Mod9: Obj5-9; Pregnancy and development Flashcards
Fertilisation: combining chromosomes of ova & sperm
Millions of sperm leak from the vagina almost immediately
- Many killed by vaginal acid
- Many get trapped in the thick mucous at the cervix
- Even if sperm reaches the ova, it needs to wait for capacitation to occur
Capacitation
Occurs when secretions from the female start to deteriorate the outer membranes of the sperm
- Takes 8-10 hours
Fertilisation requires capacitation
- Break-down of the membranes surrounding the acrosome, eventually result in the hydrolytic enzymes being released
- Acrosomal enzymes are necessary for penetration into the egg, but would cause negative effects if they were released as the sperm was released from the male reproductive tract
- Sperm have olfactory sensors to detect where to fertilise
Steps to fertilisation
- Sperm weave through granulosa cells of the corona radiata
- Sperm bind to zona pellucida, causing increased calcium levels in the sperm, this leads to the breakdown of membrances (acrosomal reaction)
- Acrosomal enzymes digest holes in the zona pellucida
- Acrosomal process is formed at the tip of the sperm, allowing it to bind to receptors of the oocyte
- Sperm and oocyte membranes fuse, releasing sperm DNA into oocyte
- Entry of sperm, causes increased calcium levels in oocyte. Cortical reaction destroys sperm receptors and hardens the zona pellucida (preventing polyspermy)
Digestion of complete holes through the zona pellucida requires acrosomes to be released, consequently…
Sperm that arrive later, are more likely to fertilise the oocyte
Once sperm has entered the cortical response, other sperm are:
unable to enter (prevents polyspermy)
What happens if polyspermy occurs?
There are too many chromosomes and fertilized oocyte dies
How do twins result?
Identical: One fertilised zygote divides into two embryos
Fraternal: Two oocytes are released and fertilised individually
Zygote to blastocyte then implantation
Embryonic development: Zygote to blastocyte
As soon as the oocyte is fertilised, it becomes a zygote
- Zygote undergoes continuous division (2 cells, 4 cells, 8 cells etc): this occurs in the absense of growth (cleavage)
- Once the collection of cells exceeds 16, the mass is known as a morula
- Morula cells divide. A fluid filled cavity forms and the zona pellucida (outside layer) starts to break down (blastocyte)
- Blastocyte consists of an inner cell mass, fluid filled cavity and a layer of trophoblast cells
The blastocyte implant
The zygote/morula/blastocyte is free floating and travels down the fallopian/uterine tube into the uterus
- Nourished by glycogen, produced by uterine glands
- Implantation of blastocyte occurs about 6-7 days after fertilisation
Integrins on the trophoblast cells allow…
The blastocyst to attach to receptors in the endometrium
- If the endometrium is not mature, the blastocyst detatches and floats to a lower level of the uterus (tries to implant again)
The blastocyst implants steps
- Trophoblast adheres to site in the endometrium with mature receptors and chemical signals
- Trophoblasts proliferate and form 2 layers
- Cytotrophoblast: inner layer of cells
- Syncytiotrophoblast: cells in the outer layer lose their plasma membranes, invade and digest the endometrium
(digestion occurs until the blastocyte has buried itself in the rich endometrial environment)
If implantation is successful
Successful implantation takes about 5 days (usually 12th day after ovulation)
- The trophoblast cells signals the corpus luteum to continue to produce progesterone and estrogen (prevent sloughing of the endometrium)
- Signalling occurs via the hormone human chorionic gonadotropin (hCG)
hCG
Levels are present in the mother’s blood one week after fertilisation and continue to rise until the end of the second month
- Important as the placenta takes over hormonal control after the second month, so before this period, hCG maintains the corpus luteum, to support the developing embryo
Implantation failure
At least 2 out of 3 blastocyts fail to implant (a further third of implanted embryos can miscarry)
- Embryo is passed, lack of hCG decreases estrogen and progesterone and sloughing occurs
Reasons for implantation failure
- Scarring of the uterus (pelvic inflammatory disease)
- Hormonal imbalances (PCOS, endometriosis)
- Non-receptive endometrium (lack of appropriate receptors=PCOS, endometriosis)
Other implantation issues
Ectopic pregnancy
- Implantation occurs outside the uterus
Placenta praevia
- Blastocyst implants near opening of cervix
- Placenta may cover opening to vagina
- Placenta may separate pematurely either before term or during labour
Embryonic Development: Gastrula to Fetus
Germ layers
- During implantation, the blastocyst starts to convert to a gastrula
- Inner cell mass develops into the embryonic disc
