Mod9: Obj5-9; Pregnancy and development

Question 1

Fertilisation: combining chromosomes of ova & sperm

Answer 1

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

Question 2

Capacitation

Answer 2

Occurs when secretions from the female start to deteriorate the outer membranes of the sperm

• Takes 8-10 hours

Question 3

Fertilisation requires capacitation

Answer 3

• 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

Question 4

Steps to fertilisation

Answer 4

1. Sperm weave through granulosa cells of the corona radiata
2. Sperm bind to zona pellucida, causing increased calcium levels in the sperm, this leads to the breakdown of membrances (acrosomal reaction)
3. Acrosomal enzymes digest holes in the zona pellucida
4. Acrosomal process is formed at the tip of the sperm, allowing it to bind to receptors of the oocyte
5. Sperm and oocyte membranes fuse, releasing sperm DNA into oocyte
6. Entry of sperm, causes increased calcium levels in oocyte. Cortical reaction destroys sperm receptors and hardens the zona pellucida (preventing polyspermy)

Question 5

Digestion of complete holes through the zona pellucida requires acrosomes to be released, consequently…

Answer 5

Sperm that arrive later, are more likely to fertilise the oocyte

Question 6

Once sperm has entered the cortical response, other sperm are:

Answer 6

unable to enter (prevents polyspermy)

Question 7

What happens if polyspermy occurs?

Answer 7

There are too many chromosomes and fertilized oocyte dies

Question 8

How do twins result?

Answer 8

Identical: One fertilised zygote divides into two embryos

Fraternal: Two oocytes are released and fertilised individually

Question 9

Zygote to blastocyte then implantation

Answer 9

Question 10

Embryonic development: Zygote to blastocyte

Answer 10

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

Question 11

The blastocyte implant

Answer 11

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

Question 12

Integrins on the trophoblast cells allow…

Answer 12

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)

Question 13

The blastocyst implants steps

Answer 13

1. Trophoblast adheres to site in the endometrium with mature receptors and chemical signals
2. 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)

Question 14

If implantation is successful

Answer 14

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)

Question 15

hCG

Answer 15

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

Question 16

Implantation failure

Answer 16

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

Question 17

Reasons for implantation failure

Answer 17

• Scarring of the uterus (pelvic inflammatory disease)
• Hormonal imbalances (PCOS, endometriosis)
• Non-receptive endometrium (lack of appropriate receptors=PCOS, endometriosis)

Question 18

Other implantation issues

Answer 18

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

Question 19

Embryonic Development: Gastrula to Fetus

Answer 19

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

Question 20

Embryonic development

Answer 20

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)

1. Ectoderm (top layer): ​Forms epidermis (hair, nails and skin), nervous system
2. Mesoderm (middle layer): Forms spine, kidneys, gonads, heart, digestive system, blood vessels, muscle and connective tissue
3. Endoderm (bottom layer): Epithelial lining of glands, digestive and respiratory tracts

Question 21

Embryonic period:

Answer 21

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

Question 22

Before birth

Answer 22

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)

Question 23

Placenta

Answer 23

• discoid,* weighs 500grams and is heaviy supplies with fetal and maternal blood vessels
• Temporary organ the originates from fetal and maternal tissue

Question 24

The placenta has 2 components

Answer 24

1. Fetal component
2. Maternal component

Question 25

Placenta: Fetal component

Answer 25

*_Chorionic villi_* develops from _trophoblast and mesoderm cells. Contain fetal vasculature_

Question 26

Placenta: Maternal component

Answer 26

Erosion of *endometrial cells* causes _pooling of maternal blood around chorionic villi_

Question 27

Placental function

Answer 27

* 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_

Question 28

Placental permeability

Answer 28

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*

Question 29

Amniotic fluid

Answer 29

*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*

Question 30

Fetal lungs

Answer 30

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)

Question 31

Fetal circulatory system

Answer 31

* 2xumbilical arteries * 1xumbilibal vein * 3xvascular shunts *(to factilitate distribution of oxygenated blood from placenta and to avoid lungs/liver)*

Question 32

Fetal circulatory system: 3 vascular shunts

Answer 32

1. *Ductus venosus:* Blood bypasses liver on way from placenta to heart 2. *Foramen ovale:* Blood is shunted from R atrium to L atrium to avoid R ventricle and pulmoary circ 3. *Ductus arteriosus:* Most of the blood entering pulm circ is shunted into aorta to bypass lungs

Question 33

Post-natal circulation

Answer 33

* 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

Question 34

Fetal circulatory system

Answer 34

Question 35

Maternal adaptations to pregnancy: Cardiovascular changes

Answer 35

Increase blood volume & increase cardiac output (volume of blood pumped out of the heart per minute)-more blood to fetus

Question 36

Maternal adaptations to pregnancy: Respiratory changes

Answer 36

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

Question 37

Maternal adaptations to pregnancy: Metabolic changes

Answer 37

Increase lactogenic hormones; decreased insulin sensitivity, increase glucose-leads to greater concentration gradient at placenta for transfer of glucose to fetus

Question 38

Maternal adaptations to pregnancy: Kidneys

Answer 38

Increased filtration at the kidneys; increased fluid retention-leads to increased blood volume

Question 39

Maternal adaptations to pregnancy: Gut

Answer 39

Increased nutrient uptake; decreased gut motility-increased nutrients available for fetus

Question 40

Hormonal changes during pregnancy

Answer 40

* 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