A trilaminar embryo is an early stage in the development of triploblastic
organisms, which include humans and many other animals.
It is an embryo, which exists as three different germ layers - the ectoderm, the
mesoderm and the endoderm. These layers are arranged on top of each other
like a stack of paper, giving rise to the name trilaminar, or "three-layered".
These three layers arise early in the third week (after gastrulation) from the
epiblast (a portion of the mammalian inner cell mass).
Gastrulation is a phase early in the embryonic development of most animals,
during which the single-layered blastula is reorganized into a trilaminar ("three-
layered") structure known as the gastrula. These three germ layers are known as
the ectoderm, mesoderm, and endoderm.
Gastrulation takes place after cleavage and the formation of the blastula and
primitive streak. Gastrulation is followed by organogenesis, when individual
organs develop within the newly formed germ layers. Each layer gives rise to
specific tissues and organs in the developing embryo.
Ectoderm is one of the three primary germ cell layers in the very early embryo.
The other two layers are the mesoderm (middle layer) and endoderm (most
proximal layer), with the ectoderm as the most exterior (or distal) layer. It
emerges and originates from the outer layer of germ cells.
In vertebrates, the ectoderm has three parts: external ectoderm (also known as
surface ectoderm), the neural crest, and neural tube. The latter two are known as
At the start of this process, the developing embryo has divided into many cells
separating the embryo, which is now a hollow sphere of cells called the blastula,
into two parts, the animal hemisphere and vegetal hemisphere. It is the animal
hemisphere of the blastula that will eventually become the ectoderm. Endoderm:
Endoderm is one of the three primary germ cell layers in the very early embryo.
The other two layers are the ectoderm (outside layer) and mesoderm (middle
layer), with the endoderm as the innermost layer. Cells migrating inward along
the archenteron form the inner layer of the gastrula, which develops into the
The endoderm consists at first of flattened cells, which subsequently become
columnar. It forms the epithelial lining of multiple systems.
In all bilaterian animals, the mesoderm is one of the three primary germ layers in
the very early embryo. The other two layers are the ectoderm (outside layer) and
endoderm (inside layer), with the mesoderm as the middle layer between them.
The mesoderm forms mesenchyme (connective tissue), mesothelium, non-
epithelial blood cells and coelomocytes.
Three Components of Mesoderm:
There are three important components, the paraxial mesoderm, the intermediate
mesoderm and the lateral plate mesoderm. The paraxial mesoderm forms the
somitomeres, which give rise to mesenchyme of the head and organize into
somites in occipital and caudal segments. Somites give rise to the myotome
(muscle tissue), sclerotome (cartilage and bone), and dermatome (subcutaneous
tissue of the skin).
The intermediate mesoderm connects the paraxial mesoderm with the lateral
plate, eventually it differentiates into urogenital structures consisting of the
kidneys, gonads, their associated ducts, and the adrenal glands.
The lateral plate mesoderm give rise to the heart, blood vessels and blood cells of
the circulatory system as well as to the mesodermal component of the limbs.
A somite is a division of the body of an animal. In vertebrates, somites are
bilaterally paired blocks of mesoderm that form along the anterior-posterior axis
of the developing embryo in segmented animals. In vertebrates, somites give rise
to skeletal muscle, cartilage, tendons, endothelial cells, and dermis.
The mesoderm forms at the same time as ectoderm and endoderm. The
mesoderm that is lateral to (at the side of) the neural t