BGDA Lecture - Development of the Embryo/Fetus 1 1 Introduction 1.1 UNSW Embryology 1.2 The Developing Human: Clinically Oriented Embryology 1.3 Larsen's Human Embryology 1.4 BGDA Practical Classes 2 Human Reproductive Cycle 3 Gametogenesis 3.1 Male 3.2 Female 4 Fertilization 4.1 Fertilization Site 4.2 Fertilization - Spermatozoa 4.3 Fertilization - Oocyte 5 Week 1 and 2 5.1 Week 2 Implantation 5.2 Early Placenta 6 Week 3 Gastrulation 7 Notochord 8 Somitogenesis 9 Week 4 9.1 Neuralation 10 Cardiogenesis 10.1 Blood Islands 10.2 Blood Formation 10.3 Red Blood Cells 11 Early Placentation 12 Abnormalities 13 Glossary Links
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BGDA Lecture - Development of the Embryo/Fetus 1 · 1.4 BGDA Practical Classes 2 Human Reproductive Cycle 3 Gametogenesis 3.1 Male 3.2 Female 4 Fertilization 4.1 Fertilization Site
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BGDA Lecture - Development ofthe Embryo/Fetus 1
1 Introduction1.1 UNSW Embryology1.2 The Developing Human: Clinically Oriented Embryology1.3 Larsen's Human Embryology1.4 BGDA Practical Classes2 Human Reproductive Cycle3 Gametogenesis3.1 Male3.2 Female4 Fertilization4.1 Fertilization Site4.2 Fertilization - Spermatozoa4.3 Fertilization - Oocyte5 Week 1 and 25.1 Week 2 Implantation5.2 Early Placenta6 Week 3 Gastrulation7 Notochord8 Somitogenesis9 Week 49.1 Neuralation10 Cardiogenesis10.1 Blood Islands10.2 Blood Formation10.3 Red Blood Cells11 Early Placentation12 Abnormalities13 Glossary Links
In medicine foundations you were given a broad overview of humandevelopment. Now in BGDA we will be working through the humandevelopment process in more detail, focussing on key events.
2017 Lecture PDF
Begin by reviewing the recent Foundations Lecture and Practical.This BGDA lecture covers conceptus development from fertilization toimplantation to trilaminar embryo formation.
Note that fertilization and week 1 concepts have already beencovered in an earlier BGDA lecture.
The lecture will also introduce early fetal membranes andplacentation.
1 Minute Embryology | UNSW theBox
Lecture Archive
Textbooks
UNSW Embryology
The Developing Human: ClinicallyOriented Embryology
Schoenwolf, G.C., Bleyl, S.B., Brauer, P.R., Francis-West,P.H. & Philippa H. (2015). Larsen's human embryology(5th ed.). New York; Edinburgh: Churchill Livingstone.(links only function with UNSW connection)
More Textbooks?
BGDA Practical Classes
Practical 3 - Fertilizationto Implantation
Practical 6 -Implantation to 8Weeks
Practical 12 -Fetal Period
Practical 14 - Placenta and FetalMembranes
Human Reproductive Cycle
Meiosis in gonad produces haploid gametes (egg and sperm)
Female Male
Menstrual Cycle a regular cycle ofreproduction (28 days)begins at puberty, release of 1 egg(oocyte) every cycleEndocrine controlled (HPG axis)Hypothalamus - Pituitary -Gonad
continuous production of sperm(spermatozoa)begins at puberty, release millionsof spermatozoaEndocrine controlled (HPG axis)Hypothalamus - Pituitary -Gonad
Seminiferous tubule cross-section and supporting cells
Human spermatozoa take about 48 days from entering meiosis untilmorphologically mature spermatozoa.
