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EMBR YOLOGY Eilidh Middleton ILOS List List the three developmental stages of life before birth and give their timings Describe Describe important early embryological events:cleavage,blastocyst formation and implantation Explain Explain gastrulation and list the major derivatives of the three germ layers Describe Describe the extraembryonic membranes,the amnion,the chorion and the yolk sac Summarise Summarise the key steps in development between weeks 1-3 Explain Explain the concept of cell potency ILOS List List the three developmental stages of life before birth and give their timings Describe Describe important early embryological events:cleavage,blastocyst formation and implantation Explain gastrulation and list the major derivatives of the three germ layers Explain Describe Describe the extraembryonic membranes,the amnion,the chorion and the yolk sac Summarise Summarise the key steps in development between weeks 1-3 Explain Explain the concept of cell potencyThree developmental stages of life before birth Week 1 – Preimplantation stage (before it is implanted into the uterus) Weeks 2-8 – Embryonic stage (Organogenesis (formation of organs)) Weeks 9-38 (term) – Foetal stage (Growth and development) ILOS List List the three developmental stages of life before birth and give their timings Describe Describe important early embryological events:cleavage,blastocyst formation and implantation Explain Explain gastrulation and list the major derivatives of the three germ layers Describe Describe the extraembryonic membranes,the amnion,the chorion and the yolk sac Summarise Summarise the key steps in development between weeks 1-3 Explain Explain the concept of cell potencyWEEK 1 Fertilisation occurs Fusion of the nuclei of the sperm cells and the nuclei of the oocyte creating the zygote. DAY 0 Zygote is defined as a fertilised oocyte Occurs at the ampulla (widest part of the fallopian tube) DAY 0 -> 7 After fertilization the zygote will travel through the remainder of the fallopian tubes,dividing as it goes (cleavage) until it gets to the uterus Cleavage is defined as the mitotic division of the fertilisedoocyte progenies (the progenies of the mitotic division are known as blastomeres) Each blastomere is half the size as the fertilisedoocyte in order for the overall physical size of the zygote to remain the same to allow passage of the zygote down the isthmus (the narrowest 1 2 4 8 16 part of the fallopian tube) MORULA 16-cell stage is called the morula All the cells are still identical and totipotent Occurs around the day 4 mark Because the overall zygote size does not get bigger – the space between the blastomeres is minimal they start to form a cluster of cells Between the cells in the cluster are tight junctions – function in cell communication Around the same time the morula enters the uterus BLASTOCYST Occurs around day 5 mark Occurs when the morula (16 cell zygote) continues to undergo mitotic divisions A zygote with greater than 16 cells is known as a blastocyst Blastocyst is a rapidly mitotic dividing cluster of cells Cell differentiation occurs The zona pellucida starts to disappear in preparation for implantation Eventually, the zona pellucida will burst and the blastocyst will “hatch” from it. Blastocyst Trophectoderm Inner cell mass Blastocyst cavity • Form the outer layer of the blastocyst • Located in the inner part of the • Fluid filled cavity located in the blastocyst inner part of the blastocyst • Will go on and contribute to the formation of the placenta. • Will go on to form the embryo and • Contains amino acids, growth factors extraembryonic tissues and water • Exerting pressure onto the Remember there is a fluid filled blastocyst cavity surrounding components, including the zona pellucida. BLASTOCYST HATCHING The blastocyst will eventually ”hatch” from the zona pellucida for two reasons 1. Pressures from the blastocyst cavity 2. cell proliferation increases the blastocyst in size Why is this necessary? 