Development of placodes in the epithelium involves multiple inductive interactions. In the case of the sensory placodes, the neural tissue induces placode formation. In the case of ectodermal appendages, the mesenchyme is a source of inductive signals. In all cases, there is reciprocal signaling between the epithelium and the mesenchyme. In addition to positive signals that induce placode formation (Shh, Wnts, FGFs, BMP antagonists), there are negative signals (such as BMPs) that are important for allowing spacing of ectodermal appendages like teeth and hair follicles.
The developing brain induces the overlying ectoderm to develop into sensory organs. The neural crest arises at the posterior border of the neural plate and the epidermis; ectodermal placodes arise at the anterior border of the neural plate and the epidermis. VIa invagination, ectodermal places then develop into ganglia (nerve bundles) and parts of the ear, eye and nose (sensory organs of the head).
Epithelial placodes do not properly develop in Individuals with ectodermal dysplasia, thus retarding development of ectodermal appendages. Mice and humans with ectodermal dysplasia have little or no hair, fewer and smaller teeth, few or no sweat glands and small nails. Cloning of mutated genes in ectodermal dysplasia patients led to the discovery of a secreted factor called ectodysplasin-A (EDA) and its receptor (EDAR). EDA is expressed throughout the epidermis, and EDAR is induced by signals from the mesenchyme (dermis). Islands of EDAR expression are induced by further signals to grow downward into the mesoderm and form placodes.
Epidermal ectoderm gives rise to teeth, hair, nails, mammary glands, scales and feathers. Via epithelio-mesenchymal interaction, underlying mesenchyme determines which structures are formed from the epidermis. For example, if chicken thigh mesoderm is grafted beneath wing ectoderm, then the wing ectoderm will form thigh feathers rather than wing feathers. Epithelio-mesenchymal interaction occurs in three steps:
| Process | Overview |
| Initiation | Signals provide positional information so that organs form in the correct place. For example, secreted signals control initiation of tooth development so that the right number of teeth develop and with proper spacing. |
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| Morphogenesis | Epithelial and mesenchymal cells interact to form a rudiment, for example a tooth bud or hair follicle. |
| Differentiation | Cells differentiate to form specific structures. For example: tooth bud epithelial cells differentiate into enamel-producing cells; and hair follicle epithelial cells differentiate into hair-producing cells. |
In birds and mammals, the endoderm is a disk that invaginates, starting at its anterior and posterior ends, to form a closed epithelial cylinder surrounded by a thin layer of splanchnic mesoderm. The splanchnic mesoderm later becomes smooth muscle. The endoderm gives rise to the pancreas, liver and lungs via branching of endodermal tubes. Regional morphological differences amongst epithelial cells demarcates the three endodermal segments, each of which evaginate:
| Segment | Overview |
| Foregut | The most anterior region of the foregut broadens and becomes the pharynx. The lateral sides evaginate, forming pharyngeal pouches which give rise to: gill slits (fish and amphibians) or part of the jaw, ear and neck (reptiles, birds and mammals). The esophagus forms as a dorsal extension of the pharynx. At its posterior end, the esophagus widens into the stomach. At its anterior end, the esophagus forms a lung bud (aka tracheal bud) that gives rise to lungs in a manner similar to hepatic (liver) and pancreatic development. |
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| Midgut | The midgut becomes the small intestine. Epithelium develops finger-like outgrowths called villi. Villi hugely expand the gut’s surface area, thus aiding nutrient absorption. Between villi, the epithelium sinks to form crypts. The foregut and hindgut also form crypts, but lack villi. Undifferentiated endodermal stem cells proliferate at the neck of each crypt (the proliferation zone). Enterocytes (absorptive cells) migrate into the villi as they differentiate. Gland cells migrate deeper into the crypt as they differentiate. This polarized pattern is maintained throughout life. In the anterior of the midgut, hepatic and pancreatic buds arise; these will form the liver and pancreas, respectively. |
Regulation of the constant rapid division in the intestine is import to avoid cancer. Intestinal cell differentiation depends on Cdx2 expression. If cells do not differentiate correctly, they continue to proliferate. As expected, Cdx2+/- knockout mice develop colon tumors at a high rate. Defects in the APC gene (encoding adenomatous polyposis coli or APC) are the most common cause of human colorectal cancer. APC targets β-catenin for degradation; β-catenin interacts with TCF/LEF factors in the Wnt signaling pathway, which maintains stem cell proliferation. APC defects reduce β-catenin degradation, causing excessive Wnt signaling and thus overabundant proliferation. Similarly, colorectal cancers sometimes arise from mutations that stabilize β-catenin or increase TCL/LEF factor levels.
