Basic Histology: A Color Atlas & Text V Subhadra Devi
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Introduction to HistologyChapter 1

 
GENERAL INTRODUCTION
  • Origin of the term: The word histology is derived from the Greek words,
    Histos = tissue; logos = study.
  • Sub-division: It is a branch of anatomy that deals with the microscopic structure of cells and tissues, their relation to one another and their functional role in the maintenance of normal health.
  • Process: It includes collection and preparation of living tissue so that their appearance will be close to their true nature for observation under microscope.
  • Microscopic anatomy: It is called microscopic anatomy as it uses light microscope or electron microscope for observing thin sections of tissues for understanding the structural and functional relationship between different components of the tissue.
  • Histological stains: To facilitate visualization or identification of microscopic structures, histological stains are used to enhance colors of different components in tissues.
  • Histotechniques: The tissues are prepared using special processes called histological techniques.
 
IMPORTANCE OF HISTOLOGY
  • Understanding of normal structure and function:
    • Structure and function of tissues are closely related and knowledge of it is important for understanding the structure and function of organs, organ systems and the complete organism.
    • Histology facilitates understanding of minor details of tissues of human body that facilitates the understanding of mechanism of different processes taking place in specific organs by medical professionals.
    • Histology is an important tool both in the field of biology and in medicine.
  • Understanding of organization of different tissues:
    • Knowledge of histology provides explanation for organization of different cells to form tissues according to their functional demands.
    • Histology helps in understanding biochemistry, molecular biology, physiology, disease processes and the effects of diseases on the tissues.
  • Diagnosis of abnormal tissues in diseases for appropriate treatment (Biopsy):
    • Knowledge of appearance of normal cells and tissues is essential for recognizing or diagnosing abnormal or diseased structures.
    • 2It is an important tool for diagnosis of diseases in the field of medicine. Diseases of various organs like liver, kidney, bone and connective tissue can be diagnosed at cellular level using the histological techniques.
    • Knowledge of histology facilitates pathologists in the diagnosis of certain diseases like cancers based on the structural differences in the observed cells and tissues.
    • The diagnosis of stage of the disease enables medical specialists in understanding patient's condition and for planning appropriate treatment or management of the patient.
  • Forensic autopsies: In legal medicine it provides explanation for the sudden unexpected death and other related issues.
  • Archeology: Study of tissues collected from archeological sites provides information about ancient history and the state of preservation of biological material during ancient times.
  • Developmental anatomy: Changes in histological architecture and composition in growth and development research.
 
RECENT ADVANCES
 
In-situ Hybridization (ISH)
  • It is a technique used for localization of specific nucleic acid in fixed tissues and cells to obtain information on gene expression and genetic loci.
  • It is a method to localize either mRNA within cytoplasm or DNA within chromosome by complimentary pairing of labeled DNA or RNA probes with normal or abnormal nucleic acid sequence in intact chromosomes, cells or tissue sections. The probes can be radioactive or nonradioactive.
  • Principle of ISH: The underlying principle in ISH is that nucleic acids in a histological section can be detected by application of complementary strand of nucleic acid to which a reporter molecule is attached. Visualization of the reporter molecule facilitates localization of DNA or RNA sequences in a tissue sample that contains heterogeneous cell population. This allows localizing and assessing degree of gene expression.
  • Techniques: There are two basic methods to visualize DNA or RNA. They are fluorescence in situ hybridization (FISH) technique and chromogenic in-situ hybridization (CISH) technique.
    • FISH is used to assess chromosomal integrity, aberrations, gene expressions in embryonic tissues and abnormal gene expression in tumours. It is used in the diagnosis of number of infective agents like human papilloma virus (HPV), human immunodeficiency virus and in establishing the link between virus and carcinogenesis as in the case of HPV and carcinoma cervix, hepatitis B virus and liver cancer.
    • Chromogenic in-situ hybridization (CISH) is an alternative to FISH and is useful in evaluation of gene amplification, gene deletion, chromosome translocation and chromosome number.
 
Immunohistology
  • It is also known as immune histochemistry (IHC). The word immune refers to use of antibodies and histo refers to tissue.
  • It is the process of detecting antigens or epitopes (e.g. proteins) in the cells of a tissue by applying the principle of specific antibodies binding to the specific antigen in tissue.
  • This technique facilitates identification of exact location of specific protein in the tissue.
  • The various steps in IHC are:
    • Step I: The tissue is examined by histological examination.
    • Step II: It is then subjected to IHC for further differentiation or to identify certain antigens. The specificity of the technique is improved by secondary antibody directed against the primary antibody. The primary antibody is obtained from mice or rabbits. The secondary antibody is obtained from a different species, i.e. goats to target the immunoglobulins of mice or rabbits.
    • Step III: The secondary antibody is conjugated with an enzyme or can be made visible by a dye. For example, the antigen-antibody reaction is visualized by conjugating the antibody with an enzyme, for example, peroxidase (immunoperoxidase staining) or tagged to fluorescein (immunofluorescence).No reaction indicates absence and a positive reaction confirms presence of antigen in the tissues.
  • 3Uses: IHC can be used as:
    • Diagnostic aid in differentiation of tumors by observing the undifferentiated cells.
    • Prognostic aid in certain tumors, e.g. lymphomas.
    • As an aid in designing appropriate therapy in certain tumors, e.g. cancers of colon and rectum.
 
Molecular Histology
  • It includes observation of distribution and localization of molecules in cells, tissues and organs.
  • It provides insight into the structure and function.
  • Molecular pathology focuses on study and diagnosis of disease through examination of molecules in tissues and body fluids. It focuses on sub microscopic aspects of disease. It includes molecular and genetic approaches for the diagnosis of disease, individual's genetic susceptibility to development of disease, designing of biomarkers for treatment, response of disease to treatment and its progression.
 
Histology vs Pathology
Appearance of cells and tissues in normal state vs pathological state
  • Histology: To observe the condition of cells and tissues in disease-free condition.
  • Pathology: To observe the changes in cells and tissues caused by the disease processes. Pathos = disease or suffering.
 
Histopathology
  • It is the scientific study of disease at cellular and tissue levels for accurate diagnosis of different stages in different types of cancers and disorders of tissues.
  • Pathologist is the expert who performs histopathological examination of tissue in detail from the sample collected from the patient and gives diagnostic report.
  • Histopathology helps in diagnosis of the disease, in determining severity and progress of the diseased condition and in evaluating response of certain therapies for the disease.
 
Histotechnicians
The trained paramedical personnel who perform the preparation of histological sections are called histotechnicians/histology technicians/ histology technologists/medical laboratory technicians. Their field of study is called histotechnology.