Spermatogonia - are the first cells of spermatogenesisPrimary spermatocytes - large, enter the prophase ofthe first meiotic divisionSecondary spermatocytes - small, complete the secondmeiotic divisionSpermatid - immature spermatozoaSpermatozoa - differentiated gamete
Links: spermatozoa | oocyte | MBoC - Figure 20-18. Influence of Sryon gonad development | Endocrinology - Comparative anatomy ofmale and female reproductive tracts
Fertilization
Oogenesis - 1 gameteproduced/meiosis + 3 polarbodies, meiosis is slow, 1 eggproduced and released atovulationSpermatogenesis - 4 gametesproduced/meiosis, meiosis isfast, 200-600 million spermreleased at ejaculation
Fertilization Site
Fertilization usually occurs in first 1/3 of uterine tube (oviduct,Fallopian tube)Fertilization can also occur outside uterine tube associated withAssisted Reproductive Technologies (IVF, GIFT, ZIFT...) and ectopicpregnancyThe majority of fertilized eggs do not go on to form an embryo
Capacitation - alteration of the spermatozoa metabolism andsurface proteinsSperm Binding - zona pellucida protein ZP3 acts as receptor forspermAcrosome Reaction - exocytosis of acrosome contents (Calciummediated) MBoC - Figure 20-31. The acrosome reaction that occurswhen a mammalian sperm fertilizes an egg
enzymes to digest the zona pellucidaexposes sperm surface proteins to bind ZP2
Membrane Fusion - between sperm and egg, allows sperm nucleipassage into egg cytoplasm
Fertilization - Oocyte
Membrane Depolarization - caused by sperm membrane fusion,primary block to polyspermyCortical Reaction - IP3 pathway elevates intracellular Calcium,exocytosis of cortical granules MBoC - Figure 20-32. How the corticalreaction in a mouse egg is thought to prevent additional sperm fromentering the egg
enzyme alters ZP3 so it will no longer bind sperm plasmamembrane
Meiosis 2 - completion of 2nd meiotic divisionforms second polar body (a third polar body may be formed bymeiotic division of the first polar body)
nodal cilia establish the embryo left/right axisaxial process extends from the nodal epiblast
Primitive streak - region of cell migration (gastrulation) from theepiblast layer forming sequentially the two germ cell layers (endodermand mesoderm)
Gastrulation, (Greek = belly)
Means the formation of gut, but has been used in a more loosersense to to describe the formation of the trilaminar embryo. Theepiblast layer, consisting of totipotential cells, derives all 3embryo layers:
1. ectoderm2. mesoderm3. endoderm
The primitive streak is the visible feature which represents thesite of cell migration to form the additional layers. Historically,gastrulation was one of the earliest observable morphologicalevent occurring in the frog embryo.
The notochord is a structure whichhas an early mechanical role inembryonic disc folding and a majorsignaling role in patterningsurrounding embryonic tissuedevelopment. This signaling rolepatterns many different tissues(neural plate, neural tube, somites,endodermal organs). It has its ownsequence of development from aprimitive axial process and is a developmental feature not present in theadult anatomy.
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axial process an initial epiblast hollow epithelial tube which extendsin the midline from the primitive pit, cranially in the embryonic disc(toward the oral membrane).
neuroenteric canal is a transient communication between theamnionic cavity and the yolk sac cavity formed by the axialprocess.
notochordal plate forms from the axial process merging with theendoderm layer.notochord forms from the notochordal plate which then separatesback into the mesoderm layer as a solid column of cells lying in themidline of the embryonic disc and running rostro-caudally (head totail).
An alternate name for the notochord is "axial mesoderm".
Mesoderm means the "middlelayer" and it is from this layer thatnearly all the bodies connectivetissues are derived. In earlymesoderm development a numberof transient structures will form andthen be lost as tissue structure ispatterned and organised. Humansare vertebrates, with a "backbone",and the first mesoderm structure wewill see form after the notochordwill be somites.
Coelom, meaning "cavity", and major fluid-filled cavities can be seen toform both within the embryo (|intraembryonic coelom) and outside theembryo (extraembryonic coelom).
The intraembryonic coelom is the single primitive cavity that lieswithin the mesoderm layer that will eventually form the 3 majoranatomical body cavities (pericardial, pleural, peritoneal).
Somite initially forms 2 maincomponents
sclerotome - (ventromedial)forms axial skeleton - vertebralbody and intervertebral discdermomyotome -(dorsolateral) forms dermisand skeletal muscle
The Human Heart from day 10 to 25 (scanning electron micrograph)
forms initially in splanchnic mesoderm of prechordal plate region -cardiogenic region
growth and folding of the embryo moves heart ventrallly anddownward into anatomical position
week 3 begins as paired heart tubes that fuse to form single heart tubebegins to beat in Humans- day 22-23
Blood Islands
2 populations of cellsperipheral- form endothelial cells that form thelining of all blood vesselscore- form blood cells (haemocytoblasts)
all vessels (arteries and veins)appear initially the same
Blood Formation
blood formation from stemcells occurs initially in theextra-embryonic mesoderm ofthe yolk sacthen later (week 5) throughout embryonic mesenchymeblood stem cells then migrate into the liver
The only cells in the blood areinitially entirely fetal red blood cells(RBC).
These red blood cells differ fromadult red blood cells in:
may remaining nucleatedcontain fetal haemoglobin - hasdifferent oxygen and carbondioxide binding characteristics
Links: Basic CardiacEmbryology
Early Placentation
The trophoblast layer has nowdifferentiated into twomorphologically distinct cellularlayers.
Syncitiotrophoblasts - forma multinucleated cytoplasmicmass by cytotrophoblast cellfusion and both invade thedecidua and secrete hCGCytotrophoblasts - form a cellular layer around the blastocyst,proliferates and extends behind syncitiotrophoblasts
Early Utero-Placental exchange - transfer of nutrition from maternallacunae filled with secretions from uterine glands and maternal blood fromblood vessels. The development of trophoblast villi extending into the
Cite this page: Hill, M.A. (2018, May 3) Embryology BGDA Lecture -Development of the Embryo/Fetus 1. Retrieved fromhttps://embryology.med.unsw.edu.au/embryology/index.php/BGDA_Lecture_-_Development_of_the_Embryo/Fetus_1