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Muscular System Introduction • Develops from mesoderm, except for: – iris muscles: • develop from neuroectoderm – esophagus muscles • believed to develop by transdifferentiation from smooth muscle Day 17 Paraxial Mesoderm Thick plate of mesoderm on each side of midline Becomes organized into segments known as somitomeres: form in a craniocaudal sequence Somitomeres 1–7 do not form somites contribute mesoderm to head and neck region (pharyngeal arches) Remaining somitomeres further condense to form 42–44 pairs of somites: Somites closest to caudal end eventually disappear: giving rise to a final count of about 35 pairs of somites Day 19 Somites Differentiate into three components: 1. Sclerotome: • cartilage and bone component 2. Myotome: • muscle component 3. Dermatome: • dermis of skin component Myotomes • Part of a somite that divides into: – 1 dorsal epaxial division – 2 ventral hypaxial division • Developing spinal nerves divide and send a branch to each myotome division: – dorsal primary ramus supplies: • epaxial division – ventral primary ramus supplies: • hypaxial division 1 2 Development of Skeletal Muscle • First indication of myogenesis (muscle formation) is: – elongation of nuclei and cell bodies of mesenchymal cells: • differentiate into primordial muscle cells called myoblasts • Myoblasts fuse to form: – elongated, multinucleated, cylindrical structures called myotubes • During or after fusion of myoblasts: – myofilaments develop in cytoplasm of myotubes • organize into myofibrils • Because muscle cells are long and narrow: – they are usually called muscle fibers Limb Musculature • Derived from myotomes in: – upper limb bud region – lower limb bud region • This mesoderm migrates into limb bud and forms: – posterior condensation – anterior condensation • Posterior condensation develops into: – extensor and supinator musculature of upper limb – extensor and abductor musculature of lower limb • Anterior condensation develops into: – flexor and pronator musculature of upper limb – flexor and adductor musculature of lower limb Development Of Smooth Muscle • Smooth muscle fibers differentiate from: – splanchnic mesenchyme surrounding endoderm of primordial gut and its derivatives • Somatic mesoderm provides: – smooth muscle in walls of many blood and lymphatic vessels ~20 Days • Muscles of the iris (sphincter and dilator pupillae) and myoepithelial cells in mammary and sweat glands are derived from: – mesenchymal cells that originate from ectoderm ~21 Days • Development of Cardiac Muscle (slide Develops from the 1 of 2) lateral splanchnic mesoderm – gives rise to mesenchyme surrounding developing heart tube • Cardiac myoblasts differentiate to form primordial myocardium – Heart muscle is recognizable in fourth week • Cardiac muscle fibers arise by differentiation and growth of single cells – unlike striated skeletal muscle fibers that develop by fusion of cells 18 days 17 days 22 days Lecture Summary • Skeletal muscle is derived from? • Myotome regions of somites • Cardiac muscle and most smooth muscle are derived from? • Splanchnic mesoderm Development of Bone and Cartilage • As notochord and neural tube form, intraembryonic mesoderm (lateral to these structures) thickens to form: Dorsal view ~18 days Transverse Section ~18 days – two longitudinal columns of paraxial mesoderm • Toward end of third week, paraxial mesoderm: – becomes segmented into blocks of mesoderm called somites sclerotome • Externally, appear as beadlike elevations along dorsolateral surface of embryo Transverse Section ~22 days Bone Formation • All bones are derived from either direct or indirect conversion of preexisting connective tissue called mesenchyme (embryonic connective tissue) into bone – This process is called ossification • During development, two types of ossification occur: 1.Intramembranous ossification 2.Endochondral ossification Endochondral Ossification (slide 2) • Ossification of limb bones begins at end of embryonic period – makes demands on maternal supply of calcium and phosphorus • Pregnant women are advised to maintain an adequate intake of these elements to preserve healthy bones and teeth • At birth, diaphyses are largely ossified, but: – most of epiphyses are still cartilaginous • Secondary ossification centers appear: – in the epiphyses of most bones during first few years after birth • In most bones, epiphyses have fused with diaphysis by: – 20 years of age mesenchyme Cartilage Formation • Cartilage develops from mesenchyme : – first appears during fifth week • Mesenchymal cells differentiate into chondroblasts: – secrete collagenous fibrils and ground substance (extracellular matrix) – Form isogenous groups Development of the Axial Skeleton • Composed of: – – – – cranium (skull) vertebral column ribs sternum • During fourth week, sclerotome cells surround: – neural tube (primordium of spinal cord) – notochord (structure about which primordia of vertebrae develop) Transverse section through 4-week embryo Development of the Vertebral Column • During precartilaginous or mesenchymal stage, mesenchymal cells from sclerotomes are found in three main areas: 1. 