Bone refilling in cortical bone multicellular units: Insights into tetracycline double labelling from a computational model

Bone remodelling is carried out by `bone multicellular units' (BMUs) in which active osteoclasts and active osteoblasts are spatially and temporally coupled. The refilling of new bone by osteoblasts towards the back of the BMU occurs at a rate that depends both on the number of osteoblasts and on their secretory activity. In cortical bone, a linear phenomenological relationship between matrix apposition rate (MAR) and BMU cavity radius is found experimentally. How this relationship emerges from the combination of complex, nonlinear regulations of osteoblast number and secretory activity is unknown. Here, we extend our previous mathematical model of cell development within a single BMU to investigate how osteoblast number and osteoblast secretory activity vary along the BMU's closing cone. MARs predicted by the model are compared with data from tetracycline double labelling experiments. We find that the linear phenomenological relationship observed in these experiments between MAR and BMU cavity radius holds for most of the refilling phase simulated by our model, but not near the start and end of refilling. This suggests that at a particular bone site undergoing remodelling, bone formation starts and ends rapidly. Our model also suggests that part of the observed cross-sectional variability in tetracycline data may be due to different bone sites being refilled by BMUs at different stages of their lifetime. The different stages of a BMU's lifetime depend on whether the cell populations within the BMU are still developing or have reached a quasi-steady state while travelling through bone. We find that due to their longer lifespan, active osteoblasts reach a quasi-steady distribution more slowly than active osteoclasts. We suggest that this fact may locally enlarge the Haversian canal diameter (due to a local lack of osteoblasts compared to osteoclasts) near the BMU's point of origin.Comment: 16 pages, 6 figures, 3 tables. V3: minor changes: added 2 paragraphs (BMU cavity in Section 2 and Model Robustness in Section 4), references [52,54

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Changes of land use on peatlands in Poznań region

Celem badań było ustalenie zmian w sposobie użytkowania torfowisk Wielkopolski. Badania, wykonane w 2002 r., prowadzono na 124 torfowiskach o łącznej powierzchni ok. 4000 ha. Prace realizowano za pomocą badań terenowych oraz interpretacji zdjęć lotniczych i satelitarnych, stosując programy komputerowe do prac graficznych w celu określenia obszaru zajmowanego przez poszczególne użytki. Wydzielano łąki jedno- i dwukośne, łąki niekoszone, łąki zakrzewione, ekstensywne pastwiska, szuwary trzcinowe, wody stojące, grunty orne oraz lasy i zadrzewienia. Potwierdzono znaczne zmiany w sposobie użytkowania obiektów w stosunku do stanu z lat: 1957-1969, określonego na podstawie dokumentacji geologicznej torfowisk, 1993-1998 na podstawie zdjęć lotniczych oraz 1989-1991 na podstawie zdjęć satelitarnych. Przy opracowaniu wniosków, obok omawianych badań, wykorzystano wcześniej publikowane wyniki badań własnych [Restoration ..., 2002]. Ustalono, iż na torfowiskach dużych (śr. 2 524 ha) łąki niekoszone i częściowo zakrzewione zajmują 23,3% ich powierzchni na obiektach średnich (śr. 34 ha) - 10,2% i małych (śr. 8,4 ha) - 10,7%. Udział powierzchni gruntów ornych w tych grupach torfowisk odpowiednio: 2,4, 6,7 i 18%. W dolinach rzek Cybina i Główna zmiany sposobu użytkowania są największe. Wynikają one z wyłączenia z użytkowania dużej powierzchni podmokłych łąk, pokrytych obecnie szuwarami trzcinowymi i rzadkimi zadrzewieniami, oraz z budowy dużego zbiornika retencyjnego. Wykazano małą przydatność archiwalnych zdjęć lotniczych i satelitarnych do określenia aktualnego sposobu użytkowania torfowisk.The objective of studies carried out in 2002 was to determine changes in the peatland use in Poznań region. The studies included 124 peatlands covering a total area of 4000 ha. The work was accomplished by field studies and by interpretation of aerial and satellite images using graphical computer programmes to determine particular areas. The following land use types were distinguished: meadows mown once, meadows mown twice, not mown meadows, meadows with shrubs, extensive pastures, reed rushes, stagnant waters, arable lands, forests and thickets. Significant changes in the land use were found when geological documentation of peatlands made in the years 1957-1969, aerial photographs (1993-1998) and satellite images (1989-1991) were compared. Apart from the mentioned studies, earlier publications were used (Ilnicki, 2002) when formulating conclusions . It was found that on large peatlands (mean area 2524 ha), not mown meadows and meadows partially covered by shrubs occupied 23.3 % of the total area; in medium size objects (34 ha) and in small peatlands (8.4 ha) they covered 10.2 and 10.7 %, respectively. The share of arable land increased from 2.4 to 6.7 and 18 %, respectively. In the valleys of the Cybina and Główna rivers, the changes in land use were the greatest. They resulted from abandoned land use on large areas of wet meadows, currently covered by reed rushes and thickets and from the construction of a large retention reservoir. The archival aerial and satellite photographs proved to be of little value for the determination of peatland use