Developmental Biology notes (copy)

General Developmental Terms

14-day rule – An ethical guideline stating that human embryos should not be cultured in vitro beyond 14 days post-fertilization, as this is when the primitive streak appears, marking the beginning of gastrulation.
Embryogenesis (in humans) – The process of early human development from fertilization to the formation of basic body structures.
Extraembryonic tissues – Structures that support embryonic development but do not form part of the embryo, such as the placenta and amniotic sac.
Zona pellucida – A glycoprotein layer surrounding the oocyte, crucial for sperm binding and preventing polyspermy.
Implantation – The attachment and invasion of the embryo into the uterine wall, typically occurring around day 6-7 post-fertilization.

Molecular and Cellular Processes

Activin – A protein involved in mesoderm induction and other signaling pathways during early development.
Adhesion – The ability of cells to stick to each other and extracellular matrices, essential for tissue formation.
Aggregates – Clusters of cells that interact and function as a unit in development.
Asymmetric gene expression – Unequal expression of genes in different cells, leading to cellular differentiation and body asymmetry.
Asymmetry – The establishment of distinct left-right, anterior-posterior, or dorsal-ventral axes in the developing embryo.
Apoptosis – Programmed cell death, crucial for shaping organs and removing unnecessary structures.
Bending – The morphogenetic process by which epithelial sheets curve during development, as seen in neural tube formation.
BMP (Bone Morphogenetic Proteins) – A group of growth factors involved in patterning and differentiation, particularly in the dorsal-ventral axis.
Chemotactic signal – A molecular cue that directs cell movement toward or away from a source, important in neural crest migration.
Cilia – Hair-like structures on cells that generate fluid flow and help in left-right patterning.
Closure – The process of sealing a developing structure, such as neural tube closure.
Collective migration – Coordinated movement of a group of cells, often observed in neural crest or epithelial cell migration.
Convergence – The movement of cells toward a midline during gastrulation and other developmental processes.
Cross-repressive relationship – Mutual inhibition between genes or pathways to refine developmental decisions.
Default model – The idea that, in the absence of external signals, cells follow a default developmental pathway, such as neural tissue formation in the absence of BMP signaling.

Gastrulation and Germ Layers

Blastocoel – The fluid-filled cavity inside a blastula.
Blastopore – The opening in the gastrula that will become the mouth or anus, depending on the species.
Dorsal blastopore lip – The site of involution during gastrulation, critical in forming the body axis.
Ectodermal cells – Cells that form the outer layer of the embryo and give rise to the nervous system and skin.
Endocrine pancreas – The hormone-secreting part of the pancreas, including insulin-producing β-cells.
Exocrine pancreas – The enzyme-secreting portion of the pancreas that aids in digestion.
Fibroblast growth factor (FGF) – A key signaling molecule involved in limb development, neural induction, and cell proliferation.
Flow – The movement of extracellular fluids, often guided by cilia, to establish left-right asymmetry.
Flow model – A hypothesis explaining how cilia-driven fluid movement contributes to left-right axis specification.
Gastrula (chick) – A stage in chick development where germ layers form through cell movements like invagination and involution.

Neurulation and Neural Development

Hinge points – Sites where the neural plate bends during neural tube formation.
Hox genes – A family of transcription factors that determine body segment identity along the anterior-posterior axis.
Induction – The process by which one group of cells influences the development of another, often via signaling molecules.
Inductive – Describes factors that initiate developmental changes in responding cells.
Inner cell mass (ICM) – The group of cells in the blastocyst that will form the embryo proper.
Left-right organizer (LRO) – A specialized embryonic structure that establishes left-right body asymmetry.
Neural plate – A thickened ectodermal region that gives rise to the central nervous system.
Neurula – The stage of development during which the neural tube forms.
Neurulation – The process of neural tube formation, which later develops into the brain and spinal cord.
Nodal – A signaling molecule in the TGF-beta pathway involved in mesoderm formation and left-right asymmetry.
Node – The organizer region in mammals that helps establish body axes.
Notochord – A rod-like structure derived from mesoderm that provides axial support and signals for neural development.

