Sukumal Chongthammakun,Ph.D. Department of Anatomy

Faculty of Science, Mahidol University Email: scsct@mahidol.ac.th

http://intranet.sc.mahidol/AN/

Germ layer contribution

Chondrogenesis

• Begins week 5

• Condensation and differentiation

• Centers of chondrification

• Secretion of the extracellular matri

x

• Entrapment of chondroblast

• Formation of chondrocyte

Interstitial growth

Appositional growth

Growth of cartilage

Osteogenesis

Mechanism of bone formation

• Endochondral ossification

• Intramembranous ossification

Endochondral ossification

• 3Ossification begins at the start of fetal period (mo. )

• PPPPPPPPPPPPP PPPPPPPPPPPPPP PPPP P PPPPP PP PPPPPPPPPPPPPPP PPPPP

• P PPPPPPPPPPPPPP PPPP PPPPPPPPPPP PPP PPPPPPPPP PP P PPPPP PP PPPPPPP

• PPPPPPPPPPPPP PPP PPPP PPP P PPPP PPPPPP

• PPP PPPPPPP PPPPP PP PPP PPPPPPPPP PPPPPPPPPPP PPP , PPPPPPPPPP

• Penetration by the vascular bud

Intramembranous ossification

• Condensations of mesenchymal cells • Secretion of osteoid

• Spicules are formed

• Networks of trabeculae

• Entrapment of osteocytes

The Axial Skeleton

1 . The vertebral column 2 . The attached rib 3. The sternum 4 . The skull

PPPPPPPPP PPP PPPP PPP PPPP PP PP PPPPPPPPPPPP ossification of the sclerotome regions of somites

• PPPPPPPPP P PPPPP PPPPPPPP PPP PPPPPPPPPP PPPPPP PPPP

• PPPP P PPPPPPPPPPPP P PPPPPP PPP PPPPPPPPP( )

• Dorsal portion forms costal processes laterally - P PPP PPPPP(1 1 2 : )

• Intervertebral disks are formed

• P PPPPPPPP PPP PPPP PP PPP PPPPPPP PPPPPPPP

• The fibrocartilage is formed around N. pulposus ( )

All somites contribute to the formation of t he axial skeleton of the body and head

42 44to pairs of somites develop• 4 1occipital ( degenerates)• PPPPPPPP8• 12 thoracic• PPP PPP5• 5 sacral• - - 810 57coccygeal (last degenerate)

Development of the skull

Neurocranium

• membranous neurocranium

• cartilaginous neurocranium

Viscerocranium

• membranous viscerocranium

• cartilaginous viscerocranium

Membranous Neurocranium

Cartilaginous Neurocranium

Membranous Viscerocranium

Cartilaginous Viscerocranium

• Somatic lateral plate mesoderm forms the skelet

on of the appendages : the limb bones and appendic

ular girdles

• The appendicular skeleton is formed by endochondral ossification of cartilage models

Appendicular Skeleton

Development of the Joints

Synovial Joints

interzonal mesenchyme differenciates :

1. Peripherally: to capsular and other ligaments

2. Centrally: disappears and becomes the joint cavity

3. Where it lines the capsule and articular surfaces: PPPPP PPP PPPPPPPP PPPPPPPP

Development of the Joints

Cartilaginous Joints

Interzonal mesenchyme differenciates into:

• Hyaline cartilage eg. Costochondral joints

• Fibrocartilage eg. Symphysis pubis

Development of Joints

Fibrous Joints

Interzonal mesenchyme differenciates into:

• Dense fibrous connective tissue eg. Sutures of the skull

• Limbs form by “budding” out from the ventrolateral body surface.

• Limbs are formed by ectoderm coverin g a

solid mesoderm core, with no endoder m

contributions. There is no coelom.

Embryonic features unique to the limbs

1. Somatic lateral plate mesoderm: cartilage, bone & CNT

2. Skeletal muscle is outsider migrating in from the somites.

3. Surface ectoderm: epidermis of skin & its specialization

4. Nerves & blood vessels grow in from the body using connective as a guide.

Germ layer origins of the limbs

1. Limb mesoderm induces surface ectoderm to form “ apical ectodermal ridge (AER)”.

2. AER induces mesoderm underneath to continue proliferation then commit to specific derivatives.

-3. Limb mesoderm also induces the formation of limb specific mesoderm specializations eg. Nails.

Limb mesoderm and surface ectoderm interaction

• Directed by AER

• The first deposit is programmed to form humerus

• The last deposit is programmed to form the bone s

of phalanges

- Proximal Distal Limb Axis

• Dorsal side is continuous with the dorsal side surface of the body.

• Ventral side is continuous with the ventral side surface of the body.

- Dorsal Ventral Limb Axis

• is established by a gradient of inducing signals released from cells of the “zone of polarizing activity (ZPA)”

• The most caudal bones (eg. Little finger) form in response to exposure to a maximum of inducer.

• The most cranial bones (eg. Thumb) form in response to PP PPP PPP PPP PPPPPPPPPPPPP PP PPPPPPPP.

