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Learning Objectives
At the end of the lecture, the student should be able to know:
• Development and differentiation of Intraembryonic Mesoderm.
• Division of Paraxial Mesoderm.
• Different parts of Transverse Process of vertebra.
• Development of Ribs.
• Development of Sternum.
• Congenital anomalies.
The Human Skeletal System
The axial skeleton is composed of the spine, rib cage, and
Vertebral column
• Vertebrae, sacrum, coccyx
– 7 cervical vertebrae
– 12 thoracic vertebrae
– 5 lumbar vertebrae
– Sacrum and coccyx are fused
Development of Mesoderm
During the third week of development, embryo has three germ layers.
Intermediate germ layer is called Paraxial Mesoderm which is
divided into:
Sclerotome -- Cartilages & Bones
Myotome – Muscles
Dermatome – Skin
Cells migrate through the primitive streak to form mesodermal layer.
Extraembryonic mesoderm lies adjacent to the trilaminar embryo.
Development of Paraxial Mesoderm
Paraxial mesoderm:
Accumulates under neural plate with thinner mesoderm laterally.
This forms 2 thickened streaks running the length of the embryonic
disc along the rostrocaudal axis.
In humans, during the 3rd week, this mesoderm begins to segment.
Neural plate folds to form a neural groove.
Development of Somites
Segmentation of the paraxial mesoderm into somites.
First somite is developed in cervical region at 14 day of
development and continuous downward
• These somite surround the notochord.
• Divide into three elements these are Sceleratom, Myotome &
• Differentiate to form the bones, cartilages & ligaments.
• Sclerotomes which surround the notochord develops the vertebrae.
• The vertebrae has a body & some processes. One pair of
processes is called Transverse processes, which has two
• Costal element
• Transverse element
Elements of Transverse Process
• Transverse element will develop into transverse process proper.
• While costal element in thoracic region form the ribs.
• While in cervical, lumbar and sacral region the transverse elements
form the rudimentary structures
Different Parts Paraxial
• Scelerotome
• Dermatome
• Myotome
Differentiation of Paraxial Mesoderm
• Scelerotome forms the vertebral column
• Dermatome forms the overlying skin
• Myotome forms the muscles.
Thoracic Cage
• Ribs: twelve pair of ribs attached to each thoracic vertebrae.
• Seven pairs: true ribs and attach to the sternum by costal
• Two pairs: false ribs that attach to cartilage.
• Two pairs: floating ribs that do not attach to the sternum or its
• Sternum: the manubrium, the body, and the xyphoid process.
Development of Ribs
Formed from the ventral or costal processes of the primitive vertebra,
Processes extending between the muscle-plates.
In thoracic region of vertebral column, the costal processes grow
lateral ward to form a series of arches, the primitive costal arches
Transverse process grows out behind the vertebral end of each arch.
It is at first connected to the costal process by continuous mesoderm,
but this becomes differentiated later to form the costotransverse
between the costal process and the tip of the transverse process the
costotransverse joint is formed.
Self Assessment:
• Identify which one are
Cervical Vertebrae.
– Thoracic Vertebrae.
– Lumber Vertebrae.
 Which part of the body is shown in this figure?
 Identify Sacrum and Coccygeal vertebrae?
• Name the groove ?
• Name the different parts of Vertebrae?
• What does sclerotome form?
 Classify the Ribs?
 Name the parts of Sternum?
• Name the parts of a Rib?
• Which structure of the thoracic cage is shown in this
figure? Identify its parts.
• The congenital abnormality located above the normal first rib