Inability of muscles in the obese mouse (ob/ob) to respond to changes in body weight and activity.

The fat-free carcass weight of the obese mouse (ob/ob) is generally less than that of wild-type siblings. The aim of this investigation was to examine the effect of obesity on muscle weights and histochemistry and to determine whether any effects could be eliminated when the obesity was largely prevented or reduced by limiting food intake. For 5 muscles examined the weights were significantly greater (except for biceps brachii) in the wild-type than in obese mice. Although there was a significant correlation between muscle weight (except for soleus) and body weight in the wild-type mice, no such correlation held for the obese mice. No remarkable differences between groups of mice were found in the histochemistry of the biceps brachii and soleus muscles except that fibre sizes were generally smaller in the obese mice. It is concluded that the skeletal muscles of obese mice cannot respond to the increased activity associated with prevented or reduced obesity.

Effects of pituitary dwarfism in the mouse on fast and slow skeletal muscles.

The Snell dwarf mouse exhibits impaired growth of the anterior pituitary resulting in reduced levels of growth hormone and thyroid stimulating hormone. Ten dwarf mice and 10 phenotypically normal littermates were killed at 33 days of age. M. biceps brachii (a predominantly fast muscle) and m. soleus (a relatively slow muscle) were removed from each animal and complete frozen transverse sections obtained. Serial sections were reacted for various enzyme activities in order to identify muscle fibre types. There was no difference in the total number of muscle fibres in m. biceps brachii but a small difference in m. soleus between normal and dwarf mice. There were marked differences in the size of all fibre types between normal and dwarf mice with the largest differences in m. soleus. The percentage of slow oxidative fibres was similar (about 32%) in both groups of mice for m. soleus but there was a marked difference for this fibre type in m. biceps brachii being about 1.5% in normal mice and 8.0% in dwarf mice. This may be related to a difference in levels of thyroid hormone. Nuclear density was very significantly greater in dwarf muscles although total nuclear numbers were less than in normal muscles. These differences are most likely due to growth hormone levels. Differences in nuclear content were much greater in m. soleus than in m. biceps brachii.

The effect of maternal undernutrition on the growth and development of the guinea pig placenta.

Fetal growth is known to be correlated with the size of the placenta and the exchange surface area. Reduction in the growth of the materno-fetal exchange surface areas may be a mechanism by which the effects of maternal undernutrition on fetal growth are mediated. In the compact placenta of the guinea pig the exchange surface is equivalent to the peripheral labyrinth. The effect of a 40% reduction in maternal feed intake on the growth of the peripheral labyrinth was investigated in pregnant guinea pigs between gestational days 25 and 65. Fetal and placental weights were significantly reduced in the last trimester by 32% and 38% respectively (P < 0.01). Placental efficiency in early gestation was significantly impaired in restricted animals but equivalent to ad lib. fed controls by the last trimester. The volume of the peripheral labyrinth increased as a percentage of the total placental volume with gestational age. Restricted placentae tended to be composed of a smaller volume of peripheral labyrinth tissue in early gestation. It is suggested that maternal undernutrition results in an impaired or delayed expansion of the peripheral labyrinth in early gestation causing a reduction in placental efficiency. By the last trimester the weight of the peripheral labyrinth of restricted animals was reduced by 33% (P < 0.05). The weight of the peripheral labyrinth was also significantly correlated with fetal weight is limited by the size of the peripheral labyrinth in the later stages of gestation.

The effect of temperature on myogenesis in embryonic development of the Atlantic salmon (Salmo salar L.).

From fertilisation to hatching one group of salmon embryos was reared at ambient temperatures (fluctuating around 1.6 degrees C) and another at 10 degrees C. At Gorodilov stages 28, 30 and 33 transverse sections of whole embryos were obtained for light and electron microscopy. Total cross-sectional areas, fibre numbers, fibre diameters and myofibrillar areas of the white muscle of m. lateralis were measured. At hatching (stage 33, which occurred much earlier at the higher temperature), the higher temperature embryos had significantly larger (P less than 0.01) but fewer (P less than 0.05) muscle fibres. These larger fibres contained significantly more myofibrillar material (P less than 0.05) than the smaller fibres of the lower temperature embryos. Lesser differences were found at pre-hatching stages. Higher temperatures caused myofibre hypertrophy to increase at a greater rate than hyperplasia. Hence, the cellularity of the tissue produced under the different temperature regimes was quite different.