Paraxial mesoderm, also known as presomitic or somitic mesoderm is the area of
mesoderm
The mesoderm is the middle layer of the three germ layers that develops during gastrulation in the very early development of the embryo of most animals. The outer layer is the ectoderm, and the inner layer is the endoderm.Langman's Medical Emb ...
in the
neurulating embryo
An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...
that flanks and forms simultaneously with the
neural tube
In the developing chordate (including vertebrates), the neural tube is the embryonic precursor to the central nervous system, which is made up of the brain and spinal cord. The neural groove gradually deepens as the neural fold become elevated, ...
. The cells of this region give rise to
somite
The somites (outdated term: primitive segments) are a set of bilaterally paired blocks of paraxial mesoderm that form in the embryonic stage of somitogenesis, along the head-to-tail axis in segmented animals. In vertebrates, somites subdivide ...
s, blocks of
tissue running along both sides of the neural tube, which form
muscle
Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of mus ...
and the tissues of the back, including connective tissue and the
dermis
The dermis or corium is a layer of skin between the epidermis (with which it makes up the cutis) and subcutaneous tissues, that primarily consists of dense irregular connective tissue and cushions the body from stress and strain. It is divided ...
.
Formation and somitogenesis
The paraxial and other regions of the mesoderm are thought to be specified by
bone morphogenetic protein
Bone morphogenetic proteins (BMPs) are a group of growth factors also known as cytokines and as metabologens. Originally discovered by their ability to induce the formation of bone and cartilage, BMPs are now considered to constitute a group of ...
s, or BMPs, along an axis spanning from the center to the sides of the body. Members of the
FGF family also play an important role, as does the
WNT pathway. In particular,
Noggin, a downstream target of the Wnt pathway, antagonizes BMP signaling, forming boundaries where antagonists meet and limiting this signaling to a particular region of the mesoderm. Together, these pathways provide the initial specification of the paraxial mesoderm and maintain this identity.
This specification process has now been fully recapitulated ''in vitro'' with the formation of paraxial mesoderm progenitors from
pluripotent stem cells Pluripotency: These are the cells that can generate into any of the three Germ layers which imply Endodermal, Mesodermal, and Ectodermal cells except tissues like the placenta.
According to Latin terms, Pluripotentia means the ability for many thin ...
, using a
directed differentiation approach.
[ ]
The tissue undergoes convergent extension as the
primitive streak
The primitive streak is a structure that forms in the early embryo in amniotes. In amphibians the equivalent structure is the blastopore. During early embryonic development, the embryonic disc becomes oval shaped, and then pear-shaped with the br ...
regresses, or as the embryo
gastrulates. The
notochord
In anatomy, the notochord is a flexible rod which is similar in structure to the stiffer cartilage. If a species has a notochord at any stage of its life cycle (along with 4 other features), it is, by definition, a chordate. The notochord consi ...
extends from the base of the head to the tail; with it extend thick bands of paraxial mesoderm.
As the primitive streak continues to regress, somites form from the paraxial mesoderm by "budding off" rostrally.
In certain model systems, it has been shown that the daughter cells of stem cell-like progenitor cells which come from the primitive streak or site of gastrulation migrate out and localize in the posterior paraxial mesoderm. As the primitive streak regresses and somites bud off anteriorly, new cells derived from these stem-cell like precursors constantly enter the posterior end of the paraxial mesoderm.
Derived tissues
Many kinds of tissue derive from the segmented paraxial mesoderm by means of the somite. Among these are:
*the
sclerotome
The somites (outdated term: primitive segments) are a set of bilaterally paired blocks of paraxial mesoderm that form in the embryonic stage of somitogenesis, along the head-to-tail axis in segmented animals. In vertebrates, somites subdivide ...
, which forms cartilage,
*the syndetome, which forms tendons,
*the
myotome, which forms skeletal muscle,
*the
dermatome, which forms the dermis as well as skeletal muscle,
*and
endothelial
The endothelium is a single layer of squamous endothelial cells that line the interior surface of blood vessels and lymphatic vessels. The endothelium forms an interface between circulating blood or lymph in the lumen and the rest of the ve ...
cells.
Head mesoderm
A particular kind of tissue deriving from the paraxial mesoderm is the head mesoderm. This tissue derives from the unsegmented paraxial mesoderm and prechordal mesoderm. Tissues derived from the head mesoderm include connective tissue and the muscles of the face.
The head mesoderm forms through a separate signaling circuit than the segmented paraxial mesoderm, though also involving BMP and
fibroblast growth factor (FGF) signaling. Here,
retinoic acid interacts with these pathways.
See also
*
Somitomere
*
Chordamesoderm
Axial mesoderm, or chordamesoderm, is the mesoderm in the embryo that lies along the central axis under the neural tube.
* will give rise to notochord
* starts as the notochordal process, whose formation finishes at day 20 in humans.
* important ...
*
Intermediate mesoderm
Intermediate mesoderm or intermediate mesenchyme is a narrow section of the mesoderm (one of the three primary germ layers) located between the paraxial mesoderm and the lateral plate of the developing embryo. The intermediate mesoderm develop ...
*
Lateral plate mesoderm
*
Mesenchyme
Mesenchyme () is a type of loosely organized animal embryonic connective tissue of undifferentiated cells that give rise to most tissues, such as skin, blood or bone. The interactions between mesenchyme and epithelium help to form nearly every ...
*
Triploblasty
Triploblasty is a condition of the gastrula in which there are three primary germ layers: the ectoderm, mesoderm, and endoderm. Germ cells are set aside in the embryo at the blastula stage, which are incorporated into the gonads during organo ...
*
List of human cell types derived from the germ layers
This is a list of cells in humans derived from the three embryonic germ layers – ectoderm, mesoderm, and endoderm.
Cells derived from ectoderm
Surface ectoderm Skin
* Trichocyte
* Keratinocyte
Anterior pituitary
* Gonadotrope
* Corti ...
References
External links
Overview at nih.govDiagram at lww.com
{{Authority control
Embryology
Mesoderm