Placental Development and Structure Notes

Blastocyst structure and implantation

• The placenta’s structure can be understood by first reviewing the blastocyst.
• Blastocyst components:
  • Outer trophoblast cells (trophoblast) form the placenta.
  • Inner cell mass (ICM) forms the organism.
• Implantation begins when the blastocyst contacts the endometrium (uterine lining) and begins to invade.
• As implantation initiates, the trophoblast differentiates into two regions:
  • Syncytial trophoblast: a multinucleated, boundary-free layer formed by fusion of trophoblast cells; it aggressively burrows into the endometrium.
  • Cytotrophoblast (cellular trophoblast): a layer of individual trophoblast cells that retain their boundaries and surround the hollow blastocyst cavity and the inner cell mass.
• Implantation progresses until the conceptus is completely buried within the endometrium, after which the surface epithelium of the endometrium regenerates to cover the invasion tracks.
• Hormonal activity during early implantation: the syncytial trophoblast produces human chorionic gonadotropin (HCG), the pregnancy hormone.

Trophoblast differentiation: Syncytial vs cytotrophoblast

• Syncytial trophoblast features:
  • Multinucleated cytoplasmic mass with no clear cell boundaries.
  • Frontline invaded into the endometrium.
  • Produces HCG to support early pregnancy.
• Cytotrophoblast features:
  • Retains cell boundaries.
  • Surrounds the blastocyst cavity and inner cell mass, contributing to placental development.

HCG and placental support in early pregnancy

• HCG (human chorionic gonadotropin) is produced by the syncytial trophoblast and sustains the corpus luteum.
• Corpus luteum maintenance preserves high levels of progesterone (and estrogen) in early pregnancy.
• HCG levels rise rapidly at the start of pregnancy, then decline sharply around the third month as placental hormone production takes over.
• Once the placenta forms sufficiently, it secretes estrogen and progesterone, reducing dependence on the corpus luteum.

Transition from corpus luteum dependence to placental hormone production

• Early pregnancy: corpus luteum, sustained by HCG, maintains progesterone to support the uterine lining.
• By around the third month: placenta-derived estrogen and progesterone take over, making the corpus luteum less essential.
• This hormonal switch is part of placental maturation and functional establishment of the maternal-fetal unit.

Inner cell mass, embryonic disc, and gastrulation

• Inner cell mass organizes into an embryonic disc.
• The embryonic disc undergoes gastrulation, forming the mesoderm layer.
• Not all mesoderm is contained within the embryo:
  • Extraembryonic mesoderm lines the cellular trophoblast and contributes to the placental structures.
• As mesoderm lines the trophoblast, the term
  • chorion begins to be used for the combined fetal membranes (later sections clarify terminology).

Extraembryonic mesoderm and the chorion

• Extraembryonic mesoderm lines the cellular trophoblast.
• Once mesoderm associates with these layers, the combined structure is referred to as the chorion (fetal portion of the placenta).
• The chorion consists of:
  • Mesoderm core
  • Cellular trophoblast
  • Syncytial trophoblast

Chorion, lacunae, and intervillous spaces

• The chorion continues to invade the endometrium as implantation progresses.
• Lacunae: small cavities formed around invading chorionic tissue, filled with maternal blood from damaged endometrium.
• Chorionic villi grow outward like roots, embedding into the endometrium to anchor the conceptus.
• Intervillous spaces: pools of maternal blood surrounding the chorionic villi.
• The mesoderm lines these spaces and will give rise to fetal blood vessels, enabling blood flow to and from the conceptus.
• Implantation is complete when the endometrium surrounds and contains the developing conceptus.
• The endometrium (now called the decidua basalis on the maternal side) remains in contact with the placental tissue; the myometrium lies beneath.

Decidua basalis and endometrium anatomy

• The maternal portion of the placenta is the decidua basalis.
• The endometrium portion involved in placental contact used to be the functional layer, historically called the stratum functionalis; it is now referred to as the decidua basalis.
• The chorionic villi grow into the maternal endometrium, forming the fetal portion of the placenta, while maternal blood remains in the intervillous spaces.
• The placenta functions as an interface for maternal and fetal tissues without direct blood mixing.

