Final answer:
The osteoid rim is part of the initial bone tissue that osteoblasts create, which later becomes mineralized to form mature bone. These osteoblasts are located at the secretory ends, where they function in bone formation and growth.
Step-by-step explanation:
The osteoid rim is adjacent to the secretory ends of osteoblasts. These osteoblasts are responsible for the synthesis and secretion of osteoid, which is the unmineralized, organic portion of the bone matrix that forms prior to the maturation of bone tissue. Osteoblasts deposit osteoid around the capillaries in trabecular bone. When osteoblasts become embedded in the osteoid they secrete, they transition into osteocytes. The periosteum is formed by osteoblasts on the surface of the spongy bone which then lays down compact bone superficially. Meanwhile, the medullary cavity is lined by the endosteum, which also plays a role in bone growth and repair.