Development of Selected Mesodermal Tissues:
Muscle, Bone, Circulatory System
Types of Vertebrate Mesoderm (Figs. #1, #2)
Composed of cells ingressing at Hensen's node.
= temporary cartilage-like rod that plays a role in neural tube formation.
: forms urinary tract and reproductive system.
Lateral plate mesoderm
: differentiates into the heart, blood vessels, blood cells, lining of
: facial connective tissues (bones, cartilage) and muscle of the face.
Paraxial (somitic dorsal) mesoderm
: forms the
(Fig. #3), which are temporary
blocks of cells that produce bones, muscle, cartilage and dermis of the back.
Formation and Differentiation of Somites along Neural Tube
a. Paraxial mesoderm "buds off" groups of cells which form
, epithelial cells that surround a
cavity (Fig. #3.)
1. Somite formation is initiated, in part, by expression of N-cadherin; this converts cells
from "loose" mesenchyme to a structured epithelium.
2. Somites form from anterior to posterior; stage of embryonic development often is
referred to by the number of somites developed.
3. Quail and chick transplantation experiments suggested Notch-expressing cells are
critical for formation of individual somites; Notch role confirmed in transgenic
experiments (Fig. #4.1) and in mice carrying mutation in the Notch receptor
gene, which have defects in rib formation (Fig. #4.2)
b. Somite differentiates into 2 different regions, based on
. (Figs. #2, #5,
These regions of the somite are called the
A. Differentiation of sclerotome cells is induced by
from notochord and ventral floor plate (Fig. #6.)
Sonic hedgehog signal induces
expression in sclerotome cells.
B. Eventually, sclerotome cells migrate out from somite as mesenchyme cells
C. Sclerotome cells migrate toward notochord, and become
, which will
form the axial skeleton (vertebrae, ribs, and associated cartilage and ligaments.)
D. Where vertebrae form, most notochord cells die; some notochord cells survive to
form the elastic