5. Foetal Membrane & Placenta (HB35)

Embryology Overview

  • Title: Foetal Membranes and Placenta

  • Professor: Dr. Mohamad Aris Mohd Moklas

  • Department: Human Anatomy, FPSK, UPM

  • Email: aris@upm.edu.my

Foetal Membranes

  • Definition:

    • Any tissue or structure developed from the zygote; does not form part of the embryo except for parts of the yolk sac and allantois.

Functions of Foetal Membranes

  • Purpose:

    • Protection

    • Provide nutrition, respiration, and excretion.

Components of Foetal Membranes

  1. Yolk sac

  2. Amnion

  3. Chorion

  4. Allantois

  5. Umbilical cord

  6. Foetal part of placenta (chorion frondosum)

Yolk Sac

Primary Yolk Sac

  • Formation:

    • Day 8: Cells from the hypoblast begin to migrate over the cytotrophoblast.

    • The layer is Heuser’s membrane; the cavity is the primary yolk sac.

Secondary Yolk Sac

  • Formation:

    • Day 12: The primary yolk sac decreases, and a smaller secondary yolk sac forms from new hypoblast cells.

Significance of Yolk Sac

  1. Nutrient transfer to the embryo (weeks 2 & 3).

  2. Blood development starts in the yolk sac wall (week 3), transitioning to the liver (week 6).

  3. Primitive gut incorporation into the embryo (week 4).

  4. Migration of primordial germ cells to gonads (week 3).

Fate of the Yolk Sac

  • Week 10:

    • Atrophies and becomes small (vitelline duct).

  • Detachment:

    • Usually detaches from the midgut loop by week 6.

Anomalies Related to Yolk Sac

  1. Meckel's diverticulum:

  • Remnant of the vitelline duct.

  1. Umbilical fistula:

  • Rare instance due to a patent vitelline duct.

Amnion

  • Description:

    • Fluid-filled, membranous sac surrounding the embryo; fused with decidua parietalis and chorion laeve.

Development of Amnion

  • Initial Formation:

    • Some fluid secreted by amniotic cells; most fluids derived from maternal tissue by diffusion.

  • Growth:

    • The amniotic cavity appears on Day 8 and expands by week 8.

Functions of Amnion

  1. Acts as a protective cushion against shocks.

  2. Permits symmetrical growth of the foetus.

  3. Provides a barrier to infection.

  4. Equalizes pressure within the cavity.

  5. Prevents adherence of the membranes to the embryo.

  6. Regulates embryo body temperature.

  7. Aids in fetal movement and muscular development.

  8. Dilation of the cervix before delivery.

Amniotic Fluid Volume

  • Normal Amount:

    • About 1000 ml.

  • Conditions:

    • Oligohydramnios: <400 ml; Polyhydramnios: >1500-2000 ml.

Chorion

  • Description:

    • Outer membrane that contains the amnion and supports the placenta. Its components include the chorion frondosum (foetal part) and chorion laeve (non-vascular).

Allantois

  • Description:

    • Small diverticulum from the yolk sac, appearing on Day 16.

Importance of Allantois

  1. Blood formation occurs in its wall (weeks 3-5).

  2. Forms umbilical vessels.

  3. Fluid diffusion from amniotic cavity to foetal circulation.

  4. Forms urachus during tail folding, which regresses and forms the median umbilical ligament.

Anomalies of Allantois

  1. Urachal cyst:

  • Persistence of the central part.

  1. Urachal fistula:

  • Open connection remaining between the allantois and bladder.

  1. Urachal sinus:

  • Part closes but the distal part remains.

Umbilical Cord

  • Description:

    • Attaches near the center of the placenta; typically 1-2 cm in diameter and 30-90 cm in length.

  • Structure:

    • Contains two arteries and one vein surrounded by Wharton’s jelly.

  • Development:

    • Forms through primitive umbilical ring; includes connection to yolk sac and vessels within the connecting stalk.

Common Issues with Umbilical Cord

  • Twisting and bending of vessels can lead to false knots, and in rare cases, true knots that can pose risks to the fetus.

Development of the Placenta

  1. Definition:

  • The placenta is a fetomaternal organ facilitating nutrient and gas exchange.

  1. Structure:

  • Discoid, 15-20 cm in diameter, 2-3 cm thick, weighs 500-600 grams.

  1. Components:

  • Large foetal portion (chorion frondosum) and small maternal portion (decidua basalis).

Placental Development Stages

  • After implantation, syncytiotrophoblast invades lacunar spaces leading to utero-placental circulation.

  • Primary to tertiary villi development and formation of cytotrophoblastic shell.

Placental Functions

  1. Exchange of metabolic and gaseous products.

  2. Production of hormones (e.g., HCG, progesterone).

  3. Transmission of maternal antibodies for passive immunity.

  4. Anchoring of the fetus via the umbilical cord.

  5. Maternal heat transfer.

Anomalies of Placenta

  • Variation involves attachment, implantation site, and degree of adhesion (e.g., accreta, increta).

  • Types of Placenta:

    1. Battledore placenta: cord attached to the margin.

    2. Velamentous insertion: cord does not reach the placenta.

Foetal Membranes in Twins

  1. Types of Twins:

  • Dizygotic (fraternal): 2 zygotes.

  • Monozygotic (identical): single zygote splits.

  1. Dizygotic Twins:

  • Each has its own placenta and membranes, may fuse.

  1. Monozygotic Twins:

  • May share a common membrane structure, varying by timing of zygote splitting (two-cell, early blastocyst, or bilaminar germ disc).

Conjoined Twins

  • Caused by incomplete separation during development, classified as thoracopagus, pygopagus, and craniopagus based on fusion areas.