Anatomy is the study of structure and function of the body. Aristotle (384–322 BC) was the first person to use the term ‘anatome’, a Greek word meaning ‘cutting up or taking apart’. The Latin word ‘dissecare’ has a similar meaning.
Anatomy is one of the oldest basic medical sciences; it was first studied formally in Egypt. Human Anatomy was taught in Greece by Hippocrates (460–377 BC) who is regarded as the ‘Father of Medicine’. He has written several books on Anatomy.
MEDICAL AND ANATOMICAL TERMINOLOGY
Although students entering the new world of Medicine are familiar with the common terms for many parts and regions of the body (e.g. heart, brain, liver, lung), they should learn to use the internationally adopted nomenclature, the Nomina Anatomica.
Anatomical terminology is important because it introduces the student to a large part of Medical Terminology. Since most terms are derived from Latin and Greek, medical language can be difficult at first, but as the student learns the origin of medical terms, the words make sense.
Example: Levator palpebrae superioris muscle (the muscle which elevates the upper eyelid).
- Levator = one which elevates
- Palpebrae = eyelid
- Superioris = superior or upper.
Clear communication is fundamental in Clinical Medicine. To describe the body clearly and to indicate the position of its parts and organs relative to each other, anatomists and clinicians use the same descriptive terms of position and direction.
The Anatomical Position (Fig. 1.1)
All descriptions in Human Anatomy and Clinical Medicine are expressed in relation to ‘anatomical position’.
A person in the anatomical position is standing erect (or lying supine) with the head, eyes and toes directed forward, the upper limbs by the sides with the palms facing anteriorly. The student must always visualize the anatomical position in his ‘mind's eye’ when describing patients lying on their backs, sides or fronts. Always describe the body as if it were in the anatomical position.
The Anatomical Planes
Anatomical descriptions are also based on four imaginary planes that pass through the body in the anatomical position. They are as follows:
Median Plane (Fig. 1.2)
This is the imaginary vertical plane passing longitudinally through the body from front to back, dividing it into right and left halves.
These are parallel to the median plane. They are named after the sagittal suture of the skull (Fig. 1.3). The sagittal plane that passes through the median plane can be called the midsagittal plane; those passing parallel to the midsagittal plane and away from the median plane may be called the parasagittal planes.
These are imaginary vertical planes passing through the body at right angles to the median plane, dividing it into anterior (front) and posterior (back) portions. These planes are named after the coronal suture of the skull, which is in a coronal plane (Fig. 1.3).
These are imaginary planes passing through the body at right angles to both the median and coronal planes (they are parallel to the ‘horizon’). A horizontal plane divides the body into superior (upper) and inferior (lower) parts. A horizontal plane is also referred to as the transverse plane (Fig. 1.4).
Terms of Relationship (Table 1.1)
Various terms (adjectives) are used to describe the relationship of parts of the body in the anatomical position.
Various terms are used to describe the different movements of the limbs and other parts of the body. Movements take place at joints where two or more bones meet or articulate with one another.
Figs 1.5A to D: Relations of organs. A. Heart and lungs; B. Gross anatomy of eye and histology of cornea; C. Transverse section of thigh, showing femur and muscles; D. Kidneys and vertebral column
The Meaning of Terms
Most of the anatomical terms are derived from Greek and Latin. Some of them are translated to English (e.g. musculus = muscle). Many anatomical terms indicate the shape, size, location and function or resemblance of a structure to something.
- According to shape
- Deltoid—delta or triangular
- According to the number of heads of origin
- Biceps—2 heads
- Triceps—3 heads
- Quadriceps—4 heads
- According to function
- Depressor anguli oris—muscle which depresses the angle of mouth
- Tensor tympani—muscle which tenses the tympanic membrane
- According to size
- Gluteus maximus—largest among the gluteus muscles
- Gluteus minimus—smallest among the gluteus muscles
- According to length
- Abductor pollicis longus—long abductor of thumb
- Abductor pollicis brevis—short abductor of thumb
- According to consistency
- Pancreas—pan = throughout, kreas = flesh, fleshy throughout
- Dura mater—dura = tough, mater = mother, tough mother
- According to location
- Biceps brachii—biceps muscle of arm
- Biceps femoris—biceps muscle of thigh
- Triceps brachii—triceps muscle of arm
- According to sites of attachment
- Sternocleidomastoid muscle—attached to sternum, clavicle and mastoid
- Omohyoid—muscle extending from scapula (shoulder blade) to hyoid (omos = shoulder).
Some of the commonly used anatomical and clinical abbreviations are given in Table 1.3.
APPROACHES IN STUDYING ANATOMY
The three main approaches are as follows:
- Regional anatomy
- Systemic anatomy
- Clinical anatomy
Regional Anatomy or Topographical Anatomy
It is the study of the body by regions such as head, neck, thorax, abdomen and limbs.
Systemic Anatomy (Table 1.4)
It is the study of the body systems, e.g. digestive system, cardiovascular system, nervous system.
Correlation of anatomy with clinical signs and symptoms to arrive at a diagnosis.
Gross Anatomy and Histology
Microscopic study of a tissue.
