ANA301 Human Embryology - Implantation & Early Intra-Embryonic Development PDF

: room name - BENTLEYANATOMY ANA301 – Human Embryology Dr. Danielle Bentley, PhD Assistant Professor, Teaching Stream...

: room name - BENTLEYANATOMY ANA301 – Human Embryology Dr. Danielle Bentley, PhD Assistant Professor, Teaching Stream Division of Anatomy, Department of Surgery, Faculty of Medicine University of Toronto Unit 1 – Foundations Topic 3: Implantation and Early Intra-Embryonic Development : [email protected] : ATLASresearchLab.ca ANA301H. Dr. Bentley. KIN*1030-2015. D. Bentley.  Resources KL Moore et al. (KLM) The Developing Human Clinically Oriented Embryology. 121h edition. All images and readings are from this book, unless otherwise stated TW Sadler (TWS) BM Carlson et al. (BMC) RL Drake et al. (Gray’s) Langman’s Medical Human Embryology and Gray’s Anatomy for Embryology Developmental Biology Students 14th edition. 6th edition. 4th edition. Especially useful for the first 8weeks Especially useful for molecular General anatomy resource of development signalling pathways ANA301H. Dr. Bentley.  T3: learning outcomes By the end of this learning activity, students will be able to: – list and explain the sequential changes that occur in the formation of a blastocyst from a zygote – summarize the extra-embryonic events of implantation – summarize the intra-embryonic events of bilaminar disk formation – describe the sequential cell movements during gastrulation distinguish and compare the epiblast from the ectoderm distinguish and compare the hypoblast from the endoderm – list and describe each step in the formation of the notochord – describe the sequential cell movements during neurulation – align events of gastrulation, notochord formation, and neurulation to specific dates ANA301H. Dr. Bentley. Intra-embryonic Development - Week One: Travel from the site of fertilization to the site of implantation while the zygote cleaves - Week Two: Implantation, with bilaminar disk, cavity formation - Week Three: Gastrulation and early Neurulation Image: RCC Fertility, used with permission Week One  Cleavage of the Zygote Mitotic divisions produce blastomeres. Each blastomere is totipotent. – def: has the ability to develop into both intra- 2-cell-stage, ~30hrs embryonic and extra-embryonic structures. Blastomeres are surrounded by the Zona Pellucida (ZP) as they travel along the uterine tube towards the uterine cavity. 4-cell-stage, ~40hrs After the 8-cell stage, compaction begins… 8-cell-stage, ~2.5 days ANA301H. Dr. Bentley. Image: KLM 2.18  Compaction Begins after the 8-cell-stage (between the 3rd and 4th cleavage). Loosely arranged blastomeres “compact” and adhere to form a tightly packed ball; outer cells flatten and surround the inner cells. Two Four ~Twelve blastomeres blastomeres blastomeres Late compaction ANA301H. Dr. Bentley. Image: BMC 4.3  Compaction results in: 1. Trophoblast (aka, outer cell mass) Will form extra-embryonic tissues such as the placenta. Cells make no contribution to the embryo proper. 2. Embryoblast (aka, inner cell mass) Will form the intra-embryonic tissues such as the embryo proper, amniotic membrane, and the lining of the primitive umbilical vesicle. compaction ANA301H. Dr. Bentley. Image: TWS 3.9  = when there are ~16 blastomeres ANA301H. Dr. Bentley. Image: KLM 2.17 Uterus Fundus Uterine Tube Isthmus Ampulla Body Infundibulum Frimbriae ANA301H. Dr. Bentley. Image: BMC, 2.2  Formation of the Blastocyst (outer cell mass) ~day 3 morula enters uterine cavity ~day 4 fluid enters morula, creating blastocyst cavity (aka, blastocoele) Conceptus is now called a blastocyst – Cavity displaces the embryoblast to the embryonic pole Blastocyst is still contained with the (degenerating) zona pellucida ANA301H. Dr. Bentley. Image: KLM 2.17  https://www.youtube.com/watch?v=ExYZ3JZQhUo Blastocyst (~5 days), showing hatching Between ~day 4 and ~day 6 the blastocyst hatches. Facilitated by proteolytic enzymes from the trophoblast which degenerate the zona pellucide. ANA301H. Dr. Bentley. Image: BMC 3.3 Week Two  Implantation Events of blastocyst implantation occur between day 6 and day 10 Day 6 trophoblast overlying embryonic pole contacts the uterine