Reproductive cycles + hormones

• Reproductive cycles

  • The menstrual cycle and ovarian cycle occur within the female body and are under the control of hormones secreted by endocrine glands.
  • Hormones also play important roles in the male reproductive system but to a lesser extent.
• Reproductive hormones

  • The pituitary gland is located in the brain just above the roof of the mouth. It secretes four hormones essential for reproductive regulation.

  1. Follicle stimulating hormone
  2. Luteinising hormone
  3. Prolactin
  4. Oxytocin
• Luteinising Hormone (LH)

  • Promotes the final maturation of the ovarian follicle, ovulation and the formation of the corpus luteum (females)
  • It also stimulates the secretion of testosterone by the interstitial cells AND oestrogen and progesterone (from follicular cells and the corpus luteum respectively) (men)
• Follicle stimulating hormone (FSH)

  • Stimulates the development and maturation of the ovarian follicle in females and sperm production in the seminiferous tubules.
  • In females, the secretion of FSH is reduced as oestrogen levels increase in the blood.
• Prolactin

  • Acts on the breasts for the production of breast milk.
• Oxytocin

  • Acts on the uterus to simulate contraction of smooth muscle during labour
  • Promotes contraction of muscle cells surrounding breast lobules during breast feeding
• Sex steroids

  • Testosterone (secreted by interstitial cells) is responsible for development of the male reproductive system and secondary sexual characteristics (characteristics that come at puberty e.g. facial hair).
  • Oestrogen (secreted by the ovaries and corpus luteum) is responsible for development of the female reproductive system and secondary sexual characteristics
  • Progesterone (secreted by the corpus luteum) acts upon:

  1. The uterus to maintain and thicken the endometrium
  2. The placenta to develop and maintain it
  3. The breasts for the development of milk secreting glands.
• Human Chorionic Gonadotropin (HCG)

  • Secreted by the placenta developing placenta
  • Maintains the corpus luteum until the placenta is able to secrete sufficient oestrogen and progesterone.
• The ovarian cycle

  • A series of events that take place within the ovaries each month
  • Includes egg maturation, follicle development and the formation of the corpus luteum.
  • Ranges from 20-40 days, average of 28 days.
• The ovarian cycle: Primary follicle

  • At birth, the ovaries contain about 400 000 immature eggs that started to form during foetal development.
  • Other cells within the ovary surround each immature egg to produce a sphere composed of a single layer of cells
  • This is called the primary follicle
  • These immature eggs remain in a resting phase until puberty.
• The ovarian cycle: Secondary follicle

  • At puberty, some of the primary follicle undergo further development.
  • During the first half of the ovarian cycle the levels of FSH and LH slowly increase prompting growth and maturation of the follicle over 10-14 days where:

  1. Cells that form the wall of the follicle begin to enlarge and divide and they secrete a fluid hat fills the follicle and forces the egg to the edge of the follicle
  2. Known as the secondary follicle
• The ovarian cycle: Mature follicle

  • As more fluid accumulates within the follicle it enlarges and moves towards the surface of the ovary, producing a bulge.
  • At this stage it’s known as the mature follicle
  • Several secondary follicle commence development in each ovarian cycle however only one completes development. The other are reabsorbed by the ovary.
• The ovarian cycle: Ovulation

  • As the follicles grow they secret oestrogen and low levels of progesterone
  • At approx day 14 the level of oestrogen is high enough to stimulate a spike in LH and FSH. The surge in LH causes ovulation of the most mature follicle (the others degenerate, causing a decrease on oestrogen secretion)
  • Ovulation - the mature follicle bursts and expels the egg
  • Usually only one follicle ruptures at a time, however sometimes 2 or more may burst.
  • With the help of fimbrae, the egg is swept into the uterine tube.
• The ovarian cycle: Corpus Luteum

  • After ovulation, the follicle collapses and the blood within it forms a clot.
  • The clot is gradually absorbed by remaining follicle cells which enlarged and turn a cream-coloured body, the corpus luteum.
  • The corpus luteum secretes oestrogen and progesterone which maintains the endometrium and prepares for implantation of a zygote
  • It also inhibits the release of FSH and LH which prevent other follicles developing.
• The ovarian cycle: Corpus Luteum and Pregnancy

  • If fertilisation takes place and pregnancy follows the corpus luteum continues to develop and the ovarian cycle ceases
  • The corpus luteum is maintained by HCG
  • Once the placenta is established and can secrete it sown oestrogen and progesterone the corpus luteum slowly degenerates and is usually still present in the ovary at childbirth
  • Ovarian cycle resumes after breastfeeding has ceased.
• The ovarian cycle: Corpus albicans

  • If fertilisation has not occurred, the corpus luteum reaches it maximum development about 8-20 days after ovulation
  • It then degenerates to form a fibrous mass of scar tissue, the corpus albicans which eventually disappears.
  • Another ovarian cycle begins due to the lack of oestrogen and progesterone.
• The menstrual Cycle

  • While the ovarian cycle is occurring in the ovaries, another cycle of changes is happening in the uterus and vagina —-> the menstrual cycle
  • Changes occur in the endometrium to prepare for implantation of an embryo.
• Menstrual cycle: Endometrial Thickening

  • While the follicle is till maturing, the endometrium becomes thicker, softer, more vascular and there are more mucus secreting glands. This is due to the secretion of progesterone
  • After ovulation the endometrium continues to thicken, and its glands secrete a watery mucus rich is glycogen.
• Menstrual cycle: Menstruation

  • If the egg is not fertilised the corpus luteum degenerates which reduces the amount of progesterone causing the endometrium to break down.
  • 14 days after ovulation blood from broken down capillaries, mucous secretions and cell debris are lost through the vagina —-> menstruation
  • This is known as the menstrual period and takes place over a few days
  • As this point is the most recognisable point of the menstrual cycle, it is taken to be day 1.
• Menarche and Menopause

  • Menarche - when menstruation begins (this marks the commencement of puberty)
  • Menopause - when the menstrual cycle becomes irregular and eventually stops (begins at 45-55 years of age)