The oculomotor nucleus and extraocular muscles in a mutant anophthalmic mouse

The object of this study was to determine whether the oculomotor nucleus and the extraocular muscles are present in the adult of a mutant mouse strain in which eye formation fails early in development (Chase and Chase, '41). Neuronal counts for the oculomotor (IIIrd) nucleus were obtained from serial paraffin sections made through the midbrains of seven mice of the anophthalmic mutant group (ZRDCT-An) and seven mice of the same strain in which eyes were present (ZRDCT-N). The orbital structures of the anophthalmic and control groups were reconstructed from 1-micrometer plastic serial sections. There was in the anophthalmic mouse, in the same location as in the normal, an oculomotor nucleus whose mean neuronal cell number was 206 (+/- 40). The normal mouse had a mean value of 262 (+/- 63) oculomotor neurons. On the basis of these results, this difference between the two groups was not statistically significant (P = 0.073). The neuronal number per cubic millimeter was similar in the two groups (P = 0.81). In the orbit of the anophthalmic mouse, several bundles of striated muscle occupied a location comparable to that of extraocular muscle in the normal mouse. Neuromuscular junctions were present on fibers of this orbital muscle. It is concluded that the early failure of eye formation does not prevent the development of extraocular muscles and the oculomotor nucleus, and their retention in the adult.