COMPARATIVE OSSIFICATION OF THE SKULL IN THREE NIGERIAN BREEDS OF SHEEP: AN ALIZARIN TECHNIQUE

This study describes the ossification time, sequence, and morphogenic pattern of the neuro-cranial and viscera-cranial bones of the skull in three Nigerian breeds of sheep. A total number of four hundred and ninety (490) wasted fetuses were collected from abattoirs. The heads of the fetuses were severed and processed using the Alizarin technique. The ossifying bones were identified, and their morphology was mapped using FastStone Image software. In the three breeds of sheep, the neuro-cranial and viscera-cranial bones developed via intra-membranous ossification. Generally, bony spicules appeared to radiate from the ossification centers of the skull bones faster in Yankasa than Balami and Uda. The frontal, parietal, and the zygomatic process of frontal bones were the first of the neuro-cranial bones to begin ossification. The premaxilla, zygomatic, and maxilla were the first of the viscera-cranial bones to begin ossification. The earliest morphogenic pattern of the neuro-cranium, the occipital condyle was comma-shaped, the parietal and frontal were irregular, and the zygomatic process of temporal bones and frontal process of zygomatic were finger-like projections. Whereas among viscera-cranium, the premaxilla, nasal, and zygomatic were spindle-shaped, the maxilla and lacrimal were triangular and cube-shaped in all the three breeds of sheep. The ossification centers of the skull bones started from the calvarium and developed faster in Yankasa and the shape of the developing bones-varies as the bone spicules radiates and fetuses advanced with age. The temporal and interparietal ossification did not occur in the 1st and early 2nd trimester fetuses (42-67 days of gestation) of the three breeds of sheep. It was concluded that the frontal and premaxilla were the first bones of the skull to ossify and ossification of the viscera-cranium occurred earlier in Yankasa than in Balami and Uda and the ossification centers of all the neuro-cranial bones and nasal bones were eccentric.