Cell and TissuesCHAPTER 1

It is the basic functional unit of all living systems. It can be described as a mass of protoplasm enclosed within a membrane (plasma/cell membrane). The cells are of different types performing specialized functions, but they have the same common characteristics. The cell is made up of water, protein, carbohydrates, lipids, and inorganic salts.

The various structures present within the protoplasm are nucleus, cytoplasm, and cell membrane (Fig. 1.1).

It is a somewhat rounded structure present at the center of the cell. It contains genetic materials of the cells. It is bounded by two membranes called as nuclear membrane. The outer membrane contains large number of minute pores called as nuclear pores which are open to the cytoplasm. The nucleus contains a colloidal solution of proteins called nucleoplasm. It contains nucleic acid, DNA and RNA. DNA plays important role in the cell division and transmission of hereditary characters. RNA plays important role in the synthesis of various proteins. RNA is mostly concentrated in a small spherical body present within the nucleus and is called as nucleolus. Aggregations of granules are scattered throughout the nucleus and are called as chromatin granules. Chromosomes appear during cell division and are small thread like structure.

It is a watery and homogenous solution of proteins, sugars and various salts. It contains other organelles like endoplasmic reticulum. Golgi apparatus, mitochondria, centrosome, lysosome, and cytoplasmic inclusions.

Both mitosis and meiosis are types of cell divisions. Mitosis occurs in both reproductive cells and body or somatic cells while meiosis occurs in germ cells or reproductive cells. The former is called as mutiplication, division or replica division as the two daughter cells produced resemble the parent cells. Mitosis occurs in order to favor growth and 3differentiation of an organism. Meiosis called as reduction division as it results in reduction of total number of chromosomes in the daughter cells, in order to maintain a constant chromosome number thereby race is maintained.

Mitosis is the visible part of cell division (Fig. 1.3). By the time mitosis begins all the ‘heavy lifting’ in the form of DNA replication and production of elements necessary for division has already been done. At the start of mitosis, the duplicated DNA exists as chromatin joined at its centromere, but has not yet been packaged in its chromosomal form.

Meiosis only occurs in germ cells, and produces ova in females and sperm in males. In the process of meiosis, the genetic material is reshuffled, and the chromosomes are reduced to their haploid number. During fertilization when the ovum and sperm unite, the diploid number is restored. There are two phases of meiosis viz: meiosis I and meiosis II (Fig. 1.4).

Meiosis I: Meiosis I separates homologous chromosomes, producing two haploid cells (23 chromosomes, N in humans), so meiosis I is referred to as a reductional division. A regular diploid human cell contains 46 chromosomes and is considered 2N because it contains 23 pairs of homologous chromosomes. However, after meiosis I, although the cell contains 46 chromatids it is only considered as being N, with 23 chromosomes.

Meiosis II: Meiosis II is the second part of the meiotic process. Much of the process is similar to mitosis. The end result is production of four haploid cells (23 chromosomes, 1N in humans) from the two haploid cells (23 chromosomes, 1N each of the chromosomes consisting of two sister chromatids) produced in meiosis I. The four main steps of meiosis II are: Prophase II, metaphase II, anaphase II, and telophase II.

Meiosis facilitates stable sexual reproduction. Without the halving of ploidy, or chromosome count, fertilization would result in zygotes that have twice the number of chromosomes as the zygotes from the previous generation. Successive generations would have an exponential increase in chromosome count. In organisms that are normally diploid, polyploidy, the state of having three or more sets of chromosomes, results in extreme developmental abnormalities or lethality. Polyploidy is poorly tolerated in most animal species. Plants, however, regularly produce fertile, viable polyploids. Polyploidy has been implicated as an important mechanism in plant speciation.

Cell metabolism is the process by which living cells process nutrient molecules and maintain a living state. Cell metabolism involves extremely complex sequences of controlled chemical reactions called metabolic pathways.

