Animal Tissues and their Types: Connective Tissues

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In this post, I'm presenting you concise description Connective Tissues.

Bone connective tissue
Darshani Kansara, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
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Some tissues bind different types of cells or tissues together in an organ, called connective tissues. They are major supporting tissues of the body. They develop from the mesoderm. These tissues constitute about 30% of the body.

Structure of connective tissue

Basically, three components – cells, fibres, and matrix – constitute a connective tissue.

1. Cell

Cells are living components of connective tissues. These are widely distributed in the matrix. The different types of cells are fibroblasts, macrophages or histocytes, mast cells, and plasma cells.

Fibroblasts: They are spindle shaped cells with long protoplasmic processes. They have oval nucleus. They form fibres.

Macrophages or histocytes: These are large irregular-shaped or amoeboid polymorphic cells with a kidney-shaped nucleus. They are actively phagocytic cells.

Mast cells: These are large oval-shaped cells that produce heparin and histamine. Heparin prevents coagulation of blood and histamine causes inflammation.

Plasma cells: They are small, round or irregular cells produced by the division of migratory lymphocytes. They produce antibodies that help in defence.

2. Fibres

Fibres are the non-living products of the cells. They are widely distributed in the matrix. They are collagen or white fibres, elastic or yellow fibres, and reticulate fibres.

Collagen or white fibres: These are the long unbranched and white fibres containing collagen, a protein. They are present in groups or bundles and are flexible but inelastic.

Elastic or Yellow fibres: These are the long, branched and yellow fibres containing a protein elastin. They are present singly and are flexible and elastic.

Reticulate fibres: These are very short, fine, thread-like fibres containing a reticulin protein. They are interconnected to form a network or reticular fibres.

3. Matrix

Matrix is a non-living, transparent fluid or semisolid intercellular fluid containing various organic and inorganic substances.

Figure 14 Different types of cells and fibres in connective tissue

Types of Connective tissues

Connective tissues are categorized as connective tissue proper, supportive connective tissues or skeletal tissues, and fluid connective tissue.

I.        Connective tissue proper

Connective tissue proper consists of a jelly-like substance containing white collagen fibres or yellow elastic fibres or both, together with the cells. These fibres are formed by fibroblasts. Based on the softness, rigidity, and degree of toughness, the connective tissue proper is divided as lose connective tissue and dense connective tissue.

1.      Loose connective tissue

Areolar tissue and adipose tissue are grouped under loose connective tissue.

a. Areolar tissue

Areolar tissue is the simplest and most widely distributed connective tissue in the body. It consists of a jelly-like soft matrix having fibres. Fibres and other cells in it are white collagen fibres (made of collagen protein), yellow elastic fibre (made of elastin protein), fibrocytes (large flat cells that make white and yellow fibres), macrophages (the irregular-shaped cells with granules and are actively phagocytic to engulf bacteria and other foreign bodies), and mast cells (produce heparin and histamine; heparin is for the coagulation of plasma and histamine is for the inflammation to cell at the time of infection).

Areolar tissue acts as supporting and packing tissues. It also helps in combating foreign toxins released by pathogenic organisms.

b. Adipose tissue

Adipose tissue is the modified areolar tissue consists of a large number of fat cells or adipocytes. It is also called fat tissue. Adipocytes are rounded or polygonal with a thin layer of cytoplasm. The nucleus lies at one side due to fat. Fibres are absent or reduced. It is present below the skin (subcutaneous fat), between internal organs (visceral fat) and in the inner cavities of bones (bone marrow adipose tissue).

 Adipose tissue serves as an insulating layer against heat loss. It is a fat reservoir. It acts as cushion in the orbits or eye sockets so that it prevents the eye from injury.

2.      Dense connective tissue proper

White fibrous tissue (tendon) and yellow elastic tissue(ligament) come under dense connective tissue proper.

a. White fibrous tissue (tendon)

White fibrous tissue is a modified areolar tissue consists of thickly packed white collagen fibres. The fibres are tough and non-elastic. The fibres run parallel to one another and form tendons. Tendons connect muscles with the bones. Its matrix contains fibroblasts which are arranged between the rows of the fibres.

