Early Embryogenesis II - Canvas.pptx

Early Embryology II The Second and Third Week of Development Sarah Greene, PHD Outline for Today + Week 2: Bilaminar Disc + Week 3:Trilaminar Disc Ectoderm Mesoderm Endoderm  First Week Fertilization Cleavage Transport to Uterine Cavity...

Early Embryology II The Second and Third Week of Development Sarah Greene, PHD Outline for Today + Week 2: Bilaminar Disc + Week 3:Trilaminar Disc Ectoderm Mesoderm Endoderm  First Week Fertilization Cleavage Transport to Uterine Cavity Blastocyst Formation Beginning of Implantation The Big Second Week Picture Trophoblast Differentiation Bilaminar Embryo Amniotic Cavity Formation Implantation Continues Placenta Develops, Circulation Begins Chorionic Cavity Forms Primitive Streak Appears Third Week Gastrulation (Form Trilaminar Germ Disk) Notochord Formation Neural Plate, Groove and Crest Formation Appearance of Somites Third-Eighth Week Embryonic Period Placental Maturation Organogenesis (all systems) Greatest Sensitivity to Teratogenesis Ninth Week to Birth Fetal Period  Week 2 Trophoblast Trophoblast cells differentiate into: + Cytotrophoblast: Mitotically active cells that form outer wall + Syncytiotrophoblast: Located at site of implantation Produce enzymes to allow blastocyst to penetrate endometrium Produce Human Chorionic Gonatotropin (HCG) – basis for pregnancy tests HCG maintains corpus luteum (secretes estrogen and progesterone to maintain Moore, K. L., & Persaud, T. V. N. (2003). The Developing pregnancy) Human: Clinically Oriented Embryology: Saunders.  Week 2 Implantation + Embryonic pole leads the way + Syncytiotrophoblast invades uterine wall dissolving maternal tissues. + As walls of maternal blood vessels are dissolved, blood spills into spaces called lacunae. + Uterine lining closes behind implanting embryo. Moore, K. L., & Persaud, T. V. N. (2003). The Developing Human: Clinically Oriented Embryology: Saunders.  Week 2 Day 8 + Blastocyst has penetrated the endometrium about half way + Continued growth of syncytiotrophoblast + Formation of Amnionic Cavity Fluid-filled space appears within embryoblast Moore, K. L., & Persaud, T. V. N. (2003). The Developing Amnion encloses amnionic cavity Human: Clinically Oriented Embryology: Saunders.  Week 2 Day 8 Formation of Bilaminar Disk + Epiblast Gives rise to embryo proper and amnion + Hypoblast Gives rise to yolk sac and allantois Exocoelomic membrane will form from hypoblast cells Moore, K. L., & Persaud, T. V. N. (2003). The Developing Human: Clinically Oriented Embryology: Saunders.  Week 2 Day 9 + Deeply embedded Closed by fibrin coagulum + Formation of lacuna Enzymes break down syncytiotrophoblast  form fluid- filled spaces Bring oxygen and nutrients from maternal blood via diffusion + Exocoelomic cavity becomes primary yolk sac (Blastocyst cavity gave rise to Langman's Medical Embryology, 15e, 2024 exocoelomic cavity)  Week 2 Day 10 + Implantation is complete + Epithelial lining beginning to regenerate and grow over site of implantation  Week 2 Days 11-12 + Surface epithelium of uterus nearly fully covers site of implantation + Syncytiotrophoblast continues penetrating more deeply + Lacunae are forming a network and communicating with maternal sinusoids Establishes uteroplacental circulation Langman's Medical Embryology, 15e, 2024  Week 2 Days 11-12 + Cells of yolk sac form extraembryonic mesoderm + Develop cavities that coalesce to form extraembryonic cavity + Extraembryonic mesoderm forms: Splanchnic (visceral) layer (inner layer) Somatic (parietal) layer (outer layer) Langman's Medical Embryology, 15e, 2024  Week 2 Day 13 + Site of implantation has healed + Trophoblast is rapidly increasing in size Increased blood flow to lacuna can lead to bleeding (can be confused with menstrual cycle) Primary villi: cells of cytotrophoblast invade syncytiotrophoblast to form Langman's Medical Embryology, 15e, 2024 columns  Week 2 Day 13 + Hypoblast cells migrate to form secondary (definitive) yolk sac Primary yolk sac is pinched off + Formation of chorionic cavity from extraembryonic coelom Moore, K. L., & Persaud, T. V. N.. The Developing Human: Clinically Oriented Embryology: Saunders.  