COT Embryology - Embryogenesis PDF

Early Embryogenesis Carnegie Stages 1-9 Weeks 1-3 Post Fertilization Jeannine Foley, PhD AN 722 COT Lecture Outline Definitions Staging embryonic development Reproductive Anatomy Overview Sperm meets oocyte Embryonic development Week 1 – Cleavage, implantation, formati...

Early Embryogenesis Carnegie Stages 1-9 Weeks 1-3 Post Fertilization Jeannine Foley, PhD AN 722 COT Lecture Outline Definitions Staging embryonic development Reproductive Anatomy Overview Sperm meets oocyte Embryonic development Week 1 – Cleavage, implantation, formation of the epiblast Week 2 – The week of 2’s (the bilaminar disc, and two cavities) Week 3 – Gastrulation (formation of the three primary germ layers) Early development of the three primary germ layers Ectoderm – most dorsal layer neuroectoderm Mesoderm – middle layer Notochord, paraxial mesoderm, intermediate mesoderm, lateral plate mesoderm, extraembryonic mesoderm Endoderm – most ventral layer Definitions Caudal: towards the tail C(o)elom: fluid-filled cavity Definitive: final structure Differentiate: distinct change in cell Dorsal: towards the back Lumen: cavity or channel within a hollow organ Primitive: early stage in development Primordial: early stage in development Rostral: towards the head Syncytium: multinucleated cell or network of fused cells Totipotent: capable of giving rise to any cell Saggital or midsaggital Ventral: towards the belly wikipedia Moore, KL; Persaud, T. V. N.; Torchia, Mark G. (2020) Before We Are Born, 9th ed. Fertilization and Gestational (Clinical) Age Embryonic period (Gestational age) Carlson, B.M. (2019) Human Embryology and Developmental Biology, 6th ed.  Carnegie Stages 23 stages cover the first 8 weeks post ovulation Based on morphological (actual) development Appearance of structures https://embryology.med.unsw.edu.au/embryology/index.php/Carnegie_Stages#Introduction  Carnegie Stages 1-4 Week 1 to early week 2 ~0.1mm in size Stage 1: Zygote. 2 pronuclei Stage 2: Morula. Stage 3: Blastocyst. Stage 4: Implantation begins are visible Blastomeres form via Embryoblast and mitosis trophoblast are present  Carnegie Stages 5-6 Week 2 Bilaminar disc ~0.1-0.2mm in size Secondary yolk sac Stage 6: Amnionic cavity, secondary yolk sac, and Stage 5: Embryo is fully implanted chorionic cavity are present. Bilaminar disc. in endometrium. Bilaminar disc. Primitive streak is visible. Gastrulation begins. Primitive yolk sac. Carnegie Stages 7-9 Week 3 Trilaminar disc 0.4mm to 2.5mm in size Stage 7: Gastrulation Stage 8: Notochord present. Stage 9: 1-3 somites stage. Head continues. Mesoderm forms. Formation of neural folds fold. Early heart. Early gut tube. Trilaminar disc. and groove. Primitive pit. Trilaminar disc.  congenital Critical Period of Development anomalies Teratogen – a substance that can cross the placental membrane and disrupt embryonic development Critical period – timeframe where embryo is most sensitive to developing congenital anomalies from teratogen exposure Weeks 3-8 is the period of organogenesis Weeks 2 is when rostral-caudal and left-right axes form Embryonic period Sadler, T.W. (2019) Langman’s Medical Embryology, 14th ed. Sagittal Section of the Pelvic Cavity Hansen, J. (2022) Netter’s Clinical Anatomy, 5th ed. Frontal Section of the Uterus Fundus Body (normal site of implantation of an embryo) Cervix Layers: Endometrium – mucosa (epithelium + underlying loose connective tissue) Myometrium – thick layer of smooth muscle Perimetrium – connective tissue (mostly serosa/some adventitia) Hansen, J. (2022) Netter’s Clinical Anatomy, 5th ed. The Endometrium Has Two Sub-layers 1. Functional layer –built and shed with each menstrual cycle Simple columnar epithelium Lining contains ciliated columnar cells Glands contain columnar secretory cells Lamina propria – loose, spongy connective tissue with rich ground substance 2. Basal layer – more cellular loose connective tissue, contains basal ends of glands, remains constant, contains stem cells for regeneration of the shed functional layer, not shed Mescher, A. L. (2021) Junqueira’s Basic Histology, 16th ed. Endometrium Prepares for Implantation 28  Ovaries and Uterine Tube Ovaries are the site of oogenesis Oocytes (ova) develop within growing follicles Haploid oocytes are produced via meiosis