 9% in normal, 2.1 +/- 0.3% in hypothyroid rats, P < 0.001). In contrast, although the size of neurons was reduced in hypothyroid young and adult rats, there was no reduction in the percentage of CR-positive neurons. These results showed that thyroid hormone deficiency altered differentially the development of neurons expressing CR protein in the central and peripheral nervous systems. This suggests that central and peripheral neurons are heterogeneous in their sensitivity to thyroid hormone.

Effect of exogenous thyroxine on the development of the Purkinje cell in fetal alcohol effects in the rat

The amelioration of fetal alcohol effects on the postnatal development of the Purkinje cell by exogenous L-thyroxine was investigated in the neonatal rat. Time-pregnant rats were divided into three groups. Group A (n = 6) received 35% liquid ethanol diet; Group B (n = 6) was fed a liquid diet in which maltose dextrins replaced alcohol isocalorically, constituting the pair-fed group; Group C (n = 6) received the 35% liquid ethanol diet and, in addition, received exogenous thyroxine (5 microg/kg/day) subcutaneously. After the pups were born, the mothers were removed and the pups of each were surrogate fostered by dams who were fed normal rat chow and water ad libitum. An average of six pups, one from each litter, were killed at days 7, 14, 21, and 28 for each of the above three groups. Light and electron microscopic observations of lobule II/III revealed a delayed alignment of Purkinje cells (Pc) in alcohol-exposed pups compared to pair-fed pups. The Pc of the pair-fed group showed a single-layer arrangement at 7 days which was seen only at day 14 in the alcohol group. However, in the alcohol + T(4)-exposed pups a single-layer arrangement was quite often seen at 7 days. Morphological observations showed impaired evidence of protein synthesis at all time sequences in the pups of Group A compared to Group B. A most interesting finding was the morphological evidence of greater protein synthesis in the Pc of the alcohol + T(4) group at all times as indicated by a hypertrophied nucleus, abundant ribosomal collection, and numerous Nissl bodies.

Microglial development is altered in immature spinal cord by exposure to radiation

Previous studies in this laboratory have documented that the microglial environment of the immature spinal cord is altered by exposure to ionizing radiation. As a result, the lumbosacral spinal cord is markedly depleted of both oligodendrocytes and astrocytes, while leaving axons and the overall cytoarchitecture intact. The status of the microglia in the irradiated region is unknown and is of interest given the interactions between microglia and astrocytes recently elucidated by others. This study uses both in vivo and in vitro approaches to examine the microglial population in normal and irradiated immature spinal cord. The lectin, Griffonia (Bandeiraea) simplicifolia, was selected since it marks microglia both in paraffin embedded sections and in cell cultures. Light microscopic examinations of spinal cord sections revealed a reduced microglial population in the irradiated region when compared to littermate controls, and a change in morphology of the remaining microglia to that described by others as "activated". Cultures prepared from lumbosacral spinal cords harvested from 3-day-old rats within 2-4 hr following irradiation were compared with cultures derived from their non-irradiated littermates after 8 days in vitro. Cultures from the irradiated spinal cords revealed trends similar to those observed in vivo, i.e. a reduced microglial population and altered morphology. Although all glial cell types were reduced in cultures from irradiated spinal cords, the few microglia present were usually positioned atop astrocytes. The consistency of reduction in all glial populations in this model shows the microglia to be a novel microenvironment for further studies of roles of microglial within the spinal cord.