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Slide #DMS 069 [Fetal bone]. Referring to the diagram (below), first identify the epiphysis, metaphysis, and diaphysis regions of the bone. Note the hyaline cartilage covering the articular (free) surface of the epiphysis. Cartilage will persist here into adulthood. Note also a portion of the attached ligament of the head of the femur.

Beneath the articular cartilage is seen the cancellous bone of the epiphysis that has formed from the "secondary" or "epiphyseal" ossification center. Much of this bone is still immature (woven bone), but some mature (lamellar) bone may be seen. How may these two types of bone be distinguished? Osteoblasts, contributing to well-defined endosteal layers, as well as numerous osteoclasts may been seen in this section.

At the margins and near the epiphyseal disk, cancellous bone is being converted to denser, compact bone. The histological "snapshots" provided by this slide offer an excellent opportunity to study the development of the earliest Haversian systems (osteons) typical of this type of bone.

Look next at the epiphyseal disk. Note its intimate union with the bone of the epiphysis and the now only very narrow zone of reserve cartilage. What is meant by "closure" of the epiphysis? Identify, as in the previous slide, the zones of proliferating, maturing, and calcifying cartilage and their junction with the metaphysis.

In the metaphyseal region, note again the remnants of calcified cartilage matrix upon which bone is being deposited by osteoblasts. Also identify some of the many osteoclasts. Can you define any resorption bays (Howship's lacunae)? Understand the process of bone resorption occurring in the resorption bay.

NOTE: The terms spicule and trabecula are interchangeable.

This is a low power view of the proximal end of a young femur. Indicated are the ligament of the head of the femur (L), the articular cartilage (AC), the epiphysis (Ep), the epiphyseal growth plate (GP) and the proximal metaphysis (M).

This low power image shows the proximal-most portion of the femur with the articular cartilage (AC) indicated. Most of the rest of the epiphysis has undergone endochondral ossification, with just residual calcified cartilage matrix remaining as an indicator of that process. Both immature woven bone and mature lamellar bone may be found as seen in subsequent images.

This is a medium power view of woven (immature, primary) bone. Note the fairly random distribution (i.e. non-lamellar) of osteocytes (compared to the next slide). In this specimen, the endosteum is seen to much better effect than in previous images.

This medium power image is an example of some of the lamellar (mature, secondary) bone that may be observed. Note the ordered disposition of osteocytes embedded in a more orderly matrix.

This specimen also provides some snapshots of the process of compaction, the filling in of the spaces between bony trabeculae in regions where the adult bone is destined become compact bone. The next slide will focus in on the region indicated by the arrow.

All bone, when first formed during development, is laid down as woven trabecular bone. If the bone is destined to become compact bone, then additional bone deposition occurs amongst the spaces between the trabeculae. This process is called compaction, and would be occurring in the image seen here. This compaction results in the formation of primary osteons, a process seen to better effect in the next image.

In this image of 'compacting' bone, one can see the roughly concentric organization of osteocytes around a central space, the future haversion canal. While these developing primary osteons are clearly reminiscent of the adult form, the primary osteons are comprised of poor-quality woven bone and will subsequently be replaced by mature lamellar bone also organized into concentric lamellae.

View the growth plate at medium power and identify the epiphyseal bone, the epiphyseal cartilage (growth plate), and the newly forming metaphyseal bone. The various zones of cartilage described in previous slides are still present but somewhat condensed at this late stage of development.

At medium power, observe the very thin zone of reserve cartilage. When this population of cells is exhausted and replaced by bone tissue, the bone can no longer growth in length at this site. This is known as closure of the epiphysis.

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