The skeletal system develops from mesenchyme tissue which originates from the mesoderm. Mesenchyme cells migrate and form cartilage and bone. Bones develop through two types of ossification - membranous and endochondral. The axial skeleton develops from somites which form vertebrae and ribs. Limbs develop from limb buds which are outgrowths that form the bones of the arms and legs. Joints form as cartilage develops between bone ends. Developmental anomalies can occur in bones, vertebrae, ribs and limbs.

1. EMBRYOLOGY OF
SKELETAL SYSTEM
PRESENTER : DR. BIJAY MEHTA

2. Contents
• Introduction
• Formation of Bones and Cartilages
• Development of Axial Skeleton
• Development of Limbs
• Developmental Malformations

3. • The skeletal system develops from
mesenchyme, which is
Mesodermal in Origin.
• Mesenchyme or the embryonic
Connective Tissue migrate to form
• Chondroblasts
• Osteoblasts
• Fibroblasts
Introduction

4. Formation of Bones and Cartilages
• At a site where cartilage is to be formed, mesenchymal cells become
closely packed. This is called a mesenchymal condensation.
• The mesenchymal cells then become rounded and get converted into
cartilage forming cells or chondroblasts.
• Under the influence of chondroblasts, the intercellular substance of
cartilage is laid down.
• Mesenchymal cells surrounding the surface of the developing
cartilage form a fibrous membrane, the perichondrium.

5. Ossification
• Bone develops through two
types of ossifications:
• Membranous Ossification, in
which mesenchymal tissues will
directly convert into bone ,eg
flat bones of the skull.
• Endochondral Ossification, in
which mesenchymal tissues
first give rise to hyaline
cartilaginous model of the
bone and then, the osteoblasts
convert them into the bone,eg
Long bones, Vertebra.

6. • Subdivisions of intraembryonic
mesoderm are paraxial mesoderm,
intermediate mesoderm and lateral plate
mesoderm.
• Paraxial Mesoderm forms a segmented
series of tissue block on either side of
the Neural tube,the Somites.
• These Somites differentiate into:
• dermatome which forms the dermis of the
skin;
• myotome which forms skeletal muscle;
• sclerotome which helps to form the
vertebral column and ribs.
Formation of Axial Skeleton

7. • The vertebral column and
ribs develop from the
sclerotome compartments of
the somites,and the sternum
is derived from mesoderm in
the ventral body wall.
• A definitive vertebra is
formed by condensation of
the caudal half of one
sclerotome and fusion with
the cranial half of the
subjacent sclerotome .
Vertebra

8. • A typical vertebra consists of a
vertebral arch and foramen
(through which the spinal cord
passes), a body, transverse
processes, and usually a
spinous process .
• During the fourth week,
sclerotome cells migrate
medially and surrounds the
notochord .
• The mesenchyme then extends
backward on either side of the
neural tube and surrounds it .

9. • Extensions of this mesenchyme also take place laterally forming
transverse processes, and ventrally in the body wall,forming ribs.
• Mesenchymal cells from the sclerotomes, also migrate cranially to
surround the notochord, where they form the intervertebral disc.
• As development progresses, the notochord degenerates and
disappears .
• Between the vertebrae, the notochord expands to form the
gelatinous centre of the intervertebral disc – the nucleus pulposus.
• This nucleus is later surrounded by circularly arranged fibres that
form the anulus fibrosus.
• The anulus fibrosus and nucleus pulposus together constitute the
intervertebral disc.

10. Ribs and Sternum
• Ribs are formed from the ventral extensions of the sclerotomic
mesenchyme that form the vertebral arches.
• The Sternum is formed from the two sternal bars on either side of the
midline.

11. Formation of Limbs
• The bones of the limbs,
including the bones of the
shoulder and pelvic girdles,
are formed from mesenchyme
of the limb buds.
• With the exception of the
clavicle (which is a membrane
bone), they are all formed by
endochondral ossification.

12. • The limb buds are paddle-shaped outgrowths that arise from
the side wall of the embryo at the beginning of the 2nd
month of intrauterine life .
• Each bud is a mass of mesenchyme covered by ectoderm.
• The mesenchyme of limb buds is derived from the parietal
layer of the lateral plate mesoderm. This mesenchyme gives
rise to bones, connective tissue and some blood vessels. The
muscles of the limbs are derived from myotomes of somites
which migrate into the limbs.

13. • The forelimb buds appear a little
earlier than the hindlimb buds. As
each forelimb bud grows, it
becomes subdivided by
constrictions into arm, forearm and
hand. The hand itself soon shows
outlines of the digits.
• The interdigital areas show cell
death because of which the digits
separate from each other . Similar
changes occur in the hindlimb.
• While the limb buds are growing,
the mesenchymal cells in the buds
form cartilaginous models, which
subsequently ossify to form the
bones of the limb.

15. • The limb buds are at first directed forward and laterally from the body
of the embryo . Each bud has a preaxial (or cranial) border and a
postaxial border. The thumb and great toe are formed on the preaxial
border.
• The forelimb bud is derived from the part of the body wall belonging
to segments C4, C5, C6, C7, C8, T1 and T2. It is, therefore, innervated
by the corresponding spinal nerves.
• The hindlimb bud is formed opposite the segments L2, L3, L4, L5, S1
and S2.

16. Formation of Joints
• The tissues of joints are derived from mesenchyme intervening
between developing bone ends. This mesenchyme may
differentiate into fibrous tissue, forming a fibrous joint
(syndesmosis), or into cartilage forming a cartilaginous joint. In the
case of some cartilaginous joints (synchondrosis or primary
cartilaginous joints), the cartilage connecting the bones is later
ossified, with the result that the two bones become continuous.
• This is seen, typically, at the joints between the diaphyses and
epiphyses of long bones.

17. • At the site where a synovial joint is
to be formed, the mesenchyme is
usually seen in three layers.
• The two outer layers are
continuous with the
perichondrium covering them
cartilaginous ends of the
articulating bones.
• The middle layer becomes loose
and a cavity is formed in it. The
cavity comes to be lined by a
mesothelium that forms the
synovial membrane . The
capsule and other ligaments are
derived from the surrounding
mesenchyme.

18. Anomalies of the Vertebra
• Absent Vertebra
• Parts of Vertebra may be absent
• Spina Bifida
• Hemivertebra
• Sacrococcygeal teratoma
• Congenital Scoliosis
• Fusion of Vertebra : In the cervical region,
Occipitalioztion of Atlas, in the lumbosacral
region,sacralization of the lumbar Vertebra.

19. Anomalies of Sternum and Ribs
• Missing Ribs
• Additional Ribs
• Pigeon Chest
• Funnel Chest

20. Anomalies of the Limbs
• Phocomelia
• Clubfoot(CTEV)
• Syndactyly
• Polydactyly
• Achondroplasia

21. Timetable for some events

22. References:
• Langman’s Medical Embryology, Fourteenth Edition.
• Inderbir Singh’s Human Embryology, Eleventh Edition.
• Various Websites

23. Thank You