Embryology Introduction Behrouz Mahmoudi 1 Gross Form Correlation

Embryology Introduction Behrouz Mahmoudi 1

Gross Form, Correlation Errors in development Structural Molecular (Clinical aspects) Real life aspects (“non medical”) Embryology Cellular and Molecular phenomena Evolutionary mirror 2

Fertilisation • • • Ovum and Oocyte Second meiotic division Site of fertilization Sperm : Acrosomal reaction Zona pellucida Single sperm… 3

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Fertilisation • Completion of meiosis II – Polar bodies (Also dealt with in Week 11) • • Restoration of diploidy Initiation of cleavage Chromosomal sex determination Genetic and evolutionary implications – Paternal and maternal chromosomes – Further mixing during next gametogenesis (crossing over) 5

Cleavage Nuclear-cytoplasmic ratio Division by reduction of cell volume Planes of cleavage Polarity…? Inner cell mass and trophoblast formed later • Transport through the uterine tube • Zona pellucida… retained until implantation • • 6

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Blastocyst • Inner cell mass • Trophoblast – Invasive nature • Zona pellucida disappears • Implantation 8

Trilaminar Embryo • • • Epiblast and Hypoblast ≠ entoderm! Primitive streak Cell migration Mesoderm and entoderm Y E H H 9

Notochord & Neural Tube • Notochord – Axis – Induction… • Neural plate – Neural groove – Neural tube and crest 10

Divisions of Mesoderm • Paraxial n Intermediate n Lateral plate 11

Segmentation • Paraxial mesoderm • “Somites” • Each somite – Sclerotome – Dermomyotome 12

Fate of Germ Layers • Ectoderm – Epidermis (not skin!) ++ • Mesoderm – Muscle, Connective (including skeletal) tissues – Some epithelial structures • Parts of urogenital systems • Endothelium of heart and blood vessels • “Serous membranes” • Endoderm – Lining epithelium of digestive system 13

Head, Tail and Lateral Folds • Flat three-layered embryo … three dimensional tube • Endoderm encloses a cavity …digestive system • Changes in the relative positions of certain regions • Further correlation with the development of the heart and digestive system 14

The Developing Human – Three Phases • • Early development Embryonic phase – organogenesis The foetal period Differentiation, Histogenesis and Functional development 15