2. 3. • around the notochord surrounding the neural tube in the body wall In a frontal section of a 4-week embryo: – sclerotomes appear as paired condensations of mesenchymal cells around notochord Frontal section Transverse section through 4-week embryo Each Condensation of Sclerotomal Cells • Consists of: – loosely arranged cells, cranially – densely packed cells, caudally • Some densely packed cells move cranially, opposite the center of the myotome: – form intervertebral (IV) disc Transverse section through 5-week embryo Frontal section Development of the Vertebral Body • Remaining densely packed sclerotome cells: – fuse with loosely arranged cells of the immediately caudal sclerotome to form: • mesenchymal centrum – primordium of vertebral body – each centrum develops from two adjacent sclerotomes • Nerves lie close to intervertebral discs and intersegmental arteries lie on each side of vertebral bodies: – In the thorax, intersegmental arteries become intercostal arteries Cartilaginous Stage of Vertebral Development • During sixth week, chondrification centers (site of earliest cartilage formation) appear in each mesenchymal vertebra – Two chondrification centers in each centrum: • fuse at end of embryonic period to form cartilaginous centrum – Concomitantly, chondrification centers in the neural arches: • fuse with each other and the centrum – Spinous and transverse processes develop from: • extensions of chondrification centers in the neural arch • Chondrification spreads until: – cartilaginous vertebral column is formed Mesenchymal vertebra at 5 weeks Mesenchymal vertebra at 6 weeks Bony Stage of Vertebral Development • Ossification of typical vertebrae: – begins during embryonic period – usually ends by 25th year • There are two primary ossification centers for the centrum: – ventral and dorsal • fuse to form one ossification center • Three primary ossification centers are present by end of embryonic period – week 8: – one in centrum – one in each half of neural arch Primary ossification centers in a cartilaginous vertebra at 7 weeks Vertebrae Ossification • Ossification becomes evident in: – neural arches during eighth week • At birth, each vertebra consists of: – three bony parts connected by cartilage • Bony halves of vertebral arch usually fuse: – during first 3 to 5 years – first unite in lumbar region and progresses cranially • Vertebral arch articulates with centrum at: – cartilaginous neurocentral joints: • permit vertebral arches to grow as spinal cord enlarges • These joints disappear when vertebral arch fuses with centrum during third to sixth years Development of the Appendicular Skeleton • Consists of: – pectoral and pelvic girdles – limb bones • Mesenchymal bones form during fifth week as: – condensations of mesenchyme in the limb buds ~28 days Formed from Somatic lateral mesoderm!!!! Longitudinal section ~33 days Appendicular Skeleton: Mesenchyme to Cartilage • During sixth week, mesenchymal bone models in the limbs undergo chondrification to form hyaline cartilage bone models – Clavicle initially develops by intramembranous ossification, and it later forms growth cartilages at both ends • Models of the pectoral girdle and upper limb bones appear: – slightly before pelvic girdle and lower limb bones • Bone models appear in a proximodistal sequence 6 weeks Later in sixth week Summary of Lecture (slide 1 of 2) • What is the main component in which the skeletal system develops? • Mesenchyme, derived from mesoderm • In most bones, such as long bones in the limbs, the condensed mesenchyme undergoes what process to form cartilage bone models? • Chondrification • When do ossification centers appear in these cartilage bone models? • By the end of the embryonic period • Later, these bones ossify which named process? • Endochondral ossification • Flat bones ossify by which named process? • Intramembranous ossification • The vertebral column develop from mesenchymal cells