**3. Physiology and endocrinology of the estrual cycle of the goat**

The ovarian cycle is classically divided into two phases: follicular phase and luteal phase (**Figure 1**). The follicular phase corresponds to the wave of follicular development that will provide the ovulatory follicle and involves the maturation of follicles that are dependent on the gonadotropins until ovulation [6]. During the follicular phase, FSH secreted by the anterior pituitary stimulates follicular growth. A cohort of antral follicles, which are gonadotropin dependent and with a diameter of 2–3 mm, is recruited, and the follicles enter their

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

the more goat is exploited at latitudes more distant from the equator, the breeding season will be shorter [2]. However, in latitudes nearer from the equator, the presence of estrus will depend on the availability of nutrients and the environment. On average, the goat's estrus cycle is 21 days, and the high frequency of short estrual cycle is characteristic and tends to occur at the beginning of the reproductive season and in young animals [1, 2]. The average duration of standing estrus is 36 hours but can range from 24 to 48 hours depending on age,

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

**3. Physiology and endocrinology of the estrual cycle of the goat**

The ovarian cycle is classically divided into two phases: follicular phase and luteal phase (**Figure 1**). The follicular phase corresponds to the wave of follicular development that will provide the ovulatory follicle and involves the maturation of follicles that are dependent on the gonadotropins until ovulation [6]. During the follicular phase, FSH secreted by the anterior pituitary stimulates follicular growth. A cohort of antral follicles, which are gonadotropin dependent and with a diameter of 2–3 mm, is recruited, and the follicles enter their

breed, season, and presence of a male [3].

**2. Folliculogenesis**

88 Goat Science

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 waves [8, 9].

Also, the luteal phase begins when the corpus luteum is formed after follicle luteinization (duration 16 days). During this phase, LH is released pulsatile and, its frequency is negatively correlated with progesterone. Now, progesterone has a negative feedback effect on LH. Luteolysis begins around day 16–17 of the estrus cycle, releasing uterine prostaglandins, influenced by oxytocin. With the above, the concentration of progesterone decreases, causing a strong increase in the frequency of pulses of LH and its amplitude, which causes ovulation [10].

medroxyprogesterone. In the 1980s, controlled internal drug releasing (CIDR) began to be used, which is an inert silicone elastomer containing 0.3 g of natural progesterone (P4) [16]. These devices are inserted intravaginally for a period of 5–14 days (see **Figure 2**) to create a luteal phase and then accompany it with a luteolytic agent, as well as the application of a hormone that synchronizes ovulation (equine chorionic gonadotropin (eCG), estradiol benzoate (EB), gonadotropin*-*releasing hormone (GnRH)), which have been

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Prostaglandin F2α (PGF2α) and its analogues have also been used to synchronize estrus by controlling luteal function since PGF2α was discovered to have a luteolytic effect in sheep. During the estrous cycle, PGF2α is secreted by the nonpregnant uterus to 16 days after estrus [5]. Administration of PGF2α after removal of a CIDR mimics the secretion of PGF2α by the uterus, causing lysis of the CL and the onset of a new follicular phase [18]. Administration of PGF2α is effective from approximately d 3 to d 14 of the estrous cycle in sheep [19]. Analogues of prostaglandins can also induce luteolysis and are often more cost-effective. The effectiveness of PGF2α is limited to the active period of cyclicity in small ruminants. The lack of ovulation of the follicle during seasonal anestrous causes a lack of luteal development [19]. A single administration of PGF2α can induce luteolysis, and two PGF2α injections at an interval of 10–12 days have been used to synchronize estrus (see **Figure 3**). These treatments can be used on cycling goats only, limiting its application during the extended periods with nonfunctional corpora

applied at the time, 24 and 36 hours to remove the device, respectively [17, 18].

**Figure 2.** Short protocol at 5 days and ovulation synchronization with OB or GnRH.

**Figure 3.** Protocol of synchronization of estrus with prostaglandins within the reproductive season.

*4.1.1. Prostaglandin-based synchronization*

lutea [20].

#### **3.1. Extern symptom of the estrus**

The external symptoms of estrus that can be mentioned as important are goats move the tail, increase vocalizations, decrease appetite, mount between them, increase urine excretion, inflammation of the vulva, and discharge of vaginal mucus.
