William A Beresford MA, D Phil ©
Professor of Anatomy
Anatomy Department, West Virginia University, Morgantown, USA
l Occurs in only one way - by the appositional or surface-depository
action of osteoblasts; soon accompanied by the selective destructive
action of osteoclasts in a remodelling process, continuously
adapting the growing bone to developing soft tissues and dynamic mechanical
forces, whilst meeting metabolic mineral demands.
2 Growth by remodelling is necessary because no interstitial growth is
possible (except in growth cartilages).
3 Bone formed in the fetus is woven: only later is it mostly replaced
by lamellar bone.
4 Dependent on whether bone is formed de novo in a soft tissue area, or in a
site already taken by an established cartilaginous model, two situations of
bone formation are noted - intramembranous and endochondral.
A INTRAMEMBRANOUS OSSIFICATION
l Seen in the skull vault, facial skeleton, and parts of the clavicle.
2 In one or more ossification centres for a given bone, mesenchymal
cells become osteoblasts and start to lay skeletal claim to territory by
forming branching trabeculae/struts of bone. The initial thin struts
may be called spicules.
3 This trabecular bone becomes denser by widening of the trabeculae, and is
then remodelled externally and internally, e.g., in the skull vault to two
denser plates, tables, with spongy bone - diploë - between
4 The remodelling plates expand from their centres, but during growth remain
separated by CT sutures for better adjustment to the enlarging brain,
eyes, nasal cavities, etc. Skull bones grow by complex interactions and
remodelling patterns that must cope also with, for instance, more teeth in
the older child's jaws and the need for articulating cartilages on the
B PREPUBERTAL LONG BONE
l Diaphysis is the long tubular shaft containing marrow. The dense
bone is the cortex, the marrow constitutes the medulla.
2 Epiphyses lie at each end of the bone. Each has:
... (a) a cap of hyaline articular cartilage over a
... (b) cushioning lattice of secondary-ossification-centre bone;
... (c) this bone on its deeper aspect is fused with an epiphyseal
plate/growth disc of hyaline cartilage.
3 Metaphysis is a lattice of bone trabeculae (primary ossification
bone) with cross-struts, that joins each end of the shaft to an epiphyseal
4 Endosteum lines all internal bony surfaces.
5 Periosteum ensheaths the bone, except for a small circumferential
perichondrium around the epiphyseal plates, and where tendons and ligaments
fasten to the bone. The articulating surfaces are bare.
6 Longitudinal growth, while the bone is under the stresses of use,
is provided for by the interstitial growth of cartilage in the growth plates.
C ENDOCHONDRAL/INTRACARTILAGINOUS OSSIFICATION
l Mesenchymal cells retracting their processes round up to become
chondroblasts, which form a minute hyaline cartilage precursor having
roughly the shape of the eventual bone, e.g., the femur. Other mesenchymal
cells differentiate and make a perichondrium.
2 In the central, shaft, region of the cartilage:
- (a) Cartilage cells hypertrophy.
- (b) Matrix around them becomes basophilic, then calcifies.
- (c) Perichondral cells close by become osteoblasts; i.e., the
perichondrium becomes a periosteum.
- (d) Periosteal bone deposition forms a subperiosteal bony collar or
ring around the centre of the shaft.
- (e) At the same time, through gaps in the bone, osteogenic/vascular buds
invade the calcified cartilage matrix to form the primary ossification
centre of bone forming in, and on the calcified walls of, spaces eroded in
3 Cells and functions of an osteogenic bud are:
(a) macrophages |
(b) chondroclasts |- for selective cartilage erosion
(c) endothelial cells? |
(d) progenitor cells of osteoblasts and osteoclasts/chondroclasts;
(f) marrow cells - to populate intertrabecular spaces;
(g) endothelial cells - to form capillaries and sinusoids.
4 Primary ossification zone establishes itself across the width of the shaft
and starts extending in both directions towards the epiphyses, resulting in
two transverse fronts of ossification across the diaphysis. At each
front is the cartilaginous growth plate.
5 Epiphyseal plate. (This only becomes plate-like after secondary
ossification has started within the epiphysis.) Starting farthest from the
front, the zones are:
- (a) Reserve/quiescent zone of hyaline cartilage with small cells
and few cell divisions.
- (b Proliferative zone where chondrocytes multiply and arrange
themselves in ordered parallel columns (palisades) of disc-shaped cells.
Growth, interstitial, in the long axis of the bone occurs mainly here.
- (c) Hypertrophic/maturing zone has the chondrocytes no longer
dividing, but enlarging at the expense of their matrix.
- (d) Calcification zone, where the matrix stains basophilic
and is impregnated by crystals of calcium salts. Mineralization may be
triggered by the seeding action of calcium-rich matrix vesicles
released from chondrocytes. Whether or not these chondrocytes then
all die is disputed.
- (e) Erosion zone encroaches into the calcification zone. Vascular
bud elements destroy some cartilage, but leave longitudinal spicules
as a scaffold on which bone is laid down. In HE preparations the
calcified cartilage cores are blue, the superficial bone red.
6 Within the cartilage of the young epiphysis, a secondary ossification
centre develops, again by processes of cartilage cell hypertrophy,
matrix calcification, and its erosion by vascular elements penetrating from
the perichondrium. However, orderly columns of chondrocytes and a defined
marrow cavity are lacking.
