Musculoskeletal Development

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What is the axial skeleton comprised of?	Skull, vertebrae, ribs and sternum
What is the axial skeleton derived from?	Mesoderm, organised into somites
When and how do the somites appear?	Days 20-30, in a programmed plan of gene initiation, in intervals set by a 'clock' of genes being switched on and off
What does each somite divide into?	Sclerotome and dermomyotome
What does the sclerotome give rise to?	The vertebrae and ribs
What doe the dermomyotome give rise to?	The muscles and the dermis of the skin
What is the more recently discovered layer between the sclerotome and the dermomyotome, and what does it give rise to?	The syndetome, which becomes tendons
Describe how the vertebral bodies form?	Sclerotome condenses around the notochord, so that cells from one side meet with cells from the other and fuse to form a vertebral body. The sclerotome then splits into 2 segments, and the cranial segment of one will fuse with the caudal segment of the one above it to give the final vertebral body.
How does the vertebral canal, transverse, costal and spinous processes form?	From the vertebral body, sclerotome cells spread dorsally to surround the developing spinal cord, thus forming the vertebral canal. The cells meet dorsally to form the spinous processes, and the cells from the vertebral arches spread transversely to form the transverse and costal processes.
How do the intervertebral discs form?	The densely packed sclerotome cells right around the notochord become the annulus fibrosus of each intervertebral disc, and the notochord itself becomes the nucleus pulposus, the soft centre of each intervertebral disc
How do spinal nerves form?	The presence of a somite induces the growth of a spinal nerve from the neural tube at the level of each somite. It grows between the 2 halves of the sclerotome and innervates the dermotome and myotome that originated from that somite. This segmental pattern continues through life.
What do the costal processes become?	Ribs
What can be some abnormalities of the sclerotome? (3)	1. Abnormal segmentation (Scoliosis, Kyphosis, Hemivertebrae) 2. Brevicollis (reduced number of cervical vertebrae) 3. Spina Bifida (failed fusion of neural arches)
How does the sternum form?	The sternum originates as a pair of cartilagenous bars in the body wall, which fuse to become the manubrium and then ossify.
What do the first 5 somites form?	The occipital bone
What are the limb buds made from?	Lateral mesoderm and its overlying ectoderm
What do the limb buds form?	Appendicular skeleton and connective tissue
What do the limb buds NOT form, and so where does it come from instead?	The muscles, which come from the somites
What are the 3 stages of limb development?	1. Initiation 2. Patterning 3. Digit patterning and sculpting
What cells lie at the most distal part of the limb buds and what do they form?	Ectodermal cells which form the apical ectodermal ridges
What is their function?	They secrete factors to induce the proliferation and differentiation of the mesoderm underneath so that the limb elongates
What is the basic method of cell signalling here?	Factors such as growth factor are released from the apical ectodermal ridges, and bind to their receptors on the surface of the mesodermal cells. This induces an intracellular cascade which ultimately will alter gene transcription.
Apart from the apical ectodermal ridges (AER), which other area of the limb bud secretes signalling molecules?	Zone of Polarising Activity (ZPA)
In what direction are the limbs patterned?	Proximal to distal
Which specific set of genes are involved in limb and digit patterning?	Hox genes
How are the digits separated?	Apoptotic signals induce the breakdown of the mesenchyme
What are some defects in limb development? (4)	1. Loss of whole elements (e.g. as a result of Thalidomide) 2. Abnormal digits (Loss of/extra/fused) 3. Cleft hands/feet 4. Clubbed feet
How do the dermatomes develop?	Spinal nerves at each somite innervate the areas originating from that somite, so at the stage of early development this present as the 'striped' segmental pattern. As the limb rotate, they give the pattern we maintain in life.
What does the myotome form?	1. Epimere 2. Hypomere 3. Limb muscles
What is the epimere? What is its innervation?	Dorsally located, this becomes the extensor muscles of the trunk (e.g. erector spinae). Innervated by the dorsal rami of the spinal nerves.
What is the hypomere? What is its innervation?	Ventrally located, forming the flexor muscles of the trunk as well as the muscles of the thoracic and abdominal walls. Innervated by the ventral rami of the spinal nerves.
How do the limb muscles form?	Cells migrate away from the hypomere to the proximal limb buds and form 2 masses around the developing appendicular skeleton, the ventral flexor muscles and the dorsal extensor muscles.
What is the innervation of the limb muscles?	Ventral rami of the spinal nerves branch into ventral and dorsal branches, with the ventral branches supplying the flexors and the dorsal branches supplying the extensor muscles. The brachial plexus (C4-T1) innervates the upper limbs, and the lumbosacral plexus (L4-S3) innervated the lower limbs.
What controls the differentiation of the somites into the different sections?	Signalling molecules produced by the adjacent tissues
What process is the development of muscle fibres in embryogenesis similar to in the adult body?	The injury/repair mechanism of muscle growth
How does the NMJ form?	ACh receptors are already present on muscle fibres in a pre-defined pattern. When the nerve terminals reach the muscle, signalling molecules are released which cause the receptors to concentrate in the areas of contact between nerve and muscle, to form synapses.

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	Creado por Hannah Tribe hace alrededor de 10 años