Anabolism: Anabolism is a constructive metabolic process whereby energy is consumed to synthesize or combine simpler substances, such as amino acids, into more complex organic compounds, such as enzymes and nucleic acids.6

Catabolism: Catabolism is a type of metabolic process occurring in living cells by which complex molecules are broken down to produce energy. On balance, catabolic reactions are normally exothermic.

Carbohydrate catabolism: Carbohydrate catabolism is the breakdown of carbohydrates into smaller units. The empirical formula for carbohydrates, like that of theirs monomer counterparts, is C_X(H_2YO_Y). Carbohydrates literally undergo combustion to retrieve the large amounts of energy in their bonds.

Fat catabolism: Fat catabolism, also known as lipid catabolism, is the process of lipids or phospholipids being broken down by lipases. The opposite of fat catabolism is fat anabolism, involving the storage of energy, and the building of membranes.

Protein catabolism: Protein catabolism is the breakdown of proteins into amino acids and simple derivative compounds, for transport into the cell through the plasma membrane and ultimately for the polymerization into new proteins via the use of ribonucleic acids (RNA) and ribosomes.

Tissue is a group of cells having similar origin and structure, which performs specific functions. A group of different tissues results in the formation of organ of the body like stomach, lungs, esophagus, etc. Various organs may join together to perform a vital function of the body and are called system like respiratory system, digestive system, etc.

Epithelial tissues form the covering of the body and lining of cavities and tubes. It is also found in glands. The cells of epithelial tissues are very closely packed and the intercellular (matrix) substance is very less.

Simple epithelium consists of a single layer of similar cells. Depending upon the shape and size of the cells they are further divided into four types. It is usually found on absorptive or secretory surfaces where single layer increases these processes. The types are named according to the shape of the cells, which differ according to their functions. The more active the tissue, the taller are the cells.

It consists of flat and plate like cells. The cells fit closely together like flat stones forming a thin and very smooth membrane. Each cell has a polygonal shape and is filled with cytoplasm with a flattened nucleus (Fig. 1.5). Complete layer of squamous epithelium rest on a basement membrane. Diffusion takes place freely through this thin, smooth and inactive lining. It is present lining the following organs-heart, blood vessels, lymph vessels, alveoli of the lungs, Bowman's capsule, membranous labyrinth of internal ear, etc.

Functions: Diffusion of the substances and protection of organs.

The cuboidal epithelium consists of cube shaped cells fitting closely together lying on a basement membrane (Fig. 1.6). These cells are present in the form of a single layer in the internal lining of uriniferous tubules in the kidney, ducts of the internal ear and some glands. Function: They perform the function of protection, secretion, absorption and excretion.

As the name indicates, columnar epithelium consists of single layer of elongated, rectangular cells resting on a basement membrane. Cells have prominent nucleus. They are found lining the following organs: Stomach, small intestine, large intestine, rectum, gallbladder, alveoli and secretary glands. They perform the functions of absorption of digested products, and secretion of mucus (Fig. 1.7).

As the name indicates it consists of columnar cells having many hairlike processes called cilia at their free edges. The cilia consist of microtubules inside the plasma membrane that extends from the free border of the columnar cells. By the wave like movement of the cilia, these epithelium helps in propelling the contents of the tube, which they line, in one direction only. Example in oviduct, cilia helps to push the egg towards the uterus, while in nasal passage, they prevent the entry of dust, foreign particles and mucus into respiratory tubes and lungs. Ciliated epithelium is found lining the organs like nasal lining, trachea, bronchial tubules, oviducts and uterus (Fig. 1.8).

Stratified epithelium consists of several layers of cells of various shapes. The superficial layers grow up from below. Basement membranes are usually absent. The main function of stratified epithelium is to protect underlying structures from mechanical wear and tear. There are two main types of stratified epithelium namely stratified squamous epithelium and transitional epithelium.

It consists of several layers of cells of different shapes. In the deepest layer the cells are columnar and as they grow towards the surface, they become flattened and are then shed (Fig. 1.9).

Non-keratinized stratified epithelium: It is mostly found on the moist surfaces like buccal cavity, pharynx, esophagus, anal canal, lower portion of urethra, and vagina, conjunctiva of the eyes etc. where they protect the organs from drying, wear and tear.

