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Cellular Studies of Human Tissues & Organ Systems
UNIT 1 CELLULAR STUDIES OF
HUMAN TISSUES & ORGAN
SYSTEMS
Structure
1.1 Introduction Objectives 1.2 Cells and Cell Structure Components of the Cell Activities of Cell 1.3 Tissue Epithelium Tissue Muscular Tissue Nervous Tissue Connective Tissue 1.4 Circulatory System 1.5 Skeletal System and Joints The Skeleton Joints: Their Composition and Movement 1.6 Summary 1.7 Terminal Questions 1.8 Answers
1.1 INTRODUCTION
In this unit you will study about the composition of the human body. You will study how the cells in the body form tissues and tissues form organs and various organ form systems and the system together form the entire human body. This unit first describes the structure of the cell and the cell cycle. It also describes the important functions of the cell. This unit then describes the various types of tissues and their functions in the human body. After this, the circulatory system and the skeletal system have been described. Emphasis has been given on the general composition of these organ systems and their physiological role.
Objectives
After reading this unit, you should be able to:
- describe the structure and function of the cells,
- write about the various stages of the cell cycle,
- describe the properties and functions of the four major animal tissue types in the human body;
- elaborate about the architecture of the circulatory system,
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- describe the various components of the circulatory system and write about their role in circulation of blood,
- describe the various components of the skeletal system; and
- discuss the different types of joints in the body.
1.2 CELLS AND CELL STRUCTURE
The human body, on an average may have as many as 10^14 cells. All cells are composed of: (i) a cell membrane which is the thin protective layer of the cell, (ii) the cytoplasm which forms the fluid of the cell, in which various structures called organelles, like the nucleus (consisting of nucleolus and chromatin network enclosed in a nuclear membrane), endoplasmic reticulum , mitochondria , lysosomes , golgi-apparatus , ribosomes , centrosomes and other cytoplasmic inclusions are suspended (Fig. 1.1a&b).
(a)
(b) Fig.1.1: Human cell (a) a cutaway section of the cell showing its features (b) diagrammatic representation of the cell
The plasma membrane has structural and functional continuity with the elaborate membrane system consisting of nuclear membrane, endoplasmic reticulum, mitochondria, chloroplasts and the Golgi apparatus. However the plasma membrane has more cholesterol than membranes of organelles.
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membrane. Generally, the peripheral proteins are exposed only on one side of the bilayer.
The lipid layer functions as a barrier to the entry or exit of charged or polar substances. Some of the protein present in the plasma membrane act as gate keepers and regulate the passage of the polar molecules and ions into and out of the cell.
II. Cytoplasm
The cytoplasm lies inside the cell membrane and is composed mainly of 90 percent water. Most of the vital chemical reactions of the cell (metabolism) take place in the cytoplasm. The cytoplasm can be divided into two components: (i) cystol which is the fluid portion of the cytoplasm and consists mainly of water plus dissolved solutes and suspended particles and (ii) organelles which are highly organized subcellur (part of cell) structures, each with a characteristic shape and specific functions for example the nucleus, endoplasmic reticulum(ER ), centrosomes, lysosomes, mitochondria and Golgi complex.
III. Organalles
All the following organelles that are present in the cytoplasm will be briefly described:
i) Nucleus: It is an important structure of the living cell and governs the activities of the cell. The nucleus is surrounded by a nuclear membrane (identical to the cell membrane) which encloses the nuclear fluid called nucleoplasm in which are contained , the chromatin which is composed of the hereditary material of Deoxyribose Nucleic Acid (DNA) ) that is combined with proteins called histones. Chromatin appears as tangled threads. However chromatin during the cell division is visible as clearly visible structures called chromosomes (Fig. 1.3). Each human cell has 46 chromosomes. This is called diploid chromosomes number (2 n). The ovum and sperm have only half the number that is 23 chromosomes (haploid number = n) each. Union of sperm and egg ensures that the embryo has 46 chromosomes. The nucleolus is a dense, tiny spherical structure is also present in the nucleolus.
ii) Endoplasmic Reticulum (ER): It is an intricate system of membranes that divides the cell into its numerous compartments. ER are of two types: (1) smooth ER, and (2) rough ER. Smooth ER is concerned with metabolism and synthesis of steroids and glycogen and the rough ER, so called because of the presence of ribosomes is concerned with proteins synthesis.
iv) Secretory Granules: They store the secretory products of the cell.
iv) Centrosomes: All cells have centrosomes which consist of a pair of tiny, cylindrical structures, located in the cytoplasm near the nuclear
A hydrolytic enzyme catalyzes a chemical reaction of a compounds like protein or carbohydrate or fat molecules by causing the removal or elimination of water and consequent breakdown of the compound into their simplest units.
