The os penis (), or baculum, is a feature of most mammals (). It is always present in the male dog. It forms from a paired ossification center in the corpora cavernosa at 35 days of age (). In large dogs it is approximately 10 cm long, 1.3 cm wide, and 1 cm thick. The bone forms a rigid axis of the glans penis, through which it passes. The caudal part, or base of the os penis, is truncate and
attached to the termination of the corpora cavernosa within which it formed (). The cranial part, or apex, tapers gradually and ends in a cartilaginous tip, which is attached by a fibrous strand to the deep surface of the corona of the glans. The body of the os penis is straight and long having as its most distinctive feature a urethral groove (sulcus urethrae), which runs ventrally along the base and body of the bone. The urethral groove is of clinical importance because the narrow entrance to the groove at its base may obstruct the passage of urinary calculi through the urethra and require surgical intervention. The groove is approximately 7 mm deep and 4 mm wide near the base and gradually becomes narrower and shallower distally until a groove no longer exists, although the urethra still lies ventral to the bone. The urethra, along its entire course in the penis, is surrounded by the corpus spongiosum, which at approximately midpoint along the bone sends vascular channels out of the urethral groove on both sides () that enter the bulbus glandis. The os penis usually has indentations on the margins of the groove caused by the passage of these vessels.
The term os genitale has been used to include the os penis of male mammals (ospriapi in Greek) and the less frequent os clitoridis of the female. The presence of a bone or baculum in the penis is a characteristic feature of males in most mammalian orders and its morphologic characteristics have been used by mammalogists as a diagnostic feature for genera of Bats, Rodents, Insectivores, and Carnivores (). The bone may be bipartite or trifurcate in rodents and have synovial joints (). An os penis is not present in monotremes, marsupials, ungulates, elephants, or man. The homologous os clitoridis of the female is less frequently present in mammals and only occasionally reported in the dog ().
The first indication of external sexual differentiation in the dog fetus is seen approximately halfway through gestation (). By 35 days of gestation (35 mm) the penis and clitoris are similar in shape and structure but differ in position relative to the anus (closer together in the female). During fetal life there is a condensation of tissue at approximately 34 days and a chondrification at 46 days to form distinctive corpora cavernosa. By the end of gestation external genital features are well differentiated but it is not until 35 days postpartum that the formation of an os penis can be distinguished by alizarine staining (). Urgel and Lacalle (1970) observed ossification histologically in a 30-day-old puppy.
Os Penis: Development of the Os Penis
In the month-old Beagle puppy, each corpus cavernosum at midlevel of the penis appears as a slightly compressed column, which dips ventrally as it passes through the bulbus glandis. The internal fibrous trabeculae of the corpus cavernosum appear as transverse striations in cleared specimens (). At the level of the bulbus glandis the striated trabecular columns become homogeneous fibrocartilages, without trabecular spaces, which fuse on the midline dorsal to the urethra and reach almost to the tip of the glans.
The first ossifications to form in the penis of the dog appear on the surface of cartilaginous nodules that form within the corpora cavernosa at 35 days after birth (). The rate of ossification may not be the same in all littermates. In four littermate Beagles stained with alizarine and cleared in potassium hydroxide and glycerine (), one had no ossification, whereas the other three had bilateral single or double ossifications within cartilaginous nodules. The fact that the pup without any ossification did not have a cartilaginous nodule indicates that the bone and the cartilage develop almost simultaneously. One pup already had trabeculae joining right and left ossifications (). By 55 days postpartum () the right and left ossifications in each fibrous column of the corpus cavernosum have elongated, flattened, and begun to fuse on the midline dorsal to the urethra. This results in the formation of a prominent urethral groove. As lengthening of the bone progresses, the base widens and the groove deepens.
In the adult male the base of the os is firmly attached to the tunica albuginea surrounding the corpora cavernosa because it developed within this sheath. The widest region of the os is at the level of the bulbus glandis, which surrounds it. The ventral margins of the os are roughened by the passage of vascular channels over the surface that connects the corpus spongiosum to the bulbus glandis. These channels can be seen developing in the 55- and 65-day pup (). Thus the sequence of bone formation for the os penis in the dog is progressive fibroplasia of the terminal portions of the corpora cavernosa, followed by cartilage formation, perichondral bone formation, endochondral ossification, fusion of right and left ossifications, and subsequent bone growth by secondary cartilage ossification at the ends and deposition by an osteoblastic periosteum.
The apex of the os remains cartilaginous () throughout life and has a fibrous continuation to the surface of the glans dorsal to the urethral orifice. It is this fibrous connection that during intromission deforms the glans within the fornix of the vagina and causes it to flatten dorsally as a “corona glandis” ().