Development of endodermal organs is dependent on interaction between the endoderm and the surrounding mesoderm. Endodermal tissue cultured in vitro does not differentiate without its surrounding mesoderm; endodermal tissue co-cultured in vitro with splanchnic mesoderm undergoes organ-specific differentiation. Furthermore, organ development depended on the position of the mesoderm — not the endoderm — along the antero-posterior axis. For example, tracheal bud endoderm cultured with mesoderm from different regins differentiates according to location from where mesoderm derived. Mesoderm fromnear liver leads to liver-like tubues in the tracheal bud. Mesoderm outside the immediate splanchnic layer also instructs the endoderm.
For example, as we will discuss in more detail below, signals from the notochord (dorsal of the endoderm) and the heart primordium (antero-ventral of the endoderm) play an essential role in the specification of the pancreas and liver, respectively. Inductive signals also pass from the endoderm to the mesoderm. In other words, signaling between the mesoderm and endoderm is reciprocal. Thus, endoderm co-cultured with somitic mesoderm (which normally forms muscle and bone) will induce the mesoderm to become smooth muscle.
Hedgehog, TGFβ and FGF are families of regulatory genes involved in endo-mesodermal interactions and are critical for region-specific endodermal differentiation.
| Genes | Source | Overview |
| Ssh | Notochord | Hedgehog genes (Shh and Ihh) are expressed in the endoderm and are regulated by signals from the surrounding mesoderm. Ssh-/- mice have overall smaller guts, and regional abnormalities as well: stomach epithelium (specialized crypts with acid-producing cells) is replaced by intestine-like epithelium (villi containing absorptive cells). FGF and TGFβ genes repress Shh expression in the region of dorsal endoderm that gives rise to the posterior stomach, spleen and pancreas. In regions of the endoderm where Ssh is not repressed, it upreguates BMP expression in the splanchnic mesoderm. |
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| TGFβ | Notochord | Loss of notochord-derived TGFβs allows ectopic posterior Ssh expression in the dorsal endoderm: the spleen and pancreas are reduced or absent. |
| FGF | Cardiac | FGF signaling from the cardiac mesoderm (aka heart) is critical for liver formation. In contrast, signals from the notochord (such as Shh) suppress liver formation. Thus, placing notochord tissue next to ventral endoderm results in absence of a liver. Liver formation is monitored via albumin, a marker of liver tissue that is expressed even before the liver bud forms. |
| BMP | Splanchnic Mesoderm |
BMPs are required in the splanchnic mesoderm for proliferation and differentiation as smooth muscle. |
| Layer | Vertebrates | Insects | Overview | ||||||
| Ectoderm | Gut, Liver, Lungs |
Gut | The ectoderm gives rise to the skin and its differentiated structures: hair, nails, feathers, scales, mammary glands and teeth. Ectodermal placodes give rise to the eye, ear and nose. Much of this organogenesis requires interaction between the ectoderm and the underlying mesoderm, referred to as epithelio-mesenchymal interactions. | ||||||
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| Mesoderm | Muscle, Heart, Blood, Skeleton, Kidney |
Muscle, Heart, Blood |
In amphibians, signals from the vegetal portion of the egg establish mesoderm; various growth factors play a role in signaling to the marginal zone cells to cause them to become mesoderm precursors. For example, we discussed the role of Xnr (Xenopus nodal-related) in mesoderm formation. Expression of nodal-related is also required for proper function of the node in birds and mammals, specifically for induction of axial mesoderm. We have seen that, in vertebrates, the most dorsal mesoderm forms the notochord, and that BMP inhibitors such as chordin and noggin are required for notochord formation. |
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| Endoderm | Nervous System, Skin |
Nervous System, Cuticle |
After gastrulation, the endoderm is the innermost germlayer.
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The mesodermal layer of the early embryo forms as a result of gastrulation. This mesodermal layer of cells initially constitutes an epithelium; after gastrulation the cells of this epithelium lose their close association with each other (undergo an epithelial-mesenchymal transformation).
| Mesoderm | Position | Overview |
|---|---|---|
| Axial Mesoderm | Most Dorsal | Forms the notochord. |
| Paraxial Mesoderm | Along Dorsal | Positioned on either side of the axial mesoderm. Gives rise to somites. |
| Intermediate Mesoderm | More Lateral | A mesenchyme that forms the ducts that will form the kidney and internal sexual organs. |
| Lateral Mesoderm | Most Lateral | Extends from either side of the embryo to the ventral midline. Gives rise to blood, blood vessels, smooth muscle and heart. |
Hensen’s Node is present in birds and mammals. The Spemann Organizer is present in Xenopus. The Node establishes l
For example, we learned that the mesoderm that comes to occupy the most dorsal position in the embryo, the dorsal mesoderm, will become the notochord. Mesoderm that occupies more ventral positions go on to become other derivatives. For example, we also saw that the endoderm becomes surrounded by mesoderm, and that mesoderm is a more ventral type (the splanchnic mesoderm).