Fig.1.3: A chromosome
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Cellular Studies of Human Tissues & Organ Systems
envelope, called centrioles. Centrioles are important during the process of cell division.
v) Lysosomes: These are the “suicide bags” of the cells which contain powerful hydrolytic enzymes. At the time of death of the cell the lysosome membrane ruptures, spilling out, its enzymes which results in and self-digestion (or autolysis) of the cell.
vi) Mitochondria: These are known as the energy power house of the cell. They are involved in the generation of energy molecule, Adenosine triphosphate (ATP) in the cell. ATP is used for various energy dependent processes of the cell.
vii) Golgi complex: It consists of 3-20 flattened membranous sacs with bulging edges, called cisterns that resemble a stack of naan bread. The cisterns are often curved, giving the Golgi complex a cuplike shape. Most cells have only one Golgi complex although some may have several. The Golgi complex is more extensive in cells that secrete proteins into the extracellular fluid. Almost all proteins secreted by the cells are glycoproteins. The main function of the golgi body is to add a carbohydrate group to the proteins manufactured by rough ER. ‘Shuttle’ vesicles containing proteins are budded off from the ER .These vesicles fuse with the cisternae at the bottom of the golgi body. The protein is quickly transferred to the series of stacked, disc-shaped, flattened membranes of the golgi body. Thus protein is transferred between the cisternae layers towards the top of the stack. The proteins as they move towards the ‘top’ of the stack are progressively converted to glycoproteins. At the top of the stack the secretory vesicles are budded off. These vesicles pass through the cytoplasm, and fuse with the cell membrane which discharges its contents to the outside ( exocytosis ).
1.2.2 Activities of Cell
Some general activities of cells are as follows:
i) Irritability: Cells have the ability to detect and respond to environmental changes.
ii) Nutrition: Cells are capable of selectively absorbing fluids and dissolved substances through the cell membrane which can be used by the cell for energy, growth or repair.
iii) Respiration: Cells have the ability to respire and as a result produce energy, by combining oxygen with nutrients, resulting in the formation of carbon dioxide (CO 2 ) and water.
iv) Excretion: Cells are able to discard waste materials through the cell membrane.
v) Growth and Reproduction: Cells have the ability to increase in size. When cells reach the limit of their growth they are capable of dividing and so reproducing into two daughter cells.
The membranes of the Golgi complex lack the ribosomes found on rough endoplasmic reticulum (ER) and so are called smooth- endoplasmic reticulum.
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1.3.1 Epithelium Tissue
The epithelium generally provides a covering to tissues. It covers the body surfaces and lines body cavities. The epithelium helps in protection, absorption, excretion and secretion of various substances. The main histological features common to all epithelial tissues is that:
The epithelial tissue consists of cells that are situated very close to each other (there is little extracellular material between epithelial cells).
Several types of junctional specializations unite adjacent epithelial cells (tight junctions, desmosomes and gap junctions).
All epithelia except for endocrine glands have one free surface, called the apical surface, which is exposed at the body surface or at the lumen (space) of the body cavity, duct, tube or vessel.
The lower surface of an epithelium (or basal surface) rests on a basement membrane: a non-living adhesive material secreted by the epithelium and the underlying connective tissue.
There are no blood vessels within the epithelial layer.
Epithelial cells are often characterized by frequent cell division because they are exposed to wear and tear and injury, necessitating replacement.
Classification of epithelium on basis of their functions
There are two basic types of epithelial tissues on basis of their function:
I) Glandular epithelia that make up most of the glands in the body.
II) Covering and lining epithelia form a continuous layer over all the free surfaces of the body.
