The aim of this study is to present selected issues concerning highly complex mechanisms regulating processes of ossification (formation of bone) and the remodeling of bones in poultry as well as to discuss the influence of various factors of the breeding and feeding environment on the development of diseases of the skeletal system, so often diagnosed in poultry practice. The bone tissue is comprised of large amounts of extracellular matter - consisting of organic matrix and bone mineral which is synthesized, maintained and remodeled by three types of cells: osteoblasts, osteocytes, osteoclasts. These cells, along with their receptors for many hormones (PTH, calciferol, glycocorticosterides, sex hormones, iodotyronines, GH and its matabolites) and cytokines (PGE₂, IL-1, IL-3, IL-6, IL-11, TGF-β, TNF) as well as numerous synthesized enzymes known as their biochemical markers, play the main role in the metabolism of bone tissue. Osteoclastogenesis involves three groups of factors: RANK receptor located in the cell membrane of osteoclasts and their precursors, osteoprotegrin (OPG) and the ligand for RANK (RANK/OPGL) receptor anchored in the cell membrane of osteoblasts or cells of the bone marrow stroma. Adrenergic impulses, the concentration of calcium ions, IL-6, TGF-β, leptin, PTH, calciferol and glucocorticoids also play a significant role. Organic matrix (mainly synthesized by osteoblasts) that constitutes about 20% of bone dry weight is made of collagen (80-90%), noncollagenous proteins (5-10%), and a small amount of proteoglicans and glycoproteines. The rest of bone dry weight is bone mineral, which is mainly composed of calcium phosphates in the form of hydroxyapatite crystals, and provides stiffness and compressional strength to the bone. Collagen is the main component of the organic matrix, contributing to the tensile strength of bone and providing oriented support to the mineral matrix. The arrangements of collagen fibers in bone with respect to the bone axis can also influence its strength. In the intensive breeding conditions, where birds grow very fast, osteogenesis and bone rebuilding processes - owing to their unusual complexity - may be disturbed by numerous factors related to the technology of breeding and feeding as well as to pathological conditions of various organs. Skeletal development in birds depends on their physical activity, bioavailability of Ca, P, Zn, Cu, Mg, Mn, Se, biotin, vitamins D, E, C and B in food, high level of metionin in relation to vitamin B₆, quality of lipid acids Ω6:Ω3, fat quality, presence of mikotoxins, cadm contaminations as well as pathological conditions of the intestinal mucous membrane. The appropriate balance of cations (Ca, Mg, Na and K) and anions (PO₄, SO₄ and Cl) in food and tissues is necessary for the homeostasis of the organism. Owing to the fact that bone tissue is a huge “container” of mineral elements and easily exchanged ions, it plays a major role in maintaining the water-electrolyte and acid-base balance of the organism. In a bird breeding environment, this balance can be easily disturbed by, for instance, a high ammoniac concentration and hyperthermia, which lead to disorders in expelling CO₂ and, in consequence, to respiratory acidosis and disorders in bone mineralization. Also an excess of protein and exogenous aminoacids in food can result in methabolic acidosis and an increased Ca loss, which leads to osteopathy.
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