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The First Three Weeks of Human Embryogenesis Department of Histology, Cytology and Embryology Kharkiv State Medical University Week 1-3: Early Stages: • 1. Fertilization • 2. Cleavage • 3. Gastrulation • 4. Formation of the embryo body • Late stages: Histogenesis, Organogenesis Week 1 • 1.Fertilization – is the fusion of the sperm and ovum = Zygote formation Fertilization. Cleavage Cleavage 2 cells stage 3-5 cells stage Morula Blastula . . uterine tube Fertilization (positive rheotaxis, gyno-androgamones, capacitation, acrosomal reaction) = zygote uterus Implantation Day 6 - 8 Week 1 • 2.Cleavage – is the division of the zygote in the uterine tube = Blastula formation (2 cell stage – 4 cell stage …16-32 cell stage (morula) – blastocyst) • During cleavage the germ is moved to the uterus At the end of the cleavage outer cells (trophoblast) involve nutritive fluid, which forms cavity, moving inner cells to the periphery (embryoblast). Blastula is formed. Inner Cell Mass (embryoblast) Trophoblast Blastocoele Week 2. 3. Gastrulation Gastrulation by the delamination and migration leads to formation of three germ layers: ectoderm, endoderm and mesoderm, and axial organs: notochord, neural tube, somites Early Gastrulation. At the beginning of gastrulation germ sinks into the uterine wall – implantation. Gastrulation begins from delamination – division of embryoblast into two germ layers - ectoderm and endoderm, forming embryonic disc and two sacs – ectoblast and endoblast Delamination Trophoblast Ectoblast Embryonic disc: Ectoderm Endoderm Endoblast Migration - extraembryonic mesoderm formation From embryonic disc cells begin to migrate which form extraembryonic mesoderm, surrounding upper and lower sacs, and underlying trophoblast As a result appear amnion, yolk sac and chorion (extraembryonic organs) Week 2 Extraembryonic Mesoderm Trophoblast Ectoderm Chorion Amnion Endoderm Yolk Sac Late gastrulation – migration continues within embryonic disk and leads to formation of the embryonic mesoderm (third germ layer) and axial organs (neural tube, notochord and somites - 43, 44 pairs) Transverse section. Moving ectodermal and endodermal cells form group of cells = primitive streak Amniotic Cavity Embryonic mesoderm Ectoderm Primitive streak Yolk Sac Endoderm • Notochord is formed from invaginated ectoderm and primitive streak Notochord formation (longitudinal section) Development of the Neural Tube by the invagination of ectoderm Development of the Neural Tube Neural groove Development of the Neural Tube Development of the Neural Tube Development of the Neural Tube Surface Ectoderm Neural Crest Neural Tube Neural tube formation Neural tube formation • Somites locate between ectoderm and endoderm, arise from primitive streak and notochord and • consist of mesoderm Axial organs –neural tube, notochord, somites (mesoderm) Neural tube Somite Notochord Formation of the Body Axis Left – Right Sidedness Left – Right sidedness • Situs inversus –20% (Kartagener syndrome) • Dysfunctional cilia • Respiratory problems • Male fertility problems 4. Formation of the embryo body (20 days) Body flexion, head and tail folds formation. Amnion accumulates fluid and increases, yolk sac decreases. Formation of a gut, allantois Body flexion formation Body flexion formation (Gut formation. The upper part of yolk sac forms tube inside the forming body = gut - primitive digestive tube gut Body flexion formation (Gut. Allantois – is extention of hindgut) Allantois FUTURE PLACENTA Body formation hindgut foregut midgut Cloacal Plate Oral Plate Heart Allantois Yolk Sac Body formation Chorionic plate participates in the formation of placenta Uterine Wall Chorionic Cavity Yolk Sac and allantois participate in the formation of ambilical cord What should we study by heart ? EXTRAEMBRYONIC ORGANS chorion Amnion • Allantois Yolk sac • FUTURE PLACENTA Extraembryonic organs (supportive, nutritive) • Amnion – protective bag of water • Yolk sac – gut, germs of gametes first blood vessels, cells • Allantois – urinary bladder • Chorion – protection, hormones, placenta • Placenta – main nutritive, protective, hormonal Differentiation of GERM LAYERS: 1. Differentiation of Ectoderm A. Surface Ectoderm B. Neural Tube 2. Differentiation of Endoderm A. G.I. Tract B. Respiratory Tree C. Endocrine glands 3. Differentiation of Mesoderm A. Somites-dermatome, myotome, sclerotome B. Intermediate- nephrotome C. Lateral-splanchnotome D. Mesenchyme Surface Ectoderm differentiates to skin, its derivatives, oral, rectal epithelium, corneal epithelium, tooth enamel amnion ectoderm proctodeum stomatodeum Ectoderm forms neuroectoderm –neural tube (neurons and neuroglia of the brain and the retina) and neural crests (nerve ganglia, neuroglia, adrenal medulla, melanocytes, APUD-system).---15-20 weeks Surface Ectoderm Neural Crest Neural Tube Endoderm differentiates to epithelium of the stomach, intestine, liver, pancreas, respiratory, endocrine systems -- 3-4 weeks - gut endoderm gut Endoderm (gut, allantois) foregut hindgut midgut Cloacal Plate Heart Oral Plate Stomatodeum Allantois Yolk Sac Mesoderm - formation of the first blood vessels in the wall of yolk sac and allantois blood vessels Mesoderm Ectoderm Amniotic Cavity (Somite) Intermediate mesoderm (nephrotome) Lateral plate mesoderm (somatopleuric, splanchnopleuric mesoderm) Endoderm Yolk Sac Notochord dermatome - dermis of skin Somite myotome - muscles, sclerotome skeleton, except skull Mesoderm urogenital system including kidneys, gonads, ducts, and accessory glands Intermediate Mesoderm. Nephrotome Lateral Mesoderm - serous membranes of pleura, pericardium and peritoneum Mesenchyme (loose part) – connective tissue of viscera and limbs, blood and lymph cells, cardiovascular and lymphatic systems Chorionic plate participates in the formation of placenta Uterine Wall Chorionic Cavity Yolk Sac and allantois participate in the formation of ambilical cord Late embryonic stages • Histogenesis • Organogenesis