Cell Biology

OBJECTIVES:

1. Recognize cell organelles, which are visible by regular light microscopy (Nucleus, nucleolus, basophilic rough endoplasmic reticulum)
2. Know different functions that are associated with different types of eukaryotic cell organelles.
3. Review the secretory pathway and the function and structure of the organelles involved.
4. Compare the three different cytoskeletal systems
5. Know the different types of cell-cell and cell-substrate adhesion complexes and apical specializations (microvilli and cilia) and their association with cytoskeletal elements.
6. 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.

I. Nuclei and Nucleoli

239 Ovary, monkey H&E Webscope Imagescope
065-1 Spinal cord, cross section Masson Webscope  Imagescope

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.

II. Rough Endoplasmic Reticulum and Golgi Complex

WU 98 pancreas, thin section H&E Webscope Imagescope
065-1 Spinal cord, cross section Masson Webscope  Imagescope

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.

III. Secretory Vesicles

WU 98 pancreas, thin section H&E Webscope  Imagescope

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.

IV. Lysosomes/Endosome

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 .

V. Cilia, Microvilli and Stereocillia

040 trachea H&E Webscope  Imagescope

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).

VI. Cell Junctions (Desmosomes)

112 thick skin, sole of foot Webscope  Imagescope

Most cell junctions are not visible using light microscopy. The spines in the stratum spinosum of the skin are one notable exception.   These spines represent desmosomes that hold skin epithelial cells together.

Electron Micrographs