JOURNAL OF MORPHOLOGY 244:69–77 (2000) Anatomy of the Baculum–Corpus Cavernosum Interface in the Norway Rat (Rattus norvegicus), and Implications for Force Transfer During Copulation D.A. Kelly* Department of Biomedical Sciences, College of Veterinary Medicine, Cornell University, Ithaca, New York ABSTRACT The baculum is a nonappendicular bone from the distal end of the glans to the tensile wall of the found in the glans tissue of members of five orders of corpus cavernosum. Forces on the distal end of the penis mammals. Its function during copulation is unknown. An- during copulation press the baculum against the corpus atomical examination of the baculum and corpus caverno- cavernosum, reducing its internal volume and increasing sum in the Norway rat (Rattus norvegicus) shows that the intracavernosal pressure and corpus cavernosum wall two structures are connected by a layer of fibrocartilage, strains. Because the wall of the erect corpus cavernosum and that the distal tip of the corpus cavernosum swells is reinforced with inextensible collagen fibers, an increase during erection to surround the proximal end of the bac- in wall strain will also increase wall tissue stiffness, and ulum. Microradiographs of bacula from sexually experi- thereby increase the flexural stiffness of the corpus cav- enced males show that regions of the bone may be remod- ernosum. J. Morphol. 244:69–77, 2000. eling; these data suggest that the baculum is load-bearing. © 2000 Wiley-Liss, Inc. On the basis of this anatomy, I propose that the baculum increases the overall flexural stiffness of the penis during copulation by transferring bending and compressive forces KEY WORDS: penis; baculum; copulation The penises of all male therian mammals are sup- Some authors have suggested that the baculum is ported during copulation by a pair of hydrostatic an ossification of the distal end of the corpus caver- skeletal elements: the corpus cavernosum and the nosum (Schummer et al., 1979; Smith and Madkour, corpus spongiosum. The corpus cavernosum is a 1980; Evans, 1993), but recent work on rodents has high-pressure system (Beckett et al., 1972, 1974; shown that the bone has a more complex origin. The Purohit and Beckett, 1976; Hanyu et al., 1987) de- baculum and the corpus cavernosum originate from signed to increase the flexural stiffness of the penis the same mesenchymal mass in the embryonic penis during erection (Kelly, 1999) and to maintain its in both mice (Glucksmann et al., 1976) and rats shape during copulation (Kelly, 1997). The corpus (Murakami and Mizuno, 1984; Vilmann and Vil- spongiosum is a low-pressure system (Purohit and mann, 1986), but the two structures develop sepa- Beckett, 1976) that inflates the glans during erec- rately. The rat (Rattus norvegicus) baculum has tion and supports its tissue during copulation three developmentally distinct parts: a proximal (Schummer et al., 1979; Williams-Ashman and portion derived from cartilage, a central portion de- Reddi, 1991; Holmes et al., 1991). But some mam- rived from membranous bone, and a distal portion mals (specifically, some carnivores, insectivores, ro- made up of nonlamellar bone. The proximal and dents, bats, and primates) have an additional skel- central portions fuse and begin to ossify within five etal element in their penises (Romer, 1970; days of birth, but the distal portion does not ossify Williams-Ashman, 1990). The baculum, or os penis, until puberty (Ruth, 1934; Murakami and Mizuno, is a bone found in the glans tissue at the distal end 1984, 1986; Williams-Ashman and Reddi, 1991). In- of the penis, dorsal to the urethra, with its proximal creases in baculum length at puberty are derived end abutting the distal end of the corpus caverno- from an endochondral growth plate located at the sum (Vilmann and Vilmann, 1979; Williams- proximal end of the bone, against the distal end of Ashman, 1990). Although the species-specific shape of the baculum has long been used as a diagnostic taxonomic character (see, for example, Burt, 1936; Wade and Gilbert, 1940; Hooper and Hart, 1962; Contract grant sponsor: NIH NRSA; Contract grant number 5 F32 Long, 1969; Patterson, 1980; Mondolphi, 1983; Thor- DK09605-03. ington et al., 1996), little is known about either its *Correspondence to: D.A. Kelly, Section of Neurobiology and Be- detailed anatomical relationship to other penile ele- havior, 1140 Comstock Hall, Cornell University, Ithaca, NY 14853. ments or its functional significance. E-mail: [email protected] © 2000 WILEY-LISS, INC. 70 D.A. KELLY the corpus cavernosum (Ruth, 1934; Vilmann and tion could be any one or a combination of those Vilmann, 1986). Postnatal baculum growth and os- proposed. sification are highly androgen-dependent; the bone A detailed knowledge of the baculum’s anatomical does not develop its adult form without induction by relationship to other penile structures is required to testosterone (Lyons et al., 1950; Glucksmann et al., test hypotheses of