White fibrous tissue provides mechanical protection against stretch.

b. Yellow elastic tissue (ligament)

Yellow elastic tissue consists mainly of thick and branched yellow elastic fibres. Sometimes, white fibres are present. The matrix contains scattered fibrocytes. Yellow elastic tissues may form the ligament which binds one bone with another bone.

Yellow elastic tissue provides stretch and elasticity.

Figure 15 Different tissues of connective tissue proper

II. Supportive connective tissue or skeletal tissues

Supportive connective tissue forms the endoskeleton of the vertebrates. It supports the body and protects the delicate and vital organs from different injuries. These tissues are cartilage and bone.

1. Cartilage

Cartilage is the soft skeletal tissue in the vertebrates. A typical cartilage consists of semi-solid matrix containing chondrin. This matrix is secreted by oval-shaped small group of cartilage cells called chondroblasts or chondrocytes. Chondrocytes are scattered in the matrix singly or in groups. They are enclosed in the fluid filled spaces called lacunae. Cartilage is covered by perichondrium, a thin and stiff membrane. The perichondrium produces and adds new chondrocytes to the matrix.

Based on the nature of matrix, cartilages are hyaline cartilage, elastic cartilage, fibro-cartilage, and calcified cartilage.

Hyaline cartilage: It is flexible. Its matrix is transparent, homogenous and tough. It covers joint surfaces, endo of ribs, nose, and tracheal rings. It forms skeletal cartilage in the embryos of all vertebrates.

Elastic cartilage: It is more flexible. Its matrix consists of a network of numerous yellow elastic fibres. It is found in the external ear (pinna), external auditory canal of ear, epiglottis, eustachian tubes and tip of the nose.

Fibro-cartilage: Abundant white collagen fibres make its matrix. It is the most resistant cartilage having less cells and matrix. It is found in the intervertebral discs (pads) between the mammalian vertebrae and in the pubic symphysis around the joints. In such places, it acts as cushion.

Calcified cartilage: When the hyaline cartilage is impregnated with calcium in the matrix, calcified cartilage is formed. It is a very hard, stiff and non-elastic cartilage. It is found in the suprascapular of pectoral girdle, in the pelvis of old frogs, in the heads of humerus and femur and in the vertebrae of sharks.

Figure 16 Different types of cartilages

2. Bone

Bone is specialized connective tissue. It is very hard due to the deposition of lime (CaCO₃) salts. It forms the two third of the skeleton of the body. It provides a framework of the body. Its matrix has bone cells, called osteocytes. Its matrix is formed by a protein called ossein. The matrix contains 70% inorganic bone salts and 30% fibres and glycoprotein.

Structure of Bone

A typical bone has periosteum, outer layer of osteoblast, endosteum, matrix, and Haversian system.

Periosteum: It is the outermost covering of bone. It consists of undifferentiated bone cells.

Outer layer of osteoblast: It lies just below the periosteum. These are flat connective cells that give rise to the bone cells.

Endosteum: It is a thin layer that covers the narrow cavity of the bone.

Matrix: It covers the large part of the bone. It consists of various layers, called lamellae. Each lamella has numerous small spaces, called lacunae. Each lacunae encloses only one osteocyte. Each lacuna gives off branching channels radiating in all directions, called canaliculi. These canaliculi anastomose with other canaliculi forming an interconnected system in the matrix, called Haversian system.

Haversian system: It consists of lamellae arranged around the central canal, called Haversian canal. Each canal with lamellae and canaliculi together forms a haversian system. Many haversian systems are connected together by the canals of Volkmann. It is found in the long bones.

Types of Bones

On the basis of the type of matrix present, bones are classified as spongy bone and compact bone.

Spongy bone: Spongy bone is present on the expanded ends of the long bones. It is filled with red bone marrow, a soft tissue. It lacks haversian system.

Compact bone: It is present on the shaft of the long bones. It is filled with a fatty tissue called yellow bone marrow. It has many haversian systems.

Bone marrow

The bone contains two types of soft and spongy bone marrow – yellow bone marrow and red bone marrow.

Yellow bone marrow possesses high fat content and is mostly found in the long bones.

Red bone marrow is found in the short bones and on the surfaces of the long bones. It produces RBC and WBC.

Ossification

Ossification is the process of bone formation. It is of two types – endochondral and intramembranous ossification.

Endochondral ossification is the transformation of a cartilage into a bone. The bone formed by such process is called cartilage bone or replacing bone.

