Physiology of Pregnancy, Parturition and Lactation Flashcards
List the events that occur from fertilisation to implantation
- Fertilisation: ampulla of fallopian tube
- Cleavage
- Morula
- Blastocyst
- Implantation
Which cells of the blastocyst form the foetus?
Inner cell mass
Which cells of the blastocyst form foetal placenta?
Trophoblast
What occurs on day 3 to 5 post fertilisation?
Transport of blastocyst into the uterus
What occurs on day 5 to 8 post fertilisation?
Blastocyst attaches to the lining of the uterus
Describe the process of implantation by the blastocyst
When the free-floating blastocyst adheres to the endometrial lining, cords of trophoblastic cells begin to penetrate the endometrium.
Cords of trophoblastic cells tunnel deeper into endometrium, carving out a hole for the blastocyst.
The boundaries between cells in the advancing trophoblastic tissue disintegrate.
By which day post fertilisation is implantation complete
Day 12
What is the placenta?
Composite structure of foetal and maternally derived tissue that supplies nutrients to the developing embryo
What are the two types of tissue from which placenta is derived?
Trophoblast and decidual
List the parts of the foetal placenta
- Chorion
- Amnion
- Umbilical cord
- Cotyledon (lobule)
What is the chorion?
Embryonic derived portion of the placenta - trophoblasts
Outer of the two fetal membranes
In contact with maternal decidua on outer aspect and amnion on inner aspect
Describe the role of the chorion in the formation of placental villi
Trophoblast cells (chorion) differentiate into multinucleate cells (syncytiotrophoblasts) which invade decidua and break down capillaries to form cavities filled with maternal blood
Developing embryo sends capillaries into the syncytiotrophoblast projections to form “placental villi”
What is the amnion?
inner of the two foetal membranes, covering the foetus and containing amniotic fluid
What is the structure of the umbilical cord?
2 veins and an artery entwined surrounded by a mucopolysaccharide substance called Wharton’s Jelly
What is a cotyledon (lobule)?
Unit which contains a main villous tree
What is the composition of the maternal placenta?
Decidua Basalis
- non-glandular, non-vascular cells of endometrium
- Produced by steroid- dependant differentiation of maternal uterine stromal cells
Describe the structure and function of a placental villus
Each villus contains foetal capillaries separated from maternal blood by a thin layer of tissue – no direct contact between foetal & maternal blood
Function: 2 way exchange of respiratory gases, nutrients, metabolites etc between mother and foetus, largely down diffusion gradient.
What is the intervillous space?
Space surrounding the chorionic villi containing maternal blood where exchange between maternal and fetal blood occurs
Formed by the membrane separating fetal capillaries from maternal decidua preventing direct contact b/w maternal
Which part of the placenta acts as an arteriovenous shunt?
Intervillous space
Which gestational age does placenta become functional?
5th week of pregnancy
Describe the role of hormones in placental development
Human chorionic gonadotropin (HCG) signals the corpus luteum to continue secreting progesterone
Progesterone stimulates decidual cells to concentrate glycogen, proteins and lipids
Describe the role of placenta in oxygen transport b/w mother and fetus
The exchange takes place between maternal (oxygen-rich) blood and the umbilical blood (mixing of arterial and venous blood, oxygen-poor).
Oxygen diffuses from the maternal into the fetal circulation system (PO2 maternal > PO2 fetal).
Carbon dioxide, (partial pressure is elevated in fetal blood) follows a reversed gradient.
Fetal, oxygen saturated blood, returns to the fetus via the umbilical vein, while maternal, oxygen-poor blood, flows back into the uterine veins.
List the three factors that facilitates efficient oxygen transport across placenta?
- Fetal Hb - increased affinity for 02
- Higher conc. of Hb in fetal blood (50% > than in adults)
- Bohr effect - Fetal Hb can carry more oxygen in low pCO2 than in high pCO2
List the nutrient/waste product and the mechanism by which it is transported across placenta
- Water - diffusion along conc. gradient (increases during pregnancy upto 35th week)
- Electrolytes - follow H20
- Glucose - simplified transport (increases in 3rd trimester due to high need)
- Fatty acids - free diffusion
- Waste products - based on conc. gradient
List the teratogens that can cross the placenta
Thalidomide, carbamazepine, coumarins, tetracycline
Alcohol, heroin, nicotine, cocaine, caffeine
drugs - 3% of all congenital malformation
List the factors affecting rate of diffusion
Maternal/foetal capillaries arrangment Blood flow rates on either side Foetal villous exchange areas Distance of diffusion (membrane thickness) Concentration gradient
List the placental hormones responsible for maintenance of pregnancy
- Human Chorionic Gonadotrophin (hCG)
- Progesterone
- Oestrogen
- Lactogen (hPL)/chorionic somatomammotropin (hCS)
When does hCG production begin and peak?
Begins from day 8 after ovulation and peaks at week 10
What is the function of hCG in pregnancy?
