Circannual changes in morphological, ultrastructural and hormonal activities of the ovary of an estuarine grey mullet, Mugil cephalus L.

Abstract

The objective of this study was to investigate the circannual ovarian cycle month-wise round the year at morphological, ultrastructural and hormonal levels in an estuarine teleost, the grey mullet Mugil cephalus L. The gonadosomatic index and follicular population per ovary began to rise from September to October (pre-breeding), reached peak during November and December (breeding) and declined from January to August (post-breeding phase). Oocyte diameter increased during pre-breeding, attained maximum size in breeding and drastically reduced in post-breeding phase. But the follicular populations per milligram of ovary per microscopic field were moderately high in pre-breeding, least in breeding and highest in post-breeding. Histologically, the ovarian lobules were moderately large and contained small and large follicles in pre-breeding, maximally enlarged with mostly large follicles in breeding and became narrow with very small follicles in post-breeding. Ultrastructurally, three types of oocytes (1) small early primordial oocytes with a thin plasma membrane, (2) large pre-vitellogenic type with numerous lipid droplets and (3) early vitellogenic oocyte with vitelline membrane and microvilli were identified in pre-breeding. In the breeding phase, two types of oocytes, the late vitellogenic oocyte, showed zona interna and zona externa with long microvilli penetrated in between the oocyte and zona externa and interna. These features were no longer seen in post-vitellogenic oocytes. Serum follicle stimulating hormone (FSH), prolactin (PRL), estradiol (E2) and testosterone levels reached peak in pre-breeding, and luteinizing hormone (LH) and 17α,20β-dihydroxy-4-pregnen-3-one (DHP) in the breeding phase. It is concluded that FSH, PRL, E2 and testosterone trigger oocyte growth in pre-breeding, and LH and DHP cause further growth and oocyte maturation in breeding.