However, simple ovarian histology provides a limited, binary (i.e., whether ovulation has occurred or not) information about pubertal ovarian maturation.
We propose herein a novel method (that we name Pub-score) to assess pubertal development based on the antral follicle developmental stages for still non-ovulating animals, and retrospective timing of the first ovulation in already ovulating ones, based on the dating of corpora lutea (CL) through evaluation of the maturational and regressive steps of functional and regressing CL.
In rats, it has been clearly established that VO and FE are tightly coupled with the first ovulation, and consequently, the age of VO is an indirect, but reliable, marker of puberty onset under physiological conditions.
Therefore, coupling of VO or vaginal epithelial cell cornification with the first ovulation might not be directly assumed under certain experimental settings in rats.
In female laboratory rodents, external signs of puberty, such as vaginal opening (VO) and epithelial cell cornification (i.e., first vaginal estrus, FE), are indirectly related to the maturational state of the ovary and first ovulation, which is the unequivocal marker of puberty.
Whereas in rats, VO and FE are almost simultaneous with the first ovulation, these events are not so closely associated in mice.
Puberty, as the developmental stage when reproductive capacity is achieved and sexual maturation completed, is a crucial event in the lifespan.
Timing of puberty, defined by the full (re)-awakening of the elements of the hypothalamic-pituitary-gonadal (HPG) axis, is under the control of numerous internal and external cues, and hence is determined by the complex interplay of genetic and environmental factors.
We propose herein the Pubertal Ovarian Maturation Score (Pub-score), as novel, reliable method to assess peripubertal ovarian maturation in rats and mice.Thus, in a previous study, Safranski and co-workers aimed to analyze the relationship between VO, FE and post-mating vaginal plug (VP) in mice; they reported that none of the animals had ovulated at the age of VO, only 15% did at the age of FE, whereas 91% had ovulated at the age of VP.Though the presence of VP appears as the most accurate external sign of ovulation, it requires mating at first estrus, which is obviously undesirable in most experimental designs.Rats and mice are the most widely used models in biomedical research.
Although they display similar ultra-short estrous cycles, relevant differences exist between these two rodent species regarding the sequence of puberty events.
Importantly, both VO and FE are indirect markers of puberty, as they are caused by the rise in estradiol levels during peripubertal period, associated to the first wave of follicular maturation, and can be induced in juvenile rats by estrogen administration.