mucosa – Blastocyst is captured from the uterine cavity by the uterine epithelium. ANA301H. Dr. Bentley. Image: KLM 2.20 Day 7 Trophoblast cells adjacent to the uterine mucosa proliferate and penetrate uterine tissue, fusing to form a multinucleated mass of cytoplasm called the syncytiotrophoblast – The remaining trophoblast, the cytotrophoblast, is mitotically active. Newly formed cells either remain in the cytotrophoblast OR migrate into syncytiotrophoblast ANA301H. Dr. Bentley. Image: KLM 2.20 Day 8 The syncytiotrophoblast continues to erode uterine tissue via proteolytic enzymes, enabling the blastocyst to burrow itself. It will: – erode local uterine blood vessels in uterine tissue when it contacts them. – produces Human Chorionic Gonadotropin (hCG), which is absorbed into maternal blood and excreted in maternal urine ANA301H. Dr. Bentley. Image: KLM 3.1 Day 9 Vacuoles appear in the syncytium – Vacuoles will fuse to form lacunae – Called the lacunar stage of trophoblast development ANA301H. Dr. Bentley. Image: KLM 3.1 Day 10 Blastocyst embeds deep into uterine tissue = official “implanted”” – Surface defect in endometrial epithelium is preliminarily closed by closing plug (fibrin coagulum of blood) ANA301H. Dr. Bentley. Image: KLM 3.2  Cross section of embryo with surrounding uterine tissue, day 12 Days 10 and 11 – adjacent lacunae fuse, forming lacunar networks, filled with embryotroph (blood + gland cellular debris). Days 11 and 12 – maternal blood from eroded endometrial BVs flows through lacunar networks. This initiates uteroplacental “circulation”. Days 12 and 13 – defect in endometrial endothelium is repaired. ANA301H. Dr. Bentley. Image: KLM 3.2  ~day 27 (gestational age) endometrial endothelium repaired Ovarian cycle * Menstrual cycle * ~day 21 (gestational age) first contact ANA301H Dr. Bentley Image: KLM 2.11 Clinical Case - Ectopic Pregnancy Frontal (coronal) section. Anterior view. Uterus, left uterine tube, and left ovary, illustrating ectopic pregnancy locations and the likelihood of each When the embryo implants outside the uterine cavity. Incidence: 1 in 100 pregnancies, >95% are in the uterine tube. ANA301H. Dr. Bentley. Image: BMC 3.19 Tubal Pregnancy Hatched blastocyst implants in the ampulla or isthmus of uterine tube. Typically caused by uterine tube pathologies which delay tubal transit time. Risk of rupture and life-threatening maternal haemorrhage. Treatment: surgical removal. Frontal (coronal) section. Anterior view. Uterus, left uterine tube, and left ovary, illustrating an ectopic pregnancy in the ampulla of the uterine tube. ANA301H. Dr. Bentley. Image: KLM 3.8  The Bilaminar Embryonic Disc This flat, two-layered disk will ultimately become all intra-embryonic tissues Formation begins as the blastocyst is implanting; between ~day 7 and day 14 Once complete, the bilaminar embryonic disk with have two fluid-filled spaces on either side of it ANA301H. Dr. Bentley. Image: KLM 2.20, 3.2 Day 8 While the trophoblast layers are working on implantation, the… …embryoblast (inner cell mass) differentiates into two layers: – Hypoblast Layer: small cuboidal cells adjacent to the exocoelomic cavity Migrating hypoblast cells form the exocoelomic membrane (adjacent to the cells of the cytotrophoblast) The exocoelomic cavity is lined by the exocoelomic membrane and the hypoblast – Epiblast Layer: high columnar cells adjacent to the amniotic cavity Migrating epiblast cells separate form the amnion (adjacent to the cells of the cytotrophoblast) The amniotic cavity is lined by the amnion and the epiblast ANA301H. Dr. Bentley.  Also called: Primary umbilical vesicle Day 8 ANA301H. Dr. Bentley. Image: KLM 3.1 Day 9 While adjacent lacunae are fusing… A fine, loose connective tissue, first appears from the exocoelomic membrane, called extraembryonic mesoderm – Surrounds the amnion and the exocoelomic cavity/primary umbilical vesicle ANA301H. Dr. Bentley.  = epiblast + hypoblast Day 10 ANA301H. Dr. Bentley. Image: KLM 3.1 Day 12 We know that blastocyst is almost completely embedded, although the defect is not yet fully healed While maternal blood is flowing through lacunar networks… Cavities form within the extraembryonic mesoderm, called extraembryonic coelomic spaces ANA301H. Dr. Bentley.  