| Axial | It gives rise to the notochordal process which later becomes the notochord. |
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| Paraxial | On either side of the neural tube lie bands of paraxial mesoderm. Paraxial mesoderm gives rise to somites. Somites form the vertebral column, dermis and skeletal muscle. Paraxial mesoderm also gives rise to branchial arches, which develop into facial muscle and cartilage. Paraxial mesoderm is exposed to BMP antagonists, but at a lesser concentration than dorsal axial mesoderm. Posterior paraxial mesoderm expresses high levels of FGF, thus keeping it in a proliferative and undifferentiated state. As the primitive streak regresses posteriorly, cells further from the node are no longer under the influence of FGF. Outside the reach of FGF, the paraxial mesoderm cells begin to compartmentalize into somites. Cells within an individual somite become compacted (which involves an increase in cadherin expression). These changes in cell adhesion cause the newly forming somite to separate from the rest of the paraxial mesoderm. |
| Intermediate | Intermediate mesoderm is located between the paraxial mesoderm and the lateral plate. It develops into the part of the urogenital system (kidneys and gonads). |
| Lateral Plate | This is ventral mesoderm and gives rise to limbs. |
| Discovery | |
| Transplant | Transplanting a small group of cells from a region that will eventually form a somite boundary, into a region of unsegmented paraxial mesoderm that normally would not be part of a boundary. |
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| Result | Transplanted cells instruct the cells anterior to them to undergo a mesenchymal-epithelial transition and to separate from the unsegmented mesoderm. |
| Boundaries are signaling centers, and the somite boundary instructs neighboring cells to undergo a mesenchymal-epithelial transition. Nonboundary cells can acquire this ability to induce boundary formation if the Notch pathway is activated in them, for example, by introducing an activated Notch receptor. | |
| Mutations in Notch signaling lead to defects in somite formation. For example, mice lacking the Notch ligand Delta-like 3 (Dll3) have serious vertebral and rib defects. As we will see below, these structures are derived from somites. In Dll3-/- mice, somite formation is irregular and delayed. As a result the structures that form from the somites are abnormal. | |
Notch controls somite size and segmentation in a negative feedback pathway. The Notch pathway establishes an oscillating pattern in somites, and one cycle of the oscillation corresponds to the budding off of one somite from the unsegmented paraxial mesoderm. Notch signaling activates a transcription factor (RBJ) that activates the expression of Hes.
Hes is a transcriptional repressor that has two functions. First, it represses expression of itself. This limits the duration of the Notch response. Second, Hes represses expression of an inhibitor of the Notch receptor, lunatic fringe (Lfng). Thus, this activity of Hes would serve to activate the Notch pathway. The oscillations are created because Hes has a very short half-life, leading to transient repression of Notch.
When the somite first separates from the presomitic mesoderm, it can give rise to any somite-derived structure. As the somite matures, its various regions become committed to forming certain cell types.
| Somite | Region | Overview | ||||
| Sclerotome | Medial | Ventral-medial cells are farthest from the back but closest to the neural tube. These undergo mitosis and an epithelial-mesenchymal transition. They eventually become chondrocytes (cartilage cells) of the vertebrae and most (if not all) of each rib. The sclerotome is induced by paracrine factors, especially Shh, secreted from the notochord and neural tube floor plate. If any source of Shh is transplanted next to other regions of the somite, they too will become sclerotome cells. Sclerotome cells express Pax1, which induces them to differentiate into cartilage; also, pax1 is necessary for formation of the vertebrae. Sclerotome cells also express I-mf, an inhibitor of the myogenic bHLH family of transcription factors that initiate muscle formation. | ||||
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| Dermamyotome | Lateral |
Cells in the two lateral portions of the epithelium (closest and farthest from the neural tube) give rise to dermamyotome, a double-layered structure composed of myotome in the lower layer and dermatome in in the upper layer.
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| Streak | Cells ingress into the streak via an epithelial→mesenchymal transition. |
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| Somites | Somites go mesenchymal→epithelial to separate from the unsegmented mesoderm. |
| Sclerotome | Sclerotome goes epithelial→mesenchymal to generate chondrocytes of the vertebrae and most (if not all) of the ribs. |
| Dermatome | Dermatome goes epithelial→mesenchymal to generate mesenchymal connective tissue of the back dermis. |
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Conclusion: Pax6 is required in the lens ectoderm for lens induction (required for competence to respond). | ||||||||||||||||||
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