Classification of epithelium on basis of their cell types
Epithelial tissues are classified according to the shape of the cell forming the tissue into three types, which are:
i) Squamous epithelium is flattened cells
ii) Cuboidal epithelium is cube-shaped cells
iii) Columnar epithelium consists of elongated cells
Classification of epithelium on basis of their cell layers
In addition to this, the epithelium is also classified on the basis of the number of cell layers into two subtypes of epithelia:
I. Simple epithelium which has only a single cell layer. II. Stratified epithelium which has more than one layer of cells
1.3.2 Muscular Tissue
Muscle tissue is the excitable (able to contract and relax) tissue of the body and possesses contractile ability hence it is also called a contractile tissue Muscle
Blood, cartilage, and bone are usually considered connective tissue, but because they differ so greatly from the other tissues in this class, the phrase “connective tissue proper,” is commonly used to exclude those three.
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tissue is composed mainly of muscle cells (also called muscle fibers ). Within the cells are myofibrils ; that contain sarcomeres. These sarcomeres are composed of actin and myosin. The actin and myosin form the contractile filaments that move past each other and change the size of the cell. The function of the muscle tissue is to produce force and cause motion. Muscles can cause either locomotion of the organism itself or movement of internal organs. They are classified (Fig. 1.5) as:
i) Skeletal ii) Smooth muscle, and iii) Cardiac muscles
i) Skeletal
Skeletal Muscles are so called because they are usually attached to bones. These muscles are under conscious control therefore they are also known as voluntary skeletal muscle. Each skeletal muscle fiber is an enormous, multi-nucleate (consisting of a number of nuclei) cell, formed by the fusion of hundreds of myoblasts (muscle cells) end-to-end. Each fiber is surrounded by a thin membrane called sarcolemma. Fibres of skeletal muscle are long, cylindrical and unbranched. These muscle fibres also show distinct striations (having transverse streaks) due to which they are also called striated muscles (Fig. 1.5a).
ii) Smooth Muscle
Smooth muscle or involuntary muscle tissue consists of small, unbranched, long, narrow, spindle shaped cells. Each smooth muscle cell has a single, elongated, centrally located nucleus. The nucleus of the cell is surrounded by a small amount of cytoplasm. These cells are arranged parallel to one another and do not show any striations microscopically hence this type of muscle is also called smooth muscle (Fig.1.5b). This type of muscle is also called “involuntary muscle” because it acts to contract and relax without being under our conscious control (as you know you just cannot will your intestines to contract, they just do – this is what causes that embarrassing grumbling while in public when you're hungry!). Functionally, smooth muscle cells contract as a single unit.
iii) Cardiac Muscles
Cardiac muscle is unique as it shows some features of skeletal muscle and some features of smooth muscle. Cardiac muscle as the name implies, is the muscle that makes up the wall of the heart. Cardiac muscle is similar to skeletal muscle in that it is striated and multinucleate. It is similar to smooth muscle in that its nuclei are centrally located and many cells are required to span the length of the muscle. It differs from both skeletal muscle and smooth muscle as its cells branch and is joined to one another via intercalated discs (Fig.1.5c). Intercalated discs allow communication between the cells due to which there is a sequential contraction of the cells from the bottom of the ventricle of the heart to the top, facilitating maximal
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(i) cell body (ii) dendrites, and (iii) axon, as shown in Fig.1.6 and described below.
i) Cell body
A neuron consists of a large cell body called perikaryon or soma, that contains (i) a nucleus and (ii) cytoplasm called perikaryon. Clusters of ribosomes and rough endoplasmic called Nissl or Nissl bodies or Nissl substance are present in the cell body. These Nissl bodies are basophilic (take up basic dye) in nature.
ii) Dendrites
Dendrites are protoplasmic projections that arise as numerous short projections from the nerve cell body. They constitute the receptor area of the neuron and so receive the information which they transmit into (conduct input) the neuron. Dendrites thus transmit information towards the nerve cell body. Dendrites are not myelinated (Dendrites are without myelin sheath).
iii) Axon
The axon is a long protoplasmic process which stretches from the cell body to a distance ranging from several millimeters up to one meter. The part of the axon where it emerges from the soma is called the axon hillock. Axons transmit nerve signals away from the nerve cell body towards other nerves, muscle, or glands. Neurons usually have only one axon. Many, but not all, axons are sheathed in a multiple layer called myelin sheath. The myelin sheath functions as a kind of electrical “insulation” greatly facilitating nerve impulse transmission. The greater the amount of myelin around an axon, the faster is the rate of nerve impulse transmission. Gaps occur in the myelin sheath and are called as Nodes of Ranvier. The terminal end of the axon away from the cell body is called the terminal bouton or axon terminal or end bulb. This terminal bouton will be associated with another neuron in a synapse.
