Morphological and histological changes in the brains of turbot (Scophthalmus maximus) with gonadal development
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Abstract: The brain plays a critical role in controlling reproduction through the hypothalamus-pituitary-gonadal (HPG) axis in vertebrates. Turbot (Scophthalmus maximus) has become an economically important marine fish in Europe and North China. Previous research investigating turbot reproduction has focused on the role of the HPG axis in regulating egg and sperm production. However, the morphology and histology of the organs in the HPG axis have not been studied. In this study, we investigated the morphology and histology of brains in female and male turbot at different stages of gonadal development. The results showed that the brains of both female and male turbot were composed of seven parts that are typical of advanced teleosts: the telencephalon, diencephalon, cerebellum, hypothalamus, pituitary gland, myelencephalon, and olfactory bulbs. The telencephalon was well-developed and contained five distinct lobes, with the contiguous diencephalon at the caudal portion. The torus longitudinales and rostral torus semicircularis of the mesencephalon flattened along the dorsal surface, and the rostral corpus cerebellum was located in the dorsal portion. The actual total brain volume in mature males was significantly greater (p<0.05) than that of females with gonadal development. Notably, the pituitary volume in male turbot significantly increased (p<0.05) from immature to mature stage, but this difference did not occur in females. The data together illustrate a distinct sex difference in the turbot brain during gonadal development, providing insight into their HPG axes.
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Key words:
- turbot /
- brain /
- pituitary /
- gonadal development /
- morphology /
- histology
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Figure 1. Characteristics of turbot at 8, 16, and 24 months post-hatching (mph). a. Total weight; b. total length. Error bars represent mean±SD (N=3). Different uppercase/lowercase letters represent statistical significance (p<0.05) between the two sexes.
Figure 2. Illustration of the ellipsoid method to assess brain volumes of turbot. a. Lateral view; b. dorsal view; c. ventral view; d. dorsal view. The measurements used to determine the sizes of the brain structures are indicated. W represents width; H, height; L, length; 1, telencephalon; 2, diencephalon; 3, cerebellum; 4, hypothalamus and saccus vasculosus; 5, pituitary; 6, myelencephalon; 7, olfactory bulb.
Figure 3. Morphological observation of turbot brain. a–b. Brain in turbot head; c. dorsal view; d. lateral view; e. ventral view. 1, telencephalon; 2, diencephalon; 3, cerebellum; 4, hypothalamus and saccus vasculosus; 5, pituitary; 6, myelencephalon; 7, olfactory bulb. Scale: 2 mm.
Figure 4. Transverse sections of brain of turbot (I). a–d. Telencephalon, diencephalon, mesencephalon, and cerebellum. Tissue abbreviations: Ac represents anterior commissure; Vp, ventral posterior nucleus; Dc, dorsocentral nucleus of the telencephalon; Dd, dorsodorsal nucleus; Dl, dorsolateral nucleus of the telencephalon; Dm, dorsomedial nucleus of the telencephalon; DM, dorsal medial nucleus of the thalamus; Dp, dorsal posterior nucleus of the telencephalon; G, nucleus glomerulosus; Ha, habenula; III, oculomotor nerve; LL, lateral lemniscus; LR, lateral recess of the inferior lobe; MLF, medial longitudinal fasciculus; ND, nucleus diffusus of the inferior lobe; NPTec, pretectal nucleus; OB, olfactory bulb; PM, magnocellular nucleus; PP, preoptic nucleus; Vcb, valvula cerebelli; SV, saccus vasculosus; Tec, tectum of the mesencephalon; Tl, torus longitudinalis of the mesencephalon; Ts, torus semicircularis of the mesencephalon. Scale: 1 mm.
Figure 5. Transverse sections of brain of turbot (II). a–d. Cerebellum and myelencephalon. Tissue abbreviations: BC represents brachium conjunctivum; CC, crista cerebellaris; CCb, corpus division of the cerebellum; VIII, auditory/vestibular nerve complex; Tec, tectum of the mesencephalon; EG, eminentia granularis; LL, lateral lemniscus; MLF, medial longitudinal fasciculus; RF, reticular formation; Vm, motor nucleus of the trigeminal nerve; Ts, torus semicircularis of the mesencephalon. Scale: 1mm.
Figure 6. Morphology of turbot brain at 8 months post-hatching (mph) (a, d), 16 mph (b, e) and 24 mph (c, f). 1, telencephalon; 2, diencephalon; 3, cerebellum; 4, hypothalamus and saccus vasculosus; 5, pituitary; 6, myelencephalon. The pituitary gland is marked with a red dotted circle in the lateral view. Scale: 2 mm.
Figure 7. The actual brain structure volumes at different reproductive stages in both sexes. a. 8 months post-hatching (mph); b. 16 mph; c. 24 mph. d. the actual total brain volumes at different stages in both sexes. Abbreviations: Te, telencephalon; Me, mesencephalon; Ce, cerebellum; Hy, hypothalamus; Pi, pituitary glands; Mo, myelencephalon; Ob, olfactory bulb. Error bars represent mean±SD (N=15). Statistical significance (p<0.05) marked with A, B in female group, and A', B' in male group. Asterisk (*) indicates significant difference (p<0.05) between the sexes.
Figure 8. Comparison of relative brain structure volumes in female (a) and male (b) turbots at different stages. Abbreviations: Te, telencephalon; Me, mesencephalon; Ce, cerebellum; Hy, hypothalamus; Pi, pituitary glands; Mo, myelencephalon; Ob, olfactory bulb. Error bars represent mean±SD (N=15). Different letters represent significant differences among the stages (p<0.05).
Figure 9. Histology of ovaries and testes at 8 months post-hatching (mph) (a, e), 16 mph (b, f), and 24 mph (c, g). d. Genadosomatic Index (GSI) of females; h. GSI of males. II, primary vitellogenic oocytes; III, IV, large growth of vitellogenic oocytes; V, mature oocytes. Abbreviations: A-SG, A type-spermatogonia; B-SG, B type-spermatogonia; PSC, primary spermatocytes; SSC, secondary spermatocytes; ST, spermatids; SZ, spermatozoa. Scale: a–c, 100 μm; e–g, 50 μm. Error bars represent mean±SD (N=3). Different letters represent significant differences among stages (p<0.05).
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