Changes in material and architectural properties of rat femoral diaphysis during ontogeny in hypophysectomized rats

Adenohypophyseal hormones are important for normal growth and maintenance of skeletal mass. From a mechanical point of view, the skeleton can be viewed as a biomechanically regulated structure, the strength of which depends on the intrinsic stiffness (material properties) and the spatial distribution (architectural properties) of the mineralized tissue. Both material and architectural properties of bone change during ontogeny. Both muscle strength and body weight, the main two types of load producing strain, remain practically unchanged in a young rat after hypophysectomy (HX). Therefore, this study was performed to analyse the effect of HX on bone development in a model in which the main loads on bone are almost constant from infancy to adulthood. Female rats were HX at 30 days of age. A basal control group (BC) was sacrificed on the same day. An age-matched control group (CON) was also included. HX and CON rats were sacrificed 5 months later. Mechanical properties of the right femur were determined using a three-point bending mechanical test. HX rats failed to gain body weight and body and femur length. Femur weight was about 1.09 times increased and femoral calcium content (left femur) was 2.25 times greater in HX than in BC rats. The load-capacity extrinsic properties (stiffness, elastic limit, and ultimate load) were 3.47, 3.42, and 3.11 times increased, respectively, in HX rats in relation to BC ones. Focussing on the intrinsic material properties, the cortical elastic modulus (E) was 10.73 and 2.54 times higher in HX rats than in BC and CON rats, respectively. In summary, HX in the rat induced the transformation of the juvenile femur to a type of adult bone characterised by its high mineral density and its unusual large modulus of elasticity in spite of cessation of lineal growth. These effects are probably due to secondary hypothyroidism induced by TSH deficiency and associated to increased collagen maturation and cross-linking, and degree of bone calcification.