Main Difference – Trophoblast vs Inner Cell Mass
An embryo is the early stage of development of animals. It travels from ampulla, oviduct to the uterus for implantation. The embryo at the 16 to 32 cells stage is called the morula, and it develops into the blastocyst after 5 days of fertilization. A blastocyst may contain 70-100 cells that are differentiated into trophoblast and the inner cell mass (ICM). A fluid-filled cavity is also formed inside the blastocyst. Therefore, trophoblast and inner cell mass are two types of cells in the blastocyst. The main difference between trophoblast and inner cell mass is that trophoblast is the tight, adherent layer of cells that form the outer layer of the blastocyst whereas inner cell mass is the internally located, rounded cells.
Key Areas Covered
1. What is Trophoblast
– Definition, Facts, Role
2. What is Inner Cell Mass
– Definition, Facts, Role
3. What are the Similarities Between Trophoblast and Inner Cell Mass
– Outline of Common Features
4. What is the Difference Between Trophoblast and Inner Cell Mass
– Comparison of Key Differences
Key Terms: Bilaminar Embryonic Disc, Embryoblast, Embryonic Stem Cells, Chorion, Chrionic Villi, Inner Cells Mass, Pluriblast, Trilaminar Embryonic Disc, Trophoblast
What is Trophoblast
Trophoblast refers to the layer of cells on the outside of the mammalian blastula. The major function of the trophoblast is the nourishment of the embryo by forming the major part of the placenta. It gives rise to the extra embryonic structure called chorion. The chorion is the fetal part of the placenta. Prior to implantation, trophoblast is protected by the zona pellucida. Zona pellucida prevents the implantation of the embryo in the oviduct. The release of zona pellucida or zona hatching in the uterus facilitates the implantation as the exposed trophoblast layer is sticky. It occurs at the sixth day after fertilization in humans. The adherence of the trophoblast initiates the implantation of the embryo to the endometrium. The implantation starts at the seventh day after fertilization. The structure of the blastocyst is shown in figure 1.
Figure 1: Blastocyst
Approximately at the sixth day after fertilization, the differentiation of cells in the trophoblast produces two cell layers: syncytiotrophoblast (ST) and cytotrophoblast (CT). The syncytial trophoblast secretes proteolytic enzymes, allowing the blastocyst to penetrate into the endometrium. The cells of the cellular trophoblast develop chorionic villi. The primary chorionic villi are shown in figure 2.
Figure 2: Primary Chorionic Villi
The spaces created by the digestion of the endometrium are called lacunae. Mother’s blood flows into lacunae and washes the chorionic villi. The chorion is made up of chorionic villi. It is the fetal part of the placenta.
What is Inner Cell Mass
Inner cell mass (ICM) refers to the embryonic pole of the blastocyst. The ICM forms the body of the embryo. It also forms some of the extra-embryonic tissues such as amnion and yolk sac. The cells of the ICM are pluripotent, giving rise to all cell lineages in the body. On that account, ICM is used as a source of embryonic stem cells. The ICM is also called the pluriblast and embryoblast. The structure of the embryo is shown in figure 3.
Figure 3: Embryo
The formation of the bilaminar embryonic disc or bilaminar blastoderm form two distinct cell lineages in the ICM. They are epiblast and hypoblast. The epiblast is the dorsal epithelial layer while the hypoblast is the ventral epithelial layer. Gastrulation is the subsequent development stage of the ICM in which a trilaminar embryonic disc is formed by the waves of migrating cells. The trilaminar embryonic disc is composed of endoderm, mesoderm, and ectoderm.
Similarities Between Trophoblast and Inner Cells Mass
- Both trophoblast and inner cell mass are types of cells in the blastocyst.
- Both trophoblast and inner cell mass are developed prior to implantation.
- Different structures of the embryo are developed from both trophoblast and inner cell mass.
Difference Between Trophoblast and Inner Cell Mass
Definition
Trophoblast: Trophoblast refers to the layer of cells on the outside of the mammalian blastula.
Inner Cell Mass: Inner cell mass refers to the embryonic pole of the blastocyst.
Significance
Trophoblast: Trophoblast is the outer layer of the blastocyst.
Inner Cell Mass: Inner cell mass is the innermost cells of the blastocyst.
Develop
Trophoblast: Trophoblast develops the chorion.
Inner Cell Mass: Inner cell mass develops the embryo.
Structure
Trophoblast: Trophoblast consists of tight cells.
Inner Cell Mass: Inner cell mass consists of rounded cells.
Function
Trophoblast: The major function of the trophoblast is the nourishment of the embryo by forming the major part of the placenta.
Inner Cell Mass: The inner cell mass forms the body of the embryo.
Conclusion
Trophoblast and inner cell mass are the two types of cells in the blastocyst. The differentiation of both cell types occurs prior to implantation. Trophoblast is the outermost cell layer of the blastocyst while inner cell mass is the innermost cell mass. Trophoblast gives rise to the chorionic villi that supply nutrients to the developing embryo. The inner cell mass eventually gives rise to the tissues of the embryo and some of the extra-embryonic tissues. The main difference between trophoblast and inner cell mass is their role in the developing embryo.
Reference:
1. “10.1 Early development and implantation.” The trophoblast, Available here.
2. “Inner Cell Mass – Development and Stem Cells.” LifeMap Discovery®, Available here.
Image Courtesy:
1. “Blastocyst English”By Seans Potato Business – derived from Blastocyst.png (CC BY-SA 3.0) via Commons Wikimedia
2. “Gray36″By Henry Vandyke Carter – Henry Gray (1918) Anatomy of the Human Body, Bartleby.com: Gray’s Anatomy, Plate 36 (Public Domain) via Available here.
3. “Gray21” By Henry Vandyke Carter – Henry Gray (1918) Anatomy of the Human Body, Bartleby.com: Gray’s Anatomy, Plate 21 (Public Domain) via Available here
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