Term tissue by BICHAT (1771 -1802) [French]

Term Histology by Mayer (1819)

• ''Specialized masses of cell with similar morphology, physiology and embryonic origin. These are developed from undifferentiated cells of the primary germ layers''. Tissues are developed from undifferentiated cells of the primary germ layers though the specialised process of ''cell differentiation''.
  
  


• Earlier it was thought that genetic composition of cell undergoes changes in such a way that one daughter cell inherits different set of genes from other daughter cells i.e., some of the genes are lost during the process. But, today from experiments it is very clear that differentiation results not from the loss of genes but from selective repression of selective genetic operans. Infact, electron micrographs suggest that some segments of DNA Helixes wound around histone cores become so condensed that they will no longer uncoil to form RNA molecules. Therefore, the repressed genes will never function again. (Regardless of the mechanism most mature cells of human beings produce about 8000 to 10,000 proteins rather than the potential 100,000 or more, if all genes are active.)
  
  


• Certain cells of embryo control the differentiation of other cells. For example, the primordial chordamesoderm is called the primary organizer of embryo and forms a focus around which rest of the embryo develops. It differentiates into a mesodermal axis that contains segmentally arranged somites and as a result of inductions in the surrounding tissues, causes formation of essentially all the organs of the body.
  
  


• Therefore, a large share of the embryo develops as a result of such inductions, one part of the body affecting another part, and this part affecting still other parts.
  
  


• Tissues can be categorised to -
  
  
  
  1. Epithelial Tissue
  
  
  2. Connective Tissues
  
  
  3. Muscular Tissues
  
  
  4. Nervous tissue
  
  
  
  

EPITHELIUM

Term epithelium by RUSCH (Dutch)

• Epithelium is the tissue which is present as the lining of-
  
  
  
  1. External and Internal exposed parts of the body.
  
  
  2. Open and closed cavities of the body.
  
  
  3. All blood and Lymph vessels of the body.
  
  
  4. All glands and their ducts.
  
  
  
  


• Epithelium is the tissue which has triplet origin i.e.,
  
  
  
  1. Ectoderm- eg. Skin epidermis
  
  
  2. Mesoderm -eg. Peritoneum
  
  
  3. Endoderm-eg. Gut epithelium
  
  
  
  


• Epithelium can be



• Before going in detail about the epithelial cells it is better to understand about the Junctional complexes present between the epithelial cells to bear mechanical stress. These complexes can visualized as -
  
  
  
  

Fig. 1.1 Junctional complexes 1. Tight Junctions; 2. Fine filaments; 3. Belt desmosomes; 4. Adherence web; 5. Core microfilaments; 6. Spot desmosomes; 7.Gap junction; 8. Hemidesmosomes; 9. Tonofilaments; 10. Microvillus

• Junctional complexes are of following 4 types -
  
  
  
  1. The tight junction or zonula occludens.
  
  
  2. The belt desmosome or intermediary junction (zonula adherens).
  
  
  3. The spot desmosome or macula adherens.
  
  
  4. The gap junction or nexus.
  
  
  
  

1. Tight junctions or Zonula Occludens(also called terminal bar.) |
   
   
   
   • In the region of tight junctions plasma Membrane of two adjacent cells fuses at a series of point containing sealing strand (Lines of attachment).
   
   
   • Blocks the passage of substances between cells.
   
   
   • Tight ness depends upon no of sealing strands, Connected to cytoplasmic strands for reinforcement as shown below -
   
   
   
   
   
   Fig. 1.2 Showing tight junctions and belt desmosomes along with sealing strands


2. Desmosomes - For cell adherence which are of following two types -
   
   
   
   
   1. Belt Desmosomes or Intermediary junction or Zonula adherence
      
      
      
      • Just below the tight junctions as a band on the Inner side of cell membrane.
      
      
      • Junctional zone is filled with fine filaments of 70A° diameter & contain actin (Contractile)
      
      
      • Helpful in closing gaps resulting from death and sloughing away of cells.
      
      
      • Also responsible for movement & changes in shape.
      
      
      • These filaments might help in filling gaps in the epithelium resulting from death and sloughing away of cells, and may be responsible for movement and changes in shape of epithelial cells in the developing embryo during organ formation..
        
        
      
      
      
      
   
   
   2. Spot Desmosomes or macula adherence
      
      
      
      • These are disc shaped points of contact between the plasma membranes of adjacent cells.
      
      
      • Here the intercellular gap is about 300-500A0.
      
      
      • These consist of a disc-shaped plaque (0.5mm diameter) on the cytoplasmic surface of each cell membrane.
      
      
      • Non-contractile 100A0filaments called tonofilaments, are found attached to the plaques.
      
      
      • These form the structural framework of the cytoplasm.
      
      
      • Thin transmembrane linkers arising from the plaques fill the intercellular gap between the membranes.
      
      
      • The filaments are bisected by a dense line called the central stratum.
      
      
      • Their complete structure looks like -
      
      
      
      Fig. 1.3 showing Spot Desmosomes
      
      
      • In the basal region of some epithelial cells only half desmosomes may be seen. Such desmosomes are called HEMIDESMOSOMES. These serve as anchoring sites for bundles of tonofilaments, and also for joining the cell membrane to the underlying basement membrane.
        
        
      
      
      
      
   
   
   
   


3. Gapjunctions or Nexus.
   
   
   
   
   • The Gap junction are patch-like areas of close intercellular contact.Here, the intercellular space is narrow (about 30A0).
   
   
   • These junctions consists of a disc shaped area on each cell membrane containing several hollow, cylindrical particles. The particles are about 70A0in diameter, and form intercellular pipes through which, exchange of certain substances takes place.
   
   
   • The channels in the particles are about 20A0in diameter and permit the passage of molecules of molecular weight upto 1,000 daltons, including ions, most sugars, amino acids, nucleotides, vitamins, steroid hormones and cyclic AMP.
   
   
   • Such a connecting system is important for cells, especially in the cells of embryos before establishment of the circulatory system.
   
   
   • Ca ions are found to regulate the activity of these gap junctions.
   
   
   • In electrically excitable tissue, gap junctions are responsible for the transmission of electrical activity without a neurotransmitter. In such cases they are known as ELECTRONIC SYNAPSE. The contraction of the muscle cells of the heart and of smooth muscles is synchronized by gap junctions.
   
   
   • These junctions looks like -
   
   
   
   
   
   Fig. 1.4 showing Gap junction