1. Demand for nutrients 2. Implantation Summary ofWeek One • Fertilisation occurs at the ampulla of the fallopian tube • The oocyte undergoes mitotic division (identical cells are form) from 1 to 2 to 4 to 8 to 16 • At the 16 cell stage the oocyte is known as a morula • It is at the morula stage that the cells are clustered together and tight junctions form between them • This 16 cell ball divides further forming the blastocyst • At the blastocyst stage the cells differentiate into trophectoderm and inner cell mass. A cavity forms in the blastocyst (known as the blastocyst cavity) • As the blastocyst increases in physical size it burst from the zona pellucidaWEEK 2 Prior to Implantation Occurs just prior to implantation The inner cell mass organise themselves into two layers: 1. 1) Epiblast 1. Outer layer that consists of columnar cells 2. 2) Hypoblast 1. Inner layer composed of cuboidal cells A basement membrane presents itself between the layers Implantation Takes place between day 7-12 after fertilisation Signalling factors from the implanting embryo and endometrium Trophoblast cells are the first to invade The trophoblast cells are very invasive and “bury” themselves into the endometrium, via the release of enzymes This is the most you need to know about the placenta formation at this moment What’s happening with the inner cell mass Already organised into two layers (hypoblast and epiblast) Together these layers are known as the bilaminar embryonic disc Two cavities form around either side of the bilaminar disc (two of the three extra- embryologic membranes) ILOS List List the three developmental stages of life before birth and give their timings Describe Describe important early embryological events:cleavage,blastocyst formation and implantation Explain Explain gastrulation and list the major derivatives of the three germ layers Describe Describe the extraembryonic membranes,the amnion,the chorion and the yolk sac Summarise Summarise the key steps in development between weeks 1-3 Explain Explain the concept of cell potency Extra -Embryologic Membranes Amnion Yolk Sac Chorion • Continuous with the epiblast • Continuous with the hypoblast • Made up of trophoblast and • Fills with amniotic fluid that • Important for nutrient transfer to extraembryonic mesoderm (forms functions to protect the developing the embryo in weeks 2-3 (until the from the epiblast layer) embryo placenta is fully established and th• Important in the formation of the • When this burst – first sign of labourembryo then gets the nutrients from foetal component of the placenta the mother) • Disappears during development • Disappears during development (around week 20) Summary ofWeekT wo TROPHOBLAST CELLS INNER CELL MASS • First to invade the endometrium • Organise themselves into two layers (epiblast starting off the process of and hypoblast) – together known as the implantation bilaminar embryonic disc • Differentiate into two layers: • Extra embryonic membranes start to form: • Syncytiotrophoblast • Amnion (continuous with epiblast layer) • Yok Sac (continuous with hypoblast layer) • Cytotrophoblast • Chorion ILOS List List the three developmental stages of life before birth and give their timings Describe Describe important early embryological events:cleavage,blastocyst formation and implantation Explain gastrulation and list the major derivatives of the three germ layers Explain Describe Describe the extraembryonic membranes,the amnion,the chorion and the yolk sac Summarise Summarise the key steps in development between weeks 1-3 Explain Explain the concept of cell potencyWEEK 3 Gastrulation Rule of 3 • Week 3 • Formation of the three layers (trilaminar) from 2 (bilaminar) Gastrulation is defined as the process of cell division and migration resulting in the formation of the three germ layers Derivatives from the germ layers Ectoderm Mesoderm Endoderm (Makes the outer layer of the (Makes the MIDDLE layer of (Makes the INNER layer of the body) the body) body) Epidermis of skin Dermis of skin Epithelial lining of the Muscular system digestive tract, respiratory Nervous system (CNS and PNS) system Epithelial lining of mouth and Skeletal system Gut and digestive organs (liver, anus Notochord pancreas) Cornea and lens of eye Thymus Reproductive system Thyroid and