Courtesy -Dr S A Karunathilake

TABLE 1 — Placenta: Big-Picture Functions

TABLE 2 — Early Conceptus: Transport, Cleavage & Control

TABLE 3 — Pregnancy Terminology (Table 15.1 Expanded)

TABLE 4 — Morula → Blastocyst: Structural & Cellular Changes

TABLE 5 — Metabolism, Trophoblast Origin & Genetics

TABLE 1 — Implantation: Big Picture & Invasion Balance

TABLE 2 — Trophoblast: Unique Cellular Characteristics

TABLE 3 — Trophoblast: Core Functions

TABLE 4 — Implantation Phases: Overview

TABLE 5 — Phase A: Apposition (Uterine Preparation)

TABLE 6 — Phase B: Adhesion (Attachment)

TABLE 7 — Phase C: Penetration (Invasion)

TABLE 8 — VEGF & Decidual Vasculature

TABLE 9 — Trophoblast Differentiation

TABLE 10 — Lacunae & Intervillous Space

TABLE 11 — Deep Invasion & Trophoblast Shell

TABLE 12 — Villous Development Timeline

TABLE 13 — Primary Villi Formation

TABLE 14 — Chorion Laeve vs Chorion Frondosum

TABLE 15 — Placental Lobules

1. Basic Concept of Placental Lobule (Cotyledon)

2. Structural Components of a Placental Lobule

3. Maternal vs Fetal Parts of a Lobule

4. Maternal Blood Flow Through a Placental Lobule

5. Functional Role of Placental Lobule

6. Clinical Correlations of Placental Lobules

If you want, I can also give a high-yield exam table comparing placental lobule vs placental villus vs cotyledon vs intervillous space, which often confuses students in anatomy and obstetrics exams.

Fetal cotyledon vs Maternal cotyledon — COMPLETE TABLE

TABLE 16 — Placental Growth & Weight

TABLE 17 — Placentation Types (Barrier Layers)

TABLE 18 — Maternal Blood Flow Increase: Species Contrast

TABLE 19 — Spiral Arteries: Before vs During Pregnancy

TABLE 20 — Spiral Artery Remodeling: Mechanism

🟥 TABLE 1: Constant Placental Architectural Findings in Pre-eclampsia

🟧 TABLE 2: Big Picture — Why Immunology Matters in Human Pregnancy

🟨 TABLE 3: Decidua — Structural & Immune Features

🟩 TABLE 4: Uterine NK (uNK) Cells — Identity & Timing

🟦 TABLE 5: Uterine NK Cells — Functional Characteristics

🟪 TABLE 6: Uterine NK Cells — Secretory Profile

🟥 TABLE 7: Proposed Functions of uNK Cells at Implantation

🟧 TABLE 8: Clinical & Exam Pitfall — NK Cell Testing

🟨 TABLE 9: Spectrum of Trophoblast Invasion

🟩 TABLE 10: Too Little Invasion → Pre-eclampsia Logic Chain

🟦 TABLE 11: Evolutionary Trade-off Concept

🟪 TABLE 12: Why the Fetus Is Not Rejected (Allograft Contrast)

🟥 TABLE 13: MHC Expression Rules on Trophoblast

🟧 TABLE 14: Why HLA-C Does NOT Cause Rejection

🟨 TABLE 15: NK–Trophoblast Receptor–Ligand Systems

🟩 TABLE 16: Maternal–Fetal Genetic Interaction

🟦 TABLE 17: Final Exam-Lock Summary

🟥 TABLE 1: Placenta as a Barrier — Compartments + What Must Be Crossed

🟧 TABLE 2: Placental Transfer — When Simple Diffusion Is “Enough” vs When It Is Not

🟨 TABLE 3: Why Diffusion Gets Easier Later — Terminal Villus Remodeling

🟩 TABLE 4: Gas Exchange Drivers — Maternal Physiologic Changes That Create Gradients

🟦 TABLE 5: Fetal Oxygen Carriage — Why It Can Be “About Equal” to Maternal

🟪 TABLE 6: Double Bohr Effect — Placental pH Shifts That Push O₂ Mother → Fetus

🟥 TABLE 7: Glucose & Carbohydrates — Maternal Metabolic Strategy + Transport Mechanics

🟧 TABLE 8: Placental Glucose Metabolism — Lactate as a Fetal Fuel

🟨 TABLE 9: What Mostly Controls Fetal Glucose Use

🟩 TABLE 10: Amino Acids — Maternal Nitrogen Conservation (Shift Toward Fetus)

🟦 TABLE 11: Amino Acids — Trophoblast Handling, Transport Systems, and Gradients

🟪 TABLE 12: Urea — Direction and Why It Happens

🟥 TABLE 13: Water & Electrolytes — Sites, Permeability, and Forces

🟧 TABLE 14: Sodium Transfer — Mechanisms Shown by Isotope Studies

🟨 TABLE 15: Iron Transfer — Why Fetal Iron Is Higher

🟩 TABLE 16: Calcium & Phosphate Transfer — Active, Inhibitable System

🟦 TABLE 17: Amniotic Fluid — Volume Curve Across Gestation

🟪 TABLE 18: Amniotic Fluid — Composition, Origin Hint, and Turnover

🟥 TABLE 19: Amnion — Origin, Layers, and Cavity Relationship

🟧 TABLE 20: Chorion — Layers, Trophoblast Thickness/Relations, and Villous Note

🟨 TABLE 21: Must-Know Negative — What Amnion and Chorion Do NOT Have

🟥 TABLE 1: Human Chorionic Gonadotrophin (hCG) — Core Role

🟧 TABLE 2: hCG — Source and Timing

🟨 TABLE 3: hCG — Structure and Molecular Features (as stated)

🟩 TABLE 4: hCG — Relationship to Pituitary Glycoprotein Hormones

🟦 TABLE 5: Pregnancy Test Logic (hCG Specificity)

