Formation of Germ Layers

The endoderm (inner layer), ectoderm (outer layer), and mesoderm (middle layer) are the three primary cell layers that emerge in the early stages of embryonic development (middle layer).

Types of Germ Layers

During embryogenesis, a primary germ layer of a cell called the germinal layer is formed. Three germinal layers are -

  • Ectoderm layer

  • Mesoderm layer

  • Endoderm layer

At the gastrulation stage, these germ layers differentiate into animal embryos, and later, different tissues and organs are formed from these layers. Animals are categorized as diploblastic or triploblastic based on their germ layers. At the tissue level, the diploblastic body plan is the most basic organization. The epidermis is formed by the outer layer ectoderm, while the internal structures are formed by the inner layer endoderm. Mesoglea is a non-cellular gel-like matrix that exists between the ectoderm and the endoderm. A three-layered embryo gives rise to triploblastic species. These species have a third layer, called mesoderm, in addition to ectoderm and endoderm.

In the case of coelomates, acoelomates, and pseudocoelomates, the relationship between germinal layers and the development of the body cavity.

The body of all adult multicellular species has a concentric arrangement of tissues. These tissues are derived from the germinal layers of embryonic cells.

  1. The ectoderm is the outer layer, and the mesoderm is the middle layer.
    The development of the stomach, colon, liver, pancreas, urinary bladder and other essential organs in the body is connected with the endoderm layer.

  2. Mesoderm is responsible for the formation of the body's key structural elements, such as skeletal muscles, the skeleton, the dermis of the skin connective tissue, and so on.

  3. The ectoderm is involved in the development of the central nervous system, the eye lens, ganglia, nerves, and glands.

  4. The coelom is the name for the body cavity lined with mesoderm, and coelomates are species that have a coelom. Annelida, Mollusca, Arthropoda, Echinodermata, Hemichordata, and Chordata are examples of phyla.

In certain species, the body cavity is not lined by mesoderm, but rather by ectoderm and endoderm in the form of scattered pouches in between ectoderm and endoderm. This type of body cavity is known as a pseudocoelom, and animals with it are known as Pseudocoelomates.

Acoelomates are species that lack a body cavity completely. Platyhelminthes, for example.

The germ layers form during the gastrulation process, when the blastula, a hollow ball of cells, begins to differentiate into more specialized cells that layer across the developing embryo. In embryonic growth, the germ layers comprise some of the first lineage-specific (multipotent) stem cells (cells destined to contribute to specific types of tissue, such as muscle or blood). As a result, each germ layer gives rise to specific tissue types in the body.

Since it is the innermost of the three germ layers, it is called endoderm. Many of the internal linings of the body, including the lining of most of the gastrointestinal tract, the lungs, the liver, the pancreas, and other glands that open into the gastrointestinal tract, as well as some other organs, such as the upper urogenital tract and female vagina, are formed by cells derived from the endoderm. The colon, the stomach, the intestines, the lungs, the liver, and the pancreas are all formed from endoderm cells. The ectoderm, on the other hand, ultimately forms the epidermis (outermost skin layer) and hair, as well as other “outer linings” of the body. The mammary glands, as well as the central and peripheral nervous systems, are all born from the ectoderm.

FAQs (Frequently Asked Questions)

1. Write the Special Characteristics/Features of Germ Layers with an Example.

Ans - In a human fetus, there are layers of germs. Germ cell layers characteristics include:

During gastrulation, two layers are formed first: inner endoderm and outer ectoderm. Later on, the mesoderm, a middle layer, is formed. The ectoderm is responsible for the development of hair, skin, nails, and mammary glands, among other things. The endoderm is responsible for the lining of the digestive tract, lungs, thyroid, and other organs. The types of mesoderm are responsible for the formation of skeletal muscle, bones, the urogenital tract, connective tissue, the heart, and other organs. There are only two germ layers in diploblastic animals. There are three germ layers in triploblastic animals.

2. What is the relationship between Germinal Layers and the Formation of the Body Cavity in the Case of Coelomates, Acoelomates, and Pseudocoelomates?

Ans: The body of all adult multicellular species has a concentric arrangement of tissues. These tissues are derived from the germinal layers, which are three embryonic cell layers. (i) The ectoderm is the outermost layer, the mesoderm is the middle layer, and the endoderm is the innermost layer. (ii) The development of the stomach, colon, liver, pancreas, urinary bladder and other essential organs in an organism is associated with the endoderm layer. (iii) Mesoderm is responsible for the formation of the body's key structural elements, such as skeletal muscles, the skeleton, the dermis of the skin connective tissue, and so on. (iv) The ectoderm is involved in the development of the central nervous system, the eye lens, ganglia, nerves, and glands. (v) The mesoderm-lined body cavity is known as coelom, and animals with coelom are known as coelomates. Annelida, Mollusca, Arthropoda, Echinodermata, Hemichordata, and Chordata are examples of phyla. (vi) In certain species, the body cavity is not lined by mesoderm; instead, mesoderm is present in the form of dispersed pouches in between the ectoderm and endoderm. This type of body cavity is known as a pseudocoelom, and animals that have it are known as pseudocoelomates, such as Ascaris. (viii) Acoelomates are species that lack a body cavity completely. e.g., Platyhelminthes.

Q3 - Describe the Criteria for Classification considering the Number of Germinal Layers.

Ans - During embryogenesis, a primary layer of cells called the germinal layer is formed. Three germinal layers are -

  • Ectoderm layer

  • Mesoderm layer

  • Endoderm layer

At the gastrulation stage, these germ layers differentiate into animal embryos, and later, different tissues and organs are formed from these layers. Animals are categorized as diploblastic or triploblastic based on their germ layers. At the tissue level, the diploblastic body plan is the most basic organization. The epidermis is formed by the outer layer ectoderm, while the internal structures are formed by the inner layer endoderm. Mesoglea is a non-cellular gel-like matrix that exists between the ectoderm and the endoderm. A three-layered embryo gives rise to triploblastic species. These species have a third layer, called mesoderm, in addition to ectoderm and endoderm.