Note that an unedited audio version of this practical has now been uploaded and is available here Embryology Face and Ear Audio 1:15:16 34 Mb. Please also read associated recording notes.
Gastrulation/Neuralation: In the third week, gastrulation establishes the 3 germ cell layers, endoderm, mesoderm and ectoderm. All 3 layers will later contribute specific components of the face and ear.
Also present from these early weeks is the buccopharyngeal membrane or oral membrane. The membrane forms in the midline towards the cranial end of of the trilaminar embryo. Initially as a small circular region during gastrulation where ectoderm and endoderm associate closely together, without mesoderm between the layers (like the cloacal membrane at the other end of the disc). Folding of the embryonic disc will change its relative position.
Human Embryonic Disc (stage 7, 15 - 17 days)
The primitive streak extends from the primitive node towards the connecting stalk end of the disc. The primitive node (Hensen's node) is also the beginning of an axial process that extends in the opposite direction within the mesoderm layer. The axial process can extend cranially only as far as the buccopharyngeal membrane an differentiates to form then notochord. The notochord regulates development, both differentiation and folding, in the surrounding tissues. The regulation is initially physical, as a dense column of cells (folding) and then by secretion of developmental factors (differentiation) that pattern the surrounding ectoderm, mesoderm and endoderm.
Now consider how each of the germ layers will eventually contribute specific components of the face and ear. The list below is a simple overview, by the end of todays pracrical you should be able to identify the germ layer embryonic origin of many different components.
Endoderm
Mesoderm
Ectoderm
As we discussed in gastrointestinal tract development, initial folding of the embryonic disc occurs ventrally around the notochord, which forms a rod-like region running rostro-caudally in the midline.
Folding in relation to the notochord:
Ventral endoderm (yellow) is folded to form a simple tube. (Movie in new window) |
Carnegie Stage 11
Facts: Week 4, 23 - 26 days, 2.5 - 4.5 mm, Somite Number 13 - 20 View: This is a dorsolateral view of embryo. Amniotic membrane removed. |
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Two other rapid growth events influence folding in the third to forth week and define the location of the early face:
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Link to next page in this Practical - Early Embryo The next page will look at the early embryo and the developmental appearance of pharyngeal arches. |
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 |
amnion - An extraembryonic membrane ectoderm and extraembryonic mesoderm in origin and forms the innermost fetal membrane, produces amniotic fluid. This fluid-filled sac initially lies above the trilaminar embryonic disc and with embryoic disc folding this sac is drawn ventrally to enclose (cover) the entire embryo, then fetus. The presence of this membane led to the description of reptiles, bird, and mammals as amniotes. (More? Placenta Notes | Week 2 Notes)
amniotic fluid - The fluid that fills amniotic cavity totally encloses and cushions the embryo. Amniotic fluid enters both the gastrointestinal and respiratory tract following rupture of the buccopharyngeal membrane. The late fetus swallows amniotic fluid.
anterior neuropore - (rostral, cephalic, or cranial neuropore) The initial "head end" or brain end opening of the neural tube before it closes (humans approximately 24 days postfertilization). The opening at the other "tail end" of teh neural tube is the posterior neuropore. The anatomical location of the anterior neuropore in later brain development is the lamina terminalis, lying behind the pituitary gland. Failure of anterior neuropore to close leads to the neural tube defect (NTD) anacephaly. (More? Neural Abnormalities | Neural Tube Defects)
axial mesoderm - Alternative name for the notochord, an early embryonic structure lying in the midline of mesoderm within the early trilaminar embryo. (More? notochord | Neural Notes | Week 3 Notes)
axial process - The precursor to the notochord. In the early embryo (week 3) epiblast structure extending from the primitive node (Hensens node) crainally, which will eventuall differentiate to for the notochord, the mesoderm structure that later replaces the axial process. (More? notochord | Neural Notes | Week 3 Notes)
buccal - (Latin, bucca = cheek) A term used to relate to the mouth (oral cavity). (More? GIT Notes)
buccopharyngeal membrane - (oral membrane) (Latin, bucca = cheek) A membrane which forms the external upper membrane limit (cranial end) of the early gastrointestinal tract (GIT). This membrane develops during gastrulation by ectoderm and endoderm without a middle (intervening) layer of mesoderm. The membrane lies at the floor of the ventral depression (stomadeum) where the oral cavity will open and will breakdown to form the initial "oral opening" of the gastrointestinal tract. The equivilent membrane at the lower end of the gastrointestinal tract is the cloacal membrane. (More? GIT Notes | Face Notes | Head Notes)
coelom - Term used to describe a space. There are extraembryonic and intraembryonic coeloms that form during vertebrate development. The single intraembryonic coelom will form the 3 major body cavities: pleural, pericardial and peritoneal. (More? Coelom Notes)
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)
first trimester - Clinical term used to describe and divide human pregnancy period (9 months) into three equal parts of approximately three calendar months. The first trimester corresponds approximately to embryonic development (week 1 to 8) of organogenesis and early fetal. The second and third trimester correspond to the fetal period of growth in size (second trimester) and weight (third trimester), as well as continued differentiation of existing organs and tissues. (More? Embryo Stages | Human Fetal Period | Development Week by Week)