🟪 TABLE 6: hCG — Gestational Pattern

🟥 TABLE 7: hCG — Evidence for Luteotrophic Action

🟧 TABLE 8: Progesterone — Source Shift and Independence

🟨 TABLE 9: Progesterone — Evidence of Trophoblastic Synthesis

🟩 TABLE 10: Progesterone — Synthesis Characteristics

🟦 TABLE 11: Estrogen in Pregnancy — Dominant Hormone

🟪 TABLE 12: Estriol — Requirement for a Fetus

🟥 TABLE 13: Fetoplacental Unit — Estriol Synthesis Pathway

🟧 TABLE 14: Estriol — Clinical Correlations

🟨 TABLE 15: Sulphation–Desulphation Logic (High-Yield)

🟩 TABLE 16: Estrogen Levels at Term

🟦 TABLE 17: Human Placental Lactogen (hPL) — Timing and Role

🟪 TABLE 18: hPL — Structural Properties (as stated)

🟥 TABLE 19: hPL — Hormonal Similarities

🟧 TABLE 20: hPL — Concentration Profile and Functional Caveat

🟨 FINAL EXAM LOCK (One-Glance Integration)

🟥 TABLE 1: Zona Pellucida — Role in Early Development

🟧 TABLE 2: Very Early Conceptus Division → Monozygotic Twins

🟨 TABLE 3: Timing of Division → Chorionicity & Amnionicity

🟩 TABLE 4: Monochorionic Placentation — Blood Flow Pattern

🟦 TABLE 5: Twin-to-Twin Transfusion Syndrome (TTTS)

🟪 TABLE 6: Hydatidiform Mole — Core Concept

🟥 TABLE 7: Complete Mole — Genetic Mechanism & Outcome

🟧 TABLE 8: Partial Mole — Genetic Mechanism & Outcome

🟨 TABLE 9: Shared Features — Complete & Partial Mole

🟩 TABLE 10: Uniparental Disomy — Mechanism

🟦 TABLE 11: Uniparental Disomy — Chromosome 15 Examples

🟪 FINAL EXAM LOCK (One-Look Summary)

Placenta — what it does (big picture)

• Anchors the pregnancy in the uterus by attaching fetal tissues to the maternal endometrium so the conceptus stays implanted and can grow.
• Nutrient exchange (mother → fetus) occurs across the placental interface, delivering oxygen and nutrients needed for fetal metabolism and growth.
• Waste exchange (fetus → mother) removes fetal carbon dioxide and metabolic waste products into the maternal circulation for clearance.
• Prevents maternal immune rejection by helping the mother not recognise / not reject paternally derived antigens carried by fetal tissues (immune tolerance at the maternal–fetal interface).
• Endocrine organ: produces hormones that adapt maternal physiology so the mother can support pregnancy and survive childbirth.
• Involved in signalling to initiate labour: contributes to biochemical signalling that helps trigger the onset of labour.
• Clinical importance / disease links: abnormal placental development or function is implicated in:
• Miscarriage
• Fetal growth restriction
• Pre-eclampsia
• Placental abruption
• Chapter focus statement: the text will discuss mechanisms behind some of these placental functions, and provides key definitions in Table 15.1.

The early conceptus — transport + cleavage timing

• Fertilised egg movement in the fallopian tube
• The fertilised egg is moved along the tube by cilial action and muscular action (tubal peristalsis).
• First cleavage division
• Occurs in the ampulla.
• Occurs within 30 hours of fertilisation.
• Subsequent cleavage divisions
• Continue every 12 hours.
• Progresses to the 16-cell stage.
• Blastomere totipotency
• The daughter cells (blastomeres) remain totipotent until the 16-cell stage is reached.
• Why totipotency matters clinically
• This “redundancy” allows preimplantation diagnosis in assisted conception:
• A single cell can be removed for testing while development can continue (because cells are still totipotent up to that point).
• Control of early divisions (maternal control)
• These early cleavage divisions are directed by maternally derived proteins and RNA (maternal stores drive early embryonic programming before later embryonic gene control dominates).
• Entry into the uterus
• The embryo enters the uterus at the eight-cell stage.
• Species differences
• The text notes that early embryo development differs by species and is summarised in Table 15.2 (the details aren’t shown in what you pasted).

Table 15.1 — Terminology of pregnancy (each term elaborated)

• Morula
• Defined as the conceptus up until approximately the 16-cell stage.
• It is prior to:
• Fluid collection, and
• Development of the inner cell mass.
• Conceptually: it is the “solid ball” stage before a cavity forms.
• Blastocyst
• The next stage after the morula.
• Key defining change: fluid separates:
• The inner cell mass from
• The outer cells
• Those outer cells become trophoblast.
• Conceptually: a cavity (fluid space) appears, and the embryo starts separating into “inner embryo” vs “outer placenta-forming” layers.
• Blastomere
• Defined as individual cells of the blastocyst.
• Conceptually: the single cellular units produced by cleavage divisions (the “daughter cells”).
• Zona pellucida
• A glycoprotein layer surrounding the oocyte.
• It persists to the morula/blastocyst stage.
• Functional meaning in the text:
• It allows growth in cell number without loss of cells prior to trophoblast invasion of the decidua.
• (So the embryo can keep dividing while still enclosed, before invasion into the uterine lining begins.)
• Trophoblast
• Derived from the outer cell layer of the blastocyst.
• Forms:
• Cytotrophoblast: cells retain a single nucleus per cell.
• Syncytiotrophoblast: a true syncytium (cells fuse into a multinucleated continuous layer).
• Key placental cell for (as explicitly stated):
• Synthesis
• Invasion
• Anchoring the pregnancy in the uterus
• Decidua
• Specialised modulation of the uterine endometrium in pregnancy.
• Includes species-specific alteration of:
• Blood vessels
• Connective tissue
• Leucocyte content
• Conceptually: the maternal endometrium becomes a specialised pregnancy-supporting tissue.
• Haemochorial placenta
• Defined as: maternal blood is in direct contact with the chorionic (syncytio)trophoblast.
• Meaning: the maternal blood bathes the trophoblast surface directly (the maternal blood space is not separated from trophoblast by an intact maternal endothelium layer at the interface).

