7C Week 1

Totipotent cell

A cell that can give rise to every cell type in the body plus supporting structures outside the embryo (e.g., placenta, membranes)

Zygote

The single fertilized egg, surrounded by a tough outer glycoprotein layer

Cleavage divisions

Early mitotic divisions of the zygote that occur without normal cell growth in between, so total volume stays the same while cell number increases

Morula

A solid ball of cells formed ~4-5 days after fertilization, by which point it has reached the uterus

Blastocyst

A hollow sphere of a few thousand cells formed by continued division of the morula

Inner cell mass

The clump of cells inside the blastocyst that will become the actual body of the embryo

Gastrula

Structure formed after implantation when inner cell mass cells reorganize and sort into three germ layers

Ectoderm

Germ layer that becomes the outer skin layer, brain, spinal cord, peripheral nerves, and pigment cells

Mesoderm

Germ layer that becomes the inner skin layer, muscle, bone, and blood

Endoderm

Germ layer that becomes the gut lining, lung lining, liver, and pancreas

Developmental sequence order

Zygote → 2-cell → 4-cell → morula → blastocyst → gastrula → three germ layers and derivatives

Gene regulation hypothesis

Idea that all cells keep the same full set of genes, but unneeded genes are turned off (repressed) as a cell differentiates

Genome reduction hypothesis

Idea that cells physically delete DNA for genes they no longer need as they differentiate

Gurdon's nuclear transfer experiment

Showed intestinal cells retain a complete, functional genome, proving differentiation works via gene regulation, not gene loss

Regenerative medicine

Field that tries to harness stem cells to repair or replace damaged tissue

Embryonic stem cells

Pluripotent cells from the inner cell mass; ethically controversial because harvesting destroys a blastocyst

Induced pluripotent stem (iPS) cells

Ordinary adult cells reprogrammed back to a pluripotent state by activating a small set of genes (2006 breakthrough)

Limitations of iPS cells

Low success rate (about 1 in 1000) and reprogramming originally required viruses that can cause cancer

Apoptosis

Programmed, orderly cell self-destruction that is a normal, controlled part of development and tissue maintenance

Pruning

Programmed death of many neurons in the developing nervous system after they initially form

Digit separation example

Cells between developing fingers/toes die off via apoptosis to separate the digits

Necrosis

Messy, uncontrolled cell death caused by injury or toxins; cell swells and bursts, potentially damaging neighbors

Apoptosis process

Cell shrinks and condenses, DNA fragments into small pieces, and debris is cleanly removed by phagocytosis

Broader roles of apoptosis

Regulates adult organ size (balance of division vs death rate) and helps prevent cancer

Caspases

A family of protease enzymes that carry out apoptosis by cutting other proteins at aspartic acid residues

Procaspases

Inactive precursor forms of caspases; become active only when cleaved by another caspase

Caspase cascade

Amplifying process where one active initiator caspase cleaves several caspases of the next tier, and so on

Executioner caspases

End-stage caspases that carry out cell destruction by cutting cytoplasmic/nuclear proteins, breaking down the cytoskeleton, and fragmenting DNA

Tissue

A collection of cells working together to perform a specific function

Organ

Structure formed when two or more tissues combine and function together as a unit

Three defining features of multicellular organisms

Cells adhere to each other, cells communicate with each other, and cells are specialized through differentiation

Epidermis

Outer skin layer; water-resistant protective barrier made mainly of keratinocytes plus melanocytes

Dermis

Inner skin layer; connective tissue that supports the epidermis, contains blood vessels and nerve endings

Basal lamina

Specialized extracellular matrix layer anchoring the bottommost keratinocytes of the epidermis

Fibroblast

Main cell type of the dermis; synthesizes the extracellular matrix

Microfilaments

Smallest cytoskeletal filament (7 nm), made of actin monomers in a helix; involved in shape, movement, cytokinesis, vesicle transport, muscle contraction

Intermediate filaments

Cytoskeletal filament of intermediate diameter (10 nm) whose protein type differs by cell (keratins, vimentins, neurofilaments, lamins); provide mechanical strength

Microtubules

Largest cytoskeletal filament (25 nm), made of alpha/beta tubulin dimers; radiate from the centrosome, involved in shape, movement, division, transport

Lamins

A type of intermediate filament found inside the nucleus, supporting the nuclear envelope

Plus end vs minus end

Ends of microfilaments/microtubules; the plus end assembles quickly, the minus end assembles slowly

Dynamic instability

Property of microtubule plus ends undergoing rapid, random cycles of shrinking and slower regrowth

Myosin

Motor protein that associates with actin microfilaments; powers muscle contraction and cargo transport, fueled by ATP

Kinesin

Motor protein that carries cargo toward the plus end of microtubules

Dynein

Motor protein that carries cargo toward the minus end of microtubules

Nonmotile cilia

Cilia that don't move and often serve sensory functions (e.g., olfactory neurons, photoreceptors)

Motile cilia

Cilia that actively move, propelling the cell or surrounding fluid (e.g., sperm, respiratory tract epithelium)

Endosymbiotic theory

Theory that mitochondria and chloroplasts originated as free-living prokaryotes that entered a symbiotic relationship with an early eukaryotic cell

Cadherins

Calcium-dependent transmembrane adhesion proteins; bind only to matching cadherin type, explaining cell self-sorting

E-cadherin

Cadherin type found on embryonic epidermal cells, letting them stick together

N-cadherin

Cadherin type found on neuronal cells, letting them stick together

Integrins

Transmembrane adhesion proteins that attach cells to the extracellular matrix and act as receptors relaying ECM information into the cell