• Retinoic acid may be the ZPA inducer

- Cranial Caudal Limb Axis

4Limb buds appear at the end of week and t he beginning of week 5 and become externall

y constricted into regions by week 8

6Week : oo ooo ooooo ooo oooooooooo ooooooo oo(

rential constriction) - 7 8 :Week

Formation of finger and toe rays (longitudinal constriction)

8Week : Formation of 2nd circumferential constriction

Constriction and Regression

Final feature of limb development : Limbs rotate about their long axis.

90The arms rotate o dorsolaterally The legs rotate 90 o ventrolaterally

Rotation

Rotation of the limbs results in : 1. The final orientation of the joints 2. The final location of muscle groups. 3. The mature patterns of sensory innervation of the skin called “dermatome”.

• Germ layer origins : All skeletal muscle is derived from somitic myotomes

• Connective tissue directs muscle mass formation

• Formation of multinucleated skeletal muscle cells occurs by fusion of myoblasts with each other.

• Skeletal muscle cell fusion, formation of contractile machinery, and specific fiber types do not require nerve input.

Histogenesis of skeletal muscle

Distiguishing between trunk, head and limb muscles

1. Trunk muscles form directly from myoblasts remaining in the myotomes.

2. The cells that migrate form intrinsic limb muscles, and the muscles of shoulder and pelvic girdles.

3. Cells from the most cranial myotomes form head and neck muscle after they migrtae into the pharyngeal arches.

Innervation : the key to the origin of skeletal muscles

Cranial nerves : muscles of pharyngeal arches

Spinal nerves : muscles of neck, body and limbs Dorsal primary rami Ventral primary rami

Axial muscles of the trunk

Trunk (axial) muscles form directly from myotomes.

Dorsal portion : epimere epaxial muscles

Ventral portion : hypomere hypaxial muscles

Appendage muscles: limbs and appendicular girdles

Limb muscles and appendicular girdle muscle s all

originate from myotomal cells that migrate into

the limbs.

Dorsal limb bud mases extensors

supinatorsabductors

Ventral limb bud massesflexorspronatorsadductors

The intrinsic limb muscles are formed from both li mb muscle masses.

All muscles of the appendicular girdles are derive d from the dorsal limb muscle mass.

Innervation: Dorsal muscle mass :

dorsal branches of ventral 1o rami Ventral muscle mass :

ventral branches of ventral 1 o rami

Appendage muscles: limbs and appendicular girdles

Muscles of the head and neck

PPPPPPPPPP P PPPPPP PPPP PPPPPPPPPP PPPPPP PPPPPP forming muscles.

Innervation helps distinguish the origins.

Origins of the Craniofacial Muscles--------------------------------------------------------------------------------------------------------

Mesodermal Origin Muscles Innervation--------------------------------------------------------------------------------------------------------

1 2Somitomeres , PPP PPPPPPP, , recti ( ) 3Somitomere Superior oblique ( ) 4Somitomere PPP -PPPPPPP P PPPPPP Trigeminal (V) 5Somitomere PPPPPPP PPPPPP ( ) 6Somitomere - Jaw opening and other 2nd arch muscles ( ) 7Somitomere PPPPPPPPPPPPPPP ( )

1 2Somites , Intrinsic laryngeals Vagus (X) -P25 Tongue muscles ( )--------------------------------------------------------------------------------------------------------

Congenital Defects of the Skull

Caused by genetic and environmental factors and disturbing neural crest migration into/and proliferation in pharyngeal

arches

Anencephaly : failure of cranial neuropore to close in week 4.

Craniofacial synostosis : premature closure of sutures between flat bones of neurocranium

coronal suture : oxycephaly/turricephaly sagittal suture: scaphocephaly

Craniofacial dysostosis : underdevelopment of arch facial bones, including maxill and mandible

Scaphocephaly

A. Oxycephaly B. Plagiocephaly

Congenital Defects of Vertebrae and Ribs

Hemivertebra : results in scoliosis

Spina bifida : vertebral arches fail to fuse dorsally

Congenital Defects of the Limbs

Causes : genetic/environmentHyperthermia

Drugs eg. Aspirin, anticonvulsant dimethadione, excess RA

Achondroplasia (chondrodystrophy) : premature ossification in epiphyseal plate of long bones

a. Short vertebral column . Short limbs with thick diaphyses in long bones

c. Sunken midfacial region . Normal to superior mental capacity

Achondroplasia

Congenital Defects of the Limbs

Amelia : complete absence of limb(s)

PP PPP PPPP: ( ) ( )

Phocomelia : a form of meromelia in which proximal structures are small or absent

Polydactyly : extra digits (incomplete and useless)

Syndactyly : fused or web digits (lobster claw)

A. Unilateral amelia B. Meromelia

A. Syndactyly B. Lobster claw

Congenital Defects of the Skeletal Muscles

Muscular dystrophy : a family of genetic diseases in which there is postnatal degeneration of various muscle groups

-Duchenne muscular dystrophy : caused by a lack of actin binding protein called dystrophin

Absent or underdeveloped muscle groups :• associated with bone defects• unassociated with bone defects