• Morula → blastocyst transition — what changes in the cells
• Time window referenced
• The development from the eight-cell stage to implantation is referenced as being shown in Figure 15.1 (not provided in your paste).
• From the 16-cell morula stage: subcellular differentiation
• Organelles redistribute: greater concentration at the apex compared with the basal pole.
• The apical surface develops numerous microvilli.
• Outer cells (future trophoblast/placenta) develop tight junctional connections
• Outer morula cells (destined to become trophoblast/placenta) develop tight intercellular junctions, specifically:
• Gap junctions
• Desmosomes
• Fluid collection and inner cell mass separation
• Fluid collects in the outer layer, and this fluid:
• Separates the outer layer from the cells that will become the inner cell mass.
• The inner cell mass ultimately becomes the fetus.
• Metabolic activation at this stage
• The morula becomes much more metabolically active, with:
• Increased protein synthesis
• Increased oxygen consumption
• Energy substrate shift
• Energy for these processes is derived from pyruvate rather than glucose metabolism.
• Preimplantation trophoblast development
• New trophoblast develops from the base of the inner cell mass (called polar trophoblast).
• This corresponds to the blastocyst stage.
• X-chromosome inactivation timing
• At the morula–blastocyst stage, in a female embryo:
• One of the two X chromosomes becomes deactivated
• It becomes condensed as a Barr body.

Implantation & Early Placenta — Zero-Omission Elaborated Points

1) Big picture: why implantation depth matters

• Successful human reproduction needs the placenta to attach firmly to the maternal uterine wall.
• Too little trophoblast invasion into decidua is linked to pregnancy complications because the placenta cannot establish a strong, low-resistance maternal blood supply.
• Too much invasion can harm the mother (because invasion into deeper maternal tissues and vessels can compromise maternal health), and that secondarily threatens the infant as well.

2) Trophoblast cell characteristics (what makes them “special”)

• Paternal X-chromosome inactivation
• Trophoblast cells show a distinctive epigenetic pattern where the paternally derived X chromosome is inactivated (a feature often contrasted with other embryonic tissues).
• Unmethylated DNA
• They have relatively unmethylated DNA, reflecting a unique epigenetic state that supports rapid growth, differentiation, and invasion programs.
• Ability to form multinucleated cells
• Trophoblast can fuse to form multinucleated syncytial cells (the syncytiotrophoblast), which is crucial for early implantation and later exchange functions.
• Immune antigen expression pattern
• They show variable expression of MHC class I antigens.
• They show no MHC class II antigen expression.
• This combination is part of how the conceptus can exist at the maternal interface without triggering a typical “foreign graft” rejection response.

3) Trophoblast functions (what trophoblast does)

• Attaches placenta to uterine wall
• Provides the cellular machinery for implantation and for anchoring the developing placenta.
• Transport to fetus
• Enables transfer of nutrients, oxygen, and maternal immunoglobulins to the fetus.
• Eliminates fetal waste
• Moves fetal waste products away from the fetus toward maternal clearance.
• Endocrine and protein secretion
• Synthesizes and secretes proteins and hormones that support pregnancy maintenance and maternal adaptation.
• Barrier function
• Acts as a barrier between maternal and fetal circulations, controlling what passes and protecting fetal development.
• Immune interface
• Forms the contact site between the maternal immune system and the conceptus, balancing tolerance with defense.

4) Implantation has 3 phases

A) Apposition (initial “positioning” and uterine preparation)

• Implantation begins at the site of contact between the polar trophoblast and the endometrium.
• Then changes spread more widely in the endometrium.
• Endometrial response includes:
• Increased mitotic activity (cells proliferate).
• Localised changes in:
• Stromal cell morphology (stromal cells change shape/behavior).
• Intercellular matrix composition (extracellular matrix is remodeled).
• Influx of host-defense cells, including natural killer (NK) cells.
• This whole maternal stromal transformation is called decidualisation.
• The endometrium develops pinopodes:
• Pinopodes remove fluid from the uterine cavity by pinocytosis.
• By removing fluid, they allow closer apposition between the blastocyst and the decidua (less space/fluid separating them, so contact becomes physically easier).

B) Adhesion (true attachment)

• Adhesion is associated with destruction of the zona pellucida:
• The zona normally prevents direct cell contact; its loss allows maternal cells and trophoblast cells to touch.
• Within a few hours after attachment:
• The surface epithelium adjacent to the trophoblast becomes eroded (the epithelial layer is locally broken down).
• Mechanisms enabling adhesion and epithelial erosion include:
• Metalloproteases (enzymes that digest extracellular matrix and aid tissue remodeling).
• Adhesion molecules on trophoblast membranes
• These are specific for ligands on the forming decidua.
• This specificity helps the trophoblast “recognise and bind” to the correct uterine surface environment during implantation.

C) Penetration (invasion into decidua and beyond)

• Trophoblast produces metalloproteases that digest extracellular matrix (ECM).
• ECM digestion does two key things:
• Facilitates invasion (creates paths through tissue).
• Releases substrates that trophoblast can use:
• Lipids
• Proteins
• Nucleotides
• Sugars
• These become metabolic substrates and fuel during this early phase.
• Early trophoblast nutrition and transport capabilities:
• They can actively transport small molecules across their membranes.
• They can take up large molecules and cellular fragments via:
• Pinocytosis
• Phagocytosis
• This early nutrition is independent of placental circulation:
• Meaning: trophoblast can support early growth before the mature maternal–fetal blood flow system is fully established.
• Cytoskeletal support for invasion:
• Trophoblast contains microfilaments in its internal cytoskeleton.
• Microfilaments enable trophoblast cells to squeeze between endometrial cells.
• Depth and pattern of implantation:
• The trophoblast invades so deeply into underlying stroma that the surface epithelium becomes restored over it.
• This is termed interstitial implantation (the conceptus becomes embedded within the endometrial tissue rather than remaining superficially attached).
• Signalling control note (important concept):
• The biochemical signalling is incompletely understood.
• There may be biochemical redundancy:
• Because knockout mice lacking genes for supposed key control cytokines can still reproduce successfully.
• This suggests multiple overlapping pathways can compensate if one is missing.

