ANHB2216 Study Guide - Final Guide: Anencephaly, Thyroid Hormones, Human Chorionic Gonadotropin
3 Jun 2018
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Course
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Pregnancy
Initiation of Pregnancy and Implantation:
• Initiation of pregnancy
o Cleavage
▪ 1st cleavage → 30 hrs → 2 cells
▪ 12-16 cell stage by 3 days → called a morula
o Blastocyst
▪ Cells become wedge shaped and tight junctions form
▪ Cells become polarized → important for differentiation of
cells in blastocyst
▪ Fluid filled cavity → blastocoele
▪ 2 regions
• Outer trophoblast → forms embryonic part of
placenta
• Inner cell mass → forms embryo
▪ Blastocyst hatches from zona pellucida
• Implantation
o Uterine receptivity
▪ Very narrow window of implantation → ~ day 20-23
▪ Characterized by change in epithelial surface properties
• Appearance of pinopodes → absorb uterine fluid
• Change of surface charge
• Loss of glycoalyx
▪ Facilitate apposition of trophoblast and uterine epithelium
▪ Progesterone dominates → stimulates glandular
production and promotes decidualization
▪ Estrogen signal
• Growth of endometrium and stimulates glandular
secretion
• Stimulates cytokine release from endometrium
o Messages mediating attachment
o Renders endometrium responsive to
blastocyst
o Results in trophoblast-endometrial cross talk
o Messages mediating attachment
▪ Inhibitory factors must be turned off
• Muc1 glycoprotein complet on epithelial surface
cells
• Expression declines during receptive phase
▪ Stimulatory factors must be turned on
• Leukaemia inhibitory factor and interleukin II →
cytokines critical for implantation
• Produced by endometrial gland cells → estrogen
induced
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• Promotes epithelial cell receptivity, stromal
decidualisation, differentiation of trophoblast
Placental Development and Function:
• Placenta → site of nutrient, gas and waste exchange between mother and
foetus
• Bring maternal and foetal tissue into close proximity
• Consists of
o Foetal portion → villous chorion
o Maternal portion → decidua basalis
• Chorionic villi formation
o Outpocketing of syncytiotrophoblast increases surface area
o One side becomes more dominant, other side forms smooth
chorion
o Blood vessels grow along allantois through connecting stalk into
chorionic villi
o Development of embryonic circulatory system
• Decidualization
o Differentiation of endometrial stromal cells
o Initially localized to implantation site → rapidly spreads
o Decidua controls trophoblast invasion
▪ Inadequate invastion implicated in pre-eclampsia
▪ Promoted by progesterone
o Increased vascular permeability and angiogenesis
o Recruitment of uterine natural killer cells
o Formation of maternal portion of placenta
• Placental development
o Anatomical arrangement of human placenta achieved by 3-4
weeks of pregnancy
o Mature maternal blood flow established at 10-12 weeks
o Embryo/foetus develops in reduced oxygen environment for 1st
trimester → oxygen can be dangerous to a developing embryo
o Extra-villous trophoblast plugs spiral arteries → blocks blood
flow
o Human placenta is
▪ Discoid
▪ Chorioallantoic
▪ Haemochorial
• Placental transport
o Simple diffusion → oxygen/carbon dioxide → increased by growth
of placenta
o Facilitated diffusion → glucose
o Active transport → amino acids
o Receptor-mediated endocytosis → IgG
o Diffusion dependent on
▪ Surface area
▪ Diffusion distance
▪ Concentration gradient
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o Facilitated diffusion dependent on presence/availability of
transporter molecules
o The placenta is not passive
▪ Responds to environmental stressors
▪ Have a sex
▪ Finite lifespan
• Endocrine function
o Major placental endocrine signals
▪ Human chorionic gonadotrophin
▪ Progesterone
▪ Estrogen
▪ Human chorionic somatomammotrophin
▪ Corticotrophin releasing hormone
o Estrogen synthesis
▪ Placenta → main source of estrogen production after luteal-
placental shift
▪ Placenta lacks 17a-hydroxylase → required for androgen
formation
▪ Foetal adrenal has specialized foetal zone → produces
larger quantities of androgens → DHEA and DHEAS
Foetal Growth and Development:
• Maternal constraint
o Shown by embryo transfer experiments
o Genetic potential constrained → maternal strategy to constrain
growth so that she can
▪ Survive
▪ Conserve and divide resources among more offspring →
maximizing reproductive success
o Achieves this by
▪ Size of uterus and abdomen
▪ Placental weight and blood flow
▪ Unequal influence of paternal-maternal genomes
o Haig Hypothesis
▪ Maternal strategy to constrain foetal life
▪ Paternal strategy to maximize in-utero growth of offspring
→ offspring can out-compete those sired from other males
to maximize reproductive success
• Hormonal control of foetal growth
o Maternal hormones
▪ Thyroxine → stimulatory, particularly neural tissues
▪ Glucocorticoids → inhibitory → foetus protected by
placental glucocorticoid barrier
o Placental hormones
▪ Placental peptides
▪ Estrogen
▪ Progesterone
o Foetal hormones
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