Individuals differ in physical appearance. Similarly variations can be seen in the size, shape, weight of organs; origin, course and termination of arteries, nerves and veins. So, individual variation must be considered while examining a patient and in the diagnosis and treatment of that patient.
The smallest functional unit of our body is the cell. It was Robert Hook who first coined the term ‘cell’ in 1665. The size of a cell can vary from a minimum of 6 μm (that of a resting lymphocyte) to a maximum of 80 μm (that of a mature ovum).
Definition: The study of cells is called ‘Cytology’ and the study of tissues is called ‘Histology’.
Structure of a Cell
A eukaryotic cell consists of the following structures:
- A cell membrane or plasma membrane, which is impermeable to large molecules like proteins, and selectively permeable to small molecules like ions and metabolites.
- A nucleus, which contains the genetic machinery.
- Cytoplasm and the organelles.
Cell Membrane (Plasma Membrane)
The cell membrane is a protective sheath that envelops the cell body. It separates the fluid outside the cell called extracellular fluid (ECF) and the fluid inside the cell called intracellular fluid (ICF). It is a semipermeable membrane and allows free exchange of certain substances between ECF and ICF.
Composition of Cell Membrane
The cell membrane is composed of three types of substances:
- Proteins (55%)
- Lipids (40%)
- Carbohydrates (5%).
Structure of Cell Membrane
When examined by electron microscope (EM) the average cell membrane is seen to be about 7.5 nm thick. It consists of two densely stained layers separated by a lighter zone, thus creating a trilaminar appearance (Fig. 1.7A).
Lipids in Cell Membranes
The major lipids in cell membrane are (Fig. 1.7B):
It is now known that the trilaminar structure of membranes is produced by the arrangement of lipid molecules (predominantly phospholipids) that constitute the basic framework of the membrane.
The phospholipid molecules are formed by phosphorus and fatty acids. Each phospholipid molecule resembles the headed pin in shape. The outer part of the phospholipid molecule is the head portion which is water-soluble (hydrophilic) and the inner part is the tail portion that is not soluble in water (hydrophobic) (Fig. 1.7B). The hydrophobic tail portions meet in the center of the membrane). The hydrophilic head portions of outer layer face the ECF and those of the inner layer face the cytoplasm.
The cholesterol molecules are arranged in between the phospholipid molecules. As phospholipids are soft and oily in nature cholesterol helps to pack the phospholipids in the membrane and maintain the structural integrity of cell membrane (Fig. 1.7C).
Functions of Lipid Layer
The lipid layer is semipermeable in nature and allows only the fat-soluble substances like oxygen, carbon dioxide and alcohol to pass through it. It does not allow the water-soluble materials like glucose, urea and electrolytes to pass through it.
Figs 1.7A to C: A. Diagram showing the fluid mosaic model of membrane; B. Phospholipid molecule (schematic representation); C. Arrangement of lipids and proteins in cell membrane (schematic representation)
Protein in Cell Membrane
The protein layers of the cell membrane are the electron dense layers situated on either side of the central lipid layer. The protein substances present in these layers are mostly glycoproteins. These protein molecules are classified into two categories (Fig. 1.7C).
- Integral proteins
- Peripheral proteins.
The integral proteins are also known as transmembrane proteins, are tightly bound with the cell membrane. These protein molecules pass through the entire thickness of the cell membrane from one side to the other side.
The peripheral proteins are also known as peripheral membrane proteins do not penetrate the cell membrane but are embedded partially in the outer and inner surfaces of the cell membrane. These protein molecules are loosely bound with the cell membrane and so dissociate readily from the cell membrane.
Functions of Protein Layer
- Integral proteins serve as receptors which bind with various hormones. The interaction of a hormone with the extracellular protein receptor produce certain conformational changes within the receptor protein. As a result, the intracellular part of the integral protein becomes enzymatically active and initiate a number of reactions inside the cell.
- Integral proteins help in the transport of substances across the cell membrane. They serve as pores and channels through which water and water-soluble ions can diffuse freely.
- Some of the integral proteins serve as carrier proteins which can facilitate transport of certain molecules across the membrane.
- Some of them act as pumps which can actively transport substances against their concentration gradient, e.g. Na+K+ATPase pump, Ca++ATPase pump. Pumps are actually enzymes which can hydrolyze adenosine triphosphate (ATP) for the release of energy. This energy can be utilized by the cell for various purposes like active transport. These membrane bound enzymes are richly seen in intestine.
- Integral proteins can act as cell-adhesion molecules which are responsible for the particular shape, growth and differentiation of cells, e.g. integrins, cadherins, etc. They help to attach the cell with the surrounding extracellular matrix (cell-matrix adhesion) and also with the neighboring cells (cell-cell adhesion). In metastatic tumor cells, there is loss of cell-matrix as well as cell-cell adhesion.
- Peripheral proteins are seen on the inner part of cell membrane, attached to one of the integral proteins. They attach loosely to the lipid bilayer, but not embedded in it. Some of them act as enzymes, which can control intracellular functions.
One most important peripheral protein is spectrin that gives some peripheral proteins, along with integral proteins, form a part of cytoskeleton which give strength and structural integrity to the cell, e.g. spectrin, ankyrin, actin, etc. which are seen on red blood cell (RBC) membrane. The spectrin gives the biconcave shape and strength to the RBC. In persons with hereditary spherocytosis, there is mutation of genes coding for spectrin. As a result, their RBCs become extremely fragile and spherical in shape. These RBCs are unable to withstand stress, which leads to hemolyticanemia.