5) VEGF and decidual vascular changes

• Vascular endothelial growth factor (VEGF) is produced in the endometrium.
• Its time course of production suggests it has a key role in vascularity changes within the decidua.
• Conceptually, this supports the idea that maternal tissues actively remodel blood supply in synchrony with implantation.

Anatomy of the developing placenta (timeline + structure)

6) Trophoblast differentiates into two layers

• Cytotrophoblast (inner layer)
• Cells remain recognisably individual (distinct cell boundaries).
• Syncytiotrophoblast (outer layer)
• Cellular walls are largely lost.
• Creates a continuous multinucleated layer important for invasion and exchange.

7) Lacunae formation and the future intervillous space

• Before implantation completes (around 11–12 days):
• Lacunae form in the polar syncytiotrophoblast.
• These lacunae are trophoblast-lined spaces.
• They will eventually become the intervillous space:
• The space that later allows maternal blood to circulate in the placental bed.

8) Deep invasion for attachment (trophoblast shell contribution)

• By about 13 days after conception:
• Some cells from the trophoblast shell invade deeply through decidua into myometrium.
• Purpose: supportive/attaching function (anchoring the developing placenta more securely).

9) Villous development: primary → secondary → tertiary (and what becomes placenta)

End of week 2: primary villous stems

• By end of week 2, primary villous stems form.
• Composition described in the text:
• Outer syncytiotrophoblast
• Core of cytotrophoblast
• Core of extraembryonic mesoderm (noted as part of the villous core in the description)

Week 3: vascularisation and connection to umbilical vessels

• Differentiation of extraembryonic mesoderm leads to vascularisation of the stem villus.
• This vascular system becomes continuous with vessels of the body stalk during week 3.
• The body stalk vessels will become the umbilical vessels.

Primary villi (buds from stems)

• Primary villous stems give rise to syncytiotrophoblast buds.
• These buds also have:
• Cytotrophoblast
• Mesodermal core
• These buds become primary villi.

Chorion laeve vs chorion frondosum (regional villous fate)

• Villi adjacent to the uterine cavity regress:
• This forms the chorion laeve (the non-placental part of the chorion).
• Villi on the decidual side of the conceptus:
• This is the chorion frondosum.
• These villi undergo further growth and branching as:
• Secondary villi
• Tertiary villi
• They fill the intervillous space to form the definitive placenta.
• True placental villi arise from the tertiary villi.

10) Placental lobules (and what they are NOT)

• Villi elaboration centred on a single primary stem villus is called a placental lobule.
• The placenta has approximately 200 lobules.
• This lobule-forming process is completed by about 8–10 weeks of gestation.
• Important distinction:
• There is no relation between placental lobules (or groups of lobules) and the cotyledons (lobes) seen on the maternal surface of the term placenta.

11) Placenta growth timing and weight relationships

• By 12 weeks, the placenta has reached its definitive form.
• After 12 weeks, it enlarges mainly laterally.
• Weight comparisons:
• The placenta exceeds fetal weight until about 17 weeks.
• At term, the placenta typically weighs about one-sixth of the fetus.

Chorionic villus and types of placentation (comparative + human mechanism)

12) Classification by layers between maternal and fetal blood

Placental types can be classified by which cell layers separate maternal and fetal blood.

A) Epitheliochorial placentation

• Trophoblast attaches to uterine surface epithelium.
• There is no invasion.

B) Endotheliochorial placentation

• Trophoblast invades uterine epithelium.
• But there is no invasion of maternal blood vessels.

C) Haemochorial placentation (humans)

• Shows the greatest invasion.
• Maternal blood vessels are invaded by trophoblast.
• Trophoblast becomes bathed in maternal blood (key haemochorial feature).

13) How maternal blood flow increases: species contrast

• In epitheliochorial species:
• Maternal blood flow increases through decidual angiogenesis:
• Increased number of similar-sized vessels.
• In human haemochorial pregnancy:
• Blood flow increases mainly through trophoblast invasion of spiral arteries in:
• The decidua
• The outer myometrium

Spiral artery remodeling (core human physiology + mechanism)

14) Spiral arteries before pregnancy

• In the nonpregnant state, spiral arteries are:
• Muscular
• High-resistance vessels

15) Spiral arteries in pregnancy

• Trophoblast invasion converts spiral arteries into:
• Wider bore
• Capacitance vessels
• Functional result:
• Uterine blood flow increases 10–15 fold.

16) How trophoblast invasion remodels arteries (mechanistic sequence)

• The mechanism is described as only beginning to be understood.
• Likely steps include:
• Trophoblast recognises vascular smooth muscle and endothelial cells via specific adhesion molecule expression.
• This recognition triggers an intracellular sequence of events that:
• Alters trophoblast motility (helps them move/invade appropriately).
• Induces apoptosis in:
• Vascular myocytes (smooth muscle cells)
• Endothelial cells
• Produces the characteristic spiral artery changes:
• Reduced muscular wall
• Endothelial lining replaced by trophoblast cells
• These structural changes are the hallmark of haemochorial placentation, and they explain the low-resistance, high-flow uteroplacental circulation.

Preeclampsia: consistent placental architecture findings

17) Two constant observations in preeclampsia (as described)

• Observation 1: Villous development is reduced
• Placentas from pre-eclamptic pregnancies show villi that are:
• Less developed
• With fewer branches
• With less complex vascular loops
• Compared with placentas from normal pregnancy outcomes.
• Observation 2: Spiral artery remodeling is reduced/patchy
• When placental bed is examined histologically (e.g., placental bed biopsy):
• The expected spiral artery architectural change is less evident or patchy.
• This implies incomplete/uneven conversion of the uterine circulation into the normal low-resistance, high-flow state.

