Developmental Biology: Patterning and Axis Determination Study Guide

Embryonic Patterning - Definition of Embryonic Patterning: A morphological process controlled by genes that gives rise to the complex organizations of cells, tissues, organs, and organ systems. It is orchestrated by inductive signals acting as developmental cues that govern tissue patterning.

• Regional Specification:

  • Refers to any mechanism telling a cell where it is in relation to the entire embryo, enabling it to behave appropriately for its position.

  • Allows a cell to adopt the correct spatial organization, essential for pattern formation.

  • Example: Hepatocytes: These cells differentiate ONLY in the liver-forming region of the foregut. If presumptive hepatocytes are transferred to another region, they will not continue to differentiate into hepatocytes; they may develop into another cell type or fail to differentiate entirely, depending on the developmental stage.

  • Example: Vertebral Column: Bone cells are found throughout the entire length of the embryo but behave differently at different positions. Thoracic vertebrae have large lateral processes for rib attachment, while lumbar vertebrae have shorter processes.

• First Coordinates: Polarity:

  • Development is progressive, moving from a simple to a detailed body plan. Pattern formation is a step-by-step process requiring a basic framework.

  • Basic Framework/First Coordinates: Establish principal axes as a guide for positional coordinates.

  • Three Types of Axes: Anteroposterior, dorsoventral, and left and right.

  • Polarity Determination: Determined by cytoplasmic determinants or external cues, such as in the case of Drosophila anteroposterior axis established by maternal gene products present in the egg during oogenesis.

• Concept of Positional Information:

  • Proposes that cells acquire positional values (positional identity or "cell address") and differentiate accordingly to specific spatial patterns.

  • Once principal axes are established, a cell's location can be defined by its position along those axes.

Morphogen Gradients - The Turing Model (1952): Alan Turing proposed that positional information is acquired through gradients of diffusible substances called morphogens.

• Morphogen Definition: Substances whose effects differ according to their concentration, playing a decisive role in embryogenesis, morphogenesis, and differentiation.

• Intracellular Morphogens:

  • Include cytoplasmic mRNAs, proteins, or transcription factors forming a gradient by diffusion within a single cell (zygote) or syncytium (as seen in Drosophila cleavage).

  • Example: The VegT-nodal morphogen gradient in the early Xenopus embryo influences which germ layer is formed in specific areas.

• Extracellular Morphogens:

  • Act as "morphogen evocators" that travel from cell to cell and differ in effect based on concentration.

  • Their diffusion away from a source sets up a gradient influencing gene expression.

  • Gene Activation Logic: Different concentrations induce different patterns. High concentrations may activate the most genes (e.g., Z, Y, and X). As concentration decreases, the number of activated genes decreases (e.g., only Z and Y). The lowest concentration activates the least number of genes (e.g., only Z).

• Models of Morphogen Gradient Formation:

  • Simple Diffusion: The inducer releases the morphogen, which diffuses from high to low concentration.

  • Restricted Diffusion: Extracellular transport where functional morphogens interact with cell surface molecules, specifically Heparin Sulfate Proteoglycans (HSPGs).

  • Planar Transcytosis: Alternate cycles of exocytosis and endocytosis of morphogens between adjacent cells.

  • Argosomes: Lipoprotein vesicles that encase morphogens during diffusion, consisting of a core of neutral lipids and an outer layer of polar phospholipids, cholesterol, and large embedded proteins called apolipoproteins.

  • Cytonemes: Long cytoplasmic extensions, also called signaling filopodia, that extend from the receiver cell to the inducer cell.

• Segmentation:

  • Refers to the division of anteroposterior axis into segments where each unit has its own anteroposterior polarity.

  • Metamerism: Repetitive segmentation seen in larvae and adults, such as those in Drosophila.

Axis Formation in Drosophila - General Governance: Governed by both maternal and zygotic genes.

• Maternal Genes:

  • Transcribed in the nurse cells of the ovary; products (tRNA and mRNA) are deposited into the oocyte in specific regions. These act as morphogens establishing the anteroposterior axis.

  • Bicoid and Hunchback mRNAs: Critical for anterior structures (head and thorax). Bicoid is located at the anterior portion; Hunchback is distributed throughout.

  • Nanos and Caudal mRNAs: Critical for posterior structures (abdominal segments). Nanos is bound to the cytoskeleton at the posterior end; Caudal is distributed throughout.

• Maternal-effect Proteins (Post-Fertilization):

  • Bicoid Protein: A transcription factor with the highest gradient at the anterior pole. It inhibits translation of caudal mRNA anteriorly and binds to enhancers of the hunchback gene to promote anterior transcription.

  • Nanos Protein: Highest gradient at the posterior pole. Together with Pumilio protein, it binds hunchback mRNA at the posterior to prevent its translation there.

