- Recognize cell organelles, which are visible by regular light microscopy (Nucleus, nucleolus, basophilic rough endoplasmic reticulum)
- Know different functions that are associated with different types of eukaryotic cell organelles.
- Review the secretory pathway and the function and structure of the organelles involved.
- Compare the three different cytoskeletal systems
- Know the different types of cell-cell and cell-substrate adhesion complexes and apical specializations (microvilli and cilia) and their association with cytoskeletal elements.
- Being able to recognize various cell organelles and cell junctions in electron micrographs and being able to identify striated or brush borders and cilia by light microscopy.
Due to their size and the limited resolution of light microscopy, most cellular organelles are not visible or their detailed structure can't be studies in regular stained tissue sections. The major exception is the cell nucleus of all nucleated cells. This is mainly due to its size and to its content, as nucleic acids are highly basophilic. In larger cells, such as oocytes and many neurons, additional details and substructures of nuclei can be analyzed by light microscopy. Look in the following two slides for Oocytes in the ovary: #239 Oocytes Webscope ,#239 Oocytes Webscope and for large motor neuron cell bodies in #065-2 grey matter of the ventral horn Webscope and in #065-2 dorsal root ganglia Webscope . Find large nuclei and study the distribution of euchromatin and heterochromatin. Note that due to the overall size of these cells, the nucleus may not always be in the plane of sections. In some nuclei you may also be able to detect an intensely stained basophilic nucleolus.
The detailed structure of the rough endoplasmic reticulum can't be studied by light microscopy. However, the abundance of membrane-bound ribosomes makes areas of rER extremely basophilic and therefore visible, especially in cells that are highly secretory. Good examples are exocrine pancreatic acinar cells and neurons, in which it is referred to as Nissle substance #065-1 Nissle substance Webscope . In some histological sections Golgi complexes appear as a pale or slightly eosinophilic (=eosin "loving", an area rich in membranes containing basic amino acids, syn. = acidophilic) regions adjacent to the cell nuclei. In some plasma cells it can sometimes be found in the form of a fine crescent adjacent to the nucleus. However, it takes some practice to recognize it.
In some secretory cells secretory vesicles can be visualized, especially if their content can be stained with regular histological dyes. View acinar cells in the exocrine pancreas and also the example of some paneth cells in the small intestine #029-1 paneth cells Webscope . Note the accumulation of secretory vesicles towards the lumen of the spaces, into which the secretory products will be released.
Again, individual endosomes and lysosomes are not visible using regular light microscopy. However, in some cell types, such as macrophages, these cellular compartments show up in regular histological sections as granular inclusions in the cytoplasm. Study the examples of #194 macrophage cells in liver Webscope macrophage cells in the liver and especially in the lung #130-1 macrophage Webscope .
Some apical specializations of epithelial cells are visible by light microscopy. Specifically when they are abundant. Due to their size, most cilia are easily recognizable. Identify cilia on the surface of respiratory epithelium. Mobile cilia contain a characteristic set of 9+2 microtubules, which can easily be recognized by electron microscopy and convey an ATP-driven active mobility to these cilia. Most eukaryotic cells also have non-mobile cilia (primary or monocilia), which appear to have a sensory function and have a 9+0 microtubule pattern lacking the central microtubule dimer, e. g. the kinocilium of hair cells. 029-1 Small Intestine (simple columnar epithelium, simple squamous epithelium) H&E Webscope Imagescope Single microvilli are usually too small to be visible by regular light microscopy. However, they become visible as a collective when they forming a dense layer at the apical surface of mostly absorptive epithelia (e. g. kidney tubules and intestine). These structures are referred to as brush or striated border. #270 orientation Webscope A second type of actin-containing apical protrusion are stereocilia. In size they approach the dimension of cilia and are readily visible by regular light microscopy. They can be found on sensory hair cells and the epithelium of the epididymis (Look at the orientation slide to find the region of the epididymus).
21 Plasma cell Webscope Imagescope
This electron micrograph shows a typical secretory cell, a plasma cell, which secretes immunoglobulin protein. Many of the major types of cellular organelles are visible in this image. In the nucleus, areas of euchromatin and heterochromatin can easily be identified. Use these micrographs to review the structure of organelles. Be sure you recognise favourable sections of the nucleas, mitochondria, and rough ER.
13 Golgi apparatus - Exocrine Pancreas Webscope Imagescope
The Golgi apparatus looks rather unusual in this electron micrograph. This is due to the enlarged stacks of cisternae (Golgi vacuoles), which distort the appearance of the Golgi complex.