Immunology of pregnancy tolerance (big picture)

• Human placenta is haemochorial, meaning maternal blood directly bathes trophoblast in placental villi.
• Advantage: close contact improves nutrition delivery and waste removal.
• Disadvantage: maternal and fetal circulations are not separated by an epithelial barrier, so trophoblast is exposed to potential maternal allogeneic immune responses.
• Core idea: pregnancy “works” by special immune rules at the placenta–uterus interface, especially where trophoblast meets decidual immune cells.
• Why the trophoblast–decidua relationship is the focus
• Trophoblast = outermost fetal cell layer that forms the key interface with the mother.
• It contacts uterine cells that have undergone decidualisation (highly specialised differentiation).
• Key decidual features:
• Enlargement of uterine stromal cells.
• A distinct uterine lymphocyte population dominated by uterine natural killer (NK) cells.
• Species concept: uterine NK cell presence is widely seen in species that show a decidual response with invasive placentation, and these NK-like cells are not seen in other tissues (i.e., they are uterine-specialised).
• Uterine NK cells (uNK): roles + characteristics (expanded point-by-point)
• Phenotype and dominance at implantation
• CD56 high, CD16 negative (CD56hiCD16–) uNK cells form the major population.
• They represent about 70% of leukocytes at the implantation site.
• They increase in late luteal phase (just before/around implantation timing) and in early pregnancy.
• Example: immune “preparation” of the endometrium late in the cycle aligns with implantation readiness.
• Similarity to blood NK cells, but important differences
• uNK cells resemble a minor fraction (~10%) of blood NK cells, but they are phenotypically different.
• Differences include unusual adhesion molecule expression and poor cytotoxicity.
• Meaning: they behave less like “killers,” more like “local regulators/organisers.”
• Origin is unclear
• It is uncertain whether decidual NK cells are:
• a discrete lineage, or
• differentiated from circulating NK cells that migrate into the uterus.
• Secretory profile (what they release)
• They produce soluble mediators, including:
• Angiogenic cytokines such as angiopoietin-2 and vascular endothelial growth factor (VEGF).
• Immunoregulatory cytokines and chemokines (local immune shaping + cell recruitment).
• Lytic enzymes (present even if killing is low).
• Example: VEGF supports vessel growth/remodelling needed for placental bed development.
• Why their cytotoxicity is reduced
• Reduced cytotoxicity may relate to lack of intracellular cytoskeletal and lytic granule organisation.
• Meaning: even if they contain “killing tools,” they are not arranged/primed to kill efficiently.
• Clinical testing pitfall (exam trap)
• Peripheral blood NK cell tests in women with reproductive failure provide no information about decidual NK function.
• Exam line: blood NK ≠ decidual NK; do not assume equivalence.
• Proposed functions at the implantation site
• Maintain mucosal and blood vessel stability.
• Modify spiral arteries in the decidua (key for uteroplacental circulation).
• Regulate trophoblast invasion into decidua and its structures.
• Example: spiral artery remodelling supports low-resistance high-flow placental perfusion.
• Decidua vs trophoblast invasion: “permit AND control”
• The decidual response is argued to both:
• permit invasion (allow placentation), and
• control invasion (prevent excessive damage).
• Both can be true: the goal is enough invasion for a functional placenta, but not so much that maternal health is compromised.
• Spectrum of invasion problems (with examples)
• Too much invasion (overinvasion)
• Seen when decidua is deficient or absent.
• Examples given:
• Tubal ectopic pregnancy (no proper decidua like in uterine cavity).
• Implantation on uterine scar tissue (deficient/abnormal decidual environment).
• Trophoblast implanted at distant ectopic sites (animal experiments / in vitro culture) shows inherent invasive growth.
• Maternal consequence: overinvasion can cause haemorrhage, and without intervention can compromise maternal health.
• Too little invasion (inadequate spiral artery invasion)
• Decidua may prevent adequate invasion into spiral arteries.
• This shifts the fetal–maternal interface in favour of the mother → reduced blood supply to placental bed.
• Fetal consequence: growth restriction and hypoxic fetus.
• Clinical association in the text: pre-eclampsia is linked to this “inadequate placentation” end of the spectrum.
• Epidemiology stated: pre-eclampsia incidence 5–10% in first-time mothers, with high maternal and fetal mortality.
• Evolutionary trade-off concept (why selection hasn’t removed it)
• The text frames pre-eclampsia as a possible trade in human reproduction:
• humans have highly invasive trophoblast tendencies
• balanced against maternal haemorrhage/death risk
• and neonatal survival consequences if the balance shifts.
• How invasion degree is controlled (what is known vs unknown)
• Not fully understood how trophoblast invasion is precisely controlled.
• There is evidence that apoptosis in invasive trophoblast may be a limiting/control factor.
• Example logic: apoptosis can act like a “brake” on how far invasive trophoblast progresses.

• Placenta–uterus immune interaction: why mother doesn’t reject fetus
• A mother generally does not reject pregnancy biologically.
• Yet she would reject an allograft from a genotypically dissimilar donor (e.g., kidney/heart transplant).
• Allograft rejection mechanism (contrast)
• Driven by strong antibody-mediated and T-cell-mediated responses to allogeneic MHC molecules on a vascularised graft.
• Key pregnancy difference stated
• The fetus is not directly in contact with the maternal immune system.
• It is shielded by trophoblast at the placental interface.
• MHC rules on trophoblast (major tolerance mechanism)
• Trophoblast cells never express MHC class II molecules.
• Therefore they cannot present antigen to maternal CD4+ T cells (a central arm of classic adaptive rejection).
• Two main maternal immune contact sites with trophoblast (humans)
• (1) Villous syncytiotrophoblast
• It is bathed by maternal blood.
• (2) Extravillous cytotrophoblast
• It interacts with uterine tissue (decidua) during invasion/anchoring.
• Syncytiotrophoblast: “immunologically neutral” surface
• It expresses no MHC antigens.
• This supports the concept that the placenta here is immunologically neutral.
• This also fits with the observation that:
• fetal cells entering maternal circulation can provoke immune responses
• examples given: rhesus isoimmunisation and haemolytic disease of the newborn
• but no systemic T-cell or B-cell immune response to trophoblast cells has been described.
• Extravillous trophoblast: unusual HLA pattern (critical detail)
• Unlike syncytiotrophoblast, extravillous trophoblast expresses an unusual combination of:
• HLA-C
• HLA-G
• HLA-E
• It does not express polymorphic:
• HLA-A
• HLA-B
• Why that matters: HLA-A and HLA-B are key initiators of classic allograft rejection responses; their absence changes the immune “signal.”
• But HLA-C is polymorphic—why no rejection? (hypotheses listed)
• HLA-C is expressed but does not lead to allograft rejection, and the text offers explanatory hypotheses:
• Mechanism difference from classic allograft rejection
• In pregnancy, uterine CD8+ regulatory T-cell populations are described as specific for a carcinoembryonic antigen-like ligand, rather than being MHC restricted.
• Meaning: regulation can be driven by non-classical specificity, dampening destructive responses.
• HLA-G → leukocyte immunoglobulin-like receptors (LILRs) on myelomonocytic cells
• Trophoblast HLA-G binds with high avidity to LILRs on myelomonocytic cells.
• Increased expression of these receptors is associated with graft tolerance.
• Possible mechanism: reduced MHC class II pathways.
• This may matter in decidua because macrophages and dendritic cells expressing MHC class II are present there.

