Foundations Practical - Week 3 and 4
Germ Layers and Dynamic Processes
- From 1 layer of cells to 3 layers that define all tissues of the entire embryo.
The conceptus is now fully implanted in the uterine wall. This time is now when a small cluster of cells (the inner cell mass) will differentiate into two simple cell layers (epiblast and hypoblast). From the single epiblast layer cells will migrate through a narrow region (primitive streak) to form firstly two layers epiblast and endoderm, then later the three layers ectoderm, mesoderm and endoderm. These final three layers (germ layers) are the trilaminar embryo and will form all the tissues of the body.
During implantation cells on the outside (trophoblast layer) of the conceptus are forming part of the early placenta. The cells on the inside (inner cell mass) now begin to form the three germ layers that will form the entire embryo. Note that as the conceptus also forms the fetal component of the placenta, not all cells from our blastocyst will form part of the developing embryo.
The images below show the embryo (week 3 between 15 to 17 days) as a disc 0.4 mm diameter in size.
Please just make note of the events that occur at this time, we do not have time to discuss these events in detail, this will occur in BGD.
The first migration of cells forms the endoderm germ layer. An epithelial layer of cells which contributes all the epithelial lining (and glands) of the gut as well as the respiratory tract.
The later migration of cells forms the mesoderm germ layer. An embryonic connective tissue (mesenchyme) which forms nearly all the connective tissues of the body (the head is different). Somitogenesis is when part of this layer segments during week 3 to form balls of mesoderm called somites.
|The first organ to begin to form as a simple muscular tube in the mesoderm layer. This heart tube will need to undergo growth, folding and septation before it achieves a structure close to that seen in the adult.|
| The remaining cells that do not migrate from the epiblast layer remain and form the ectoderm.
Neurogenesis begins towards the end of week 3, when the neural tissues separate from this germ cell layer.
| There are two major folding processes that take place during this time.
Folding of the ecoderm will form a neural groove, then closing to form a neural tube, separating the neural ectoderm from the embryo surface ectoderm.
Folding of the whole embryonic disc ventrally, separates the endoderm to form the epithelial lining of the gut.
The animation on the left shows a mid-line section of the embryo as it also folds from the head (cranially) and tail (caudally) to form a "C" shape around the heart.
- blastocyst - (Greek, blastos = sprout + cystos = cavity) Term used to describe the hollow cellular mass taht forms in early development. In humans, this stage occurs in the first and second weeks after the zygote forms a solid cellular mass (morula stage) and before implantation. The blastocyst consists of cells forming an outer trophoblast layer, an inner cell mass (embryoblast) 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.
- conceptus - The entire product of conception, that is all the structures derived from the zygote and includes not only the embryo, but also the placental and membrane components.
- 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 epithelia covering the embryo.
- endoderm - (Greek, endo = inside + derma = skin) One of the initial 3 germ cell layers, which will form the epithelial lining of the gastrointestinal tract (GIT), accessory organs of GIT and respiratory tract in the embryo.
- extraembryonic mesoderm - Cells from the conceptus that contribute to placenta and fetal membranes. Described as "extraembryonic" because it is tissue lying outside the embryonic trilaminar disc (ectoderm, mesoderm and endoderm) and "mesoderm", because of the connective tissue cellular organization.
- folding - a general term for the process of bending (something flexible and relatively flat) over on itself so that one part covers another.
- 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)
- genesis - the origin or mode of formation of something, used as a suffix (at the end of a word) for the structure being formed.
- gonad - (Greek, gonos = seed) A gamete-producing (germ cell) organ. A non-sexual term which is used to describe both the female ovary and male testis.
- hCG - An acronym for the hormone human Chorionic Gonadotrophin.
- human chorionic gonadotrophin - (hCG) Placental hormone initially secreted by cells (syncitiotrophoblasts) from the implanting conceptus during week two, supporting the ovarian corpus luteum, which in turn supports the endometrial lining and therefore maintains pregnancy. Hormone can be detected in maternal blood and urine and is the basis of many pregnancy tests. Hormone also stimulates the onset of fetal gonadal steroidogenesis, high levels are teratogenic to fetal gonadal tissues.
