Recent studies have challenged the traditional view that all tendons and ligaments are alike. It is postulated that tendons vary in structure and biochemical composition. Moreover, each tendon shows regional hcterogenicity along its Icngth
(Harwood and Arniel, 1992; Vogel et aI., 1993). The study of the detailed structure of tendons has gained special attention as it has important clinical implications. Tendons may need to be carefully investigated and selected for particular surgical transfers and joint reconstructions (Benjamin et al., 1995).
The Achilles tendon is of particular interest. since it is the strongest tendon in the human body with a tensile length of 50 - lOON I rnm. It is formed from the joining of the two tendons of soleus and gastrocnemius (Williams et al., 1995). Despite of its strength, the Achilles tendon is the most frequently ruptured tendon in the human body. Tendon fibers begin to disrupt after length increase of 3 - 4% and rupture after an increase of 8%. Rupture is common in males in the fourth decade. it usually occurs between 2 - 6 em from its insertion into the calcaneus. The greatest risk of rupture is when the tendon is obliquely loaded and the muscle is contracting maximally, as a result of pushing off with feet against resistance (Cetti et al, 1993 & Maccellan and Maffulli, 2002).
Moreover, rupture is a common sequence of steroid therapy (Hersh and Heath, 2002), quinolone antibiotic therapy (Poon and Sundaram, 1997) levoflaxacin and other fluoroquinoiones (Mathi et aI., 2003). Rupture of the Achilles tendon
has been also noticed after renal transplantation (Hestin et al., 1993) and in cases of chronic obstructive pulmonary diseases (Khurana et al., 2002). Furthermore, in-  creased activity and velocity sports might increase the incidence of rupture (Mazzone and McCue, 2002). The exact etiopathogenesis of Achilles tendon rupture is unknown, but it isclaimed that both ruptured and tendinopathic tendons show evidences of histological degeneration compared to the normal tendons (Tallon et aI., 2001). Understanding the anatomy and histology of the ruptured Achilles tendon and contiguous structures is essential in the diagnosis and treatment of Achilles tendon injuries (Schepsis et al.,2002). Thus the purpose of the present study was to determine the detailed ultrastructure of the Achilles tendon in rat. Moreover, it aimed at illustrating and comparing the degenerative changes resulting from both aging and following steroid administration with the degenerative changes in the experimentally ruptured tendon.