Fertilization And Implantation

Question 1

What is the ovarian cycle?

Answer 1

• it is the name given to the physiological change that occur in the ovaries essential for the preparation and release of a oocyte
• the ovarian cycle consists of three phases, all of which are under the control of hormones

Question 2

Describe the follicular phase of the ovarian cycle

Answer 2

• the formation of oogonia in the germinal epithelium of the ovaries is known as oogenesis
• primordial germ cells differentiate into oogonia in the ovaries during fetal life
• these diploid stem cells divide mitotically and proliferate into millions of germ cells
• most of the germ cells degenerate by atresia, however some develop further into primary oocytes and enter the prophase of meiosis 1 cell division
• meiotic arrest occurs and the process does not continue until after puberty
• whilst in this arrested prophase stage of meiosis 1 the primary oocyte is surrounded by follicular cells and is known as the primordial follicle
• there are up to 2 million oocytes in each ovary at birth, this number reduces to around 40000 at puberty and around 400 of these will mature and ovulate during the woman’s lifetime
• following puberty FSH and LH further stimulate the development of primordial follicles into primary and secondary follicles and subsequently into large preovulatory or graafian follicles by folliculogenesis
• low levels of oestrogen and progesterone stimulate the hypothalamus to produce GnRH
  - -> this hormone causes the production of FSH and LH by the anterior pituitary gland
  - -> FSH controls the growth and maturity of the Graafian follicles
  - -> the Graafian follicles begin to secrete oestrogen, which compromises of oestradiol, oestrone, and oestriol
  - -> rising levels of oestradiol cause a surge in LH
  - -> when oestradiol reaches a certain peak, the secretion of FSH is inhibited
  - -> the reduced FSH secretion causes a slowing in follicle growth and eventually leads to follicle death (atresia)
• the largest and dominant follicle secretes inhibin which further supresses FSH
  - -> this dominant follicle prevails and forms a bulge near the surface of the ovary and soon becomes competent to ovulate

Question 3

Describe the ovulation phase of the ovarian cycle

Answer 3

• high oestrogen levels cause a sudden surge in LH around day 12-13 of a 28 day cycle which lasts for approx 48 hours
• this matures the oocyte and weakens the wall of the follicle and causes ovulation to occur on day 14
• ovulation is the process whereby the dominant Graafian follicle ruptures and discharges the secondary oocyte into the pelvic cavity
• fimbrae guide it into the uterine tube where it awaits fertilization
• during the time of ovulation meiotic cell division resumes and the diploid oocyte becomes haploid
• some women may experience abdominal pain and there may be some light bleeding, stringy clear mucus appears in the cervix ready to accept the sperm from intercourse
• following ovulation the fertilized or unfertilized oocyte travels to the uterus

Question 4

Describe the luteal phase of the ovarian cycle

Answer 4

• the luteal phase is the process whereby the cells of the residual ruptured follicle proliferate and form a yellow irregular structure known as the corpus luteum
• the corpus luteum produces oestrogen, relaxin, inhibin and progesterone for approx 2 weeks to develop the endometrium of the uterus which awaits the fertilized oocyte
• small amounts of relaxin cause uterine quiescence which is a ideal environment for the fertilized oocyte to implant
• the corpus luteum continues its role until the placenta is adequately developed to take over
• during the luteal phase the cervical mucus becomes sticky and thick
• in the absence of fertilization the corpus luteum degenerates and becomes the corpus alibcans and as a result progesterone, oestrogen, relaxin an inhibin levels decrease
• in response to low levels of oestrogen and progesterone the hypothalamus produces GnRH
  
  • -> the rising levels of GnRH stimulate the anterior pituitary gland to produce FSH and the ovarian cycle comments again

Question 5

What is the menstrual cycle?

Answer 5

• it is the name given to the physiological changes that occur in the endometrial layer of the uterus and which are essential to receive the fertilized oocyte
• it consists of three phases

