**2. Folliculogenesis**

In mammals, germ cells originate from the extraembryonic endoderm and migrate by amoeboid movement into the coelomic cavity, to reach the urogenital mesodermal crest. Subsequently, the germ cell is transformed into oogonium, which should populate the gonad by mitotic processes. At the end of mitosis, the oogonium enters the meiotic cycle until prophase I, where it acquires the primary oocyte state [4]. Therefore, folliculogenesis begins when the primordial follicle is formed, due to the union of the primary oocyte and granulosa cells. In primordial follicles, the primary oocytes leave their state of latency spontaneously and continue to other phases of growth during which the differentiation and proliferation of the oocyte coexist with the surrounding cells and by the effect of the growth factors synthesized in the microenvironment ovarian; all these events are independent of the gonadotropins [5]. Different types of follicles are present during folliculogenesis. Primordial follicle: This follicle begins the process of follicular growth and maturation, to guarantee increasingly mature units that can lead the ovary toward ovulation or atresia. Primary follicle: It is characterized by a significant change occurring when the flattened cells surrounding the oocyte increase in size into a more cuboidal form, grow in diameter, and consolidate as a structure with oocytes having a diameter greater than 22.12 mm and a layer of 25–40 cuboidal cells called granulosa cells. It also increases the volume of the oocyte and the formation of the zona pellucida. Secondary follicle: The transition to this stage depends on the FSH stimulus. At this stage, granulosa cells develop the ability to synthesize growth factors and steroids [5].

terminal phase of growth [7]. Only two to three of these follicles reach a size of 4 mm in diameter and are selected to enter the dominance phase. Under the influence of LH, the follicles reach the preovulatory stage (6–9 mm), while the subordinate follicles degenerate (follicular atresia). The increase in the peripheral concentrations of estradiol 17β causes a positive feedback effect on the hypophysis-gonadotropin axis, due to the follicular growth inducing the goat's estrus behavior [8]. The consequent increase in gonadotropin*-*releasing hormone (GnRH) secretion induces an increase in the preovulatory LH peak, which will induce ovulation between 20 and 26 hours later, and finally the luteinization of follicular cells will occur. The beginning of the follicular phase, before the estrual behavior is observed, is known as proestrus. The estrus phase includes the events of the sexual behavior of the goat until ovulation. Consequently, a response to estrus and ovulation will mainly depend on the time of pregnancy and nutrition. Whereas providing the goat with short energy supplementation can increase the rate of ovulation. During the estrus cycle, the ovaries undergo a series of morphological (follicular recruitment and growth), biochemical (follicle maturation), and physiological (endocrine regulation) changes, which lead to ovulation. These cyclic changes that take place in the gonads are known as ovarian cycles. For follicular growth, it develops in the form of waves throughout the cycle (**Figure 1**). A follicular wave is characterized by the sequence of three gonadotropin-dependent events: recruitment, selection, and dominance. When performing repeated ultrasound studies, one can mention and/or suggest that there are between two and six waves of follicular development during the estrus cycle; in goats, there are usually three–four waves. The last follicular wave is the one that is going to give rise to the ovulatory follicle. When double ovulations occur, they are due to follicles derived from the same wave but two consecutive follicular

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**Figure 1.** Schematic representation of the estrual cycle of the goat (modified by Fatet et al. [3]).

waves [8, 9].
