Anatomical, Histological, and Morphometrical Features of the Auditory Ossicles in Dog Fetuses at 55-56 Days of Gestation.

This study examines the auditory ossicles of 55-56-day gestational age dog fetuses and shows that the basic three-bone middle ear pattern is already present, but the bones remain partly cartilaginous and are actively ossifying. The tympanic cavity contained a gelatinous, mesenchyme-like material, making dissection difficult. The malleus was the largest and most laterally located ossicle, with an oval head, short neck, curved manubrium, and notable variability in the anterior process. The incus had a rectangular body with short and long processes of similar length, and the lenticular process plus a synovial incudo-malleal joint were already present. The stapes was the smallest ossicle, triangular in shape, with a shorter posterior crus and early ossification in the distal crura and footplate. Histology showed active bone formation with medullary cavities, trabeculae, and osteoclast activity in specific regions, especially the malleal neck, the incudal long process, and the distal stapes, as the metrical data may reflect early mechanical specialization. Overall, the paper concludes that fetal dogs already possess the mammalian ossicular plan, but the structures remain immature, reflecting a clear histological evolution from cartilage-dominant tissue toward organized bone.