- Three primary germs layers and the extraembryonic membranes develop
Embryonic development
As the blastocyst is implanting, the inner cell mass starts to form into 3 layers (germ layers, give rise to all tissues in the developing embryo)
- Ectoderm (top layer): Forms epidermis (hair, nails and skin), nervous system
- Mesoderm (middle layer): Forms spine, kidneys, gonads, heart, digestive system, blood vessels, muscle and connective tissue
- Endoderm (bottom layer): Epithelial lining of glands, digestive and respiratory tracts
Embryonic period:
Conception to the end of the 8th week. After this time, it is a fetus
- Rapid cell division occurs during embryonic period and is the most critical time
- All major organs and systems are formed by end of 8th week
- During embryonic period the embryo is more susceptible to damage via external factors such as alcohol
Before birth
Growth and development of the fetus is independent of fetal lung and gut
- Fetus can survive without either of these due to the functions of the placenta (strongly dependent)
Placenta
- discoid,* weighs 500grams and is heaviy supplies with fetal and maternal blood vessels
- Temporary organ the originates from fetal and maternal tissue
The placenta has 2 components
- Fetal component
- Maternal component
Placenta: Fetal component
Chorionic villi develops from trophoblast and mesoderm cells. Contain fetal vasculature
Placenta: Maternal component
Erosion of endometrial cells causes pooling of maternal blood around chorionic villi
Placental function
- Provides substrates for fetal metabolism and excrete waste
- Act as a fetal lung (gas exchange)
- Produce hormones important for pregnancy (progesterone, estrogens, relaxin and protein hormones, such as hCG, hPL)
- Provide a partial barrier of cells from mum to fetus immunilogical defense
Placental permeability
Placenta is permeable to alcohol, nicotine, drugs and some maternal pathogens (fetal complications)
- Placenta is impermeable to large proteins (e.g. hormones and imune defence, except igG)
- normally impermeable to blood cells
Fetal and maternal blood supplies are in close proximity to each other, but remain separate
Amniotic fluid
Cushions fetus from trauma and provides space to exercise growing muscles
- Prevents _uterus from compressing the fetus and allows normal lung development (_allows symmetrical growth)
- Maintains constant temperatue
- Provides a fluid reservoir from which the fetus swallows (stimulates gut development)
- Consists mostly of fetal urine
- 10ml at 8wks to 1L at 32 wks
Oligohydramnios & Polyhydramnios both have negative effects on fetal development
Fetal lungs
During fetal life the lungs are liquid filled (amniotic fluid)
- Do not function in gas exchange (placenta instead)
- Growth and structureal maturation of the fetal lung are largely dependent upon the degree to which it is expanded by liquid (decreased growth with oligohydramnios&congenital diaphragmatic hernia)
Fetal circulatory system
- 2xumbilical arteries
- 1xumbilibal vein
- 3xvascular shunts (to factilitate distribution of oxygenated blood from placenta and to avoid lungs/liver)
Fetal circulatory system: 3 vascular shunts
- Ductus venosus: Blood bypasses liver on way from placenta to heart
- Foramen ovale: Blood is shunted from R atrium to L atrium to avoid R ventricle and pulmoary circ
- Ductus arteriosus: Most of the blood entering pulm circ is shunted into aorta to bypass lungs
Post-natal circulation
- Deoxygenated blood returns to R side of heart via the R atrium
- Then goes to R ventricle to be pumped to the lungs
- Oxygenated blood returns to L side of heart via the L atrium
- Flows into L ventricle to be pumped around rest of the body
Fetal circulatory system
Maternal adaptations to pregnancy: Cardiovascular changes
Increase blood volume & increase cardiac output (volume of blood pumped out of the heart per minute)-more blood to fetus
Maternal adaptations to pregnancy: Respiratory changes
Increased breathing (hyperventilation); altered amounts of oxygen and carbon dioxide dissolved in blood-Leads to greater concentration gradients at placenta for exchange of oxygen and carbon dioxide
Maternal adaptations to pregnancy: Metabolic changes
Increase lactogenic hormones; decreased insulin sensitivity, increase glucose-leads to greater concentration gradient at placenta for transfer of glucose to fetus
Maternal adaptations to pregnancy: Kidneys
Increased filtration at the kidneys; increased fluid retention-leads to increased blood volume
Maternal adaptations to pregnancy: Gut
Increased nutrient uptake; decreased gut motility-increased nutrients available for fetus
Hormonal changes during pregnancy
- Human chorionic gonadotropin increases in the first two months and then decreases in need
- Both estrogen and progesterone increases, however estrogen is morre predominant than progesterone
