W7 - PREGNANCY AND PLACENTATION

Question 1

Describe the pre- and post-implantation nutrition of the placenta

Answer 1

• Pre-implantation (up to blastocyst-stage) - nutrients are derived from uterine secretion (histiotrophic nutrition)
  
  • Includes gases and simple sugars
• Post-implantation (up to 3.2kg foetus) - nutrients derived from maternal blood (haemotrophic nutrition)
  
  • Achieved via haemochorial placenta in humans (development starts week 2 - fully functional by week 12)
    
    • Nutritive, respiratory, excretory, immunological and endocrine functions established
  • Nutrient supply and excretion occur through maternal blood

Question 2

Describe the functions of the placenta

Answer 2

• Transports nutrients from maternal circulation to foetus
• Exchange gases between the foetus and mother
• Excretes foetal waste into maternal compartment
• Immunomodulatory role in maternal acceptance of foetus
• Delivery of maternal antibodies to foetus (passive immunity)
• Produces hormones that regulate maternal and foetal organs

Question 3

Explain the maturation of the placenta

Answer 3

• Extra-embryonic mesoderm lines cytotrophoblast and syncytiotrophoblast to collectively form the chorion and chorionic villi
  
  • Chorion and villi of the embryo combine with the maternal decidua basalis (stratum functionalis of the endometrium) to form the true placenta
  • Maternal component includes the decidua on the basal side and the blood supply
• Villi will become highly vascularised and project into the maternal blood supply
  
  • Maternal and foetal blood separated by syncytiotrophoblast, cytotrophoblast, villus connective tissue and foetal capillary endothelium
  • Chorion in decidua capsularis compressed - discoidal haemochorial placenta
  • Placenta is fully functional by the end of week 12 after fertilisation
  • As the foetus grows, it expands into the decidua basalis and results in a concentrated region of supply

Question 4

Explain the formation of the chorionic villi

Answer 4

• Primary stem cell villi (11-13 days after fertilisation)
  
  • Cytotrophoblast expands into syncytiotrophoblast projections
• Secondary stem villi (16 days)
  
  • Extraembryonic mesoderm proliferates into projection
• Tertiary stem villi (21 days)
  
  • Mesoderm differentiates into connective tissue and blood vessels

Question 5

What are the placental hormone? Explain their functions

Answer 5

• hCG
  
  • Maintains corpus luteum, suppresses maternal immune system
• Oestrogen
  
  • Enlarges uterus, relax pelvic ligaments and pubic symphysis, develops breasts
• Relaxin
  
  • Relax pelvic ligament and pubic symphysis in preparation for birth
• Progesterone
  
  • Maintains functional endometrium, quietens uterus, develops breast, increases respiration tidal volume
  • Stops contractibility of the myometrium, do not want a premature foetus expelled
  • Develop breasts and branding of the alveoli
• hCS/hPL
  
  • Decrease glucose and increase fatty acid metabolism in mother, develops breasts
  • Important in parturition in the developing foetus
  • Increases glucose to foetus and fatty acids for mother
• hCT
  
  • Increases maternal metabolism
• CRH
  
  • Stimulates cortisol production via foetal hypothalamus-pituitary-adrenal axis

Question 6

Describe the anatomical changes during pregnancy

Answer 6

• Increased vascularisation of reproductive organs
• Breast development and milk synthesis - progesterone, oestrogen, hCS/hPL, prolactin
• Dramatic uterine enlargement - oestrogen
  
  • Fist-size to filing abdominal cavity
  • Pressure on internal organs
  • Alters centre of gravity - lordosis and backache
• Pelvis and ligaments relax and widen (relaxin, oestrogen)
  
  • Causes a waddling gate
• Considerable weight gain - 13 kg
  
  • Other factors apart from the weight of the uterus - fluid retention

Question 7

Describe the metabolic changes during pregnancy

Answer 7

• Foetus requires proteins, calcium, iron and energy for development
  
  • Increased appetite (cortisol) - elevated fat storage and blood glucose levels
  • Increased fatty acid consumption and decreased glucose metabolism in mother (hCS/hPL - anti-insulin affects = antagonist for insulin receptors) - free-up glucose in blood for foetal metabolism
    
    • 10% risk of gestational diabetes due to increased blood glucose
  • Increased rate of maternal metabolism (hCT) - elevated calcium for foetal bone development

Question 8

Describe the physiological changes during pregnancy

Answer 8

• Gastrointestinal system
  
  • Initial nausea (elevated progesterone, oestrogen)
  • Reflux and constipation - due to an overcrowded abdominal cavity - reflux
    
    • Progesterone suppresses smooth muscle contraction - constipation
• Urinary/renal system
  
  • Increased frequency and volume of urination - because of reduced bladder size due to overcrowding
  • Na+ and water retention (renin, angiotensin II, aldosterone) - fluid up-regulation due to oestrogen and progesterone
• Respiratory system
  
  • Increased tidal volume - greater need for O2 (progesterone) puts extra demand on mother
• Cardiovascular system
  
  • Increased blood volume 25-40% and period increase in cardiac output 20-40% (thyroxine)

Question 9

Explain the sexual differentiation of the reproductive tract

Answer 9

• Determination of sex
  
  • Sex of offspring depends on sex chromosomes (X and Y)
    
    • Females - XX
    • Males - XY
  • A single gene on the Y-chromosome carries sex determining region - SRY on the Y-chromosome
    
    • Controls maleness
    • Anti-female coding
  • Genetic Sex (XY or XX) determines gonadal sex (testis or ovaries); determines phenotypic sex (internal and external genitalia)

Question 10

Which gamete determines sex?

Answer 10

• Spermatogenesis
  
  • 2 X-bearing
  • 2 Y-bearing
• Oogenesis
  
  • 1 X-bearing
• Sex is determined by sperm
  
  • X-chromosome is much larger than the Y-chromosome

Question 11

Explain the development of the reproductive tract

Answer 11

• During week 5 of embryo development, gonadal tissue (gonadal ridge) develops - future ovaries or testes
• Primordial germ cells migrate into gonadal ridge - future oogonia or spermatogonia
• Testis or ovary development regulated by the presence (XY) or absence (XX) of the SRY gene
  
  • SRY produce testis determining factor (TDF) - testis (if TDF and SRY are present) and sertoli cells (differentiate)
  • Sertoli cells produce mullerian inhibitory factor/anti-mullerian hormone (MIF/AMH)
    
    • Differentiation to form leydig cells
  • Leydig cells produce testosterone - male genitalia

Question 12

Explain the formation of the male embryo

Answer 12

• MIF is produced in sertoli cells
  
  • Production of leydig cells
• Large rise in hCG stimulates leydig cells
  
  • Increase in testosterone output
  • Causes the formation of the male reproductive tract
• At birth, the testis, scrotum and penis develop
• The testes descend from the gubernaculum and the prostate and accessory glands are still developing

Question 13

Explain the formation of the female embryo

Answer 13

• Absence of TDF leads to a delay compared to male differentiation before the normal pathway takes place
• No sertoli cells producing MIF
• No leydig cells or male androgens (testosterone)
• Ovaries descend to their resting position