Structure of the Cell Lecture (Part III) 2024 PDF

Faculty of Medicine Histology & Cell Biology The Structure of the Cell “Part III” By Dr. Amany Abd El Fattah Mohamed Lecturer of Medical Histology & Cell Biology  Learning outcomes ❑ At the end of the lecture,...

Faculty of Medicine Histology & Cell Biology The Structure of the Cell “Part III” By Dr. Amany Abd El Fattah Mohamed Lecturer of Medical Histology & Cell Biology  Learning outcomes ❑ At the end of the lecture, the students will be able to: 1. Mention components and function of cytoskeleton. 2. Describe structure and function of microtubules. Describe L.M & E.M of centriole. Describe structure of cilia & flagella. 1. Describe L.M & E.M of cytoplasmic filaments. Mention the cytoplasmic inclusions and their types.  Case Scenario A 15-year-old male patient came complaining of persistent low-grade fever for 2 months and pain in the throat since 10 days. His clinical examination revealed pallor, cervical lymphadenopathy, and splenomegaly. The physician asked for CBC which revealed anemia (HB 5.5 gm/dl), leucopenia (3000/cmm). Bone marrow biopsy provides a provisional diagnosis of acute lymphoid leukemia. He starts chemotherapy and was given vincristine. Q: What is the mechanism of action of this drug?  Learning outcome 1 Mention components and function of cytoskeleton.  The Cytoskeleton The structural framework of the cell. It is formed of: Microtubules. Microfilaments (actin). Intermediate filaments. They are important for: Determining cell shapes. Helping cell movement. Movement of organelles and vesicles inside cytoplasm.  Learning outcome 2 Describe structure and function of microtubules.  Microtubules Non-membranous organelles which are pipe-like structures of unfixed length but fixed diameter (25 nm). Structure: Formed of tubulin protein molecules (tubulin dimer). By polymerization →one protofilament. 13 protofilaments → 1 microtubule.  Polymerization is directed by microtubule organizing centers (MTOCs) e.g. centriole. Microtubules show dynamic instability Polymerization ↔ depolymerization.  L.M.: not seen. E.M.: - Hollow tubules, consist of 13 protofilaments.  Function: 1.Cytoskeleton. 2.The main structural component of centriole, cilia & flagella. 3.Tracks for organelles and chromosome movement (mitotic spindle). N.B: Cytotoxic drugs (used in ttt of cancer) prevent the polymerization of new microtubules → arrest mitosis of cancer cells.  Learning outcome 3 Describe L.M & E.M of centriole.  Centriole Non-membranous organelle important for cell division (each cell contains one pair). L.M: ✓H&E: not seen. ✓Special stain (Iron H.): appear as two dark blue-stained granules.  E.M: ✓2 short cylinders, perpendicular to each other. ✓Its wall is composed of 27 microtubules arranged in 9 peripheral triplets (3 microtubules).  Function: MTOC → formation of new microtubules (mitotic spindle in cell division). Form basal body of cilia  Learning outcome 4 Describe structure of cilia & flagella.  Cilia A non membranous organelle helps movement of materials (mucous) on its surface. L.M.: Hair-like processes project from the free surface of certain cells (e.g. respiratory tract). 10-15 µm long. They are several hundred/cell.  E.M.: - Surrounded by the cell membrane. - Formed of a shaft, basal body, and rootlets. - Basal body is a typical centriole. - Shaft is formed of 20 microtubules arranged in 9 peripheral doublets & 2 central singlets. Cilia (SEM)  Flagella Long cytoplasmic process presents in tail of a sperm that help sperm movement. One flagellum/sperm. Much longer than cilia (200 µm). E.M: - Similar to shaft of cilia.  Learning outcome 5 Describe L.M & E.M of cytoplasmic filaments.  Microfilaments Non-membranous organelles which are thread-like structures (6-8 nm in diameter). Structure: Formed of actin protein molecules (G- actin). By polymerization →one filament (F- Actin).  L.M.: not seen. E.M.: - Thin filaments.  Function: 1.Cytoskeleton. 2.Duringcell division, separate the daughter cells. 3.In muscles → contraction. N.B: Another type of filaments called myosin is present together with actin. Interactions between F-actin and myosin form the basis for various cell movements and for muscle contraction.  Intermediate filaments Thread-like structures (10 nm in diameter)→ intermediate. Characters: Stable → mechanical stability to cell structure. Made up of different proteins →different types: 1. Keratin in epithelial cells. 2. Desmin in muscle. 3. Neurofilaments in neurons.  Learning outcome 6 Mention the cytoplasmic inclusions and their types.  Cell Inclusion Non-living components of the cytoplasm. Not essential for the life of cells → not present in all cells. Types: 1. Stored food e.g. glycogen, lipids. 2. Pigments: naturally coloured substance → exogenous as dust, minerals, and tattoo. → endogenous as hemoglobin, and melanin.  Case discussion This is a case of acute lymphoid leukemia. Vincristine is a drug that blocks cell growth via stopping mitosis. It interferes with microtubule polymerization →prevent formation of mitotic spindle → prevent cancer cell division.  Summary and wrap up Cytoskeleton is the structural framework of the cell. It is formed of microtubules, microfilaments & intermediate filaments. Microtubules are tiny tubules formed of tubulin dimer. Centriole is MTOC important for cell division. Microfilaments are very thin threads formed of actin protein. Intermediate filaments have many types like keratin & desmin. Cytoplasmic inclusions include stored food as glycogen and pigments as melanin.  References Junqueira's Basic Histology Text & Atlas (16th ed.). https://www.youtube.com/watch?v=oIrnecBfHzg https://www.youtube.com/watch?v=tO-W8mvBa78 https://www.youtube.com/watch?v=YTv9ItGd050