Non-Membranous Organelles Lecture 10 PDF

Faculty of Medicine Histology Department NON-MEMBRANOUS ORGANELLES Lecture 10 In Block 102PMS Dr/ Maha Mahmoud Abd El Rouf Lecturer of Histology Histology...

Faculty of Medicine Histology Department NON-MEMBRANOUS ORGANELLES Lecture 10 In Block 102PMS Dr/ Maha Mahmoud Abd El Rouf Lecturer of Histology Histology and cell biology Department/ Faculty of Medicine/Assiut University 2021- 2022 Learning objectives After the lecture, students should be able to: - Define cytoskeleton and enumerate its components. - Discriminate the structure and function of microtubules and centrosomes. - Illustrate the structure and function of microfilaments. - Recognize structure and types of intermediate filaments. - Describe the structure and types of ribosomes. 1  Non- Membranous organelles They include: 1- Cytoskeleton which includes (Microfilaments, Intermediate filaments and Microtubules) 2- Proteosomes 3- Centrosomes 4- Ribosomes The Cytoskeleton The term cytoskeleton collectively refers to three separate classes of proteins seen as fine cytoplasmic filaments. In filament diameter; (1) the smallest class is the microfilaments (also called actin filaments); they are about 6 nm in diameter; (2) the next largest class is the intermediate filaments; they are about 8 to 10 nm in diameter; (3) the third and the largest class is the microtubules, which are about 25 nm in diameter. Function: 1-Determine the shapes of cells. 2- Play an important role in the movements of organelles and cytoplasmic vesicles. 3- Allow the movement of entire cells. 2  1-Microtubules - They are fine tubular structures within the cytoplasm of all eukaryotic cells. -- Most of which are highly dynamic in length. Structure: - Each microtubule is hollow, a rigid structure with an outer diameter of 25 nm, that help to maintain cell shape. - The protein subunit of a microtubule is a heterodimer of α and β tubulin. - Under appropriate conditions the tubulin polymerizes to form the microtubules. - The tubulin subunits align as protofilaments, with 13 parallel protofilaments forming the circumference of each microtubule wall. - Microtubules vary in length by a process of polymeraization or depolymerization of tubulin subunits. Functions: 1-Microtubules maintain the cell shape. 2- Help in intracellular transport of membranous vesicles, macromolecular complexes, and organelles. 3- Microtubules are organized to form dynamic structures such as mitotic spindle of cell division, centrioles, basal bodies and axoneme of cilia and flagella. 3 4  2-Microfilaments (Actin Filaments) - Actin filaments are thin (6 nm diameter), shorter and more flexible than microtubules. - The cytoplasmic actin filament is a thin 6-nm wide, very long polymer of a globular actin monomer; it is a homopolymer in that all its protein subunits are the same. - The actin filaments in a cell are highly dynamic (they are in a constant state of assembly and disassembly). Functions: 1- Actin filaments are found in great abundance in muscle cells (they make up about 60% of the protein in these cells); actin filaments integrated with myosin (a motor protein) permit very forceful contractions. 2- They are found as a major protein (10% to 15%) in essentially all nonmuscle cells as well, where they play a central role in "cell locomotion", "maintenance of cell shape", "translocation of cell organelles", "formation of the contractile ring in mitosis", and numerous other activities. 3-Intermediate Filaments - They are intermediate in size between the other two components, with a diameter averaging 10 nm. - Unlike microtubules and actin filaments, these intermediate filaments are stable. 5 - Intermediate filament proteins have particular biological, histological, or pathological importance. - They are made up of different protein subunits in different cell types. - Intermediate filaments can be localized in various cells by immunohistochemistry. There are 6 major classes of intermediate filaments class intermediate filament protein Cell distribution I. Acidic cytokeratin Epithelial cells II. Basic cytokeratin Epithelial cells III. Vimentin Mesenchymal cells desmin Muscle cells GFAP Astrocytes IV. Neurofilaments ( NF) Neurons V. Lamins Nuclei of all cells VI. Nestin Neural stem cells  Keratins or cytokeratins: in all epithelial cells. Intermediate filaments of keratins form large bundles (tonofibrils) that attach to certain junctions between epithelial cells. In skin epidermal cells, cytokeratins accumulate during differentiation in the process of keratinization producing an outer layer of non-living cells.  