1. Dr.Haythem Ali Alsayigh Department of Human Anatomy and Histology
The Skeletal System
Dr.Haythem Ali Alsayigh
Department of Human Anatomy and Histology
UNIVERSITY OF BABYLON
COLLEGE OF MEDICINE

2. The Skeletal System summary
1-The skeletal system develops from mesenchyme, which is derived from the mesodermal germ layer and from neural crest.
2-Some bones, such as the flat bones of the skull, undergo membranous ossification; that is, mesenchyme cells are directly transformed into osteoblasts
3-In most bones, such as the long bones of the limbs, mesenchyme condenses and forms hyaline cartilage models of bones.
4-Ossification centers appear in these cartilage models, and the bone gradually ossifies by endochondral ossification

3. a membranous portion, which forms the cranial vault,
The skull consists of the neurocranium and viscerocranium (face).
The neurocranium includes
a membranous portion, which forms the cranial vault,
and a cartilaginous portion,the chondrocranium, which forms the base of the skull.
Neural crest cells form the face, most of the cranial vault,
and the prechordal part of the chondrocranium (the part that lies rostral to the notochord).
Paraxial mesoderm forms the remainder of the skull.

4. Limb
Limbs form as buds along the body wall that appear in the fourth week.
Lateral plate mesoderm forms the bones and connective tissue, while muscle cells migrate to the limbs from the somites.
The AER regulates limb outgrowth, and the ZPA controls anteroposterior patterning.
Many of the genes that regulate limb growth and patterning have been defined (Fig. 9.17).

5. vertebra
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 .

6. The many abnormalities of the skeletal system include vertebral (spina bifida), cranial (cranioschisis and craniosynostosis), and facial (cleft palate) defects. Major malformations of the limbs are rare, but defects of the radius and digits are often associated with other abnormalities (syndromes).

7. Musculo system Most muscles arise from the mesoderm.
Skeletal muscles are derived from paraxial mesoderm,
including (1) somites, which give rise to muscles of the axial skeleton, body
wall, and limbs, and

8. Musculo system
(2) somitomeres, which give rise to muscles of the head.
Progenitor cells for muscle tissues are derived from the ventrolateral (VLL) and dorsomedial (DML) edges (lips) of the prospective dermomyotome.

9. Musculo system
Cells from both regions contribute to formation of the myotome.
Some cells from the VLL also migrate across the lateral somitic frontier into the parietal layer of the lateral plate mesoderm. This frontier or border separates two mesodermal domains in the embryo: (1) the primaxial domain that surrounds the neural tube and contains only somitederived cells (paraxial mesoderm) and (2) the abaxial domain that consists of the parietal layer of lateral plate mesoderm in combination with somite-derived cells that migrate across the frontier into this region. Abaxial muscle precursor cells differentiate into infrahyoid, abdominal wall (rectus abdominus, external and

10. internal obliques, transversus abdominus), and limb muscles
internal obliques, transversus abdominus), and limb muscles. Primaxial muscle precursor cells form muscles of the back, some muscles of the shoulder girdle, and intercostal muscles
(Table 10. Muscles of the back (epaxial muscles) are innervated by dorsal primary rami;

11. muscles of the limbs and body wall (hypaxial muscles) are innervated by ventral primary rami. Molecular signals for muscle cell induction arise from tissues adjacent to prospective muscle cells. Thus, signals from lateral plate mesoderm (BMPs) and overlying ectoderm (WNTs) induce VLL cells; while signals from the neural tube and notochord (SHH and WNTs) induce DML cells. Connective tissue derived from somites, parietal mesoderm, and neural crest (head region) provides a template for establishment of muscle patterns.
Most smooth muscles and cardiac muscle fibers are derived from splanchnic mesoderm.
Smooth muscles of the pupil, mammary gland, and sweat glands differentiate from ectoderm.