parathyroid glands Summary ofWeekThree Gastrulation! Gastrulation begins with the formation of the primitive streak at the caudal end (moves towards the cranial end) The epiblast layer of the bilaminar disc invaginates at the primitive streak – these cells either: Move down and outwards -> forming the mesoderm layer Displace the hypoblast cells -> forming the endoderm layer ILOS List List the three developmental stages of life before birth and give their timings Describe Describe important early embryological events:cleavage,blastocyst formation and implantation Explain Explain gastrulation and list the major derivatives of the three germ layers Describe Describe the extraembryonic membranes,the amnion,the chorion and the yolk sac Summarise Summarise the key steps in development between weeks 1-3 Explain Explain the concept of cell potency Cell potency = ability of the cell to differentiate into other cell types Totipotent Pluripotent Multipotent Completely undifferentiated More differentiated than Most differentiated of the totipotent but not as much as three Can become everything multipotent All cells of the body+ Placenta Can become cells of a All cells of the body particular cell type i.e. embryonic and extra- embryonic structures Example: inner cell mass Example: Haematopoietic cells Example: zygoteHAEMATOPOIETIC CELLSQuiz Q1: What is the name of the first developmental stages of life before birth A) Foetal stage B) Blastocyst stage C) Implantation stage D) Embryonic stage E) Pre-implantation stage Q2: In what week does implantation occur A)Week 1 B)Week 2 C)Week 3 D)Week 4 E)Week 5 Q3: Which layer of the bilaminar disc differentiates into the 3 germ layers A) Epiblast layer B) Hypoblast layer C) Mesoderm layer D) Cytotrophoblast layer E) Inner cell mass Q4: The primitive streak begins to form initially at which end of the embryo A) Distal end B) Proximal end C) Posterior end D) Caudal end E) Cranial end Q5: The yok sac is continuous with which layer of the bilaminar disc A) Epiblast layer B) Hypoblast layer C) Mesoderm layer D) Cytotrophoblast layer E) Inner cell mass Q6: The yok sac is continuous with which layer of the bilaminar disc A) Epiblast layer B) Hypoblast layer C) Mesoderm layer D) Cytotrophoblast layer E) Inner cell mass Q5: The yok sac is continuous with which layer of the bilaminar disc A) Epiblast layer B) Hypoblast layer C) Mesoderm layer D) Cytotrophoblast layer E) Inner cell mass Q6: Which structure surrounds the zygote preventing it from preimplantation A) Zona glomerulosa B) Zona pellucida C) Zona reticularis D) Zona fasciculata E) None of the above Q7: The epidermis of the skin is derived from which germ cell layer A) Ectoderm B) Endoderm C) Mesoderm D) Epiblast E) Hypoblast Q8: Which of the following is an example of a pluripotent cell A) Zygote B) Blastomere C) Inner cell mass D) Mesenchymal stem cells E) Haematopoietic stem cellsAnswers Q1: What is the name of the first developmental stages of life before birth A) Foetal stage B) Blastocyst stage C) Implantation stage D) Embryonic stage E) Pre-implantation stage Q2: In what week does implantation occur A)Week 1 B)Week 2 C)Week 3 D)Week 4 E)Week 5 Q3: Which layer of the bilaminar disc differentiates into the 3 germ layers A) Epiblast layer B) Hypoblast layer C) Mesoderm layer D) Cytotrophoblast layer E) Inner cell mass Q4: The primitive streak begins to form initially at which end of the embryo A) Distal end B) Proximal end C) Posterior end D) Caudal end E) Cranial end Q5: The yok sac is continuous with which layer of the bilaminar disc A) Epiblast layer B) Hypoblast layer C) Mesoderm layer D) Cytotrophoblast layer E) Inner cell mass Q6: Which structure surrounds the zygote preventing it from preimplantation A) Zona glomerulosa B) Zona pellucida C) Zona reticularis D) Zona fasciculata E) None of the above Q7: The epidermis of the skin is derived from which germ cell layer A) Ectoderm B) Endoderm C) Mesoderm D) Epiblast E) Hypoblast Q8: Which of the following is an example of a pluripotent cell A) Zygote B) Blastomere C) Inner cell mass D) Mesenchymal stem cells E) Haematopoietic stem cellsGood ResourceTHANKYOUAND GOOD LUCK!