**3. Ovarian follicular population**

The follicular population may vary between individuals, and some factors such as genetics, breed, age, species, and hormone levels can influence the number of follicles present in the ovaries [13]. For bovine females, it is estimated that the number of follicles at birth is about 235,000 [14].

An increase in the number of antral follicles present in the ovary and stimulated by gonadotrophin secretion is influenced by body development in heifers [15]. Endocrine activity at first seems to be controlled by suppressing negative feedback mechanisms until the heifer has a good body condition to initiate the estrous cycle and reproduction activities [16–18].

The first ovulation in heifers is marked by a gradual increase in LH secretion, which leads to the development of the antral follicles and the secretion of estrogen. At birth, antral follicles are not typically present in the ovaries, and the number of follicles increases when heifers reach 2 months of age. After that, the number of follicles declines at 5 months of age, and some changes continue to occur throughout the productive life of the female [19–21].

## **4. Antral follicle count and fertility in dairy cattle**

Ovarian follicular population is highly variable among species, a concept that is well established [20–22] and has been already reported in cattle [23]. In recent

years, numerous studies have focused on the ovarian follicular population and its influence on reproductive activities, as well as on animal reproduction biotechniques [22, 24]. The antral follicle count (AFC) is a strategy to identify different profiles of cows, performing transrectal B-mode ultrasonography and counting all follicles larger than 3 mm [25]. The total number of follicles counted in the pair of ovaries is added up, and the cow is classified as low, medium, or high AFC. A feature in cattle is the high variability of AFC between animals, but it is known that there is high repeatability in the same individual [19, 26, 27]. The appearance of the ovary on ultrasound examination of cows with high and low AFC is presented in **Figure 2**.

Furthermore, according to [28], it has already been established that the concentrations of anti-müllerian hormone (AMH), which is released by the granulosa cells of growing ovarian follicles, are positively related to the entry into puberty of bovine females. In other words, the higher serum concentrations of this hormone are, the higher AFC will be [27]. AMH is a glycoprotein that belongs to the TGF-β growth factor family [29] and it is correlated with follicular growth [30]. AMH is in the granulosa cells and it is responsible for the growth of preantral and antral follicles [31] and follicular growth modulator through the control of ovarian follicular reserve depletion [32].

The intrafollicular AMH expression increases until the follicle reaches 5 mm in cows and then decreases as the follicle reaches the antral stage and increases in size [31]. The positive correlation of AMH with the ovarian follicular population has already been described in previous studies [33, 34]. Thus, the measurement of AMH can be a method of predicting AFC [35]. In *Bos taurus taurus*, *Bos taurus indicus*, and *taurus* × *indicus* crosses, animals with a high plasma concentration of AMH present a greater number of antral follicles than those with a low concentration of this hormone [36].

Reproductive biotechniques, such as embryo transfer (ET) and IVEP, depends on the population of antral follicles present in the ovary of donor females to succeed. Among other factors that interfere with ET and IVEP, it is important to mention genetics, breed, and age [37, 38]. High AFC bovine females have been described to have a greater number of viable embryos produced *in vivo* per animal [39–41]. Similarly, in IVEP—ovum pick-up (OPU) procedures, high AFC animals resulted in a higher rate of blastocyst production than low AFC females [24, 42].

In contrast, a high conception rate was observed after the use of TAI in low AFC females *Bos taurus indicus* compared to high AFC animals [43, 44]. Additionally,

#### **Figure 2.**

*Aspect of the ovary on ultrasound examination of cows with different counts of antral follicles (AFC). On the right (A) cow with low AFC, and the left (B) cow with high AFC. Images were generated via the transrectal route with the equipment model S8v (SonoScape®) with a frequency of 8.6 MHz and a linear transducer of 5–10.0 MHz.*

*Folliculogenesis, Fertility and Biotechnology in Dairy Cattle DOI: http://dx.doi.org/10.5772/intechopen.101243*

other authors [45] observed that high AFC *Bos taurus taurus* females had lower fertility and shorter reproductive life than females with low AFC, but contrasting data have been reported [46].

Donors with a high number of antral follicles have been selected, mainly for OPU-*in vitro* fertilization (IVF) procedures, due to the quantitative advantages for producing *in vitro* embryos. Because of the high number of calves generated from IVF, there is an increasing interest in studying the relationship between AFC and reproductive characteristics.

In summary, several studies have tried to verify the influence and the correlation of AFC in the reproductive performance of bovine females. So far, it is not possible to establish the role of AFC in fertility parameters due to the controversial results. Although it is quite predictable that AFC may be related to reproductive efficiency, a better understanding of the subject is necessary. Furthermore, considering basic research, it is necessary to elucidate some aspects of follicular physiology that remain unknown [47].
