Female Reproductive System 2: Regulation of the Ovarian Cycle

-

Hormones

Oestrogens are responsible for female secondary sexual characteristics and bone and muscle growth. Oestrogens influence the production of cervical mucus and the structure of the vaginal epithelium. This encourages growth of bacteria, which are responsible for the acidity of the vaginal fluid. Estrogens cause the proliferation of the uterine endometrium, encourage fluid retention and inhibit FSH. Estradiol is the main oestrogen, although others include estrange and estriol.

Progesterone only acts on tissues which have been previously acted on by oestrogen. Progesterone causes endometrial growth, secretory changes in the lining of the uterus and causes the body temperature to rise half a degree after ovulation.

Follicle stimulating hormone increases follicle development and stimulates inhibit and oestrogen secretion. Before day 10 of the ovarian cycle, the secretion of oestrogen inhibits the secretion of LH. However, after day 10, oestrogen stimulates the release of luteinizing hormone from the anterior pituitary gland, stimulating meiosis 1, ovulation and corpus luteum formation. The secretion of both oestrogens and luteinizing hormone stimulates progesterone secretion, reading the endometrium for implantation.

How do hormones affect the ovarian cycle?

  1. On day one, GnRH (gonadotrophin releasing hormone) from the hypothalamus stimulates the production and secretion of follicle stimulating hormone, as well as the secretion of luteinizing hormone (from the pituitary glands). Different pulses of GnRH cause the production of these hormones. FSH stimulates the beginning of development of the follicle (follicular phase).
  2. The follicular cells (theca and granulosa cells) produce oestrogens.
  3. Low oestrogen levels from the developing follicle inhibits the release of luteinizing hormone.
  4. As the secondary follicle develops, follicle stimulating hormone is inhibited through inhibin release (negative feedback).
  5. As the follicle grows, more oestrogen is released.
  6. On day 10, oestrogen release increases. This stimulates the release of LH.
  7. Day 14 is when ovulation occurs, involving a sudden surge of LH which triggers the completion of meiosis 1. The follicle wall ruptures and the oocyte is released into the fallopian tube.
  8. The luteal phase begins where LH causes the secretion of progesterone and the formation of the corpus luteum.
  9. GnRH pulses begin to stimulate the release of more LH than FSH, maintaining the structure of the corpus luteum and building the endometrial wall.
  10. As progesterone levels rise, oestrogen levels fall slightly. Progesterone is the primary hormone of the luteal phase and continues preparation of the uterus for pregnancy, enhancing its blood supply.
  11. If no pregnancy occurs, the corpus luteum degenerates. Progesterone and oestrogen levels fall.

Comments

Post a Comment

Popular posts from this blog

Growth and Development 2: Placentation and Hormones

-
How does the placenta form? The placenta forms from the embryonic trophoblastic/mesoderm tissues, as well as the maternal endometrial tissues. Vessels develop in the embryonic tissues which then supply nutrients to the foetus from the maternal blood supply. The chorion develops fingerlike villi which have become vascularised and extend to the embryo as umbilical arteries and veins. The embryo, amnion and yolk sac float in a fluid-filled chamber attached to the placenta by a body stalk. The placenta grows and begins to develop into organised regions, bulging into the uterine cavity as it grows. It becomes fully developed by the end of the third month (end of first trimester), where the foetus moves farther from the placenta connected by the umbilical cord. By 12 weeks, the placenta is providing nutrients and removing wastes, meaning it takes on the role of oestrogen and progesterone which used to be released from the corpus luteum. Decidua capsularis: endometrium surroundi...
Read more

Growth and Development 4: Labour, foetal circulation and lactation

-
What happens when we give birth? During the last week of pregnancy, oestrogen reaches a peak. This causes myometrial weakness and irritability. Then, weak 'Braxton Hicks' contractions occur, which become uterine contractions stimulated by oxytocin and prostaglandins closer to birth. Emotional and physical stress establishes a positive feedback mechanism with the hypothalamus, increasing oxytocin release. The stages of labour are summarised below:   1.   Dilation Stage (8 hours) Onset of labour Cervix dilates Foetus moves towards birth canal Contractions begin and increase in frequency Water eventually breaks   2.    Expulsion Stage (2-3 hours) Cervix pushed open by foetus Contractions increase in intensity, frequency and length Continues until foetus emerges from vagina (delivery) Surgical intervention possible: episiotomy (cutting of perineal membrane) or caesarean section Newborn health assessed immediately   3.   Plac...
Read more

Lymphatics and Immunity 3: Cell-Mediated Immunity

-
Image
What is adaptive immunity then? Adaptive or specific immunity responds to specific antigens through the coordinated action of T and B cells. It is only developed after birth. Adaptive immunity involves both cell-mediated and antibody-mediated immunity. Cytotoxic T cells provide cell-mediated immunity, providing defence against abnormal cells and pathogens inside  cells. B cells provide antibody-mediated immunity, providing defence against antigens and pathogens in body fluids. There are four properties of immunity: Specificity - response to only a specific type of antigen Versatility - production of many types of lymphocytes Memory - active lymphocytes stay in circulation and provide immunity against new exposure Tolerance - immune system ignores normal antigens What is cell-mediated immunity? T cells only recognise antigens that are bound to glycoproteins in a plasma membrane. These are termed major histocompatibility complex proteins. These proteins differ betwee...
Read more