BIOLOGYINDEEP

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

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