Oocyte is ovulated with zona pellucida and corona radiata Ovulation occurs on day 14 of a 28-day cycle Uterine tube contains fimbriae Ampulla is the enlarged region of the uterine Corona tube where fertilization often takes place radiata cells Sadler, T.W. (2019) Langman’s Medical Embryology, 14th ed. Sperm must make it to the oocyte within 24 hours of ovulation Carlson, B.M. (2019) Human Embryology and Developmental Biology, 6th ed. Zygote Formation Sperm penetrates corona radiata cells, zona pellucida, and finally the oocyte Female pronucleus is formed Sadler, T.W. (2019) Langman’s Medical Embryology, 14th ed. Male pronucleus is formed Male and female pronuclei Polar bodies copy their DNA and merge A zygote is a diploid cell, Male and female pronuclei formed by the fusion of two Zona pellucida haploid gametes Carlson, B.M. (2019) Human Embryology and Developmental Biology, 6th ed.  Early Embryogenesis C. Sperm and egg pronuclei replicate their DNA and merge D. & E. Diploid (23 from sperm, 23 from egg) cell undergoes mitosis - chromosomal sex of the embryo is established (XX or XY) F. Cell cleavage occurs – rapid cytokinesis without cell growth (zona pellucida restricts embryo size) – blastomeres become smaller with each division (blastomeres are totipotent) Sperm pronucleus cleavage Zona pellucida Egg pronucleus Polar bodies Sadler, T.W. (2019) Langman’s Medical Embryology, 14th ed.  Week 1 of Development Day 0 – Fertilization Day 1 – Cleavage Day 2.5 – 8 cell stage and compaction Day 3 – Morula (solid ball of 16+ cells) (in ampulla) Day 4 – Blastocyst (inner and outer cell mass are apparent) Blastocyst “hatches” out of zona pellucida Day 6 – Implantation of blastocyst between uterine glands Endometrium in secretory phase Fimbriae Carlson, B.M. (2019) Human Embryology and Developmental Biology, 6th ed.  Normal and Ectopic Sites for Implantation Schoenwolf, G.C. (2021) Larsen’s Human Embryology, 6th ed. Hansen, J. (2022) Netter’s Clinical Anatomy, 5th ed.  Week 1 of Development Zona pellucida (arrowheads) Cleavage 8 cell stage blastomeres Compaction – cells are held together by tight junctions Zona pellucida is gone Blastocele (cavity) 16-cell morula within a blastocyst Schoenwolf, G.C. (2021) Larsen’s Human Embryology, 6 th ed. Components of the Blastocyst Embryoblast (inner cell mass) gives rise to definitive embryo Trophoblast (outer cell mass) invades endometrium and contributes to placenta Day 4.5 Day 6 Sadler, T.W. (2019) Langman’s Medical Embryology, 14th ed. Components of the Bilaminar Disc Bilaminar disc – two-layer stage of embryonic development (week 2) - Epiblast – columnar dorsal layer - Hypoblast – cuboidal ventral layer Trophoblast: Cytotrophoblast – cells surround the developing embryo Syncytiotrophoblast – cells lose their membranes and become part of an interconnected syncytium Sadler, T.W. (2019). Langman’s Medical Embryology. Philadelphia: Wolters Kluwer. Week 2 of embryonic development: the week of 2’s Embryoblast forms two layers Epiblast – tall columnar cells adjacent to the amniotic cavity Hypoblast – cuboidal cells adjacent to the primitive/primary yolk sac Trophoblast forms two layers Cytotrophoblast Syncytiotrophoblast Two cavities form Amniotic cavity Primary (primitive) yolk sac forms first Schoenwol f, G.C. (2021) Larsen’s Human Embryology, 6 th ed.  Late Week 2: Connecting stalk hypoblast Trophoblast Extraembryonic (chorionic) cavity Secondary (definitive) yolk sac Forms early blood islands Contributes to early angiogenesis Sadler, T.W. (2019) Langman’s Medical Embryology, 14th ed.  Placenta: Embryonic Contributions End of 3rd week Primary villi have a cytotrophoblast core and are surrounded by an interconnected syncytiotrophoblast Secondary villi have a core of mesoderm in addition to the trophoblastic cells Tertiary villi mesodermal cells differentiate into blood vessels and contain blood End of 4th week Sadler, T.W. (2019) Langman’s Medical Embryology, 14th ed.  Placenta: Embryonic Contributions Progressive development of chorionic villi: Primary villi – outgrowth of cytotrophoblast Secondary villi – infiltration of extraembryonic mesoderm Tertiary villi – chorionic arteries and veins Schoenwol f, G.C. (2021) Larsen’s Human Embryology, 6th ed. Placenta: Parental Contributions Parental vessels penetrate the trophoblast to fill intervillous spaces (sinusoids) with blood Decidua basalis – the part of the endometrium that underlies the placenta Mescher, A. L. (2021) Junqueira’s Basic Histology, 16th ed.  