Oocytes in cattle are formed during embryogenesis and develop within individual follicles in the cortex of the ovary. Dormant primordial follicles become active and undergo progressive development at regular intervals commencing during the late fetal stage and continuing throughout adulthood. Once activated, follicles and oocytes in a cohort either grow to maturation and ovulation or undergo atresia, ultimately depleting the ovaries of germ cells.
It takes an estimated days for a follicle and its oocyte to reach the mature ovulatory stage. Within an individual, number of follicles in one ovary is similar to number in the other ovary; however, there are large differences among individuals in total number of follicles present in both ovaries.
In follicles that are recruited into the preovulatory pool, the fluid filled antrum enlarges and growth can be monitored by ultrasonography. Preovulatory follicles grow in waves rather than in a continuous stream, and number of follicles detected in a wave is related positively to number of microscopic follicles populating the ovarian cortex.
Number of follicles in a wave is highly repeatable and the estimated heritability of number of follicles in heifers is approximately 0. Monitoring preovulatory follicle numbers by ultrasonography can be used to identify females that produce more embryos following superovulation or more natural twin births. Number of follicles in a wave can be influenced by energy balance and exogenous hormones, and quality of oocytes within those follicles can be influenced environmental factors, hormones and vitamins.
The potential exists to increase genetic progress through repeatedly harvesting oocytes from preovulatory follicles of genetically superior females. During development of the embryo, cells from the inner cell mass undergo differentiation to become specialized stem cells that form various tissues of the fetus.
Some stem cells become primordial germ cells, which are precursors of oogonia in the female and spermatogonia in the male. Primordial germ cells migrate from the embryonic yolk sac to the genital ridge and populate the developing gonads -- the bilateral ovaries of the female and the bilateral testes of the male.
Primordial germ cells in the bovine female fetus differentiate into oogonia during the first trimester of gestation. Oogonia divide mitotically well into the second trimester of gestation, generating a peak of approximately 2. During the second trimester, oogonia form egg nests in the ovarian cortex and enter the first prophase of meiosis where they become arrested in the diplotene stage.
Somatic germinal epithelial cells from the surface of the ovary invade the egg nests and form a single flattened layer of epithelial cells around each oocyte. The primordial follicle comprises the arrested oocyte surrounded by the single layer of epithelial cells. Once the primordial follicle is formed, the follicular epithelial cells that are in direct contact with the oocyte interact through localized cellular signals to regulate oocyte maturation. Follicle growth is regulated by signals between the oocyte and surrounding epithelial cells and between the epithelial cells and other somatic cells in the ovarian cortex and by signals emanating from systemic circulation.
Some primordial follicles that populate the fetal ovary begin to grow during the second trimester of gestation, but most remain in an arrested state. Growth is characterized first by enlargement of the flattened epithelial cells to become cuboidal-shaped granulosa cells. Next, the arrested oocyte increases in size and surrounding granulosa cells divide to form multiple layers.
Ultimately a fluid-filled central antrum forms and partially surrounds a cluster of cumulus granulosa cells surrounding the oocyte. Once follicles start growing there are two possible fates, atresia and elimination of the follicular cells and oocyte through apoptosis and other mechanisms or full maturation and ovulation of the oocyte.
Ovulation only occurs in postpuberal females that are undergoing estrous cycles, so most oocytes are lost through atresia. In fact, about two-thirds of the original population of germ cells is the fetus are lost by the time of birth. Nevertheless, hundreds of thousands of oocytes are present in a neonatal calf's ovaries and thousands can remain in the ovaries of mature cows Erickson b. Thus there is an opportunity to capitalize on this underused source of germ plasm to accelerate rate of progress in genetic selection and to enhance fecundity in livestock.
This paper will focus on some of the practical aspects of managing follicle growth to produce more oocytes, embryos and live births from cattle.
The microscopic appearance of follicles in a histological section of ovarian tissue from a mature cow is illustrated in Figure 1. The cytoplasmic membrane of the oocyte in the secondary follicle appears thicker because the zona pellucida layer has been secreted by the oocyte. The estimated time course of folliculogenesis in the bovine is 80 to days Britt, ; Figure 2. Primordial follicles may undergo progressive development soon after they are formed or they may remain dormant for years before beginning further development.
