Tissue Level of Organization

Chapter 2

Tissue Level of Organization

 Introduction

• Though cell is considered as the structural and functional unit of the body, it cannot function in isolation. The cells group and work together. A group of cells similar in structure (due to their common embryonic origin) and function is known as tissue. The science that deals with the microscopic structure of the tissues is known as Histology.

• According to the structural and functional characteristics, the body tissues are classified into four fundamental types.

• 1. Epithelial 2. Connective 3. Muscle 4. Nervous

• In a tissue, the cells remain in place due to their attachment with other cells, basement membrane and binding connective tissue. Different tissues organize to form organs and organs organize to form systems of the body. Every cell in the body is bathed in a fluid called extracellular fluid (ECF). This provides a medium for transporting the substances and carrying out chemical reactions. Thus, there are microscopic spaces between the cells in a tissue, where this extracellular fluid (also called as interstitial fluid) is present.

Epithelial Tissue

Origin

• The embryonic tissues from which all tissues and organs of the body develop are known as the germ layers, i.e. ectoderm, mesoderm and endoderm. The epithelial tissue develops from all three types of germ layers.

Characteristics

• Most important feature of epithelial tissue is that the cells are closely packed together with very little intercellular substance. The epithelial cells are arranged in continuous sheets in either single or multiple layers. The cells are attached to a basement membrane, which is composed of mucopolysaccharides, proteins and collagen fibrils. The basement membrane is attached to connective tissue. The cells are held together by means of cell junctions on the lateral surface; close to the outer surface of cells. The epithelial tissue is vascular and the cells are dependent on underlying connective tissue for their nourishment.

• Epithelial tissue covers the body surface, lines the body cavities, forms the inner coat of blood vessel and lymphatic system and composes of both the endocrine and exocrine glands.

Stratified Epithelium

• (a) Simple Squamous Epithelium

• Formed by a single layer of flat cells, it appears as a thin sheet. In general, it appears in organs where filtration and diffusion occur, e.g. lining of blood vessels, lining of alveoli and the glomerular capsule of nephron.

• (b) Simple Cuboidal Epithelium

• Cuboidal epithelial cells are in a single layer of cube like cells with a large spherical central nucleus. Simple cuboidal epithelia are found on the surface of ovaries, lining of Nephron, the walls of renal tubules, the parts of the eye and the thyroid. On these surfaces the cells perform secretion and absorption.

• (C) Simple columnar epithelium

• This is formed by a single layer of rectangular cells; it lines the gastrointestinal tract and at this location their apical surfaces possess microvilli. It also lines the ducts of many glands. Its function is secretion and absorption.

Connective Tissue

Origin

• The connective tissue develops from mesoderm.

Characteristics

• The most important feature of connective tissue is that the cells are widely separated from each other by an intercellular substance. The intercellular substance is known as matrix; and consists of ground substance and fibers. The matrix is secreted by the immature cells of connective tissue such as fibroblast, osteoblast and chondroblasts. Once the matrix is formed the immature cells differentiate and become mature cells. The mature cells are named by the suffix-cite, e.g. Osteocytes, chondrocytes etc.

• Muscle Tissue.

• Functions of connective tissue: As body organs are formed by different tissues bound together by connective tissue; it is most widely distributed in the body as connecting tissue. In addition, it supports and strengthens other tissues in the body. It has an important role in protection, transport and repair activities of the body. Adipose tissue acts as stores of reserve food of the body.

MUSCLE TISSUE

• The cells of muscle tissue are elongated and often taper at their ends; hence these cells are called muscle fibers. The fires are typically designed and constructed to produce a force of contraction. Thus, the tissue is contractile and able to produce movements.

• On the basis of structural and functional characteristics, the muscles are classified into three types:

• 1. Striated or voluntary muscle;

• 2. Smooth or involuntary muscle; and

• 3. Cardiac muscle.

• (a) Striated Muscle

• It is described as skeletal, striated, striped or voluntary muscle. It works under the control of the will. It is found in skeletal muscle. The cells of voluntary muscle, about 10-40 millimeters in length are roughly cylindrical in shape. Sarcolemma is the fine sheath surrounding each muscle fiber and several nuclei are situated under it. The muscle fibers lie parallel to one another and show transverse dark and light bands.

• (b) Smooth Muscle

• It is described as involuntary, plain or visceral muscle. It is not under the control of our will. It is made up of spindle shaped cells, with only one central nucleus. There are no distinct sarcolemma but a very fine membrane surrounds each fiber. The bundles of cells form a sheet. During contraction and relaxation, the cells glide over one another. It is found in the walls of the blood vessel, lymph vessels, alimentary canal, respiratory tract, the urinary bladder and uterus.

• (c) Cardiac Muscle

• It is found exclusively in the wall of the heart. The cardiac muscle cells show cross- stripes similar to those of voluntary muscle. Each cell has a nucleus and one or more branches. The ends of the cells and their branches are in close contact with each other forming "intercalated discs" which look like thicker and darker lines. This arrangement gives cardiac muscle an appearance of a sheet of muscle rather than fibers. Each fiber does not need to be stimulated as the impulse spreads from cell to cell across intercalated discs.  

Nervous Tissue

• The nervous tissue carries out the special function of carrying messages of stimuli within the body. The properties of 'irritability' and 'conductivity' are specially developed in the nervous tissue. The impulses are conducted along the special cells 'neurons’ which form a unit structure of the nervous tissue. The neurons are supported by a special type of connective tissue called neuralgia. Each neuron consists of a nerve cell and its processes are called axon and dendrites.

Nerve cells:

• The nerve cells considerably vary in size and shape. They form gray matter of the nervous system and are found at the periphery of the brain, in the center of the spinal cord, in groups called ganglia outside the brain and the spinal cord and as single cells in walls of organs.