Seasons and cycles

• Estrous Cycle


  A phenomenon in adult female mammals recurring at regular intervals and similar sequence accompanying changes in the reproductive tract from one period to the next
• Estrous Cycle


  • Excludes human beings
  • Starts after puberty in females
  • Interrupted by anestrous phases of pregnancies
  • Involves periodic regression of follicles and activity associated with instinct of sexual receptivity known as estrus
• Menstrual Cycle


  The succession of endometrial changes observable in female humans and some primates, that recur regularly during the reproductive years
• Menstruation


  The physiologic shedding of the uterine endometrium accompanied by uterine bleeding that occurs at approximately monthly intervals from menarche to menopause
• Menstrual Cycle


  1. Begins at the end of luteal phase and prior to the follicular phase

  2. Removes the stratum functionale, i.e. the stratum compactum and part of stratum spongiosum, which build up during proliferative Phase, so that there can be a new sloughable layer in the next cycle
• Ovarian Cycle


  The sequence of physiologic changes in the ovary, including development and rupture of the follicle, discharge of the ovum as well as corpus luteum formation and regression
• Ovarian Cycle


  1. Follicular or proliferative phase (overlapped by desquamative phase)

  2. Ovulatory phase

  3. Luteal phase or progestational phase
• Ovarian Cycle in non-pregnant females

  Accompanied by periodic sloughing of uterine lining and consequent bleeding called menstrual cycle
• Corpus luteum of pregnancy (high progesterone condition) inhibits the progression of any cycle
• These cycles exist only in non-pregnant females
• Categories of Estrous Cycle

  • Polyestrous (more than one per year)
  • Monoestrous (one estrous cycle per year)
• Types of Polyestrous Cycle

  • Non-seasonal (cycles all year round)
  • Seasonal (cycles at only certain times of the season)
• Estrous (ovulation)


  Period of ovulation
• Metestrous


  Immediately after estrous when females are no longer receptive to male
• Early diestrus


  Corpus luteum is formed, and progesterone secretion increases
• Late diestrus


  Corpus luteum is being destroyed, progesterone secretion drops
• Proestrus


  Estrogen level is high, stimulates high LH
• If pregnancy ensues, estrous cycle ceases and a period of anestrous follows until parturition or termination of the pregnancy
• Proestrous Phase


  1. One or several follicles start to grow

  2. Lining of the endometrium starts to develop

  3. Some animals may have bloody vaginal discharge (not menses)

  4. Females are not sexually receptive in this phase
• Estrus Phase


  • Female is sexually receptive with GnRH, ovarian follicles develop and are maturing and estrogen secretions exert their biggest influence
  • Sexually receptive behaviour is one of the observable features of estrus, such as the lordosis reflex and reddened labia
• Types of Ovulators


  • Spontaneous ovulators (human beings)
  • Induced ovulators (felines, camels)
• For monogamy, the loss of estrous serves to promote sexual receptivity and helps to maintain monogamous pair bonds by encouraging repeated sexual contact
• Human beings are one of very few species with menstrual cycle without the display of estrous behaviour
• In Estrous Cycle


  The endometrium is resorbed if conception does not occur during the cycle
• In Menstrual Cycle


  Endometrial shedding occurs
• In Estrous Cycle


  Females are generally only sexually active during the heat period
• In Menstrual Cycle


  Females can be sexually active at any time in their cycle
• Human beings have concealed ovulation (no obvious external sign to receptivity at ovulation)