12.5 Placenta

Structure of the mature human placenta

Gross morphology

  • The mature human placenta is a discoid organ, typically 22cm in diameter and 2-2.5 cm thick at term, weighing around 500 grams

  • It comprises fetal and maternal components

Fetal side

  • Derived from the chorionic plate, which gives rise to chorionic villi

  • Villi are organised into lobules (also known as cotyledons), and each lobule contains a core of fetal capillaries

  • Covered by:

    • Syncytiotrophoblasts: Multinucleated outer layer in direct contact with maternal blood; facilitates exchange

    • Cytotrophoblasts: Inner layer of trophoblasts that gives rise to syncytiotrophoblasts

Maternal side

  • Originates from the decidus basalis of the endometrium

  • Forms septa, which divide the placenta into lobes

  • Maternal blood enters from the spiral arteries, flows into the intervillous space, and bathes the villi

Histological layers (interhemal membrane)

  1. Syncytiotrophoblast

  2. Cytotrophoblast

  3. Basal lamina of trophoblast

  4. Villous stroma (contains fetal connective tissue, Hofbauer cells)

  5. Endothelium of fetal capillaries

Circulation

  • Maternal blood enters through spiral arteries, circulates in the intervillous space, and exits via uterine veins

  • Fetal blood runs through capillaries inside the villi, separated from maternal blood by several thin diffusion layers

Main functions of the placenta

1. Nutrient, gas, and waste exchange

  • Oxygen supply: O2 from maternal blood diffuses into fetal capillaries

  • Carbon dioxide removal: CO2 diffuses from fetal to maternal blood

  • Nutrient transport:

    • Glucose: via facilitated diffusion

    • Amino acids, fatty acids, vitamins: via active transport or diffusion

  • Waste removal:

    • Urea, uric acid, bilirubin pass into maternal blood for excretion

  • Filtration and selective transport: allows for efficient but selective exchange between fetal and maternal systems

2. Hormone production

  • The placenta acts as a temporary endocrine organ producing multiple hormones vital for pregnancy maintenance and maternal physiological adaptation.

  • Human chorionic gonadotropins (hCG)

    • Secreted by embryonic trophoblasts, then chorion

    • Peaks around week 9

    • Maintains corpus luteum, which produces progesterone in early pregnancy

    • Detected by home pregnancy test (in urine)

  • Progesterone

    • Initially from corpus luteum; taken over by the placenta from ~week 9

    • Relaxes uterine muscles to prevent contractions

    • Prepares breasts for lactation

    • Maintains endometrial lining

  • Oestrogens

    • Promote uterine growth, blood flow, and breast tissue development

  • Human placental lactogen (hPL)/ Human somatomammotropin

    • Alters maternal metabolism:

      • Promotes insulin resistance, ensuring glucose availability for the fetus

      • Promotes fat deposition as energy reserve for lactation

    • Promotes breast development for lactation

  • Placental growth hormone

    • Increases maternal nutrient availability, similar to hPL

  • Relaxin

    • Mainly produced by corpus luteum; the placenta also contributes in some species

    • Effects:

      • Vascular: Systemic and renal vasodilation

      • Cardiac: Increases stroke volume, cardiac output, and lowers blood pressure

      • Renal: Increases glomerular filtration rate

      • Skeletal: Softens pubic symphysis, sacroiliac, and sacrococcygeal joints

      • Prepares cervix for labour by softening it

Protection from the environment

  • Physical barrier: Syncytiotrophoblast layer blocks many large pathogens

  • Selective permeability:

    • Many bacteria blocked

    • Some viruses cross (e.g. Zika, Rubella, Varicella)

  • Drug and hormone metabolism

    • e.g. converts cortisol to cortisone to prevent fetal exposure

  • Limitation: Does not block alcohol or certain drugs — risk of fetal exposure and damage (e.g. fetal alcohol syndrome)

Immune protection

  • produces immunodulatory hormones that promote maternal immune tolerance to the semi-allogenic fetus

  • Allows exposure to fetal (paternal) antigens without immune rejection

  • Transports maternal IgG antibodies across the placenta:

    • Mainly in late pregnancy

    • Provides passive immunity to the neonate (e.g. protection against tetanus, diphtheria)