Animal Cell Shape Regulation

Lecture 5: Regulation of Animal Cell Shape

• The lecture focuses on animal cells.
• The major topics covered are the cytoskeleton, cell junctions, and the extracellular matrix.

Lecture 4 Practice Questions

• Pinocytosis is the uptake of extracellular fluid.
• Two key roles of the smooth endoplasmic reticulum (sER) required naming in a practice question.
• The purpose of tagging vesicles leaving the Golgi complex.

Lecture 5 Objectives

• Identify the major components of the cytoskeleton (microtubules, microfilaments, intermediate filaments).
• Explain how these structures regulate cell shape.
• Outline the importance of cell junctions (tight, gap and desmosomes).
• Describe the composition and origin of the extracellular matrix.

The Cytoskeleton

• Helps maintain cell shape and the position of organelles within cells.
• It rapidly disassembles and reassembles, unlike the body's skeletal system.
• This dynamic nature facilitates rapid changes in cell shape.
• The cytoskeleton provides stability despite its dynamic nature.
• The three main components are microtubules, microfilaments, and intermediate filaments.

Microtubules

• Composed of tubulin subunits.
• They radiate out from an organizing center called the centrosome.
• They resist compression and maintain cell shape.
• They provides cell motility.
• Flagella exhibit "snake-like" motion, while cilia exhibit "rowing-like" motion.
• Beating of cilia moves fluid past cells if the cells are fixed in place.
• Microtubules facilitate organelle motility within the cell.
• ATP-powered motor proteins "walk" organelles along microtubules.
• Vesicles, or other organelles, are transported to specific targets within the cell.

Microfilaments

• Double chains of actin subunits.
• They form linear strands and 3-dimensional networks (using branching proteins).
• They resist tension.
• The cortical network under the plasma membrane helps reduce fluidity and maintain cell shape.
• Interactions between actin and motor proteins like myosin support cell movement.
• Actin-myosin interactions allow muscle contraction.
• Non-animal examples include amoeboid movement and cytoplasmic streaming in plants.

Intermediate Filaments

• Made of various proteins, including keratins in hair, lamins in the nucleus, and neurofilaments in neurons.
• They are supercoiled into “cables.”
• Less dynamic than microtubules or microfilaments.
• Contribute to maintenance of cell shape and anchoring of organelles.
• Relatively permanent and can remain after the cell dies, such as in hair and the outer layer of skin.

Cell Junctions

• Three major types: tight junctions, desmosomes, and gap junctions.
• Each type differs in structure and function.

Tight Junctions

• Hold neighboring cells tightly pressed together.
• They form a continuous seal, preventing fluid movement across cell layers.

Desmosomes

• Anchoring junctions that provide attachments between sheets of cells, e.g., muscle.
• Act like rivets; a “torn muscle" indicates a torn desmosome.
• Connected into the cell by intermediate filaments.

Gap Junctions

• Represent a point of cytoplasmic contact between two cells.
• Ions and small molecules can pass from cell to cell allowing rapid cell-to-cell (intercellular) communication.

The Extracellular Matrix (ECM)

• In many tissues, cells do not make direct contact with other cells, instead, cells lie within the extracellular matrix (ECM).
• The ECM composition varies between tissues.
• Composed of materials secreted by cells via constitutive exocytosis.
• Most ECM proteins are glycoproteins (proteins with added carbohydrates).
• The most abundant ECM glycoprotein is collagen.
• Collagen fibers have great tensile strength.
• Collagen fibers are embedded in a proteoglycan complex matrix (green in figure).
• Proteoglycans are proteins with extensive sugar additions.
• Proteoglycans trap water within the ECM, resisting compression and helping retain tissue shape.
• ECM is important in providing strength and resilience to skin.
• As one ages, collagen (strength) and elastin (stretch) break down.
• Hyaluronic acid is a glycosaminoglycan (part of proteoglycans) and is involved in retaining moisture to 'plump' skin.
• Fibronectins attach cells to ECM.
• Integrins connect the ECM to the cytoskeleton, providing a communication link from ECM to the cell interior.

Lecture 5 Summary:

• The cytoskeleton is dynamic, and provides structure and stability and has three main components.
• These components (microtubules, microfilaments and intermediate filaments) have distinct structures and therefore distinct functions within the cell.
• There are three main types of cell junctions (tight junctions, desmosomes and gap junctions) which are important for adhesion and communication between cells.
• The extracellular matrix (ECM) is the non-cellular part of tissues and is secreted via constitutive exocytosis.