12. skeletal system develops from:
1. Paraxial mesoderm.
2. Lateral plate mesoderm; somatic layer :
Shoulder bones
Pelvic girdle
Bones of limbs
Sternum
3. Neural crest;
Bones of face
and skull
The skeletal system develops from:
1. The paraxial mesoderm.
2. The somatic layer of lateral plate of mesoderm (as the bones of the shoulder and pelvic girdle, bones of the limbs).
3. The neural crest cells; in the head region they differentiate into mesenchyme and participate in formation of bones of the face and skull

13. The skeletal system develops from:
1. The paraxial mesoderm.
2. The somatic layer of lateral plate of mesoderm (as the bones of the shoulder and pelvic girdle, bones of the limbs).
3. The neural crest cells; in the head region they differentiate into mesenchyme and participate in formation of bones of the face and skull

14. The paraxial mesoderm:
At 3rd week
Paraxial develops into:
Somitomeres: head region
Somites: from occiput to caudal end of trunk.
first pairs of somites ;occipital region at 20th day
proceeds cephalocaudally forming 3 somitic pairs each day
End of 5th week: somitic pair as :
4=occipital =cervical.
12=thoracic =lumber.
5=sacral =coccygeal.
During the 3rd week, the mesoderm on the side of the midline i.e. Paraxial, develops into segmented tissue blocks on the sides of the neural tube, these are called the somitomeres , they develop in the head region and form the head mesenchyme. These blocks develop more to form the somites in the regions from the occiput to the caudal end of the trunk. The pairs of somites occurs first in the occipital region at the 20th day, and then proceeds cephalocaudally forming 3 somitic pairs each day. Finally, somitic pair develops at the end of the 5th week as following:
4=occipital,
8=cervical.
12=thoracic.
5=lumber.
5=sacral.
8-10=coccygeal.
The first occipital and the last 5-7 coccygeal disappear. The other somites form the axial skeleton.

15. During the 3rd week, the mesoderm on the side of the midline i. e
During the 3rd week, the mesoderm on the side of the midline i.e. Paraxial, develops into segmented tissue blocks on the sides of the neural tube, these are called the somitomeres , they develop in the head region and form the head mesenchyme. These blocks develop more to form the somites in the regions from the occiput to the caudal end of the trunk. The pairs of somites occurs first in the occipital region at the 20th day, and then proceeds cephalocaudally forming 3 somitic pairs each day. Finally, somitic pair develops at the end of the 5th week as following:
4=occipital, =cervical =thoracic. 5=lumber.
5=sacral =coccygeal.
The first occipital and the last 5-7 coccygeal disappear. The other somites form the axial skeleton.
During the 4th week, each of the somites has three parts:

16. The paraxial mesoderm:

17. Parts of Somites 4th week Ventromedial part:
(sclerotome) : differentiate to loosely C.T.
Surrounding notochord and spinal cord.
This mesenchymal tissue differentiates later to:
Fibroblast
Chondroblast
Osteoblast
The first occipital and the last 5-7 coccygeal disappear. The other somites form the axial skeleton.
During the 4th week, each of the somites has three parts:
A. Ventromedial part; that is called the sclerotome which differentiate into loosely woven C.T. of the embryonic mesenchyme surrounding the notochord and the spinal cord. This mesenchymal tissue differentiates later forming the fibroblast, chondroblast, and osteoblast of the vertebral column.
vertebral column

18. Parts of Somites Ventrolateral part;
Form muscle of body wall and limbs.
Dorsomedial part;
form back muscles
Cells between these two groups ;
form dermatome which forms dermis and subcutaneous tissues
B. ventrolateral part; that migrate to form the muscle of the body wall and limbs.
C. Dorsomedial part; that forms the muscles of the back.
D. Cells between these two groups ; form the dermatome which forms the dermis and subcutaneous tissues of the skin having segmental nerve supply.
Cells from both muscle precursor groups become mesenchymal and migrate beneath the dermatome to create the dermomyotome.

19. of the skin having segmental nerve supply.
Cells from both muscle precursor groups become mesenchymal and migrate beneath the dermatome to create the dermomyotome.

21. Skull
The skull can be divided into two parts: the neurocranium, which forms a protective case around the brain, and the viscerocranium, which forms the skeleton of the face.