Adherens junctions

Continuous belt-like anchoring junctions around a cell's circumference, linking actin cytoskeletons of neighboring epithelial cells via cadherins

Desmosomes

Discrete buttonlike anchoring junctions built from cadherins that connect to intermediate filaments, reinforcing epithelial strength

Hemidesmosomes

Junctions built from integrins that anchor epithelial cells to the underlying basal lamina, connecting to intermediate filaments

Tight junctions

Junctions built from claudins and occludins that create a true seal between cells, forcing substances to cross by passing through cells

Apical membrane vs basolateral membrane

The two membrane zones (top vs bottom/sides) created by tight junctions dividing the epithelial cell membrane

Gap junctions

Communicating junctions made of connexin proteins that allow ions and small molecules to pass directly between cells

Plasmodesmata

Passages through plant cell walls connecting adjacent cells, with continuous membranes and openings large enough for RNA and proteins

Extracellular matrix (ECM)

Insoluble meshwork of proteins and polysaccharides that provides structural support and signaling cues to cells

Middle lamella

First-formed layer of the plant cell wall; a gluelike carbohydrate layer that is the main way plant cells stick together

Primary cell wall

Plant cell wall layer formed while the cell is still growing; mainly cellulose plus pectin, thin and flexible

Secondary cell wall

Plant cell wall layer built after growth stops, made largely of cellulose plus lignin, which hardens and waterproofs the wall

Cellulose assembly

Unlike most ECM, cellulose is assembled outside the cell at the membrane surface, with building blocks and enzymes delivered by microtubules

Collagen

Most abundant protein in the animal ECM (~25% of body protein); three polypeptides wound into a triple helix, bundled into fibrils then fibers

Elastin and laminin

Fibrous ECM proteins (along with collagen) that give the ECM tensile strength

Connective tissue

Tissue type dominated by extracellular matrix with low cell density; provides physical connection and support

Benign tumor

Encapsulated tumor that grows continuously but stays contained, pushing outward against surrounding tissue

Malignant tumor

Dangerous tumor whose cells can metastasize, breaking away and spreading to distant body parts

Metastasis and the basal lamina

Cancer cells must cross a basal lamina twice (into and out of the bloodstream) to metastasize, using integrins to adhere

Bissell/Caron ECM experiment finding

Laminin specifically (not type IV collagen or HSPG) induces albumin gene expression in hepatocytes grown in 3D culture

Epithelial tissue

Tissue lining internal/external body surfaces; tightly packed cells joined by junctions, no blood vessels of its own

Simple vs stratified epithelium

Classification by layering: single layer = simple, more than one layer = stratified

Squamous, cuboidal, columnar

Classification of epithelial cells by shape: flat, round/square, and tall respectively

Muscle tissue

Tissue made of contractile fibers containing actin and myosin filaments; includes skeletal, cardiac, and smooth muscle

Nervous tissue

Tissue made mostly of neurons that take in sensory information, process it, and send signals to maintain homeostasis

Sensory neurons

Neurons that receive and transmit information about the environment or the body's internal state

Interneurons

Neurons that process information from sensory neurons and relay it to produce a suitable response

Motor neurons

Neurons that carry out the response, such as stimulating muscle contraction or adjusting internal physiology

Ganglia

Clusters of nerve cell bodies that act as relay stations, processing sensory input and sending signals to motor neurons

Nerve net

Simplest true nervous system, found in cnidarians; diffuse, with no ganglia and no central brain

Dendrites

Fiberlike neuron extensions that receive signals and carry them toward the cell body

Cell body

Part of the neuron containing the nucleus, where dendritic signals converge

Axon hillock

Junction between the cell body and axon where incoming signals are summed; triggers an action potential if strong enough

Axon

Neuron's output extension that carries the action potential away from the cell body to the axon terminals

Synapse and synaptic cleft

The junction between a presynaptic axon terminal and postsynaptic cell, separated by a 10-20 nm gap

Neurotransmitters

Chemical messengers released from the axon terminal that diffuse across the synaptic cleft and bind postsynaptic receptors

Glial cells

Non-signal-transmitting support cells that provide nutrition, physical support, insulation, and developmental guidance to neurons

Astrocytes and the blood-brain barrier

Glial cells that support endothelial cells joined by tight junctions, forming a selective barrier protecting brain tissue

Membrane potential

The electrical charge difference across a cell's membrane, measured in volts

Resting membrane potential

Baseline membrane potential (about -40 to -85 mV) when no signal is being sent, with the inside negative relative to outside

Sodium-potassium (Na+-K+) pump

Active transporter that moves 3 Na+ out for every 2 K+ in, using ATP, contributing to the resting potential

Potassium leak channels

Channels that allow K+ to passively diffuse out of the cell at rest, contributing to the negative resting potential

Depolarization

A shift in membrane potential toward less negative (or positive), typically triggered by neurotransmitter binding

Threshold potential

The critical level of depolarization (~15 mV above resting) that must be reached at the axon hillock to trigger an action potential

Action potential

A rapid, brief, all-or-nothing spike in membrane potential that propagates down the axon

Rising phase of action potential

Voltage-gated Na+ channels open rapidly, Na+ rushes in, creating a positive feedback loop that peaks around +40 mV

Falling phase (repolarization)

Na+ channels close/inactivate while voltage-gated K+ channels open fully, letting K+ diffuse out and lowering membrane potential

Hyperpolarization / refractory period

Brief dip below resting potential caused by prolonged K+ efflux, during which the neuron cannot fire another action potential

All-or-nothing principle

Once threshold is crossed, every action potential in a neuron is identical in size and shape regardless of stimulus strength

Frequency coding

Information is encoded by the rate and timing of action potentials, not by variation in a single action potential's size