Balbiani body formation and cytoplasmic zonation during early oocyte development in two Clupeiform fishes

Abstract

The Balbiani body (Bb) was examined in primary growth phase oocytes for the first time in two clupeoid fish species, the Mediterranean sardine, Sardina pilchardus, and the European anchovy, Engraulis encrasicolus, which belong to different families, Clupeidae and Engraulidae, respectively. Cytoplasmic morphological changes of early secondary growth oocytes were also investigated using confocal laser scanning microscopy, light and transmission electron microscopy. The ultrastructural observations showed that the two species develop a distinct spherical Bb. However, differences in the cytoplasm, mainly in the perinuclear area, were observed. Briefly, in sardine the Bb coexists with a thick perinuclear ring containing mitochondria, nuage, endoplasmic reticulum and small vesicles, while in anchovy this perinuclear ring is thinner, consisting of complexes of nuage and mitochondria. After the disassembly of the Bb, a prominent cytoplasmic zonation develops in the secondary growth oocytes of sardine and anchovy, although with different organelle distribution between the two species. Sardine oocytes exhibit a thick zone of endoplasmic reticulum around the nucleus, whereas in those of anchovy, a thick mitochondria‐rich ring surrounding the nucleus was observed. The cytoplasmic characteristics, such as the perinuclear ring in primary oocytes in sardine and the mitochondria‐rich ring of early secondary oocytes in anchovy, are also discernible in histological sections by standard procedures and could thus be used as indicators of maturity or imminent spawning period in routine light microscopy observations, providing a valuable tool for applied fisheries biology.

Quantitative Analysis of Ovarian Dynamics of European Sardine Sardina pilchardus (Walbaum, 1792) during Its Spawning Period

“…The division of primary growth and cortical alveolar oocytes into two oocyte stages each, limited mean oocyte diameter variance, the most influential factor for OPD estimates [9]. As far as we know, such a division in PG oocytes was used for the first time in routine light microscopy observations of sardine oocytes, and it was achieved due to the use of the methylene blue/azure II/basic fuchsin stain that unveiled structures not always apparent [23]. Correction for histological oocyte shrinkage was also applied, providing more realistic estimates [10,25].…”

“…Oocytes were classified into nine developmental stages (Figure 2) according to published descriptions of sardine oocyte developmental stages and characteristics of early oocyte development [1,3,20,23]. Oocytes at primary growth phase (PG) were divided into two stages, PG1 and PG2.…”

“…Early primary growth oocytes (PG1) were smaller with homogenous, densely stained cytoplasm, while late primary growth oocytes (PG2) were larger and characterized by the appearance of zona radiata and a granular asymmetric cytoplasm. The appearance of the latter has been found to be associated with the formation of the Bb complex and the adjacent perinuclear ring [3,23]. Cortical alveoli oocytes (CA) were classified into early cortical alveoli (CA1), with vesicles appearing at the periphery of the cytoplasm forming a single layer, and late cortical alveoli oocytes (CA2), with larger vesicles forming multiple layers and fully formed follicular layers [1].…”