Amniotic Cavity The amniotic membrane surrounds the developing embryo and fetus The amniotic cavity contains amniotic fluid Amniotic fluid volume is balanced and critical for normal development Early in development, amniotic cells and maternal tissues contribute to amniotic fluid Later in development, fetus drinks amniotic fluid and urinates to keep volume constant Standring, S. (2021) Gray’s Anatomy, 42nd ed.  The Bilaminar Disc Sets the Stage for Gastrulation Formation of primitive streak on dorsal surface of epiblast (facing amniotic cavity) Elevated primitive node at cephalic end surrounds primitive pit Gastrulation: epiblast gives rise to the three primary germ layers Ectoderm Mesoderm Endoderm Schoenwol f, G.C. (2021) Larsen’s Human Embryology, 6th ed. Bilaminar disc Trilaminar disc Week 3: Gastrulation Cells delaminate from the epiblast and undergo an epithelial-to-mesenchymal transition (EMT) Cells migrate ventrally (ingress) through Schoenwol f, G.C. (2021) Larsen’s Human Embryology, 6 th ed. the primitive streak Cells initially displace the hypoblast to become the definitive endoderm More cells migrate to make the mesoderm Cells in the mesoderm layer are mesenchymal cells Epiblast cells that are left behind become the definitive ectoderm Carlson, B.M. (2019) Human Embryology and Developmental Biology, 6th ed. Week 3: Gastrulation (Video)  R Week 3 of Development: Gastrulation C D Continues through the end of the 4th week The primitive streak reveals the three body axes: cranial-caudal, dorsal-ventral, left- V Sadler, T.W. (2019) Langman’s Medical Embryology, 14th ed. right Oropharyngeal membrane – future oral cavity Cloacal membrane – future anal canal Schoenwolf, G.C. (2021) Larsen’s Human Embryology, 6 th ed.  Establishment of the Notochord and other Important structures Mesoderm fills in between ectoderm and endoderm, but not entirely Oropharyngeal and cloacal membranes are where ectoderm and endoderm remain apposed Notochord (axial mesoderm) induces formation of the neural plate and other structures Prechordal mesoderm contributes to head development Extraembryonic mesoderm contributes to the chorion and connecting stalk Sadler, T.W. (2019) Langman’s Medical Embryology, 14th ed.  Formation of Embryonic Mesoderm – Fate Map of the Epiblast As epiblast cells migrate through the primitive node and streak, they populate different portions of the mesoderm layer Notochord (axial mesoderm) Paraxial mesoderm Intermediate mesoderm Lateral plate mesoderm Extraembryonic mesoderm Sadler, T.W. (2019) Langman’s Medical Embryology, 14th ed.  Extraembryonic mesoderm Mesoderm Differentiation in the trunk Notochord (axial mesoderm) Neural tube Paraxial mesoderm Intermediate mesoderm Lateral plate mesoderm - Somatic - Splanchnic Schoenwol f, G.C. (2021) Larsen’s Human Embryology, 6 th ed.  Induction of the Neuroectoderm Notochord – mesoderm structure that induces surface ectoderm to become neuroectoderm Surface ectoderm (becomes epidermis and other structures) Neuroectoderm (begins as the neural plate and becomes nervous system and other structures) Schoenwol f, G.C. (2021) Larsen’s Human Embryology, 6 th ed. Embryonic Lengthening During Gastrulation Oriented cell division Oriented cell migration Convergent extension Schoenwol f, G.C. (2021) Larsen’s Human Embryology, 6 th ed. Regression of the Primitive Streak Carlson, B.M. (2019) Human Embryology & Developmental Biology 6th ed. Schoenwol f, G.C. (2021) Larsen’s Human Embryology, 6 th ed. Congenital Anomalies Potentially Linked to Issues with Gastrulation Caudal Dysplasia/Caudal Regression Syndrome Sirenomelia Zion Clark Sacrococcygeal Teratoma Learning Objectives describe the basic anatomy of the female reproductive system relevant to pregnancy compare methods of staging human development describe the progression from oocyte to implanted blastocyst (week 1 of development) describe changes in embryonic and placental structures (week 2 of development) diagram the formation and regions of the trilaminar germ disc, amnion and yolk sac discuss anatomical factors that relate to embryonic nutrition and risk of birth defects