Carbohydrates of the Cell Membrane
Carbohydrate molecules form a thin loose covering over the entire surface of the cell membrane called glycocalyx. Some carbohydrate molecules are attached with proteins and form glycoproteins and some are attached with lipids and form glycolipids (Fig. 1.8).
Fig. 1.8: Glycolipid and glycoprotein molecules attached to the outer aspect of cell membrane (schematic representation)
Functions of Carbohydrates
- The carbohydrate molecules are negatively charged and do not permit the negatively charged substances to move in and out of the cell.
- The glycocalyx from the neighboring cells helps in the tight fixation of cells with one another.
- Some of the carbohydrate molecules form the receptors for some hormones.
Functions of Cell Membrane
- Protective function: Cell membrane protects the cytoplasm and the organelles present in the cytoplasm.
- Selective permeability: Cell membrane acts as a semipermeable membrane which allows only some substances to pass through it and acts as a barrier for other substances.
- Absorptive function: Nutrients are absorbed into the cell through the cell membrane.
- Excretory function: Metabolites and other waste products from the cell are excreted out through the cell membrane.
- Exchange of gases: Oxygen enters the cell from the blood and carbon dioxide leaves the cell and enters the blood through the cell membrane.
- Maintenance of shape and size of the cell: Cell membrane is responsible for the maintenance of shape and size of the cell.
Role of Cell Membrane in Transport of Material into or out of the Cell
Some molecules can enter cells by passing through passive channels in the cell membrane. Large molecules enter the cell by the process of endocytosis (Fig. 1.9). In this process the molecule invaginates a part of the cell membrane, which first surrounds the molecule, and then separates (from the rest of the cell membrane) to form an endocytic vesicle. This vesicle can move through the cytosol to other parts of the cell.
The term pinocytosis is applied to a process similar to endocytosis when the vesicles (then called pinocytotic vesicles) formed are used for absorption of fluids (or other small molecules) into the cell.
Some cells use the process of endocytosis to engulf foreign matter (e.g. bacteria). The process is then referred to as phagocytosis.
Molecules produced within the cytoplasm (e.g. secretions) may be enclosed in membranes to form vesicles that approach the cell membrane and fuse with its internal surface. The vesicle then ruptures releasing the molecule to the exterior. The vesicles in question are called exocytic vesicles, and the process is called exocytosis or reverse pinocytosis (Fig. 1.10).
Fig. 1.9: Three stages in the absorption of extracellular molecules by endocytosis (schematic representation)
The cytoplasm is the fluid present inside the cell. It contains a clear liquid portion called cytosol which contains various substances like proteins, carbohydrates, lipids and electrolytes. Apart from these substances, many organelles are also present in cytoplasm. The cytoplasm is distributed as peripheral ectoplasm just beneath the cell membrane and inner endoplasm between the ectoplasm and the nucleus.
We have seen that (apart from the nucleus) the cytoplasm of a typical cell contains various structures that are referred to as organelles (Table 1.5). They include the endoplasmic reticulum (ER), ribosomes, mitochondria, the Golgi complex, and various types of vesicles (Fig. 1.11). The cytosol also contains a cytoskeleton made up of microtubules, microfilaments and intermediate filaments.
Endoplasmic reticulum is made up of tubules and microsomal vesicles. These structures form an interconnected network which acts as the link between the organelles and cell membrane.
Types of Endoplasmic Reticulum
The endoplasmic reticulum is of two types, namely rough ER and smooth ER.
Rough Endoplasmic Reticulum
Rough ER is the one to which the granular ribosome is attached. This gives the rough appearance and so, it is called the rough ER. Attachment of the granular ribosome also gives the beaded or granular appearance and so it is also called granular ER (Fig. 1.12).
Smooth Endoplasmic Reticulum
Smooth ER is also called as agranular ER because of its smooth appearance without the attachment of ribosome. It is formed by many interconnected tubules. So, it is also called tubular ER.
Fig. 1.11: Cell organelles (schematic representation)Abbreviations: TS = Transverse section; LS = Longitudinal section
Functions of Endoplasmic Reticulum
- Rough (granular) ER is concerned with synthesis of proteins.
- Smooth (agranular) ER lacks ribosomes and is concerned with the synthesis of lipids, steroids, cholesterol and carbohydrates. It acts as a surface for the attachment of many enzyme systems and helps in detoxifying drugs, alcohol, metabolic by-product, etc.
- Highly specialized ER is present in some cells. In striated muscle cells, where it is called sarcoplasmic reticulum, it is involved in the storage and release of Ca2+ to initiate muscle contraction.
Golgi apparatus (Golgi body or Golgi complex) is present in all the cells except RBCs. It consists of 5–8 flattened membranous sacs called cisternae (Fig. 1.13).
The Golgi apparatus is situated near the nucleus. It has two ends or faces namely, cis face and trans face. The cis face is positioned near the ER. The reticular vesicles from ER enter the Golgi apparatus through cis face. The trans face is situated near the cell membrane. The processed substances make their exit from Golgi apparatus through transface.