227 Pancreas - Exocrine, detail of acinus Organelles of the Secretory Pathway Webscope Imagescope
Pancreatic acinar cells as depicted in this electron micrograph are cells that are highly specialized for protein secretion. Therefore, all the organelles of the protein secretory pathway are well-represented and are clearly visible in this micrograph.
14 Centrioles Webscope Imagescope
This image shows two centrioles, which represent the central structure of the microtubule-organizing center (MTOC). Some microtubules are also visible in the vicinity.
156 Cilia - Cross-sectioned in human trachea Webscope Imagescope
Cross sections of cilia. The typical 9+2 arrangement of the microtubules is especially evident. The basal bodies are centrioles and have 9 triplets of microtubules with no central pair.
170 Kidney - Cortex, proximal tuble Brush Border Webscope Imagescope
This EM micrograph depicts the typical appearance of microvilli on the apical surface of two types of cells with a striated or brush border. Shown is the epithelial lining cell of a proximal tubule in the kidney.
16 Epithelium - Desmosome and Intermediate Filaments Webscope Imagescope
A desmosome can be seen in the upper right corner of this transmission electron micrograph. The cytoplasm is full of intermediate filaments (tonofilaments), some of which are attached to the desmosomal plaque.
Click on a question to reveal the answer.
In which cell organelle(s) occur(s) the addition and modification of carbohydrate moieties of secretory and membrane proteins
N-linked glycosylation usually happens co-translational at the level of the rough endoplasmic reticulum. O-linked glycosylation and modifications of N-linked carbohydrate moieties are functions of the Golgi complex.
Describe the two major pathways of protein degradation in cells.
Extracellular and membrane proteins are taken up by cells using a vesicle- mediated process (usually coated vesicles) and are degraded via the endosome and lysosomal pathway. Many intracellular proteins, which are destined for degradation, are first covalently modified with multiple ubiquitin peptides. Subsequently they are recognized and proteolytically degraded by cytosolic proteasomes.
1. Which of the following cell organelles is NOT involved in the synthesis, processing and export of a secretory protein?
- Golgi complex
- Trans-Golgi network
- Secretory granules
- Plasma membrane
- Rough endoplasmic reticulum
Endosome - Endosomes are part of the endocytosis pathway
2. Which of these microscopy methods has the highest resolution (is able to depict the smallest structures)?
- Differential interference light microscopy
- Bright field light microscopy
- Fluorescence microscopy
- Transmission electron microscopy
- A magnifying glass
- Phase contrast microscopy
Transmission electron microscopy - Transmission electron microscopy as it uses very short wave electrons, rather than longer wave electromagnetic radiation (visible light or UV)
3. Which type of protein is NEVER synthesized by free ribosomes in the cytoplasm?
- Nuclear proteins
- Secreted proteins
- Cytoplasmic proteins
- Most Mitochondrial proteins
- Most Chloroplast proteins
- Peroxisomal proteins
Secreted proteins - secreted proteins are synthesized by rough ER membrane-bound ribosomes
4. Which of the following structures or terms is NOT associated with the proteolytic destruction of cellular proteins?
Ribosomes - Ribosomes synthesize proteins, not degrade
5. Which of the following statements about mitochondria is false?
- Mitochondria have two membranes/lipid bilayers
- The inner mitochondrial lipid bilayer contains a complex of electron-transporting proteins
- Mitochondria contain ribosomes, which synthesize mitochondrial proteins
- A proton gradient across the inner mitochondrial lipid bilayer drives the generation of ATP
- Mitochondria may be derived from fungal eukaryotic cells that have been taken up by a eukaryotic cell
Mitochondria may be derived from fungal eukarotic cells that have been taken up by a eukaryotic cell. - Fungi are deukaryotic cells that have their own mitochondria. A The endosymbiotic hypothesis states that cyanobacteria were taken up by early eukaryotic cells and converted into mitochondria and chloroplasts
6. Which of the following terms does NOT describe a cytoskeletal structure/protein complex?
- Nucleosomes (Histones)
- Microfilaments (Actin)
- Membrane skeleton (Spectrin-ankyrin)
- Microtubules (Tubulin)
- Intermediate filaments (Keratin, Neurofilaments etc. )
Nucleosomes (Histones) - Nucleosomes (Histones) are responsible for DNA packaging