baculum function during copula- 1976; Murakami, 1986). For a more complete review tion. As the first part of a biomechanical study of of baculum development, see Edwards (1997). baculum–corpus cavernosum interaction during In adult rodents, the baculum is embedded in the copulation, I performed a morphological study of the glans tissue, and seems to be anchored to the distal interfaces of the baculum with the corpus caverno- end of the corpus cavernosum (Vilmann and Vil- sum and corpus spongiosum in the penis of the Nor- mann, 1979; Sportono, 1992). But although several way rat (Rattus norvegicus). small ligaments attach the baculum to folds in the glans tissue surrounding the urethra (Sportono, MATERIALS AND METHODS 1992), no muscles or ligaments connecting the prox- imal end of the baculum to the corpus cavernosum Twenty-one fresh-frozen sexually mature male have been described. The corpus spongiosum and Norway rats (Rattus norvegicus) were obtained from corpus cavernosum are held together by a thin layer a supplier (Southwest Rodents, Tucson, AZ). Rats of connective tissue and skin, but these tissues are ranged in mass from 181–603 g; each rat was highly elastic and mobile and do not provide much thawed for 4 h before its penis was used for study. mechanical support during erection. At present, the The length, dorsoventral diameter, and lateral di- anatomical position of the baculum in relation to the ameter of each penis and its baculum were mea- erect corpus cavernosum and the nature of its at- sured with a pair of digital calipers (Mitutoyo) to a tachment are unknown. precision of 0.01 mm. For each measurement, the The functional relevance of a free-floating bone in penis was first removed from the rat by dissecting the penis has been debated for some time. The pos- proximally to the penis’ ischial attachments and sibility that the bone is a pleiotrophic artifact of carefully separating the penile crurae from the bone. The intact, isolated penis was placed on a flat sur- penile development is disputed by Patterson and face and the calipers were closed on either side of a Thaeler’s (1982) morphometric analysis of baculum pair of landmarks until they just touched the tissue. variability; their data suggest that the baculum is External penile length was defined as the distance subject to direct selection and therefore has an adap- between the distal opening of the urethra and the tive, functional role in copulation. This hypothesis is proximal split of the crura of the corpus cavernosum. supported by anatomical data from studies of mu- External dorsoventral and lateral diameters of the tant Hoxd-13 mice: Hoxd-13 is expressed in the de- soft portion of the penis were measured near the veloping penis but not in germ cells or gonads, and midpoint of the corpus cavernosum, approximately 5 Hoxd-13 mutant males have anatomically normal mm proximal to the bacular attachment. External corpora cavernosa but malformed bacula. Mutant baculum length was defined as the distance between females are fertile, but mutant males cannot father the distal and proximal ends of the baculum, both of offspring, although they copulate normally (Dolle´et which can be felt through the surrounding tissue; al., 1993). These data suggest that the successful external dorsoventral and lateral diameters of the impregnation of a female requires either a specific baculum were measured approximately 2 mm distal bacular shape or specific bacular movements during to the flared proximal end of the bone. Three re- copulation. peated measurements of these variables were within Despite evidence that the baculum has a func- 0.05 mm in all cases. External flaccid corpus caver- tional role during copulation, the identity of that nosum length was estimated by subtracting mean role remains a mystery. The baculum’s bony nature baculum length from mean total penile length. has led some authors to suggest that it provides additional support to the ancestral hydrostatic skel- etons in the penis by bracing the corpus spongiosum Gross Morphology and urethral tissue in the glans (Romer, 1970; Dix- Gross dissections of thawed penises from four in- son, 1987, 1995). It has also been suggested that the dividuals were used to examine the spatial relation- baculum facilitates penetration, acting as a wedge to ship of the baculum and corpus cavernosum and to open the female’s vaginal tract (Ruth, 1934; Long try to identify muscular or ligamentous attachments and Frank, 1968), or that it stimulates the female between them. Penile tissue distal to the ischial during copulation, eliciting a neuroendocrine re- attachment of the corpus cavernosum was removed sponse that could either enhance sperm transport, from each individual and examined under a dissect- induce ovulation, or prepare the uterus for implan- ing microscope. Two penises were dissected whole, tation (Patterson and Thaeler, 1982; Eberhard, while two were first cut into 5-mm thick serial trans- 1985, 1996).