Functions of Golgi Apparatus
- It is concerned with the processing and delivery of substances like proteins and lipids to different parts of the cell.
- It functions like a post office because, it packs the processed materials into the secretory granules, secretory vesicles, and lysosomes and dispatch them either out of the cell or to another part of the cell.
- It also functions like a shipping department of the cell because it sorts out and labels the materials for distribution to their proper destinations.
The cytoplasm of a cell may contain several types of vesicles (Fig. 1.14). The contents of any such vesicle are separated from the rest of the cytoplasm by a membrane which forms the wall of the vesicle.
Vesicles are formed by budding off from existing areas of membrane. Some vesicles serve to store material. Others transport material into or out of the cell, or from one part of a cell to another. Vesicles also allow exchange of membrane between different parts of the cell.
Details of the appearances of various types of vesicles will not be considered here. However, the student must be familiar with their terminology given below.
Phagosomes: Solid ‘foreign’ materials, including bacteria, may be engulfed by a cell by the process of phagocytosis. In this process the material is surrounded by a part of the cell membrane. This part of the cell membrane then separates from the rest of the plasma membrane and forms a free floating vesicle within the cytoplasm. Such membrane bound vesicles, containing solid ingested material are called phagosomes (also see lysosomes).
Pinocytotic vesicles: Some fluid may also be taken into the cytoplasm by a process similar to phagocytosis. In the case of fluids the process is called pinocytosis and the vesicles formed are called pinocytotic vesicles.
Exocytic vesicles: Just as material from outside the cell can be brought into the cytoplasm by phagocytosis or pinocytosis, materials from different parts of the cell can be transported to the outside by vesicles. Such vesicles are called exocytic vesicles, and the process of discharge of cell products in this way is referred to as exocytosis (or reverse pinocytosis).
Lysosomes are membrane bound spheroidal bodies containing hydrolase enzymes capable of degrading a wide variety of substances. They are present in all cells except mature RBC. They are dominant in neutrophils. The different types of lysosomes are given in Flowchart 1.1.
Functions of Lysosomes
- Digestion of unwanted substances
- Removal of excess secretory products in cells
- Secretory function: Recently, lysosomes having secretory function called secretory lysosomes are found in some cells, e.g. melanocytes, mast cell, etc.
Genetic defects can lead to absence of specific acid hydrolases that are normally present in lysosomes. As a result some molecules cannot be degraded, and accumulate in lysosomes.
Examples of such disorders are lysosomal glycogen storage disease, in which there is abnormal accumulation of glycogen, and Tay-Sach's disease, in which lipids accumulate in lysosomes and lead to neuronal degeneration resulting in seizures, muscle rigidity and blindness.
Peroxisomes are small, spherical, membrane bound organelle that closely resemble lysosomes. However, they contain entirely different set of enzymes—oxidases and catalases. Large peroxisomes are found in liver and kidney cells.
They help in the detoxification and oxidation of a wide variety of compounds.
The centrosome is situated near the center of the cell close to the nucleus. It consists of two cylindrical structures called centrioles which are responsible for the movement of chromosomes during cell division.
Mitochondria are membrane bound organelles and are called the power-generating units of the cell. Each mitochondrion consists of two layers of membrane:
- An outer smooth membrane, which contains many pores, allowing free passage of small molecules.
- An inner membrane, which is thrown into folds that increase the surface area. The folds or cristae project into the inner cavity, which is filled with an amorphous matrix. Amount of cristae is proportional to the metabolic activity of the cell (they are more in cardiac muscle) (Fig. 1.15).
Matrix contains enzymes, Ca2+, glycogen, ribonucleic acid (RNA), deoxyribonucleic acid (DNA), etc. Mitochondria have several unique features among other cell organelles—they can move, change their size and shape and divide within the cells. Also they synthesize 37 of their own constituent proteins.
Functions of Mitochondria
- It is the chief site of tricarboxylic acid (TCA) cycle, electron transport chain and fatty acid metabolism.
- Release of energy from ATP and guanosine triphosphate (GTP).
- It concentrates Ca2+.
Sperms do not contribute mitochondria to the zygote. Therefore it comes from the ovum and is completely maternal in origin.
The cytoskeleton of the cell is a complex network that gives shape, support and stability to the cell. It is also essential for the cellular movements and the response of the cell to external stimuli. The cytoskeleton consists of three major protein components (Table 1.6).
- Intermediate filaments
Nucleus is present in those cells which divide and produce enzymes. The cells with nucleus are called eukaryotes and those without nucleus are known as prokaryotes (e.g. RBCs). Prokaryotes do not divide or synthesize the enzymes.
Most of the cells have only one nucleus (uninucleated). Few types of cells like skeletal muscle cells have many nuclei (multinucleated). Generally the nucleus is located near the center of the cell. It is mostly spherical in shape. However, the shape and situation of nucleus vary in different cells.
- Nuclear membrane
The nucleus is covered by a double-layered membrane called nuclear membrane. It encloses the fluid called nucleoplasm. Nuclear membrane is porous and permeable in nature and it allows nucleoplasm to communicate with the cytoplasm (Fig. 1.16).
