Dog Reproduction

Clinical Anatomy and Physiology of the Reproductive System in Dogs and Cats

Dog Reproductive Anatomy

It is not uncommon for a client to call regarding some aspect of reproduction in pet animals. As animals move more into the home and are treated more like humans, their physiological characteristics are also interpreted with the knowledge of similar function in humans. This causes considerable confusion in the pet owner when faced with a perplexing and often embarrassing condition involving the reproductive sytem of their pet. It is your responsibility to handle these situations as a professional. Never make light of or fun of these situations. This client is very serious about their concern and may be at the disadvantage of not knowing or being too embarrassed as to how to describe a particular situation. Never make the client feed dumb or inadequate and never speak arrogantly to a client in this situation. This will most certainly alienate the client and not be of service to the pet.

The Male Dog Anatomy


The male dog has reproductive organs that are analogous with any other male animal. The testicles are located in the scrotum. This scrotal sac hangs caudal between the rear legs of the pet. The testicular artery, vein and ductus deferens (vas deferens) are contained in the spermatic cord that passes into the abdomen through the inguinal ring. The ductus deferens carries sperm to the urethra from the testicles. The sperm is deposited in to the urethra just before the prostate gland. The prostate gland surrounds the urethra just caudal to the urinary bladder. This gland produces prostatic fluid, which feeds the sperm and helps maintain them in their passage into the female dog. The male dog urethra therefore carries sperm from the testicles into the penis. The urethra also carries urine from the bladder to the penis. Since urine is toxic to sperm, there is a reflex mechanism that prevents both from being transmitted by the urethra simultaneously. The prepuce is the external covering of the male dogs penis. The penis of the male dog has several anatomically important characteristics. The penis is composed of three components one is the os penis which is a bone that is v-shaped and extends the length of the penis. The urethra is the second component ans the penile portion of the urethra lies in the tough of the v-shaped os penis. This bone helps to protect the urethra from trauma. The third component is the erectile tissue that surrounds the os penis and urethra. This erectile tissue in the dog expands at the base of the penis to form the bulbus glandis.

During an erection this tissue can expand greatly forming two large knots that is very concerning to the uninformed pet owner. The function of this tissue of the penis is to expand inside the vaginal vestibule of the female dog. During copulation, commonly called a “tie” by dog breeders, the male dog’s bulbus glandis expands within the female dog’s vaginal vestibule. Ejaculation will occur, then the male dog will step off the back of the female dog. The two dogs are then joined rear to rear by the male dog’s penis. The “tie” will continue for 20 to 30 minutes and helps to ensure the passage of sperm through the convoluted cervix of the femal dog. These pets should not be pulled apart or hosed down with water. This is a natural process and although the dogs may occasionally cry in pain there is nothing that can be done to lessen the apparent discomfort. Male dogs may have an erection in the absence of a female dog. This erection may occur within the prepuce or outside the prepuce. Either circumstance is concerning to the pet owner. When the erection occurs outside the prepuce it appears as if the dog cannot retract the penis and that the penis may be “bloody” or damaged. This is a normal reaction and the client should be told to observe the dog for another 30 to 45 minutes before becoming alarmed. There are instances where the penis will not easily go back into the prepuce and medical intervention is necessary but this is the exception and not the rule. In cases where the dog has an erection inside the prepuce, the prepuce skin will stretch and the client will notice what appears as two knots under the skin. Most clients think this is abnormal but few realize what the cause is. Speculation by the client may be that the knots are tumors, enlarged lymph nodes or a hernia. Clients take this very serious and so should you. A calm professional discussion of the normal anatomy using the correct terminology allows the client to know that you understand what is happening. The client also learns the correct terms to use for more in depth discussion.

The Female Dog Anatomy

the female dog anatomy

The female dog has a typical anatomical structure but has an exceptional reproductive cycle that can be confusing to the pet owner. The ovaries of the dog are located in close proximity to each kidney. The ovary is enveloped by a “catchers mitt” tissue called the infundibulum. This basket or mitt “catches” the eggs and transports them into the fallopian tubes. The fallopian tubes lead to the uterus where the fertilized egg will implant and grow. The uterus has paired ovaries, paired fallopian tubes and paired uterine horns. These uterine horns unite to form the body of the uterus. The body of the uterus is joined to the vagina. The cervix is valve or opening at the junction of the uterus with the vagina. The vagina is the receptacle for the male dog’s penis. The most caudal portion of the vagina has a dilated area called the vaginal vestibule. This portion is the receptacle for the male dog’s bulbus glandis. Engorgement of the erectile tissue of the male dog’s penis in the vestibule prevents the male dog from removing his penis during copulation. This result is a “tie” as described above. The vaginal vestibule also contains the urethral orifice which is the opening of the urinary tract into the vagina. This orifice lies on the floor of the vagina at the junction of the vagina and the vestibule. Caudal and ventral to this area is the clitoral fossa that contains the erectile homologue of the male penis. The external opening is called the vulva.

