Animal Histology

Introduction

     A tissue is a group of cells having a common origin, similar structure and performing a common function. Tissues are characteristic of advanced multicellular animals, commonly called as eumetazoans. They are true multicellular animals where the body cells aggregate to form tissues, organs and even organ systems.

Epithelial Tissue

     It is normally found as a covering either externally or internally. It is the simplest and the least specialised animal tissue. It is described as avascular, since a direct blood supply is absent. The tissue depends upon the other underlying tissues for its nourishment. The epithelial tissue can originate from any of the three germ layers-ectoderm, mesoderm or endoderm.

Simple Epithelium

     Simple epithelium is classified into the following types based on the shape of the cells.

Stratified Epithelium (Multilaminar Epithelium)

     Here, the cells are arranged in more than one layer. Stratified epithelium is classified into the following types based on the shape of the constituent cells.

Muscular Tissue

     The muscular tissue is a tissue that is capable of bringing about different types of movements in the body. It is one of the highly specialized animal tissues. It is a derivative of mesoderm. The muscular tissue exhibits a unique property called contractibility. It is the capacity of the cells to exhibit regular contractions and relaxations. Hence, it is also known as contractile tissue.

Smooth Muscle

     The smooth muscle always occurs in the form of thin sheets. Each sheet has a large number of muscle fibres that are held together by a transparent connective tissue covering. The individual muscle fibres in the sheet are elongated and spindle shaped.

Striated Muscle

     The striated muscle occurs in bundles called fascicles. Each fascicle has a large number of muscle fibres that are held together by connective tissue. The individual muscle fibre is elongated, cylindrical and unbranched. Each fibre is bound by a thick and distinct limiting membrane called sarcolemma in addition to the cell membrane.

Cardiac Muscle

     The cardiac muscle fibres do not form fasciles. They are arranged in the form of a network. The muscle fibres are elongated, cylindrical and branched. The branches become inter-connected with one another to form a network. The space between the muscle fibres is filled with connective tissue.

Connective Tissue

     It is another highly specialised animal tissue. It is a derivative of mesoderm. The specialisation in connective tissue is for various specific functions.

Connective Tissue Proper

     It is type of connective tissue in which the matrix is soft and homogenous and encloses cells and fibres. It is also known as binding tissue or loose connective tissue.

Supporting Tissue

     It is a type of connective tissue in which the matrix is hard and rigid due to the presence of inorganic salts. The matrix encloses few cells. Fibres may be present or absent. It is also known as skeletal tissue since it forms the internal supporting framework (endoskeleton) of the body.

Fluid Connective Tissue

     It is a type of connective tissue in which the matrix is in the liquid form. It is known as plasma. The plasma encloses different types of cells. Fibres are absent.

Blood

     It is a type of fluid connective tissue, which forms the chief circulating fluid in the body. As a circulating fluid, blood is involved in a wide variety of functions in the body. As a type of connective tissue, blood shows two essential components, namely matrix and cells.

Lymph

     It is another type of fluid connective tissue. It is characterized by the absence of RBC. Hence, it has a plasma containing only WBC. Some of the plasma proteins found in blood, are absent in the lymph. Lymph flows only inside definite vessels called lymph vessels or lacteals. In certain places of the body, lymph vessels aggregate to form lymph nodules.

Nervous Tissue

     It is a highly specialised animal tissue that exhibits two unique properties irritability (capacity to respond to the stimulus) and conductivity (capacity to transfer the response from one region to another). Nervous tissue is of ectodermal origin.

Summary

     Epithelial tissue (epithelium) is the simplest. It is avascular and develops from all the three primary germ layers.