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Flashcard study product for Dr. Martin's powerpoint lecture on musculoskeletal development.

What forms skeletal muscles?

Somites and Somitomeres

What three germs layers is the embryo composed of?

Externally: ectoderm
Middle: mesoderm
Internally: endoderm

What does paraxial mesoderm form?

Head region: somitomeres
Caudally: somites

What do the somites divide into?

Ventral
Dorsolateral

What is the ventral portion called?

Sclerotome

What does sclerotome become?

Cells surround the neural tube and form the vertebral column.

What is the dorsolateral portion called?

Dermomyotome

What does the dermomyotome differentiate into?

1. Dermis of the skin
2. Myotome portion--> skeletal muscles

How does the paraxial domain come about?

Derived from the DML portion of the dermomyotome
Surrounds the neural tube and contains only somite derived (paraxial mesoderm).

What does the paraxial domain give rise to?

Intercostals
Intrinsic muscles moving the vertebral column

How does the abaxial domain come about?

Derived from the VLL portion of the dermomyotome
Which have migrated across the lateral somitic border and merged with cells of the lateral plate mesoderm.

What does the abaxial domain give rise to?

Lateral body wall muscles
Limb muscles

Where does endochondral ossification occur?

Long bones
*except clavicle (intramembranous ossification)

What are the steps of endochondral ossificatin?

Week 5- cartilage models are present
Week 9- ossification begins in the mid-shaft (diaphysial primary center)
Week 12- all primary centers have formed
Birth- shafts of long bones are completely ossified, but the ends (epiphyses) are still cartilaginous.

How does growth occur in bones?

On both side of the epiphyseal plates
Appositional growth of hyaline cartilage by SLOW proliferation of chondrocytes facing the epiphysis.
More rapid appositional growth of cartilage followed by destruction and replacement, initially with spongy bone, at the metaphysis side of the plate.

When full growth is attained que pasa?

The plates disappear!
Ends and shaft unite and growth ceases!

When does the clavicle begin and cease ossifying?

Begins in 5th to 6th week.
Age 25.

What are the secondary ossification centers?

Humerus
Ulna
Femur
Tibia

What does damage to a secondary ossification center cause?

Stunted growth
Knee area of limb is most at risk.

How does the development of joints begin?

Interzonal mesenchyme forms between developing bones and is the primordium of a joint, its ligaments, capsule, synovial membrane (if present), and articular discs.

What is the timeline in joint development?

Week 6- Joints start to develop
Week 8- all are present

What are some examples of synovial joints?

Shoulder, Elbow, Hip, Knee

What are some examples of cartilaginous joints?

Primary- epiphyseal plate
Secondary- IV discs

What are some examples of fibrous joints?

Interosseous membrane of forearm and leg
Sutures of the skull

As the somites segment into sclerotomes they are organized how?

Loosely arrange rostral portion.
Densely packed caudal portion.

How is a vertebra initially formed?

Caudal portion fuses with the rostral portion.

What is the position of the myotome in vertebrae?

Spans the developing vertebrae, able to move the vertebral column.

What is the anulus fibrosus formed from?

Remaining mesoderm.

What is the nucleus pulposus formed of?

Remnant of the notochord.

How is the C1 vertebrae formed?

Caudal portion of C1 sclerotome fuses below with cranial portion of C2 sclerotome.

When does limb growth begin?

Week 4- limb buds are visible as outpunching's from the ventrolateral body wall.

Initially what do the limb buds consist of?

Mesenchyme core

What is the mesenchyme core derived from?

Lateral plate mesenchyme

What will the mesenchyme core form?

The bones, connective tissues of the limb covered by a layer of ectoderm.

What forms the apical ectodermal ridge?

Limb thickening at the tip of the ectoderm.

What is the function of the apical ectodermal ridge (AER)?

Inductive influence on the adjacent mesenchyme cells causing it to remain as a population of undifferentiating, rapidly proliferating cells called the PROGRESS ZONE.

How does the AER influence the differentiation into cartilage and muscle?

As the limb grows cells FURTHEST from the influence of the AER differentiate into cartilage and muscle.

How does the limb develop?

Proximodistally.

How are fingers and toes formed?

When cell death, apoptosis, in the AER separates the ridge into 5 parts.

What happens at week 7 of limb growth and development?

Limbs begin to rotate in opposite directions.
Lower limb --> medially 90 degrees

What is so special about week 8 of limb growth and development?

Teratogens are unable to cause MAJOR congenital anomalies.

What do dorsal divisions of the limb plexus innervate?

Limb extensor musculature.

What do ventral divisions of the limb plexus innervate?

Limb flexor musculature.

Describe Prune Belly Syndrome.

A congenital partial or complete absence of abdominal wall musculature.

Describe the causes of Prune Belly Syndrome.

Caused by abnormal and or non-migration of myoblasts from the VLL of the somites into the Lateral Plate Mesoderm --> forming the abaxial domain.

What are the muscles affected in Prune Belly Syndrome?

External/Internal obliques
Transversus abdominus
Rectus abdominus

Describe the effects of Prune Belly Syndrome.

Wall becomes so thin that organs can be seen and palpated easily.

What is Prune Belly Syndrome associated with?

Polyhydramnios

What is the congenital lack of diaphragm muscle entail?

Distinguished eventration of diaphragm (stretching due to muscle tone)
Congenital Diaphragmatic Hernia

Both lead to: incomplete expansion of lungs

What is the nerve that keeps the diaphragm alive?

Phrenic Nerve C3, C4, C5

So whats the problem with CDH?

Well the pleuroperitoneal canals normally close at week 6, if not closed by week 10 when the abdominal contents are return to the abdomen the viscera can be forced into the thorax.

If untreated intestines push into the thorax compressing initially the left lung but eventually compressing all thoracic contents.

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