Placenta Formation (Extra Embryonic Mesoderm, Yolk Sac)

Placenta formation is an important topic in anatomy, as it is a complex process that involves the development and function of the placenta during pregnancy. The placenta is a vital organ that connects the developing fetus to the mother's uterine wall, allowing for the exchange of nutrients, oxygen, and waste products. The placenta also plays a crucial role in hormone production, immune protection, and fetal development.

In the NEET PG Exam, questions related to placenta formation may be asked to test the student's understanding of the anatomy and physiology of the placenta, as well as its role in fetal development and pregnancy. A thorough understanding of placenta formation is essential for medical students. 

Read this blog further to get a quick overview of this important anatomy topic for NEET PG/NExT exam preparation.

• EMBRYOBLAST CONSISTS OF
  • HYPOBLAST → dorsal, degenerate & contributes to extra embryonic structures
  • EPIBLAST → ventral columnar cells
  • DORSAL AMNIOTIC CAVITY
  • VENTRAL YOLK SAC CAVITY

Extra Embryonic Mesoderm

• EXTRA EMBRYONIC MESODERM splits to form EXTRA EMBRYONIC coelomic CAVITY which is connected by CONNECTING STALK

Amnion

• Covers amniotic cavity
• Contributed by
• Amniogenic cells
• Somatopleuric layer of EEM

Chorion

• CONTRIBUTED BY
• Cytotrophoblast
  • Syncytiotrophoblast
  • somatopleuric layer of EEM
• Trophoblast divides to form
• form placental membrane
  • cytotrophoblast
  • syncytiotrophoblast

Extra Embryonic Coelomic Cavity

• It divides the extra embryonic mesoderm into two parts.
• One towards the yolk sac is called as visceral or splanchnopleuric extra – embryonic mesoderm and beyond that lines the amniotic cavity and outside is called the parietal or somatopleuric extraembryonic mesoderm (towards body wall)
• Between somatopleuric extraembryonic and visceral/ splanchnic mesoderm is the extra embryonic coelomic cavity.
• Chorion formed by trophoblast with a somatopleuric layer of extraembryonic mesoderm.
• Amnion formed by amniogenic cells lining the amniotic cavity and somatopleuric layer of extraembryonic mesoderm.
• Connecting stalks later become umbilical cord components.

YOLK SAC

• 1° YOLK SAC →  lined by flattened Endodermal cells
• 2° YOLK SAC →   lined by cuboidal Endodermal cells 
• 3°YOLK SAC → formed during cephalo caudal development of Embryo →forms gut tube → Part of the yolk sac remaining outside the Embryo - 3 YOLK SAC
• In the beginning, we had a primary yolk sac. Later it will become secondary yolk sac and then tertiary yolk sac. Tertiary yolk sac communicates with midgut as vitello – intestinal duct.
• Extra – Embryonic coelomic cavity forming around the baby, there is cephalo- caudal folding of baby. Head comes towards the tail and in this process an amniotic cavity surrounds the body of the baby all around.
• Allantois and the Vitello intestinal duct [midgut diverticulum] will enter the umbilical cord to become its contents.
• PLACENTAL COMPONENTS
  • Placenta formed by fetal & maternal contributions
  • Placenta formation: Decidua basalis from the maternal side and from the fetal side is chorion frondosum. Chorion layer develops some villi called chorionic villi and they will penetrate into decidua basalis of the maternal side.
  • Maternal placenta is decidua basalis [endometrium of uterus] and chorionic villi from fetal placenta component.

UTERUS

• MATERNAL PLACENTA
• DECIDUA BASALIS [DB]  →  The endometrium where the embryo implants
   →forms the maternal /uterine placenta
• DECIDUA CAPSULARIS  →  Surrounds the embryo on luminal side 
  DONOT FORM PLACENTA
• DECIDUA PARIETALIS   →The rest of the gravid endometrium

• FETAL PLACENTA
  • Derived from chorion
  • CHORION FRONDOSUM - chorion towards DB forms layer like projections into it
  • CHORION LAEVE -  chorion on the side of D. capsularis, DO NOT FORM PLACENTA
  • Uterine cavity = Space b/w decidua basalis & Parietalis 
• Chorionic cavity = Chorion & Amnion  

CHORIONIC VILLI

PRIMARY VILLUS [Day 12]	Core of cytotrophoblast cells, covered by syncytio TB
SECONDARY VILLUS [Day 13-15]	Cytotrophoblast layer invaded by extra Embryonic mesoderm
TERTIARY VILLUS [Day 17-21]	Fetal blood vessels invades the mesoderm

Learn more about this important anatomy topic with the help of our detailed video lectures on the PrepLadder app. You will also get access to comprehensive study notes and high-yield MCQs for practice.