development of muscular system

Question 1

Precursors for muscle tissue

Answer 1

Three types: skeletal and cardiac are striated
Smooth is not
Axial groups and appendicular group

Question 2

origin of muscle precursors

Answer 2

all skeletal muscle is derived from paraxial mesoderm (mesoderm surrounding the notocord)

Muscles of the body wall and limbs form from somites, muscles in head/neck region of the embryo originates from head mesoderm

Cardiac and visceral smooth muscle originate from splanchnic mesoderm (mesoderm on the side of paraxial mesoderm),

Vascular smooth muscle originates from local mesenchyme (ectoderm -derived neural crest mesenchyme)

Smooth muscle of the constricotr and dilator papillae and myoepithelial cells of mammary and sweat glands originate ectoderm)

Question 3

Characteristics of muscle precursor: mesenchyme

Answer 3

muscle forming mesenchyme often migrates or is displaced from its site of origin

Development is mediated by specific transcription factors and other signaling factors

Question 4

Formation of skeletal muscle precursors

Answer 4

Skeletal muscles from trunk and limbs originate from somites

Somite is a transient structure in the embryo in which precursor cells for skeletal muscle, bone, cartilage and CT can be grouped and redistributed to other areas of the embryo

Differentiation and redistribution of cells in the somite is controlled by signaling molecules from surrounding tissues

Cells in dorsolateral/dorsum of somite are commanded by Wnts from neural tube and surface ectoderm, maintain their epithelial structure–> dermamyotome

Question 5

Fates of dermamyotome (DM)

Answer 5

Cells of DM exposed to Bmp4 from lateral mesoderm modulated by noggin (Bmp inhibitor) from dorsal and ventral neural tube

Other Dm cells are influenced by Wnt and Shh from the neural tube and undergo an epithelium–>mesenschyme transformation (EMT) and relocate beneath the epithelial portion of the DM.

Cells then become myogenic cells forming a myotome, the precursor for the sk muscle of the trunk and limbs, cells in the middle of the DM remain epithelial. Either become more myogenic cells or satellite cells or become dermis of the skin on back and neck (dermatome)

Question 6

Lateral somite frontier

Answer 6

Boundary between the lateral edge of the somite and the medial edge of the LM
Interface set up by the interaction of different signaling molecules originating from the somite (LM and surrounding tissues)

Cells originating from the dorsomedial portion of the DM remain in the region adjacent to the neural tube and notochord influenced by Wnt and Shh signals from the tube and chord. The myogenic cells express the myogenic regulatory factors (Myf5 and MyoD)–> form intrisic muscles of the back and prevertebral and intercostal muscles–Pimaxial domain

Myotome derived cells from the ventrolateral edge of the DM migrate across the boundary–> somatic mesoderm where they influence Wnt and Bmp4–> myogenic cells express MyoD and become abdominal wall and limbs and infrahyoid muscles (called the abaxial domain)

Question 7

Differentiation of skeletal muscle precursors

Answer 7

once cells become part of the myotome portion of the DM their potency becomes restricted to the skeletal muscle lineage

Myogenic cells or pre myoblasts (they can proliferate, migrate and begin to up regulate certain myogenic regulatory factors (transcription factors)

Once at the location of the embryo they seecrete adhesive glyco protein fibronectin

Align in chains of myoblasts and fuse–>multinucleated myotubes ( via M cadherin)

Troponin and topomyosin –> myofiber and sarcomere formation – differentiated muscle cell or muscle fiber

Question 8

Primary and secondary muscle fibers

Answer 8

The first muscle fibers to form are primary muscle fibers

Secondary muscle fibers appear later forming around the primary fibers at a time when branches of spinal nerves approach and innervate groups of muscle fibers–>pre muscle masses

Question 9

Basic plan for somatic motor innervation of sk muscle

Answer 9

Muscles in the head and neck are derived from head mesoderm, occipital myotomes, they are innervated by cranial nerves

In the trunk sk muscle originates from the ventrolateral and ventromedial edges of the myotome

muscles of the trunk and limbs are innervation, the myotome is subdivided in dorsal epaxial and ventral hypaxial portions

Muscles forming from the epaxial portion of myotome (back muscles) receive motor innervation from dorsal primary rami

Muscles from the hypaxial portion (front and limbs) receive motor innervation from the ventral primary rami of spinal nerves

Question 10

Development of smooth muscle

Answer 10

serum response factor and its co activator (myocardin)

Question 11

Malformation of muscular system

Answer 11

Absence of skeletal muscle unilateral and often asymptomatic
Poland sequence (no pec major an minor) no subQ fat and axillary hair, upper lim anomylies (more right side)

Prune belly- no abs, high testes, bladder and UT anomilies

Question 12

Congenital muscular torticollis

Answer 12

fixed rotation and tilting of the head to side

Question 13

Muscular dystrophy

Answer 13

family of genetic diseases progressive weakness no CNS or PNS path,

Duchenne type most common X linked (beckers type less)