It is a gel-like ground substance and contains large quantities of the genetic material in the form of DNA. The DNA is made up of chromatin threads. These chromatin threads become the rod-shaped chromosomes just before the cell division.
One or more nucleoli are present in each nucleus. The nucleolus contains RNA and some proteins, which are similar to those found in ribosomes. The RNA is synthesized by chromosomes and stored in the nucleolus.
Functions of Nucleus
- Controls all the activities of the cell
- Synthesizes RNA
- Forms subunits of ribosomes
- Sends genetic instruction to the cytoplasm for protein synthesis through messenger RNA
- Controls the cell division through genes
- Stores the hereditary information in chromosomes (and genes) and transforms this information from one generation to the next.
Chromosomes form the physical basis of inheritance. They are located in the nucleus. Chromosomes are made up of genes. They become visible during cell division. Number of chromosomes is species-specific.
Normal human cells contain 23 pairs of chromosomes, i.e. the total being 46. One member of each pair is inherited from each parent.
Body characters and functions are regulated by genes on 22 pairs of chromosomes known as autosomes.
The 23rd pair consists of sex chromosomes. Sex chromosomes are of two types, namely X and Y, based on their role in sex determination. Females have 22 pairs of autosomes and XX chromosomes, while in males, there are 22 pairs of autosomes and XY chromosomes (Fig. 1.17).
The body is composed of only 3 basic elements, i.e. cells, intercellular substances and body fluids. The cells are derived from the 3 cellular layers (ectoderm, endoderm and mesoderm) of the embryo. These cells continue to divide and gradually specialize structurally and functionally.
- A primary or basic tissue may be defined as a collection of cells and associated intercellular materials specialized for a particular function or functions.
- Organs are formed from these tissues, and usually, all four basic tissue types are present in a single organ.
There are 4 primary or basic tissues in the body. They are:
- Connective tissue
- Muscle (described in Chapter 3 The Muscular System)
- Nervous tissue (described in Chapter 4 The Nervous System).
Epithelium is one of the 4 primary tissues of our body.
Definition: It is a layer or layers of cells covering body surfaces and all the body cavities opening onto it.
- Epi = above; Thelos = nipple
- Epithelium = covering of nipple
- This term originally referred to the cellular covering of the nipple.
The cells of the epithelium are derived from the 3 germ layers of the embryo—the ectoderm, endoderm and mesoderm.
- Ectodermal cells give rise to epidermis, glandular tissue of breast, cornea, junctional zones of buccal cavity and anal canal.
- Endodermal cells give rise to the inner lining of alimentary canal and its glands, most of the respiratory tract and distal urogenital tract.
- Mesodermal cells gives rise to the lining of internal cavities (pleura, peritoneum and pericardium) and a part of the urogenital tract.
It is the lining of blood vessels and lymphatics.
Functions of Epithelium
- Form selective barriers
- Protection from dehydration, chemical and mechanical damage
- Sensory function, e.g. smell, touch, taste, etc.
Classification of Epithelium
For classification of epithelium (Table 1.7).
Unilaminar or Simple
Simple Squamous Epithelium (Pavement Epithelium)
It is composed of a single layer of flattened interlocking, polygonal cells or squames (resembling the scales of fish). Nucleus usually bulges into the overlying space (Fig. 1.18).
- Because it is so thin, the squamous epithelium allows rapid diffusion of gases and water.
- Active transport of molecules.
Cuboidal and Columnar Epithelia
They are found at sites where there is a high metabolic activity. These types consist of single, regular rows of cuboidal or columnar epithelial cells.
The cells are square in vertical section. Commonly, microvilli are present on their free surfaces, providing a large absorptive area.
Sites of occurrence: Proximal and distal convoluted tubules, thyroid follicles.
The cells are taller than their diameter. Commonly microvilli are present. The cells may be ciliated or non-ciliated.
Sites of occurrence:
- Most of the respiratory tract. Here the cells are ciliated.
- Uterine tubes—ciliated columnar cells
- Gastrointestinal tract.
Some columnar cells are glandular, their apices are filled with mucus, giving a characteristic appearance. They are called goblet cells (shaped like a wine-glass, e.g. large intestine (Fig. 1.18D).
Pseudostratified epithelium (Figs 1.19A and B) is a simple columnar epithelium in which nuclei lie at different levels, giving a false impression that it is a stratified epithelium. Not all cells extend through the whole thickness of the epithelium. Some cells constitute a basal layer. They are able to replace damaged mature cells.
- Much of the ciliated lining of respiratory tract is of pseudostratified type.
Figs 1.19A and B: A. Pseudostratified ciliated columnar epithelium (trachea)—schematic; B. Photomicrograph—pseudostratified ciliated columnar epithelium (trachea)
- Sensory epithelium of olfactory area.
- Parts of male urethra.
Sensory epithelia are seen in olfactory (smell), gustatory (taste) and vestibulocochlear (hearing and balance) receptor systems. They are highly specialized cells.
Myoepitheliocytes or Basket Cells
- Myoepitheliocytes or basket cells are specialized simple epithelial cells.
- They are star-shaped, containing actin and myosin filaments.
- They surround glands and ducts of the glands, squeezing out their contents.
- They are seen in mammary, salivary and sweat glands.
