Sex and Reproductive Cycles

the morphological processes related to reproduction that recur periodically in mature female animals. Reproduction in animals can be monocyclic, in which one cycle occurs during the reproductive season, or polycyclic, in which several cycles occur.

Most invertebrates, fishes, amphibians, and reptiles have the simplest reproductive cycle. This cycle consists of a single follicular stage, during which the egg grows, matures, and is released into the environment; examples are spawning in fish and amphibians and oviposition in reptiles. Monocyclic reproduction prevails in the northern and temperate latitudes and polycyclic reproduction in the southern latitudes.

Birds have a reproductive cycle consisting of three stages: the follicular stage, in which the eggs grow, mature, and ovulate in the ovaries; the incubation stage; and the nestling period.

A complex reproductive cycle of four stages is typical of female mammals. The first stage is the follicular stage, during which the growth of the ovarian follicles is induced by the follicle-stimulating hormone (FSH), secreted by the anterior pituitary. The ovarian follicles release estrogenic hormones into the female’s blood. These hormones act on the central nervous system and under the appropriate external conditions sexually excite the female to mate. The general changes that occur in the body during this estrous cycle are also manifested by a strong rush of blood to the oviducts, uterus, and vagina that results in some morphological changes in the female’s genitalia. The luteinizing hormone (LH), secreted by the anterior pituitary, acts with FSH to stimulate the completion of the growth of the follicles, as well as their maturation and ovulation. The released ovum first enters the oviducts, where fertilization may occur, and then enters the uterus.

The second stage of the reproductive cycle starts after ovulation and is called the luteinic stage, or the corpus luteum stage. LH induces the conversion of the empty ovarian follicles into yellow bodies. Progesterone is secreted into the blood by the corpus luteum and inhibits the secretion of FSH. It also causes the mammary glands and the walls of the uterus to enlarge considerably.

The third stage is the pregnancy stage, which starts after the fertilization of the ovum and its implantation into the uterine wall. The lactation stage is the fourth stage. The yellow bodies continue to inhibit the maturation of ovarian follicles during the third and fourth stages.

Each stage of the reproductive cycle in female mammals requires specific stimuli in order to be adequately completed: the first and second stages are frequently stimulated by the male’s courting of the female followed by coitus, the third stage is stimulated by the developing embryo and placenta, and the fourth stage is stimulated by suckling.

The reproductive cycle consists of only two stages if fertilization does not occur. The cycle is then called a sex cycle and is usually divided into several phases: proestrus, estrus, metestrus, and diestras. The first two phases correspond to the follicular stage and the last two to the luteinic stage.

The sex cycle in women is called the menstrual cycle.