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F. J. Schweigert and D. Schams

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 μmol 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. Changes in the chemical composition of follicular fluid and the morphological findings indicate a continuous development of the dominant follicle throughout the lactational oestrus in grey seals.

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F. J. Schweigert and H. Zucker

Summary. The degree of atresia of the follicle had no influence on the intrafollicular concentrations of β-carotene, vitamin E and cholesterol. This might result from the passive transfer of these substances from blood to follicular fluid bound to high density lipoproteins. However, concentrations of vitamin A in follicular fluid were significantly (P < 0·001) influenced by follicle quality, with highest concentrations (0·32 μg/ml) in non-atretic follicles and lowest values (0·15 μg/ml) in greatly atretic follicles. The higher concentrations of vitamin A in healthy follicles might be due to a local conversion of β-carotene into vitamin A in follicular structures. By influencing hormone and protein synthesis, vitamin A may have a potential for local modulation of follicular development and therefore be one of the factors controlling recruitment, selection and growth of the dominant follicle in cattle.

Keywords: follicular fluid; vitamin A; β-carotene; vitamin E; fertility; cattle