FUNCTIONAL CHANGES

It is often stated that bitches come in heat twice a year, in spring and autumn. In fact, three heats are not uncommon, although even when this is so, the greater part of the year is occupied by periods of anestrus.

The reproductive organs, quiescent during anestrus, develop rapidly in proestrus when, over a period of a week, a batch of follicles enlarges. The uterus now increases in length and in thickness; its endometrium proliferates, and the entire reproductive tract becomes hyperemic. A thickened, edematous vulva discharges the serous uterine secretion, which is tinged with blood, the result of diapedesis from the widened endometrial vessels. Estrus also lasts about a week and can be dis­tinguished from proestrus by the female’s readiness to accept a male. The endometrial hypertrophy and hyper­emia continue, but the discharge gradually becomes less blood-stained. Ovulation, which occurs about the second day of estrus, is succeeded by very rapid forma­tion of corpora lutea, which may be mature by the end of estrus. The separation of diestrus and metestrus is difficult to determine because there is often a period (2 to 8 weeks) of pseudopregnancy during which the bitch exhibits the usual physical and behavioral signs of pregnancy, even though fertilization has not occurred; pseudopregnancy can perhaps be likened to a greatly extended period of diestrus. The cervix is tightly closed during diestrus and metestrus, and secretions that would have been utilized for embryo nutrition then accumulate in amounts that may distend the uterus; infection often supervenes, producing a condition (pyometra) that may necessitate hysterectomy.

The responses of the vaginal epithelium to changes in hormonal levels are more pronounced than in other domestic species, and smears taken from the vagina provide evidence of the stage within the cycle. Both cor­nified epithelial cells and erythrocytes are present in large numbers during proestrus, but while the former persist through estrus, the latter gradually become fewer as leu­kocytes become present. The stages of the cycle are also reflected in the gross appearance of the vaginal lining, including that covering the dorsomedian fold. In proes- trus the lining becomes edematous and forms prominent soft folds. As estrogen levels drop rapidly during estrus, the vaginal wall becomes less oedematous and the lining wrinkles until about 4 days after ovulation, when the surface is said to resemble crepe paper. A few days later the mucosa becomes flat and patchy; with the desquama­tion of the cornified superficial layer of epithelium the blood vessels are able to shine through once more.

Ova enter the uterus about the sixth day after ovula­tion. If fertilized, they implant after a further 10 days, and this delay also allows appropriate spacing. An omphalovitelline (yolk sac) attachment is first estab­lished, but though effective in early pregnancy, it is later replaced by the definitive chorioallantoic placenta (Figure 15-13/d). This develops through the invasion of the endometrium by villi growing from a broad band of the chorion encircling the trunk of the fetus, as a con­tinuation of the erosion that started in the nonvascular (chorioamniotic) regions and about the yolk sac attach­ment. The erosion leads to the interdigitation of thin plates of fetal tissue, and endometrial lamellae are reduced to little more than the maternal capillary endo­thelium (Figure 5-70, E-H). The tissue barrier of this basically chorioendothelial placenta is further reduced at the margins of the zonary band, where blood extrava- sated from maternal vessels directly bathes the fetal tissue.

*Ovulation is not spontaneous in the cat but is induced by coitus.

Figure 15-13 The feline fetal membranes in transverse and longitudinal section, schematic. 1, Amnion; 2, amniotic cavity; 3, yolk sac; 4, chorioallantois; 5, allantoic cavity; 6, zonary placenta.

of placenta consists of three zones: a transfer zone (around the embryo for nutrient transfer), a pigmented zone at either end of the transfer zone (maternal hema­tomas, probably important for iron transport from dam to fetus), and a relatively nonvascular zone, the allanto- chorion that is thought to be responsible for resorption from the uterine lumen. Only a certain proportion of the antibodies the pup receives from the dam penetrates the placenta; the greater share (about 75%) of the passive immunization of the newborn is dependent on the colostrum.