• Maternal Terminal Genes:

  • Torso Gene: Encodes for receptor tyrosine kinase (RTK) protein. While proteins are evenly distributed in the oocyte membrane, they are only activated by ligands at the anterior and posterior parts of the vitelline membrane, leading to the formation of the acron (head) and telson (tail).

• Maternal Dorsal Gene:

  • Transcribed by nurse cells; translated dorsal proteins are initially throughout the oocyte but translocate only into ventral nuclei to establish the dorsoventral axis by activating or repressing specific genes.

• Zygotic Genes:

  • Functions as maternal materials are depleted.

  • Segmentation Genes: A cascade defining segment number and polarity.

  • Gap Genes: First transcribed; define broad segmental zones. Their proteins activate pair-rule genes.

  • Pair-rule Genes: Create a striped pattern of seven vertical bands.

  • Segment Polarity Genes: Divide the embryo into 14 segment-wide units.

  • Homeotic Genes (Hox Genes): Regulate development after segmentation to determine the developmental fate of each segment.

  • Hox Gene Abdominal-B (Adb-B): Establishes left/right asymmetry by activating the myosin ID (myo1D) gene for dextral orientation and controlling sinistrally oriented organs.

Axis Formation in Frog Embryos (Xenopus) - Dorsoventral Axis: Established after fertilization, before first cleavage, influenced by BMP4, goosecoid, noggin, chordin, and follistatin.

• Anteroposterior Axis: Established during gastrulation when mesoderm involutes. Head mesoderm forms anterior structures; chordamesoderm forms trunk and tail structures under the influence of Cerberus, frzb, and dickkopf.

• Left-Right Asymmetry: Established by a cascade of signaling in the left lateral plate mesoderm during neurulation.

• Specific Signaling Molecules:

  • Bone Morphogenetic Proteins (BMPs): Members of the TGF-$\beta$ superfamily that ventralize the embryo. In areas where BMP4 is active, ventral structures form, while inactivated areas give rise to dorsal structures (neural tissue).

  • Goosecoid Gene: Expressed in the primary organizer, encodes a transcription factor that activates noggin, chordin, and follistatin to inactivate BMP4 and promote dorsalization.

  • Noggin, Chordin, and Follistatin: Secreted from the notochord to bind and inactivate BMP4, allowing the dorsal ectoderm to follow its "default fate" and become the neural tube.

  • Cerberus Gene (Cer1): Expressed in pharyngeal endoderm and prechordal mesoderm, inactivating both BMP4 and Xwnt8 to promote head development.

  • Frzb and Dickkopf: Secreted by prechordal mesoderm; they inhibit Xwnt8 to promote anterior neural tube development.

  • Nodal Family: A TGF-$\beta$ morphogen secreted on the left side, inducing its own transcription, signaling the expression of Lefty and PitX2.

  • Lefty (Antivin): Exerts negative feedback to inhibit Nodal.

  • PitX2 (Paired Like Homeodomain 2): Promotes morphogenesis of asymmetric organs.

Axis Formation in Chick and Mammalian Embryos - Chick Axis Formation:

• Dorsoventral Axis: Established during cleavage, where blastoderm cells transport $Na^+$ and water from the albumen (basic, $pH \approx 9.5$) to the subgerminal space (acidic, $pH \approx 6.5$). The epiblast side facing the negative/basic albumen becomes dorsal; the side facing the positive/acidic subgerminal space becomes ventral.

• Anteroposterior Axis: Established by gravity as the ovum rotates in the shell gland during passage through the reproductive tract.

• Posterior Marginal Zone (PMZ): Equivalent to the amphibian Nieuwkoop center, where cells have localized $\beta$-catenin and promote formation of the primitive streak and Hensen’s node.

• Hensen’s Node: Equivalent to the frog dorsal blastopore lip, a site of gastrulation initiation, precursor to chordamesoderm/notochord, and secretes noggin, chordin, and nodal, containing Fibroblast Growth Factors (FGF) for hindbrain and trunk neural tube development.

  • Mammalian Axis Formation:

• Dorsoventral Axis: Specified during inner cell mass (ICM) formation, with the dorsal side near the trophoblast and the ventral side facing the blastocoel.

• Anteroposterior Axis: Established by two signaling centers: the Node and the Anterior Visceral Endoderm (AVE).

• Anterior Visceral Endoderm (AVE): Extraembryonic structure that secretes Wnt inhibitors (Cerberus and dickkopf) and nodal inhibitors (Lefty1) to prevent primitive streak formation in the anterior region.

• Node: Embryonic structure equivalent to Hensen’s node, secreting BMP inhibitors (noggin and chordin).