Development of the amniotic and yolk sacs, chorion

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Introduction

During the second week of human development, a set of extraembryonic membranes forms that are essential for embryonic survival, protection, nutrition, gas exchange, and waste removal. These membranes include the amniotic sac, yolk sac, and chorion. Although these structures do not contribute directly to the definitive body of the fetus, they play indispensable roles in early development and placentation. Their formation is closely linked to differentiation of the inner cell mass, trophoblast, and extraembryonic mesoderm.

Development of the Amniotic Sac

Amniotic Cavity

The amniotic sac is a fluid-filled sac that contains a developing fetus. It consists of the amniotic membrane and amniotic fluid. It is formed very early in pregnancy and surrounds the embryo as a protective shell. As the festus grow, the amniotic cavity expands which results in the displacement of the uterine cavity and chorionic cavity.

• development: 2nd week of development through migration of epiblast cells
• components:
  • Lined with amniotic epithelial cells
  • Filled with amniotic fluid, which is produced by amniotic epithelial cells

Origin and Formation

The amniotic sac begins to form at the beginning of the second week of development.

• As the inner cell mass differentiates, it separates into:
  • Epiblast
  • Hypoblast

A small cavity appears within the epiblast called the amniotic cavity.

• Epiblast cells adjacent to the trophoblast differentiate into amnioblasts
• Amnioblasts line the inner surface of the cytotrophoblast
• The remaining epiblast forms the embryonic disc

Together:

• Amnioblasts + epiblast form the amnion

Structure

The amniotic sac consists of:

• Amniotic cavity (filled with amniotic fluid)
• Amniotic membrane (amnion)

As development progresses:

• The amniotic cavity expands rapidly
• By the end of the first trimester, it surrounds the embryo entirely
• Eventually, the amnion fuses with the chorion, obliterating the chorionic cavity

it is worth to note that the amniotic sac is composed of maternal (decidua) and fetal components (charioamniotic membranes) that surround the fetus and provides mechanical support.

• Amnion ( A membrane that lines the inside of the amniotic cavity. Consists of a layer of cuboidal epithelial cells and an underlying layer of fibrous tissue. Derived from epiblast cells.)
  • Inner amniotic membrane
  • Develops from the embryoblast
  • 

    Diagrammatic representation of the human fetal membranes. The amnion and chorion are attached together by the sponge layer. The amnion is formed of a single-layered amniotic epithelial cell and compact and fibroblast layers. The chorion is formed of multilayered trophoblast cells and reticular layer. The fibroblast layer of the amnion and the reticular layer of the chorion contain mesenchymal cells. The mesenchymal cells exhibit plasticity among fibroblast/myofibroblast cells and macrophages.
    Secretes amniotic fluid
• Chorion
  • Middle amniotic membrane
  • Secretes amniotic fluid
• Decidua
  • outermost membrane
  • develops from the decidua capsularis, which lies above the site of implentation

Amniotic Fluid

Amniotic fluid is a medium essential for normal embryonic and fetal development, and its origin changes as pregnancy progresses. During the early first trimester, amniotic fluid is derived primarily from maternal plasma, entering the amniotic cavity by diffusion and filtration across the amnion and chorion before fetal organ systems become functional. As development advances, the fetal kidneys begin urine production around weeks 10–12, leading to a gradual shift in the source of amniotic fluid. By the mid-second trimester (approximately 16 weeks onward), fetal urine becomes the principal contributor, with additional input from fetal lung fluid secretions, which begin during the second trimester and support pulmonary development. Throughout later pregnancy, amniotic fluid volume is maintained by a balance between fetal urine production, fetal swallowing, and intramembranous absorption across the amniotic membrane. This evolving regulation explains the close association between abnormalities of fetal renal or gastrointestinal function and disorders of amniotic fluid volume.

