LECTURE 8 (Ovulation to Implantation) Flashcards
Describe the first week of development
1) Ovulation
2) Fertilisation
3) Cleavage
4) Cleavage continues
5) Implantation
Which organ controls the Menstrual cycle?
The hypothalamus
Describe the Ovarian cycle
HYPOTHALAMUS produces GONADOTROPIN-RELEASING HORMONE (GnRH) which acts on cells of the anterior lobe of the pituitary gland (ADENOHYPOPHYSIS) which secrete GONADOTROPINS -> GONADOTROPINS, FOLLICLE-STIMULATING HORMONE (FSH) and LUTEINIZING HORMONE (LH) stimulate and control cyclic changes in the ovary -> At the beginning of each ovarian cycle, 15-20 primary-stage-preantral follicles are stimulated to grow under the influence of FSH -> Only one follicle reaches full maturity & only one OOCYTE is discharged -> Other follicles degenerate and become ATRETIC
What symptoms can be felt during ovulation?
- Slight pain called MITTELSCHMERZ
[occurs near the middle of the menstrual cycle] - Rise in basal temperature
What can be given to women who fail to ovulate?
Women that fail to ovulate due to a low concentration of GONADOTROPINS can be given an agent ti stimulate gonadotropin release
What happens to Granulosa cells after ovulation?
GRANULOSA CELLS in the wall of the ruptured follicle with cells from the THECA INTERNA are vascularised by surrounding vessels -> Under the influence of LH, the cells develop a YELLOWISH PIGMENT and change into LUTEIN CELLS which form the CORPUS LUTEUM and SECRETE OESTROGENS AND PROGESTERONE -> PROGESTERONE & OESTROGEN cause the UTERINE MUCOSA to enter the PROGESTATIONAL/SECRETORY STAGE in preparation for implantation of the embryo
How long does it take the fertilised oocyte to reach the uterine lumen?
3 to 4 days
Describe how Oocytes move from the ovaries into the uterine tube
FIMBRIAE of the uterine tube sweep over the ovary surface and contract RHTHMICALLY -> Oocyte surrounded by GRANULOSA CELLS is carried into the tube by sweeping movements of FIMBRIAE and by motion of cilia on epithelial lining -> Once in tube, CUMULUS CELLS withdraw their cytoplasmic processes from ZONA PELLUCIDA and lose contact with the oocyte -> Oocyte is propelled by MUSCULAR CONTRACTIONS of the tube and CILIA
What happens if fertilisation does not occur?
CORPUS LUTEUM reaches maximum development around 9 days after ovulation -> CORPUS LUTEUM shrinks because of degeneration of LUTEIN CELLS and forms the CORPUS ALBICANS -> PROGESTERONE production decreases causing menstrual bleeding
What happens if fertilisation does occur?
CORPUS LUTEUM continues to grow and forms the CORPUS LUTEUM OF PREGNANCY -> Yellowish LUTEAL CELLS continue to secrete progesterone until the end of the fourth month -> Luteal cells regress slowly as secretion of progesterone component of the placenta becomes adequate for maintenance of pregnancy
Degeneration of the corpus luteum is prevented by which hormone secreted by embryo?
Human chorionic gonadotropin
Describe what happens to Primary oocytes during meiosis
One PRIMARY OOCYTE gives rise to four daughter cells (each with 22 plus 1 X chromosome) -> Only one of these develops into a mature gamete (OOCYTE) -> The other three (THE POLAR BODIES) receive little cytoplasm and degenerate during development
Describe what happens to Primary spermatocytes during meiosis
One PRIMARY SPERMATOCYTE gives rise to four daughter cells (two with 22 plus 1 X chromosomes and two with 22 plus 1 Y chromosomes) -> All four develop into mature gametes
What is Fertilisation?
The process by which female and male gametes fuse
Where does Fertilisation occur?
In the AMPULLARY REGION of the uterine tube (the widest part of the tube and is close to the ovary)
How does movement of sperm from the cervix to the uterine tube occur?
By muscular contractions of the uterus and uterine tube and very little by their own propulsion
How long does the trip from the cervix to the oviduct take?
30 mins to 6 days
Fertilisation can only occur after the spermatozoa undergoes which processes?
1) Capacitation
2) The acrosome reaction
What is Capacitation?
A period of conditioning in the female reproductive tract that lasts 7h
What happens in Capacitation?
A glycoprotein coat and seminal plasma proteins are removed from the plasma membrane that overlies the acrosomal region of the spermatozoa
[Only capacitated sperm can pass through the corona cells and undergo the acrosome reaction]
What happens in the acrosome reaction?
The acrosome reaction occurs after binding to the ZONA PELLUCIDA and is induced by ZONA PROTEINS. The reaction culminates in the release of enzymes needed to penetrate the ZONA PELLUCIDA including ACROSIN-AND-TRYPSIN LIKE substances
What are the three phases of Fertilisation?
PHASE 1 = Penetration of the CORONA RADIATA
PHASE 2 = Penetration of the ZONA PELLUCIDA
PHASE 3 = Fusion of the OOCYTE and SPERM CELL MEMBRANES
Describe PHASE 1 of fertilisation
PENETRATION OF THE CORONA RADIATA
Only one sperm fertilises the egg and the capacitated sperm pass freely through corona cells
What is the Zona?
