Histology of Reproduction and the Estrus Cycle in the Female Fur Seal, Callorhinus ursinus
Authors: Craig A. M. 
Published: April 2011

Gross and histological analyses of the reproductive tracts from 72 multiparous females, 12 nulliparous females, and 28 non-pregnant females, taken pelagically in the eastern Pacific and on the Pribilof Islands between 1959 and 1961, have been used to determine the histology and physiology of the estrus cycle, the age of sexual maturity, and certain causes of prenatal mortality.The ovaries alternate in function, one ovary ovulating in one breeding season, the opposite ovary ovulating in the next. An undetermined factor suppresses follicular development in the ovary containing a corpus luteum of pregnancy; suppression maintains for 6 months after parturition. Graafian follicular development is greatest in June, with an average of 15 follicles in the ovary destined to ovulate. About four of these will enlarge abruptly in July; one will reach ovulation size (10 mm or greater in diameter) and rupture; the rest will become atretic.Ovulation occurs spontaneously 3–5 days after parturition in mid July. If fertilization is accomplished, the resulting blastocyst remains free in the uterus until early or mid November, when it implants in the mucosa.The newly formed corpus luteum is actively secretory for a month after ovulation. During this time, follicular development is suppressed in both ovaries, and the uterine mucosa prepares for implantation. From the middle of August, the corpus luteum regresses; the luteal cells are vacuolated, and secretion is minimal. This is reflected in a recrudescence of follicular activity in the ovary containing the corpus luteum, and regression of the associated uterine mucosa. Immediately prior to implantation the corpus luteum resumes secretion, and the mucosa is prepared for implantation. Placentation is established during December. Luteal degeneration begins in January and is progressive until parturition, when the corpus luteum appears to be non-secretory. Luteal degeneration has no effect on the pregnant uterine horn; it is assumed that the placenta secretes hormones sufficient to replace the luteal hormones.A nulliparous female ovulates for the first time in late August or September, [Formula: see text] months later than subsequent ovulations. On the basis of gross reproductive examination, approximately 70% of females ovulate for the first time at 4 years of age. Since the pregnancy rate of 5-year-olds is consistently about 50%, probably a number of 4-year-olds do not mate, either because ovulation occurs after the breeding season, or because breeding males are not available.Prenatal mortalities take 3 forms: "missed" pregnancies, abortions, and resorptions. On the basis of histological examinations, "missed" pregnancies are ultimately due to failure of implantation. Three conditions are responsible: failure of fertilization, failure of implantation, or malfunction of the reproductive tract which precludes pregnancy. Abortion results in the expulsion of a conceptus from the uterus before term; resorption is a gradual process of dissolution of the conceptus within the uterus. On the basis of histological evidence, three causative agents of abortion and resorption are apparent: degeneration of the corpus luteum after implantation, malfunction of the uterine mucosa, and inter-uterine mortality of the conceptus.