Fetal-maternal exchange and placental vasculature

• Placental function is twofold:
  • Exchange: oxygen, nutrients, wastes, and gases diffuse across the placental barrier; maternal and fetal blood do not mix.
  • Endocrine: the placenta acts as an endocrine gland, secreting several hormones.
• Fetal vasculature in the chorionic villi:
  • Within the villi are fetal blood vessels connected to the umbilical cord.
  • The umbilical cord contains $2$ arteries and $1$ vein:
  • Two umbilical arteries carry deoxygenated blood away from the fetus to the placenta.
  • One umbilical vein carries oxygenated blood from the placenta to the fetus.
• The intervillous space surrounding the villi contains maternal blood and serves as the exchange surface.

The placenta as an endocrine gland: hormones and roles

• Estrogen and progesterone: secreted by the placenta, essential for maintaining pregnancy and supporting uterine/embryo development.
• Human placental lactogen (hPL): secreted by the placenta; promotes maturation of mammary alveolar tissue in preparation for milk production.
• Relaxin: relaxes the pubic symphysis to aid in parturition and pelvic remodeling.
• These placental hormones shift the hormonal control from the corpus luteum to the placenta as pregnancy progresses.

Amnion, amniochorionic membrane, and smooth chorion

• The amnion surrounds the developing fetus and, as pregnancy progresses, fuses with the chorion to form the amniochorionic membrane.
• The chorion near the lumen of the uterus becomes the smooth chorion as the villi near the luminal interface are stretched and reduced.
• The amnion eventually fuses with the smooth chorion to create the amniochorionic (amniotic) membrane that surrounds the fetus.

Placental development timeline and morphological changes

• Early placenta: syncytial trophoblast invasion and establishment of the decidua basalis.
• Formation of chorionic villi and lacunae, with maternal blood entering the intervillous spaces.
• Mesoderm derivatives develop vasculature within chorionic villi, enabling fetal blood flow.
• As the placenta grows, the villi near the placental surface remain extensive while those projecting into the lumen may be reduced to form the smooth chorion.
• Placental endocrine function becomes the dominant source of maternal hormones (estrogen, progesterone, hPL, relaxin).

Key concepts and quick recap

• Two major placental components:
  • Fetal portion: chorionic villi, fetal vessels, and the chorion.
  • Maternal portion: decidua basalis and intervillous spaces.
• Two primary functions of the placenta:
  • Exchange of gases, nutrients, and wastes without mixing of maternal and fetal blood.
  • Endocrine production of hormones essential for maintaining pregnancy and preparing for lactation.
• Hormonal transition in early to mid-pregnancy: reliance on HCG and corpus luteum decreases as placental hormone production rises.
• Structural maturation includes formation of the amniochorionic membrane via fusion of the amnion with the smooth chorion, and expansion of placental vasculature within chorionic villi.
• Important terms to remember:
  • Syncytial trophoblast
  • Cytotrophoblast
  • HCG (human chorionic gonadotropin)
  • Chorion and chorionic villi
  • Lacunae and intervillous spaces
  • Decidua basalis
  • Extraembryonic mesoderm
  • Amnion and amniochorionic membrane
  • Relaxin, human placental lactogen (hPL)

Glossary of key terms (quick reference)

• Syncytial trophoblast: multinucleated, boundary-free trophoblast layer that invades the endometrium and secretes HCG.
• Cytotrophoblast: inner layer of trophoblast cells preserving cell boundaries.
• HCG: pregnancy hormone that maintains the corpus luteum and progesterone production in early pregnancy.
• Chorion: fetal component formed by trophoblast layers and extraembryonic mesoderm; gives rise to chorionic villi.
• Chorionic villi: projections into the maternal endometrium that facilitate gas and nutrient exchange through the intervillous spaces.
• Intervillous spaces: maternal blood-filled spaces surrounding the chorionic villi.
• Decidua basalis: maternal portion of the placenta; site of placental attachment.
• Extraembryonic mesoderm: embryonic mesoderm located outside the embryo that contributes to placental vasculature.
• Amnion: membrane surrounding the fetus; fuses with the chorion to form the amniochorionic membrane.
• Smooth chorion: portion of the chorion where villi are reduced as growth proceeds and contact with the lumen forms.
• Estrogen, Progesterone, hPL, Relaxin: placental hormones essential for pregnancy maintenance, lactation preparation, and pelvic relaxation.