Histology and histochemistry are useful tools
to study reproductive mechanisms in fish and they have
been applied in this study. In the bluefin tuna, Thunnus
thynnus L., oocyte development can be divided into 4
principal phases based on the morphological features of
developing oocytes and follicles. The primary growth
phase includes oogonia and basophilic or previtellogenic
oocytes classified as chromatin-nucleolus and
perinucleolus stages. The secondary growth phase is
represented by vitellogenic oocytes at early (lipid
globule and yolk granule 1), mid (yolk granule 2) and
late (yolk granule 3) vitellogenesis stages. The
maturation phase involves postvitellogenic oocytes
undergoing maturation process. During the spawning
period, both postovulatory follicles, which indicate
spawning, and atretic follicles can be distinguished in
the ovary.
Carbohydrates, lipids, proteins and specially those
rich in tyrosine, tryptophan, cystine, arginine, lysine and
cysteine, as well phospholipids and/or glycolipids and
neutral glycoproteins were detected in yolk granules.
Moreover, affinity for different lectins (ConA, WGA,
DBA and UEA) was detected in vitellogenic oocytes
(yolk granules, cortical alveoli, follicular layer and zona
radiata), indicating the presence of glycoconjugates with
different sugar residues (Mannose- Man- and/or Glucose
-Glc-; N-acetyl-D-glucosamine- GlcNAc- and/or sialic
acid- NANA-; N-acetyl-D-galactosamine- GalNAc-; LFucose
-Fuc-). Histochemical techniques also
demonstrated the presence of neutral lipids in globules
(vacuoles in paraffin sections) and neutral and
carboxylated mucosubstances in cortical alveoli. By
using anti-vitellogenin (VTG) serum, immunohistochemical
positive results were demonstrated in yolk
granules, granular cytoplasm and follicular cells of
vitellogenic oocytes. Calcium was also detected in yolk
granules and weakly in follicular envelope.
In females, the gonadosomatic index (GSI) increased progressively from May, during early vitellogenesis,
until June during mid and late vitellogenesis, where the
highest values were reached. Subsequently, throughout
the maturation-spawning phases (July), GSI decreased
progressively reaching the minimal values during
recovering-resting period (October)