Changes

* Long before spawning, seasonal cues begin the process of maturation.  

* Long before spawning, seasonal cues begin the process of maturation.  

** In many fish, this can take up to a year.

** 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.

* When the gametes have matured, an environmental stimulus may signal the arrival of optimal conditions, triggering [[The_Ovary_-_Ovulation_-_Anatomy_%26_Physiology|ovulation]] and spawning.

** Examples of environmental stimuli are:

** Examples of environmental stimuli are:

*** Changes in photoperiod

*** Changes in photoperiod

*** Rainfall

*** Rainfall

*** Food availability.

*** Food availability.

* A variety of sensory receptors detect these cues, including the eye, [[Endocrine_System_-_Pineal_Gland_-_Anatomy_%26_Physiology|pineal gland]] (an organ in the dorsal part of the [[Forebrain_-_Anatomy_%26_Physiology|forebrain]] that is sensitive to light), [[Special_Senses_-_Olfactory_-_Anatomy_%26_Physiology|olfactory organs]], [[Oral_Cavity_-_Tongue_-_Anatomy_%26_Physiology#Taste_Buds|taste buds]], and thermoreceptors.

* A variety of sensory receptors detect these cues, including the [[Special_Senses_-_Visual_-_Anatomy_%26_Physiology#Structure|eye]], [[Endocrine_System_-_Pineal_Gland_-_Anatomy_%26_Physiology|pineal gland]] (an organ in the dorsal part of the [[Forebrain_-_Anatomy_%26_Physiology|forebrain]] that is sensitive to light), [[Special_Senses_-_Olfactory_-_Anatomy_%26_Physiology|olfactory organs]], [[Oral_Cavity_-_Tongue_-_Anatomy_%26_Physiology#Taste_Buds|taste buds]], and thermoreceptors.

* The [[Endocrine_System_-_Hypothalamus_-_Anatomy_%26_Physiology|hypothalamus]], located at the base of the brain, is sensitive to signals from sensory receptors and releases [[Endocrine_System_-_Hypothalamus_-_Anatomy_%26_Physiology#Outputs|gonadotropin releasing hormones (GnRH)]] in response to environmental cues.

* The [[Endocrine_System_-_Hypothalamus_-_Anatomy_%26_Physiology|hypothalamus]], located at the base of the brain, is sensitive to signals from sensory receptors and releases [[Endocrine_System_-_Hypothalamus_-_Anatomy_%26_Physiology#Outputs|gonadotropin releasing hormones (GnRH)]] in response to environmental cues.

* GnRH travels from the [[Endocrine_System_-_Hypothalamus_-_Anatomy_%26_Physiology|hypothalamus]] to the [[Endocrine_System_-_Pituitary_Gland_-_Anatomy_%26_Physiology|pituitary gland]].  

* GnRH travels from the [[Endocrine_System_-_Hypothalamus_-_Anatomy_%26_Physiology|hypothalamus]] to the [[Endocrine_System_-_Pituitary_Gland_-_Anatomy_%26_Physiology|pituitary gland]].  

** '''GTH-II''' has LH-like activity.

** '''GTH-II''' has LH-like activity.

* The gonadotropic hormones travel to the gonads, which synthesize steroids responsible for final maturation of the gametes.

* The gonadotropic hormones travel to the gonads, which synthesize steroids responsible for final maturation of the gametes.

** GTH-I binds to theca and granulosa cells of follicles.  This induces testosterone production by theca cells.  Testosterone then travels to the granulosa cells, where it is converted to oestrogen (as in mammals). Oestrogen induces vitellogenin (egg yolk precursor protein) production in the liver.

** GTH-I binds to theca and granulosa cells of follicles.  This induces testosterone production by theca cells.  Testosterone then travels to the granulosa cells, where it is converted to oestrogen (as in mammals). Oestrogen induces vitellogenin (egg yolk precursor protein) production in the [[Liver_-_Anatomy_%26_Physiology|liver]].

** GTH-II binds to granulosa cells and induces production of progestins.   

** GTH-II binds to granulosa cells and induces production of progestins.   

* 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.

* 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.

** Finally, the walls of the germinal vesicle break down, releasing the chromosomes into the cell.

** Finally, the walls of the germinal vesicle break down, releasing the chromosomes into the cell.

* After the egg has matured, prostaglandins are synthesized.

* After the egg has matured, prostaglandins are synthesized.

**  Stimulate ovulation

**  Stimulate [[The_Ovary_-_Ovulation_-_Anatomy_%26_Physiology|ovulation]]

* The egg is then released into the body cavity or ovarian lumen, where it may subsequently be released to the outside environment.

* 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.

* Following [[The_Ovary_-_Ovulation_-_Anatomy_%26_Physiology|ovulation]], the viability of the eggs can decrease rapidly.

= Sex Determination =

= Sex Determination =