Compound or Multilaminar Epithelia
They are found at surfaces subject to mechanical wear and tear or other potentially harmful conditions.
Stratified Squamous Epithelium (Fig. 1.20)
These are multilayered epithelia in which there is a constant formation, maturation and loss of cells. This type of epithelium is found in sites which are subject to wear and tear.
The cells are formed at the innermost or basal layer; gradually move superficially and eventually shed from the surface. The basal cells are columnar; intermediate cells are cuboidal or polygonal; superficial cells are flattened. The 2 major types of stratified squamous epithelia are:
- Keratinizing or keratinized
- Non-keratinizing or non-keratinized.
Found at sites which are subject to drying, mechanical stresses and high levels of abrasions.
Sites: Skin, mucocutaneous junctions of lips, nostrils, distal anal canal, outer surface of tympanic membrane, parts of gums, hard palate, etc.
- The superficial cells of the epithelium are flattened; they synthesize a protein, keratohyaline.
- Most superficial cells lose their nuclei; the cells are dead and flattened (squames).
- The keratin filaments form a coat on the cells surface.
- This arrangement makes this type of epithelium an excellent barrier against different types of injury and water loss.
It is present at surfaces subject to stress, but protected from drying; includes the buccal cavity, oropharynx, esophagus, part of the anal canal, vagina, distal part of uterine cervix, cornea, conjunctiva and inner surfaces of eyelids.
- There are 6–10 layers of cells.
- Basal cells are columnar.
- Surface cells are flattened, resembling simple squamous cells.
- In between, the cells are polygonal.
- Except in cornea, the underlying connective tissue is raised into ridges and folds that appear like finger-like processes.
Stratified cuboidal epithelium is seen in the ducts of sweat glands.
Stratified columnar epithelium is rare. May be seen in some parts of male urethra.
Transitional Epithelium or Urothelium (Figs 1.21A and B)
Transitional epithelium or urothelium is so called, because originally it was believed to represent a transition between the stratified squamous and stratified columnar types. This type of epithelium exclusively lines the urinary tract (from renal pelvis, ureter, bladder and part of the urethra). It is exposed to internal pressure and capacity. Its appearance varies with the degree of distension.
- There are 4–6 layers of cells.
- Basal layer of cells is cuboidal or columnar.
- Intermediate layers are polyhedral or piriform.
- Superficial cells are cuboidal, when relaxed and squamous, when distended.
- Cells of the superficial layers are often binucleate. The superficial cells are sometimes known as ‘umbrella cells’.
- Cell membrane of superficial cells are protected by a glycoprotein—lipid complex. The adjacent cells are held together by tight junctions. So, the urothelium forms an effective barrier, preventing urine and toxic substances present in it from passing into the epithelium or the underlying tissues.
In addition to protection and absorption, many cells of the epithelium secrete materials. Such cells, present singly or in groups are called glands (Flowchart 1.2).
- Endocrine glands discharge their secretions directly into the bloodstream, i.e. they are ductless glands. There are some unicellular endocrine cells, e.g. The enteroendocrine cells in the mucosa of GIT.
- Exocrine glands discharge their secretions via a duct. They may be mucous or serous glands.
- Unicellular glands lie among other cells of columnar or pseudostratified epithelium. For example—goblet cells, which secrete mucus, are situated between non-secretory epithelial cells.
- Simple glands—when all the secretory cells of an exocrine gland discharge into one duct, the gland is called a simple gland.
- Compound glands have a branching duct system. A group of secretory cells open into a small duct. These ducts unite to form larger ducts, which ultimately open on an epithelial surface.
Figs 1.22A to J: Scheme to show various ways in which the secretory elements of a gland may be organized. A and B are examples of unicellular glands. All others are multicellular. Glands with a single duct are simple glands, while those with a branching duct system are compound glands
Based on the arrangement of secretory cells, the glands can be classified into different types:
- Secretory unit may be tubular. The tube may be straight, coiled or branched.
- The cells may form rounded sacs or acini.
- They may form flask-shaped structures called alveoli.
- Glands which have greatly distended secretory elements are called saccular glands.
Combinations of the above four types may be present in a single exocrine gland. Thus, the glands may be (Fig. 1.22):
- Simple tubular
- Simple alveolar/simple acinar
- Compound alveolar
- Compound tubuloalveolar or racemose.
Two types of arrangement of cells are seen. In some glands, the cells are arranged in cords or clumps. There are large numbers of capillaries or sinusoids which come in contact with these cells, so that, their secretions are discharged directly into the bloodstream.
In some endocrine glands, as in thyroid gland, the cells form a rounded vesicle or follicle having a cavity. The secretions are stored in this cavity; when required, they are released into the bloodstream.
There are some glands having both exocrine and endocrine functions, e.g. pancreas.
Classification of glands based on the manner in which the secretions are poured out of the cells.
- Apocrine: Apical parts of the cells are shed off to discharge the secretions.
- Holocrine: The entire cell disintegrates to release the secretions.
(Merely releases - merocrine; Apex gone - apocrine; whole cell disintegrates - holocrine).
THE CONNECTIVE TISSUE
The connective tissue (CT), one of the four primary tissues of the body, connects various structures of our body with each other. It is also called the supporting tissue or communicative tissue. It is derived from the mesodermal layer of the embryo.