The “heat cycle” of the dog is a commonly misunderstood process. Estrus is used as a synonym for “heat”. The female dog’s ovaries are grape size and produce smaller nodular follicles in response to Follicle Stimulating Hormone (FSH) which is produced by the pituitary gland in the brain. FSH causes the follicles to grow and to produce the eggs that will be ovulated. Ovulation is the process by which the follicle ruptures and squirts the egg along with the follicular fluid into the infundibulum of the fallopian tubes. Ovulation occurs as a result of another hormone from the pituitary gland called Luteinizing Hormone (LH). This hormone will cause the follicles to ovulate within 24 hours of its release from the pituitary gland. Coinciding with this hormonal action and ovulation is the psychological changes in behavior in the female dog that will permit a male dog to mount. Dog breeders call this “standing heat”. This means the bitch will stand and allow the male dog to mount her for copulation. Before this time the female dog will sit down when the male tries to mount her. She will also avoid the male and may be aggressive. Before ovulation, while the follicle is forming the egg, estrogen is released from the follicle. Estrogen from the developing follicles prepare the vagina and uterus for breeding and pregnancy. The vaginal cells flatten and toughen in preparation to receive the force of the male penis. Vaginal smears examined microscopically during this time show these changes and can be used to roughly predict breeding time. The uterus prepares by increasing the uterine mucosal glands and blood supply that will feed the fetus. This process is so vigorous in the dog that blood leaks from the uterus during this time. This causes the bloody discharge that is typical for pro-estrus (before heat). This discharge will occur for 7 to 10 days prior to ovulation. After ovulation the discharge stops. This bleeding confuses many clients. They may think the pet is out of “heat” when the discharge stops when actually they are ready to breed. Estrogen influence therefore causes the uterus and vagina to prepare. This estrogen is being produced by the follicles that are growing the eggs. Once LH causes ovulation, the estrogen level drops and behavior changes result. Ovulation of all ripe eggs in the ovary will occur over a 24 hour period. These eggs must then mature over the next 48 hours in the fallopian tubes before they can be fertilized. The bitch will start “standing heat” 24 hours after LH release and at the time of ovulation but the eggs are not mature enough at this time to be fertilized. After copulation however, the male dog’s sperm lives 96 hours in the uterus and fallopian tubes of the female dog. So these sperm may wait until the eggs are ready. Standing heat may last for 7 to 10 days with the possibility of there being copulation with many different male dogs. This results in multiple fathers of the same litter. Once the eggs are fertilized the embryos will implant in the wall of the uterus. The follicles change from producing estrogen after ovulation to producing progesterone (promote gestation). This progesterone factory of the ovary is now called a corpus luteum. Progesterone is produced to hormonally maintain the viability of the pregnancy. Dogs differ from most other species in the production of progesterone. In most species that do not become pregnant, the progesterone level will fall and the uterus will go through a recycling stage. Therefore in any animal other that the dog, if the progesterone level is high – this indicates pregnancy. This is the basis of the pregnancy test for people as well as for horses. In dogs however progesterone remains high regardless of whether the dog is pregnant or not. The corpus lentum continues to produce progesterone, which causes the mammary glands to develop, the uterus to enlarge and the dog to act as if she is pregnant. Therefor hormonally every female dog becomes “pregnant” or false pregnant. This cycle occurs over an over with each heat and can ultimately result in increased numbers of uterine glands. These glands produce increased uterine secretions. These secretions may ultimately feed bacteria and cause pyometra (pus filled uterus) which is life threatening in the dog. If a female dog is not to be used for breeding, ovariohysterectomy is recommended to prevent pyometra.

The similarities of the dog female cycle to the human female cycle are in appearance only. In the dog, as the uterus is becoming ready for implantation, blood leaks out causing the bloody discharge. This discharge occurs for 7 to 10 days and stops just as the uterus is ready. In the human the uterus become ready for the egg silently by producing an engorged mucosal lining of the uterus. If there is no implantation of the egg in the human uterus, it will slough this mucosal lining resulting the menstrual period.


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