Intramembranous ossification is the formation of bone directly from the connective tissue. The bone is called dermal investing or membrane bone.

Figure 17 Different structures in bone

III. Fluid Connective Tissue (Haemopoitic Tissue)

Fluid connective tissue consists of both myeloid and lymphoid tissues. Myeloid tissues form blood while lymphoid tissues form lymph. They do not have fibres. These tissues circulate in the body.

1. Blood

Blood consists of plasma and corpuscles. A normal human adult contains 5 to 8 litres of blood in the body. So, it constitutes 5-8 % of the body weight. It is composed of blood plasma and blood corpuscles.

a. Blood plasma

Blood plasma is a pale-yellow fluid. It forms 55% of blood by volume. It is slightly alkaline. It contains various organic and inorganic materials. Its components are water (90 to 92 %), dissolved solids (8 to 10%), proteins (7%; serum albumin, serum globulin, and fibrinogen), organic constituents (1-2%; nonprotein nitrogenous substances such as urea, ammonia, amino acids, neutral fats, glucoses), respiratory gases (O2 and CO2), and internal secretions (antibodies and various enzymes).

Functions of blood plasma

1. Blood transports oxygen from the lungs to all the parts of the body and brings carbon dioxide from the tissues to the lungs.
2. It transports soluble organic substances from the small intestine to the different parts of the body.
3. It transports soluble excretory products from the tissues, in which they are produced, to the organs of excretion.
4. It transports metabolic by-products from the areas of production to other parts of the body.
5. It transports hormones from the glands to the target organs.
6. It regulates the body temperatures and maintains the constant blood osmotic pressure and pH.
7. It provides defense to the body against diseases by the phagocytosis, by the immune mediated by antibodies or lymphocytes, and by the clotting of the blood.

b. Blood corpuscles (cells)

The three types of blood cells are erythrocytes or red blood corpuscles (RBCs), leucocytes or white blood corpuscles (WBCs), and thrombocytes or blood platelets.

i. Erythrocytes

Erythrocytes are biconcave discs and non-nucleated but in fishes, frogs, reptiles, and birds, they are oval and nucleated. They are 7.5 μm thick and are about 5,400,000 per mm3 of blood in men and about 4,800,000 in women.  Their average lifespan in human is around 110-120 days. They contain red pigment the haemoglobin.  They are formed in the red bone marrow by the process of haemopoiesis. In foetus, they are formed in the liver and in the spleen. They transport O2 and assist CO2 transport.

ii. Leucocytes

Leucocytes are nucleated and non-pigmented cells. They have diameter of 8-20 μm and are 7,000-10,000 per mm3 of blood. They show amoeboid movement. They are produced in the bone marrow and in the lymph glands. They normally live for 1-4 days inside blood. The excessive blood formation is called leukemia, type of blood cancer. They fight against infection by the process of phagocytosis.

Based on the size, granules, staining reaction, number and shape of nuclei, WBCs are classified as granulocytes and agranulocytes.

Granulocytes

Granulocytes are with lobated nuclei and fine granules. They are formed from the red bone marrow. They form 72% of total WBC. They are of three types – eosinophils (acidophils), basophils, and neutrophils.

Eosinophils (acidophils): These are the spherical cells having bilobed nucleus. They are stained in acidic dyes. In human, they are with 9-12 μm diameter. They make 2-3% of total leucocytes. They mainly destroy and detoxify the toxins. The excessive formation of eosinophils causes eosinophilia.

Basophils: These are spherical cells with S-shaped nucleus. They are stained in basic dyes. In human, they are with 9-12 μm diameter. They constitute 0.5-4% of WBCs. They also release heparin and histamine.

Neutrophils: These are spherical cells with multi-lobed nucleus. They are stained in neutral dyes. In human, they are with 9-12 μm diameter. They constitute 60-70% of total WBCs. The engulf bacteria or foreign bodies.

 Agranulocytes

Agranulocytes have oval nuclei without granules. They are formed in the red bone marrow and lymph nodes. They form 28% of total WBCs. They are of two types – lymphocytes and monocytes.

Lymphocytes: They are small cells having large nuclei. In human, they are with 8-10 μm diameter. They form 24% of total WBC. They are non-motile and non-phagocytic in nature. They produce antibodies to destroy microorganisms. They are of two types – B-lymphocytes and T-lymphocytes.