Initially the uterus enlarges locally, and each concep­tus is confined within a globular swelling that is bounded by regions of constriction. The separate ampullae persist until about the 40th day (in a gestation that aver­ages 63 days, measured from the date of ovulation*),

Figure 15-14 A, Ultrasonographic (transabdominal) view of a 33-day (after a single mating) Beagle fetus in its ampulla; the scale on top is in centimeters. 1, Head of fetus; 2, thorax of fetus; 3, yolk sac; 4, uterine wall. B, Pregnant bitch with several almost full-term fetuses. Note the gas in the rectum.

when there begins to be a gradual relaxation of the constrictions, eventually creating an almost uniformly expanded uterus. The positions of the individual fetuses are still obvious on inspection of the exposed organ as the whole thickness of the uterine wall is very vascular at the placental sites.

Pregnancy diagnosis by abdominal palpation is pos­sible from 18 to 21 days of gestation onward, first by the presence of round swellings of approximately 1 cm in diameter and later, between 24 and 32 days, by palpa­tion of swellings of about 2.5 to 4 cm in diameter. From 35 to 45 days of gestation, the swellings enlarge, elon­gate, become flaccid, and are found ventrally in the abdomen. For a few days, starting from about the 50th

Figure 15-15 Pregnant uterus of bitch, dominating the abdominal topography.

day, it is no longer possible to palpate individual swell­ings, but from the 55th day of gestation individual fetuses are easily palpable.

In the later stages of pregnancy abdominal radio­graphs not only serve to determine the number of pups in the litter but also provide a means of assessing fetal age, thus predicting the date of parturition. Mineraliza­tion commences in the axial skeleton by about the 45th day and is soon followed by the progressive mineraliza­tion of the appendicular skeleton in proximodistal sequence (Figure 5-74; Table 15-1). Mineralization of the skeleton of kittens follows the same pattern, but each element makes its appearance a few days earlier than in pups.

For some time now, ultrasonography has provided an alternative or additional means of diagnosing preg­nancy and predicting term. Its advantages and disad­vantages for these purposes, when compared with radiography, are dependent to a large extent on the stage of pregnancy when the examination is made.

Parturition is facilitated by pelvic rotation at the sac­roiliac joints and by elevation of the tail, which are both maneuvers that significantly increase the dimensions of the pelvis. In both dogs and cats some 60% to 80% of fetuses present the head toward the cervix at term, which is a bias that has yet to receive a satisfactory explanation of how it is achieved. Fetuses tend to be delivered from each horn in alternation, and when each is delivered, the emptied segment of the uterus contracts and brings those littermates left behind closer to the

Modified from Concannon P, Rendano V: Radiographic diagnosis of canine pregnancy: onset of fetal skeletal radi­opacity in relation to times of breeding, preovulatory leu- teinizing hormone release, and parturition, Am J Vet Res 44:1506-1512, 1983; and Yaeger AE, Mohammed HO, Meyers-Wallen V et al: Ultrasonographic appearance of the uterus, placenta, fetus and fetal membranes throughout accurately timed pregnancy in beagles, Am J Vet Res 53:324-329, 1992.

Table 15-2

exit. When expelled, each fetus is still attached to its placenta, from which it is freed by the dam’s biting through the umbilical cord. The “afterbirth,” with which considerable maternal tissue is shed, is normally consumed.

Although less often useful to the clinician, some information on the development of certain external fea­tures of fetuses will be found in Tables 15-2 and 15-3.

The cat is sexually mature at 6 to 9 months of age. The proestrus stage, the nonacceptance of a male, lasts 12 to 48 hours. In cats, pea-sized swellings can be palpated at 21 days of gestation. By 28 days, the swellings are firm and are about 2.0 to 2.5 cm in diameter. The uterus is evenly distended during days 35 and 50 and may be difficult to differentiate from pyometra.

Potentially embarrassing mistakes in the determina­tion of the sex of newborn kittens are relatively easily made. The difficulty arises from the orientation of the penis. This brings the anal and genital openings rela­tively close together in the tom, and the spacing is inconveniently similar to that in the female (Figure 15-16).