Estrus

ESTRUS in animals, analogous to human menstruation, occurs in the female organism in connection with ovulation. Ovulation and the associated estrus in animals occur regularly at specific intervals, and the entire period from one ovulation to the next is called the sexual cycle. Throughout the sexual cycle, the ovary and the entire sexual system undergo a series of changes. At various moments of the sexual cycle, quite specific changes appear in the oviducts, uterus, vagina, vaginal secretions, external genitalia, and mammary glands. By biological methods, it has been established that the sexual cycle consists of four phases: I phase of rest-dioestrus, II phase of proestrus-prooestrus, III phase of estrus-oestrus, IV phase of post-estrus-metaoestrus. Each of these phases has a specific duration and is accompanied by characteristic changes in the ovary and sexual organs. The sexual cycle has been best studied in rodents, its duration in mice and rats being determined as 102 hours. The longest phase is the resting phase, which in rats continues for 57 hours, whereas the proestrus phase has 12 hours, estrus-27 hours, and the post-estrus phase lasts only 6 hours. The oestrus phase-estrus-has the following characteristics in the development of the ovary and sexual organs. In the ovary, follicular maturation is completed and ovulation occurs. The oviducts and uterus are maximally enlarged and have increased secretory activity. Leukocytes are absent in the uterus. In the vagina, there is enhanced development of the epithelium, which changes from single-layered to multi-layered. The upper layer of the epithelium quickly undergoes degenerative processes, turning into keratinized, anucleated scales, so-called Schollen, which desquamate and collect in masses in the vaginal secretions. The external genitalia swell, redden, and their mucous membrane becomes dry. The mammary glands enlarge. During estrus in animals, sexual desire appears, which in most animals makes mating possible, since at other times the female shows no desire for mating. Exceptions are mice, rabbits, guinea pigs, and cats, in which mating can occur regardless of the stage of the sexual cycle, since in them follicular maturation and ovulation are caused by the act of copulation. In contrast to estrus, dioestrus-the resting phase-is characterized by the following processes. In the ovary, young growing folliculi and rapidly regressing yellow bodies are observed. In the oviducts and uterus, secretion is absent. In the uterus, individual leukocytes are found, its lumen is narrow, and the glands are atrophic. In the vagina, the cylindrical epithelium is covered with 1-2 rows of polygonal cells and 1-2 rows of mucous functional cells. The external genitalia have a moist mucous membrane. The proestrus phase has transitional stages of development in the ovary and sexual organs from rest to estrus. The metaoestrus phase represents the reverse development from the phenomena of estrus to the resting phase. In many mammals, amphibians, reptiles, and fish, the winter period is characterized by a phase of sexual rest, from which the animal can be brought out only by experimental intervention. Research by Zondek and Aschheim (1928-1930) showed that the hormone of the anterior pituitary promotes egg maturation and stimulates the ovary to produce follicular hormone, which causes all the phenomena of estrus in the genital system. The pituitary hormone has no direct effect on the sexual apparatus but acts on it only through the ovary, as evidenced by experiments with castrates. In castrates, phenomena of estrus occur only under the influence of follicular hormone, whereas the pituitary hormone, even in large doses, has no effect on the sexual organs. Utilizing the properties of the pituitary hormone as a stimulator of the sexual gland, it is possible in animals that have had a long period of rest to induce a shift to estrus and thereby increase their productivity. Wolf (1929), Dubowik (1930), and Houssay (1932) caused premature egg-laying in frogs, snakes, and fish, 3-4 months before the normal time. Similar experiments also gave positive results in mammals. Method of obtaining vaginal smears and determining estrus cycles. Vinal secretions at various stages of the sexual cycle have quite specific histological changes. Stockard and Papanicolaou (1917) proposed using vaginal smears to determine the stage of estrus. A smear from the vagina is made on a slide in a drop of distilled water. After drying, the smear is fixed in absolute alcohol and stained with methylene blue and eosin. After staining, the smear is washed in running water, dried, and can be considered ready for analysis. Analysis of vaginal smears taken at various stages of the sexual cycle development reveals the following histological picture: 1) during the resting period, many leukocytes, epithelial cells with nuclei, and much mucus are found in the smear; 2) during the transitional period to proliferation-prooestrus-the number of leukocytes decreases, the number of epithelial cells increases, and mucus decreases; 3) the proliferation stage is characterized by the absence of leukocytes in the smear, a sharp decrease in the number of epithelial cells, and traces of mucus remain; 4) in the transitional period to estrus, leukocytes and mucus are absent in the smear, individual epithelial cells are found, and keratinized anucleated scales appear; 5) the estrus stage is characterized by the presence of only scales in the smear; leukocytes, epithelial cells, and mucus are completely absent; 6) the transition from estrus to the post-estrus period is marked by the presence of leukocytes, epithelial cells, and scales in the smear with complete absence of mucus; 7) the post-estrus stage differs from the previous one by a decrease in the number of scales and an increase in the number of leukocytes and epithelial cells; mucus is absent; 8) the transition to the resting stage is characterized by an increase in the number of leukocytes and epithelial cells, complete absence of scales in the smear, and appearance of mucus. The reaction of vaginal smears has wide application because it allows detection of estrus, which determines the breeding period in most animals. Timely determination of estrus is important because ovulation usually occurs at the beginning of estrus, and as shown in sheep, the egg retains its fertilization ability for only 4-5 hours. Therefore, for successful fertilization, it is necessary that mating occurs in the first hours of estrus. In addition, this method makes it possible to determine the degree of activity of ovarian preparations, which is extremely important for clinical purposes.

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