Biomechanical evaluation of retrieved massive allografts: preliminary results.

Allograft bone is the primary source of graft material for large skeletal defects. No study has determined the physical characteristics of such grafts after various periods of time in vivo for incorporation and remodeling. The purpose of this pilot study was to obtain allograft tissue and biomechanically evaluate the tissue to assess allograft bone material properties. The mechanical properties of the retrieved allograft tissue were compared to allograft bone prior to transplantation. Histological analysis of the retrieved allograft tissue is currently underway to correlate degree of incorporation, allograft porosity, and microfracture density with allograft material properties. After allograft retrieval, radiographs were used to plan sectioning for histological and biomechanical analyses. Rectangular sections of uniform dimensions (50 x 3 x 3 mm) were mapped and machined from the bulk specimens. The samples were loaded in bending in the medial to lateral direction using a 4-point bending fixture to obtain flexural elastic modulus and breaking strength. Preconditioning was applied to each specimen by cycling through 5 submaximal loading cycles (maximum deflection = 1% specimen length). After preconditioning, the specimens were loaded to failure at a rate of 1 mm/min. Retrieved specimens consisted of 1 tibia, 2 femurs, and 2 humeri ranging from 2 to 13 years in vivo. Two control tibia specimens were also tested. Assuming that material properties of cortical bone are consistent regardless of skeletal site, the preliminary data indicates that allograft modulus and strength decline with time in vivo. Testing and analysis of more specimens continue in order to corroborate these initial results.

Development of the eye-antenna imaginal disc and morphogenesis of the adult head in Drosophila melanogaster.

We have studied the organization and development of the eye-antenna imaginal disc of Drosophila melanogaster. We examined the pattern of gynandromorph mosaicism and determined the "sturt distances" between 42 different structures of the head, antenna, and maxillary palpus. A morphogenetic map based on these sturt distances resembles more closely in size and shape that of a single thoracic segment than that of two or more adjacent segments, suggesting that the eye-antenna disc is derived from a single embryonic body segment. We examined the morphology of the eye-antenna discs in situ in late third-instar larvae in serial cross sections. The two discs are connected medially by a thin cellular membrane that probably serves to join the two discs during evagination and morphogenesis of the adult head. A fate map of the imaginal disc was established by cutting the mature disc into fragments and transplanting the fragments into host larvae for metamorphosis. The peripodial layer of the eye-antenna disc is thickened in several regions, and our data suggest that these thickened areas represent primordia of adult head structures. A comparison of the location of precursors in the imaginal disc with those of the differentiated structures of the adult head revealed the nature of the morphogenetic movements that must occur during evagination and differentiation.

The maternal and zygotic roles of the gene Polycomb in embryonic determination in Drosophila melanogaster.

The mutation Polycomb (Pc) is known to cause a variety of intersegmental transformations in homozygous and heterozygous individuals of Drosophila melanogaster; Pc+ is thought to act as a negative regulator of genes of the bithorax complex. The function of this gene in the maternal germ line has been assessed by examining the variation in expression of these homoeotic phenotypes in individuals derived from a maternal germ line with a single or no dose of the Pc+ allele. Mosaic individuals with a homozygous or heterozygous Pc germ line were produced by transplantation of pole cells, the embryonic precursors of the germ line. By employing an X-linked dominant female-sterile mutation, the identification of mosaic females and the study of progeny derived from the exogenous germ line were greatly simplified; the advantages of this system for the transplantation of pole cells for such analyses are described. In general, all thoracic and abdominal segments of homozygous Pc embryos differentiate characteristics of the eighth, most posterior, abdominal segment. The extent and uniformity of this transformation as well as other manifestations of the homozygous Pc genotype are described and shown to be correlated with the maternal germ line genotype; homozygous Pc embryos derived from a homozygous Pc maternal germ line show greater expression of these phenotypes than do genetically identical embryos derived from a heterozygous Pc maternal germ line. The expression of some homoeotic phenotypes typical of heterozygous Pc adults shows only a slight correlation with the maternal genotype, while no homoeotic transformations are clearly evident in heterozygous larvae of either origin. Thus, the maternal effect of Pc is rescuable. The results suggest that the Pc+ gene is active in the maternal germ line but that the absence of the maternally derived Pc+ product can be largely compensated by the introduction of a wild-type allele upon fertilization; this rescue indicates that the maternal activity of Pc+ plays no major role in the normal process of embryonic segmental determination. The normal fertility of males and females with a homozygous Pc germ line and of their progeny suggests that Pc+ plays no role in the determination or development of the germ line in either the maternal or zygotic genome.