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Early Development of the Zygote to a Blastocyst (1st Week)

  • Ovulation releases ovum/secondary oocyte
  • Fertilization in the oviduct → produces zygote
  • Zygote undergoes cleavage as it moves along oviduct → produces morula
    • As zygote divides, cells become smaller \ Morula stays same size
    • Movement by cilia and peristalsis present in oviduct walls
  • Morula develops into a blastocyst
    • Trophoblast (outer layer of blastocyst) → nourishes future embryo
    • Inner cell mass → will become fetus
    • Fluid filled cavity → for protection (absorbs shocks, resists compression, ...)
  • Blastocyst (≈100cells) implants itself in uterus lining
    • Nourished by secretion from uterus
    • Microvilli provide large surface area (→gas + nutrients exchange)

Implantation Of The Blastocyst Into The Uterine Lining (2nd Week)

  • Trophoblast secretes enzymes → digest tissues and blood vessel of endometrium
  • Embryo uses released nutrients/products from digestion
  • Blastocyst becomes buried within endometrium
  • Microvilli are replaced by placenta
  • Trophoblast secretes human chorionic gonadotrophin (hCG) hormone

The Developing Fetus

  • During gestation (→length of pregnancy) growth rate is in excess
  • Placenta is the first organ to develop when blastocyst embeds itself in uterine lining
    • Growth faster than embryo in early pregnancy
  • Development increases in complexity
    • Differentiating of inner cell mass of blastocyst
    • First month → beginning of a gut, developed kidney, brain, beating heart
    • Second month → all main organ systems present; embryo is called a fetus
  • At the end of gestation placenta is discarded, but essential for the 1st 9 month of life

Features of the Circulatory System of the Developing Fetus:

Placenta is the Fetal Gas Exchange Organ → Fetal Lungs are Non-Functional

  • Umbilical vein carries oxygenated blood from placenta to vena cava
  • Blood in the heart bypasses through foramen ovale
  • Oxygenated blood flows from right into left atria / flap valve prevents back flow of blood
  • Some blood in right atria passes to right ventricle into pulmonary artery (to lungs)
  • Blood bypasses lungs through ductus arteriosus
  • Oxygenated blood flows from pulmonary artery into aorta
  • Umbilical arteries carry deoxygenated blood from aorta to placenta

Importance of Pulmonary Circulation

  • Carry oxygenated blood to lungs
  • To allow respiration in lungs / be ventilated before birth

At Birth the Placenta is Replaced by Lungs as the Organ of Gas Exchange

  • Umbilical vein constricts → prevents blood loss
  • Ductus arteriosus constricts → blood leaving right ventricle is sent to lungs
  • Blood pressure in left atrium exceeds that in right atrium
  • Valve closes foramen ovale which fuses within atrial wall
    • Prevents mixing of de- and oxygenated blood
    • Deoxygenated blood in right ventricle is oxygenated in lungs
  • [EXAM] Replacement of fetal to adult Hb takes ≈3months
    • Each polypeptide/globin chain is coded by a separate gene
    • Gene for fetal (gamma) globin is suppressed
    • Gene for adult (beta) globin becomes active

Structure of the Placenta

  • Originates from fetal tissues and endometrium
  • Fully developed ≈20cm across and ≈3cm thick
  • Umbilical cord connects placenta with fetus
    • 1 umbilical vein → oxygenated blood from placenta to fetal vena cava
    • 2 umbilical arteries → deoxygenated blood from fetal aorta to placenta
  • FICK'S LAW: (surface area x difference in conc)/thickness of surface µ rate of diffusion
    • Microvilli grow into endometrium
      • Each villi contains a network of fetal capillaries
      • Surrounded by thin pool of maternal blood
      • Supplied by uterine arteries and drained by uterine vein
    • Max difference in concentration
      • Fetal Hb has a greater affinity for O2 than adult Hb
      • Flow of maternal and fetal blood in opposite direction
        • Uterine artery to umbilical vein
        • Maintains gradient/prevents concs reaching eqm
    • Short diffusion path (≈3.5μm)
      • Fetal and maternal blood supply is separated by 3 layers
        • Capillary endothelium
        • Thin layer of connective tissue
        • Epithelium covering villi
      • Exchange surface only one cell thick
      • Maternal and fetal blood come close together but never mix
        • Maternal blood may be genetically different from fetal blood

