Chapter 11 Reproductive system
Reproduction is the means by which a species is able to perpetuate itself. If animals lived for ever, there would be no need for another generation to take over from previous ones; in reality, all animals become old or ‘worn out’ and die and must be replaced if the species is not to become extinct.
All species of mammal have evolved separate sexes and they reproduce sexually. This is in contrast to less highly evolved species, which may reproduce asexually – producing offspring that are identical to the parent. Sexual reproduction involves the transfer of genetic material. After mating specialised germ cells – spermatozoa from the male and ova from the female – fuse to form a single-celled zygote. The zygote undergoes cell division to form the embryo. The offspring resulting from sexual reproduction are genetically different from each other and from their parents.
The male dog is known as a dog; the male cat is known as a tomcat. The reproductive system of the dog and the tomcat are generally similar – any differences will be described where appropriate (Figs 11.1, 11.2). The parts of the male reproductive tract are:
There is a pair of testes, which, in the adult animal, lie outside the body cavity in the scrotum. – a sac of relatively hairless and often pigmented skin. Spermatogenesis occurs most efficiently at temperatures below that of the core body temperature, so the testes are carried outside the body cavity in a cooler environment. In the dog, the scrotum lies between the hind limbs; in the cat it is attached to the perineum, ventral to the anus. Internally, the sac is divided into two, each part containing one testis; the left testis often hangs lower than the right one. Within the wall of the scrotum is the dartos muscle. In cold weather this contracts and thickens the scrotal skin, raising the temperature; in warm weather, the muscle relaxes and the scrotum becomes thinner and thus cooler. A constant temperature for spermatogenesis is therefore maintained.
Each testis is an oval-shaped structure wrapped in a double layer of peritoneum known as the tunica vaginalis (Fig. 11.3). The testicular tissue consists of numerous blind-ending tubules known as seminiferous tubules which are lined by two types of cells:
Lying between the tubules are the cells of Leydig or interstitial cells. They secrete testosterone and are under the control of interstitial cell stimulating hormone (ICSH) (see Ch. 6), produced by the anterior pituitary gland.
The coiled seminiferous tubules make up most of the testicular tissue and eventually combine to form slightly larger efferent ducts. These drain into the epididymis, lying along the dorsolateral border of the testis. The cauda epididymis or tail is attached to the caudal extremity of the testis and is the point at which the temperature of the testis is lowest. It is here that sperm are stored and undergo a period of maturation ready for fertilisation (Fig. 11.4).
Fig. 11.4 A normal spermatozoon. The acrosome protects the head of the sperm and contains enzymes which aid penetration of the ovum. The head contains the haploid number of chromosomes. The midpiece contains enzymes and mitochondria to provide energy for movement. The tail produces a powerful propulsive force.
The blood supply to the testis is via the testicular artery. This leaves the aorta in the abdomen, just caudal to the renal artery. As it enters the scrotum, the testicular artery runs alongside the epididymis and then divides to form the convoluted pampiniform plexus. This complicated capillary network ensures that the blood is cooled before it enters the testicular tissue.
In the early embryo, the undifferentiated gonads develop inside the abdomen close to the kidney. In the male, the gonad becomes the testis and a band of tissue known as the gubernaculum forms and runs from the caudal end of each testis to the inside of the developing scrotal sac. During late gestation, the testes are pulled caudally by the contraction of the gubernaculum and they migrate through the abdomen. The testes leave the abdominal cavity viathe inguinal canal – a channel between the fibres of the external abdominal oblique muscle in the groin or inguinal area (see Ch. 4). As each testis with its associated blood capillaries, nerve and deferent duct passes through the inguinal canal into the scrotum, it becomes wrapped in a double fold of peritoneum which forms the tunica vaginalis (Fig. 11.3).
The testes begin their descent into the scrotum during early neonatal life and should be palpable within the scrotum by 12 weeks of age in the puppy and 10–12 weeks in the kitten. Failure of the testes to descend is described as cryptorchidism; the testes may be retained in the abdomen or within the inguinal canal.
Retention of the testis may be a hereditary condition and affected dogs should not be used for breeding. Bilaterally cryptorchid dogs are usually sterile but may have normal sexual libido (desire). As the dog grows older, the retained testis is more likely to develop a Sertoli cell tumour.
The epididymis continues as the deferent duct (also called the vas deferens or ductus deferens; Figs 11.1, 11.2), which passes out of the scrotum into the abdominal cavity via the inguinal canal within the spermatic cord. The spermatic cord is wrapped in the tunica vaginalis and also contains the testicular artery and vein and the testicular nerve. Lying within the cord is a strip of muscle derived from the internal abdominal oblique muscle and known as the cremaster muscle. Contraction of this muscle raises the testis closer to the body in response to cold and works in conjunction with the Dartos muscle to maintain a constant temperature for the testes.
