**1. Introduction**

The reproductive system of the bovine female includes several organs such as ovaries, genital pathways or tubular portion that surround the oviducts, uterus, vagina, and vulva, the attached glands, embryonic vestiges, blood vessels, and nerves. The ovaries have two differentiated portions: a medullary zone formed by connective tissue, fibroelastic and vessels (arteries, veins, and lymphatics), and nerves, all together responsible for the conservation and nutrition of the organ. The other portion is the cortical zone, which is surrounded by the germinal epithelium and the tunica albuginea within it. The follicles and corpora lutea in their different stages of development and regression are located in this last portion. In adult females, this structure of the cortical zone undergoes cyclical changes according to the regulation of the sexual cycle or of pregnancy. The female sexual organs include two functions essential to the reproduction of females, a gametogenic function, composed by folliculogenesis and oogenesis, and an endocrine function in which the main produced hormones are estrogen, progesterone, and relaxin. The ovary of the cow or heifer, relatively to its weight is small when compared to other species, has an ovoid shape and varies in size and contour at each cycle due to the projection of follicles and corpus lutea on its surface (**Figure 1**).

#### **Figure 1.**

*The ovary where the following can be observed: A follicle (A) and a corpus luteum (B). In the upper right corner, these ovarian structures are emphasized.*

Its weight ranges from 3 to 20 g and can reach 37 mm in length. In the cortical area of the ovaries, structures called follicles are present, which cyclically enter the growth phase. On average, if cow or heifer is not pregnant, a follicle grows and ovulates every 21 days. When follicles reach maturation, they rupture and release the oocyte within. The primordial follicle is an oocyte surrounded by a single layer of low follicular cells. When these cells multiply by mitosis, this is called the secondary follicle. When it reaches its maximum development, it is called the vesicular, mature, or antrum follicle. Follicular development is observed in the fetal period, in prepubescent heifers, heifers, and cyclic cows and during gestation. The oocytes are conceived from germ cells that originate the female sex cells, which contain in their cytoplasm ribosomes, mitochondria, glycogen granules, large lipid droplets and endoplasmic reticulum, and poorly developed Golgi complex. During oocyte growth, changes in the distribution, number, and size of cytoplasmic organelles occur [1]. The oocyte is surrounded by the zona pellucida (**Figure 2**), which is a dense membrane with multiple functions, being primordial for the normal development of the follicle [2].

#### **Figure 2.**

*Fine detail of a bovine oocyte zone pellucida captured by transmission electron microscopy. The bar represents 1 μm.*

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**Figure 3.**

*Bovine Embryonic Development to Implantation DOI: http://dx.doi.org/10.5772/intechopen.80655*

**2. Oocyte maturation and ovulation**

ovulatory cycle occurs until the tenth day.

time of its expulsion.

mature oocyte (**Figure 3**).

*Bovine typical oocyte's aspect after maturation.*

During the evolution of the oocyte, the nucleus that had entered the prophase of the first meiotic division will support the reductive divisions. Two daughter cells appear that contain half of the chromosome load in the first division, where each of the cells gets a large part of the cytoplasm, called the secondary oocyte. The smaller one is called the first polar body. Throughout the second meiotic division, the secondary oocyte divides into two (ovoid and second polar body). The corpus luteum is an endocrine gland that occurs by cycles in the ovary of females and has a short secretory activity during the sexual cycle. In the bovine species, it has an ovoid or spherical shape. Its main function is the production of progesterone, which is responsible for the preparation of the endometrium and the blastocyst for implantation. According to Gordon [3], it takes 11 days for the corpus luteum to develop and reach a 4 g weight in a female of beef cattle; in a female of milk breed, its weight is higher. The fast growth of the yellow body in the first phase of the

The infundibulum is a funnel-shaped tube that encloses the ovary during the ovulation. It serves to capture oocytes and channels them to the oviduct, where fertilization occurs in the presence of viable spermatozoa [4]. Finally, the uterus is a muscular, cavitary, pelvic abdominal organ and with great capacity of dilation and displacement to welcome the development of the embryo. This organ is divided into three parts, uterine horns and posteriorly, through the cranial orifice of the cervical canal, cervix which is the caudal portion of the uterus with a well-individualized structure due to its thick wall, constricted light and full of protrusions and recesses, the cervical rings [1]. The body of the cow's uterus is short and undeveloped. Its size varies with age and number of deliveries and can reach 5 cm in length. It has several functions like assisting the transport of the spermatozoa to the oviduct and helping in the expulsion of the newborn. In this organ the placenta that will allow nutrition and protection to the fetus also develops [4]. The cervix is a unique structure within the reproductive apparatus of the cow. It has thick walls and attaches the vagina to the uterus. Its main function is to protect the uterus from the external environment. The vagina is a copulatory organ that has a thin, elastic wall that allows its distension during mating and delivery. It serves as a free passage for the calf at the

Folliculogenesis begins with the formation of primordial follicles, progressing to primary, secondary, tertiary, and preovulatory, and ends with the ovulation of a

*corner, these ovarian structures are emphasized.*

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*1 μm.*

**Figure 2.**

**Figure 1.**

*Fine detail of a bovine oocyte zone pellucida captured by transmission electron microscopy. The bar represents* 

Its weight ranges from 3 to 20 g and can reach 37 mm in length. In the cortical area of the ovaries, structures called follicles are present, which cyclically enter the growth phase. On average, if cow or heifer is not pregnant, a follicle grows and ovulates every 21 days. When follicles reach maturation, they rupture and release the oocyte within. The primordial follicle is an oocyte surrounded by a single layer of low follicular cells. When these cells multiply by mitosis, this is called the secondary follicle. When it reaches its maximum development, it is called the vesicular, mature, or antrum follicle. Follicular development is observed in the fetal period, in prepubescent heifers, heifers, and cyclic cows and during gestation. The oocytes are conceived from germ cells that originate the female sex cells, which contain in their cytoplasm ribosomes, mitochondria, glycogen granules, large lipid droplets and endoplasmic reticulum, and poorly developed Golgi complex. During oocyte growth, changes in the distribution, number, and size of cytoplasmic organelles occur [1]. The oocyte is surrounded by the zona pellucida (**Figure 2**), which is a dense membrane with multiple functions, being primordial

*The ovary where the following can be observed: A follicle (A) and a corpus luteum (B). In the upper right* 

for the normal development of the follicle [2].
