Postnatal growth of the extra- and intrafusal fibers in the soleus and medial gastrocnemius muscles of the cat.

Development of intrafusal and extrafusal fibers in the soleus and medial gastrocnemius muscles in kittens was studied in sections stained by van Gieson or histochemical procedures. Cross-sectional areas of intrafusal and extrafusal fibers overlapped extensively in the two weeks following birth, with the means for intrafusal fibers in the medial gastrocnemius muscle being larger than those for the extrafusal fibers. A definite increase in extrafusal fiber size was seen at 20 days and in intrafusal fibers at 29 days. In the one mother cat studied, mean areas for soleus and medial gastrocnemius fibers were 12 and 59 times larger than those for a kitten sacrificed the day after birth, while the intrafusal fiber areas were 3.0 and 4.8 times greater. The equatorial region in nuclear bag fibers showed no significant growth in cross-sectional area, but almost doubled in length. In a newborn kitten, all extrafusal fibers in the soleus and medial gastrocnemius demonstrated a high level of nicotinamide adenine dinucleotide diaphorase activity, but with the adenosine triphosphatase reaction, fibers had either low or high activity. After one month, soleus fibers with high adenosine triphosphatase activity were scarce, whereas in the medial gastrocnemius fibers with both low and high levels of activity were present. The intrafusal fibers at birth were already morphologically and histochemically differentiated into nuclear chain and bag fibers, and two varieties of the latter could be recognized in slides treated for adenosine triphosphatase activity.