<P> Bone marrow, also known as myeloid tissue in red bone marrow, can be found in almost any bone that holds cancellous tissue . In newborns, all such bones are filled exclusively with red marrow or hematopoietic marrow, but as the child ages the hematopoietic fraction decreases in quantity and the fatty / yellow fraction called marrow adipose tissue (MAT) increases in quantity . In adults, red marrow is mostly found in the bone marrow of the femur, the ribs, the vertebrae and pelvic bones . </P> <P> Bone is a metabolically active tissue composed of several types of cells . These cells include osteoblasts, which are involved in the creation and mineralization of bone tissue, osteocytes, and osteoclasts, which are involved in the reabsorption of bone tissue . Osteoblasts and osteocytes are derived from osteoprogenitor cells, but osteoclasts are derived from the same cells that differentiate to form macrophages and monocytes . Within the marrow of the bone there are also hematopoietic stem cells . These cells give rise to other cells, including white blood cells, red blood cells, and platelets . </P> <P> Osteoblasts are mononucleate bone - forming cells . They are located on the surface of osteon seams and make a protein mixture known as osteoid, which mineralizes to become bone . The osteoid seam is a narrow region of newly formed organic matrix, not yet mineralized, located on the surface of a bone . Osteoid is primarily composed of Type I collagen . Osteoblasts also manufacture hormones, such as prostaglandins, to act on the bone itself . The osteoblast creates and repairs new bone by actually building around itself . First, the osteoblast puts up collagen fibers . These collagen fibers are used as a framework for the osteoblasts' work . The osteoblast then deposits calcium phosphate which is hardened by hydroxide and bicarbonate ions . The brand new bone created by the osteoblast is called osteoid . Once the osteoblast is finished working it is actually trapped inside the bone once it hardens . When the osteoblast becomes trapped, it becomes known as an osteocyte . Other osteoblasts remain on the top of the new bone and are used to protect the underlying bone, these become known as lining cells . </P> <P> Osteocytes are mostly inactive osteoblasts . Osteocytes originate from osteoblasts that have migrated into and become trapped and surrounded by bone matrix that they themselves produced . The spaces they occupy are known as lacunae . Osteocytes have many processes that reach out to meet osteoblasts and other osteocytes probably for the purposes of communication . Osteocytes remain in contact with other cells in the bone through gap junctions--coupled cell processes--which pass through small channels in the bone matrix called the canaliculi . </P>

What do you call the young bone-forming cells