Journal of Structural Biology 175 (2011) 451–456 Contents lists available at ScienceDirect Journal of Structural Biology journal homepage: www.elsevier.com/locate/yjsbi The canine baculum: The structure and mechanical properties of an unusual bone ⇑ A. Sharir a, D. Israeli a, J. Milgram a, J.D. Currey b, E. Monsonego-Ornan c, R. Shahar a, a Koret School of Veterinary Medicine, The Robert H. Smith Faculty of Agriculture, Food and Environment, The Hebrew University of Jerusalem, Israel b Department of Biology, University of York, UK c School of Biochemistry and Nutrition, The Robert H. Smith Faculty of Agriculture, Food and Environment, The Hebrew University of Jerusalem, Israel article info abstract Article history: The baculum is an extraskeletal bone located in the penis of a few species in several orders of mammals Received 25 March 2011 such as carnivores, insectivores, rodents, bats and primates. Received in revised form 9 June 2011 This study aims to describe the structure, architecture and mechanical properties of the canine bacu- Accepted 13 June 2011 lum. To this end canine bacula from castrated and uncastrated dogs were collected and examined by light Available online 25 June 2011 microscopy, micro-computed tomography (microCT) scanning, histological staining, and mechanical test- ing. Their mineral density and mechanical properties were compared with those of a typical skeletal bone Keywords: (the radius) in the same dog. Furthermore, a numerical model of a representative baculum was created Baculum and its mechanical performance analyzed using the finite element method, in order to try to elucidate its Mechanical properties Bone mineral density function. Examination of light microscopy images of transverse sections shows that the baculum consists of a typical sandwich structure, with two cortical plates separated, and joined, by loose cancellous bone. MicroCT scans reveal that the mineral density is lower in the baculum than in the radius, both in cas- trated as well as in uncastrated dogs, resulting in much lower stiffness. Castration was found to decrease the mineral density in both the baculum and the radius. The most likely function of the baculum of the dog is to stiffen the penis to assist intromission, and its much lower mineral density compared to that of the radius may be a mechanism designed to decrease the stiffness somewhat, and thus reduce the risk of fracture during copulation. Ó 2011 Elsevier Inc. All rights reserved. 1. Introduction used as a diagnostic taxonomic character; however its structure and material characteristics have not yet been described in detail. The success of the process of copulation in all mammals re- The baculum is a heterotopic (extra-skeletal) bone that is de- quires that the male achieves erection and high flexural stiffness rived from connective tissue. The development of the baculum of his penis. These features are achieved by the synergistic function has been investigated primarily in rats and mice. It has been shown of a high-pressure system (corpus cavernosum) and low pressure that this process consists of two stages. In the first stage mesen- system (corpus spongiosum). However, some mammals, belonging chymal condensation occurs in the penile organ; it is independent to such different orders as carnivores, insectivores, rodents, bats of estrogen, and is observed in both males and females. In the sec- and primates, also have a penile bone – the baculum (os penis) (Ro- ond stage, differentiation into chondrocytes and osteocytes occurs mer, 1970; Williams-Ashman, 1990). This bone is found in the only in males (and incidentally, also in androgen-treated females, glans tissue at the distal end of the penis, dorsal to the urethra. Murakami, 1986). Usually its proximal end abuts the distal end of the corpus cav- The function of the baculum is not well understood and its pres- ernosum. The morphology and size of the bacula in different orders ence in different orders, and only in some species within these or- and species vary greatly, so that its species specific-shape is often ders, is puzzling. Furthermore, the variability in form in the different orders is extreme. Thus in carnivores, for instance, it is present in bears and dogs, but absent in cats. In the dog it serves as a channel for the urethra, in the wolverine it is forked at the Abbreviations: microCT, micro-computed tomography; BMD, bone mineral tip and in the raccoon it is s-shaped and terminates in an enlarged density; GPa, gigapascal. condyle. ⇑ Corresponding author. Address: Laboratory of Bone Biomechanics, Koret School The most obvious biomechanical explanation for the function of of Veterinary Medicine, The Robert H. Smith Faculty of Agriculture, Food and Environment, The Hebrew University of Jerusalem, Israel. Fax: +972 39604079. the baculum is to stiffen the penis, thus assisting intromission. Yet E-mail address: [email protected] (R. Shahar). many species