Development of the Nervous and Musculoskeletal Systems for Limb Formation Flashcards
Begins soon after gastrulation, with differentiation of the ectoderm to form the neural tube
Neurulation
Forms the nervous system
Neural tube
Occurs with induction of ectoderm by factors from the underlying notochord (a midline rod of mesoderm)
Neurulation and neural tube formation
Neuroectodermal cells organize in the midline as a thickening called the
Neural plate
Proliferation of neuroectoderm cells of the neural plate results in buckling of the tissue to form a depression called the
Neural groove (w/ lateral neural folds)
With further proliferation and more buckling, the edges of the neural folds approximate and fuse in the dorsal midline to form the
Neural tube
Neural tube fusion starts in the cervical region and occurs as a
bi-directional “Zipper”
Neural tube fusion starts in the cervical region and occurs as a bi-directional “Zipper” that closes the neural tube with fusion moving from
Cranial to caudal directions
The open (unfused ends) of the neural tube are alled
Neuropores
The final step of neurulation, which occurs by the end of week 4, is
Neuropore closure
What day do the following neuropores close?
- ) Cranial
- ) Caudal
- ) Day 25
2. ) Day 28
Forms the brain and spinal cord of the CNS
Neural Tube
Detach from the dorsolateral edges of the neural tube and migrate throughout the body to form a wide variety of cell types, including ganglia of the PNS
Neural crest cells
Defects which mostly result from failure of the neuropores to close
Neural tube defects
Failed closure of the cranial neuropore results in
Anencephaly
Failure to form the cranial vault around the cerebral hemispheres
-often lethal
Anencephaly
Failed closure of the caudal neuropores results in
Spina bifida
A defective fusion of the vertebral arches, commonly at L4-S1
Spina bifida
A defect in the fusion of vertebral arches without involvement of underlying neural tissue
Spina bifida occulta
A defect in the fusion of vertebral arches WITH involvement of underlying neural tissue
Spina bifida Cystica
In spina bifida cystica with meningocele, there is a protrusion of
Meninges
In spina bifida cystica with meningomyelocele, there is protrusion of
Neural tissues w/ protruding meninges
In spina bifida cystica with rachischisis (or myeloschisis), exposed nerve tissue is permanently damaged by
Amniotic fluid
Most neural tube defects can be prevented by supplementation with
Folate
The spinal cord’s conus medullaris ends at different locations when comparing the
Fetus, newborn, and adult
Up until the 3rd month in utero, the spinal cord extends the
Entire length of the embryo
Up until the 3rd month in utero, spinal nerve roots pass through intervertebral foramina located at the
Same level as their segmental origin
For example, up until the 3rd month in utero, the short S1 spinal nerve root exits through the
S1 intervertebral foramen
At birth, the conus medularis ends at the
L3-L4 vertebra
With further growth during adulthood, the spinal cord ends at the
L1-L2 vertebrae
Cells of the neural tube that form neurons and neuroglia of the CNS
Neuroepithelial cells
Primitive neurons that lose the ability to divide as soon as they are formed from neuroepithelium
Neuroblasts
Neuroblasts move from neuroepithelium to the mantle layer to form nuclei in
Gray matter of spinal cord
Neuroblast migration into the mantle layer and subsequent differentiation forms which two prenatal thickenings?
- ) Alar plates (become dorsal sensory horns)
2. ) Basal plates (become ventral & lateral motor horns)
Myelinated axon processes (“fibers”) extend from
neuroblasts in mantle layer and into marginal layer to form
White matter of spinal cord
What do the following prenatal structures of the spinal cord become postnatally?
- ) Mantle layer
- ) Marginal layer
- ) CNS gray matter
2. ) CNS white matter
Aggregations of neuron cell bodies in the PNS
Ganglia
Sensory neurons that project processes from receptors and into spinal cord
Dorsal root ganglia
What are the primary tissues of the musculoskeletal (MSK) system?
- ) Epithelium
- ) Connective tissue
- ) Muscle
- ) Nerve
Made up of large, cylinder-shaped cells that are striated and multinucleate
Skeletal muscle
What gives rise to skeletal muscle and most connective tissue in MSK?