Fig.1.6: Structure of a neuron
Interneurons: Interneurons are the neurons between the sensory and motor neurons. 99.9% of all neurons are interneurons. The vast majority of neurons are interneurons. Interneurons are also called internuncial neurons
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Cellular Studies of Human Tissues & Organ Systems
1.3.4 Connective Tissue
Connective tissue is one of the four types of tissues in traditional classifications (the others being, epithelial, muscle, and nervous tissue ). The main function of the connective tissues is to support the body and to bind or connect together all types of tissue. The connective tissue also provides a mechanical framework (the skeleton) which plays an important role in locomotion.
Connective tissues consist of cells and intercellular material. Fibroblasts are the most common cell types and are responsible for producing the fibres and other intercellular materials forming the connective tissue. Other cells that occur in the connective tissue generally in the loose connective tissue are adipose cell (adipocytes), mast cells, macrophages, leucocytes, and plasma cells.
The connective tissue exists in a number of forms however all types of connective tissues have three basic structural elements (Fig. 1.7): (i) cells, (ii) non-cellular fibres, and (iii) non-cellular intercellular substance (matrix or ground tissue). The types of fibres that occur in the connective tissue are: (i) collagen (collagenous) and (ii) elastic fibres and (iii) reticular fibres. Collagen fibres are for strength while the elastic ones are for elasticity of the tissue. Reticular fibers are for forming delicate structural framework. The consistency of matrix is highly variable from gelatin-like to a much more rigid material and so may be liquid (e.g. blood), semi-solid (e.g. connective tissue) or solid (e.g. bone). The other characteristic feature of all or most connective tissues is that they are:
- Involved in structure and support.
- Usually derived from mesoderm (middle layer of the embryonic layer).
- Characterized largely by the traits of non-living tissue.
Fig.1.7: A generalized structure of connective tissue showing its various components
Cellular Studies of Human Tissues & Organ Systems
I) Loose Connective Tissue
Loose connective tissue has abundant cells among few or loosely arranged fibers and a sparse to abundant gelatinous ground substance. Loose connective tissue is the most common type of connective tissue. It occurs beneath the epithelium in skin and in many internal organs such as lungs, arteries and the urinary bladder. This type of tissue also forms a protective layer over muscles, nerves and blood vessels. The loose connective tissues is furthermore classified into
i) Areolar tissue ii) Adipose tisse, and iii) Reticular tissue
i) Areolar Tissue
Areolar connective tissue holds organs and epithelia in place and consists of matrix, within which lie two kinds of proteinaceous fibers: (a) white or collagenous fibres, and (b) yellow or elastic fiber. These fibers intercross, thus making a network. The space within the network is occupied by the matrix and various types of cells such as fibroblasts, histiocytes, basophils, plasma cells, pigment cells, mast cells, lymphocytes, etc.
ii) Adipose Tissue
Adipose tissue is a fairly loose connective tissue, containing large numbers of round shaped, fat-storing cells called adipocytes which make up 90% of the tissue. Adipose tissue has a rich supply of blood and a high metabolic activity. Adipose Tissue may develop anywhere, but it tends to accumulate beneath the skin, where it can act as a shock- absorber and insulator. Women tend to have more adipose tissue than men.
There are two types of adipose tissue, white adipose tissue (WAT) which is composed mainly of white fat cells and brown adipose tissue (BAT) composed primarily of brown fat cells.
iii) Reticular Tissue
Reticular tissue consists of loosely arranged, branching, very thin, white, reticular, collagen fibres with a glycogen coating, present in a gelatinous ground substance. Reticular connective tissue is moderately rich in ground substance, and often has numerous undifferentiated, mesenchymal cells. Reticular tissue provides a delicate structural framework for organ stroma (e.g. bone marrow, lymph nodes, spleen).
II) Dense Connective Tissue
Dense connective tissue is also called dense fibrous tissue. It has collagen fibers as its main matrix element. Dense connective tissue contains
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relatively few cells with much greater numbers of collagen fibers. It is divided into two sub-categories:
Dense irregular connective tissue which has bundles of collagen fibers that appears to be fairly randomly orientated (as in the dermis).
