EMBRYOLOGY/MEDICAL GENETICS
SUBDIVISIONS OF INTRAEMBRYONIC MESODERM
Intra-embryonic mesoderm is formed by proliferation of cells in the primitive streak separating
the ectoderm and the endoderm, except in the following regions:
(a) Prochordal plate
(b) Cloacal membrane, and
(c) The part occupied by notochord in the midline caudal to the prochordal plate.
Cranial to the prochordal plate, the mesoderm of the two sides meets in the midline. At the edges
of the embryonic disc, the intraembryonic mesoderm is continuous with the extra-embryonic
mesoderm.
The intraembryonic mesoderm now becomes subdivided into three parts:
(A) Paraxial mesoderm: The mesoderm on either side of the notochord, becomes thick
and is called the paraxial mesoderm.
(B) Lateral plate mesoderm: The mesoderm forms a thinner more lateral layer called the
lateral plate mesoderm.
(C) The intermediate mesoderm: Lying between these two, are a longitudinal strip called
the intermediate mesoderm.
, DEVELOPMENT AND FATE OF SOMITES
The paraxial mesoderm now becomes segmented into cubical masses called somitomeres which
give rise to somites (also called metameres or primitive segments). The first somites are seen on
either side of the midline, a little behind the prochordal plate. More somites are formed caudally,
on either side of the developing neural tube.
Fate of Somites
Paraxial mesoderm becomes segmented to form a number of somites that lie on either side of the
developing neural tube. A crosssection through a somite shows that it is a triangular structure and
has a cavity. The somite is divisible into three parts:
Sclerotome (ventromedial part): The cells of the sclerotome migrate medially. They
surround the neural tube and give rise to the vertebral column and ribs.
Dermatome: (lateral part): The cells of this part also migrate and come to line the deep
surface of the ectoderm covering the entire body. These cells give rise to the dermis of
the skin and to subcutaneous tissue.
Myotome (intermediate part): It gives rise to striated muscles.
FORMATION OF INTRA-EMBRYONIC COELOM
While the paraxial mesoderm is undergoing segmentation, to form the somites, changes are also
occurring in the lateral plate mesoderm.
• Small cavities appear in the lateral plate mesoderm.
• These coalesce (come together} to form one large cavity, called the intraembryonic
coelom.
The cavity has the shape of a horseshoe. There are two halves of the cavity (one on either side of
the midline} which are joined together cranial to the prochordal plate.
• At first, this is a closed cavity but soon it comes to communicate with the extra-
embryonic coelom}.
With the formation of the intraembryonic coelom, the lateral plate mesoderm now splits into:
1. Somatopleuric or parietal, intra-embryonic mesoderm that is in contact with ectoderm.
2. Splanchnopleuric, or visceral, intra-embryonic mesoderm that is in contact with endoderm.
SUBDIVISIONS OF INTRAEMBRYONIC MESODERM
Intra-embryonic mesoderm is formed by proliferation of cells in the primitive streak separating
the ectoderm and the endoderm, except in the following regions:
(a) Prochordal plate
(b) Cloacal membrane, and
(c) The part occupied by notochord in the midline caudal to the prochordal plate.
Cranial to the prochordal plate, the mesoderm of the two sides meets in the midline. At the edges
of the embryonic disc, the intraembryonic mesoderm is continuous with the extra-embryonic
mesoderm.
The intraembryonic mesoderm now becomes subdivided into three parts:
(A) Paraxial mesoderm: The mesoderm on either side of the notochord, becomes thick
and is called the paraxial mesoderm.
(B) Lateral plate mesoderm: The mesoderm forms a thinner more lateral layer called the
lateral plate mesoderm.
(C) The intermediate mesoderm: Lying between these two, are a longitudinal strip called
the intermediate mesoderm.
, DEVELOPMENT AND FATE OF SOMITES
The paraxial mesoderm now becomes segmented into cubical masses called somitomeres which
give rise to somites (also called metameres or primitive segments). The first somites are seen on
either side of the midline, a little behind the prochordal plate. More somites are formed caudally,
on either side of the developing neural tube.
Fate of Somites
Paraxial mesoderm becomes segmented to form a number of somites that lie on either side of the
developing neural tube. A crosssection through a somite shows that it is a triangular structure and
has a cavity. The somite is divisible into three parts:
Sclerotome (ventromedial part): The cells of the sclerotome migrate medially. They
surround the neural tube and give rise to the vertebral column and ribs.
Dermatome: (lateral part): The cells of this part also migrate and come to line the deep
surface of the ectoderm covering the entire body. These cells give rise to the dermis of
the skin and to subcutaneous tissue.
Myotome (intermediate part): It gives rise to striated muscles.
FORMATION OF INTRA-EMBRYONIC COELOM
While the paraxial mesoderm is undergoing segmentation, to form the somites, changes are also
occurring in the lateral plate mesoderm.
• Small cavities appear in the lateral plate mesoderm.
• These coalesce (come together} to form one large cavity, called the intraembryonic
coelom.
The cavity has the shape of a horseshoe. There are two halves of the cavity (one on either side of
the midline} which are joined together cranial to the prochordal plate.
• At first, this is a closed cavity but soon it comes to communicate with the extra-
embryonic coelom}.
With the formation of the intraembryonic coelom, the lateral plate mesoderm now splits into:
1. Somatopleuric or parietal, intra-embryonic mesoderm that is in contact with ectoderm.
2. Splanchnopleuric, or visceral, intra-embryonic mesoderm that is in contact with endoderm.
