The Placenta

• Early gestation (10-12 weeks) is the period of placentiation– Fetus is nourished by secretions of uterine

endometrial glands in early gestation

• Placenta is a metabolically active tissue– Responsible for exchange of nutrients,

gases & metabolic waste products between maternal and fetal circulation

– Glucose is predominant energy source for both placenta and fetus

Placental Architecture

• Maternal and fetal blood do not mix: “placental barrier”– Fetal blood flows through capillary networks

within highly branched terminal chorionic villi– Maternal blood flows through intervillous

space• Uterine arteriols bring blood in• Uterine venules drain blood

Figure 29.6a

Figure 29.6 A Three-Dimensional View of Placental Structure

• Chorionic villi extend into maternal tissue– Forms intricate branching network for

maternal blood

• Umbilical cord connects fetus to placenta

Placentation

Placental Capacity Increases During Gestation

• Expression of transporters increases

• The “brush border” microvilli develop to:– increase surface area – impede maternal blood flow

• Flow through the placenta at term is 500 ml/minute

Mechanisms of Nutrient Transfer Across the Placenta

Maternal to Infant Nutrient Transportation Across The Placenta

Substance Primary Mechanism

Water, electrolytes, urea, free fatty acids, steroids, fat soluble vitamins

Passive diffusion

Glucose Facilitated diffusion

Amino acids, water soluble vitamins, calcium, iron, iodine

Active transport

Globulins, phospholipids, lipoproteins

Pinocytosis and endocytosis

Water, electrolytes Bulk flow (due to changes in hydrostatic or osmotic forces), solvent drag

Fetal to Maternal Transport

• Carbon dioxide

• Water & urea

• Hormones

Factors Affecting Placental Transfer

• Placental size• Diffusion distance –

– diabetes and infection cause edema of the villi– distance decreases as pregnancy progresses and

fetal needs increase

• Maternal-placental blood flow• Blood saturation with gases and nutrients

Factors Affecting Placental Transfer (cont)

• Maternal-placental metabolism of the substance

• Disorders in expression or activity of nutrient transporters

• Maternal use of tobacco, cocaine, alcohol

• Trophoblast secretes hormones to maintain pregnancy– HCG– Estrogens– Progesterone– hPL– Placental prolactin– relaxin

Hormones of the placenta