Webslide 0301, developing finger, long. sect.
[DigitalScope]
The bones shown in Webslide 301 (distal end of a metacarpal, and the 1st, 2nd, and part of the 3rd phalangeals) are still undergoing the process of endochondral ossification, but there are many general features of long bones that can be appreciated. The shaft of each bone (the diaphysis) consists of a rigid cylinder of 'compact' or 'cortical' bone surrounding a hollow marrow cavity. Within the marrow cavity are hematopoietic cells and some adipocytes amongst spicules of 'trabeculated,' or 'spongy,' bone. As most long bones mature, the hematopoietic cells are replaced by the adipocytes and the marrow transitions from being 'red' (hematopoietic) to 'yellow' (fatty). in addition to the terms 'compact' and 'spongy,' bone can also be classifed as 'lamellar' (mature) or 'woven' (immature). As the name implies, woven bone consits of many osteocytes within an irregularly organized ossified matrix that has a woven appearance. As the bone matures, it becomes organized into lamellar bone that features lamella of sparse osteocytes separated by distinct layers of ossified matrix.

Outside of the cylinder of compact bone is a connective tissue sheath known as the periosteum. The periosteum is further subdivided into an inner, cellular periosteum (close to the bone), which is less dense in nature and made up of fibroblasts and osteoprogenitor cells and an outer, fibrous periosteum made up of dense, irregular connective tissue. The periosteum adheres to the bone by way of Sharpey's fibers that extend from the fibrous layer into the compact bone.

The articular surfaces at the ends of the bones (the epiphyses) are comprised of hyaline cartilage. Active growth plates (or epiphyseal plates) may be observed at the proximal ends of the phalanges and the distal end of the metacarpal bone whereas the articular cartilage at the distal ends of the phalanges exhibits much less proliferation, which is consistent with the general pattern of growth observed these and other long bones.

050_HISTO_40X, fibula, cross sect.
[DigitalScope]
This is a cross section of a fibula from an adolescent primate and therefore exhibits characteristics of more mature bone. The marrow cavity still contains a great deal of hematopoietic cells and would therefore be considered ‘red’ --however, note that there is a significant amount of adipose tissue that will increase over time. The shaft of the fibula consists of compact bone which is organized into inner and outer circumferential lamella, made up of bone laid down in concentric layers. Sandwiched between the inner and outer lamellae are collections of osteons, or Haversian systems. Within the bone, you can see the individual osteocytes within their lacunae and the small canaliculi through which the osteocytes extend processes to communicate with each other. The periosteum is relatively thin and consists primarily of an outer fibrous layer of dense irregular connective tissue. Outside the periosteum are fibers of skeletal muscle associated with the extensor and flexor compartments of the leg.

There is a metaphyseal tumor arising in the distal femur. The tumor has broken through the cortex to involve the soft tissue. In addition, the tumor has extended through the epiphyseal plate. There is a periosteal reaction.

The tumor is composed of sheaths of loosely packed fibroblastic and osteoblastic cells, which have large hyperchromatic nuclei and scanty eosinophilic cytoplasm. In the intercellular spaces of the tumor there is deposition of pink homogeneous osteoid tissue which frequently transforms into irregular, bony trabeculae. New periosteal bone formation may be seen in the section.