| Thymus histogenesis |
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| • | During
downward (caudal) growth lumina are lost. |
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| Epithelium
is transformed into solid branching |
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| cords:
future thymic
lobules |
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| • | Densely
packed epithelium develops into loose |
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| reticulum
into which lymphocytes appear |
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| • | 12th week: development of well-defined medulla |
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| and cortex |
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| • | Hassall’s corpuscles in medulla: ectodermally |
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| derived
(third pharyngeal groove?) |
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