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. The developmental processes that occur form this time onward will now be separated into development of the embryoic and extra-embryonic (placenta and fetal membranes). This introductory class will focus mainly on embryonic development.
The conceptus inner cell mass (small inner cluster of cells) will form teh embryo and also contribute to extra-embryonic tissues. The conceptus trophoblastic shell will contribute to placenta and fetal membranes.
The inner cell mass initially differentiates into two simple cell layers (epiblast and hypoblast). From the single epiblast layer cells will later migrate through a narrow region (primitive streak) to form two layers epiblast and endoderm, then later the three layers ectoderm, mesoderm and endoderm. These final three layers will form all the tissues of the embryo.
During implantation cells on the outside (trophoblast layer) of the conceptus are forming part of the early placenta and are undergoing rapid growth and interacting with the uterine endometrium and hormonally throught the maternal bloodstream. 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 and fetal membranes, therefore not all cells from our blastocyst will form part of the developing embryo.
These notes and linked materials have been prepared for Educational purposes only. Please email Dr Mark Hill if you wish to make a comment about this current project.
This is a view of embryonic disc, showing the epiblast viewed from the amniotic (dorsal) side.
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.
Human embryonic disc (Stage 7) showing the primitive streak (rotated 90 degrees from above image)
Dev Biol: Gastrulation
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).
Cells migrate through the primitive streak to form mesodermal layer. Extraembryonic mesoderm lies adjacent to the trilaminar embryo totally enclosing the amnion, yolk sac and forming the connecting stalk.
Paraxial mesoderm accumulates under the neural plate with thinner mesoderm laterally. This forms 2 thickened streaks running the length of the embryonic disc along the rostrocaudal axis. In humans, during the 3rd week, this mesoderm begins to segment to form mesoderm balls (somites). The neural plate folds to form a neural groove and folds.
Segmentation of the paraxial mesoderm into somites continues caudally at 1 somite/90minutes and a cavity (intraembryonic coelom) forms in the lateral plate mesoderm separating somatic and splanchnic mesoderm.
Movie: Fate of the Somite (315Kb)
The remaining cells that do not migrate from the epiblast layer remain and form the ectoderm. An epithelial layer of cells which contributes all neural and external epithelium of the embryo.
Ectoderm showing neural plate region
Cells in the central part of the ectoderm (neural plate) generates all the central neural tissues (brain and spinal cord). Cells at the edge of the neural plate are neural crest cells which migrate throughout the body and head forming many different tissues (peripheral nervous system, skin menaocytes, adrenal medulla......). Cells lying outside the neural crest, at the edge of the ectoderm, form the external epithelia of the body.
There are two major folding processes that take place during this time.
Folding of the ecoderm above the primitive streak to form a neural groove then closing to form a neural tube.
Folding of the whole embryonic disc ventrally to enclose the endoderm of the gut.
Link to next page in this Practical - Week 1 to 8
An overview of the Embryonic Period of development.
Use the alphabetical list below to find definitions of terms that are new to you.
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. (More? Week 1 - Blastocyst | Week 2 Notes | Stem Cell Notes)
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. (More? Week 1 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 epithelia covering the embryo. (More? Week 3 Notes)
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. (More? Week 3 Notes)
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. (More? Placenta | Week 2 Notes | Gastrointestinal Tract 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)
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. (More? Week 1 Notes)
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. (More? Placenta Notes | Week 2 Notes)
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. (More? Week 2 - Implantation | Week 2 Abnormalities)
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. (More? Neural Notes | Neural Crest Notes)
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. (More? Neural Crest Notes)
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. (More? Neural Notes)
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. (More? Neural Notes)
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. (More? Neural Notes)
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. (More? Carnegie Stage 7 | Week 3 Gastrulation)
somitogenesis The process of segmentation of the paraxial mesoderm to form somites, or 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? Musculoskeletal Notes | Week 3 Notes)
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. (More? Genital System - Female Uterus | Human Menstrual Cycle | Genital Notes)
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)