• Uterine NK recognition by trophoblast: receptor–ligand control system
• Key premise
• Uterine (not peripheral) NK cells express an array of receptors for trophoblast MHC class I-related antigens.
• HLA-E pathway (inhibitory)
• A C-type lectin receptor CD94–NKG2A binds HLA-E.
• This interaction reduces NK cell cytotoxicity.
• Example: inhibitory signalling helps prevent NK-mediated damage at implantation.
• HLA-G pathway (signalling/angiogenic shift)
• All NK cells express KIR2DL4, which can bind HLA-G.
• This binding leads to upregulation of proinflammatory and proangiogenic cytokines.
• Meaning: instead of killing, NK cells can be pushed toward inflammation + blood-supply support locally.
• Soluble HLA-G and systemic pregnancy changes (important concept)
• The text suggests soluble HLA-G entering systemic circulation might:
• interact with blood NK cells
• contribute to systemic vascular and inflammatory changes of pregnancy.
• HLA-C ↔ KIR system: activating vs inhibitory balance
• Polymorphic HLA-C on trophoblast interacts with NK cell KIR ligands, which come in:
• Activating receptors: KIR2DS
• Inhibitory receptors: KIR2DL
• Therefore, two polymorphic gene systems interact at the maternal–fetal interface:
• Maternal KIR genotype
• Fetal HLA-C genotype
• Consequence stated: trophoblast invasion can vary across pregnancies, because some ligand–receptor combinations may:
• favour invasion (support placentation), while others may
• limit invasion (risk inadequate placentation).

• Exam-focused “connect-the-dots” (directly from the concepts above)
• Placental tolerance is largely local, not systemic, because trophoblast shields fetal tissues and uses non-classical HLA patterns at the interface.
• Syncytiotrophoblast = no MHC, so it stays “quiet” despite being bathed in maternal blood.
• Extravillous trophoblast = HLA-C/G/E (not A/B), shaping a controlled immune dialogue with decidual immune cells.
• Decidual NK cells are not killers here; they are regulators of angiogenesis, spiral artery remodelling, and invasion balance.
• Mismatch/imbalance in invasion control maps to the spectrum:
• too much invasion → haemorrhage risk
• too little invasion → reduced placental perfusion → growth restriction/hypoxia → association with pre-eclampsia

Placenta as a barrier + how exchange happens (core principles)

• The placenta sits between maternal circulation (intervillous space) and fetal circulation (villus capillaries), so exchange must cross the placental barrier.
• Simple diffusion is only really significant for:
• Low–molecular-weight substances: gases, sodium, urea, water.
• Nonpolar molecules: unconjugated steroids and fatty acids.
• For many other substances, transfer needs carriers or active transport systems (explained below), not just “free diffusion.”

Why diffusion gets easier as pregnancy advances (terminal villus remodeling)

• Early pregnancy terminal villi:
• Villi are large: about 150–200 micrometres in diameter.
• Fetal vessels are central.
• Beneath about 10 micrometres of syncytiotrophoblast.
• Meaning: metabolites must diffuse a relatively long distance to reach fetal blood.
• Later pregnancy terminal villi:
• Villi become smaller: about 40 micrometres in diameter.
• Fetal vessels become more eccentric (closer to the surface).
• This creates a ~90% reduction in diffusional distance.
• Key consequence:
• For most molecules, diffusion rate is inversely proportional to diffusion distance → shorter distance = faster transfer.
• Oxygen is the special case:
• Oxygen is highly soluble, so its transfer depends more on blood flow than just barrier thickness.
• So improving perfusion/flow can matter more than merely thinning the barrier.

Oxygen and carbon dioxide exchange (including the “double Bohr effect”)

Maternal adaptations that drive gas exchange

• Oxygen consumption rises in pregnancy in proportion to the growing conceptus (fetus + placenta).
• Maternal cardiac output increases.
• Uterine and placental bed blood flow increases.
• Ventilation increases, which lowers:
• Maternal carbon dioxide concentration.
• Maternal bicarbonate concentration.
• These changes help create concentration gradients that:
• Drive oxygen into the fetus.
• Drive carbon dioxide out of the fetus (into maternal blood).

Why fetal oxygen carriage can be “about equal” to maternal oxygen carriage

• Fetal oxygen carriage is approximately equal to maternal blood because fetal hemoglobin has:
• Greater intrinsic affinity for oxygen.
• Reduced binding to 2,3-diphosphoglycerate (2,3-DPG) (embryonic/fetal hemoglobin interacts differently), which supports higher oxygen affinity.

Double Bohr effect (exam-critical mechanism)

• In the placenta:
• Fetal blood pH increases → fetal hemoglobin binds oxygen more strongly → better uptake of oxygen.
• Maternal blood pH decreases as it picks up carbon dioxide → maternal hemoglobin releases oxygen more readily → better delivery of oxygen.
• Net effect: both sides shift in a way that pushes oxygen from mother to fetus efficiently.

Glucose and carbohydrate transfer (and why maternal insulin resistance helps)

Maternal metabolic shift with gestation

• As gestation progresses, maternal tissues become relatively less sensitive to insulin.
• This:
• Can allow latent diabetes to appear.
• Also spares plasma glucose for the fetal–placental unit, so relatively less is taken up by maternal tissues.

