Allometric study on the relation between the growth of preantral and antral follicles and that of oocytes in bovine ovaries.

We examined the relation between the growth of preantral and antral follicles and that of their oocytes in the ovaries of Holstein cows. We recovered follicles and oocytes (419 pairs) from the ovaries of 61 cows, and examined the relative growth relating the follicle diameter to the oocyte diameter by using six regression models for only healthy oocytes and all the oocytes including degenerated ones with and/or without zona pellucida. The best fitting model was found to be a hyperbolic regression (R(2): 0.999). The differentiated equation for the hyperbolic curve in normal oocytes with zona pellucida and the follicles was found to be y'=41.0/(x+0.253) (2): y and x are diameters of oocytes (microm) and follicles (mm), respectively. When follicles grew more than 4.0 mm in diameter, the growth rate of the oocytes calculated by the differentiation equation was found to be an asymptotic depression around zero. Thus, it is suggested that when the follicles grow more than 4.0 mm in diameter, the oocytes reach full size and cease to grow. Furthermore, it is considered that the equation can be applied to the assessment of normal growth in oocytes and follicles cultured in vitro.