• composition: initially a clear liquid
  • Amount: approximately 850-1500 mL by the end of pregnancy (the amniotic fluid is completely exchanged every 3 hours)
  • pH: 7-7.5 (slightly alkaline)
  • Proteins, glucose, urea
  • Fetal urine, lung fluids, hair, dead skin, sebum
  • Vernix: a milky-white, lipid rich substance that consists of fetal dermal cells and sebaceous gland secretions. It covers the fetus’s skin (especially in the third trimester)
• Reasbsorption
  • Reabsorption by the amniotic epithelium
  • 

    Amniotic fluid ciruclation
    The fetus swallos approximately 400 mL of amniotic fluid per day, which is excreted through the kidneys

• Functions of the amniotic fluid include:
  • Cushions the embryo (shock absorption)
  • Allows free movement → normal musculoskeletal development
  • Prevents adhesion between embryo and surrounding tissues
  • Maintains constant temperature

Clinical correlations:

• oligohydramnios → renal agenesis, pulmonary hypoplasia
• Polyhydramnios → gastrointestinal obstructions, neural tube defect

Development of the Yolk Sac

An extra-embryonic membranous sac derived from endoderm and lined by mesenchyme derived from mesoderm. Connects to the embryo via a yolk stalk. Involved in blood supply to the embryo. The yolk sac starts developing very early, around days 8-12 after fertilization.

Development of the Yolk Sac and amniotic cavity

Primary Yolk Sac Formation

The yolk sac arises from the hypoblast.

• Hypoblast cells migrate and line the inner surface of the blastocyst cavity
• This forms the exocoelomic (Heuser’s) membrane
• The cavity enclosed is the primary yolk sac

Secondary (Definitive) Yolk Sac

As extraembryonic mesoderm forms and the chorionic cavity expands:

• The primary yolk sac is pinched off
• A smaller secondary yolk sac remains

This is the functional yolk sac in humans.

Structure and Location

• lined by endoderm
• surrounded by extra embryonic mesoderm
• connected to the midgut via the vitelline (omphalomesenteric) duct

Functions of the Yolk Sac

1. Early nutrition (Transfer of nutrients before placental cirulation is established)
2. Hematopoiesis (first site of blood cell formation “weeks 3-6”)
3. Germ cell origin (primordial germ cells arise in the yolk sac wall and migrate to the gonadal ridges)
4. Gut development (incorporated into the embryo to form the primitive gut)

it is worth noting that the bilaminar disc forms the dividing layer between the yolk sac and amniotic cavity.

Development of the Chorion

Origin

The chorion is the outermost fetal membrane and forms from three components:

1. Trophoblast
   • Cytotrophoblast
   • Syncytiotrophoblast
2. Extraembryonic mesoderm
3. Chorionic cavity (extraembryonic coelom)

Extraembryonic Mesoderm and Chorionic Cavity

• Extraembryonic mesoderm forms between:
  • Cytotrophoblast
  • Amnion and yolk sac

• Spaces develop within this mesoderm and coalesce to form the chorionic cavity.

The mesoderm splits into:

• Somatic (parietal) layer → lines cytotrophoblast and amnion
• Splanchnic (visceral) layer → covers yolk sac

Chorionic Plate and Chorionic Villi

Development of chorionic villi

The chorion contributes to placental formation through chorionic villi:

1. Primary villi
   • Cytotrophoblast core covered by syncytiotrophoblast
2. Secondary villi
   • Extraembryonic mesoderm invades the core
3. Tertiary villi
   • Blood vessels develop within mesoderm

These villi:

• Increase surface area
• Enable maternal–fetal exchange

Chorion laeve and chorion frondosum

Chorion Frondosum and Chorion Laeve

Two distinct parts of the outer fetal membrane (chorion) during pregnancy, differing in their villi: the frondosum (leafy) is rich in branching villi at the embryonic pole, forming the placenta's fetal side for nutrient exchange, while the laeve (smooth) is the rest of the membrane where villi degenerate, becoming a smooth surface that fuses with other membranes,

• Chorion frondosum
  • Region with abundant villi
  • Forms the fetal part of the placenta
• Chorion laeve
  • Smooth region
  • Villi regress as pregnancy advances

Integration and Fate of the Extraembryonic Membranes

• The amnion expands and fuses with the chorion
• The chorionic cavity disappears
• The chorion and amnion form the fetal membranes
• The yolk sac regresses, leaving remnants

Together, these structures support:

• Embryonic growth
• Placental function
• Protection and homeostasis of the developing fetus

Summary

• The amniotic sac provides a protected, fluid-filled environment
• The yolk sac plays essential early roles in nutrition, hematopoiesis, and germ cell development
• The chorion forms the fetal component of the placenta and enables maternal–fetal exchange

These membranes arise early, evolve dynamically, and are indispensable for normal human development.