Keratinized stratified epithelium: The dry surfaces are mostly found to be covered with keratinized stratified epithelium like skin, hair, nails, etc. They protect the surface from wear and tear. This epithelium consists of dead cells containing keratin (protein). This forms a tough, relatively waterproof protective layer that prevents drying of the underlying live cells.

It is a compound epithelium in which basement membrane is absent. It is composed of several layers of pear shaped cells which are comparatively thinner and have more elasticity. Its outermost layer is composed of pear-shaped cells and innermost layer is composed of cuboidal cells. They are found lining the pelvis of the kidney, urinary bladder, ureters and upper part of the urethra. It allows stretching of these organs when they are filled (Fig. 1.10).

These are extensively spread throughout the body. They are found connecting the various parts of the body and provide them suitable support. The cells forming the connective tissues are more widely separated from each other than those forming the epithelium. An intracellular substance, i.e. matrix is present in large amounts. Depending upon the structure and functions connective tissues are classified into following categories.

Connective tissue proper: It consists of a large amount of intercellular materials. It is further classified into loose connective tissue and dense connective tissue.

Loose connective tissue: It is a mass of widely scattered cells whose matrix is a loose weave of fibers. Many of the fibers are strong protein fibers called collagen. Loose connective tissue is found beneath the skin and between organs. It is a binding and packing material whose main purpose is to provide support and hold other tissues and organs in place. Three types of loose connective tissue are recognized. These include areolar, adipose, and reticular types.

Areolar connective tissue: It is loose connective tissue that consists of a mesh-work of collagen, elastic tissue, and reticular fibers with many connective tissue cells in between the mesh-work of fibers. The different types of cells embedded within the areolar tissue are fibroblasts, plasma cells, adipocytes, mast cells, and macrophages. The fibers and cells are embedded in a semifluid ground matrix. Areolar tissue binds skin to the muscles beneath. The key functions of areolar tissue are support, strength and elasticity (Fig. 1.11).

Adipose tissue: It is a loose fibrous connective tissue packed with many cells (called “adipocytes”) that are specialized for storage of triglycerides, i.e. “fats”. Each adipocyte cell is filled with a single large droplet of fat. As this occupies most of the volume of the cell, its cytoplasm, nucleus, and other components are pushed towards the edges of the cell which is bounded by the plasma membrane. Adipose tissue acts as an insulating layer, helping to reduce heat loss through the skin. It also has a protective function, providing mechanical protection (“padding”) and support around some of the major organs, e.g. kidneys. Adipose tissue is also a means of energy storage. Food that is excess to requirements is converted into fat and stored within adipose tissue in the body (Fig. 1.12).

Reticular connective tissue: Reticular connective tissue is named for the reticular fibers which are the main structural part of the tissue. These fibers are present in many types of connective tissue and are particularly heavily concentrated in reticular connective tissue. The cells that make the reticular fibers are fibroblasts called reticular cells.

Reticular connective tissue forms a scaffolding for other cells in several organs, such as lymph nodes, liver, spleen and bone marrow (Fig. 1.13).

Dense connective tissue: They are also called dense fibrous tissue and has fibers as its main matrix element. Dense connective tissue is mainly composed of collagen fibers. Crowded between the collagen fibers are rows of fibroblasts, fibre forming cells, that manufacture the fibers. In addition, these body tissues also contain ground substance - the material that fills in the gaps between fibroblasts and holds the fibers themselves. Ground substance contains fluids and cell adhesion proteins, which essentially act as the glue that keeps the connective tissue attached to the extracellular matrix. Dense connective tissue forms strong, rope like structures such as tendons and ligaments. Tendons attach skeletal muscles to bones; ligaments connect bones to bones at joints. Ligaments are more stretchy and contain more elastic fibers than tendons. Dense connective tissue also makes up the lower layers of the skin (dermis), where it is arranged in sheets.