foregut - The first of the three part/division (foregut - midgut - hindgut) of the early forming gastrointestinal tract. The foregut runs from the buccopharyngeal membrane to the midgut and forms all the tract (esophagus and stomach) from the oral cavity to beneath the stomach. In addition, a ventral bifurcation of the foregut will also form the respiratory tract epithelium. (More? Gastrointestinal Tract - Stomach | Gastrointestinal Tract Notes | Respiratory Notes)
gastrula - (Greek, gastrula = little stomach) A stage of an animal embryo in which the three germ layers have just formed. (More? Gastrulation)
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. Named originally by Robert Remak (1815 - 1865) a German scientist and embryologist. (More? Week 2 Notes Week 3 Notes | Robert Remak)
intraembryonic coelom - The "horseshoe-shaped" space (cavity) that forms initially in the third week of development in the lateral plate mesoderm that will eventually form the 3 main body cavities: pericardial, pleural, peritoneal. The intraembryonic coelom communicates transiently with the extraembryonic coelom. (More? Coelom Notes | Week 3 Notes)
mesenchyme - Term used to describe the cellular organisation of undifferentiated embryonic connective tissue . Mesenchymal tissue is mainly derived from mesoderm and neural crest, which will form most of the adult connective tissues. This connective tissue organization contrasts with the other main form of cellular organization, epithelial tissue. (More? Musculoskeletal Notes)
mesoderm - The middle layer of the 3 germ cell layers of the embryo. Mesoderm outside the embryo and covering the amnion, yolk and chorion sacs is extraembryonic mesoderm. (More? Musculoskeletal Notes | Week 3 Notes)
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 groove - The second 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, beginning 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)
neurogenesis - The process of formation of the neural system. This begins with segregation of the neural plate from the ectoderm of the trilaminar embryo by folding to form initially the neural groove, which then fuses to form the neural tube (the central nervous system progenitor, brain and spinal cord) and associated neural crest. (More? Neural Note)
notochord - (axial mesoderm) The rod of cells lying in the midline of the trilaminar embryo mesoderm layer ventral to the neural tube. Thought to have at least 2 early roles in development and later roles in patterning surrounding tissues. 1. Mechanical, influencing the folding of the early embryo; 2. Morphogenic, secreting sonic hedgehog a protein which regulates the development of surrounding tissues (neural plate, somites, endoderm and other organs). In humans, the notochord forms in week 3 and is eventually lost during the formation of the vertebral column. (More? Mesoderm Notes | Neural Notes | Molecular Development- Sonic Hedgehog)
paraxial mesoderm - The two lateral strips of mesoderm lying beside the axial mesoderm (notochord). This mesoderm at the body level will segment into somites, at the head level it remains unsegmented. (More? Musculoskeletal Notes | Week 3 Notes)
pharynx - uppermost end of gastrointestinal and respiratory tract, in the embryo beginning at the buccopharyngeal membrane and forms a major arched cavity within the phrayngeal arches. (More? Head Notes | GIT Notes | Carnegie stage 13 pharynx)
prosencephalon - (forebrain) the most anterior neural tube primary brain vesicle (there are 3 primary brain vesicles) that will form the two secondary brain vesicles, telencephalon and diaencephalon. These generate in the adult brain the cerebral hemispheres (neocortex, basal nuclei, palaeocortex, archicortex) and thalmus, hypothalmus and other nuclei respectively. The prosencephalon lumen (cavity of the neural tube) will form the lateral and third ventricle. (More? Neural Notes | Ventricular System)
somitogenesis The process of segmentation of the paraxial mesoderm within the trilaminar embryo body to form pairs of somites, or balls of mesoderm. A somite is added either side of the notochord (axial mesoderm) to form a somite pair. The segmentation does not occur in the head region, and begins cranially (head end) and extends caudally (tailward) adding a somite pair at regular time intervals. The process is sequential and therefore used to stage the age of many different species embryos based upon the number visible somite pairs. In humans, the first somite pair appears at day 20 and adds caudally at 1 somite pair/90 minutes until on average 44 pairs eventually form. (More? Week 3 - Somitogenesis | Musculoskeletal Notes)
splanchnic mesoderm - Gastrointestinal tract (endoderm) associated mesoderm formed by the separation of the lateral plate mesoderm into two separate components by a cavity, the intraembryonic coelom. Splanchnic mesoderm is the embryonic origin of the gastrointestinal tract connective tissue, smooth muscle, blood vessels and contribute to organ development (pancreas, spleen, liver). The intraembryonic coelom will form the three major body cavities including the space surrounding the gut, the peritoneal cavity. The other half of the lateral plate mesoderm (somatic mesoderm) is associated with the ectoderm of the body wall. (More? Week 3 | Gastrointestinal Tract Notes | Heart Notes )
stomadeum - (stomadeum) A ventral surface depression on the early embryo head surrounding the buccopharyngeal membrane, which lies at the floor of this depression. This surface depression lies between the maxillary and mandibular components of the first pharyngeal arch. (More? Gastrointestinal Tract Notes | Head Notes)
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