Day 12 ANA301H. Dr. Bentley. Image: KLM 3.1 Day 13 We know the embedded blastocyst slightly protrudes into uterine lumen While the endometrial defect is healing … Extraembryonic coelomic spaces fuse, creating the extraembryonic coelom (chorionic cavity) – Extraembryonic coelom/chorionic cavity surrounds the amnion and the primary umbilical vesicle EXCEPT at the connecting stalk Primary umbilical vesicle decreases in size, changing its name to the secondary umbilical vesicle The extraembryonic mesoderm is now named according to location: – Extraembryonic somatic mesoderm: adjacent to cytotrophoblast + covers amnion – Extraembryonic splanchnic mesoderm: covers umbilical vesicle ANA301H. Dr. Bentley.  starting to form getting squished Day 13 ANA301H. Dr. Bentley. Image: KLM 3.5 Day 14 We know that the newly pregnant person would except their period today While they (potentially) learn of the pregnancy… As the extraembryonic coelom squishes the primary umbilical vesicle into the secondary umbilical vesicle, a portion of the primary vesicle may persist as an exocoelomic cyst Intra-embryonic cells are still the shape of a bilaminar disk – However, hypoblast at the cranial end become tall columnar cells, forming the prechordal plate – site of future mouth ANA301H. Dr. Bentley.  Day 14 ANA301H. Dr. Bentley. Image: KLM 3.5  Day ??, Low Magnification Day ??, High Magnification ANA301H. Dr. Bentley. Image: KLM 3.4 Week Three  Gastrulation Most characteristic event of week 3 Transformation of the gastrula from bilaminar disk → trilaminar disk Begins with the formation of the primitive streak at the caudal end of epiblast – Clearly visible by day 15/16 – Sets up each axis of the body Exocoelomic cavity Cranial tip of the primitive streak is the primitive node, a slightly elevated area surrounding a small primitive pit Caudal Cranial end end Exocoelomic membrane ANA301. Dr. Bentley. Image: TWS, 5.1, 5.2  Epiblast cells migrate towards the primitive streak, detach from epiblast, and slip inwards – Process called invagination Cells then undergo an epithelial-mesenchymal transition (lining the amnion) Exocoelomic membrane (lining the exocoelomic cavity) Horizontal-section of the bilaminar disk, showing invagination and the cellular epithelial-to-mesenchymal transformation, ~day 16 Dynamic cell migration process that results in three germ cell layers ANA301. Dr. Bentley. Image: TWS, 5.2  Some of these invaginating/migrating cells displace hypoblast cells and form a single layer of cells called the endoderm Most of the invaginating/migrating cells end up in between endoderm and epiblast and forming the mesoderm Non-migrating epiblast cells now form the ectoderm Endoderm Cross section of early embryo ANA301. Dr. Bentley. https://www.youtube.com/watch?v=3AOoikTEfeo  Cranial During gastrulation, as cells invaginate they proliferate and migrate throughout the embryonic disk Cells that invaginate through the primitive node migrate cranially along the midline (dark yellow) Primitive streak is active until the early part of the 4th week. – It disappears by the end of the 4th week (aka day 28). Caudal Birdseye view of Dorsal Surface, during gastrulation ANA301. Dr. Bentley. Image: BMC 6.6  Cell proliferation and migration during gastrulation will elongate the (still flat) early embryo Dorsal views, during gastrulation ANA301. Dr. Bentley. Image: KLM 4.6  Notochord Formation Notochord resembles a rigid, rod-like structure that stretches along the longitudinal axis of the embryo General functions: – provides rigidity and support to the embryonic disk – serves as the primary inductor (signaling center) in the early embryo induces neurulation provides signals for future musculoskeletal development indicates the future site of vertebral bodies assists in defining embryo axis The notochord degenerates as the bodies of the vertebrae are formed – Small portions persist as the nucleus pulposus of each intervertebral disc ANA301. Dr. Bentley. Image: TWS 6.8 Formation of Notochordal Process Epiblast cells that invaginate through the primitive node will migrate cranially along the midline The cells form a cord-like structure, the notochordal process, extending between the primitive node and the prechordal plate The prechordal plate is a small, circular area of columnar endodermal cells at the cranial end of the embryo where ectoderm and endoderm are fused (no mesoderm in between) – Plate contributes endodermal cells to the oropharyngeal membrane (future site of the oral cavity) Notochordal process soon acquires a lumen: the notochordal canal ANA301. Dr. Bentley.  