7 The epiphyseal, secondary, ossification centre spreads to occupy
much of the epiphysis and forms the bony border to the cartilaginous
epiphyseal plate. The cartilage grows (thus lengthening the whole bone)
keeping pace with the front of ossification invading it from the metaphyseal
side, until puberty. Then resorption and ossification slowly overtake
halting chondrocyte proliferation, until the primary ossification front fuses
with the secondary epiphyseal bone - epiphyseal fusion/closure. The
growth plate is obliterated, but an irregularity in the trabecular bone
pattern marks its site.
8 Hyaline cartilage remains as a thin cap over the epiphysis to be the
9 Growth in width of the shaft is by a periosteal deposition
on the outside surface, coordinated with an osteoclastic resorption on
the inner, marrow, aspect. These patterns may be reversed at sites of
change in shape or drift. At the same time, shaft bone is remodelled
internally to be more lamellar and have the layers of Chapter
l The osteoid seam is a very poorly mineralized, narrow zone of
organic matrix seen sometimes with LM between the true bone and active
2 It results from a definite lag between the formation of collagen
fibrils and the later deposition of mineral crystals.
3 The presence of osteoid can be determined for sure by methods, e.g.,
von Kossa's silver, microradiography, EM, which are able to show the absence
of mineral, but certain stains for decalcified sections are reliable.
4 The seam widens significantly in osteomalacia and rickets
when too little Ca2+ is available, e.g., in kidney disease.
E PHYSIOLOGICAL FACTORS AFFECTING CONNECTIVE TISSUES
- Growth: for matrix synthesis, particularly in epiphyseal
cartilage; lack causes dwarfism, excess gigantism, or in
- Parathyroid: acts on osteoblasts to cause osteoclasts to resorb
bone, thus raising blood Ca2+; lack of hormone results in death by tetany;
large eroded spaces from an excess fill with fibrous CT in 'osteitis
fibrosa' - hyperparathyroidism. (There is also the paradox that small
doses of PTH stimulate bone formation.)
- Calcitonin: acts on the 'clast to block bone resorption and lower
blood Ca2+, but its role seems to be minor (except in treating Paget's
disease - uncoordinated 'clasts and 'blasts).
- Thyroid: acts indirectly on all cell activities by controlling
metabolic rate; lack thus slows growth; an excess favours resorption.
- Sex: affect genital tract CT, e.g., endometrial stroma; also
the timing of secondary ossifications and epiphyseal closure (premature
fusion and dwarfing follow an excess of gonadal hormone in childhood).
- Glucocorticoid: excess impairs bone and CT matrix synthesis; used
clinically to reduce inflammation, perhaps by reducing prostaglandins.
- Insulin, Norepinephrine, etc: control fat cell metabolism.
- Parathyroid hormone-related protein/PTHrP is a PTH-like peptide
released from various tumours that causes a cancer-linked destruction of bone,
and hypercalcaemia. (PTHrP acts more in a local - paracrine - manner than the
l Vitamin D: in its active form is needed for Ca2+ to be
absorbed in the gut; low blood Ca2+ from a lack of D prevents
mineralization of growth cartilage matrix, resulting in rickets, and
causes the failure of osteoid to mineralize in osteomalacia; excess
D may raise blood Ca2+ to the point where soft tissue calcifications occur.
2 Prostaglandins: stimulate osteoclastic bone resorption
3 Peptides: although thought of originally in the contexts of
immunity and haemopoiesis, the cytokines (see F below) influence matrix
formation and destruction, and the numbers and activities of all connective
tissue cells, e.g., macrophages stimulate fibrogenesis, and osteoblasts
interact with osteoclasts, etc.
l Calcium, phosphorus: see 2.l above for Ca2+ deficiency.
2 Vitamin A: excess and deficiency disturb ossification and remodelling in
3 Vitamin C: deficiency (scurvy) impairs collagen synthesis in all
4 Copper: needed for making elastin.
5 Toxic elements, e.g., Pb, 90Sr, F, may substitute for the natural
elements and ions in the mineral crystals of bone and teeth.
CTs respond to more use by making a matrix better able to withstand the
greater forces, e.g., osteoblasts build more and wider bone trabeculae;
conversely, disuse leads to the few thin and frail trabeculae of
1 The name does not indicate the only source or action; all cell
types use cytokines for signalling, including neural and epithelial cells.
2 The materials are protein or glycoprotein and express sub-types, e.g.,
acidic and basic fibroblast growth factors (aFGF & bFGF), and control cells by
binding to receptors, but comprise a system separate from hormones,
neurotransmitters, and the eicosanoids (derivatives of arachidonic acid, e.g.,
prostaglandins and leukotrienes).
3 Cytokines' actions are diverse, and not consistently stimulatory or
inhibitory, but depend on the target cell type and the action of other agents.
4 Factors that could qualify, but are already known as hormones, such as
erythropoietin & insulin, are not listed.
5 Cytokines are important for the control of renewing cell populations, in
inflammation and healing (wound and fracture), and the immune responses, and
are used clinically to influence disease or tardiness in these processes.
6 A few cytokines and their actions are listed below, but which cytokine does what
Epidermal growth factor EGF Insulin-like growth factors IGFs
Platelet-derived growth factor PDGF Fibroblast growth factors FGFs
Transforming growth factors TGF-1 Interleukins 1,2,3-- IL-1,2, --IL-10
Tumor necrosis factor-alpha TNF-a Colony-stimulating factors CSFs
Interferons IFs Stem-cell factor SCF
Help to start and/or complete cell proliferation;
Promote or inhibit differentiation;
Activate white blood cells, osteoclasts, etc.;
Raise or lower the rate of synthesis of ECM;
Alter the release of proteases or their inhibitors;
Induce chemotaxis, motility & change of cell shape;
Change sensitivity to other cytokines or hormones;