Monocytes: They are large cells having nearly bean-shaped nuclei. In human, they are with 12-20 μm diameter. They form about 4% of total leucocytes. They are motile and actively phagocytic in nature. They mainly engulf the microorganisms.

iii. Thrombocytes

Thrombocytes are irregular and non-nucleated cells. They measure 2-4 μm of diameter. In human blood, they are 250,000 per mm³ of blood. Their lifespan is about 7 days. They help in blood clotting as they contain thromboplastin, which speeds up the conversion of prothrombin to thrombin.

Function of blood

• Blood transports oxygen from the lungs to all the parts of the body and brings carbon dioxide from the tissues to the lungs.
• It transports soluble organic substances from the small intestine to the different parts of the body.
• It transports soluble excretory products from the tissues, in which they are produced, to the organs of excretion.
• It transports metabolic by-products from the areas of production to other parts of the body.
• It transports hormones from the glands to the target organs.
• It regulates the body temperatures and maintains the constant blood osmotic pressure and pH.
• It provides defense to the body against diseases by the phagocytosis, by the immune mediated by antibodies or lymphocytes, and by the clotting of the blood.

2. Lymph

Lymph is the blood without RBCs, WBCs, and platelets. It is transparent, slightly yellowish and alkaline fluid. It contains same concentration of glucose, salts, amino acids, and vitamins as in plasma; less protein than in plasma. It circulates throughout the body through lymph vessels and lymph hearts.

Functions of lymph

• Lymph is like a middle man and transports respiratory gases, food materials, hormones, etc.
• It produces lymphocytes and brings antibodies from lymph nodes to the blood.
• It destroys pathogens and foreign particles.
• Lacteals, the lymph capillaries in the intestinal villi, are associated with absorption and fat-soluble vitamins.
• It maintains the volume of the blood in the body. When the blood volume is reduced in the blood vascular system, the lymph rushes from lymphatic system to the blood vascular system to maintain the blood volume uniform.

Figure 18 Components of blood 

Animal Tissues and their Types: Epithelial Tissues

Welcome to MBC Biology! In this post, I'm presenting you concise description of Epithelium.

Simple Columnar Epithelium
Source: Berkshire Community College  Bioscience Image Library, CC0, via Wikimedia Commons
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The tissue is a group of cells that have similar structures, origin, and functions. The study of tissues is called histology. The study of the development and differentiation of tissues is known as histogenesis.

Various tissues combine to form an organ and different organs functioning together constitute an organ system. Various systems constitute a body. Those tissues are of four fundamental types (epithelial, connective, muscular, and nervous tissue) based on their structure and function.

A.    Epithelial tissues

The tissue which forms the outer and the inner coverings of various body parts (skin), external parts, internal organs and body spaces inside are called epithelial tissues or epithelium. The cells are closely packed; there is no intercellular space or matrix exists between the cells. The cells are held together by a cementing substance. Cells lie on a basement membrane, which is not supplied with blood vessels. Epithelium contains nerve endings too.

Based on the shape of the cells and the layers, epithelium is simple, stratified or compound, and modified epithelium.

1.      Simple epithelial tissues

Simple epithelial tissue is composed of single layer cells resting on the basement membrane. They are simple squamous, simple cuboidal, simple columnar, and pseudo-stratified epithelium.

a.       Simple squamous epithelium

Simple squamous epithelium is made of thin, flat, and hexagonal cells with a large central rounded nucleus. The cells are closely packed like the tiles on the mosaic floor. So, this tissue is also called pavement epithelium.

Simple squamous epithelium is found in a covering around coelom, lining of the buccal cavity, blood vessels, kidney, alveoli of lungs etc.

It helps in protection, absorption, filtration, and exchange of gases.

b.      Simple cuboidal epithelium

Simple cuboidal epithelium is composed of cuboidal cells having a centrally located small rounded nucleus in each. Cells lie on a basement membrane.

It is found in a lining on kidney tubules, sweat glands, salivary glands, gut, testis, and ovary.

It is involved in secretion, absorption, and excretion.

It is of two types – ciliated cuboidal and brush bordered cuboidal epithelium.

