3rd WEEK DEVELOPMENT Embryology 1 1.pptx

Embryology of the third week of development

1. 3 rd WEEK DEVELOPMENT : Trilaminar Germ Disc 3/16/2024 1

2. Gastrulation The most characteristic event occurring during the third week of gestation Is the process that establishes three definitive germ layers of the embryo (ectoderm , mesoderm , and endoderm ) These three germ layers give rise to all the tissues and organs in the body It begins with formation of the primitive streak on the surface of the epiblast 3/16/2024 2

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4. The primitive streak consists of the primitive groove, primitive node, and primitive pit Caudal to the primitive streak is the future site of the anus called cloacal membrane Cranially is Prochordal plate to form the future mouth 3/16/2024 4

5. Cells of the epiblast migrate toward the primitive streak Upon arrival in the region of the streak they slip beneath it. This inward movement is called invagination . 3/16/2024 5

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7. Some invaginated cells displace the hypoblast and create endoderm Others lie between the epiblast and the newly created endoderm to form mesoderm The remaining epiblast cells form the ectoderm The ectoderm, mesoderm , and endoderm of the trilaminar disk are all derivative of epiblast 3/16/2024 7

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9. CHANGES INVOLVING INTRAEMBRYONIC MESODERM Paraxial mesoderm : thick plate of mesoderm located on each side of the midline - Organized into segment to form somites - Somites differentiate into: Scleretome Myotome Dermatome 3/16/2024 9

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11. 2. Intermediate mesoderm : longitudinal dorsal ridge of mesoderm located between the paraxial & lateral plate mesoderm 3. Lateral plate mesoderm is a thin plate of mesoderm located along the lateral sides of the embryo 3/16/2024 11

12. DERIVATIVES OF THE GERM LAYER Ectoderm: The ectoderm gives rise to organs and structures that maintain contact with the outside world : ● The skin epidermis, including the hair and nails ● The central nervous system; ● The peripheral nervous system ● The sensory epithelium of the ear, nose, and eye ● Subcutaneous glands ● The mammary glands ● The pituitary gland ● Enamel of the teeth 3/16/2024 12

13. Mesoderm: Paraxial: form somites Scleretome : cartilage, ligaments, tendons and bones Myotome : skeletal muscles Dermatome: dermis and subcutaneous tissue of skin 3/16/2024 13

14. Intermediate mesoderm: longitudinal ridge This ridge forms the urogenital ridge, which is involved in the formation of the future kidneys and reproductive system Lateral plate mesoderm: Endothelium of blood and lymph vessels, Red blood cells and white blood cells, smooth muscle, and spleen 3/16/2024 14

15. 3. Endoderm: The endoderm gives rise to structures that maintain contact with the inner body ● The epithelial lining of the respiratory tract, GIT ● The parenchyma of the thyroid, parathyroids , liver , lungs and pancreas ● The reticular stroma of the tonsils and the thymus ● The epithelial lining of the urinary bladder and the urethra ● The epithelial lining of the tympanic cavity and auditory tube 3/16/2024 15

16. TUMOUR ASSOCIATED WITH GASTRULATION Sacrococcygeal teratomas Remnants of primitive streak persisting in the sacrococcygeal region The cells are pluripotent, hence contain the tissues derived from all the 3 germ layers 3/16/2024 16

17. Sacrococcygeal teratoma 3/16/2024 17

18. Sirenomelia