Sheet #7 Dr. Heba Kalbouneh Dr. Heba Kalbouneh Dr. Heba Kalbouneh
Connective tissue The differences between epithelial and connective tissue - Epithelial cells are tightly packed (no or minimal spaces between cells) - Connective tissue cells are widely separated, no cell junctions, high amount of ECM - Epithelium originates from the 3 embryonic layers (endoderm : GI tract, ectoderm : skin, mesoderm: blood vessels ) - Connective tissue originates from the mesenchyme (the middle layer of embryo (mesoderm) - Mesenchyme is a mesodermal embryonic tissue - Connective tissue is composed of cells and ECM (ground substance and fibers). It is a filing tissue - Connective tissue is highly vascularized - The Ground substance (water + stabilizing macromolecules to form gel like structure) doesn t have shape - Connective tissue has variable regeneration power. We will study different types of c.t, and each has its own regeneration power The function of the connective tissue: 1- Support: this support is mechanical support (support epithelial tissue) and nutritional (c.t has blood vessels (capillaries) and by diffusion epithelial cells get their oxygen and nutrients and eliminate their waste products) 2- defense and protection ( contains inflammatory cells ) 3- storage ( like adipose tissue= it stores lipids) 4- transport (it is a medium for transport of small molecules and gases) Refer to figures in the powerpoint presentation The connective tissue is composed of: 1-Cells 2- Extracellular matrix (ground substance and fibers). The ground substance: the medium where fibers and cells are embedded. Fibers are proteins polymerize to form the fibers The different proportions of these components give different types of the connective tissue.
Cells of the connective tissue either are fixed or transient. Transient cells: they are involved in inflammation (immune response) so that what makes connective tissue involved in the immune response. These cells leave blood by migrating between the endothelial cells to enter connective tissue (this is called diapedesis) Again. We have 2 kinds of cells in c.t * Fixed cells: always found in c.t *transient cells: come on demand if we have inflammatory process, they are recruited from the blood and enter the c.t to perform their defense function Fibroblasts - Fibroblast:-active cell that synthesizes fibers + ground substance (Extra cellular material), it is the most numerous cell in c.t - Blast in histology means active cell in synthesis - Fibrocyte is the mature/ retired cell that has done its job and it s in a resting state - Originate from undifferentiated MESENCHYMAL cells (from mesenchyme) - Fibroblasts make collagen fibers (the main component of tendons) Fibrocyte can redifferentiate to fibroblast in case of injury, in order to replace the damaged tissue with ECM Fibroblast Active large abundant cytoplasm full with organelles Prominent nucleus and rounded euchromatic nucleus Many Processes Elongated and more rounded cell Growing phase Fibrocyte Inactive Smaller Scant cytoplasm with Little organelles Nucleus dark (heterochromatic) and flattened Fewer processes Spindle-shaped Mature phase
- Rarely undergo cell division: mature cell doesn t undergo division except in the case of injury - Myofibroblast contains a high amount of actin and myosin, these cells produce contraction at the site of skin wound for example, these cells are then capable of speeding wound repair by contracting the edges of the wound. - Are closely associated with collagen bundles. - Cytoplasm is acidophilic and difficult to be differentiated from ECM. Note that the ECM is acidophilic. Since we cant see the cell membrane under the light microscope using H&E, you cant outline the cytoplasm of fibroblasts. They appear as naked nuclei Myofibroblast - Has features of both smooth muscles and fibroblasts. - Their contraction is responsible for wound contraction. Mast cells: - under EM its cytoplasm appears filled with secretory vesicles, some thought it was a phagocytic material or ingested food from the outer environment so they called it Mast cells (The word "mast" refers to food) - Large cells. - Cytoplasm is full with secretory granules. - Nucleus is rounded and euchromatic because they re active cells. - Derived from precursors produced by hematopoiesis Explanation: Blood has two main types of cells: Red blood cells: carrying o2 White blood cells: immune system (protection against invading microorganisms) Both types of cells originate from the bone marrow in a process called Hematopoiesis.
Hematopoiesis: Production of blood cells. Mast cells also originate from the bone marrow - Mast cells are alarm cells scattered within C.T, any slight disturbance of the tissue makes them release their content in order to recruit the inflammatory cells and trigger the Process of inflammation, such as: - Histamine: (important in allergic reactions), it increases blood flow which means increasing diameter of blood vessel and the permeability. - Heparin: anticoagulant at the site of inflammation - Leukotriene. - Eosinophil chemotactic factor - Neutrophil chemotactic factor - Neutrophils and eosinophils are white blood cells and part of immune system - They are seen in 2 places ( places exposed to antigens): 1. Perivascular mast cells which surround blood vessels. 2. Mucosal mast cells which are in mucous membrane lining the cavities that open to the outside like in GI tract, respiratory tract.. - They re waiting for any disturbance or allergen to secrete their granules; Allergies are caused (in part) by inappropriate sensitivity of mast cells. The symptoms are treated with antihistamines, chemicals which interfere with the action of histamine - Under LM the nucleus appear rounded and pale (euchromatic) and you can t see cytoplasm, because it s filled with secretory granules which are called metachromatic granules. - Metachromatic granules: able to change the color of the stain (ex: change the color of toluidine blue from blue to red purple (violet)).