Dense regular connective tissue (White fibrous): In this type of tissue, the collagen fibers are densely packed, and arranged in parallel. This type of tissue is found in ligaments and tendons. These are powerfully resistant to axially loaded tension forces, but allow some stretch (Fig. 1.14).

Dense irregular connective: Tissue has an irregular, somewhat disorderly, dense weave of thick collagen fibers, with bundles of fibers oriented in all directions. With its high tensile strength, dense irregular connective tissue effectively binds various tissues together to form organs and passively translates mechanical forces in all directions without tearing. It is found in several locations: The dermis of the skin, the walls of large tubular organs, such as the alimentary canal, in glandular tissue, and in organ capsules (Fig. 1.15).

Elastic connective tissue: They are thicker and do not exists as bundles. They form ligaments which join the bone with another tissue. They also form the pinna or lobe of the ear, the epiglottis and part of the tunica media of blood vessel walls (Fig. 1.16).10

Supporting connective tissues: These tissues are responsible for the formation of the skeleton of the body. They are further classified into cartilage and bone.

It is a supporting tissue. The cells are nearly rounded in shape and the intercellular spaces are filled with a substance called as matrix. The matrix is tough, gelatinous and elastic in nature formed of chondrin and hence is also called as chondrion. A cartilage consists of cells called chondroblasts which are packed in the matrix. The entire tissue is covered by a tough and fibrous membrane called as perichondrium. Cartilages are of following types hyaline cartilage, fibrocartilage, and elastic cartilage.

Hyaline cartilage: It is composed of collagen fibers which are packed in a translucent and elastic matrix. It has semi transparent and glossy matrix. It is found in the hyoid apparatus (at the base of tip tongue), larynx, trachea, sternum and the end of limb bones. It is also present at the tip of nose (Fig. 1.17).

Fibro cartilage: It has a compact matrix with large amounts of white fibers. It is present in the movable parts of the body which require extra strength like knee joints, joints between scapular and humerus bone of upper arm, intervertebral discs and pubic region (Fig. 1.18).

Elastic cartilage: It contains a network of branching and rejoining collagenous fibers which are yellowish in color. It is present at the tip of the nose, eustachian tube, and pinna. It is highly elastic in nature (Fig. 1.19).

Bone: It is hardest of all the cartilages because its matrix contains fine granular calcium carbonate. It is present in scapula, and spongy ends of the bones (Fig. 1.20).

All the bone consists of four parts periosteum, endosteum, matrix and bone marrow. Periosteum is the outermost thick and tough part made of fibrous connective tissue and a layer of bone cells called osteoblasts (bone forming cells). The endosteum the is innermost layer that lines the cavity of the bone. Matrix lies in between periosteum and endosteum and forms the major part of the bone. There are two types of bone tissues namely compact bone and spongy bone (Fig. 1.21).

Compact bone: It consists of a large number of units called Haversian systems. The Haversian system consists of a central canal called as Haversian canal which contain artery and a vein for blood supply and a nerve. This canal is surrounded by several concentric rows of lamellae. Between the lamella there are spaces called lacunae containing lymph and bone cells called osteocytes. All the lacunae are connected to each other through narrow spaces called canaliculi which permit the flow of food materials and waste matter. The lymph carrying nourishment flows through the canaliculi. In the spaces between the Haversian system there are interstitial lamellae (Fig. 1.20).

Spongy bone: It is also called as cancellous bone. The Haversian canals are much larger and there are fewer lamellae as compared to compact bone. Red bone marrow is always present with cancellous tissue.

These are those connective tissues which circulate throughout the body and transport various metabolites. Basically, they are of two types: Blood and Lymph.

Blood: It is a constantly circulating fluid providing the body with nutrition, oxygen, and waste removal. Blood is mostly liquid, with numerous cells and proteins suspended in it, making blood “thicker” than pure water. The average person has about 5 liters (more than a gallon) of blood. A liquid called plasma makes up about half of the content of blood. Plasma contains proteins that help blood to clot, transport substances through the blood, and perform other functions. Blood is composed of blood cells: Erythrocytes, i.e. red blood cells, leukocytes, i.e. white blood cells and thrombocytes, i.e. platelets (Fig. 1.22).