How glucose crosses the placenta

• Placental glucose uptake is by facilitated diffusion:
• Specific carriers move D-glucose down its concentration gradient.
• Transport characteristics:
• Maximum transfer rate: 0.6 mmol/minute/g placenta.
• Transport becomes saturated only at about 20 mmol/L glucose (so under usual physiology, it is typically not saturated).

Placental metabolism of glucose → lactate

• The placenta metabolises considerable amounts of glucose to lactate.
• Lactate becomes available to the fetus as a fuel at about one-third the availability of glucose (relative contribution as described).

What controls fetal glucose use

• The rate of fetal glucose use is largely related to fetal insulin production (fetal insulin drives fetal tissue uptake/utilization and growth effects).

Amino acids and urea (nitrogen economy shift toward fetus)

Maternal nitrogen conservation

• Maternal liver deamination of amino acids is reduced in pregnancy.
• Maternal excretion of resulting urea falls.
• This leaves more ingested/absorbed amino acids available for placenta and fetus.

What the trophoblast does with amino acids

• Some amino acids are synthesised within trophoblast cells, including examples given:
• Acidic nonessential amino acids.
• Neutral straight-chained amino acids.
• The placenta has multiple active transport systems for groups such as:
• Neutral branched-chain amino acids.
• Basic amino acids.
• Uses of transported amino acids:
• As building blocks for the placenta itself.
• For the dependent fetus (growth, protein synthesis).

Concentration gradient created by active transport

• These transport systems concentrate amino acids in trophoblast to ~5 times the maternal plasma concentration.

Urea movement (fetal → maternal)

• Fetal amino acid metabolism generates urea.
• Urea diffuses passively back across the placenta into maternal circulation.
• This is favored because maternal urea concentration is reduced in pregnancy.

Water and electrolytes (sites + mechanisms)

Where water exchange occurs

• Water exchange between mother and fetus occurs mainly at:
• The placenta
• The nonplacental chorion (remainder of chorion)

Membrane permeability + forces

• Amnion and chorion are freely permeable to water.
• Therefore, transfer must be explained by:
• Diffusion, and/or
• Hydrostatic forces.

Hydrostatic gradient logic

• A large, constant hydrostatic pressure difference between maternal and fetal circulations in the placenta does not occur.
• Likely mechanism described:
• Necessary fetal-to-maternal water flux is achieved via small and intermittent hydrostatic gradients.

Sodium trafficking

• Isotope studies show sodium crosses the placenta via:
• Co-transport linked to various active transport molecules, and also
• Paracellular diffusion.

Iron transfer (why fetal iron is higher)

• Fetal blood contains iron at 2–3 times the concentration of maternal blood.
• Iron is delivered by active transport across the placenta.
• An intratrophoblast ferritin-binding stage has been described (a handling/binding step inside trophoblast related to ferritin).

Calcium and phosphate transfer

• Calcium and phosphate are transferred to fetus against a concentration gradient by an active transport system.
• This system is sensitive to:
• Metabolic inhibitors
• Competitive inhibitors
• Meaning it is energy-dependent and can be blocked/interfered with by competing substances or metabolic poisoning.

Amniotic fluid (volume, composition, turnover)

Volume changes over gestation

• Amniotic fluid volume rises:
• About 15 mL at 8 weeks
• About 450 mL at 20 weeks
• After 20 weeks:
• Net production declines to zero by 34 weeks
• Approximate volume around that stage: 750 mL

What composition suggests about origin

• Early composition suggests it is initially a transudate (fluid filtered across membranes early on).

Late pregnancy composition trend

• Toward term:
• Total solute concentration falls
• But concentrations of urea, creatinine, and uric acid rise

Dynamic turnover (high-yield fact)

• Amniotic fluid is in a dynamic state.
• Complete exchange of its water occurs roughly every 3 hours.

Placental membranes (amnion + chorion anatomy)

Amnion: origin + layers

• Amnion arises as an epithelial layer between:
• The ectodermal disc of the inner cell mass, and
• The trophoblast (chorion)
• Amnion has five layers:
1. Cuboidal epithelium (with prominent intracellular canals and vacuoles)
2. Basement membrane
3. Compact layer
4. Fibroblast layer
5. Spongy layer of mucoid reticular tissue (remnant of extraembryonic coelom)
• The amniotic cavity lies between:
• The amnion and
• The ectodermal disc

Chorion: layers + relationships

• Chorion has four layers:
1. Fibroblasts
2. Reticular layer
3. Basement membrane
4. Trophoblast layer
• The trophoblast layer:
• Is 2–10 cells thick
• Lies immediately adjacent to the decidua
• Is continuous with placental trophoblast
• Obliterated chorionic villi have been described (villi that become regressed/obliterated).

Vessels/lymphatics/nerves (must-know negative)

• Neither amnion nor chorion has:
• Blood vessels
• Lymphatics
• Nerves

Hormonal control of placental synthesis

Human chorionic gonadotrophin (hCG)

Why it matters

• Pregnancy survival in the first trimester requires the corpus luteum not to involute.

Source + timing

• hCG is secreted by syncytiotrophoblast from as early as 6–7 days post-fertilisation.

Structure and size (as given)

• Consists of two amino acid chains:
• Alpha and beta
• Linked by noncovalent bonds
• Both chains have attached carbohydrate residues.
• Relative molecular weight given: 38 400 kDa (reported as stated in the text you provided).

Relationship to pituitary hormones

• hCG is chemically and functionally similar to luteinising hormone (LH).
• Alpha subunit similarity:
• Similar to all glycoprotein hormones: LH, FSH, thyroid-stimulating hormone
• Beta subunit:
• Similar to LH but has an additional 30 amino acids at the carboxy terminus.

Pregnancy test logic

• Immunoassays directed at the beta-subunit carboxy-terminal region are specific for hCG → basis of urinary pregnancy tests.

Pattern over gestation

• Maternal serum/urine hCG levels:
• Rise rapidly early,
• Peak around 12 weeks gestation,
• Then decline.