Mesoderm cells
What are two types of connective tissue in the musculoskeletal system?
- ) Connective tissue proper
2. ) Specialized connective tissue
Includes dermis, tendon, or ligament
Connective tissue proper
Specialized connective tissue includes
Bone and cartilage
Immature embryonic connective tissue made up of fibroblast-like cells surrounded by ECM
Mesenchyme
Differentiate into cells of connective tissue
Mesenchymal cells
Soon after gastrulation in week 3 of development, mesoderm further differentiates and sets the stage for development of the
MSK
Bilateral columns of PARAXIAL MESODERM form on on either side of the midline and further differentiate into
Somites
Within each block/segment, somitic mesoderm cells differentiate and reposition into the
- ) Sclerotome
- ) Dermatome
- ) Myatome
Forms the axial skeleton
Sclerotome
Forms the dermis of the skin
Dermatome
Form skeletal muscles of the body wall and limbs
Myotome
The mesoderm divides into the
Paraxial and lateral plate mesoderms
Bilateral columns of lateral plate mesoderm form in the lateral-most parts of the embryo and divides into which two layers?
- ) Parietal/somatic mesoderm
2. ) visceral/splanchnic mesoderm
Like the somite’s mesenchymal cells, SOMATIC MESENCHYMAL CELLS form
Connective tissues
Forms connective tissues associated with the midline
Somitic mesoderm
Forms connective tissues away from the midline that include cartilage, bones, tendons, and ligaments of the appendicular skeleton (plus sternum)
Somatic mesoderm
Skeletal muscle cells ALL initially originate from
Somite myotome cells
Thus, myotome cells (originally from somite) migrate to the somatic mesoderm to form
Skeletal muscles in anterolateral trunk and limbs
Folow the course of the migrating myotome cells
Nerves
More specifically, somatic motor axons from a spinal cord segment level extend with the migrating somite
mesoderm cells from the
Same somite level
Neural crest cells also form connective tissues in the
Head
Forms the cartilage, bone, and tendons/ligaments of the axial skeleton, but not the sternum
Sclerotome
Forms the cartilage, bone, and tendons/ligaments of the appendicular skeleton, PLUS the sternum
Somatic mesoderm
In direct ossification, or intramembranous ossification, bone-forming osteoblasts can lay down bone directly on
Mesenchymal membranes
In indirect ossification, or endochondral ossification, bone-forming osteoblasts lay down bone along a previously formed
Cartilaginous skeletal template
Sclerotome mesenchymal cells migrate to surround the neural tube and notochord. Vertebrae develop from sclerotome of
Two adjacent somites
Forms the annulus fibrosis of the intervertebral disc
Sclerotome
Forms the nucleus pulposus of the intervertebral disc
Notochord
Mesenchymal cells form chondrocytes which form a cartilaginous vertebrate that is quickly replaced by bone via
Endochronal ossification
Derived from scleretome cells that make up the coastal processes of the developing thoracis vertebrae
Ribs
Fibroblasts make
Dermis, tendons, and ligaments
Forms bone, cartilage, and tendons/ligaments associated with the vertebral column, ribs, and flat bones of posterior skull
Sclerotome
The sternum develops independently of the rest of the axial skeleton and is formed from
Somatic mesoderm
As the ribs grow laterall and ventrally from the 12 thoracic vertebrae toward the midline, they fuse with
Coastal cartilages
In 1% of the population, cervical ribs can be seen attached to
C7 vertebrae
The extra rib may impinge on nerves and arteries associated with innervation into the upper limb, this is called
Thoracic outlet syndrome
A rare defect that results from incomplete fusion between the sternal mesenchyme bars
-results in an area of soft tissue and skin between the sternal segments
Cleft sternum
In addition to neurulation, induces formation of the vertebral column around the spinal cord
Notochord
Many of the flat bones of the skull form from
Somite mesoderm
Much of the other components of the skull form from
Neural crest associated with pharyngeal arches
