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Cytoskeleton, Microvilli, Cilia, Stereocilia, and Epithelial Junctions – Vocabulary Flashcards

Cytoskeleton, Microvilli, Cilia, and Stereocilia

  • The Cytoskeleton acts as the cell's internal scaffolding, giving shape and support to special finger-like parts that stick out from the cell surface (like microvilli, cilia, and stereocilia). "Actin filaments" are a key part of this support system.
  • Cilia are like tiny, moving hairs that can beat, creating a current. They can move things on the cell's surface in one direction or back and forth.
  • Microvilli are short, non-moving "fingers" that mostly increase the cell's surface area, helping it absorb more stuff.
  • Stereocilia are also non-moving, like microvilli, but they are longer and look messy, not organized at all.
  • In short: Microvilli and stereocilia are mainly for absorption, while cilia are mostly for moving things along surfaces.

Microvilli and brush border

  • Microvilli are tiny extensions that stick out from the top (apical) surface of cells; the cell's internal jelly (cytoplasm) can reach into them.
  • Their main job is to increase the surface area so the cell can absorb more nutrients or other substances.
  • When seen under a regular microscope, they look like a fuzzy, organized "brush" on the cell surface, often called a brush border.
  • A Glycocalyx is a layer of tiny sugar chains that stick out from the microvilli. This makes the surface a bit slippery and helps cells recognize each other.
  • The organized look of microvilli is what forms the uniform "brush-border" appearance, which is really important for absorption in places like the lining of your intestines.
  • Simple way to remember: Microvilli create that "brush border" look.

Stereocilia

  • Stereocilia look similar to microvilli but are messy and disorganized, like uncombed hair. They are longer and not arranged neatly like microvilli.
  • Structure: They contain actin filaments (like microvilli) and also stick out from the cell's top surface. They do not move.
  • Compared to microvilli: Stereocilia are longer and irregular; microvilli are shorter and make a neat brush border.
  • Where you typically find them: In the epididymis (a tube on top of the testicle). Here, stereocilia help with reabsorption and other absorption processes.

Cilia

  • Cilia are bigger than microvilli and have a very specific internal structure.
  • Size:
    • Cilia: about 1~\text{µm} \times 0.1~\text{µm}
    • Microvilli: about 5-10~\text{µm} \times 2~\text{µm}
    • Note: These sizes are from the class; typical textbook numbers might be slightly different.
  • Internal Structure (Ultrastructure): Inside, they have 9 pairs of tiny rods (microtubules) arranged in a circle around two central microtubules – this is called the 9+2 arrangement.
  • Movement (Motility): Most cilia can move and beat to push fluids or substances along the cell surface. However, some cilia don't move.
  • Main jobs and examples:
    • In your respiratory system (like in your windpipe), cilia beat to push mucus and trapped dirt particles up towards your throat to be swallowed or coughed out.
    • In fallopian tubes (which carry eggs from the ovaries to the uterus), cilia beat to help move both sperm and the egg.
    • Ependymal cells line the fluid-filled spaces in your brain (ventricles) and their cilia help move the cerebrospinal fluid (CSF).
    • A single cilium on a cell, especially if it helps the whole cell move (like a sperm tail), is typically called a flagellum.
  • What they look like: In detailed pictures (SEM images), cilia look longer and more prominent. Microvilli look shorter and like a dense, fuzzy layer.

Visual and imaging considerations

  • Special cross-section views (SEM) can show the unique 9+2 arrangement inside cilia. You won't see this in stereocilia or microvilli.
  • In microvilli, the internal cell material (cytoplasm) extends into them in a less organized way, making them appear as a dense, short "brush border" when magnified.
  • The lesson really emphasizes looking at pictures! Visual clues (like organized cilia versus messy stereocilia) make it much easier to tell them apart than just reading descriptions.

Lateral surface and junctions (cell–cell and cell–basal interactions)

  • Cells that make up a tissue can work together as a team or act more individually. Special connections called junctions control how cells communicate and how tightly sealed a tissue barrier is.
  • Tight junctions (also called occluding junctions): These are like a seal around the top part of the cell, preventing anything from slipping between cells (paracellular passage). They create a strong barrier.
  • Zona adherens (adherens junctions): These junctions act like glue, holding cells together.
  • Desmosomes: These are strong anchoring points that connect the internal support fibers (intermediate filaments) of one cell to another. They are generally stronger than zona adherens.
  • Hemidesmosomes: These are like "half-desmosomes" that anchor a cell to the layer beneath it, called the basal lamina (or basement membrane), rather than to another cell.
  • Basal lamina and basement membrane:
    • The basal lamina is a thin, dense layer found right underneath epithelial cells.
    • A key protein in the basal lamina is Type IV collagen.
    • Hemidesmosomes use this basal lamina to firmly attach cells to the underlying tissue.
  • Gap junctions: These are like tiny tunnels connecting the insides (cytoplasm) of neighboring cells directly. They allow small molecules and electrical signals (ions) to pass through, helping cells talk to each other and share resources.
    • They are formed by proteins called connexins which come together to make channels called connexons.
    • Their main feature is permeability, meaning small molecules, ions, and possibly signaling chemicals can freely move between cells.
  • Summary of junction types (what they do):
    • Tight junctions: Create a barrier to stop substances from moving between cells.
    • Zona adherens: Mechanically link adjacent cells together.
    • Desmosomes: Provide strong cell-to-cell adhesion by connecting internal filaments.
    • Hemidesmosomes: Anchor cells to the underlying basement membrane.
    • Gap junctions: Allow cells to communicate and share small molecules/signals directly.

