Developmental Biology Unit 3: Patterning and Axis Determination Notes

EMBRYONIC PATTERNING

• Definition of Embryonic Patterning: A morphological process controlled by specific genes that result in the complex organization of cells, tissues, organs, and organ systems. It is orchestrated by inductive signals which function as developmental cues governing tissue patterning.

• Regional Specification: This refers to any mechanism that informs a cell of its location relative to the entire embryo.

  • Functional Importance: It allows cells to behave appropriately for their position and adopt correct spatial organization, which is essential for pattern formation.

  • Example (Hepatocytes): Hepatocytes differentiate exclusively in the liver-forming region of the foregut. If presumptive hepatocytes are transferred to a different embryonic region, they will not continue to differentiate into hepatocytes; they might develop into a different cell type or fail to differentiate entirely, depending on the developmental stage.

  • Example (Vertebrae): While bone cells are found throughout the vertebral column, their architectural behavior differs by position. For instance, the thoracic vertebrae develop large lateral processes for rib attachment, whereas lumbar vertebrae develop shorter processes.

• First Coordinates: Polarity: Development proceeds from a simple body plan to a detailed one through a step-by-step process.

  • Basic Framework: Known as the first coordinates, these establish the principal axes ($anteroposterior$, $dorsoventral$, and $left/right$) that guide positional coordinates.

  • Determination: Polarity is determined by external cues or cytoplasmic determinants. For example, the Drosophila $anteroposterior$ axis is established by maternal gene products present in the egg during oogenesis.

• Concept of Positional Information: This theory proposes that cells acquire specific "positional values" (also termed positional identity or "cell address"). They differentiate based on these values to create spatial patterns. Once principal axes are established, a cell's location can be defined by its position along those axes.

MORPHOGEN GRADIENTS

• Historical Context: In 1952, Alan Turing proposed that positional information is acquired via gradients of diffusible substances called morphogens.

• Definition of Morphogens: Substances whose biological effects vary based on their concentration, playing a decisive role in embryogenesis, morphogenesis, and differentiation.

• Types of Morphogens:

  • Intracellular Morphogens: These include cytoplasmic mRNAs, proteins, or transcription factors that form a gradient via diffusion within a single cell (the zygote) or a syncytium (as seen in Drosophila cleavage).

  • Example: In early Xenopus embryos, the $VegT-nodal$ morphogen gradient influences the type of germ layer formed.

  • Extracellular Morphogens: These are "morphogen evocators" secreted by cells that travel from cell to cell.

  • Mechanism: Once secreted, they diffuse from the source to create a gradient. Different concentrations induce distinct patterns of gene expression, meaning the same signal can trigger differentiation into varying tissue types.

  • Gene Activation Logic: Highest concentrations activate the most genes (e.g., Genes $Z$, $Y$, and $X$); intermediate concentrations activate fewer (e.g., $Z$ and $Y$); the lowest concentrations activate only the most sensitive gene (e.g., $Z$).

• Models of Morphogen Gradient Formation:

  • Simple Diffusion: The inducer releases morphogens that move from high to low concentration regions, with concentration decreasing as the distance from the source increases.

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

  • Planar Transcytosis: The movement of morphogens across adjacent cells through alternating cycles of endocytosis and exocytosis.

  • Argosomes: Lipoprotein vesicles that encase morphogens for diffusion. These lipoproteins consist of a central core of neutral lipids, an outer layer of polar phospholipids and cholesterol, and embedded proteins called apolipoproteins.

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

SEGMENTATION

• Definition: The division of the $anteroposterior$ axis into units, where each unit possesses its own $anteroposterior$ polarity. These represent the anatomical divisions found in larvae and adults.

• Metamerism: The process of repetitive segmentation.

• Example (Drosophila): Visible segmentation in both the larval and adult stages dictated by segmentation genes.

AXIS FORMATION IN DROSOPHILA

• General Control: Governed by both maternal genes (provided by the mother) and zygotic genes (produced by the embryo).

• Maternal Genes/Maternal-Effect Genes: Transcribed in the ovarian nurse cells, gene products (mRNA and tRNA) are deposited into the oocyte in specific regions to act as morphogens.

  • Maternal-Effect mRNAs (Anteroposterior Axis):

  • Bicoid and Hunchback: Critical for anterior structures (head and thorax). $Bicoid$ mRNA is localized specifically at the anterior pole, whereas $Hunchback$ mRNA is distributed throughout the oocyte.

  • Nanos and Caudal: Critical for posterior structures (abdominal segments). $Nanos$ mRNA is bound to the cytoskeleton at the posterior end, while $Caudal$ mRNA is distributed throughout the oocyte.

  • Maternal-Effect Proteins (Post-Fertilization):

  • Bicoid Protein: A transcription factor forming the highest gradient at the anterior pole. It inhibits the translation of $Caudal$ mRNA in the anterior region and binds to enhancers of the $Hunchback$ gene to promote its transcription, increasing anterior levels of $Hunchback$ protein.

