NPTEL Biotechnology Tissue Engineering. Tissue Organization

Transcription

1 Tissue Organization S. Swaminathan Director Centre for Nanotechnology & Advanced Biomaterials School of Chemical & Biotechnology SASTRA University Thanjavur Tamil Nadu Joint Initiative of IITs and IISc Funded by MHRD Page 1 of 8

2 Table of Contents 1. TISSUE ORGANIZATION Cells TISSUE TYPES Epithelial tissue Squamous epithelium Simple cuboidal epithelium Simple columnar epithelium Ciliated columnar epithelium Glandular epithelium Stratified Epithelium Pseudostratified epithelium Connective tissue Muscular tissue Nervous tissue Glandular tissue...7 Joint Initiative of IITs and IISc Funded by MHRD Page 2 of 8

3 1. Tissue organization The human body consists of ten organs and each organ performs its own physiological functions. Each organ comprises of a number of tissues like epithelial, connective, nerve, muscle and glandular. The word tissue is derived from the French meaning to weave. A tissue is a collection of cells and extracellular matrix, which perform the function that are intended to Cells There are over 200 different types of cells in the human body. The cell types fall into two main categories Epithelial and mesenchymal cells. Epithelial cells: Epithelial cells are cuboidal in shape and are grown as twodimensional sheets. These cells are polar and tightly interconnected with the neighboring cells and hence they cannot migrate individually. Basal side of the sheets is connected to the ECM molecules while the superior side is connected to a vesicle or tubule. Early embryo consists of mainly epithelial cells while mesenchymal cells occur as fillers. Mesenchymal cells: Mesenchymal cells are loosely connected cells and usually exist alone and hence they can migrate independently. They are bipolar in shape with lamellipodial extensions. Fibroblast cells, which play a major role in morphogenesis and wound healing, can grow readily in 2D surfaces. However, their growth is contact inhibited. Mesenchymal cells also have the potential to differentiate into most specialized cells like osteoblasts, chondrocytes and fibroblast. The table below summarizes the differences between epithelial cells and mesenchymal cells. Joint Initiative of IITs and IISc Funded by MHRD Page 3 of 8

4 CHARACTERISTICS EPITHELIA MESENCHYME Continuous sheets + - Polar with outer free surface + - Bipolar and stellar shape - + Migratory - + Contact inhibition - + Basal surface on ECM Tissue types The major tissue types include epithelial, connective, nerve, muscle and glandular, which perform variety of functions like covering, support, movement, control and production respectively. Let us see one by one in detail Epithelial tissue Epithelial tissue consists of sheets of epithelial cells, which may be single layer or multi-layered. In single layer, they are simple while in multi-layer they are stratified. They are very specialized cells to form lining of the all the internal and external body surfaces. The epithelial tissue is separated from the underlying tissue by a thin sheet of connective tissue or basement membrane. The basement membrane provides structural support for the epithelium and there are different types of epithelial tissue available based on geometric arrangements. They are: Squamous epithelium The name squamous indicates that the cell width is greater than its height that is they are thin, flat plates, having horizontal flattened elliptical nuclei ( fried egg Joint Initiative of IITs and IISc Funded by MHRD Page 4 of 8

5 shaped ) in its morphology. This is mainly located in the lining of cavities such as blood vessels, mouth, lungs and also outer layer of the skin Simple cuboidal epithelium They are roughly square shaped in structure (i.e.) cuboidal arrangement (width and height are equal). Each cell has spherical nuclei in its centre and is mainly found in the glands, kidney tubule linings and also in the ducts of the glands Simple columnar epithelium Cuboidal as name indicates the height of the cell is greater than the width. Hence, cells are elongated and column shaped. The nuclei are elongated and located near the base of the cells and these are mainly present in the lining of the stomach and intestines Ciliated columnar epithelium This type of epithelial cells possesses fine hair like projections, called cilia on their free surface. These structures are capable of rapid, rhythmic, wavelike beatings in a certain directions. They are mainly located in the air passages like nose and also present in the uterus Glandular epithelium Columnar epithelium with specialized cells called goblets cells are called glandular epithelium. These are capable of synthesizing certain hormones or enzymes, sweat and even saliva. As the name indicating that this is located in the lining of the glands. Joint Initiative of IITs and IISc Funded by MHRD Page 5 of 8

6 Stratified Epithelium This type of cells is composed of several layers of cells, which are mainly present in the body linings where it has to withstand wear and tear. The top layer of cells is flat and scaly and it may or may not be keratinized. This is mainly present in the skin and also in the lining of the mouth cavity Pseudostratified epithelium The epithelial layers appear stratified due to the different location of nuclei with respect to basement membrane. Multiple cell layers can also be described by various shapes like stratified squamous epithelium like the one found in esophagus or it may be in the transition state between the cuboidal and columnar based on the state of the organ as in bladder. The epithelial tissue has many functions in the body and is responsible for excretion, diffusion, cleaning, reduces friction, protection, sensation, secretion, and also absorption Connective tissue Connective tissue has variety of functions, which include mechanical support for bones and ligaments, storage of body s fluid like fat and blood, aid in immune response and tissue repair mechanism. Connective tissue originates from the mesoderm of the embryo. These include cells like fibroblasts, adipocytes, macrophages, plasma cells and also non-cellular components like collagen I and hyaluronic acid. Like epithelial cells, there are various types of connective tissues include loose connective tissue, fibrous connective tissue and specialized connective tissues. Joint Initiative of IITs and IISc Funded by MHRD Page 6 of 8

7 2.3. Muscular tissue The body consists of three types of muscle tissue skeletal, cardiac and smooth. Skeletal muscles consist of striated and multinucleated myofibers that are responsible for the movement. Cardiac muscle in heart helps to propel the blood through the blood vessels. Cardiac muscle fibers are striated but contain only one nucleus and they branch and join at junctions called intercalated disks. These junctions are responsible for the propagation of action potential. Smooth muscle fibers on the other hand are non-striated and are present in the hollow organs like intestine to squeeze the substances by alternate contraction and relaxation. Different organization of muscle tissue is important to perform functions ranging from slow, small magnitude contractions in the gut to the fast, co-ordinated depolarization of heart muscle 2.4. Nervous tissue Nervous tissue is important for the function of the nervous organs like brain and spinal cord. The nerve co-ordinates the body functions and the nervous tissue consists of mainly neurons and supporting cells like Schwann cells and astrocytes. Neurons are responsible for generating electrical impulses called action potentials. These electrical impulses are received by the soma (cell body) and are carried along axons to the other neurons via dendrites. Supporting cells have no role in conducting electrical impulses, however, they play an important role in providing nourishment, insulation and protection Glandular tissue Glandular tissue is mainly composed of polarized epithelial cells that produce and secrete the protein products from the precursors present in the microenvironment. The protein products are usually secreted in endoplasmic reticulum and are packaged into secretory vesicles in Golgi apparatus and then they are exocytosed into the cytoplasm. Joint Initiative of IITs and IISc Funded by MHRD Page 7 of 8

8 Glands are classified into exocrine and endocrine glands. Exocrine glands secrete the products either onto the body surfaces like skin or the body cavities like digestive tube. These glands may be unicellular or multicellular and they are called ductless glands. Examples of exocrine gland are the sweat glands of the skin and the liver, which secretes bile. On the other hand endocrine glands also known as duct glands and they secrete hormones directly into the blood stream. Then these hormones are carried to the target site where they perform their function. Joint Initiative of IITs and IISc Funded by MHRD Page 8 of 8