The oculomotor nucleus and extraocular muscles in a mutant anophthalmic mouse

Mouse Extraocular Muscles and the Musculotopic Organization of Their Innervation

The organization of extraocular muscles (EOMs) and their motor nuclei was investigated in the mouse due to the increased importance of this model for oculomotor research. Mice showed a standard EOM organization pattern, although their eyes are set at the side of the head. They do have more prominent oblique muscles, whose insertion points differ from those of frontal-eyed species. Retrograde tracers revealed that the motoneuron layout aligns with the general vertebrate plan with respect to nuclei and laterality. The mouse departed in some significant respects from previously studied species. First, more overlap between the distributions of muscle-specific motoneuronal pools was present in the oculomotor nucleus (III). Furthermore, motoneuron dendrites for each pool filled the entire III and extended beyond the edge of the abducens nucleus (VI). This suggests mouse extraocular motoneuron afferents must target specific pools based on features other than dendritic distribution and nuclear borders. Second, abducens internuclear neurons are located outside the VI. We concluded this because no unlabeled abducens internuclear neurons were observed following lateral rectus muscle injections and because retrograde tracer injections into the III labeled cells immediately ventral and ventrolateral to the VI, not within it. This may provide an anatomical substrate for differential input to motoneurons and internuclear neurons that allows rodents to move their eyes more independently. Finally, while soma size measurements suggested motoneuron subpopulations supplying multiply and singly innervated muscle fibers are present, markers for neurofilaments and perineuronal nets indicated overlap in the size distributions of the two populations. Anat Rec, 302:1865-1885, 2019. © 2019 American Association for Anatomy.

Extraocular eye muscles in human fetuses with craniofacial malformations: anatomical findings and clinical relevance

Anomalies of the extraocular muscles have been suspected to be present frequently in patients with craniofacial malformations. We studied the extraocular musculature in the orbits of 15 anencephalic human fetuses between the 5th and 10th months after gestation, and in one fetus with an occipital meningocele and a large defect of the left cranial base. The findings were compared to those in nine normal age-matched fetuses. All malformed fetuses exhibited special features of the levator palpebrae superioris muscle, either its absence, an anomaly of the muscle belly, or an anomalous insertion. In addition, deficient or aberrant muscle bellies of the oblique and rectus muscles were found. In anencephaly, the frontal bone is condensed, forming narrowed orbital cavities with hypotelorism. Frequent disturbance of extraocular muscle development in fetuses with craniofacial malformations is implicated. The presented specimens are examples of extreme variants of cranial and craniofacial malformation syndromes. Thus, similar findings of extraocular muscles might also be regularly found in clinically relevant craniofacial malformations. This would force new considerations about diagnostic and therapeutic approaches to patients with craniofacial malformations.

Neural tube derived signals and Fgf8 act antagonistically to specify eye versus mandibular arch muscles

Recent knockout experiments in the mouse generated amazing craniofacial skeletal muscle phenotypes. Yet none of the genes could be placed into a molecular network, because the programme to control the development of muscles in the head is not known. Here we show that antagonistic signals from the neural tube and the branchial arches specify extraocular versus branchiomeric muscles. Moreover, we identified Fgf8 as the branchial arch derived signal. However, this molecule has an additional function in supporting the proliferative state of myoblasts, suppressing their differentiation, while a further branchial arch derived signal, namely Bmp7, is an overall negative regulator of head myogenesis.

Extraocular muscles in the microphthalmic rat

The extraocular muscles in a mutant microphthalmic strain of rat were studied. The eyeball of this strain of rat is reduced to about a third in diameter of that of the normal rat. Nevertheless, in the orbit of the mutant rat, every one of the extraocular muscles was identified; their origins and courses were the same as in the normal rat, but differences existed in the insertions. These insertions could be classified into three groups: Group A (retractor bulbi): like normal insertion into the eyeball. Group B (superior rectus and superior oblique): attachment of tendonlike insertions to each other; these muscles come from opposite directions and form a loop. Group C (lateral, medial, and inferior rectus and inferior oblique): insertion into connective tissue surrounding the reduced eyeball. The volume of each muscle of the mutant rat was smaller than that of the normal rat; moreover, significant differences existed in the degree of reduction in the volume of each muscle group classified according to the change of insertion. In the group A muscle the volume was only 33% of the normal volume, whereas group B was 74% and group C was about half of normal.

Quantitative analysis of the oculomotor nuclei in the mutant microphthalmic rat

A quantitative analysis of the oculomotor, trochlear, and abducens nuclei was carried out in the mutant microphthalmic rat. In this strain of rat, the eyeball is reduced to about one-third in diameter, and there is no optic nerve. Nevertheless, this strain possesses all the types of extraocular muscles; however, the volume of these muscles is reduced from 33 to 74% of the normal values. The oculomotor, trochlear, and abducens nuclei in this mutant rat were found in the same location as in the normal ones. Moreover, the neurons in these nuclei in the microphthalmic rat did not appear to be any different from those in the controls. The neuronal population of these nuclei was reduced by 63% (oculomotor), 50% (trochlear), and 61% (abducens) of normal values, respectively. The long axis of all neurons of these nuclei in the microphthalmic and normal rats was measured. In all three nuclei of both strains, the size histograms showed a unimodal distribution ranging from 10 to 35 micron, with peaks at 20 to 25 micron. There was no significant difference between the normal and mutant strains. Consequently, the only influence of the shrinkage of the muscles which they supply was seen in the reduction of the cell populations of these nuclei.