Patterning and Organogenesis

Organizer – A group of cells that direct the development of surrounding tissues via signaling.
Oviduct – The fallopian tube, where fertilization typically occurs.
Paracrine factors – Signaling molecules that act on nearby cells rather than distant targets.
Paralog – A gene that has evolved from a common ancestor through duplication.
Primary axis – The main body axis (anterior-posterior) formed during early development.
Progesterone – A hormone that maintains pregnancy and prepares the uterus for implantation.
Protease – An enzyme that breaks down proteins, often involved in tissue remodeling.
Robust mechanism – A developmental process that maintains proper function despite variability.
Somite – A segmental block of mesoderm that gives rise to muscles, vertebrae, and dermis.
Somitogenesis – The sequential formation of somites from the presomitic mesoderm.
Sonic hedgehog (Shh) – A key morphogen in limb development, neural patterning, and axis formation.
Spemann model – A theory describing how the organizer region influences development, based on Spemann and Mangold’s experiments.
Syncytiotrophoblast – The outer layer of trophoblast cells that invade the uterine wall during implantation.
TGF-beta pathway – A signaling pathway that regulates cell differentiation and tissue formation.
Trophoblast – The outer layer of cells in a blastocyst that contributes to placenta formation.
Uterine natural killer cells – Immune cells involved in modulating maternal immune responses during implantation.
VEGF (Vascular Endothelial Growth Factor) – A signaling protein that promotes blood vessel formation.

Limb Development and Morphogenesis

Anterior-posterior axis – The head-to-tail body orientation.
Apical ectodermal ridge (AER) – A signaling center essential for limb outgrowth.
Autopod – The distal region of the limb, forming the hand or foot.
Branching morphogenesis – The process by which tissues form branched structures, such as the lungs or kidneys.
Competent – A cell’s ability to respond to inductive signals.
Definitive endoderm – The layer of cells that gives rise to the gut, liver, and pancreas.
Determination front – The boundary at which mesodermal cells commit to forming somites.

Here's a breakdown of the key developmental signaling pathways and factors from your notes:

BMP (Bone Morphogenetic Protein)

A member of the TGF-β (Transforming Growth Factor-beta) family.
Functions as a morphogen (meaning it forms concentration gradients to control cell fate).
Plays a key role in neural induction, somite formation, limb development, and apoptosis (e.g., digit separation).
Inhibited by noggin, chordin, and gremlin, which are important for neural and limb patterning.

Nodal

Another TGF-β family member crucial for left-right asymmetry.
Activates TGF-β signaling, which controls the expression of lefty and Cerberus (inhibitors of Nodal, creating feedback regulation).
Important for mesoderm and endoderm induction during early development.
Flow-based mechanisms distribute nodal asymmetrically to establish left-right asymmetry in embryos.

Notch Signaling

Involved in cell fate decisions, somite segmentation, and neural development.
Works as an oscillating "clock" gene in the presomitic mesoderm to regulate periodic somite formation.
Interacts with FGF and RA gradients to define the determination front for somite segmentation.

Shh (Sonic Hedgehog)

A key morphogen in limb development, neural tube patterning, and gut asymmetry.
Secreted from the notochord and floor plate of the neural tube to establish dorsal-ventral axis.
In limb development, it is secreted by the zone of polarizing activity (ZPA) and establishes anterior-posterior patterning (e.g., thumb vs. pinky).
Inhibits BMP signaling via noggin, ensuring proper neural tube closure.

Fgf8 (Fibroblast Growth Factor 8)

Regulates early mesoderm patterning, limb bud initiation, and neural development.
Interacts with Wnt signaling to maintain the apical ectodermal ridge (AER), which is necessary for proximal-distal limb growth.
Works with RA (Retinoic Acid) in opposing gradients to determine limb positioning.

Fgf10

Critical for limb bud formation by initiating AER formation.
Works downstream of RA-Tbx5 signaling, specifying forelimb vs. hindlimb identity.
Extra Fgf10 expression leads to ectopic limb formation.

Wnt Signaling

Functions in axis formation, neural patterning, and limb outgrowth.
Wnt7a regulates dorsal-ventral limb identity by inducing Lmx1b, which establishes dorsal fate.
Interacts with FGF and SHH to control proliferation and patterning in the neural tube and limbs.

RA (Retinoic Acid)

Establishes anterior-posterior body axis.
Works opposite to Fgf8 in somite segmentation and limb positioning.
RA induces Tbx5 for forelimb development, while Tbx4 regulates hindlimb formation.

Eph/Ephrin Signaling

Controls boundary formation and cell migration by mediating repulsive interactions.
Important in somite segmentation, neural crest migration, and vasculature patterning.
Works with Notch signaling to regulate clock-and-wave mechanisms in somite formation.