Question 6

Describe the menstrual phase of the menstrual cycle

Answer 6

• often referred to as menstruation or a period
• physiologically this is the terminal phase of the reproductive cycle of events and is simultaneous with the beginning of he follicular phase of the ovarian cycle
• reducing levels of oestrogen and progesterone stimulate prostaglandin release that causes the spiral arteries of the endometrium to go into spasm, withdrawing the blood supply to it and the endometrium dies, referred to as necrosis
• the endometrium is shed down to the basal layer along with blood from the capillaries, the unfertilized oocyte tissue fluid, mucus and epithelial cells
• failure to menstruate (amenorrhoea) is an indication that a woman may have become pregnant
• eumenorrhoea denotes normal, regular menstruation that lasts for typically 3-5 days although 2-7 days is considered normal
• the average blood loss during menstruation is 50-150 ml
• the blood is inhibited from clotting due to the enzyme plasmin contained in the endometrium
• the menstrual flow passes from the uterus through the cervix and the vagina to the exterior
• some women experience uterine cramps caused by muscular contractions to expel the tissue

Question 7

Describe the proliferative phase of the menstrual cycle

Answer 7

• this phase follows menstruation, is simultaneous with the follicular phase of the ovary and lasts until ovulation
• there is the formation of a new layer of endometrium in the uterus (proliferative endometrium)
• this phase is under the control of oestradiol and other oestrogens secreted by the Graafian follicle and consist of the re-growth and thickening of the endometrium in the uterus
• during the first few days of this phase the endometrium is reforming (regenerative phase)
• at the completion of this phase the endometrium consists of 3 layers
  
  • -> the basal layer - lies immediately above the myometrium and is approx 1mm thick, it contains all the necessary rudimentary structures for building new endometrium
  • -> the functional layer - contains tubular glands, approx 2.5mm, lies on top of the basal layer, it changes constantly according to the hormonal influences of the ovary
  • -> the layer of cuboidal ciliated epithelium covers the functional layer, it dips down to line the tubular glands of the functional layer
• if fertilization occurs the fertilized oocyte implants itself within the endometrium

Question 8

Describe the secretary phase of the menstrual cycle

Answer 8

• this phase follows the proliferative phase and is simultaneous with ovulation
• it is under the influence of progesterone and oestrogen secreted by the corpus luteum
• the functional layer of the endometrium thickens to approx 3.5mm and becomes spongy in appearance because the glands are more tortuous
• the blood supply to the area is increased and the glands produce nutritive secretions such as glycogen
• these conditions last for approx 7 days awaiting the fertilized oocyte

Question 9

Describe the process of fertilization

Answer 9

• also known as conception, it is the fusion of genetic material from the haploid sperm cell and the secondary oocyte to form the zygote
• process takes approx 12-24 hours and normally occurs in the ampulla of the uterine tube
• following ovulation the oocyte passes into the uterine tube
  - -> it is wafted along by the cilia and by the peristaltic muscular contraction of the uterine tube
• at the same time the cervix which is under the influence of oestrogen secretes a flow of alkaline mucus that attracts the spermatozoa
• in the fertile man at intercourse approx 300 million sperm are deposited into the posterior fernix of the vagina
  - -> approx 2 million reach the loose cervical mucus survive and propel themselves towards the uterine tubes while the rest are destroyed by the acid medium of the vagina
  - -> approx 200 sperm will reach the oocyte
• sperm swim through the cervical canal using their flagella
• prostaglandins from semen and uterine contractions as a result of intercourse facilitate the passage of the sperm into the uterus and beyond
• once inside the uterine tubes the sperm undergo a process called cap capacitation, this takes up to 7 hours
• influenced by secretions from the uterine tube the sperm undergo changes in the plasma membrane resulting in the removal of the glycoprotein coat and increased flagellation
• the zona pellucida of the oocyte produces chemicals that attract capacitated sperm only
• the acrosomal layer of the capacitated sperm becomes reactive and releases the enzyme hyaluronidase known as the acrosome reaction which disperses the corona radiata (outer layer of oocyte) allowing access to the zona pellucida
• other enzymes such as acrosin produce an opening in the zona pellucida
• the first sperm that reaches the zona pellucida penetrates it
• upon penetration the oocyte releases corticol granules (cortical reaction)
• the cortical reaction and depolarization of the oocyte cell membrane makes it impermeable to other sperm (important as many other sperm surrounding oocyte)
• the plasma membranes of the sperm and oocyte fuse
• the oocyte at this stage completes its second meiotic division and becomes mature
• the pro nucleus now has 23 chromosomes (haploid)
• the tail and mitochondria of the sperm degenerate as the sperm penetrates the oocyte and there is the formation of the male pronucleus
• the male and female pronuclei fuse to form a new nucleus that is a combination of the genetic material from both the sperm and oocyte (diploid cell)
• the male and female chromosomes each contribute half the complement of chromosomes to make a total of 46 chromosomes
• this new cell is called a zygote