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Fish Reproduction - Anatomy & Physiology (view source)
Revision as of 13:19, 29 July 2008
, 13:19, 29 July 2008→Reproductive Endocrinology
= Reproductive Endocrinology =
= Reproductive Endocrinology =
In fish, as with all higher animals, hormones play a critical role in the reproductive process. Hormones are chemical messengers released into the blood by specific tissues, such as the pituitary gland. The hormones travel through the bloodstream to other tissues, which respond in a variety of ways. One response is to release another hormone, which elicits a response in yet another tissue. The primary tissues involved in this hormonal cascade are the hypothalamus, pituitary gland, and gonads.
* Fish have evolved to reproduce under environmental conditions that are favorable to the survival of the young.
* Long before spawning, seasonal cues begin the process of maturation.
** In many fish, this can take up to a year.
* When the gametes have matured, an environmental stimulus may signal the arrival of optimal conditions, triggering ovulation and spawning.
** Examples of environmental stimuli are:
*** Changes in photoperiod
*** Temperature
*** Rainfall
*** Food availability.
* A variety of sensory receptors detect these cues, including the eye, pineal gland (an organ in the dorsal part of the forebrain that is sensitive to light), olfactory organs, taste buds, and thermoreceptors.
* The hypothalamus, located at the base of the brain, is sensitive to signals from sensory receptors and releases gonadotropin releasing hormones (GnRH) in response to environmental cues.
* GnRH travels from the hypothalamus to the pituitary gland.
* Gonadotroph cells of the pituitary receive GnRH and release gonadotropic hormones into the bloodstream.
* The gonadotropic hormones travel to the gonads, which synthesize steroids responsible for final maturation of the gametes.
* Maturation of the egg is a long process that involves complex physiological and biochemical changes. One important step, vitellogenesis, is a process in which yolk proteins are produced in the liver, transported to the ovary, and stored in the egg, resulting in tremendous egg enlargement. The yolk is important as a source of nutrition for the developing embryo.
* Also critical are germinal vesicle migration and germinal vesicle breakdown (GVBD).
** Before it migrates, the germinal vesicle, or nucleus, is located at the center of the egg in an arrested stage of development.
** At this stage, the egg is physiologically and genetically incapable of being fertilized, even though it has the outward appearance of a fully mature egg.
** When conditions are appropriate for final maturation, nuclear development resumes, and the germinal vesicle migrates to one side.
** Finally, the walls of the germinal vesicle break down, releasing the chromosomes into the cell.
* After the egg has matured, prostaglandins are synthesized.
** Stimulate ovulation
* The egg is then released into the body cavity or ovarian lumen, where it may subsequently be released to the outside environment.
* Following ovulation, the viability of the eggs can decrease rapidly.
= Sex Determination =
= Sex Determination =