  • Nanos Protein: Forms the highest gradient at the posterior pole. Combined with $Pumilio$ protein, it binds to $Hunchback$ mRNA in the posterior to prevent its translation there.

• Maternal Terminal Genes: A third set of genes generating the extreme ends of the embryo.

  • Torso Gene: Encodes a Receptor Tyrosine Kinase ($RTK$). Although $Torso$ proteins are evenly distributed in the membrane, they are only activated by ligands at the anterior and posterior parts of the vitelline membrane. Activation leads to the formation of the acron (head) and telson (tail).

• Maternal Dorsal Gene: Transcribed by nurse cells. Proteins are initially throughout the oocyte but translocate into ventral nuclei to activate or repress genes, establishing the $dorsoventral$ axis.

• Zygotic Genes: Activated as maternal materials deplete.

  • Segmentation Genes:

  • Gap Genes: Transcribed first under maternal influence; define broad segmental zones. Their proteins activate pair-rule genes.

  • Pair-rule Genes: Create a striped pattern of seven vertical bands perpendicular to the $anteroposterior$ axis. They activate segment polarity genes.

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

  • Homeotic (Hox) Genes: Regulate development post-segmentation, determining the developmental fate of each specific segment.

  • Hox Gene Abdominal-B ($Abd-B$): Establishes $left/right$ asymmetry. It activates the myosin ID ($myo1D$) gene, responsible for the dextral (right-handed) orientation of organs, and controls the development of sinistrally oriented organs.

AXIS FORMATION IN FROG EMBRYOS (XENOPUS)

• Dorsoventral Axis: Established after fertilization but before the first cleavage. It is influenced by $BMP4$, $goosecoid$, $noggin$, $chordin$, and $follistatin$.

  • Bone Morphogenetic Proteins ($BMPs$): Members of the $TGF-\beta$ superfamily.

  • Mechanism: $BMP4$ is initially throughout the embryo. Where active, it ventralizes the embryo. Where inactivated by dorsalizing agents, neural tissue and dorsal structures form.

  • Goosecoid Gene: Encodes a transcription factor in the primary organizer. It activates $noggin$, $chordin$, and $follistatin$.

  • Noggin, Chordin, and Follistatin: Secreted from the notochord; they bind to and inactivate $BMP4$ at the dorsal ectoderm. This allows the dorsal ectoderm to follow its "default fate" (becoming the neural tube) and dorsalizes the mesoderm.

• Anteroposterior Axis: Established during gastrulation as mesoderm involutes.

  • Head Mesoderm: Forms anterior structures under the influence of $Cerberus$, $frzb$, and $dickkopf$.

  • Chordamesoderm: Forms trunk and tail structures.

  • Cerberus Gene ($Cer1$): Expressed in pharyngeal endoderm and prechordal mesoderm. It inactivates both $BMP4$ and $Xwnt8$ (an inhibitor of neural induction) to promote head development.

  • Frizzled-related Proteins ($Frzb$) and Dickkopf: Secreted by prechordal mesoderm; they bind to and inhibit $Xwnt8$ to promote anterior neural tube development.

• Left and Right Asymmetry: Established during neurulation by asymmetric signaling in the left lateral plate mesoderm.

  • Nodal Family: A morphogen in the $TGF-\beta$ superfamily. A midline signal cascade initiates $Nodal$ secretion on the left side, which then induces its own transcription.

  • Lefty (Antivin): Exerts negative feedback by inhibiting $Nodal$.

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

AXIS FORMATION IN CHICK EMBRYOS

• Dorsoventral Axis: Specified during cleavage. Blastoderm cells transport $water$ and $sodium ions$ from the albumen to the subgerminal space.

  • Orientation: Epiblast side facing the negative/basic ($pH 9.5$) albumen becomes dorsal. Epiblast side facing the positive/acidic ($pH 6.5$) subgerminal space becomes ventral.

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

• Posterior Marginal Zone (PMZ): Located at the posterior epiblast; equivalent to the amphibian Nieuwkoop center. Cells have nuclear $\beta-catenin$ and promote the formation of the primitive streak.

• Hensen’s Node: The avian equivalent of the frog dorsal blastopore lip.

  • Functions: Site where gastrulation begins; forms the chordamesoderm (notochord); secretes $noggin$, $chordin$, and $Nodal$ (dorsalizing agents). It contains Fibroblast Growth Factors ($FGF$) that induce the hindbrain and trunk neural tube.

• Left and Right Asymmetry: Involves a $Nodal$ signaling cascade.

AXIS FORMATION IN MAMMALS

• Dorsoventral Axis: Specified during inner cell mass (ICM) formation. The dorsal side is adjacent to the trophoblast; the ventral side is exposed to the blastocoel.

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

  • Anterior Visceral Endoderm (AVE): An extraembryonic structure secreting Wnt inhibitors ($Cerberus$ and $dickkopf$) and the Nodal inhibitor $Lefty1$. This prevents primitive streak formation in the anterior region.

  • Node: An embryonic structure equivalent to Hensen's node; it secretes BMP inhibitors $noggin$ and $chordin$.