Dense regular connective tissue that consists of closely-packed densely- arranged fiber bundles with clear orientation (such as in tendons) and relatively few cell.
Elastic
Elastic connective tissue is rich in parallel bundles of elastic fibers, and is found in specialized sites (e.g. yellow ligaments of vertebral column, vocal chords).
III) Cartilage
Cartilage and bone are “rigid” connective tissues. Cartilage however though tough and hard is softer than bone. Cartilage is flexible, avascular (without blood vessels), and without nerves. Cartilage contains large quantity of clear, firm jellylike ground substance or matrix of dense and elastic nature in which living cartilage cells called chondroblasts are embedded in irregularly scattered fluid spaces called lacunae. The matrix is made up of chondrion in which there is a delicate network of collagen fibers. Cartilage, occurs in the human body in the ears, tip of the nose, and at joints such as the knee and between bones of the spinal column. Cartilage connective tissue is further subdivided into three types (Fig. 1.8):
i) Hyaline ii) Fibrous, and iii) Elastic
i) Hyaline cartilage: consists of rubbery a matrix as the ground substance and living cartilage cells called chondrocytes, which are situated far apart in fluid-filled spaces, called lacunae. The matrix also contains a number of collagenous fibers. The cells are arranged in groups of two and four. Hyaline cartilage is found in bones (articular cartilage), ribs (costal cartilage) and in long bones (epiphyseal cartilage).
ii) Fibrous cartilage: is also called fibrocartilage cartilage. It contains cells which are larger in number, arranged in groups and placed inside lacunae. White fibrocartilage is an extremely tough tissue. The amount of collagen fibres present here is more than that found in hyaline cartilage.
iii) Elastic cartilage: Basically elastic cartilage is similar to cartilage, but it contains, in addition to the collagenous fibres present in its matrix also an abundant network of branched yellow elastic fibres. This type of cartilage also consists of a number of cells in lacunae.
A tendon (or sinew ) is a tough band of fibrous connective tissue that usually connects muscle to bone and is capable of withstanding tension. Tendons are similar to ligaments except that ligaments join one bone to another. Tendons and muscles work together and can only exert a pulling force.
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The compact bone as a result is hard. Its outer layer serves as an attachment for the muscles, tendons and ligaments. It has an outer fibrous layer and an inner osteogenic layer. The osteogenic layer consists of cells like the osteoblasts, osteoclasts and the osteocytes.
ii) Spongy bone part
In contrast to compact bone tissue, spongy (cancellous) bone consists of an irregular latticework of thin blades of bone called trabeculae. The spaces between the trabeculae contain blood vessels and bone marrow which produces blood cells (Fig.1.10). The bone marrow may be red or yellow in colour depending on the amount of fat cells. The red bone marrow produces the red blood cells. With accumulation of fat, the red bone marrow changes into the yellow bone marrow. The most important function of the bone marrow is production of RBCs, reticulo- endothelial cells, platelets, etc. The spaces between the trabeculae can be seen with naked eye and give spongy bone tissue its "spongy" look.
Fig.1.10: Spongy bone tissue
V) Blood
Blood is a connective tissue consisting of cells separated by a liquid called the ground substance or plasma matrix or simply as plasma (Fig.1.11a). Blood is a sticky fluid connective tissue with a slightly salty taste. It is fluid present almost everywhere and is distributed by means of blood vessels: arteries, veins, arterioles, venules and capillaries. It has a bright red or scarlet colour when it flows in the arteries but a dark red or purple colour when it flows in the veins. It is slightly alkaline (pH 7.4). The plasma of the blood is a watery fluid and transports dissolved glucose, wastes, carbon dioxide and hormones. The plasma also regulates the water balance for the blood cells. Several types of cells are present in the plasma which are as follows (Fig.1.11):
Cellular Studies of Human Tissues & Organ Systems
(a) (b)
Fig.1.11: a) Components of human blood and their function (a) Non cellular components forming the plasma, b) cellular components
Composition of blood
Blood has of two parts (i) fluid or plasma that constitutes 55% of the blood, and (ii) suspended solid component of cells (Fig. 1.11 b) that make up upto 45% of the blood.