Week 2 Day 14 + Secondary (definitive) yolk sac + Connecting Stalk Develops from extraembryonic somatic mesoderm Future umbilical cord Moore, K. L., & Persaud, T. V. N.. The Developing Human: Clinically Oriented Embryology: Saunders.  Week 2 Day 14 + Prechordal plate Small, disk-like thickening of hypoblast Also called buccopharyngeal membrane Marks the anterior (head) end of the embryo (site of future mouth development) Moore, K. L., & Persaud, T. V. N.. The Developing Human: Clinically Oriented Embryology: Saunders.  Week 2 Day 14 + Prechordal plate Small, disk-like thickening of hypoblast Also called buccopharyngeal membrane Marks the anterior (head) end of the embryo (site of future mouth development) Langman's Medical Embryology, 15e, 2024 Small Group Discussion + Alyssa, a 30-year-old woman, has recently discovered she is pregnant. She is currently at 4 weeks of gestation (2 weeks post-fertilization) and is very curious about what is happening with her developing embryo during this stage. She has scheduled an appointment with her obstetrician to learn more about the early stages of pregnancy, specifically the second week of development. Discuss important events of the second week, including: Implantation Formation of the Bilaminar Disc Development of Extraembryonic Structures (primary yolk sac, extraembryonic mesoderm, chorionic cavity) Created with and Adapted from ChatGPT Practice Question A 39-year-old woman is undergoing an ultrasound examination in her second week of pregnancy. During this stage, the process of implantation is completed, and the blastocyst has differentiated into two distinct layers. Which of the following pairs correctly identifies these layers? A. Ectoderm and Mesoderm B. Epiblast and Hypoblast C. Mesoderm and Endoderm D. Syncytiotrophoblast and Cytotrophoblast E. Amnion and Chorion Created with and adapted from Chat GPT Practice Question During the second week of embryonic development, the bilaminar embryonic disc forms. This disc consists of two layers that will give rise to different tissues and structures. Which of the following correctly describes the role of the epiblast? A. It forms the primary yolk sac. B. It gives rise to the amniotic cavity. C. It develops into the future placenta. D. It contributes to the formation of the embryonic ectoderm, mesoderm, and endoderm. E. It produces hCG to maintain the corpus luteum. Created with and adapted from Chat GPT Practice Question The syncytiotrophoblast is an important structure that forms during the second week of embryonic development. Which of the following is a primary function of the syncytiotrophoblast? A. It forms the inner cell mass of the blastocyst. B. It produces enzymes that help the blastocyst implant into the endometrium. C. It differentiates into the epiblast and hypoblast. D. It gives rise to the extraembryonic coelom. E. It provides a structural framework for the developing placenta. Created with and adapted from Chat GPT  Week 3 Gastrulation + Gastrulation forms three primary germ layers + Sets stage for organ system development + Begins with formation of primitive streak (caudal) and primitive node (cranial) on surface of epiblast Langman's Medical Embryology, 15e, 2024 + Primitive pit is at the center of the primitive node  Week 3 Primitive Streak Day 16 + Allows for differentiation of cranial and caudal end of embryo + Cells of epiblast migrate towards primitive streak + Cells invaginate downward + Displace hypoblast cells EXCEPT at prechordal plate Langman's Medical Embryology, 15e, 2024  Week 3 Moore, K. L., & Persaud, T. V. N.. The Developing Human: Clinically Oriented Embryology: Saunders. Gastrulation + Epiblast cells give rise to all three primary germ layers: Ectoderm, Mesoderm, Endoderm + Hypoblast cells are replaced by endoderm  Week 3 Primary Germ Layers + Ectoderm Epidermis, nervous tissue, retina, and other tissue + Mesoderm Pleuripotent cells (can differentiate into Endoderm diverse cell types) Definitive Cartilage bone, muscle, connective tissue Yolk Sac + Endoderm Epithelial lining of respiratory and GI tract, glandular cells  Week 3 Primary Germ Layers Langman's Medical Embryology, 15e, 2024 + Mesoderm is between ectoderm and endoderm EXCEPT at: Oropharyngeal membrane (future oral cavity) Cloacal membrane (future site of anus)  Week 3 Notochord + Epiblast cells migrate into mesoderm layer + Form tube-like “Notochordal Process”  forms notochord + Notochord elongates, stopping its forward growth at the prechordal plate. + Transitions from tube to solid structure of cells Langman's Medical Embryology, 15e, 2024  Week 3 Notochord + Induces the development of the nervous system Langman's Medical Embryology, 15e, 2024 Degenerates at the site of development of the vertebra Remnant is nucleus pulposus within intervertebral disk (we will revisit in OS1) Moore’s Clinically Oriented Anatomy, 9e, 2023  Week 3 20 Days Ectoderm 18 Days + Induced by notochord + Neuroectoderm forms: Neural plate (forms at end of week 3)  neural tube  brain and spinal cord Neural crest cells (pleuripotent cells) Sadler et al. (2019). Langman's Medical Embryology 14E: Wolters Kluwer.  Neurulation + Fusion of the neural folds forms the neural tube and neural crest cells + We will revisit in OS1 (heart development) and OS3 (nervous system development) Sadler et al. (2019). Langman's Medical Embryology 14E: Wolters Kluwer.  18 Days Intraembryonic Mesoderm + Paraxial mesoderm 18 Days Somites and somitomeres (OS1) + Intermediate mesoderm Urogenital system (OS2) + Lateral plate mesoderm Pleural, pericardial, peritoneal cavities (OS1) Tissues of body wall, GI system, and limbs (OS1 and OS2) Moore, K. L., & Persaud, T. V. N. (2003). The Developing Human: Clinically Oriented Embryology: Saunders.  Somite Development + Paraxial mesoderm differentiates into clusters of cells (somites) on both sides + Occipital, cervical, thoracic, lumbar, sacral, and coccygeal somites AMBOSS  Somite Development + Sclerotome Cells migrate medially around notochord to form vertebral column and base of skull + Myotome Cells divide into two groups, epaxial and hypaxial and form most skeletal muscle of the body + Dermatome Cells migrate under ectoderm (epidermis) to form the dermis and subcutaneous tissue Sadler, T. W. (2004). Langman's Medical Embryology: Lippincott Williams & Wilkins.  Body Cavities + The intra-embryonic coelom is the primordium of the body cavity + The body cavity will give rise to the pericardial cavity, the pericardioperitoneal canals, and the peritoneal cavity during week 4 20 Days 19 Days Sadler, T. W. (2004). Langman's Medical Embryology: Lippincott Williams & Body Cavities + Each body cavity (pericardium, pleural, and peritoneal) is composed of both visceral and parietal serous membranes The visceral serous membrane is adhered to the surface of the organ, and is derived from splanchnic mesoderm. The parietal serous membrane lines the cavities, and is derived from somatic mesoderm. End of Week 4 Sadler, T. W. (2004). Langman's Medical Embryology: Lippincott Williams & Wilkins.  