22. Development of the skull
Neurocranium: from 2 portions:
Membranous portion:
flat bones of brain vault
Derived from neural crest and occipital paraxial mesoderm
Bony specules
Enlarged after birth by osteoblastic activity at outer and osteoclastic activity absorbing inner surface
Development of the skull:
1. Development of the brain box (the neurocranium):
It is derived from two portions:
A. Membranous portion; that forms the flat bones of the cranial vault. Derived from the cells of neural crest and the occipital paraxial mesoderm. These flat bones begin as bony specules radiating from a primary ossification center. After birth, these flat bones enlarged by osteoblastic cells adding bones at the outer surface with osteoclastic activity absorbing the bones from the inner surface.

23. Development of the skull
Neurocranium: from 2 portions:
Membranous portion:
At birth; sutures (CT seams)
Derived from NC cells and PM
Fontanelles
Allow for molding
At birth, these flat bones are separated by C.T. called the sutures that are derived from the neural crest cells ( sagittal suture) and paraxial mesoderm (coronal suture). At points where more than two bones meet, sutures are wide and are called fontanelles . This C.T. allow overlapping or called molding of these flat bones to decrease the size of the skull during its passage out of the birth canal. Remodeling of the flat bone occurs after birth resulting in a larger vault in comparison to the face. The connective tissues of the sutures and the fontanelless ossify later on. Before ossification the fontanels indicates the intracranial pressure.

24. Closure of Fontanelles
In humans, the sequence of fontanelle closure is as follows:
Posterior fontanelle closes 1-3 months after birth
Sphenoidal fontanelle close around 6 months
Mastoid fontanelle closes from 6 to 18 months
Anterior fontanelle is the last to close between 1-2 0r 3 years.

25. Development of the skull
Neurocranium: from 2 portions:
Cartilagenous portion: (chondrocranium)
Forms bones of base of skull& floor of cranial cavity.
consists of a number of separate cartilages
prechordal chondrocranium; from NC cells
chordal chondrocranium; from occipital sclerotomes
B. Cartilagenous portion; called the chondrocranium, forms the base of the skull and the floor of the cranial cavity. The cartilaginous neurocranium or chondrocranium of the skull initially consists of a number of separate cartilages. Those that lie in front of the rostral limit of the notochord, which ends at the level of the pituitary gland in the center of the sella turcica, are derived from neural crest cells. They form the prechordal chondrocranium. Those that lie posterior to this limit arise from occipital sclerotomes formed by paraxial mesoderm and form the chordal chondrocranium. The base of the skull is formed when these cartilages fuse and ossify by endochondral ossification.

26. Development of the skull
Viscerocranium:
From bones of face
Mainly from mesoderm of first two pharyngeal arches (from NC cells)
1st phayngeal arch:
Maxillary process:
Maxilla, zygoma and part of temporal bone
Mandibular process:
Meckel cartilage which give rise to the mandible
2nd phayngeal arch:
with dorsal tip of mandibular process gives rise to incus, the malleus, and the stapes
2. Development of the face bones (the viscerocranium):
Mainly they develop from the mesoderm of the first two pharyngeal arches (which is derived from neural crest cells). The first arch gives rise to:
A. Dorsal portion (maxillary process), which gives rise to the maxilla, the zygomatic bone, and part of the temporal bone
B. Ventral portion (mandibular process), contains the Meckel cartilage which give rise to the mandible.
The dorsal tip of the mandibular process, along with that of the second pharyngeal arch, later gives rise to the incus, the malleus, and the stapes. Ossification of the three ossicles begins in the fourth month .
At first, the face is small in comparison with the neurocranium. This appearance is caused by (1) virtual absence of the paranasal air sinuses and (2) the small size of the bones, particularly the jaws. With the appearance of teeth and development of the air sinuses, the face loses its babyish characteristics.