Function of the Placenta

  • Exchange of substances between maternal and fetal blood
    • O2 and waste products (urea, CO2) cross placenta by diffusion
    • Glucose enters fetal blood by facilitated diffusion
    • Amino acids enter fetal blood by active transport
      • Placenta contains many mitochondria
    • Maternal antibodies are taken into villi by pinocytosis
      • Infant has immunity to same diseases as its mother after birth
  • Secretes hCG (oestrogen, progesterone) → maintains pregnancy

Maternal Physiology

Effect of Pregnancy on Aspects of Maternal Physiology

  • GROWTH OF: uterus from ≈50g to 1kg / secreting tissue in breasts by progesterone
  • ENLARGEMENT OF: smooth muscle fibres of uterus wall / ducts of breast tissue by oestrogen
  • INCREASE OF:
    • Body mass/thirst/metabolic rate/ventilation rate/cardiac output/blood volume/red blood cell number
    • Ca2+ and glucose levels in bloodstream
    • Dietary requirements of Carbohydrates(energy), Protein(growth), Fe(Hb), Ca(bones), Vitamins
  • Minimises stresses imposed on female body → optimum environment for growing fetus

(1) Changes in Thermal Balance

  • Respiration + high growth rate increase heat
  • Heat excess transferred to cooler maternal blood by heat gradient
  • Mother loses this excess heat from her body

(2) Changes in Cardiac Output and Blood Volume and their Significance

  • High growth rate of fetus, placenta, maternal tissues (not just breast and uterus)
  • This increases O2 consumption/respiration
  • As maternal muscles have to work harder to move her increased size
  • Increases CARDIAC OUTPUT (= STROKE VOLUME x HEART RATE)
    • Heart beats faster
    • Increase in stroke volume
    • Increase in cardiac muscle / heart chambers enlarge / output increases by 40%
  • Increase in maternal blood volume
    • Changes in volume of plasma > increase in number of red blood cells

Birth

  • Fetus lies with its head down against stretched cervix
  • Weak contraction of uterus every ≈30min / increase in strength and frequency
  • Caused by hormone oxytoccin secreted by posterior pituitary gland
  • When cervix is fully dilated
  • Expulsion of baby by contractions of mother's abdominal muscles
    • Umbilical cord shuts down, isolating baby from mother
    • Rises CO2 content of the blood / stimulates baby's first breath
  • Expulsion of placenta → ≈30min after birth
    • Pregnancy lasts ≈38 weeks from implantation, 40 weeks from last period

Hormonal Changes During And After Pregnancy

Human Chorionic Gonadotrophin hCG Hormone

  • Secreted by trophoblast and developing placenta
  • Maintains corpus luteum past the time it normally disintegrates
    • Endometrium is maintained and menstruation does not occur
    • Female sex hormones still at high level
  • Basis for pregnancy test → hCG can be detected in urine
  • Peak in bloodstream after ≈2months followed by a slow decline

Progesterone and Oestrogen

  • Secreted by corpus luteum for first 3 months
  • Maintain endometrium, development of uterus, prevent menstruation
  • Inhibit FSH production from anterior pituitary gland
    • Prevents development of further mature ovarian follicles in ovary
  • Hormone secretion is taken over by placenta
  • Corpus luteum degenerates

Oxytocin causes Uterine Contraction (Birth)

  • Oestrogen from placenta makes muscles of uterus sensitive to oxytocin
    • End of pregnancy, level of oestrogen in blood rises, level of progesterone falls
    • Oestrogen promotes uterine contraction, progesterone inhibits it
  • Pressure of fetus against cervix stimulates stretch receptors
  • Hypothalamus stimulates posterior pituitary glands to secrete oxytocin
  • Oxytocin causes contraction of uterus

Lactation and Prolactin

  • High levels of progesterone inhibits prolactin
  • At birth, progesterone levels fall → prolactin levels increase
  • Suckling promotes production and ejection of milk
    • Nerve impulses travel to hypothalamus
    • Stimulates posterior pituitary gland to secrete oxytocin
      • Stimulates muscles in walls of milk ducts to contract, squeezing milk out
    • Stimulates anterior pituitary gland to secrete prolactin
      • Stimulates production of more milk
      • Prevents secretion of FSH and LH
      • Thus, ovulation does not happen → mother is less likely to conceive
  • Milk is bacteria free/ contains antibodies, essential nutrients, Ca2+ for bone growth, NO fibre/iron → baby will need solid food after ≈3-4months