Oocyte development in cattle: physiological and genetic aspects

Abstract

-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 100 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 0.35. 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.

Evidence that mastitis can cause pregnancy loss in dairy cows: A systematic review of observational studies

“…In the study by Moore et al (2005) (+++), the odds of PL during 35 to 41 d of gestation were 3.6 times higher in cows with DHIA test SCS >4.5 during the 30 d before AI compared with cows without mastitis (OR = 3.6; 95% CI = 1.0, 13.4). Although an explanation for this observed association was not reported in that study, we offer the following hypothetical explanation: subclinical mastitis can trigger a systemic inflammatory response that can affect follicular growth, oocyte quality, and embryo survival (Britt, 2008;Ribeiro et al, 2016). In Moore et al (2005) study, 381 pregnant cows were enrolled at the time of AI.…”

Shading effect on physiological parameters and in vitro embryo production of tropical adapted Nellore heifers in integrated crop-livestock-forest systems

“…In ruminants, studies have confirmed that it takes months for a primordial follicle to reach the preovulatory stage (Britt 2008 ). It is known that oocytes from preantral follicles are growing and with an intense synthesis of RNA (Oktem and Urman 2010 ), which makes them sensitive to environmental changes.…”

The Making of a Competent Oocyte – A Review of Oocyte Development and Its Regulation

“…The embryonic precursors of both the male and female gametes are the primordial germ cells (PGCs) -the undifferentiated cells that have the potential of developing into spermatozoa or oocytes (Edson et al, 2009). The fate of the germ cells is known during earliest stage of development stage, during cleavage or in the epiblast (Britt, 2008;Marlow, 2015;Downs, 2018). The germ cells then undergo two developmental stages.…”