Postnatal development of bones and related hormonal factors in piglets.

Piglets grow considerably in their first weeks of life. However, little information exists about the interplay of hormonal factors that orchestrate bone development. The present study aimed to investigate the development of hormonal factors in serum, bone metrics, histology and mRNA expression of genes related to growth and transformation of bone tissue from day of life (DoL) 3 to 35. Litters (n = 10) remained with their mother sows until weaning on DoL 28. Blood and metacarpal bones of 10 piglets (5 males and 5 females) were collected on DoL 3, 7, 14, 21, 28, 31 and 35. Bone geometry parameters linearly increased during the suckling phase (P < 0.05). Histological parameters, including the total width of the growth plate and its proliferative and reserve zones, in turn, quadratically changed from DoL 3 to 28 (P < 0.05), indicating greater longitudinal bone growth between DoL 7 to 14. Osteocyte and osteoclast numbers also linearly increased from DoL 3 to 28 along with increasing mRNA expression of RANKL and OPG (P < 0.05), indicating bone maturation. Serum alkaline phosphatase and bone-specific alkaline phosphatase, important factors for bone mineralization, were higher in the first week of life and linearly declined thereafter (P < 0.05). Serum levels of parathyroid hormone (PTH) and vitamin D, which respond to decreasing Ca levels in the blood, linearly increased during the suckling phase (P < 0.05). Serum levels and mRNA expression of fibroblast growth factor-23 (FGF23), that regulates the renal phosphate excretion, and of KLOTHO, needed for the action of FGF23 on the kidney, linearly decreased during the suckling phase (P < 0.05), which allowed the retention of phosphate for bone growth. Postweaning, bone geometry parameters suggested growth stagnation from DoL 28 to 35. However, quadratic effects of age on histological parameters for the growth plate indicated interrupted growth mainly on DoL 31, whereas on DoL 35 longitudinal bone growth resumed (P < 0.05). Quadratic effects of age on postweaning levels of serum Ca and phosphate and expression of KLOTHO supported reduced growth on DoL 31 (P < 0.05), whereas serum levels of vitamin D and parathormone linearly increased and decreased postweaning, respectively (P < 0.05). Overall, the present data provide valid information on bone mineralization and development of the regulatory interplay of hormonal factors and feedback loops during the suckling phase and the impact of weaning on bone growth.