Ciliated cuboidal: Ciliated simple cuboidal epithelium contains cells with cilia on the free surface. Each cell is associated with secretory goblet cells. They are located on the ducts of the nephron. Cilia help in conducting the mucus and other substances.

Brush bordered cuboidal: Cells in this epithelium have microvilli at the free ends. They are located at the proximal convoluted tubules of the nephron.

c.       Simple columnar epithelium

Simple columnar epithelium consists of tall and narrow cells that are placed side by side forming a layer like a column. Each cell has a large nucleus situated at the basal end. These cells lie on a basement membrane.

It is found in the lining of the goblet cells of the stomach, gastric glands, intestinal glands, gall bladder, ureter, and uterine wall.

It is of two types – simple ciliated columnar and brush bordered columnar epithelium.

Ciliated columnar: Cells in ciliated columnar are with numerous cilia on the free surface. They are associated with secretory goblet cells.  They are located in the oviducts, respiratory passages (bronchioles), etc.

Brush bordered columnar: This simple columnar epithelium has cells with microvilli at the free ends of the cells. They are found in the intestinal mucosa.

d.      Pseudo-stratified epithelium

Pseudo-stratified epithelium consists of the columnar cells. As the cells do not reach free surface and their nuclei appear to be at different levels, they provide a false multilayered tissue. These cells rest up on the basement membrane.

It is found in the lining of trachea, large bronchi, and urinary bladder.

It protects the dust particles entering our respiratory tract.

Figure 1-4 Simple epithelium: 1A. & 1B. simple squamous epithelium; 2A. simple cuboidal cells; 2B. ciliated cuboidal cells; 2C. brush-bordered cuboidal cells; 3A. columnar epithelium; 3B. ciliated columnar cells; 3C. brush-bordered columnar cells; 4. pseudostratified epithelium  

2.      Stratified or compound epithelial tissue

Stratified or compound epithelial tissue is made of several layers of the epithelial cells. It is multi-layered as it contains an upper layer and a lower layer of cells. Cells in a lower layer multiply and give rise to the cells of the upper layers. The cells of a lower layer are called germinative cells.

Stratified or compound epithelial tissue are stratified squamous epithelium, stratified cuboidal epithelium, stratified columnar epithelium, and transitional epithelium.

a.       Stratified squamous epithelium

The tissue in which the upper layer of cells consists of large, flat and polygonal or squamous cells, but the cells of germinative layer are either cuboidal or columnar is stratified squamous epithelium. The first formed cells are cuboidal shaped; they are pushed towards the upper surface outwards and become flattened squamous. Stratified squamous epithelium is of two types – keratinized stratified and non-keratinized squamous epithelium.

keratinized stratified epithelium: The uppermost layer consists of dead cells and is hardened due to the deposition of keratin, a protein. The deposition of keratin makes the cell layer water proof. It is located on hair, claws, and nails.

Non-keratinized squamous epithelium: The uppermost layer consists of living cells without keratin. The layer is wet due to the absence of keratin. It is found on wet surfaces like buccal cavity, pharynx, oesophagus, and vagina.

b.      Stratified cuboidal epithelium

It is a stratified epithelium in which the outermost layer consists of cuboidal cells. However, the lower layer has germinative cells either columnar or squamous.

It is found on the lining of the ducts of sweat glands, salivary glands, pancreatic gland, and female urethra, etc.

c.       Stratified columnar epithelium

The outermost layer of this tissue consists of tall columnar cells. However, the germinative cells are cuboidal shaped.

It is found on the lining of the ducts of mammary glands, lining of vasa-differentia, trachea, and bronchi.

d.      Transitional epithelium

Transitional epithelium is composed of three or four layers of cells. Cells in the uppermost layer are dome-shaped; the middle layer cells are club-shaped; and the basal layer cells are cuboidal or rounded cells. It has the capacity to stretch and relax.

It is found in the lining of the urinary bladder, ureters, and uterus, etc.

Figure 5-8 Stratified or compound epithelium: 5A. Stratified squamous epithelium; 5B.Nonkeratinized cells; 5C.Keratinized cells; 6. Stratified cuboidal epithelium; 7. Stratified columnar epithelium; 8. Transitional epithelium.   

