Histology Mentor

Bone, Questions 29-36

29. Question - Make a cross sectional drawing of compact bone illustrating the lamellar organization and vascular supply and coverings. What is a cement or reversal line?

Cement lines are specialized layers formed at any surface when osteoclasts stopped eroding bone and osteoblasts have laid down new bone. It is also called a reversal line. They surround osteons and separate bone packets in trabecular bone from underlying bone.

30. Question - Diagram the cytology of osteoblasts and osteocytes. What is their source and function?

Osteoblasts have abundant RER, a distinct Golgi but few secretion granules (constitutive secretion). Processes meet osteocyte processes with gap junctions. When surrounded by matrix it is called an osteocyte. The osteoblast is derived from stromal or bone lining cells. Osteocytes have fewer organelles but maintain the bone and provide signals to regulate the surface cells.

31. Question - Diagram the cytology of an osteoclast. Identify three events that must take place for osteoclasts to perform their function. What is the source of osteoclasts?

Osteoclasts differentiate from the monocytic line of blood cells. They must attach and hydrogen ions and lysosomal enzymes must be released through the ruffled border.

32. Question - Illustrate the structure of trabecular bone. Diagram remodeling in trabecular bone. Diagram a remodeling site in compact bone.

33. Question - Illustrate the structure of a synovial joint including the attachments of joint capsule, ligaments and tendons to bone.

34. Question - Using diagrams explain the process of endochondral bone formation.

In this process a cartilage model appears first. This grows longitudinally and in breadth by both interstitial and appositional processes. These processes continue in the epiphyseal disk until the disk "closes".

35. Question - Describe intramembranous bone formation. Identify two bones that are formed in that way. How does a long bone grow in length? In diameter? Differentiate between red and yellow marrow. What other name is given to red marrow?

Intramembranous bone begins to form on a connective tissue surface not an eroded cartilage surface. To begin only bone trabeculae form but as these get thicker the spaces between the trabeculae are obliterated to form compact bone. Often the first bone formed is woven bone but remodeling soon replaces this with lamellar bone. The maxilla, mandible and calvarium form in this way.

Long bones grow by proliferation of chondrocytes and the deposition of cartilage matrix in the epiphyseal disk so that it tends to expand in thickness. Bones grow in diameter by apposition of bone to the outsides of the bone and of cartilage to the outside of the growth plate.

Red marrow, also called myeloid tissue, is characterized by containing obvious islands of proliferating blood cells. Yellow marrow is essentially adipose tissue among bone trabeculae. Although it contains some blood stem cells their presence is not noticeable.

36. Question - Explain the difference between lamellar bone and woven bone. Where are these found? Differentiate between bone modeling and remodeling in both compact and trabecular bone.

In lamellar bone the matrix consists of layers. Within each layer the collagen fibers are parallel with each other. Osteocyte lacunae tend to be arranged between the lamellae. In woven bone there is no orientation of collagen fibers and the osteocyte lacunae are larger, closer together and lack any evidence of order.

Woven bone is first formed in intramembranous bones but most of it is replaced by lamellar bone. Some woven bone remains in the alveolar bone proper surrounding tooth sockets. Lamellar bone is found in most other locations of both compact and trabecular bone.

Modelling is the regulated process of bone erosion and formation that results in the development of the final shape of the bone and orientation of trabeculae. Remodeling is a process that continues throughout life but more rapidly in trabecular than compact bone. It replaces old more highly mineralized bone with new less mineralized bone. It can result in changes of bone shape and trabecular orientation in response to changes of forces applied to the bone.

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