Background : the calcaneus clearly
demonstrates the characteristics
of
ossification processes that are
found
in both short and long bones.
Cartilage
canals are found in the epiphysis
of long bones and in small
and
irregular bones, but their role in
ossification
process have not been
established.
Aim of the work : to study the developing
calcaneus and to clarify the
role
of cartilage canals in the boneforming
process.
Materials and Methods : Twenty
four human fetuses were used in this
study in 9
th
, 10
th
, 13
th
, 15
th
,
19
th
, 22
nd
and 24
th
weeks of gestation.
Sagittal sections were prepared from the calcaneus and stained with
haematoxylin & Eosin, alcian blue &
periodic acid Schiff reagent and Mallory
stains. Results : In 9
th
week-aged fetus,
the calcaneus showed undifferentiating
cells with no cartilage canals. In
10
th
week-aged fetus, the cartilage
canals began to originate from the
perichondrium. In 13
th
week-aged fetus,
there was an area of proliferated
cartilage
cells which were arranged
in
groups. The cartilage canals comprised
connective tissue, blood vessels
and were surrounded by collagen
fibers and exhibited +ve PAS
reaction.
In 15
th
week-aged fetus,
there was hypertrophied chondrocytes
arranged in small groups. In 17
week-aged fetus the hypertrophied cartilage cells in the center of the
calcaneus were arranged in pairs or
small groups surrounded by expanded
areas of pale basophilic matrix. In
19
th
week-aged fetus the perichondral
ossification center appeared. In
22
nd
week-aged fetus the endochondral
center of ossification appeared
in
the form of ossified matrix around
cartilage
canal with marrow spaces.
Increased
patches of basophilic matrix
around hypertrophied chondrocytes
were seen. In 24
th
week–aged
fetus the perichondral center and the
endochondral center of ossification
became more developed. The primary
center of ossification showed
large
areas of bone formation in the
form
of specules or trabeculation
separated
by areas of marrow spaces
filled with blood cells. The cartilage
canals showed significant progressive
increase in both number
and
size. Cartilage canals showed
thin
discontinuous wall at more advanced
ages of gestation.
Conclusion : Cartilage canals are
involved in the nourishment of the
cartilage cells as well as in the ossification
process. Calcanean endochondral
ossification center was detectable
at 22 nd , 24th weeks of
gestation so, it is recommended to  be used as a mean to evaluate gestational
age in late pregnancy by ultrasonography.