27. Development of the limb bone
4th week
limb buds project from ventrolateral body wall
extensions from somatic layer of LPM covered by ectoderm.
6th week; flat plates of hand and foot develop distally.
Later, constrictions appear between parts of the limb
Development of the limb bone:
The limb buds projects from the ventrolateral body wall in the 4th week, they are formed of extensions from the somatic layer of the lateral plate mesoderm and covered by ectoderm. At the 6th week, the flat plates of the hand and foot develop distally. Later on, constrictions appear between the parts of the limb.

28. Development of the limb bone
AER appearance induces elongation and formation of the limb buds.
Segmental death in AER will form digits
Development of UL precedes LL by about 1-2 days.
The tip of the limb bud shows a thickening of the ectoderm called the apical ectodermal ridge that induces elongation and formation of the limb buds. Segmental death occurs in the AER results in the formation of the digits in the hand and foot. The development of the UL precedes that of the LL by about 1-2 days.

29. Development of the limb bone
7th week, UL rotate 90 degrees laterally, and LL rotates 90 degrees medially.
Hyaline cartilaginous model of long bones forms at 6th week
Ossification occur at 12th week by presence of primary ossification centers which will form the diaphysis
completely ossification of diaphysis occur at birth
epiphysis are still cartilagenous
During the 7th week, the UL rotate 90 degrees laterally, and the LL rotates 90 degrees medially. In the 6th week, the mesenchyme of the limb buds forms the hyaline cartilaginous model of the long bones, which ossify in the 12th week by the presence of primary ossification centers which will form the diaphysis of the shaft of the bones. At birth the diaphysis of the bone is usually completely ossified,but the epiphysis are still cartilagenous.so shortly after birth secondary ossification centers appear at the end sides of the shaft of the bones ( epiphysis). Between the bone of the epiphysis and diaphysis a cartilaginous rim (epiphyseal plate) formed and plays an important role in growth in the length of the bones.

30. Development of the vertebral column
Vertebrae: derived from the sclerotomes of somites
Migrate to surround notochord and spinal cord during 4th week.
sclerotome of each somite undergoes a process called resegmentation
Resegmentation occurs when the caudal half of each sclerotome grows into and fuses with the cephalic half of each subjacent sclerotome
The mid-region form intervertebral disc:
- nucleus pulposus from notochord
- peripheral annulus fibrosis from medial regions of sclerotomes
Development of the vertebral column:
The vertebrae are derived from the sclerotomes of the somites that migrate medially and ventrally to surround the notochord and the spinal cord during the 4th week. With further development the sclerotome of each of the somite undergoes a process called resegmentation. Resegmentation occurs when the caudal half of each sclerotome grows into and fuses with the cephalic half of each subjacent sclerotome. This invasion results in the formation of the cartilaginous body of the vertebrae. The mid-region between the cephalic and caudal parts of each sclerotome forms the intervertebral disc. The notochord forms the Centrum of the vertebral body and the centrally located nucleus pulposus of the IVD. The peripheral annulus fibrosis of the IVD is derived from the medial regions of the sclerotomes

31. Development of the vertebral column

32. Development of the ribs and sternum
Bony portion from the costal processes of thoracic vertebrae (thoracic sclerotomes)
Costal cartilages from sclerotome cells that migrate across the lateral somitic frontier into the adjacent lateral plate mesoderm
sternum :
Two sternal bands formed in parietal LPM on either side of the midline
Fuse to form cartilaginous models of manubrium, sternebrae, and xiphoid process
Development of the ribs and sternum:
The ribs develop as processed from the thoracic vertebrae that are called as the costal processes. The ribs therefore are derived from the thoracic sclerotomes. The sternum is derived from the somatic layer of lateral plate of the mesoderm forming the ventral body wall.

33. Abnormality of the skeletal system
Skull Anomalies :
Cranioschisis
Anencephaly
Meningocele
Meningoencephalocele
Abnormality of the skeletal system:
Anomalies of the skull:
1.cranioschisis; opening in the cranial vault; if it was a large opening it may be associated with degeneration of the brain and is called Anencephaly. Cranioschisis is caused by failure of the cranial neuropore to close. Small opening may be associated with herniation of the brain and is called meningoencephalocele, or there may be herniation of the meninges only forming meningocele.

