Prenatal and early post-natal development of the skull in Nigerian breeds of sheep: a gross anatomical study

Abstract

The aim of this study was to bridge the gap of information on the gross morphology of the skull of sheep foetuses. A total of 240 samples consisting of fetal and day-old Balami, Uda and Yankasa breeds were used; the skulls were prepared using chemical maceration technique. In the 1st trimester, no visible ossifications were observed on the skull. In the 2nd trimester, ossification of the frontal, parietal, occipital and basisphenoid bones were observed. The nasal bones were ossified, but concaved in Balami and flat in Uda and Yankasa. In the 3rd trimester, an interparietal bone developed between the parietal and occipital bones. The paracondyloid processes, occipital squamous, and the external occipital protuberance were developed in the 3rd trimester. The interparietal bone appeared paired in Yankasa but unpaired in Balami and Uda, which developed into a paired bone in the day-old. In the day-old, the orbital walls were completely fused except in the Uda; the formation of the cornual process began; the presphenoid of the Yankasa was developed. It was concluded that grossly, ossification was observed in the 2nd trimester and it began with the neurocranial bones, however, in the 2nd trimester, the skull was partly soft and pliable and the foramina were rudimentary. In the 3rd trimester, the skull bones were hard and osseous. All the bones on the dorsal surfaces of the skull were membranous whereas some of the bones of the ventral surface were cartilaginous in origin.