Classification of connective tissue is shown in Flowchart 1.3.
Components of Connective Tissue
Any CT is made up of 3 basic elements. They are:
- An amorphous ground substance or intercellular substance. It can be compared to cement.
- Fibers (can be compared to metal or iron rods).
- Cells (can be compared to bricks or granite).
These 3 elements are bathed in tissue fluid or extracellular fluids.
- Intercellular substances are non-living, in which the cells live.
- They provide strength and support to the tissues.
- They act as a medium for diffusion of tissue fluid between blood capillaries and cells.
Composition: Composed of glycosaminoglycans and glycoproteins.
Types of Glycosaminoglycans
The different types of glycosaminoglycans are:
- Hyaluronic acid
- Chondroitin sulfate
- Dermatan sulfate
- Keratan sulfate
- Heparan sulfate.
- Largest glycosaminoglycan.
- Present in nearly all types of connective tissues.
- Important sites of occurrence: Wharton's jelly of umbilical cord, synovial fluid and vitreous body of eye.
Chondroitin sulphate is abundant in cartilage, bone and intervertebral disc.
Dermatan sulfate is abundant in skin, tendons and valves of heart.
Keratan sulfate: There are 2 types—I and II. Type I is exclusively present in cornea. Type II is a component of cartilage and intervertebral disc.
Heparan sulphate is found in relation to cell surfaces.
Glycoproteins are compound molecules of proteins and polysaccharides.
Fibers of Connective Tissue (Figs 1.23A to C)
Function: To provide tensile strength and support for the tissues. There are 3 types of fibers:
All are complex proteins, insoluble in water or neutral solvents.
Collagen Fibers (One which yields jelly or glue)
- Found in all connective tissues
- Made up of a polypeptide, collagen
- Extremely tough
- In the fresh state, white in color; so, they are also called ‘white fibers’.
- Fibers are transparent, wavy, soft and flexible.
Figs 1.23A to C: Connective tissue. A. White fibrous tissue-collagen fibers; B. Yellow elastic tissue-elastic fibers; C. Adipose tissue
Types of collagen fibers—mainly types I, II, III, IV and V. See Table 1.8 for the distribution of different types of collagen fibers.
Reticular fibers are very fine collagen fibers, arranged to form a net-like supporting framework or reticulum.
Sites of occurrence: Occur as fine network around muscle fibers, nerve fibers and fat cells, in the fine partitions of lung and lymphoid tissues.
Elastic fibers are long, thin, highly refractile cylindrical threads or flat ribbons. Tissues rich in elastic fibers appear yellow in the fresh state. So they are also called yellow elastic fibers. Composed of elastin.
Sites of occurrence: Walls of major blood vessels (e.g. aorta), elastic cartilage (pinna, epiglottis).
Cells of Connective Tissue (Fig. 1.24)
- Fibroblasts—most numerous cells of connective tissue
- Mast cells
- Undifferentiated mesenchymal cells
- Fat cells
- Blood leukocytes
- Pigment cells
Fibroblasts (Fig. 1.24)
They occur in large numbers in the connective tissue. They are responsible for the production of fibers and ground substances. They are large, flat, branching cells (when viewed from the surface) or spindle-shaped when viewed from a side (profile).
Macrophages (Fig. 1.24)
They are also called histiocytes, seen in highly vascular areas. Two types—fixed or wandering (free) macrophages. These are the 2 functional phases of the cells of same origin. When stimulated, the fixed cells become free cells. Example of fixed macrophages and Kupffer cells of liver. Wandering macrophages are found in blood. Macrophages are irregularly shaped, capable of free amoeboid movement and phagocytosis.
- Macrophages are important agents of defence. They act as scavengers. Several macrophages fuse together to form a multinucleated, foreign body giant cell to attack a large ‘foreign body’.
- Contribute to immunological reaction of the body.
Mast Cells (Fig. 1.24)
These cells occur in groups in relation to blood vessels. They are irregularly oval, with cytoplasmic granules. These granules secrete heparin, histamine and serotonin. These chemical mediators promote allergic reactions (immediate hypersensitivity reaction).
Undifferentiated Mesenchymal Cells
They are developed from mesoderm. They are pluripotent cells (capable of developing into different types of cells, when required). They are seen around blood vessels.
Fat Cells (Fig. 1.24)
They are large cells seen in groups. When seen in large groups, the tissue is called an adipose tissue. Each cell has a large glistening droplet of fat, surrounded by a thin rim of cytoplasm. Nucleus is pushed to one side and it is flattened.
Although leukocytes are transported by the bloodstream, they perform their functions outside the blood vessels. That is why they are encountered within connective tissue. The two most frequently seen cells are lymphocytes and eosinophils.
- Lymphocytes are the smallest of the free cells of connective tissue. They possess a spherical, dark staining nucleus that occupies most of the cell, surrounded by a thin rim of cytoplasm. Two types of lymphocytes are seen in connective tissue; one population with brief life span; the other living for months or years. Functionally there are 2 types:
- T-lymphocytes are long-lived, concerned with cell-mediated immunity.
- B-lymphocytes are short-lived cells, which when stimulated by an antigen, are capable of active division and differentiation into plasma cells that synthesize antibodies against the antigen.
