Observations on the Water Content of Bone.

In relation to its water content whole mammalian bone may be considered conveniently as two divisions. The lacunae, the canaliculi and the vascular spaces are unossified, and have been described collectively by Robinson (1960) as the marrow-vascular-osteocyte or M.V.O. space. The lacunae and canaliculi account, fairly constantly, for about one.tenth of the total volume of any region of bone (Robinson 1960), but the vascular spaces vary widely in size from the large cavities characteristic of developing and of osteoporotic bone to the narrow tubular spaces of mature bone. About nine-tenths of the total volume of the marrow-vascularosteocyte space consists of water, and it is probable that this figure is independent of both the character and age of the bone. The second division of whole bone is the calcified tissue which is located around and between the elements of the marrow-vascular-osteocyte space, and which will be referred to here as bone tissue proper. In the simplest chemical terms it consists of inorganic, organic and water fractions, the latter being divisible into three parts, namely the water of inorganic crystallisation, collagen-bound water, and free water. Bone tissue proper is seldom homogeneous : indeed, because of the nature of both primary osteogenesis and secondary resorption and replacement, most fields are aggregates of dissimilar microscopic regions in which the three chemical fractions are combined in different proportions. The demonstration of regional differences in the concentration of inorganic material in bone tissue proper by microradiography is now commonplace, and much is already understood of the relationship of such differences to the process of osteogenesis and the age of the tissue. Regional variations in the amount of organic material in bone tissue proper may be studied at the microscopic level by various methods including that of carbonisation (Smith 1963). The water fraction of bone tissue proper has always been the most difficult to assess. Even the determination of the water content in a macroscopic region is beset with difficulties : the proportion of bone tissue proper in the block of whole bone must be determined, and the evaporation of water from the tissue and the absorption of water from the atmosphere have to be controlled throughout the preparation of the block and subsequent procedures. Even then the method gives no information of any differences which may exist in the water contents of the dissimilar microscopic regions of which the macroscopic block is composed. This paper describes a method which, though it fails to measure the absolute amount of water in the tissue, does give a useful indication of the relative water contents in both the dissimilar microscopic subdivisions of bone tissue proper and the various elements of the marrowvascular-osteocyte space. The information obtained by this method has been correlated with that derived from microradiographs and carbonised preparations.