- Acts on the corpus luteum to prevent luteolysis and stimulate progesterone production
- Acts on placenta to enhance conversion of maternal LDL cholesterol to pregnenolone and progesterone
- Development of the foetal adrenal and gonad is regulated by foetal hCG.
- Suppress the maternal immune rejection of the foetus
Role of hCG as a tumour marker
hCG is also produced by trophoblastic neoplasm such as hydatidiform mole and choriocarcinoma and hence is used as a marker for these conditions
Function of placental progesterone in pregnancy
• Necessary for establishment of pregnancy
- Reduced uterine contractility – blockade of myometrial prostaglandin production
- Stimulates endometrial gland secretion
- Negative feedback mechanism on hypothalamus to prevent FSH secretion and thus ovulation.
• May act as an immunosuppressant – inhibits T-cell mediated tissue rejection and provides immunologic tolerance by the uterus of invading embryonic trophoblast tissue.
• Preparation for lactation
When does placental progesterone secretion begin and peak?
Begins at 6th or 7th week of gestation
Markedly increases after week 2 when hCG levels drop
In the event of a hypohysectomy progesterone production falls. True or False?
False
Secretion is independent of factors that normally regulate steroid synthesis and secretion
What is the function of placental oestrogen in pregnancy?
Precursor of estrogen - DHEA sulphate which comes from FETAL ADRENAL GLAND
Therefore, increased oestrogen level indicates increase in foetal adrenal size
Estriol- metabolic product of DHEA
Therefore, estriol levels reflect the integrity of foetal circulation and metabolism as well as adequacy of the placenta
enlargement of uterus
breast development
relaxation of ligments
Function of hCS in pregnancy
• Growth hormone like effects - protein tissue formation
• Alters maternal glucose metabolism and lipolysis
• Causes hyperinsulinemia in response to glucose loads:
- Directly stimulate pancreatic islet insulin secretion
- Contributes to the peripheral insulin resistance characteristics of pregnancy
- Decreases maternal glucose utilisation
- Diverts glucose to the foetus
Effect of placental hormones on mother
CRH from placenta: ACTH in mother –> aldosterone and cortisol
- HTN
- Oedema & insulin resistance –> gestational diabetes
HC thyrotropin in placenta: hyperthyroidism in mother
Increased Ca2+ demands: Hyperparathyroidism
List the factors that cause parturition
- Estrogen:progesterone ratio
- Oxytocin
- Fetal hormones
- Mechanical stretch of uterine muscles - increases contractility
- Stretch of the cervix - stimulate uterine contractions
Role of estrogen:progesterone ratio in parturition
Progesteron inhibits contractility while oestrogen increases contractility
Which fetal hormones are involved in parturition?
oxytocin
adrenal gland hormones
prostaglandins
^^control timing of labour
List the events that occur during the onset of labour
Braxton hicks contractions
Positive feedback: stretch of the cervix by fetal head increases contractility and causes further oxytocin release
Strong uterine contraction and pain from the birth canal cause nurogenic reflexes from spinal cord that induce intense abdominal muscle contractions
Which hormones are involved in the initiation of labour? What is the mechanism of action?
- Oestrogen
- from ovaries
- indues oxytocin receptors on uterus - Oxytocin
- from foetus and mother
- stimulates uterus to contract
- stimulates placenta to make prostaglandins - Prostaglandins
- stimulates more vigorous contractions of uterus which acts as a positive feedback signal on the placenta to produce more PG as well as on oxytocin production by mother and foetus
Describe the positive feedback loops involved in labour
- Fetus drops lower in uterus
- Cervical stretch which causes
a) oxytocin release from pituitary
b) PG from uterine wall
c) Uterine contractions
a), b) and c) all act as positive feedback stimuli on each other, and as uterine contractions increase, so does the cervical stretch (also +ve feedback)
What are the 3 stages of labour and their duration?
Stage 1: cervical dilation (8-24hrs)
Stage 2: passage through birth canal (few to 120 mins)
Stage 3: expulsion of placenta
Which hormones are involved in the production and release of milk?
Estrogen
Progesterone
Prolactin
Oxytocin - milk let down reflex
Describe the role of Estrogen and Progesterone in milk production
E: growth of ductile system
P: development of lobule-alveolar system
Both E and P inhibit milk production. At birth sudden drop in E and P
At what period of pregnancy do prolactin levels rise?
week 5 to birth
What is the function of prolactin?
1-7 days after birth prolactin induces high milk production
Stimulates colostrum (high volume, no fat)
Describe the process of milk production
Sound of child’s cry triggers higher brain centres to inhibit PIH cell.
This reduces levels of PIH and thus production of prolaction in the anterior pituitary increases.
This increase in prolactin is responsible for milk prodution
Describe the milk let down reflex
- Suckling baby stimulates mechanoreceptors in the nipple
- Impulses propagated to spinal cord
- Stimulation of hypothalamic nuclei
- Oxytocin released by posterior pituitary
- SM contraction increases and milk is ejected
