Begin by very briefly covering the first 3 weeks of development. Our story of GIT development begins in the third week with the formation of the 3 germ cell layers, one layer the endoderm will form the lining of the entire gastrointestinal tract and also contibute to some other organs.
Note that the description below, and throughout the practical, has been substantially simplified and for more details use the (More?) links.
Fertilization/Blastocyst Formation: an egg released into the uterine tube has been fertilized by a single sperm to form the first diploid cell (zygote). This cell then undergoes rapid division to form first a solid ball of cells (morula) and then a hollow ball (blastocyst) with an outer cell layer, an inner cell mass and a fluid filled cavity. All this has occurred in the uterine tube (horn, oviduct, fallopian tube) prior to implantation in the uterus. (More? See Notes Week 2)
Implantation/Placentation: The blastocyst has now reached the uterus and attaches and then implants into a wall of the uterus. The blastocyst inner cell mass will form the entire embryo and the outer cells contribute to the placenta and fetal membranes. The first steps of placenta formation begin and all development will now occur within this wall and the uterine cavity will be obliterated by fetal growth. (More? See Notes Week 2 | Implantation movie)
Animation showing the process of blastocyst implantation into the maternal uterus. (More? Week 2 Movies)
(Animation based upon images from Human Embryology, Larsen)
Gastrulation/Neuralation: The inner cell mass forms a flat sheet of cells and cells migrate through a specific region of the sheet (primitive streak) turning the single layer into first 2 then 3 layers (trilaminar embryo).
This process of forming a 3 layered disc is called gastrulation (which means "gut forming"), but is much more than that as we have now formed the 3 germ layers which will form all parts of the embryo. (More? See Notes week 3 | Development movie)
Germ Layers
In the trilaminar embryo, running rostro-caudally (head to tail) an axial process, then notochord forms in the mesoderm layer. The notochord regulates development, both differentiation and folding, in the surrounding tissues. The twofold notochord mechanisms are by secretion of developmental factors (differentiation) and the physical properties of a dense column of cells (folding).
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Link to next page in this GIT Practical - Folding Turning the flat trilaminar embryo into a tube. There are additional pages providing external online background readings for this Practical. |
Use the alphabetical list below to find definitions of terms that are new to you or use the Google search window to search UNSW Embryology site.
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 |
blastocyst - (Greek, blastos = sprout + cystos = cavity) Term used to describe the hollow cellular mass that forms in early development. The blastocyst consists of cells forming an outer trophoblast layer, an inner cell mass and a fluid-filled cavity. The blastocyst inner cell mass is the source of true embryonic stem cells capable of forming all cell types within the embryo. In humans, this stage occurs in the first and second weeks after the zygote forms a solid cellular mass (morula stage) and before implantation. (More? Week 1 - Blastocyst | Week 2 Notes | Stem Cell Notes)
ectoderm - (Greek, ecto = outside + derma = skin) One of the initial 3 germ cell layers, which will form the nervous system from the neural tube and neural crest and also generates the entire epithelial layer of the skin covering the embryo. The ectoderm is formed from the epiblast following gastrulation, with a central coulumnar epithelium (neural plate) and lateral cuboidal epithelium and is continuous with and forms the epithelium that lines the amniotic cavity. In humans, the ectoderm forms during week 3 and 4 of development.
endoderm - (Greek, endo = inside + derma = skin) One of the initial 3 germ cell layers, formed by the process of gastrulation. The endoderm forms as a cuboidal epithelium and contributes not only to the trilaminar embryo, but also lines the yolk sac. It will form the entire epithelial lining of the gastrointestinal tract (GIT), contribute to the accessory organs of GIT and also forms the epithelial lining of the respiratory tract. Note that in the GIT it contributes both epithelium and the associated epithelial glands. In humans, endoderm forms during week 3 of development. (More? Week 3 - Gastrulation | Week 3 Notes)
gastrointestinal tract - (GIT) the digestive tube extending from the oral cavity (mouth) to the anus. The digestive system includes the associated organs, which may also have other functions. (More? GIT Notes)
gastrulation - The process of differentiation forming a gastrula. Term means literally means "to form a gut" but is more in development, as this process converts the bilaminar embryo (epiblast/hypoblast) into the trilaminar embryo (endoderm/mesoderm/ectoderm) establishing the 3 germ layers that will form all the future tissues of the entire embryo. This process also establishes the the initial body axes. (More? Gastrulation)
germ layers - The first three cellular layers (ectoderm, mesoderm, and endoderm) that will form all tissues of the embryo. In humans, these layers begin to form during week 3 of development. Term should not be confused with germ cells, which are the oocyte and spermatazoa forming cells. Term originally used by Robert Remak (1815 - 1865), a German scientist and embryologist. (More? Week 2 Notes Week 3 Notes | Robert Remak | Development Week by Week)
morula - (Latin, morula = mulberry) An early stage in post-fertilization development when cells have rapidly divided to produce a solid mass of cells (12-15 cells) with a "mulberry" appearance. This stage is followed by formation of a cavity in this cellular mass (blastocyst stage). In humans, morula stage of development occurs during the first week following fertilization. (More? Week 1 Notes | Week 1 Early Cell Division)
neuralation - The general term used to describe the early formation of the nervous system. It is often used to describe the early events of differentiation of the central ectoderm region to form the neural plate, then neural groove, then neural tube. The nervous system includes the central nervous system (brain and spinal cord) from the neural tube and the peripheral nervous system (peripheral sensory and sympathetic ganglia) from neural crest. In humans, early neuralation begins in week 3 and continues through week 4. (More? Neural Notes | Neural Crest Notes)
primitive node - (Hensen's Node, primitive knot) The small circular region located at the cranial end of the primitive streak, where gastrulation occurs, and is a controller of this process. It is centered around the primitive pit, the site of epiblast extension cranially to form the initial axial process. Region is equivilant to the blastopore in amphibians. (More? Carnegie Stage 7 | Week 3 Gastrulation | Neural Notes | Nobel Laureate- Hans Spemann)
primitive streak - Visible region on the surface of the early epiblast embryonic bilaminar disc, representing the site of cell migration during gastrulation (cell migration to form endoderm then mesoderm). The primitive streak extends centrally from the primitive node (Hensen's node) to the disc periphery near teh connecting stalk. The orientation of the streak establishes teh early body axis (rostro-caudal, head-tail). In the human embryo, gastrulation occurs from week 3 through to week 4. (More? Carnegie Stage 7 | Week 3 Gastrulation)
trilaminar embryo - Term meaning three (3) layered embryo, used to describe the early embryo development following gastrulation when it now has a structure consisting of the 3 germ cell layers (ectoderm, mesoderm, endoderm). In humans, this developmental stage occurs during week 3. (More? Week 3 - Trilaminar Embryo | Week 3 | Embryo Stages | Development Week by Week)
On this first Practical page is a brief text description of the first 3 weeks of development.
There are also animations to help with the concepts of early development (how and where it occurs) and these can be played back and forth by dragging the control at the bottom of the movie.
The primitive streak, germ cell layers and notochord are examples of a transient developmental structure that exists and functions only in the embryo.
These notes and linked materials have been prepared for Educational purposes only.
Next Mark Hill Practical:
Sexual Differentiation covers early stages of male female development differences.
Movie of Human Embryo Growth (this shows a human embryo growing, all images are to scale)
Movie of Mouse Embryo Growth (this shows a mouse embryo growing)
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