During ejaculation, the sperm and fluid produced in the seminiferous tubules are propelled along the epididymis and up the deferent duct, which joins the urethra. At this junction, the walls of the deferent ducts are thickened and glandular; the whole area is surrounded by the prostate gland (Figs 11.1, 11.2).
The urethra runs through the centre of the penis and extends from the bladder to the tip of the penis. It is shared by both the reproductive and urinary systems (see Ch. 10). The penis of the dog and the cat are anatomically different.
The penis runs from the ischial arch of the pelvis, passes cranioventrally along the perineum and between the hind limbs (Fig. 11.5). The urethra lies in the centre and is surrounded by a layer of cavernous erectile tissue known as the corpus spongiosum penis. This is expanded proximally into the bulb of the penis and, towards the tip, as the glans penis. Surrounding this and serving to attach the penis to the ischial arch is a pair of erectile tissue crura (sing. crus), known as the corpus cavernosum penis. These form the root of the penis at its attachment to the ischial arch. The urethra runs in a groove between the two crura.
Fig. 11.5 Structure of the penis in three layers, with transverse sections through the urethra. A Shows the urethra only. B Shows the urethra with the layer of cavernous erectile tissue surrounding it: the corpus spongiosum penis. C Shows the corpus cavernosum penis, which forms the two crura and attaches the penis to the ischial arch.
Cavernous erectile tissue is made of connective tissue perforated by ‘caverns’ lined by endothelium. During sexual excitement these caverns fill with blood under pressure and the tissue becomes engorged and erect.
Within the tissue of the glans penis is a tunnel-shaped bone, the os penis, whose function is to aid entry of the penis into the vagina of the bitch during the early stages of mating when erection is only partially complete. The canine os penis lies dorsal to the urethra, which runs through the ‘tunnel’ in the bone. At this point the urethra cannot expand and this can be a common site for urethral blockage with urinary calculi.
Many dogs develop an infection of the lubricating glands of the prepuce. This is known as balanitis or balanoposthitis and it results in a greenish-yellow discharge. The condition is so common that it is considered almost normal and is usually only treated if the mucous membrane becomes ulcerated or smelly.
The distal part of the penis is contained within a sheath of hairy skin known as the prepuce. This is suspended from the ventral abdominal wall and covers and protects the penis. It is lined with mucous membrane and is well supplied with lubricating glands. During mating, the prepuce is pushed back to reveal the glans penis. Afterwards, the retractor penis muscle pulls the penis back into the prepuce.
The main parts of the penis are similar to those of the dog, except that the cat penis is shorter and points backwards – the external opening is ventral to the anus (Fig. 11.2). The glans penis is covered with tiny barbs, which elicit a pain reflex as the male withdraws from the female after mating. This stimulates the nerve pathway to the hypothalamus, resulting in ovulation approximately 36 hours later – known as induced ovulation. The os penis lies ventral to the urethra in the cat. During sexual excitement, the penis engorges and points cranioventrally so that the mating position in cats is similar to that seen in the dog.
The reproductive tract of the bitch and queen are similar and vary only in size (Fig. 11.6). The tract is designed to carry several fetuses during a single pregnancy and is said to be bicornuate (two horns). The bitch and the queen bear litters of young: they are multiparous.
(With permission from Colville T, Bassett JM 2001 Clinical anatomy and physiology for veterinary technicians. Mosby, St Louis, MO, p 330.)
There is a pair of ovaries, one lying on each side of the dorsal abdominal cavity, caudal to the kidney (see Ch. 10, Fig. 10.2). The ovary is held close to the kidney by the ovarian (suspensory) ligament (Fig. 11.7). The ovary is suspended from the dorsal body wall by part of the visceral peritoneum called the mesovarium, which also encloses the infundibulum of the uterine tube. Part of the mesovarium forms a pocket-like structure known as the ovarian bursa, which completely covers the ovary. Within this is a small opening allowing ova to leave the ovary – this is a potential means of entry of infection into the peritoneal cavity.
The tissue of the ovary consists of a framework of connective tissue, smooth muscle and blood capillaries, within which are a large number of germ cells and developing follicles (Fig. 11.8). In an immature animal, each ovary is oval with a smooth outline but, as sexual maturity approaches, the ovary becomes nodular as the follicles enlarge.