Target + evidence that it is luteotrophic

• hCG binds to LH receptors in the corpus luteum.
• Evidence described:
• Nonpregnant women given parenteral hCG do not have corpus luteum involution.
• Animal studies: antibodies preventing hCG binding at LH receptor cause corpus luteum involution and abortion.

Progesterone (source shift and autonomy)

• hCG maintains corpus luteum progesterone output early, but:
• Within 2 weeks of fertilisation, the conceptus is synthesising all steroid hormones needed for pregnancy.
• Although the corpus luteum may remain active throughout pregnancy:
• It is not essential after ~6 weeks
• It contributes only a small amount to total pregnancy progesterone.
• Key evidence progesterone is made by trophoblast:
• In blighted ovum pregnancies (no embryo forms),
• And in choriocarcinoma or hydatidiform moles
• Progesterone production by the conceptus is unaffected.
• Progesterone synthesis:
• Localised to trophoblast (immunocytochemical/in vitro studies).
• Uses maternal cholesterol.
• Appears autonomous to the placenta (no external control identified).
• Levels:
• Peak at term at about 10× luteal-phase ovarian-cycle levels.
• Main urinary excretion product:
• Pregnanediol.

Estrogen (fetoplacental unit; estriol dominance)

• Main estrogen in pregnancy is estriol (less potent) rather than 17-estradiol.
• Estriol synthesis is:
• Severely reduced in molar and choriocarcinoma pregnancies.
• Requires presence of a fetus (unlike progesterone production).
• Pathway (classic fetoplacental collaboration):
• Placenta synthesises estriol using DHEA from fetal adrenal cortex.
• In fetal liver, DHEA is hydroxylated to 16-hydroxy DHEA.
• In trophoblast, aromatisation of the A-ring converts substrate to estriol.
• Clinical correlation given:
• Deficient fetal adrenal activity (e.g., congenital adrenal hyperplasia, anencephaly) → low estriol levels.
• Sulphation/desulphation logic (very testable)
• In fetal blood, 16-hydroxy DHEA circulates as a sulphate conjugate.
• Placental sulphatase is essential for estriol synthesis.
• Sulphation in fetus → steroid becomes water soluble and inactive.
• Desulphation in placenta → returns it to a form that can become bioactive in mother.
• Purpose: fetus is less exposed to biological effects of a steroid that is bioactive in mother and important for pregnancy adaptations.
• Maternal estriol concentration:
• Rises to about 100× nonpregnant estrogen concentration at term.
• Estrone and estradiol:
• Also synthesised by placenta, but in smaller amounts than estriol and from maternal precursors.

Human placental lactogen (hPL)

• As trophoblast makes less hCG near end of first trimester, it synthesises human placental lactogen.
• Structure:
• Single-peptide protein
• Given molecular weight: 21 600 kDa (as stated in your text)
• 191 amino acids
• Two disulphide bridges
• Plasma half-life ~15 minutes
• Similarity:
• Chemically and functionally similar to prolactin and pituitary growth hormone, but less biologically active.
• Concentration profile:
• Plateaus after about 35 weeks.
• Functional note in text:
• May modify carbohydrate metabolism, but pregnancies lacking hPL can still appear to progress normally.

Abnormal placentation

Multiple pregnancy (timing of division → chorionicity/amnionicity)

Zona pellucida role

• One function of the zona pellucida is preventing blastomeres from falling apart during early cleavage.

If the conceptus splits early → monozygotic twins

• If division occurs very early:
• Two completely separated pregnancies occur.
• Implantations may be far apart or coincidentally close.
• Placentas are anatomically and functionally separate.

Division timing outcomes (key timeline facts from the text)

• If division occurs between 4 and 8 days after conception:
• Monochorionic diamniotic twins:
• Two fetuses
• Two amniotic sacs
• Single chorion
• Single placenta
• If division occurs after 9 days from conception:
• Either:
• Two fetuses in a single amniotic sac with a single placenta (monoamniotic twins), or
• If later still → conjoined fetuses result.

Twin-to-twin transfusion (TTTS) in monochorionic placentation

• In monochorionic placenta:
• Most cotyledons return oxygenated blood to the umbilical vein of the same cord.
• But some cotyledons drain to the other twin’s cord, so one twin can become net donor and the other recipient.
• Recipient twin features (hypervolemic state):
• Hypervolaemia
• Excessive amniotic fluid (polyhydramnios)
• Hyperdynamic circulation
• High haemoglobin concentration
• Donor twin features:
• Anaemia
• Oliguria
• Growth restriction
• This pattern is termed twin-to-twin transfusion syndrome.

Molar pregnancy (genetic mechanism → phenotype)

• Chromosome complement in trophoblast profoundly affects placental phenotype; classic example: hydatidiform mole.
• Classified as partial vs complete based on:
• Chromosome complement
• Presence/absence of fetal tissue

Complete mole

• Fertilisation of an oocyte where maternal chromosomes are lost and paternal chromosomes are duplicated.
• Total: 46 chromosomes, but all derived from a single sperm’s genome duplicated.
• Because no maternal chromosomes → no fetal tissue forms.

Partial mole

• Total: 69 chromosomes
• Two sets paternal
• One set maternal (from oocyte)
• In partial mole, fetal tissue does form.

Shared clinical/biologic behavior (as stated)

• In both complete and partial moles:
• Trophoblast is more invasive than normal.
• Secretes more pregnancy hormones per gestational stage than a normal conceptus.

Uniparental disomy (chromosome imbalance → placental + syndrome effects)

• Uniparental disomy creates imbalance between paternal and maternal chromosome contributions.
• Mechanism described:
• Conception starts with 46 chromosomes,
• But one pair becomes:
• Loss of maternal or paternal chromosome,
• Duplication of the remaining one,
• So you end with 22 from one parent and 24 from the other (as described in the text).

Best-characterised example: chromosome 15

• Loss of maternal chromosome 15 + duplication of paternal:
• Rare cause of Angelman syndrome.
• Maternal disomy for chromosome 15 (reverse situation):
• Causes Prader–Willi syndrome.