Reproductive Physiology of the Mummichog Fundulus heteroclitus—An Excellent Experimental Fish
The mummichog is a euryhaline cyprinodont fish native to North America. In this monograph, various reproductive properties, i.e. gonadal changes during ontogeny, gonadal and endocrine changes during annual and daily reproductive cycles, and environmental and endocrine control of reproduction are described. This fish is a spring to summer spawner showing distinct annual changes in gonadal states. A reared strain of this fish shows daily spawning also indicating distinct cycles of oocyte
... of oocyte development and maturation. Their annual reproductive cycle is precisely controlled by environmental factors, such as lowering temperature in autumn and winter, warm temperature in spring, and short photoperiod in early autumn. These factors induce the early phase of gonadal development, initiation of the spawning period, and termination of the spawning period, respectively. Their gonadotrophs (FSH cells and LH cells) showed prominent changes during the ontogeny and the annual reproductive cycle. FSH cells appeared at very early stages in the ontogeny, and their abundance showed good correlation to the gonadal stages in the annual cycle. LH cells appeared later, and were abundant only during the spawning period. Plasma FSH levels also showed good correlation to the gonadal stages, whereas plasma LH levels were high only during the spawning period. These findings indicate that FSH is important for gonadal development and LH is responsible for final gametes maturation. Such functional differentiation of GtHs is also indicated by studies on their receptors (FSHR and LHR). The above described researches show that the mummichog is an excellent model fish for studying environmental and endocrine control of reproductive cycles, especially for GtH related studies because of the presence of sufficient analyzing tools (specific antibodies, purified hormones, ELISA systems, receptors, etc.). Fig. 3. Photomicrographs of gonadal sections of the mummichog in various developmental stages. 1-5, 7-13: PAS and Mayer's hematoxilin staining. 6: Immunostaining with anti-vitellogenin. 1: Ovary, 3 weeks after hatching. Perinucleolus stage oocytes are observed. 2: Ovary, 6 weeks after hatching. Early cortical alveolus stage oocytes are observed. 3: Ovary, 12 weeks after hatching. Late cortical alveolus stage oocytes are present. 4: Ovary, 24 weeks after hatching. Early yolk globule stage oocytes are observed. 5: Ovary, 36 weeks after hatching. Maturing oocytes are present. 6: Ovary, 36 weeks after hatching. Strong immunoreactivities are observed in the plasma in the blood vessel (V), ovarian fluid (the arrow), granulosa cells (arrowheads), and a vitellogenic oocyte (asterisks). 7: Testis, 3 weeks after hatching. The number of germ cells (type A spermatogonia) has increased. 8: Testis, 4 weeks after hatching. Spermatocytes are observed. 9: Testis, 6 weeks after hatching. Spermatids and spermatozoa are present. 10: Testis, 8 weeks after hatching. Figures of basic spermatogenesis are seen. 11: Testis, 12 weeks after hatching. Figures of basic spermatogenesis are seen. The numbers of germ cells have increased. 12: Testis, 24 weeks after hatching. Figures of active spermatogenesis are seen. 13: Testis, 36 weeks after hatching. Figures of spermiation (the asterisk) are observed. Bars = 50 µm. Reprinted from Gen. Comp. Endocrinol., 156, Shimizu et al., Appearances and chronological changes of mummichog Fundulus heteroclitus FSH cells and LH cells during ontogeny, sexual differentiation, and gonadal development, 312-322, 2008, with permission from Elsevier.