3.      Modified epithelium ####

Some epithelial tissues are modified for the specialized functions. These are ciliated epithelium, sensory epithelium, Germinal epithelium, and glandular epithelium.

a.       Ciliated epithelium

Ciliated epithelium contains modified columnar or cuboidal cells. Those cells have cilia at their free surfaces. It forms the lining of the neck of uriniferous tubules, sperm ducts, trachea, and bronchi, etc.

b.      Sensory epithelium

Some of the columnar cells are modified with sensory fibres at their free surfaces. They form the lining of the tongue and the nasal cavity.

c.       Germinal epithelium

The modified cuboidal cells found in the lining of testes and ovary make germinal epithelium. They can divide and develop as gametes (spermatozoa and ova) by meiosis. The germinal epithelium forms the lining of the gonads (seminiferous tubule of testis and lining of ovary).

d.      Glandular epithelium

The modified columnar or cuboidal cells specialized for manufacture and secretion of certain chemical substances make glandular epithelium. The glandular epithelia form glands.

Figure 9. Modified epithelium: 9 A. Ciliated epithelium; 9 B. Sensory epithelium

Glands

The glands are grouped on the basis of number of cells present; the kind of secretion and the duct present; the shape and complexity; the mode of secretion, and the nature of secretion.

Glands based on the number of cells present

There are unicellular and multicellular glands based on the number of cells present in them.

1. Unicellular gland: A single cell scattered in the columnar cells is unicellular gland. Examples are goblet cells or mucus secreting cells.
2. Multicellular gland: It is made of many cuboidal cells that form many tubular invaginations. Examples are sweat glands and gastric glands.

Glands based on the kind of secretion and the duct present

These are exocrine and endocrine glands.

1. Exocrine glands: The glands which pour their secretions through the ducts are called exocrine glands. They secrete enzymes. Glands can be unicellular or multicellular (simple or compound). Examples are salivary, tear, gastric, and intestinal glands.
2. Endocrine glands: The glands that do not possess ducts but pour their secretions directly into the blood vessels are called endocrine glands. They are also called ductless glands. They secrete hormones. Examples are pituitary, thyroid, and adrenal glands, etc.

Glands based on the shape and complexity

Glands based on the shape and complexity or exocrine glands are of two types – simple glands and compound glands.

Simple glands

These glands have a single unbranched duct. The secretory part can be of tubular form (called tubules) or sacs (called alveolar). These can be coiled or uncoiled; branched or unbranched.

1. Simple tubular glands are found in intestinal crypts in the intestine.
2. Simple coiled tubular glands are found in simple sweat glands in the skin of mammals.
3. Simple branched tubular glands are found in the linings of gastric glands and Brunner’s glands of intestine.
4. Simple alveolar glands are found in the mucous secreting glands in the skin of frog.
5. Simple branched alveolar glands are found in the sebaceous or oil glands in the skin of mammals.

Compound glands

These glands have a number of ducts forming a branching pattern. The secretory part can be in the form of tubes (tubules), sacs (alveoli), or both.

1. Compound tubular glands are in the salivary glands.
2. Compound alveolar glands are found in the mammary glands, pancreatic glands etc.
3. Compound tubular-alveolar glands are found in the parts of salivary and mammary glands.

Glands based on the mode of secretion

Depending on the mode of secretion, the exocrine glands are of three types – merocrine glands, apocrine glands, and holocrine glands.

1. Merocrine glands: The secretions are discharged on the cell surface by diffusion or exocytosis without causing any damage or loss in the secretory cells. The cells remain intact e.g. goblet cells, salivary glands, intestinal glands, and sweat glands.
2. Apocrine glands: The secretions are discharged on cell surface causing loss or damage to some parts of secretory cells e.g. mammary glands, eyelid, and ear, etc.
3. Holocrine glands: The secretions are discharged on the cell surface by the rupture of the plasma membrane of secretory cells completely e.g. sebaceous glands in the skin of mammals.

Glands based on the nature of secretion

These are mucous glands, serous glands, and mixed glands.

1. Mucous glands: These glands secrete the mucus. The cells are called mucous cells or mucocytes. The mucus is a proteinous viscous and slimy substance. The goblet cells in the intestine are examples of mucous glands.
2. Serous glands: These glands secrete a clear watery fluid. These cells are called serocytes. Serous cells are found in the parotid salivary glands, intestinal glands, and sweat glands.
3. Mixed glands: Some glands are made of both the mucocytes and serocytes. These glands produce both kinds of secretions. Examples are gastric secretion and pancreatic secretion.