Connections and practical implications

  • How cilia, microvilli, and stereocilia are arranged directly impacts what a tissue can do: absorption, movement, or specialized functions in different body parts.
  • Where you find them and what they do:
    • Epididymis: Stereocilia here are specialized for absorption.
    • Respiratory tract (windpipe) and oviducts (fallopian tubes): Cilia help clear mucus and move eggs/sperm, respectively.
    • Brain ventricles: Ependymal cilia help circulate the fluid around the brain (CSF).
  • Problems with these cell connections (junctions) can lead to issues with how tissues form barriers or stay together, potentially causing diseases.
  • The basal lamina (with its collagen IV) is crucial for tissue structure and signals. Defects in it can affect how cells stick together and the overall health of the basement membrane.

Quick recall: key terms and relationships

  • Microvilli: Short, uniform, for absorption; create the brush border; increase surface area; don't move.
  • Stereocilia: Long, messy microvilli-like structures; don't move; found in the epididymis.
  • Cilia: Usually move, longer than microvilli; have the 9+2 internal structure; beat to move things; found in the respiratory tract, fallopian tubes, brain ventricles.
  • Flagellum: A single cilium, like a sperm tail; used for propulsion.
  • Tight junctions: Seal cells, no passage between them.
  • Zona adherens: Stick cells together.
  • Desmosomes: Strong anchors between cells.
  • Hemidesmosomes: Anchor cells to the basement membrane; look like half a desmosome.
  • Gap junctions: Channels for small molecules/signals between cells; made of connexins/connexons.
  • Basal lamina: Supportive layer under cells; contains Type IV collagen.

Observational tips for exams

  • When identifying: Look at how organized they are and their length. Cilia are usually longer and in neat rows. Microvilli form a uniform, short brush border. Stereocilia are long and messy.
  • Remember what they do: Moving structures (cilia/flagella) mean they push things. Absorbing structures (microvilli) mean they increase surface area.
  • In diagrams: Note where junctions are: tight junctions are at the very top, adherens and desmosomes are on the sides, hemidesmosomes are at the bottom attaching to the basal lamina, and gap junctions are small tunnels between cells.
  • Last advice from the lecturer: Focus on the

Interphase

  • This is the longest phase of the cell cycle, during which the cell grows and copies its DNA before entering mitosis.
  • It is divided into three main sub-phases:
    1. G1 Phase (First Gap):
      • The cell grows, carries out normal metabolic functions, and produces proteins and organelles.
      • It prepares for DNA replication.
    2. S Phase (Synthesis):
      • DNA replication occurs, meaning each chromosome is duplicated to become two sister chromatids joined at the centromere.
      • The amount of DNA in the cell doubles (e.g., from 2n to 4n if considering DNA content).
    3. G2 Phase (Second Gap):
      • The cell continues to grow and synthesizes proteins and organelles necessary for cell division.
      • It checks the duplicated chromosomes for errors and makes any necessary repairs.
      • The cell prepares for mitosis.

Mitosis (M Phase)

  • This is the process of nuclear division, resulting in two identical daughter nuclei.
  • It is generally divided into four main stages, followed by cytokinesis:
    1. Prophase:
      • Chromatin condenses into visible chromosomes, each consisting of two sister chromatids.
      • The nuclear envelope begins to break down.
      • The mitotic spindle (made of microtubules) begins to form from the centrosomes, which move to opposite poles of the cell.
    2. Metaphase:
      • Chromosomes align at the metaphase plate (equatorial plate) in the middle of the cell.
      • Each sister chromatid is attached to a spindle fiber from opposite poles.
    3. Anaphase:
      • Sister chromatids separate and are pulled apart by the shortening spindle fibers.
      • Each chromatid becomes an individual chromosome, moving towards opposite poles.
      • The cell elongates.
    4. Telophase:
      • Chromosomes arrive at opposite poles and begin to decondense.
      • New nuclear envelopes form around the two sets of chromosomes.
      • The mitotic spindle disassembles.

Cytokinesis

  • This is the division of the cytoplasm, which usually overlaps with telophase.
  • It results in the formation of two separate daughter cells, each with a complete set of chromosomes and organelles.
  • In animal cells, a cleavage furrow forms and pinches the cell in two.
  • In plant cells, a cell plate forms to create a new cell wall between the daughter cells.