Puberty in females
On the surface, the definition of puberty seems simple enough: the age at which reproductive competence is achieved. However, in farm animals, several criteria have been used to define puberty in females.
For example, age at first estrus or heat can be a defining moment in cattle. However, this may or may not be associated with ovulation. Thus, this may not necessarily reflect reproductive competence. Ovulation can be evaluated by palpation of the ovary or ultrasound imaging in larger animals. However, in practical terms, the age at which a female can conceive and support pregnancy without harm to herself is probably an excellent definition, while the physiological demands to complete follicular development, ovulation, and transport of the fertilized ova to the uterus for implantation are relatively minor. The metabolic requirements to maintain the pregnancy and initiate lactation to support a rapidly growing neonate can be daunting. Thus, from a husbandry standpoint, it is rarely an advantage for females to become pregnant at the earliest possible time.Endocrinology of female puberty
In females the neurons of the hypothalamus that secrete GnRH must acquire the ability to secrete enough GnRH in response to feedback from ovarian estrogen to stimulate ovulation. This process is influenced by body mass, management, social cues, and genetics. For example, the range in months for the onset of puberty in cattle can be marked, that is, as low as 8.5 months in Holsteins to 19.0 months in Brahman cattle. We begin by recalling the relationship between the hypothalamic hormones, anterior pituitary hormones, and specifically the impact of gonadotropin-releasing hormone or GnRH on secretion of follicle-stimulating hormone (FSH) and luteinizing hormone (LH). It is known even in prepubertal animals that exogenous GnRH is capable of stimulating secretion of FSH and LH from the anterior pituitary and that FSH and LH can stimulate follicular development in the ovary. However, sustained follicular development and maturation of selected follicles to undergo ovulation requires sustained secretion of these gonadotropic hormones.
This suggests that the primary delay is a failure of the hypothalamus to produce enough GnRH in prepubertal animals.Senger (2003) has likened this gradual process to a rheostat in control of a light. As the rheostat is gradually turned higher, the light in the room becomes more and more intense until maximal brightness is reached. You may recall that the hypothalamus has a number of specific nuclei. The secretion of GnRH is controlled by a tonically acting center located in the area of the dorsomedial nucleus. Before puberty, secretion of GnRH occurs infrequently, and the amplitude of the each of the secretory events is also low. This means that corresponding effects on secretion of FSH and LH from the anterior pituitary are also reduced. However, as ovarian development progresses, secretion of estradiol from waves of growing follicles increases. Over time, the estradiol (along with effects of environment, nutrients, and social interactions) escalates so the frequency and amplitude of bursts of GnRH secretion from this tonic center in the hypothalamus are increased. This produces more FSH and LH and, in a positive cascade, more follicular activity.
However, ovulation requires not just low-level secretion of LH but rather a marked surge in the concentration. This is called the preovulatory LH surge. This requires activation of a second population of hypothalamic nuclei called the surge center. These nuclei are located more anterior in the hypothalamus (preoptic and anterior) nuclei. Essentially, the prepubertal female is characterized by having insufficient ovarian-derived estradiol to stimulate the surge center. As she matures, her hypothalamus becomes progressively more sensitive to estradiol.