- This property is called Metachromasia refers to the ability of changing the color of the dye. Plasma cells: Remember: they are transient cells not fixed. - Origin: All blood cells are produced within bone marrow; blood contains red and white blood cells. Hematopoiesis is the formation of blood cells, hemato refers to blood, poiesis refers to formation of. - Lymphocytes (are small white blood cells with large nucleus, little cytoplasm and can enter and exit C.T easily). - These lymphocytes are very important weapons in our immune system; they give 2 types (T, B). - B lymphocytes give the plasma cells (origin). - Plasma cells are seen in chronic inflammation areas due to their function (producing Antibodies proteins ) how? - When a certain antigen enters like bacteria, these cells will synthesize antibodies against them so when dealing with the same antigen again we don t need to produce new antibodies. As they synthesize antibodies they are Protein synthesizing cells. For describing plasma cells: - Large rounded cells. - Its Nucleus is Euchromatic and Eccentric (not in the center) it is toward the periphery (near the plasma membrane of the cell.) - Cytoplasm is full with organelles (indicating the activity of the cell): - Prominent RER for the synthesis of proteins= cytoplasm is Basophilic. - Prominent Golgi apparatus for packaging of these proteins (-ve Golgi stain; it appears pale or non-stained because it s composed of lipids and can t be stained with H &E or aqueous stains). - Nucleolus presence indicates the activity of the cells. - The heterochromatin of plasma cells is typically clumped in a characteristic "spoke-wheel" arrangement which also aids plasma cell recognition. This appearance is called:
Cart wheel appearance (nucleolus at the center and there s heterochromatin lines going outside it). Or Clock-face appearance (peripheral regions of heterochromatin alternating with lighter areas of euchromatin) Macrophage - Function: phagocytosis, SO we predict the presence of lysosomes in its cytoplasm - Origin: (Monocytes from bone marrow) Stem cells in bone marrow give Monocytes which are released into blood (circulating cells), once they enter C.T they become Macrophages (fixed cells in C.T),when an antigen enters c.t, macrophages become activated. - Always in C.T you ll find scattered number of macrophages (fixed) waiting for any antigen, when the C.T is exposed to an antigen (infection), it will cause recruitment of other macrophages from blood circulation ( more differentiation of monocytes into macrophages) so it is considered also a transient cell Describing Macrophages: - Large cells with many cell membrane projections to phagocytose (pseudopodia). - Eccentric nucleus, indented (kidney shaped) or oval shaped. - Macrophages are usually difficult to distinguish under the light microscope. One way to visualize them is to inject an animal with carbon particles. Macrophages which come into contact with the circulating particles will phagocytose some of them. These particles will be visible as dark black accumulations in the cytoplasm of the macrophages - In very active macrophages which have accumulated indigestible material, the lysosomes (residual bodies) may be visible by light microscopy as dark brown granules also. The primary lysosome fuses with the phagosome that contains the engulfed material and becomes secondary lysosome. The undigested material accumulate in the cytoplasm giving a brown color (residual bodies) -
Remember: - Primary lysosome fuses with phagosome (ingested material) to become secondary lysosome. After digestion it will become residual body (the remaining undigested material). Accumulation of Residual bodies inside certain cells gives what is called the age pigment lipofuscin pigment which you ll see it in long lived cells that have undergone long processes of digestion (a lot of Residual bodies in their cytoplasm). - Same concept in Macrophages: when they have been very active in digestion they accumulate large amount of residual bodies in their cytoplasm so they appear pigmented. FOR EX: lung alveoli are lined with simple squamous epithelium with thin layer of C.T, lung macrophages (dust cells) are in direct contact with inhaled air. In the lung, macrophages "patrol" the respiratory surfaces and ingest airborne particles which settle there. They can be identified by the accumulations of fine particles in their cytoplasm. - They are part of MPS mononuclear phagocytic system : MPS is a system of phagocytic cells in different locations in our body with different names and it is part of the immune system. - In lungs : dust cells - In liver: kupffer cells - In central nervous system: microglial cells - In skin: langerhans cells - In bone : osteoclasts All these cells are characterized by phagocytic activity Some are macrophages and some look like them and have same function - They used to name this system as Reticulo-endothelial System but it is no longer used, why: Reticulo-endothelial System is a system composed of certain organs (liver, spleen, lymph nodes) composed of blood filled channels (lined with endothelium) supported by reticular fibers, so when injecting the animal with carbon material they ve seen the endothelium of these organs also colored with pigments SO they said that endothelial cells are the origin of macrophages or the same cells. But later on they ve
discovered that this is wrong and these endothelial cells only Shuttle the ingested material to the underlying C.T by endocytosis and exocytosis - Macrophages were also named Histiocytes - Also macrophages can aggregate in case of injuries to form giant cells (huge cells)