Dorsal View of embryonic disk, ~day 16 ANA301. Dr. Bentley. Image: KLM 4.8  Mid-sagittal (median) section, medial view. Early embryo showing formation of the Notochordal Process ~day 16 ANA301. Dr. Bentley. Image: KLM 4.8  Mid-sagittal (median) section, medial view. Early embryo showing formation of the Notochordal Process ~day 18 (early) ANA301. Dr. Bentley. Image: KLM 4.8  Prechordal plate Umbilical vesicle Mid-sagittal (median) section, medial view. Early embryo showing formation of the Notochordal Process ~day 18 (middle) ANA301. Dr. Bentley. Image: KLM 4.8 Transformation of Notochordal Process Floor of the notochordal process fuses with the underlying embryonic endoderm – Fused cells degenerate, creating holes that grow/merge as the floor disappears – Notochordal canal is obliterated as well Amniotic cavity (dorsally) is now in direct contact with the umbilical vesicle (ventrally) via the neurenteric canal Roof of the notochordal process becomes the notochordal plate Beginning at the cranial end, cells at the lateral edges of the notochordal plate proliferate and bilaterally fold ventrally – Fusion creates the notochord Endoderm fills in ventrally, reestablishing the continuous cellular layer ANA301. Dr. Bentley.  Mid-sagittal (median) section, medial view. Early embryo showing formation of the Notochordal Process ~day 18 (middle/late) ANA301. Dr. Bentley. Image: KLM 4.9  Mid-sagittal (median) section, medial view. Early embryo showing formation of the Notochordal Process ~day 18 (late) ANA301. Dr. Bentley. Image: KLM 4.9  Neurulation Represents the beginning of nervous system development Starts as the notochord develops; completed by the end of week 4 The notochord induces the overlying ectoderm to thicken into the neural plate ~day18, the lateral edges of the neural plate (called neural folds) elevate dorsally – Tip of each neural fold contains neural crest cells which well remain separate from the neural tube – In between the two neural folds is the neural groove The bilateral neural folds continue to elevate and bend medially towards each other. Fusion creates the neural tube, separating the: – Neuroectoderm (green) – Surface Ectoderm (blue/grey) ANA301. Dr. Bentley.  Neural plate Ectoderm ANA301. Dr. Bentley. Image: Bidmos ‘16  Neural folds Neural crest Neural groove ANA301. Dr. Bentley. Image: Bidmos ‘16  Cranial Neuropore Neural canal Neural crest Neural tube Caudal Neuropore ANA301. Dr. Bentley. Image: Bidmos ‘16  closes on day25 Cranial Migrating Neuropore Neural Crest Cells Caudal Neuropore closes on day27 ANA301. Dr. Bentley. Image: Bidmos ‘16  Title ANA301. Dr. Bentley.  Results in the separation of the Neuroectoderm from the Surface Ectoderm – Neuroectoderm: becomes the neural tube Neural Crest Cells from the tips of the neural folds migrate within mesoderm – Surface ectoderm: become the epidermis of the skin ANA301. Dr. Bentley. Image: TWS 6.8 Clinical Case: Neural Tube Defects Can result from disruption to neurulation; typically at a neuropore Failure of the cranial neuropore: meroencephaly to anencephaly – Lethal defect, detected early in pregnancy when imaging is available – Local neural tissue is exposed to amniotic fluid and degraded ANA301. Dr. Bentley.  Week Monday Lecture Friday Lecture T1: Preparation for Success in Jan 6 – 10 T2: Gametogenesis and Fertilization Embryology T3: Implantation and Early Intra- T4: Placentation and Early Extra- Jan 13 – 17 Embryonic Development Embryonic Development T5: Embryonic Folding and Body Jan 20 – 24 T6: Stem Cells (with Dr. Duval) Cavities Unit 1 CA: Infertility and Clinical Jan 27 – Jan 31 T7: Staging, Aging, and Timing Embryology T8: Development of the Musculoskeletal Feb 3 – Feb 7 Unit 1 Term Test (27%) System - back ANA301H. Dr. Bentley.