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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
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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