Extraocular muscles in the microphthalmic rat

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.

The differentiation and morphogenesis of craniofacial muscles

Unraveling the complex tissue interactions necessary to generate the structural and functional diversity present among craniofacial muscles is challenging. These muscles initiate their development within a mesenchymal population bounded by the brain, pharyngeal endoderm, surface ectoderm, and neural crest cells. This set of spatial relations, and in particular the segmental properties of these adjacent tissues, are unique to the head. Additionally, the lack of early epithelialization in head mesoderm necessitates strategies for generating discrete myogenic foci that may differ from those operating in the trunk. Molecular data indeed indicate dissimilar methods of regulation, yet transplantation studies suggest that some head and trunk myogenic populations are interchangeable. The first goal of this review is to present key features of these diversities, identifying and comparing tissue and molecular interactions regulating myogenesis in the head and trunk. Our second focus is on the diverse morphogenetic movements exhibited by craniofacial muscles. Precursors of tongue muscles partly mimic migrations of appendicular myoblasts, whereas myoblasts destined to form extraocular muscles condense within paraxial mesoderm, then as large cohorts they cross the mesoderm:neural crest interface en route to periocular regions. Branchial muscle precursors exhibit yet another strategy, establishing contacts with neural crest populations before branchial arch formation and maintaining these relations through subsequent stages of morphogenesis. With many of the prerequisite stepping-stones in our knowledge of craniofacial myogenesis now in place, discovering the cellular and molecular interactions necessary to initiate and sustain the differentiation and morphogenesis of these neglected craniofacial muscles is now an attainable goal.

Effects of prenatal alcohol exposure on the postnatal morphology of the rat oculomotor nucleus

Morphological development of the rat oculomotor nucleus was investigated on postnatal day 15 following a prenatal ethanol exposure. Analysis of toluidine blue stained plastic sections showed that the prenatal alcohol exposure caused a decrease in the density of neurons and an increase in the density of astrocytes in the center of the nucleus. There was an alcohol-induced reduction in the overall size of the cross-sectional region of the oculomotor nucleus, but no effect on the number of neurons per unit area of that total oculomotor region, indicating a delay or alteration of the migration of neurons to their normal clustered position in the center of the nucleus. The areas of the neuronal cell nucleus and nucleolus were not affected by the alcohol exposure. Analysis of Golgi-Cox-impregnated multipolar neurons showed that the alcohol exposure caused a reduction in area of the cell soma; a reduction in the number of dendritic branches; and a reduction in the complexity of the dendritic arbor relative to distance from the soma, based on concentric ring analysis. The results of this study demonstrate that gestational alcohol exposure can retard the maturation of the oculomotor nucleus.