Comparison between tract and nerve

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  Comparison between tract and nerve Nerve Nerve contain bundle of axons (nerve fibers) in the peripheral nervous system (PNS) that transmits signals between the central nervous system (CNS) and other parts of the body. It is the most important part of peripheral nervous system Location: Peripheral nervous system. Structure: Made of axons, connective tissue layers (endoneurium: axon is surrounded by it , perineurium: bundle of axons are surrounded by it, epineurium: whole nerve trunk is surrounded by it), and blood vessels. It gets myelination by Schwann cell   Function: Carries sensory signals to the CNS (afferent) from peripheral part of body or motor signals carry from from the CNS (efferent) to target organs like skeletal muscle, cardiac muscle, smooth muscle or glands Example: Sciatic nerve, median nerve, except optic nerve (though optic nerve is technically part of CNS in develop...
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Difference between somitomere and somite of paraxial mesoderm

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Difference between somitomere and somite

Somitomeres and Somites: Key Differences and Development

Both somitomeres and somites originate from the paraxial mesoderm and play crucial roles in the formation of the axial skeleton, muscles, and dermis. 



1. Somitomeres

🔹 Definition: Somitomeres are loose masses of mesoderm that appear in a cranial-to-caudal sequence along the embryonic axis before forming somites.

🔹 Characteristics:

  • First seen in the paraxial mesoderm during the third week of development.
  • Less organized than somites.
  • Found in the head and neck region (first 7 pairs do not form somites).
  • Contribute to craniofacial muscles, pharyngeal arches, and extraocular muscles.

🔹 Fate:

  • The first 7 pairs contribute to head and neck structures.
  • Caudal somitomeres condense into somites in the occipital and trunk regions.

2. Somites

🔹 Definition: Somites are well-organized, paired blocks of mesoderm that develop from somitomeres in the occipital to sacral region.

🔹 Characteristics:

  • Begin forming around day 20 in a cranial-to-caudal sequence.
  • Total of 42–44 pairs of somites form, but some later regress.
  • Highly organized compared to somitomeres.

🔹 Fate: Somites differentiate into three major components:

  1. Sclerotome → Forms vertebrae and ribs.
  2. Dermatome → Forms dermis of the skin.
  3. Myotome → Forms skeletal muscles of the body and limbs.


Somitomere

Somite

Loosely organized segmented collections of paraxial mesoderm in the cranial region

Epithelial balls of cells formed in segmented pairs along the neural tube from paraxial mesoderm

It form muscles and bones of the face and skull

It forms vertebrae, muscles of the back and body wall and dermis of skin

It has no subdivision

It has three subdivision dermatome myotome sclerotome

It is situated cranially

It is situated caudally

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