Puberty is defined as the ability to accomplish reproduction successfully (10). It can be defined in many ways in females such as the onset of estrus, the age of first ovulation, or the age at which an individual can support pregnancy without negative effects (10). 

Timing of the Estrous Cycle: 

• Length of estrous cycle = average of 17 days

• Follicular phase (proestrus & estrus) = 20% of estrous cycle

• Luteal phase (metestrus & diestrus) = 80% of estrous cycle (10)

4 Stages of the Estrous Cycle: 

1. Proestrus - 

• Lasts between 12-14 hours for ewes (10) 

• Begins when progesterone declines due to luteolysis 

• Estrogen increases

• Preovulatory follicle increases in size 

• Antral follicle matures for ovulation (14) 

2. Estrus -  

• Also referred to as "heat" 

• Period of sexual receptivity 

• Average length of 29 hours

• Estrogen levels peak and induce behavioral changes

• Behavioral signals that she is ready for mating - standing to be mounted, more frequent urination, vocalization, etc. (10)

• Surge of luteinizing hormone (LH) 

• Ovulation occurs 24-48 hours after the LH surge

• High uterine motility (contractions)

• Optimal conditions for sperm transport

• Increase in cervical mucus (14)

3. Metestrus -  

• Occurs right after ovulation 

• Time between ovulation and the formation of a functional corpus luteum 

• Estrogen and progesterone levels are low in the beginning of this stage

• Newly ovulated follicle forms the corpus luteum (luteolysis)

• Corpus hemorrhagicum (bloody body) is present 

• The corpus luteum begins to secrete progesterone

• Uterine contractions end

• FSH increases which stimulates follicle growth (14)

4. Diestrus - 

• The longest stage of the estrous cycle

• The corpus luteum (CL) is fully functional 

• High levels of progesterone secretion from the CL 

• The uterus prepares for early-embryo development

• FSH increases to promote follicle growth within the ovaries

• The CL regresses at the end of this phase if the female is not pregnant and Prostaglandin is released as a result (14)

Sheep Are Short Day Breeders 

• Increased darkness causes higher melatonin release from the pineal gland in the brain

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• Decreases RFRP-3 inhibition of kisspeptin neurons in the hypothalamus

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• High kisspeptin-10 stimulates the release of GnRH from the hypothalamus which initiates proestrus

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• GnRH acts on the gonadotropin cells of the anterior pituitary gland to increase release of FSH and LH 

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• FSH and LH travel through the bloodstream to reach the ovaries 

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• FSH promotes the growth & maturation of ovarian follicles that begin to secrete estrogen and inhibin

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• Increase in estrogen allows for behavioral changes in the ewe during estrus and stimulates a surge of LH that initiates ovulation while inhibin suppresses FSH secretion from the pituitary gland

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• After ovulation, during metestrus, the ruptured follicle forms a corpus luteum which secretes progesterone

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• During diestrus progesterone maintains pregnancy if successful implantation occurred or if there is no pregnancy, the corpus luteum regresses and progesterone levels drop

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• A new estrous cycle begins with low progesterone levels

Key Hormones in the Estrous Cycle

• • • GnRH: 

1. Controls the release of FSH and LH from the anterior pituitary which then act on the ovaries to stimulate follicle synthesis 

• • • FSH: 

1. Promotes the growth and development of follicles 

2. Increases oxygen uptake of granulosa cells to initiate protein synthesis 

3. Promotes formation of granulosa cells that secrete estrogen which are important for follicle maturation 

4. Prepares the dominant follicle for ovulation in response to surge of LH 

• LH: 

1. Aids in maturation of primordial germ cells (eggs)

2. Influences steroid hormone synthesis in the ovaries 

3. Regulates length and order of phases in the estrous cycle

• Estradiol:  

1. Promotes maturation of growing follicles

2. Causes behavioral changes that occur during estrus (heat) - signal that the ewe is ready to mate

3. Enhances blood flow and glandular secretions to prepare the uterine lining for implantation of a fertilized egg

• Progesterone:

1.  Increases during luteinization in the metestrus phase of the estrous cycle

2. Produced by luteal cells in the corpus luteum (CL) to maintain pregnancy

3. Insufficent progesterone levels can lead to miscarriage during early pregnancy

•  Prostaglandin: 

1. Receptors found on large luteal cells of the corpus luteum 

2. Controls luteolysis in the ovary

• Inhibin: 

1. Produced by follicles once FSH and LH are released from the anterior pituitary 

2. Reduced supply can lead to atresia (cell death) of the maturing follicle