What does the animal model teach us about the effects of physical activity on growing bone?

Experiments to design physical activity programs that optimize their osteogenic potential are difficult to accomplish in humans. The aim of this article is to review the contributions that animal studies have made to knowledge of the loading conditions that are osteogenic to the skeleton during growth, as well as to consider to what extent animal studies fail to provide valid models of physical activity and skeletal maturation. Controlled loading studies demonstrate that static loads are ineffective, and that bone formation is threshold driven and dependent on strain rate, amplitude, and duration of loading. Only a few loading cycles per session are required, and distributed bouts are more osteogenic than sessions of long duration. Finally, animal models fail to inform us of the most appropriate ways to account for the variations in biological maturation that occur in our studies of children and adolescents, requiring the use of techniques for studying human growth and development.

Observations of spermiogenesis and epididymal sperm maturation in the rufous hare wallaby, Lagorchestes hirsutus (Metatheria, Mammalia)

Acrosomal development in the early spermatid of the rufous hare wallaby shows evidence of formation of an acrosomal granule, similar to that found in eutherian mammals, the Phascolarctidae and Vombatidae. Unlike the other members of the Macropodidae so far examined, the acrosome of this species appears to be fully compacted at spermiation and extends evenly over 90% of the dorsal aspect of the nucleus. During spermiogenesis, the nucleus of the rufous hare wallaby spermatid showed evidence of uneven condensation of chromatin; this may also be related to the appearance of unusual nucleoplasm evaginations from the surface of the fully condensed spermatid. This study was unable to find evidence of the presence of Sertoli cell spurs or nuclear rotation during spermiogenesis in the rufous hare wallaby. The majority of spermatozoa immediately before spermiation had a nucleus that was essentially perpendicular to the long axis of the sperm tail. Nuclei of spermatozoa found in the process of being released or isolated in the lumen of the seminiferous tubule were rotated almost parallel to the long axis of the flagellum; complete parallel alignment occurred during epididymal maturation. At spermiation spermatozoa have characteristically small cytoplasmic remnants compared to those of other macropods. Unlike the majority of macropodid spermatozoa so far described, the spermatozoa of the rufous hare wallaby showed little evidence of morphological change during epididymal transit. There was no formation of a fibre network around the midpiece or of plasma membrane specializations in this region; the only notable change was a distinctive flattening of midpiece mitochondria and scalloping of the anterior mitochondrial sheath to accommodate the sperm head. Preliminary evidence from spermiogenesis and epididymal sperm maturation supports the classification of the rufous hare wallaby as a separate genus but also indicates that its higher taxonomic position may need to be re-evaluated.

Visualisation of macropinosome maturation by the recruitment of sorting nexins

We report that phosphoinositol-binding sorting nexin 5 ( SNX5) associates with newly formed macropinosomes induced by EGF stimulation. We used the recruitment of GFP-SNX5 to macropinosomes to track their maturation. Initially, GFP-SNX5 is sequestered to discrete subdomains of the macropinosome; these subdomains are subsequently incorporated into highly dynamic, often branched, tubular structures. Time-lapse videomicroscopy revealed the highly dynamic extension of SNX5-labelled tubules and their departure from the macropinosome body to follow predefined paths towards the perinuclear region of the cell, before fusing with early endosomal acceptor membranes. The extension and departure of these tubular structures occurs rapidly over 5-10 minutes and is dependent upon intact microtubules. As the tubular structures depart from the macropinosome there is a reduction in the surface area and an increase in tension of the limiting membrane of the macropinosome. In addition to the recruitment of SNX5 to the macropinosome, Rab5, SNX1 and EEA1 are also recruited by newly formed macropinosomes, followed by the accumulation of Rab7. SNX5 forms heterodimers with SNX1 and this interaction is required for endosome association of SNX5. We propose that the departure of SNX5-positive tubules represents a rapid mechanism of recycling components from macropinosomes thereby promoting their maturation into Rab7-positive structures. Collectively these findings provide a detailed real-time characterisation of the maturation process of the macropinocytic endosome.