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Orthopaedic embryology

FORMATION OF TRILAMINAR EMBRYO

Week 3

GASTRULATION is the formation of the trilaminar embryonic disc from a bilaminar disc. EPIBLAST becomes ECTODERM (dorsal), HYPOBLAST becomes ENDODERM (volar)

The PRIMITIVE STREAK at the dorsal aspect of the embryonic disc is formed. The cranial end thickens to form the PRIMITIVE KNOT, defining the cranial end. (Primitive streak remnants can form Teratomas)

The primitive streak produces MESENCHYMAL (mesoblastic) cells destined to form the MESODERM by blending endoderm and ectoderm. This will produce connective tissue, bone, blood vessels, blood cells, muscles and the G.U. system.

ECTODERM forms the NOTOCHORD - at the cranial end of the PRIMITIVE STREAK. The Notochord is the structure around which the spinal cord will develop (remnants can form Chordomas).

NEURULATION - The Notochord induces the overlying ectoderm to form the NEURAL PLATE (the primordium of the CNS). The thickened plate then invaginates centrally to form the NEURAL GROOVE, with thickened NEURAL FOLDS (day 18). The folds fuse dorsally forming the NEURAL TUBE, which will become the spinal cord.

As the neural folds fuse some of the ectodermal cells lying along the crest of each neural fold attach to neighbouring cells & become the NEURAL CREST cells, which differentiate to begin the formation of the peripheral nervous system, the automatic nervous system and Schwann cells.

SOMATISATION - SOMITES are formed from MESODERM and they begin to line both sides of the NOTOCHORD. Eventually they will form 42-44 pairs. The SOMITES continue their developmental process and soon become a lateral dermatome, a medial myotome and a ventral scleratome. This becomes in due time, the basis of the skin, muscle and skeletal elements, respectively.




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EMBRYONIC PERIOD

Weeks 4 to 6


Weeks 4 to 6


Weeks 4 to 6

The embryo FOLDS laterally & transversally forming a C-shaped cylindrical embryo.

The APICAL ECTODERMAL RIDGE exerts an inductive influence on the limb mesenchyme that promotes growth & development of the limbs.

UPPER LIMBS - At this time the limb buds also develop. The upper extremity, with pronated forearms, appears first - and begins to rotate externally. For more detail see Upper limb Congenital Disorders

LOWER LIMBS - The lower extremity appears slightly later than the upper extremity and begins to rotate internally.

Developmental disturbances during this period gives rise to major congenital malformations. Early suppresion of limb development causes AMELIA (complete abscence of a limb); Late suppresion causes MEROMELIA (partial abscence).

Early suppresion of limb development causes AMELIA (complete abscence of a limb); Late suppresion causes MEROMELIA (partial abscence).

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