Growth characteristics underlying the lack of a chin in pigs: a histomorphometric study.

OBJECTIVES: Despite similar mandibular growth to that of humans, pigs lack a chin projection as shown in most humans. To understand whether this divergence is contributed to differences in local symphyseal growth, this project characterized bone modeling activities at the symphyseal surfaces of juvenile pigs.

MATERIAL AND METHODS: Symphyseal specimens from two age groups (4- and 6-month-old, n = 10) were processed into histological sections with and without decalcification, which were assessed for surface mineral apposition and bone resorption, respectively. In a blinded fashion, measurements of four parameters (MAR: mineral apposition rate, MAZ: mineral apposition zone, ES/BS: eroded surface and OC.N/BS: osteoclast number) were obtained and tested by a multivariate two-way mixed-model analyses of variance (manova) for the differences between symphyseal regions and ages.

RESULTS: Qualitatively, pig symphyseal labial and lingual surfaces were horizontally oriented and characterized by mineral apposition and bone resorption, respectively. Quantitatively, labial mineral apposition tended to be greater rostrally than caudally at 4 months, which became greater caudally than rostrally at 6 months (region/age interactions: p = 0.127 for MAR, p = 0.012 for MAZ). Lingual bone resorption tended to be greater caudally than rostrally, but only ES/BS measurements were significant (p = 0.039) regardless of age, while OC.N/BS measurements varied with ages and regions (age/region interaction, p = 0.087).

CONCLUSIONS: Insufficient differential in symphyseal surface modeling between the labial-caudal and labial-rostral regions contributes to the lack of chin projection in the pig.