Cavities in the Embryo (by (by Dr. Dr. Paulsen) Paulsen) Blastocoel (lined by cytotrophoblast) Hypoblast cells proliferate and migrate to cover inner surface of cytotrophoblast (exocoelomic membrane) Exocoelomic cavity (Primitive yolk sac – lined by exocoelomic membrane) New layer of cells forms between exocoelomic membrane and cytotrophoblast (extraembryonic mesoderm) Primary yolk sac (lined by exocoelomic membrane) Fluid accumulates to open up spaces within extraembryonic mesoderm layer. Spaces eventually fuse to form a new space called the Extraembryonic coelom (or Chorionic cavity) between two layers of extraembryonic mesoderm called the splanchopleure and the somatopleure. Pole of primary yolk sac farthest from bilaminar germ disk pinches off and separates from original yolk sac to form a remnant of the primary yolk sac and leaving the remainder to be called the secondary yolk sac. Secondary yolk sac (lined by exocoelomic membrane) Gastrulation results in replacement of hypoblast by endoderm. Endodermal cells proliferate and migrate to replace cells of the exocoelomic membrane with endoderm. Definitive yolk sac (lined by endoderm) Blood and Blood Vessels + Vasculogenesis Mesodermal cells form clusters (blood islands) Blood islands form vessels (A-C) + Angiogenesis Vessels sprout from other vessels (D) Sadler, T. W. (2004). Langman's Medical Embryology: Lippincott Williams & Wilkins. Endoderm * * + Endoderm lining the definitive yolk sac becomes the lining of the embryonic gut tube through lateral (side-to-side) folding and cephalocaudal (head-to-tail) folding of the growing embryo. + Outgrowths of the developing gut tube form the respiratory system (lined by endoderm), the solid digestive glands, and the gall bladder. Allantois + Outpouching of yolk sac into connecting stalk + Appears at approximately Day 16 + Related to development of urinary system (we will Moore, K. L., & Persaud, T. V. N.. The Developing Human: Clinically Oriented Embryology: Saunders. revisit in OS2) Allantois + Outpouching of yolk sac into connecting stalk + Appears at approximately Day 16 + Related to development of urinary system (we will Moore, K. L., & Persaud, T. V. N.. The Developing Human: Clinically Oriented Embryology: Saunders. revisit in OS2)  End of Third Week Langman's Medical Embryology, 15e, 2024 Development of Placenta + End of second month + End of third month Amnion and chorion fuse Langman's Medical Embryology, 15e, 2024  Placenta (we will revisit in OS1) Moore, K. L., & Persaud, T. V. N. (2003). The Developing Human: Clinically Oriented Embryology: Saunders. Practice Questions During the third week of embryonic development, the process of gastrulation occurs, leading to the formation of the three primary germ layers. Which structure is primarily responsible for initiating this process? A. Notochord B. Primitive streak C. Somites D. Neural crest cells E. Amniotic cavity Created with and adapted from Chat GPT Practice Questions During the third week of embryonic development, the notochord plays a crucial role in the formation of various structures. Which of the following correctly describes the origin and function of the notochord? A. Originates from the hypoblast and induces the formation of the neural plate B. Originates from the primitive streak and provides signals for the development of the vertebral column and axial skeleton C. Arises from the epiblast and contributes to the formation of the extraembryonic mesoderm D. Develops from the trophoblast and initiates the formation of the primary germ layers E. Forms from the amnion and plays a role in the development of the umbilical cord Created with and adapted from Chat GPT Practice Questions A researcher studying early embryonic development notes the appearance of a rod-like structure that defines the body’s primary axis and provides signals for the development of surrounding tissues. Which structure is this researcher observing, and what is its significance? A. Neural tube, initiates the development of the central nervous system B. Notochord, serves as the primary signaling center for the development of the axial skeleton and central nervous system C. Somite, differentiates into skeletal muscle, vertebrae, and dermis D. Neural crest, contributes to the formation of peripheral nervous system structures E. Primitive node, organizes the formation of the primary germ layers Created with and adapted from Chat GPT Questions?