Skeletal System. Prof. Dr. Malak A. Al-yawer Department of Anatomy/Embryology Section

Transcription

1 Skeletal System Prof. Dr. Malak A. Al-yawer Department of Anatomy/Embryology Section

2 Learning objectives At the end of this lecture, the medical student will be able to: State the embryonic origin of skeletal system Define Mesenchyme ( embryonic connective tissue) Distinguish between the two types of ossification Describe the embryonic development of the skull( neurocranium and viscerocranium) State the embryonic origin of sutures State the characteristic of new born skull Describe the embryonic development of the vertebrae State the significance of re segmentation in development of vertebral column State the embryonic development of ribs and sternum State the embryonic development of limbs Define apical ectodermal ridge and progress zone Distinguish between the embryonic development of the forelimbs and hindlimbs State the significance of appearance of ossification centers in determination of bone age State some clinical correlates

3 develops from paraxial mesoderm lateral plate (parietal layer) mesoderm neural crest. Somites differentiate into Sclerotome dermomyotome. Mesenchymal cells may become fibroblasts chondroblasts, osteoblasts The skeletal system

4 parietal layer of the lateral plate mesoderm of the body wall forms bones of the pelvic and shoulder girdles limbs sternum Neural crest cells in the head region form bones of the face and skull. Occipital somites and somitomeres contribute to formation of the cranial vault and base of the skull.

5 Bone formation Intramembranous ossification Mesenchyme in the dermis differentiates directly into bone such as the flat bones of the skull Endochondral ossification Mesenchymal cells first give rise to hyaline cartilage models, which in turn become ossified. Most bones formed by this way

6 The skull can be divided into two parts: Viscerocranium forms the skeleton of the face (first two pharyngeal arches). Neurocranium forms a protective case around the brain

7 Neurocranium 1. Membranous Neurocranium Mesenchyme from neural crest cells and paraxial mesoderm invests the brain and undergoes Membranous ossification.

8 Neurocranium 2. Cartilaginous Neurocranium Prechordal chondrocranium in front of the rostral limit of the notochord / derived from neural crest cells. or Chondrocranium Chordal chondrocranium posterior to rostral limit of the notochord / arise from occipital sclerotomes formed by paraxial mesoderm.

9 Newborn Skull Sutures are derived from two sources: 1. neural crest cells (sagittal suture) 2. paraxial mesoderm (coronal suture). Several sutures and fontanelles remain membranous for a considerable time after birth After birth, palpation of the anterior fontanelle may give valuable information about 1. ossification of the skull 2. intracranial pressure

10 At first, the face is small in comparison with the neurocranium because of the (a) absence of the paranasal air sinuses (b) the small size of the bones, particularly the jaws

11 Cranioschisis: failure of cranial neuropore to close Cranial meningocele & meningoencephalocele: Small defects in the skull through which meninges and/or brain tissue herniate

12 Craniosynostosis A. Scaphocephaly : early closure of the sagittal suture. Note the frontal and occipital bossing B. Brachycephaly : early closure of both coronal sutures C. Plagiocephaly : premature closure of the coronal suture on one side of the skull

13 Vertebrae and the Vertebral Column Vertebrae form from the sclerotome portions of the somites A definitive vertebra is formed by condensation of the caudal half of one sclerotome and fusion with the cranial half of the subjacent sclerotome (re-segmentation) Mesenchymal cells between cephalic and caudal parts of the original sclerotome fill the space between two precartilaginous vertebral bodies Notochord regresses in the region of the vertebral bodies but it persists and enlarges in the region of the intervertebral disc(nucleus pulposus)

14 Re segmentation of sclerotomes into definitive vertebrae causes : Myotomes bridge the intervertebral discs Intersegmental arteries pass midway over the vertebral bodies. Spinal nerves come to lie near the intervertebral discs and leave the vertebral column through the intervertebral foramina

15 Two primary curves thoracic curvature sacral curvature Two secondary curves cervical curvature, as the child learns to hold up his or her head lumbar curvature, which forms when the child learns to walk Scoliosis (lateral curving of the spine): two successive vertebrae fuse asymmetrically or have half a vertebra missing The curvature is seen between the shoulder blades (thoracic spine)

16 Spina bifida imperfect fusion or nonunion of the vertebral arches (a) spina bifida occulta involve the bony vertebral arches Intact spinal cord (b) spina bifida cystica the neural tube fails to close vertebral arches fail to form neural tissue is exposed

17 Bony portion of each rib is derived from sclerotome cells that remain in the paraxial mesoderm Costal cartilages are formed by sclerotome cells that migrate across the lateral somitic frontier into the adjacent lateral plate mesoderm Ribs

18 Limbs Limb Growth and Development The forelimb appears first followed by the hindlimb 1 to 2 days later. limb buds (4 th week) hand- and footplates (6 th week) is separated from the proximal segment by a circular constriction. A second constriction divides the proximal portion into two segments Morphogenesis of the lower limb is approximately 1 to 2 days behind that of the upper limb. During the 7th week of gestation, the limbs rotate in opposite directions.

19 Apical ectodermal ridge (AER) AER: Ectoderm at the distal border of the limb thickens Progress zone Development of the limb proceeds proximodistally. Cell death in the interdigital spaces produces separation of the digits

20 Bone Age Radiologists use the appearance of various ossification centers to determine whether a child has reached his or her proper maturation age. Useful information about bone age is obtained from ossification studies in the hands and wrists of children. Prenatal analysis of fetal bones by ultrasonography provides information about fetal growth and gestational age.

21 Partial (meromelia) or complete absence (amelia) of one or more of the extremities A. Brachydactyly B. Syndactyly C. Polydactyly D. Cleft hand and foot (lobster claw deformity

22 Amniotic bands may cause ring constrictions and amputations of the limbs or digits.

23 Congenital hip dislocation underdevelopment of the acetabulum and head of the femur common in female newborns Although dislocation usually occurs after birth, the abnormality of the bones develops prenatally. many babies with congenital hip dislocation are breech deliveries frequently associated with laxity of the joint capsule.

24 Summary Skeletal system derives from paraxial mesoderm, parietal layer of lateral plate mesoderm and neural crest cells Bone ossification is of two types: intramembraneous ossification and endochondral ossification Skull is divided into 2 parts: neurocranium and viscerocranium which have different embryonic origin Vertebrae is derived from sclerotomes which undergo re segmentation Apical Ectodermal ridge play an important role in development of limbs Useful information about bone age is obtained from ossification studies in the hands and wrists of children