2010 BGD Lecture - Development of the Embryo/Fetus 1
- Begin by reviewing the recent Foundations Lecture and Practical.
- This lecture covers conceptus development from fertilization to implantation to trilaminar embryo formation.
- The lecture will also introduce early fetal membranes and placentation.
- Previous 2008 Lecture
BGD Cycle A 2010 Audio - Dr Mark Hill Thursday 6th May 2010 10-11am Clancy Auditorium (This recording is not the UNSW Clancy iLecture, available elsewhere).
The testes have two functions.
- produce the male gametes or spermatozoa
- produce male sexual hormone, testosterone (internal and external genitalia, sex characteristics)
- Spermatogonia - are the first cells of spermatogenesis
- Primary spermatocytes - large, enter the prophase of the first meiotic division
- Secondary spermatocytes - small, complete the second meiotic division
- Spermatid - immature spermatozoa
- Spermatozoa - differentiated gamete
Sertoli cells (support cells) Interstitial cells or Leydig cells (produce hormone)
The ovaries have two functions.
- produce the female gametes or oocytes
- produce female hormones, estrogen and progesterone (secondary sex characteristics, menstrual cycle)
Follicle cells (support cells) Theca cells (produce hormone)
- Oogenesis - 1 gamete produced/meiosis + 3 polar bodies, meiosis is slow, 1 egg produced and released at ovulation
- Spermatogenesis - 4 gametes produced/meiosis, meiosis is fast, 200-600 million sperm released at ejaculation
- Fertilization usually occurs in first 1/3 of uterine tube (oviduct, Fallopian tube)
- Fertilization can also occur outside uterine tube associated with In Vitro Fertilization (IVF, GIFT, ZIFT...) and ectopic pregnancy
- The majority of fertilized eggs do not go on to form an embryo
Fertilization - Spermatozoa
- Sperm Binding - zona pellucida protein ZP3 acts as receptor for sperm
- Acrosome Reaction - exyocytosis of acrosome contents (Calcium mediated) MBoC - Figure 20-31. The acrosome reaction that occurs when a mammalian sperm fertilizes an egg
- enzymes to digest the zona pellucida
- exposes sperm surface proteins to bind ZP2
- Membrane Fusion - between sperm and egg, allows sperm nuclei passage into egg cytoplasm
Fertilization - Oocyte
- Membrane Depolarization - caused by sperm membrane fusion, primary block to polyspermy
- Cortical Reaction - IP3 pathway elevates intracellular Calcium, exocytosis of cortical granules MBoC - Figure 20-32. How the cortical reaction in a mouse egg is thought to prevent additional sperm from entering the egg
- enzyme alters ZP3 so it will no longer bind sperm plasma membrane
- Meiosis 2 - completion of 2nd meiotic division
- forms second polar body (a third polar body may be formed by meiotic division of the first polar body)
Week 1 and 2
Week 2 Implantation
Week 3 Gastrulation
Gastrulation, (Greek = belly) means the formation of gut, but has been used in a more looser sense to to describe the formation of the trilaminar embryo. The epiblast layer, consisting of totipotential cells, derives all 3 embryo layers:endoderm, mesoderm and ectoderm. The primitive streak is the visible feature which represents the site of cell migration to form the additional layers. Historically, gastrulation was one of the earliest observable morphological event occurring in the frog embryo.
- primitive node - region in the middle of the early embryonic disc epiblast from which the primitive streak extends caudally (tail)
- nodal cilia establish the embryo left/right axis
- axial process extends from the nodal epiblast
- primitive streak - region of cell migration from the epiblast layer forming sequentially the two germ cell layers (endoderm and mesoderm)
The notochord is a structure which has an early mechanical role in embryonic disc folding and a major signaling role in patterning surrounding embryonic tissue development. This signaling role patterns many different tissues (neural plate, neural tube, somites, endodermal organs). It has its own sequence of development from a primitive axial process and is a developmental feature not present in the adult anatomy.
