Follicular fluid composition in the grey seal (Halichoerus grypus) during the oestrous cycle

During the period of lactational oestrus, the corpus luteum of ovaries of grey seals decreased in size following the birth of the pup, while on the contralateral ovary one major large follicle rapidly expanded. These large follicles had the highest concentration of oestradiol (4282 +/- 609 ng ml-1) and progesterone (499 +/- 168 ng ml-1). Osmolality (322 +/- 3 mosmol kg-1) and the intrafollicular concentration of electrolytes (Na: 126 +/- 1; Cl: 96 +/- 1; Ca: 1.3 +/- 0.1 mumol ml-1) and proteins (94 +/- 1 mg ml-1) were independent of stage of lactation and follicle size. Concentrations were lower in follicular fluid than in plasma. The concentrations of triglycerides and, to some extent, those of vitamin E, cholesterol and phospholipids were affected by the decrease in the plasma concentration of these components with the onset of lactation and the increase in follicle size. These two events resulted in a marked decrease of these components in the largest follicles at the end of lactational oestrus. Vitamin A (exclusively as retinol), although a blood-borne component in follicular fluid, was the only component with a higher concentration in small and medium follicles than in plasma and decreased with increasing follicle size despite an increase in plasma retinol. This decrease and the negative correlation with intrafollicular oestradiol might indicate a high demand of preovulatory follicle structures for vitamin A owing to its possible importance in steroid hormone or protein synthesis or in both processes.(ABSTRACT TRUNCATED AT 250 WORDS)