Each uterine tube is a narrow convoluted structure lying close to the ovary (Fig. 11.7). The open end is funnel-shaped and known as the infundibulum. It is fringed with finger-like processes known as fimbriae, which spread over the surface of the ovary to capture ova as they are released. The ova pass down the lumen of the tube, which is lined with ciliated columnar epithelium. The cilia propel the ova along the tube towards the uterine horns. The uterine tube is suspended by part of the visceral peritoneum known as the mesosalpinx.
The uterus is a Y-shaped structure lying in the midline of the dorsal abdomen (Fig. 11.6). During pregnancy, the weight of the conceptuses pulls the uterus ventrally and at full term it occupies the greater part of the abdomen. The function of the uterus is:
The uterus consists of two parts. A pair of uterine horns lead from the uterine tubes. Each horn is about five times the length of the uterine body and, during pregnancy, contains the developing embryos. The two horns join to form a short central body.
The cervix is a short, thick-walled muscular sphincter that connects the uterine body with the vagina (Fig. 11.6). Running through the centre is a narrow cervical canal, which is normally tightly closed and relaxes only to allow the passage of sperm or fetuses. During pregnancy, the canal is blocked by a mucoid plug, which protects the conceptuses from infection. In the non-pregnant animal, the cervix lies in the pelvic cavity but during pregnancy the weight of the conceptuses pulls the cervix cranially and ventrally over the edge of the pelvic brim.
Ovariohysterectomy, more commonly called ‘spaying’, is performed by the veterinary surgeon to prevent unwanted pregnancies and oestrous cycles. The surgical procedure involves the complete removal of the reproductive tract from the ovaries to a point just cranial to the cervix. Ligation of the major blood vessels is essential, particularly if the procedure is performed during oestrus or pregnancy as the vessels may be extremely well developed.
The vagina and vestibule form a channel leading to the external opening of the reproductive tract – the vulva. The vagina leads from the cervix to the external urethral orifice – the point at which the urethra joins the reproductive tract. The vestibule leads from the external urethral orifice to the vulva and is shared by both the urinary and reproductive tracts.
The lumen is lined by stratified squamous epithelium, which undergoes hormonal changes during the oestrous cycle. The lining epithelium is folded longitudinally to allow widthways expansion during parturition and is surrounded by layers of smooth muscle. These are very strong and during canine mating they tighten on the penis of the male and maintain the ‘tie’.
The epithelial cells lining the vagina undergo hormonal changes during the oestrous cycle. These changes may be used as an aid to the correct timing of mating. The technique, known as vaginal exfoliative cytology, involves making smears from vaginal swabs taken every other day during pro-oestrus and early oestrus. The smears are then stained and examined microscopically looking for diagnostic changes in the cells that provide an indication of the stages of the cycle.
Although these are not strictly part of the reproductive tract, they are essential to reproduction in the mammal. The presence of mammary glands is the defining characteristic of the class Mammalia. All mammals feed their young on milk produced by the glands during a process known as lactation.
Mammary glands are modified cutaneous glands. In the dog and cat, they are present in both sexes but are rudimentary in the male. The glands lie externally on the ventral wall of the abdomen and thorax, on either side of the midline.
Each gland consists of glandular tissue embedded in connective tissue and lined by a secretory epithelium (Fig. 11.9). The milk produced drains through a network of sinuses that eventually form teat canals. These open on to the surface of each teat, known as a teat orifice. Each gland has one teat but each teat has several orifices.
This varies between different species and is an important consideration when feeding orphaned animals. The milk produced by the bitch and the queen is more concentrated and contains more protein and twice as much fat as cow’s milk. The average composition of milk is shown in Table 11.1.
|Minerals||1.5–1: calcium phosphate, magnesium, sodium, potassium and chlorideMilk is deficient in iron and copper; traces of iodine, cobalt, tin and silica are present|
|Vitamins||A, B2, B5, E, KMilk is low in vitamins C and D|
NB. Milk from a cat contains the amino acid taurine, as cats have a specific requirement for this.
The first milk secreted by the dam following parturition is known as colostrum. It is rich in maternal antibodies, which provide the neonate with immunity to diseases to which the dam has been exposed. It is essential that the neonate takes in colostrum within the first 24 hours of life. During this time, the protein antibodies can be absorbed by the small intestine without being digested. After 24 hours, normal protein digestion starts and the antibodies are broken down and destroyed. After a few days, production of colostrum stops and the composition of the milk remains constant.
The oestrous cycle is the rhythmic cycle of events that occurs in sexually mature non-pregnant female mammals and includes limited periods of sexual receptivity known as oestrus. The function of the oestrous cycle is: