Tissues of Human Body: Types and Subtypes

Human body consists of countless cells as an essential component of different tissues. The entire lot of cells originates from typical cell, the ovum or the egg cell. Ovum or the egg cell is composed of protoplasm and contains a nucleus. After fertilization this cell multiplies to form a ball shaped structure called embryo, which by the way of differentiation develops into various tissues required to form organs and various parts of the body. The embryo or the ball of cells developed from the fertilized ovum could be divided into three layers at the very early stage:

1. Outer Layer or Ectoderm: The outer part of the skin, nails, hair follicle and sweat glands mucus membrane lining of the mouth and nasal cavities develop from the ectoderm. The nervous system also develops from the ectoderm.
2. Middle Layer or Mesoderm: Muscles, bones, fat and some parts of the cardiovascular system develop from the mesoderm.
3. Inner Layer or Endoderm: The lining of the alimentary canal and respiratory tract develop from the endoderm.

Tissue: A tissue is an aggregation of cells of a unique shape and size destined to perform a particular task. The special function of a tissue may be governed by biological substances and/or nervous system. Elementary tissues of our body are of four types:

1. Epithelial Tissue
2. Connective Tissue
3. Muscular Tissue
4. Nervous Tissue

Subtypes of Epithelial Tissue:

A). Covering and Lining Epithelial Tissue: [1. Simple Epithelial Tissue (Pavement Epithelium: Figure-1, Cuboidal Epithelium: Figure-2, Columnar Epithelium: Figure-3); 2. Stratified Epithelial Tissue; 3. Transitional Epithelial Tissue]

Fig-1: Pavement Epithelium

Fig-2: Columnar Epithelium

Fig-3: Ciliated Columnar Epithelium

B). Glands: [Exocrine Glands (Simple Exocrine Glands, Compound Exocrine Glands); Endocrine Glands]

Subtypes of Connective Tissue:

A). Loose Connective Tissue or Areolar Tissue

B). Fatty Connective Tissue or Adipose Tissue

C). Dense Connective Tissue or Fibrous Tissue. Ultrastructural view of collagenous fibres has been depicted in figure-4.

Fig-4: Ultrastructural view of Collagenous Fibres, 27000x

D). Cartilage

E). Bone

F: Blood (Blood is also a form of connective tissue suspended in liquid matrix called plasma).

Subtypes of Muscular Tissue:

A). Voluntary Muscle (Striated or striped muscle). Ultrastructural view of striated muscle has been depicted in figure-5.

Fig-4: Ultrastructural view of Striated Muscle, 6000x

B). Involuntary Muscle (Smooth or plain muscle)

C). Cardiac Muscle (Striated involuntary muscle)

Tissues of Human Body: Distribution of Various Tissues

Distribution of Various Tissues:

Epithelial Tissue:

Covering and lining epithelia has been classified according to shape and arrangement of their cells. Simple epithelium composed of flat cells as single layer attached to a basement membrane is called pavement epithelium. These are found lining the blood vessels and peritoneum. Cuboidal epithelium is characterized by cube shaped cells and is found covering the ovaries. Columnar epithelium is composed of taller cells supported by a basement membrane. This type of epithelium is found where wear and tear is little more. It is found lining the stomach and intestine. Ciliated columnar epithelium show hair like projections at the free surface of cells under microscopes and is found in the respiratory tract. A brush border is found in the cells specialized for absorption. These cells have minute fingerlike projections called microvilli. Such cells are found lining the small intestine.

Stratified epithelium is made up of many layers of cells. The deepest layer of columnar cells rests on the basement membrane and is called germinal layer. Cells of the germinal layer keep on dividing frequently and as they divide the parent cells are pushed nearer the surface and become flattened. The cells on the surface are rubbed off frequently and are replaced by new cell from below. Skin is the dry stratified epithelium and the cells of surface layer have keratin that made our skin water proof. In moist surfaces of cavities like mouth, the cells of surface layer survive until they are rubbed off and keratin is not developed in these cells.

Transitional epithelium is also like stratified epithelium. The only difference in transitional epithelium and stratified epithelium is that the surface cells are round in shape and are capable of spreading out when the organ expands. Transitional epithelium is found lining the urinary bladder.

Glands: Glands are a special type of epithelial tissue. Glands have ability to manufacture essential substances from the basic material supplied by blood. The substances produced by glands are called as secretions of glands. Gastric glands can produce hydrochloric acid from sodium chloride provided by the blood. Exocrine glands pour their secretions through a duct. Secretions of majority of exocrine glands are enzymes. Endocrine glands are ductless glands and pour their secretions into the blood stream. Secretions of endocrine glands are hormones and reach the target sites through the blood stream. Pituitary gland in the skull and thyroid gland in the neck are two important examples of endocrine glands.

Connective Tissue:

Connective tissue supports and binds other tissues. There are three main components of connective tissue: Cells, intercellular substances called matrix and fibres. Matrix and fibres form the supporting material of our body. Fibres may be collagenous fibres or elastic fibres. Collagenous fibres originate from fibroblast cells. These coarse fibres occur in bundles. Elastic fibres are fine branching fibres with elastic like property. These are found layers surrounding the organs and in fibrous tendons joining the muscles.

Areolar tissue is loose connective tissue and is composed of loose network of collagenous fibres and elastic fibres with scattered groups of fat cells and fibroblasts. Areolar tissue forms a very thin, transparent and tough layer and is found between and around the organs of our body.

Adipose tissue is a fatty tissue. It is similar to areolar tissue but the spaces of the network of fibres are filled with fat cells. It is known for its food reserve in the form of fat globules in fat cells. It retains body heat and protects our delicate organs like kidneys and eyes being the poor conductor of heat.

Dense connective tissue or fibrous tissue is composed mainly of bundles of collagenous fibres embedded with fibroblasts. It is found in the ligaments and fascia.

Cartilage consists of cells known as chondrocytes, separated by fibres. There are no blood vessels in cartilage tissue. Cartilage is tough and flexible tissue and found in the trachea, covering heads of bones, joints, between body of vertebrae, and auricles of the ear and the epiglottis.

Bone is known as a specialized type of cartilage. In bone, the collagen is impregnated with calcium. It is tough and rigid due to collagen and calcium respectively and gives support to soft tissues of our body. The cells between the fibres are called osteocytes. Bone has very rich blood supply through blood vessels.

Muscular Tissue:

Muscular tissue provides movement to our body through its specialized function of contraction. Where there is movement in the body there must be muscular tissue. Voluntary or striped muscular tissue forms the flesh of our body and supports movement of our body. It consist long cells varying in size from a few millimeters to 30 centimeters depending on the length of muscle. Each muscle cell contains numerous threads like fibres called myofibrils. The diameter of myofibrils varies from 10 micrometers to 100 micrometers. Myofibrils are symmetrically striped in alternate dark and light bands throughout their length. Each fibril is enclosed in connective tissue sheath called sarcolemma. This muscle is under the control will and need energy for contraction. Energy for contraction is supplied by conversion of adenosine triphosphate (ATP) to adenosine diphosphate (ADP). Adenosine diphosphate (ADP) is again converted into ATP by utilization of energy provided by oxygenation of glycogen. Oxygen for energy generation is supplied by the blood supply to the muscle. Blood capillaries run through voluntary muscle cells to ensure adequate blood supply. Contraction of muscle is controlled through nerves. Ultrastructural view of voluntary muscle has been depicted in Fig-1.

Fig-1: Ultrastructural view of voluntary muscle, 6000x

Cardiac muscle is involuntary but irregularly striped also. It is found in the heart wall only and is different from any other skeletal muscle. Cardiac muscle is composed of short cylindrical fibres with centrally placed nuclei. Muscle fibres of cardiac muscle have no sheath but are bound together by connective tissue. It is not under the control of will but contracts automatically in rhythmic fashion throughout life. Rhythmic contractions are controlled by nerves.

Nervous Tissue:

Nervous tissue receives information from inside as well as outside of our body through network of nerves. It is specially designed tissue to carry impulses. Nervous tissue is composed of nerve cells called neurons and supporting network called neuroglia. Each neuron has a large cell body and short processes called dendrites. Dendrites bring impulses from other cells and tissues. There is a long process called axon, which carries impulses away from the cell body.