Histology

Introduction  Definition of Histology : to study the fine structure associated with its function in the human body. Contents: cells (basic units of structure) and intercellular substance   Tissues (epithelial, connective, muscle and nervous)   more than 2 kinds of tissues  Organs   organs with related functions  Systems

Epithelial Tissue (ET)  Characteristics: 1. Membrane-like structure: crowded cells with very scanty intercellular substance. 2. Polarity : basal end rests on the basement membrane (BM),  the distal end associates with functions, e.g. absorption (microvilli by EM, striated boarder by LM), secretion (gland: goblet cell, acinus) and etc. 3. Avascular: in general ET has no blood vessels, may be rich in nerve endings.

Classification of Epithelia  1. The simple epithelium (each cell rests on BM) mainly associated with lubrication, transport, absorption or secretion function. It can be divided into:  (1). Simple squamous epithelium (e.g.endothelium)  (2). Simple cuboidal epithelium ( renal tubules)  (3). Simple columnar epithelium (intestinal inner-surface)  (4). Pseudostratifird (cilliated) columnar epithelium  ( trachea inner-surface)

 2. The stratified epithelium ( multi-layers contain deep, middle and superficial layers, only the basal cells rest on BM), mainly associated with protection function. Based on the shape of the suface cells, it can be divided into:  (1)Stratified squamous epithelium : non-keratinized (esophageal inner-surface), keratinized (skin epidermis)  (2) Stratified cuboid epithelium  (3) Stratified columnar epithelium  (4) transitional epithelium (urothelium): relaxed  stretched, may be pseudostratified.

Contacts between adjoining epithelial cells on the lateral side  From free end towards basal there are 4 types of contacts between adjoining cells on lateral side of the simple columnar epithelium:  1. Tight junctions (belt type): some fusions of cell membrane prevent large molecules from passing through intercellular space.  2.Intermediate junction (belt type): the cytoplasmic filaments are anchored to the dense material on cytoplasmic faces of the cell membrane to form terminal web, functioning as support.  3. Desmosome (plague type): it is a spot-weld type in cell to cell attachment. Some filaments connect the desmosome plaques across the intercellular space and intermediate line (fusion of the cell coats, glycocalyx ).

 4.Gap junction (spot type ) : a series of microwholes pass through the narrow intercellular space, about 3nm. It is an area of low electrical resistance to ionic flow, involved in cell to cell communication.  The combined above tight, intermediate and gap junctions may constitute a junctional complex, although the tight junction, desmosome and gap junction may exist independently.  Notice: these junctions also exist in each kind of primary tissues, not limited in the epithelial tissue.

Key points  1. How to distinguish/identify the following epithelial tissues, e.g. the simple squamous epithelium, simple columnar epithelium, pseudostratified cilliated columnar epithelium, stratified squamous epithelium and trasitional epithelium.  2. The structure and function of the tight junction, gap junction and desmosome.

Connective tissue (CT)  Characteristics:  1.The cells are scattered with no polarity.  2. A great amount of intercellular substance (IS) may differentiate into fibers and ground substance (GS, amorphous).  3. Always accompanied by blood vessels.  4. Associated with connection, support, nutrition and defense functions.

Classification: based on the character of IS in CT

The structure of mesenchyme Mesenchymal cells: stallate-shaped, undifferentiated type (with differentiation capacity) with pale stained nucleus, distinct nucleuolus and weak basophilia cytoplasm. In the adult the mysenchyme still retained in some regions, e.g.around the small blood vessels The mesenchymal cells can differentiate into the blood vessel endothelium, smooth muscle and each kind of CT, if required.

Structure of loose connective tissue There are 6 kinds of cells and 3 kinds of fibers. Cells: 1. Fibroblast (fibrocyte, inactive type ) and 2. Plasma cell are both of protein-secreting type, with developed mitochondria, Golgi complex, especially the granular endoplasmic reticulum, resulting in cytoplasm basophilia. The protein products of the fibroblast are fibers (collagen, elastic and reticular) and ground substance; the protein products of plasma cell are antibodies/ immunoglobulins, such as IgG, IgE and etc. 3. Macrophage: function as scavenger by the lysosome and involve immune response by direct way, e.g. process antigen as the antigen presenting cell (APC) and by indirect way through mediators secreted from the macrophage, e.g. IL1and CSF respectively stimulate T cell and neutrophils to proliferate.

 4. Mast cell: there are lots of heterochromatic granules (the staining color of granules is different from the color of the dye used ) in cytoplasm. The granules contain heparin, histamine, etc. The histamine can enhance permeability of the capillary. During the receptor, IgE of the mast cell combined with its specific sensitive antigen may result in anaphylaxis, such as asthma, urticaria, hay fever, and etc 5. Wandering leucocytes: e.g. the lymphocyt, eosinophil, and etc. increase in number during inflammation. The plasma cell, macrophage, mast cell and wandering leukocyte function as immune cells. 6. Fat cell (adipose cell) : synthesis and store of lipid (nutrition).

Fibers: function mainly as support 1. Collagen fiber (white fiber): with tensile strength The collagen fiber consists of collagen synthesized by the fibroblast. The fibril is made up of pre-collagen molecules arranged in stepwise way, appeared as striated with EM. The striation with periodicity of 64 nm can be identified as collagen fiber. The wavy bundle of collagen fiber can be stained with eosin in red color. 2. Elastc fiber (yellow fiber): with elasticity decreased with aging The elastic fiber consists of elastin synthesized by the fibroblast. The thicker elastic fiber also can be stained with eosin. 3. Reticular fiber (argyrophilia fiber): The reticular fiber is a premature type of the collagen fiber coated by a layer of glycoprotein. The reticular fiber can be demonstrated by silver staining.

Ground substance (GS): also secreted from the fibroblast, homogenous (amorphous) under LM and involved in barrier and nutrition function. The GS consists of proteoglycans (polysaccharide) and glycoprotin. 1. Proteoglycans: the major component of proteoglycans is the large molecule of hyaluronic acid (bottle brush type) with negative charge to combine Na + and H + OH -, besides a small amount of tissue fluid. The ultrastructure of GS functions as a molecular sieve to prevent from dispersion of deleterious materials/ bacteria. 2. Glycoprotein: only a small amount The glycoprotein contains fibronectin, laminin,etc, providing substantial base for cell recognition, adhesion and migration.

Key points  1.The fine structure associated with function of the fibroblast, plasma cell, macrophage and mast cell.  2. How to identify fibrocyte, plasma cell, macrophage and mast cell respectively.  3. How to distinguish collagen fibers from elastic fibers.