SECTIONAL TRANSACTIONS.—I. 531 
AFTERNOON. 
Visit to Messrs. Boots, Nottingham. 
Friday, September 8. 
Discussion on Ossification (10.0) :— 
Prof. R. Rosison, F.R.S. 
The development of the calcified animal skeleton may sipponded by 
two routes, one leading primarily to calcified cartilage, the other 
directly to bone. Though histologically distinct these processes manifest 
the closest biochemical relationship. Hypertrophying cartilage cells 
and osteoblasts both synthesise phosphatase; and both, at a certain 
stage in their development, secrete in the intercellular spaces a highly 
specialised ground-substance possessing properties distinct from those 
of the enzyme but also essential for normal calcification. In the animal 
the actual deposition of calcium salt may lag somewhat behind the 
formation of this specialised matrix ; but the presence and extent of the 
latter may be demonstrated by calcification in vitro. ‘Thus it is found that 
in embryonic rabbit femora and tibie the hypertrophic cartilage is fully 
calcifiable, possessing both phosphatase and the second essential mechanism 
(Niven and Robison). In the rudimentary long bones of fowl embryos, 
however, the cartilage hypertrophies and synthesises phosphatase but does 
not at once become calcifiable. The full calcifying mechanisms are subse- 
quently developed in parts of the cartilage, but part becomes eroded without 
acquiring these powers (Fell and Robison). Similar stages in the develop- 
ment of the calcifying mechanisms were noted in cultures of endosteal bone, 
grown in vitro. The area of calcifiable tissue was sharply defined and did 
not include the whole of the osteoid (Fell and Robison). 
Other experimental work (Rosenheim and Robison) has thrown further 
light on the phosphatase and second calcifying mechanisms, but has not yet 
provided an explanation of the latter. The calcification im vitro of other 
tissues, such as kidney and aorta, has also been effected by prolonged im- 
mersion in calcifying solutions (Rosenheim and Robison) ; but these experi- 
ments have emphasised yet more strongly the special properties exhibited 
by the fully developed hypertrophic cartilage and osteoid tissue. 
Dr. Honor B. FELL. 
The classical conception of the osteoblast as a specific bone-forming 
cell has recently been much questioned. It is sometimes held that 
osteoblasts and fibroblasts are identical and that ossification is caused 
by the presence of excess calcium in a young and highly vascular 
connective tissue. This view is not supported by experiments in vitro, in 
which osteoblasts from the embryonic fowl mandible and from the perios- 
teum and endosteum of the developing limb-bones, when isolated from the 
body and cultivated under standard environmental conditions, readily form 
bone in vitro, whilst similar cultures of connective tissue growing under the 
same conditions do not ossify. These results imply that the osteoblast 
possesses inherent bone-forming properties which are lacking in the 
ordinary unmodified fibroblast. 
A close histogenetic relationship exists between the osteoblast and the 
hypertrophic chondroblast, and tissue culture experiments have shown 
that the one type of cell can be directly transformed into the other. 