of what structure? • Sclerotomes of somites • Each vertebra is formed by? • Fusion of caudal half of one pair of sclerotomes with cranial half of subjacent pair of sclerotomes Summary of Lecture (slide 2 of 2) • The appendicular skeleton develops from? • Endochondral ossification of the cartilaginous bone models, which form from mesenchyme in the developing limbs • What are the three classifications of joints? • Fibrous joints, cartilaginous joints, and synovial joints • Joints develop from? • Interzonal mesenchyme between the primordia of bones • In a fibrous joint, the intervening mesenchyme differentiates into? • Dense fibrous connective tissue • In a cartilaginous joint, the mesenchyme between the bones differentiates into? • Cartilage • In a synovial joint, what forms within the intervening mesenchyme by breakdown of cells? • Synovial cavity • In a synovial joint, mesenchyme also gives rise to? • Synovial membrane, capsule, and ligaments of the joint Review of Development (slide 1 of 3) • Lateral plate mesoderm migrates into limb bud: – condenses along the central axis to eventually form: • vasculature and skeletal components of limbs • Somites of paraxial mesoderm migrate into limb bud: – condense to form: • musculature component of limbs Early Stages of Limb Development (1 of 4) • Limb development begins with: – activation of mesenchymal cells in lateral mesoderm • Limb buds form deep to: – thick band of ectoderm called Apical Ectodermal Ridge (ACR) • Toward end of fourth week, limb buds first appear as: – elevations of ventrolateral body wall • Upper limb buds: – visible by day 26 or 27 • Lower limb buds: – appear 1 or 2 days later • Each limb bud consists of: – mass of mesenchyme derived from: • somatic layer of lateral mesoderm – covered by ectoderm Early Stages of Limb Development (3 of 4) • Mesenchymal cells proximal to AER differentiate into: – blood vessels – cartilage bone models • Distal ends of limb buds: – flatten into paddle-like hand- and footplates • By end of sixth week and during seventh week: – mesenchymal tissue in handplates and footplates condense to form: • digital rays – outline pattern of digits or fingers/toes Early Stages of Limb Development (4 of 4) • Intervals between digital rays occupied by: – loose mesenchyme that breaks down, forming: • notches between digital rays – as tissue breakdown progresses: • separate digits (fingers and toes) are formed by end of eighth week • Programmed cell death (apoptosis) is responsible for: – tissue breakdown in the interdigital regions – incomplete programmed cell death (apoptosis) leads to: • syndactyly (webbing of fingers or toes) Innervation of Limbs • Motor axons arising from spinal cord enter limb buds during fifth week: – grow into dorsal and ventral muscle masses • Sensory axons enter limb buds after motor axons – use them for guidance C5 – T1 brachial plexus, L2 – S3 lumbosacral plexus!!!!! Cutaneous Innervation of Limbs (1 of 2) • Dermatome (segmental innervation) is: – area of skin supplied by a single spinal nerve and its spinal ganglion • Cutaneous nerve Upper limbs lateral areas (multisegmental rotation, lower – medial innervation) is: – area of skin supplied by a peripheral nerve that is formed by multiple spinal cord segments rotation!!!!!!!!! Summary of Limb Development • • • • • • • • • • • • • • • • • • • • When and how do limb buds appear in the embryo? Toward the end of the fourth week as slight elevations of the ventrolateral body wall What is the temporal relationship between the development of upper and lower limb bud development? The upper limb buds develop approximately 2 days before the lower limb buds What are the main sources of tissues for limb bud development? Lateral mesoderm, paraxial mesoderm, and ectoderm What does AER exerts an inductive influence on? The limb mesenchyme, promoting growth and development of the limbs In the distal part of limb development, how is apoptosis (programmed cell death) important in limb development? Formation of the notches between the digital rays, giving rise to fingers and toes Limb muscles are derived from? mesenchyme (myogenic precursor cells) originating in the somites What do the muscle-forming cells (myoblasts) form? dorsal and ventral muscle masses or condensations When do nerves grow into the limb buds? After the muscle masses have formed Most blood vessels of the limb buds arise as? Buds from intersegmental arteries Which direction do the upper and lower limbs rotate? Upper limb rotates laterally 90 degrees and the lower limb rotates medially 90 degrees