copulate successfully without it. Furthermore, its 1047-8477/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.jsb.2011.06.006 452 A. Sharir et al. / Journal of Structural Biology 175 (2011) 451–456 Fig.1. (a) 3-D drawing of the entire canine baculum derived with permission from Millers Anatomy of the dog (Evans, 1993). (b) 3-D reconstructions (based on microCT scan) of the entire baculum and cross-sectional sections at three different regions: base, mid-region, and tip of the bone. (c) Photograph of a cross section of the baculum at mid-length, showing the cortical and cancellous regions. small size compared to the corpus cavernosum in rats, makes this consent) from eight mature castrated dogs which died at the veter- hypothesis unlikely, at least in that species (Kelly, 2000). Several inary teaching hospital of the Koret school of veterinary medicine other roles have been suggested, such as providing added stimula- from causes unrelated to the skeletal system, and from eight ma- tion to the female, helping the male penetrate the hymen, or even a ture intact dogs which were euthanatized due to population con- means of functional specificity of genital morphology, inhibiting trol regulations at local animal shelters. The bacula were cross-species copulation (Meczynski, 1974; Patterson and Thaeler, prepared in the same way as the bacula in the first part. 1982). In any case, no single interpretation of bacular function seems satisfactory and well-supported (Baryshnikov et al., 2003). 2.2. Light microscopy The canine baculum is long and tapers in the proximodistal direction (see Fig. 1). The proximal end (base of the baculum) lo- The bacula evaluated in this study ranged in length from 83 to cated in the body of the penis just caudal to the bulbus glandis, 128 mm. Each baculum was transversely sectioned by handsaw to is quite broad. It is thicker dorso-ventrally than from side to side. obtain three 20 mm-long sections – from the proximal baculum, The distal end (tip) of the bone is small in diameter and it is ex- mid-baculum and distal baculum. One millimeter-thick slices were tended by a slightly curved fibrocartilagenous projection. The then cut from the centers of these sections by low-speed water- proximal two thirds of the bone are indented ventrally by a distinct cooled diamond saw (Buehler isometÓ, USA). These slices were groove in which the penile urethra and corpus spongiosum reside. ground and polished using a Minimet polisher (Buehler MinimetÒ, During erection, the corpus spongiosum receives the greater part of USA) by successively more refined abrasive paper (from 200 to the blood through the artery of the bulb of the penis. 4000 grit), followed by 3 and 1 lm cloth with diamond paste. This manuscript describes the results of a study of the micro- The polished surfaces were studied using reflected light micros- structure of the dog baculum, the mechanical properties of its copy (Olympus BX-51 microscope) and images were captured material, compares them to those of a typical long bone (radius), using a 12.1 megapixel-resolution dedicated camera attached to and evaluates the effects of early castration on its mineral density the microscope (Olympus DP-71). and distribution. 2.3. Histological staining 2. Methods Samples obtained from three bacula from the first (intact) group 2.1. Samples of dogs were fixed overnight in 4% paraformaldehyde (Sigma Chemical, St. Louis, MO, USA), then in phosphate buffered saline In the first part of this study, bacula were obtained from nine at 4 °C for 3 days, then subjected to 3 weeks of decalcification in mature medium-sized intact (non-castrated) male dogs weighing 0.5 M EDTA, pH 7.4. Following dehydration in graded ethanol solu- 25–35 kg, which were euthanatized due to population control reg- tions and histoclearÓ (National Diagnostics, USA), the tissue was ulations at local animal shelters. Bacula were removed by careful embedded in Paraplast (SPI supplies, USA), cut into 5 lm sections dissection, cleaned of soft tissue, wrapped in saline-soaked gauze and mounted on glass slides (Superfrost, Thermo Scientific). The and stored at À20 °C. In the second part of the study, both the embedded sections were deparaffinized in xylene, washed twice baculum and radius bones were harvested (after receiving owner with ethanol and rehydrated through a graded series of ethanol A. Sharir et al. / Journal of Structural Biology 175 (2011) 451–456 453 Fig.2. (a) Reflected light microscopy image of the cortical region of the baculum at mid-length. Note internal remodeled (secondary osteonal) region and external lamellar region. Scale bar 200 lm. (b) Polarized light microscopy image of the lamellar region, showing the orientation of the collagen fibers. (c). Transmission light microscopy of a transverse section of the mid-baculum, stained with Alcian blue stain. Note the inner, secondary osteonal bone, consisting of concentric lamellae (black arrows).