Erythrocytes: They are non-nucleated biconcave discs, containing hemoglobin. They are responsible for transportation of oxygen, carbon dioxide and minerals.

Leukocytes: They are part of the body's immune system; they destroy and remove old or aberrant cells and cellular debris, as well as attack infectious agents and foreign 12substances. There are several different types of white blood cells, these are granulocytes and agranulocytes.

Granulocytes (polymorphonuclear leukocytes): Leuko-cytes characterized by the differential staining of the granules in their cytoplasm. There are three types of granulocytes: neutrophils, basophils, and eosinophils which are named according to their staining properties.

Agranulocytes (mononuclear leukocytes): Leukocytes characterized by the apparent absence of granules in their cytoplasm. The cells include lymphocytes, monocytes, and macrophages.

Thrombocytes also called platelets, are responsible for blood clotting. They change fibrinogen into fibrin. This fibrin creates a mesh onto which red blood cells collect and clot, which then stops more blood from leaving the body and also helps to prevent bacteria from entering the body.

The fluid “lymph” can be described as a tissue in its own right in the same way as the fluid “blood” can be described as “blood tissue”. Lymph is a clear fluid that is similar to plasma, but contains less protein. It flows through lymphatic vessels throughout the body and includes chemicals and cells whose composition vary according to location within the body. Despite being a fluid, lymph is classified as a connective tissue. The major functions of lymph is draining interstitial fluid, transporting dietary lipids is vitamin K, and protecting the body against invasion/infection as it contains leukocytes (particularly lymphocytes and macrophages) (Fig. 1.23).

It is a type of connective tissue, which help in the movement of body parts by articulating bones with each other. It is contractile and is therefore able to produce movements. The cells, which form this tissue are different from the normal cells as they are shorten or contract. The cells are like long fibers of variable lengths. Each fiber is made of very fine fibers called myofibrils which are arranged in the form of bundles. The muscle fibers may or may not be covered by a layer of connective tissue called sarcolemma. The cytoplasm of the muscular cells is termed as sarcoplasm. There are three types of muscle fibers:

It is also called as involuntary, plain or visceral muscle. It is not under control of the will. The muscle cells are long and spindle-shaped with a nucleus present in the center of the spindle. The cells of these muscles are uninucleate and each nucleus is surrounded by sarcoplasm. These cells generally occur in the form of sheets made of loosely packed fibers of connective tissue (Fig. 1.24). The myofibrils in the sarcoplasm are also present longitudinally and the sarcolemma is absent. In its place the muscle fibers are covered by the plasma membrane. These muscles are present in the lower part of the esophagus, stomach intestine, lungs, walls of blood vessels, urinary bladder and eyes.

The striated muscles are also sometimes called as skeletal muscles, because they are attached to bones where they extend from one bone to the other and help in their movements by their contractions.13

It works under the control of will hence are called as voluntary muscle. The cells are about 10 to 40 mm in length and are roughly cylindrical in shape (Fig. 1.25).

These are found only in the heart where they are capable of constant rhythmic contractions. They are also unique in having the characteristics of both striped and unstriped muscle fibers. The muscle cells are arranged in the form of cylindrical units which are without sarcolemma. The fibers have few branches. All the fibers united and branched in such a way that a network of muscle fibers is formed. The myofibrils have regions of different densities so that their cytoplasm presents an irregular striated appearance. The transverse band-like appearance is also proved by the cell membranes of the adjacent cell walls (Fig. 1.26).

The nervous tissue carries out the special function of carrying messages of stimuli within the body. The properties of ‘irritability#x2019; and ‘conductivity’ are specially developed in the nervous tissue. The nervous tissue is made of nerve cells called neurons. The neurons are supported by a special type of connective tissue called neuroglia.

In the transmission of nerve impulse, whether sensory or motor more than one neuron is always involved. The point at which the nerve impulse passes from one to another is called synapse. Various chemicals known as transmitters are secreted in the synapse and they are involved in the transmission of information across the synapse.