A glycoprotein shell surrounding the egg that facilitates and maintains sperm binding and induces the acrosome reaction
Both binding and the acrosome reaction are mediated by the ___________________
Zona Protein
Describe PHASE 2 of Fertilisation
PENETRATION OF THE ZONA PELLUCIDA
Release of ACROSIN (acrosomal enzyme) allows sperm to penetrate the ZONA and come in contact with the plasma membrane of the OOCYTE -> Permeability of the ZONA PELLUCIDA changes when the head of the sperm comes in contact with OOCYTE surface -> Contact results in release of LYSOSOMAL ENZYMES from CORTICAL GRANULES lining the plasma membrane of the OOCYTE -> Enzymes alter properties of the ZONA PELLUCIDA preventing further sperm penetration and inactivate SPECIES-SPECIFIC RECEPTOR SITES for sperm on zona surface -> Only one sperm can penetrate the OOCYTE
Describe PHASE 3 of Fertilisation
Plasma membranes of sperm and egg fuse after adhesion -> Fusion is accomplished between the OOCYTE MEMBRANE and the HEAD MEMBRANE OF SPERM -> Both the head and tail of the spermatozoa enter the cytoplasm of the oocyte but the plasma membrane is left behind on the oocyte surface
What is the initial adhesion of the sperm to the oocyte mediated by?
The interaction of integrins on the oocyte and their ligands and disintegrins on sperm
What are three ways the egg responds after fertilisation?
- Cortical and zona reactions
- Resumption of the second meiotic division
- Metabolic activation of the egg
Describe ‘Resumption of the second meiotic division’
The oocyte finishes its second meiotic division immediately after entry of the spermatozoa -> One of the daughter cells (which receives hardly any cytoplasm) is known as the SECOND POLAR BODY -> The other daughter cell is the DEFINITIVE OOCYTE. It’s chromosomes (22 plus X) arrange themselves into the FEMALE PRONUCLEUS.
Why must each pronucleus replicate its DNA during growth of male and female pronuclei?
If it does not, each cell of the two-cell zygote has only half of the normal amount of DNA
What are the main results of fertilisation?
- Restoration of the diploid number of chromosomes
- Determination of the sex of the new individual
- Initiation of cleavage
What are two techniques to aid with infertility?
- In Vitro Fertilisation (IVF) = Fertilising eggs in a culture medium and placing them in the uterus at the eight-cell stage
- Intracytoplasmic Sperm Injection (ICSI) = A single sperm is injected into an egg’s cytoplasm
What are in vitro techniques associated with?
- Increased risk for birth defects
- Prematurity
- Low birth weight
- Multiple births
What happens once the zygote has reached the two-cell stage?
It undergoes a series of mitotic divisions, increasing the numbers of cells called BLASTOMERES
Describe the stages leading to the eight-cell stage
After the third cleavage, BLASTOMERES maximise their contact with each other forming a compact ball of cells held together by TIGHT JUNCTIONS -> COMPACTION segregates inner cells which communicate extensively by GAP JUNCTIONS from outer cells -> 3 days after fertilisation, cells of the compacted embryo divide to form a 16-cell MORULA -> Inner cell of MORULA MASS gives rise to tissues of the embryo proper and outer cell mass forms the TROPHOBLAST which later contributed to the PLACENTA
When does the human blastocyst begin to penetrate the uterine mucosa?
By the sixth day of development
What are the three layers of the uterus wall?
- ENDOMETRIUM (mucosa lining the inside wall)
- MYOMETRIUM (thick layer of smooth muscle)
- PERIMETRIUM (peritoneal covering lining the outside wall)
What are the stages the endometrium passes through during the menstrual cycle?
1) Follicular or proliferative phase
2) Secretory or progestational phase
3) Menstrual phase
Describe the Proliferative phase
- Begins at the end of the menstrual phase
- Under the influence of oestrogen
- Parallels growth of the ovarian follicles
Describe the Secretory phase
- Begins 2/3 days after ovulation in response to progesterone
- Produced by CORPUS LUTEUM
Describe the Menstrual phase
Shedding of the endometrium marks the beginning of the menstrual phase of fertilisation doesn’t occur
What happens to the Endometrium if fertilisation does occur?
The endometrium assists in implantation and contributes to formation of placenta
What happens later in gestation?
The placenta assumes the role of hormone production and the CORPUS LUTEUM degenerates
What happens at the time of implantation?
Endometrium is in the SECRETORY PHASE during which uterine glands and arteries become coiled and tissue becomes succulent -> Three distinct layers can be recognised in the endometrium (SUPERFICIAL COMPACT LAYER, INTERMEDIATE SPONGY LAYER & THIN BASAL LAYER) -> If oocyte is not fertilised, venules and sinusoidal spaces gradually become packed with blood cells and an extensive DIAPEDEDIS of blood into the tissue is seen
What happens when the menstrual cycle begins?
Blood escapes from superficial arteries and small pieces of STROMA and GLANDS break away -> During 3/4 days the compact and spongy layers are expelled from the uterus and the basal layer is the only part of the ENDOMETRIUM that is retained -> BASAL LAYER supplied by its own arteries (BASAL ARTERIES) functions as the regenerative layer in the rebuilding of glands and arteries in the proliferative phase