Vimentin: is found in most cells derived from embryonic mesenchyme. Important vimentin-like proteins include desmin found in almost all muscle cells and glial fibrillary acidic protein (GFAP) found especially in astrocytes, supporting cells of central nervous system tissue. 6  Neurofilament proteins that form the subunits of the major intermediate filaments of neurons.  Lamins (the nuclear lamina; lamin A and lamin B) are the intermediate filaments associated with the inner membrane of the nuclear envelope. The lamins help maintain nuclear shape, participate in anchoring chromatin to the nuclear envelope, are involved in gene transcription, and participate in nuclear assembly-disassembly during cell division. 7  Ribosomes Ribosomes: are small, non- membranous organelles about 20 × 30 nm in size, present in all animal cells in varying amounts depending on their activity in protein synthesis. Structure: - A functional ribosome has two subunits of different sizes (small subunit& large subunit) bound to a strand of mRNA. Chemically; ribosomes are composed of ribosomal RNA (rRNA) and proteins. - These ribosomal proteins are themselves synthesized in cytoplasmic ribosomes, but are then imported to the nucleus where they associate with newly synthesized rRNA. The ribosomal subunits thus formed then move from the nucleus to the cytoplasm where they are reused many times, for translation of any mRNA strand. 8  - During protein synthesis many ribosomes typically bind the same strand of mRNA to form larger complexes called polyribosomes, or polysomes. - In stained preparations of cells polyribosomes are intensely basophilic because of the numerous phosphate groups of the constituent RNA molecules. Types of polyribosomes: 1- Free polyribosomes: They exist as isolated cytoplasmic clusters and synthesize cytoplasmic proteins needed for cellular growth and differentiation. 2- Bound polyribosomes:  Endoplasmic reticulum (ER) Bound polyribosomes: Polyribosomes attached to membranes of the endoplasmic reticulum (ER). They are involved in the formation of membrane proteins of the ER, the Golgi apparatus, or the cell membrane; enzymes to be stored in lysosomes; and secretory proteins.  Polyribosomes associated with the outer membrane of the nuclear envelope.The outer membrane itself is continuous with the rough endoplasmic reticulum. Polyribosomes: free or bound to the endoplasmic reticulum 9  Proteasomes - Proteasomes are very small abundant protein complexes not associated with membrane, each approximately the size of the small ribosomal subunit. - Whereas lysosomes digest organelles or membranes by autophagy; proteasomes deal primarily with free proteins molecules including denatured and short-lived proteins. Structure: - It is a cylindrical structure made of four stacked rings of proteins including proteases. - At each end of the cylinder is a regulatory particle that contains ATPase and recognizes proteins with attached molecules of ubiquitin, an abundant cytosolic protein found in all cells. - Ubiquinated proteins are recognized by the regulatory particles of proteasomes, unfolded by the ATPase using energy from ATP, and then translocated into the core of the cylindrical structure and degraded by proteolytic enzymes 11 Functions: 1- Degrade denatured or nonfunctional polypeptides. 2- Remove proteins no longer needed by the cell. 3- It is considered a part of protein quality control system of the cell that prevents cell damage by aggregates of denatured proteins. In the brain, such aggregates may cause serious neurodegeneration as in case of Alzheimer disease. Centrosome Structure: - It is composed of two cylindrical centrioles surrounded by a pericentriolar matrix (PCM) that found close to the nucleus of dividing cells. - The two cylindrical centrioles have their long axes at right angles. - Each centriole is composed of nine highly organized microtubule triplets and there are no central microtubules. Before cell division, more specifically during the period of DNA replication, the centrosome duplicates itself so that now each centrosome has two pairs of centrioles. 11 Functions: 1-During mitosis, the centrosome divides into halves, which move to opposite poles of the cell, and become organizing centers for the microtubules of the mitotic spindle. Cells lack centrosomes cannot divide. 2- It serves as the basal body for cilia. Ciliated cells contain hundreds or even thousands of basal bodies or centrioles. 3- It serves as the basal body from which the microtubules of the tail of the sperm grow. References  Lippincott Illustrated Reviews: Integrated systems. 2016; 2: 41–42.  Elsevier’s Integrated Histology. 2007; 1: 24- 30.  Junqueira’s Basic Histology Text & Atlas. 2018; 1: 37 and 42- 47. 12