It is believed that primordial follicles are kept dormant by factors that reach the follicle through the vascularized ovarian cortex, because primordial follicles undergo spontaneous activation if they are isolated and cultured in vitro Cushman et al.
Once progressive follicle growth begins, there are a number of local and systemic regulatory factors that are required to sustain growth of the follicle to the fully mature Graafian stage preceding ovulation Figure 2. These factors include metabolic regulators such as insulin and insulin-like growth factor I IGF-I , steroids such as estradiol and testosterone, gonadotropins, and a broad array of growth factors that regulate cellular differentiation and multiplication.
The number of growth factors and hormones that regulate folliculogenesis are beyond the scope of this review, but a few will be mentioned in the context of manipulation of follicular development. In the last decade, much has been learned about relationships among various classes of follicles that populate the ovary.
For example, we conducted studies to determine how the population of primordial follicles in ovaries of cattle was related to populations of growing follicles and whether the population in any follicle class was related to number of ovulations after superovulation Cushman et al. Non-lactating Holstein and Jersey heifers and cows had one ovary removed for histological evaluation and were then given FSH to stimulate superovulation of the remaining ovary.
The number of ovulations corpora lutea, CL was counted approximately 6 days after ovulation. Cows that were Low responders had fewer primordial, primary, secondary and tertiary follicles than cows that were Medium or High responders Figure 3. Eventually these cells become known as 'granulosa' cells. The granulosa cells will secrete progesterone after ovulation. A thick glycoprotein layer develops between the oocyte and the zona granulosa, called the zona pellucida. Finally, the stroma around the follicle develops to form a capsule like 'theca'.
Theca is greek for 'box'. Only one of the maturing follicles completes the maturation process each month. The rest degenerate into atretic follicles. Follicular maturation takes about 3 months. This is an image of a secondary follicle. Can you identify the oocyte , theca interna and externa , follicular cells , and follicular fluid? The primary follicle develops into a secondary follicle. The secondary follicles look very similar to primary follicles, except that they are larger, there are more follicular cells, and there are small accumulations of fluid in the intracellular spaces called follicular fluid nutritive fluid for the oocyte.
These gradually coalesce to form an antrum. The surrounding granulosa cells is called the cumulus oophorus greek for 'egg bearing heap'. The surrounding theca differentiates into two layers: the Theca interna rounded cells that secrete androgens and follicular fluid and a more fibrous Theca externa - spindle shaped cells.
The androgens are converted into oestrogen by the granulosa cells. Can you identify the antrum, membrana granulosa, cumulus oophorus, theca externa and theca interna in this image of a Graafian follicle? The first meiotic division is now completed, and the oocyte is now a secondary oocyte, and starts its second meiotic division.
After the first meiotic division, most of the cytoplasm goes into one of the two daughter cells. The other becomes the polar body hard to see. The folicular fluid fills a single space, called the antrum , which is surrounded by the follicular cells - called the membrana granulosa. Second phase of meiosis is arrested till secondary oocyte is fertilized with a sperm. Soon after fertilization, secondary oocyte undergoes meiosis II and produces a mature egg cell called the ovum. The ovum nucleus fuses with the sperm nucleus and produces the zygote, which can develop into an individual.
A follicle is a fluid-filled small sac found in female ovaries. They are roughly spheroid cellular aggregations. Normally, an ovary contains 8 to 10 follicles. They can be 2 mm to 28 mm in size. Follicles contain immature eggs known as oocytes. Follicle nourishes and protects the oocyte. Each follicle has the potential to release a mature egg cell or ovum for fertilization. In a normal menstrual cycle, one follicle will grow bigger until it ruptures at ovulation to release the egg.
This takes place on the 14 th day after the beginning of the menstrual cycle. Moreover, follicles secrete hormones which influence stages of the menstrual cycle and release essential reproductive hormones. Follicle size and status are vital information during fertility assessments and treatments. A pelvic ultrasound scan can assess the size and number of follicles present in the ovaries. Egg quality and follicle count are two important factors that affect successful pregnancy.
The number of follicles within the ovaries indicates the state of fertility.
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