- implantation - The term used to describe process of attachment and invasion of the uterus endometrium by the blastocyst (conceptus). Abnormal implantation is where this process does not occur in the body of the uterus (ectopic) or where the placenta forms incorrectly.
- mesoderm - (Greek, mésos = middle + derma = skin) The middle layer of the initial 3 germ cell layers that contributes most of the connective tissues of the embryo.
- neurogenesis/neuralation - The general term used to describe the 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 and all the processes that occur therafter. 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.
- neural crest - A cell region at edge of neural plate, then atop the neural folds, that remains outside and initially dorsal to the neural tube when it forms. These paired dorsal lateral streaks of cells migrate throughout the embryo and can differentiate into many different cell types (= pluripotential). Those that remain on the dorsal neural tube form the sensory spinal ganglia (DRG), those that migrate ventrally form the sympatheitic ganglia. Neural crest cells also migrate into the somites and regions throught the entire embryo.
- neural folds - The central region of the trilaminar embryo ectoderm called the neural plate region folds dorsally, generating two neural folds, which later fuse to form the neural tube. The mid-line depression between the two folds is described as the neural groove. In humans at approximately day 18-19 post-fertilization to form the neural groove, which then fuses to form an initially open at either end hollow neural tube. The neural tube forms the central nervous system (brain and spinal cord). Developmental sequence: neural plate ->(day 18-19) neural groove -> neural tube -> Central Nervous System -> brain and spinal cord.
- neural plate - The first stage in early development of the central nervous system. In the trilaminar embryo (ectoderm, mesoderm, endoderm) the central region of the ectoderm (in the midline above the mesodermal notochord) initially forms a columnar epithelium described as the neural plate. This epithelium will fold dorsally to form the neural groove, which then fuses to form an initially open at either end hollow neural tube. The neural tube forms the central nervous system (brain and spinal cord). Developmental sequence: neural plate -> neural groove -> neural tube -> Central Nervous System -> brain and spinal cord.
- neural tube - The third stage in early development of the central nervous system. In the trilaminar embryo (ectoderm, mesoderm, endoderm) the central region of the ectoderm (in the midline above the mesodermal notochord) initially forms a columnar epithelium described as the neural plate. This epithelium will fold dorsally to form the neural groove, which then fuses to form an initially open at either end hollow neural tube. The neural tube forms the central nervous system (brain and spinal cord). Developmental sequence: neural plate -> neural groove -> neural tube -> Central Nervous System -> brain and spinal cord.
- primitive streak - Region visible on the surface of the early epiblast embryonic disc showing the region where gastrulation (cell migration to form endoderm) is occurring. In the human embryo this process occurs from week 3 through to week 4.
- somitogenesis - The process of segmentation of part of the mesoderm layer, the paraxial mesoderm, to form somites which are balls of mesoderm . The process begin cranially (humans day 20) and extending caudally at 1 somite/90 minutes until on average 44 pairs have been formed. (More? Somitogenesis | Mesoderm)
- uterine tube - (fallopian tube, oviduct) the tubular extensions connecting the ovary with the uterus body. Fertilization and the first week of development occurs in this structure. Main site of ectopic implantation.
- uterus - The female internal genital (reproductive) tract forming a hollow muscular walled organ, embryonically derived from the paramesonephric ducts. The human uterus has two uterine horns (fallopian tubes) where the first week of development occurs and a single hollow body where implantation of the blastocyst normally occurs. Following puberty, the non-pregnant uterus (epithelium and underlying stroma) undergoes cyclic changes under the influence of hormones, the menstrual cycle. This cycle of uterine changes ceases during pregnancy and it contributes the maternal component of the placenta.
Introduction | Week 1 and 2 | Week 3 and 4 | Week 1 to 8 | Week 9 to 36 | Neonatal | Critical Periods | Additional Resources | Quiz
Cite this page: Hill, M.A. 2017 Embryology Foundations Practical - Week 3 and 4. Retrieved September 24, 2017, from https://embryology.med.unsw.edu.au/embryology/index.php/Foundations_Practical_-_Week_3_and_4
- © Dr Mark Hill 2017, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G