i) Plasma
The plasma (Fig. 1.11 (a)) is composed of water (90-92%) and solids (8- 10%). The solids are in turn made up of electrolytes, plasma proteins, fats, hormones, colouring substances like bile pigments, etc. The important plasma proteins are serum albumin, serum globulin and serum fibrinogen. Serum albumin and fibrinogen are derived from the liver, whereas the serum globulins are derived from lymphocytes.
ii) Blood Cells
The cells (Fig. 1.11 b) present in the blood are:
The white blood corpuscles are also called white blood cells or WBCs, or Leukocytes. These cells, unlike RBCs are nucleated (have nucleus) and do not carry the oxygen carrying pigment haemoglobin. WBC function in the immune system. The total WBC count of blood in humans is 4000-11,000/mm^3. The WBCs are further subdivided, based
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Fig.1.13: Red blood corpuscle
Platelets are small cytoplasmic bodies derived from cells (Fig.1.11b). The platelets are cell fragments that function in blood clotting. The platelets are round or oval, non-nucleated bodies of varying size. The platelets circulate in the blood in the blood. They are one fourth the size of RBCs. They are non-nucleated with the presence of distinct granules in their cytoplasm. In normal adults the platelet count is 2, 50,000- 5,00,000/mm^3. They are required for the process for initiation blood clot formation, repair of capillary leaks, clot retraction, etc.
General Functions of Blood
Blood is an essential component of the body. It performs various functions in the body. The important functions of blood are listed below:
- Blood transports oxygen by means of Red Blood Corpuscles (also called red blood cells or RBCs), from the lungs to the tissue and transport of carbon dioxide (CO 2 ) from the tissue back to the lungs.
- It transports nutrients from the digestive tract to various tissues of the body.
- Blood acts as a vehicle for the movement of hormones, vitamins, pigments form their source to various parts of the body.
- It maintains the fluid, electrolyte and acid-base balance in the body.
- Blood helps in regulation of the body temperature.
- It helps in the excretion through urine of waste products that form in the cells.
- The white blood corpuscles (also called white blood cells or WBCs) present in the blood help in the body’s defence mechanism against infection.
VI) Lymph
Lymph is a modified fluid tissue containing 94% water and 6% solids. It differs from blood in the fact that only lymphocytes (5000-75000/cmm) of
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the white blood cells are present in it and red blood cells are absent. Lymph is thus similar in composition to blood plasma with various types of white blood cell floating in it. It flows in lymphatic vessels.
SAQ 2
Fill in the blanks:
i) ______________ cartilage is found in inter vertebral discs. ii) ______________ epithelium is found in the urinary bladder. iii) During _________, the cell divided into two daughter cells. iv) A ______ is the functional unit of the nervous system. v) ________ is the hardest connective tissue.
1.4 CIRCULATORY SYSTEM
The circulatory system is a closed circulatory transport system responsible for the transport of nutrients, gases and waste materials through the body by means of blood. Blood as you will recall is a fluid, connective tissue that flows within a closed system of vessels, namely the arteries, veins, capillaries, etc. Blood appears as a red, viscous and slightly alkaline fluid. The total quantity of blood in the human body is approximately 5-6 litres. The important components of the circulatory system are the heart, and the blood vessels namely arteries and veins.
Heart
The heart is a muscular organ, lying in the upper left part of the chest (Fig. 1.14). It pumps blood through the blood vessels. The regular contractions of the heart, or when the heart pushes in, force the blood to the various parts of the body
A normal heart lies under the sternum (breast bone). The heart consists of three layers (i) The outer, tough, fibrous layer or pericardium that forms a bag like structure around the heart and contains a fluid called the pericardial fluid (ii) The middle layer or the myocardium consisting of the muscular, contractile fibres and the (iii) innermost layer is called the endocardium.
Internal structure of the heart: The heart is made up of four chambers, viz. two atria (singular atrium) and two ventricles. The walls of the atria are thin and serve as a filling reservoir, from which the blood is pushed into the ventricles. The ventricles are thick walled and push the blood into either the pulmonary circulation (right ventricle) or systemic circulation (left ventricle). The muscle wall surrounding the left ventricle is thicker than the wall surrounding the right ventricle due to the higher force needed to pump the blood through the systemic circulation.
When the blood goes from the atria to the ventricles it goes through heart valves. When blood goes out of the ventricles it goes through valves. The valves make sure that blood only goes one way in or out.
