Slides 27 - Fetal Limb and 26 - Fetal Finger show one or more developing long bones cut in longitudinal section.  Study whichever of these two slides is in your box; then take a look at your neighbor’s slide for purposes of comparison.  With the aid of textbook or atlas illustrations, find examples on your slide of the following:

            1.     Perichondrium surrounding hyaline cartilage; in the more advanced bones,this becomes restricted toward the head (epiphyses), and is absent from the articular surface.

            2.     Periosteum surrounding shaft (diaphysis) of bone.

            3.     Periosteal bone collar around shaft, where ossification has already taken place.  This lies right under the periosteum.  Bone stains pink in H&E.

            4.    Primitive vascular and connective tissue lying within marrow cavity.

            5.   Growing hyaline cartilage with small chondrocytes, in the epiphysis.  Cartilage matrix stains blue/lavender.   

            6.   Zonation of the cartilage cells (epiphyseal growth plate) in the cartilage adjacent to the trabecular bone of the metaphysis:

           a.   zone of resting cells

           b.   zone of proliferating cells

           c.   zone of hypertrophic cells

            7.   Formation of spicules of calcified cartilage as vascular tissue invades the lower rows of hypertrophic chondrocytes.  (Note that calcified cartilage is different from, and precedes, bone.  It is more basophilic than ordinary cartilage).

            8.   Lining up of osteoblasts along the edges of spicules as they deposit bone on the spicultes.  Note that the osteoblasts become confined in lacunae, thus becoming the osteocytes of spongy bone.  (Not every spicule is covered with osteoblasts; you will have to search).  The first bone matrix laid down is unmineralized and is called osteoid .

            9.   Spicules of spongy bone in the shaft.  Bony matrix stains pink; often a spicule still contains a core of calcified cartilage (lavender).  As spicules are resorbed with growth, and new spicules laid down, the cartilage content gradually disappears from a bone.

            10.  The presence of multi-nucleated, giant cells, the osteoclasts, in depressions (Howship’s lacunae) along spicules of spongy bone.  These cells are active in the resorption of bone as part of the continual remodeling of this tissue.  Resorption of the inner surfaces of the bone (around the marrow cavity) is accompanied by further deposition at the periphery (under the periosteum).

Look at various pieces of cartilage in your section for evidence of greater or lesser degrees of ossification.  In some there may simply be enlarged chondrocytes in the center of the shaft, before the invasion of blood vessels and the conversion to osteoblastic tissue occurs.

How is growth in width of a long bone achieved?  How is growth in length of a long bone achieved?  What is the significance of the epiphyseal growth plate?  What is its eventual fate?  How old are you when your last epiphyseal growth plates disappear (growth plate closure)?

            NOTE:  The terms spicule and trabecula are interchangeable.