Embryo Tutorial 3 M2M Tutoring Slides (1) PDF

Fertilisation 🡪 Gastrulation Prenatal Blastocyst/Pre-embryonic First 2 weeks Embryonic – ORGANOGENESIS occurs, teratogenic effects impact here Week 3 to 8 Foetal Months 3 to 9  Abnormal gametes: Down syndrome GAMETOGENESIS...

Fertilisation 🡪 Gastrulation Prenatal Blastocyst/Pre-embryonic First 2 weeks Embryonic – ORGANOGENESIS occurs, teratogenic effects impact here Week 3 to 8 Foetal Months 3 to 9  Abnormal gametes: Down syndrome GAMETOGENESIS (trisomy 21), non-disjunction Each gamete is a haploid cell Two kinds of normal sperm: 23,X and 23,Y There is only one kind of secondary oocyte: 23,X SPERMATOGENESIS Type A Spermatogonia Type B Primary Spermatocytes Secondary Earl y Spermatids Lat e Residual Bodies Spermatozoa Kartagener Syndrome Oogenesis: Differentiation of PGCs into oogonia begins shortly after their arrival in the ovary. Oogonia give rise to primary oocytes that enter prophase of the first meiotic division. This prophase stage may last 40 or more years and finishes only when the cell begins its final maturation.  OVULATION Anovulation, Polycystic Ovarian Syndrome Corona radiata Zona Pellucida Corpus hemorrhagicu Secondary Oocyte m Angiogenesis of blood The secondary oocyte (arrested in metaphase vessels in theca interna of meiosis II) is released from the Graafian follicle and ruptured germinal epithelium of the ovary, flows into the uterine tubes.  2 blastomeres Main results of fertilization: Day 1 Restoration of the diploid number of chromosomes Determination of the genetic sex (x or y chromosome carrying sperm) of the individual Initiation of cleavage of zygote to form blastomeres 7 COMPACTION Compacted blastomeres cell outlines are indistinct Morula – 16 cells- same size as initial zygote Blastocyst The first sign of the trophoblast is in the blastocyst  Early implantation: ~6-7 days, the blastocyst attaches to endometrial epithelium Trophoblast: Proliferates and splits into: Synctiotrophoblast: Invades the epithelium into connective tissue stroma of endometrium Will produce human chorionic gonadotropin to maintain corpus luteum cytotrophoblast Inner cell mass epiblast Bilaminar disk hypoblast Implantation of blastocyst in abnormal site results in ectopic pregnancy Summary: Week 2 of blastocystic development – (week of two’s) 1. Embryoblast /inner cell mass splitting into two germ layers: the epiblast and hypoblast (lamination- bilaminar disk) 2. Trophoblast /outer cell mass divides into two layers: inner cytotrophoblast and outer syncytiotrophoblast 3. Two cavities appear- the amniotic cavity and the blastocyst cavity 4. Blastocyst cavity is remodeled twice to form the primary/primitive and secondary yolk sacs 5. Extraembryonic mesoderm splits into two layers: parietal and visceral layers 12  Embryonic period Week 3 of development- Gastrulation Gastrulation During gastrulation, the bilaminar embryonic disc is converted into a trilaminar embryonic disc. It is characterized by changes in cell shapes, rearrangement, movement 13  Gastrulation establishes the three definitive germ layers of the embryo (ectoderm, intraembryonic mesoderm, and endoderm), forming a trilaminar embryonic disk These three germ layers give rise to all the tissues and organs of the adult. Formation of the primitive streak is the 1st step Primitive streak consists of: primitive groove, primitive node, and primitive pit. cloacal membrane is the future site of the anus where the epiblast and hypoblast cells fuse. Ectoderm, intraembryonic mesoderm, and endoderm of the trilaminar embryonic disk are all derived from the epiblast. 14  TRILAMINAR DISK DOES NOT give rise to 3 germ cell layers because they do other things too. Only epiblast gives rise to the three layers Gastrulation (formation of all 3 germ layers/trilaminar disk) 1. Epiblast cells form walls that form the primitive streak (Presence of definitive primitive streak is first sign of gastrulation) 2. Primitive streak elongates to form: Endoderm -Primitive node -Primitive pit -Primitive groove 3. Primitive streak determines embryo’s superior-inferior, ventral-dorsal, right- left directions 15  Gastrulation- 4. Epiblast cells form a mesenchyme-like /watery-like cell matrix) and pass through primitive streak 5. First population of epiblast 🡪 primitive streak 🡪 intraembryonic endoderm Hypoblasts cells 🡪 extraembryonic endoderm to line the blastocytic cavity 🡪primary yolk sac 6. Second population of epiblast cells Endoderm 🡪primitive streak 🡪 intraembryonic mesoderm between epiblast and endoderm layers -Mesoderm is induced by the endoderm to differentiate 5. Sacrococcygeal teratoma, is a tumor that arises from remnants of the primitive streak sacrococcygeal teratoma Gastrulation- 7. Third population of medially- located epiblast cells will be induced by notochord to form neuroectoderm and detach from epiblast layer forming the neural tube and neural crest cells 8. Fourth and remaining population of epiblast cells don’t migrate and form intraembryonic ectoderm 17 Epiblast forms all 3 germ layers Modification of the Trilaminar Disc Intraembryonic mesoderm Changes: Paraxial mesoderm- Organised into Somitomeres; Sclerotome forms the cartilage and bone components of the vertebral column. Myotome forms epimeric and hypomeric muscles. Dermatome forms dermis and subcutaneous area of skin. 19  Trilaminar disk 1. Ectoderm (Ectoderm+ Neuroectoderm) A. Ectoderm= ectoderm/ epithelium of skin B. Neuroectoderm- Neural tube= CNS, retina C. Neuroectoderm- Neural crest cells= PNS, face (viscerocranium), mouth, nose, pharynx, larynx 2. Mesoderm (Paraxial, intermediate, lateral plate) A. Paraxial = somites/somitomeres divided into: Sclerotome= Bones of torso, neurocranium Dermomyotome= Back dermis and all skeletal muscles B. Intermediate mesoderm=Urogenital system C. Lateral plate mesoderm divided into: -Parietal (**)= Limb bones, sternum, rest of dermis, parietal layers of pericardium, pleura and peritoneum -Visceral (*)= Smooth and cardiac muscles, visceral layers of pericardium, pleura and peritoneum, vasculature 3. Endoderm-lining of GI and respiratory tract Layers of the trilaminar disc Summary Summary: Week 3 of embryonic development What is the importance of these 3 landmark events that happen in this week- 1. Appearance of (definitive) primitive streak in the epiblast layer 2. Development of notochord and beginning of nervous system (neurulation) 3. Differentiation of three germ layers (gastrulation) to form the trilaminar disk These 3 events allow for: Establishment of embryo orientation Neurulation Gastrulation