- axial process an initial epiblast hollow epithelial tube which extends in the midline from the primitive pit, cranially in the embryonic disc (toward the oral membrane).
- neuroenteric canal is a transient communication between the amnionic cavity and the yolk sac cavity formed by the axial process.
- notochordal plate forms from the axial process merging with the endoderm layer.
- notochord forms from the notochordal plate which then separates back into the mesoderm layer as a solid column of cells lying in the midline of the embryonic disc and running rostro-caudally (head to tail).
- An alternate name for the notochord is "axial mesoderm".
Mesoderm means the "middle layer" and it is from this layer that nearly all the bodies connective tissues are derived. In early mesoderm development a number of transient structures will form and then be lost as tissue structure is patterned and organised. Humans are vertebrates, with a "backbone", and the first mesoderm structure we will see form after the notochord will be somites.
Coelom, meaning "cavity", and major fluid-filled cavities can be seen to form both within the embryo (intraembryonic coelom) and outside the embryo (extraembryonic coelom). The intraembryonic coelom is the single primitive cavity that lies within the mesoderm layer that will eventually form the 3 major anatomical body cavities (pericardial, pleural, peritoneal).
Somite initially forms 2 main components
- ventromedial- sclerotome forms vertebral body and intervertebral disc
- dorsolateral - dermomyotome forms dermis and skeletal muscle
Week 4 Neuralation
Ectoderm - 2 parts
- midline neural plate (columnar cells)
- lateral surface ectoderm (cuboidal cells)
- sensory placodes
- epidermis of skin, hair, glands, anterior pituitary, teeth enamel
- extends from buccopharyngeal membrane to primitive node
- forms above notochord and paraxial mesoderm
- neuroectodermal cells
- broad brain plate
- narrower spinal cord
- 3 components form: floor plate, neural plate, neural crest
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 and downward into anatomical position
- week 3 begins as paired heart tubes that fuse to form single heart tube
- begins to beat in Humans- day 22-23
- 2 populations of cells
- all vessels (arteries and veins) appear initially the same
- blood formation from stem cells occurs initially in the extraembryonic mesoderm of the yolk sac
- then later (week 5) throughout embryonic mesenchyme
- blood stem cells then migrate into the liver
- then spleen, bone marrow, lymph nodes
Red Blood Cells
The only cells in the blood are nearly entirely fetal red blood cells (RBC).
These red blood cells differ from adult red blood cells in:
- often remaining nucleated
- contain fetal haemoglobin - has different oxygen and carbon dioxide binding characteristics
The trophoblast layer has now differentiated into two morphologically distinct cellular layers.
- Syncitiotrophoblasts - form a multinucleated cytoplasmic mass by cytotrophoblast cell fusion and both invade the decidua and secrete hCG
- Cytotrophoblasts - form a cellular layer around the blastocyst, proliferates and extends behind syncitiotrophoblasts
Early Utero-Placental exchange - transfer of nutrition from maternal lacunae filled with secretions from uterine glands and maternal blood from blood vessels. The development of trophoblast villi extending into the uterine decidua.
There are three stages of villi development:
- Primary Villi - cytotrophoblast
- Secondary Villi - cytotrophoblast + extraembryonic mesoderm
- Tertiary Villi - cytotrophoblast + extraembryonic mesoderm + blood vessels
There are two main types of early villi:
- Anchoring villi - attached to decidua
- Floating villi - not attached to decidua, floating in maternal lacunae.
The following material is not part of todays lecture, these are 2009 science lectures and other resources that relate to the concepts covered in much more detail. Cardiac Embryology was a 2009 medical student ILP embryology education project completed in my laboratory.
Cell Division/Fertilization | Week 1&2 Development | Week 3 Development | Mesoderm Development | Ectoderm, Early Neural, Neural Crest | Early Vascular Development | Placenta | Cardiac Embryology | Movies
- A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z | Numbers | Symbols
- 2010 BGD: Lecture 1 | Lecture 2 | Practical 3 | Practical 6 | Practical 12
- Dr Mark Hill 2013, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G