34. Abnormality of the skeletal system
Skull Anomalies :
Craniosynostosis
Scaphocephaly
brachycephaly
plagiocephaly
Microcephaly
2.craniosynostosis; premature closure of the skull sutures leading to abnormal skull shapes. Involvement of the sagittal suture leads scaphocephaly, involvement of the coronal sutures leads to brachycephaly (towered skull), involvement of both lambdoid and coronal sutures on one sides leads to plagiocephaly.
3.microcephaly; under development of the skull that if sever it may be associated with underdevelopment of the brain and mental retardation.

35. Abnormality of the skeletal system
Limb Anomalies :
Absence of limbs: -meromelia , Amelia
Micromelia
Polydactyly
Ectrodactyly
Syndactyly
Anomalies of the limb:
1. absence of the limbs; either partial meromelia, or complete Amelia (as with use of thalidomide ).
2. micromelia or short limb.
3. polydactyly; extra finger or toes.
4. ectrodactyly; absence of digits.
5. syndactyly;fusion of the fingers or toes.
6. lobester claw; cleft of hand or foot, by the absence of the 3rd metacarpus and finger.
7. club foot; the foot is adducted, planter flexed and the sole of the foot turned medially. Occurs mainly in males and it may be caused by abnormal posture inside the uterus.
8. Congenital absence or deficiency of radius
9.tearing in the amniotic membrane by infection or toxins, the tear amniotic band may encircle any part of the body as the limbs causing constriction as a ring that may lead to amputation.
10.congenital hip dislocation; underdevelopment of the acetabulum and femoral head usually during breech delivery

36. Abnormality of the skeletal system
Limb Anomalies :
Cleft of hand or foot; lobester claw
Club foot
Congenital absence or deficiency of radius
Amputation by amniotic bands.
CDH.

37. Abnormality of the skeletal system
Vertebral anomalies:
Fusion .
Scoliosis .
Abnormal number
Malunion of vertebral arches: Cleft vertebra and spina bifida
Anomalies of the vertebral column:
1.fusion of the vertebrae.
2.missed half vertebrae, leading to lateral curvature of the spine or called scoliosis.
3.abnormal number of the vertebrae as in Klippel- Field syndrome.
4.malunion of the vertebral arches; as in cleft vertebra and spina bifida.

38. The muscular system:

39. Development of smooth muscles
from visceral layer of LPM
SM of the GIT wall and the vascular walls.
Ectoderm: pupillary, breast muscles and sweat glands muscles

40. Development of cardiac muscles
From the visceral layer of LPM
Cardiac myoblasts adhere to one another by the intercalated discs.
Myoblast Not fuse together.
Later special bundle of muscles form the Conductive system

41. Development of skeletal muscles
From somites
Head region: from paraxial mesoderm (somitomeres & somites) and mesoderm of pharyngeal arches.(iris from optic cup ectoderm)
The somites form skeletal muscles in two regions:
Dorsomedial region of somites--- Epiaxial M
Ventrolateral region of somites--- Hypaxial M

42. Development of skeletal muscles
Dorsomedial region of somites:
5th week myotomes form epimeres
dorsal extensor muscles of vertebral coulumn.
supplied segmentally by dorsal rami

43. Development of skeletal muscles
Ventrolateral region of somites:
hypomere
skeletal muscles of limbs and body wall
supplied by ventral ramus.
ventral tip of hypomeres form: rectus abdominis, external, internal, transversus abdominus and infrahyoid muscles.

44. Development of skeletal muscles
7th week, limb muscles differentiate
regulated by connective tissues and bones of the limbs
derived from somites, somatic LPM and NCS
upper limb: opposite lower 5 cervical and upper 2 thoracic somites
lower limb: opposite lower 4 lumber and upper 2 sacral
Nerves: supply muscles , promote development and provide sensory innervation for the dermatomes.
Regardless of their domain, each myotome receives its innervation from spinal nerves derived from the same segment as the muscle cells originate.

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