Development of the Nervous and Musculoskeletal Systems for Limb Formation Flashcards by Joel Glotfelty

Begins soon after gastrulation, with differentiation of the ectoderm to form the neural tube

Neurulation

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Forms the nervous system

Neural tube

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Occurs with induction of ectoderm by factors from the underlying notochord (a midline rod of mesoderm)

Neurulation and neural tube formation

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Neuroectodermal cells organize in the midline as a thickening called the

Neural plate

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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)

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With further proliferation and more buckling, the edges of the neural folds approximate and fuse in the dorsal midline to form the

Neural tube

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Neural tube fusion starts in the cervical region and occurs as a

bi-directional “Zipper”

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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

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The open (unfused ends) of the neural tube are alled

Neuropores

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The final step of neurulation, which occurs by the end of week 4, is

Neuropore closure

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What day do the following neuropores close?

) Cranial
) Caudal

) Day 25

2. ) Day 28

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Forms the brain and spinal cord of the CNS

Neural Tube

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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

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Defects which mostly result from failure of the neuropores to close

Neural tube defects

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Failed closure of the cranial neuropore results in

Anencephaly

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Failure to form the cranial vault around the cerebral hemispheres

-often lethal

Anencephaly

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Failed closure of the caudal neuropores results in

Spina bifida

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A defective fusion of the vertebral arches, commonly at L4-S1

Spina bifida

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A defect in the fusion of vertebral arches without involvement of underlying neural tissue

Spina bifida occulta

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A defect in the fusion of vertebral arches WITH involvement of underlying neural tissue

Spina bifida Cystica

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In spina bifida cystica with meningocele, there is a protrusion of

Meninges

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In spina bifida cystica with meningomyelocele, there is protrusion of

Neural tissues w/ protruding meninges

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In spina bifida cystica with rachischisis (or myeloschisis), exposed nerve tissue is permanently damaged by

Amniotic fluid

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Most neural tube defects can be prevented by supplementation with

Folate

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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?

1. ) 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? 1. ) Mantle layer 2. ) Marginal layer

1. ) 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?

1. ) Epithelium 2. ) Connective tissue 3. ) Muscle 4. ) 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?

1. ) 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

1. ) Sclerotome 2. ) Dermatome 3. ) 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?

1. ) 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