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In osteology, the osteon or haversian system (/ h ə ˈ v ɜːr. ʒ ən /; named for Clopton Havers) is the fundamental functional unit of much compact bone. Osteons are roughly cylindrical structures that are typically between 0.25 mm and 0.35 mm in diameter . [ 1 ]
Haversian canals [i] (sometimes canals of Havers, osteonic canals or central canals) are a series of microscopic tubes in the outermost region of bone called cortical bone. They allow blood vessels and nerves to travel through them to supply the osteocytes .
Stromal cell-derived factor 1 (SDF-1) and CXCR4 mediate recruitment of mesenchymal stem cells. IL-1 and IL-6 are the most important cytokines for bone healing. IL-1 promotes formation of callus and of blood vessels. IL-6 promotes differentiation of osteoblasts and osteoclasts. [4] All cells within the blood clot degenerate and die.
Volkmann's canals, also known as perforating holes or channels, are anatomic arrangements in cortical bones that allow blood vessels to enter the bones from periosteum.They interconnect the Haversian canals (running inside osteons) with each other and the periosteum.
The AP U.S. History course is designed to provide the same level of content and instruction that students would face in a freshman-level college survey class. It generally uses a college-level textbook as the foundation for the course and covers nine periods of U.S. history, spanning from the pre-Columbian era to the present day.
During the American Civil War and afterwards, congressional Radical Republicans feuded with Conservative Republicans who generally opposed efforts by Radical Republicans to rebuild the Southern U.S. under an economically mobile, free-market system [25] and thrived politically on antipathy towards civil rights and black suffrage, [26] and with Moderate Republicans, who were less enthusiastic ...
The First Party System was the political party system in the United States between roughly 1792 and 1824. [1] It featured two national parties competing for control of the presidency, Congress, and the states: the Federalist Party, created largely by Alexander Hamilton, and the rival Jeffersonian Democratic-Republican Party, formed by Thomas Jefferson and James Madison, usually called at the ...
The other form, known as the gang system, was harsher. [1] The gang system forced the enslaved people to work until the planter said they were finished and allowed them almost no freedom. Whether planters organized their enslaved people on the task or gang system had much to do with the type of crops they harvested.