- Eosinophils also migrate from the bloodstream into the connective tissue. They are abundantly seen in the connective tissue of lactating breast, respiratory and gastrointestinal tracts. Nucleus is bilobed or kidney-shaped. Cytoplasm contains spherical granules. Eosinophils accumulate in blood and tissues in allergic and inflammatory conditions resulting from parasitic diseases.
Pigment Cells (Fig. 1.24)
Pigment cells are seen in the skin, the choroid coat of the eye and pia mater. The main pigment found in these cells is melanin.
- Protection of deeper tissues from ultraviolet radiation (e.g. skin).
- Prevention of light from reaching deeper tissues, e.g. choroid of eye.
Types of Connective Tissue
Depending on the proportion of cells, ground substance and fibers, different types of connective tissues are formed.
- The loose connective tissue has few fibers and more cells.
- The dense connective tissue has abundant, densely packed fibers.
Adipose tissue is made up of fat cells, reticular fibers and large number of blood vessels.
- In the subcutaneous tissue all over the body, except eyelids, scrotum and penis
- Hollow spaces like orbit, axilla (armpit)
- Bone marrow cavities of long bones
- Around abdominal organs, e.g. perinephric fat
- Peritoneal folds, mesentery.
- Store house of energy
- Mechanical support for organs. It also acts as a shock absorber.
Reticular tissue forms a framework for lymphoid organs, bone marrow and liver.
- Dense irregular connective tissue is found in fasciae, periosteum, dermis of skin and capsule of organs.
- Dense regular connective tissue is found in tendons and ligaments. Fibers are dense and parallel.
Functions of Connective Tissue
- The loose connective tissue holds together structures like skin, muscles and vessels. It binds together different layers of hollow organs. It forms the capsule for some organs.
- In the limbs the connective tissue of deep fascia provides a tight covering for deeper structures and helps to maintain the shape of the limbs.
- The ligaments hold the bone ends together at joints and prevent excess movements in unwanted direction.
- The tough dura mater provides support to the brain and spinal cord.
- Macrophages and plasma cells help the body fight against microorganisms.
- Fibroblasts help in repair of wounds by laying down collagen fibers and ground substance.
- Undifferentiated mesenchymal cells helps in the regeneration of tissues like cartilage and bone by providing cells.
- Deep fascia facilitates venous return from limbs by enabling muscles to act as pumps.
Multiple Choice Questions
- Simple squamous epithelium is seen lining the:
- Bowman's capsule
- Buccal mucosa
- Ciliated columnar epithelium forms the lining of:
- Fallopian tube
- Parotid duct
- The urinary bladder is lined by _____________ epithelium:
- Stratified squamous non-keratinized
- Stratified squamous keratinized
- Stratified cuboidal
- Pseudostratified ciliated columnar epithelium lines the:
- Alveoli of lung
- Bowman's capsule
- Follicle of thyroid
- Esophagus is lined by _____________ epithelium:
- Simple squamous
- Stratified columnar
- Stratified squamous non-keratinized
- Myoepitheliocytes are associated with:
- Blood vessels
- Most numerous cells in connective tissue are:
- Fat cells
- Mast cells
- Pigment cells
- Kupffer cells are seen in:
- Type II collagen fibers are abundant in:
- Elastic fibers are abundant in:
- Wall of aorta
- Hyaline cartilage
- Intervertebral disc
- Lymphoid tissue
- Name the basic tissues. Classify epithelium and give one site of occurrence of each.
- Describe the components of connective tissue.
- Classify connective tissue. With the help of diagrams describe the cells of connective tissue.
- Plasma membrane
- Endoplasmic reticulum
- Simple epithelia
- Stratified epithelia
- Glandular epithelium
- Fibers of connective tissue
Define the Following
- Anatomical position
- Sagittal plane
- Median plane
- Coronal plane
- Pseudostratified epithelium
- Endocrine glands
Fill in the Blanks
- The epithelial lining of trachea is ___________________
- Transitional epithelium is also known as ___________________
- Umbrella cells are seen in ___________ epithelium
- The alveoli of lung are lined by ___________ epithelium
- Fallopian tube is lined by ___________ epithelium
- ___________ cells are the contractile cells found in glands which squeeze out their secretions
- ___________ cells of connective tissue secrete histamine in allergic reactions
I. Match the Following
a. Unicellular glands
1. Umbrella cells
b. Simple squamous epithelium
c. Stratified squamous non-keratinized epithelium
3. Goblet cells
d. Ciliated columnar epithetium
4. Alveoli of lung
e. Transitional epithelium
5. Fallopian tube
6. Thick skin
II. Match the Following
a. Kupffer cells
b. Mast cells
2. Signet ring appearance
d. Elastic fibers
e. Collagen fibers
f. Pigment cells
III. Match the Following
a. Mucoid connective tissue
1. Kidney-shaped nucleus
2. Hyaline cartilage
c. Stratified squamous non-keratinized epithelium
3. Wharton's jelly
d. Collagen fibers
4. Buccal mucosa
Draw Neat Labelled Diagrams
- Cell and organelle
- Transitional epithelium
- Cells of connective tissue
- Stratified squamous non-keratinized epithelium