All of the long bones and many others of the body, are preformed embryologically in hyaline
cartilage and then replaced by bone by endochondral ossification. Such a change has begun in the
middle of the shaft of this bone, thanks to the invasion of blood vessels and their accompanying
primitive connective tissue. Pale pink cartilage is seen in the head of the bone. A dark pink
periosteal bone collar has already formed around the middle of the shaft, and ossification is
proceeding toward both ends of the cartilage model. The dark pink strands lying outside the whole
bone are dense collagenous tissue of periosteum (around the bony part) and perichondrium (around
the cartilaginous part).
H & E stain.
Endochondral ossification in greater detail. The cartilage cells (chondrocytes) near the region
of active ossification have enlarged (hypertrophied) and lined up more or less in columns. The
purplish material in the center of the shaft is primitive bone marrow, with reticular cells and
developing blood cells. The vascular elements of the marrow tissue actively invade the cartilage
above, leaving spicules of calcified cartilage, upon which bony matrix will be deposited. The dark
pink spicules here are made of bone; the paler pink, small spicules at the leading edge of the
cartilage are made of calcified cartilage.
Endochondral ossification in Mallory stain. Cartilage is light blue and bone is dark blue. A thin
layer of bone has already been laid down on the surface of the cartilage spicules along the leading
edge of cartilage. Blood cells in the marrow cavity are red. The very dark blue at the lower left
and right is spongy bone of the periosteal bone collar of the shaft. This will later be remodeled
into Haversian systems of compact bone.
Head of fetal bone still made of hyaline cartilage. Near the point where ossification is going on
(upper right corner) the cartilage cells become larger and the cartilage matrix becomes calcified
(purple instead of pale pink here, as stained in H & E). A small amount of dark pink bone has been
laid down on the surface of the calcified cartilage. Later on, a secondary center of ossification
will form in the head of the bone, and the cartilage that remains between the two centers of
ossification will be the epiphyseal plate for growth of the bone in length.
Region of ossification at higher magnification -- same stain as previous slide. Chondrocytes are
hypertrophied, degenerating, and lined up in columns at the right. As the marrow tissue invades the
cell columns, spicules of cartilage will be left. The cartilage matrix is calcified (purple), and
one small area of bone deposition, has begun on it (the red color at the upper right). The small
cells caught in the red matrix are osteocytes.
Another detail of ossification. Calcified cartilage spicules are purple-blue; bone deposits are
purple-red. Gradually the cartilaginous portions will be resorbed as the bone is constantly
reshaped, until finally there will be no trace of cartilage left. The main purpose of the cartilage
in the first place was to provide a framework upon which bone deposition could begin.
Spicules showing early endochondral ossification. In H & E stain, the centers of the spicules show
the purple of calcified cartilage; the edges are pink because of the bony matrix laid down upon the
cartilage.
Spicules of spongy bone (bright red) surrounded by a whole line-up of osteoblasts. The osteoblasts
that have previously been trapped in their own salt deposits now lie in lacunae within the spicule
and are called osteocytes. The cells of the primitive bone marrow lie between bone spicules.
Detail of bony spicule with typically acidophilic (pink in H & E) matrix. Osteoblasts are lined up
along its borders, depositing another layer of matrix. Osteocytes lie within lacunae in the spicule.
EM of active osteoblast laying down the fibers and salts of bone. The cytoplasm of the cell is to
the left and contains lots of rough endoplasmic reticulum and many mitochondria. In the lower right
corner is mineralized bony matrix containing the typical black CaPO4 (apatite) crystals. Between
this matrix and the osteoblast lies a pale area of newly secreted pre-bone (or osteoid) which
contains collagen fibrils (note their cross-striations) lying in an as yet unmineralized ground
substance.
The large central space is a resorption area where young compact bone is being actively remodeled.
This is an area where osteoclasts are resorbing the bony substance; notice to large multinucleated
osteoclasts toward the left of the cavity next to the intact Haversian system that lies in the upper
left corner of the field. Later, osteoblasts will differentiate from the primitive reticular tissue
in the resorption cavity and will begin to lay down new bony lamellae around the edge of the cavity.
As successive lamellae are laid down, the cavity will gradually grow smaller, until eventually a new
Haversian system with a narrow central canal will be formed.
Detail of an osteoclast, a giant, multinucleated cell associated with bone resorption. The shallow
bay in which it lies is a Howship's lacuna. The osteoclast is now considered to develop from a
separate stem cell in the bone marrow.
EM of an osteoclast, with its ruffled border next to the area where bony matrix is being resorbed.
The net effect of a ruffled border is to increase the cell surface area for contact with the collagen
fibrils and apatite crystals being resorbed.
Haversian systems of decalcified compact bone, mostly cut in cross section here. The one channel
cut longitudinally is a Volkmann's canal; these channels run perpendicular to both the long axis of
the bone and the central canals of the Haversian systems.
Decalcified bone with Haversian system cut longitudinally. Notice the central blood vessel and the
many concentric bony lamellae around it. As always, osteocytes are trapped in their lacunae.
End of a young long bone, with the pale epiphyseal plate lying between the primary ossification
center of the shaft and the secondary ossification center of the head. The plate and the pale area
continuous with it, up over the head, are composed of hyaline cartilage. Active ossification is
going on along the lower edge of the epiphyseal plate, allowing growth in length of the bone. There
is also active ossification along the lower edge of the cartilage that surrounds the head, thus
allowing for growth in size of the head of the bone. Bony spicules are seen throughout the centers
of ossification, making areas of spongy bone with red marrow between the spicules.
Diagram of a cross-cut chunk of wall of the shaft of a long bone. Most of the substance is compact
bone, with Haversian systems cut cross-wise on the uppermost surface and longitudinally on the
right-side surface. Volkmann's canals carry blood vessels from the inner and outer bone surfaces to
the vessels of the Haversian canals. The lamellae of the Haversian systems are pulled out here so
that you can see the lamellar rings. External (or periosteal) circumferential lamellae are seen
surrounding the whole bone. Internal (or endosteal) lamellae line the inner surface next to the
marrow cavity (to the left). Notice that the inner, endosteal wall bears many spicules of spongy
(cancellous, trabecular) bone. The dense collagenous connective tissue coat (the periosteum) looks
dark here and surrounds the whole shaft.