Figure 10 A. unicellular exocrine glands; 10 B. multicellular exocrine glands

Figure 11 A. Exocrine gland; 11 B. Endocrine gland.

Figure 12 Simple and compound glands: 12 A. simple tubular; 12 B. simple coiled tubular; 12 C. simple branched tubular; 12 D. simple alveolar; 12 E. simple branched alveolar glands. 12 F. compound tubular; 12 G. compound alveolar; 12 H. compound tubular-alveolar glands.

Figure 13 Glands based on the mode of secretion

Secondary Growth in Dicot Stem

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Here, I am posting a concise description of the secondary growth in dicot stem.

Schematic diagram of primary and secondary growth in dicot stem 
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The process of formation of extra layers or secondary tissues due to activity of vascular cambium and cork cambium for increasing girth or thickness or diameter of plant is called secondary growth. It takes place in stellar regions due to the activity of vascular cambium (see Figure 17) and in extra-stellar regions due to the activity of cork cambium (see Figure 21).

Secondary growth in stellar regions due to the activity of vascular cambium

Secondary growth due to activity of vascular cambium takes place by the formation of cambium ring, formation of secondary tissues, formation of annual rings or growth rings, and formation of heartwood and sapwood.

Formation of cambium ring

The dicot stem has a strip of cambium in between xylem and phloem in vascular bundles. This cambium is known as intrafascicular cambium. During secondary growth, some cells of medullary rays that are present at the level of intrafascicular cambium show meristematic activity to form the cambium between two adjacent vascular bundles, called interfascicular cambium. Intrafascicular cambium and interfascicular cambium join to form a cambium ring.

Formation of secondary tissues

Cambium ring acts as a meristem to add the secondary tissues both towards inner side and outer side. Cambium ring has two types of cells – elongated spindle-shaped fusiform initials and short isodiametric ray initials (see Figure 18). Fusiform initials divide to form secondary phloem towards outer side and secondary xylem towards inner side. The primary phloem and primary xylem are pushed towards periphery and centre by secondary phloem and secondary xylem, respectively. Ray initials divide to form vascular rays or secondary medullary rays on both sides. The activity of fusiform initials is more than the activity of ray initials. So, more secondary vascular tissues are formed than secondary medullary rays. In such condition, secondary medullary rays are compressed by secondary vascular tissues and form vascular rays. Vascular rays are uniseriate (one cell in breadth) or multiseriate (two or more cells in breadth). Vascular rays present in between the secondary xylem are called wood or xylem rays while the part of rays present in between secondary phloem are called phloem rays (see Figure 19).

Formation of annual ring or growth ring

The activity of cambium ring in temperate region is not uniform in spring or rainy season (favourable condition) and in cold winter or dry hot summer (unfavourable condition). Cambium ring is more active during favourable condition and form wide vessels containing more secondary xylem, called spring wood (early wood). But during unfavourable condition, a few small vessels containing secondary xylem, called autumn wood (late wood), are formed. These two kinds of wood appear together as a concentric ring in a year, called annual ring or growth ring. Such rings are formed year after year. Such type of annual ring is called ring porous wood.

The vessels in secondary xylem of the trees of tropical region are almost similar in both the spring wood and autumn wood due to the uniform activity of cambium ring in spring and autumn season. Such type of annual ring is called diffuse porous wood.

Formation of heartwood and sapwood

In old trees, secondary xylem is differentiated into centrally located dark and non-functional wood due to deposition of phytochemicals (oils, resins, gums, tannins, etc.) in cell wall and cell cavities, called heartwood or duramen and the outer light-coloured wood without phytochemicals in cell wall and cell cavities, called sapwood or laburnum (See Figure 20). Heartwood is stronger and more durable than sapwood.

Secondary growth in extra-stellar regions due to activity of cork cambium

The outermost layer of cortical cells (rarely deeper cortical cells/ pericycle cells) goes dedifferentiation and form a meristematic layer of cambium, called cork cambium (phellogen). Phellogen cells divide on both the outer side and inner side to form secondary tissues. The secondary tissue produced on the inner side of phellogen is called secondary cortex or phelloderm (See Figure 21). Phellogen produce cork cells or phellem on the outer side. It consists of dead and compactly arranged rectangular cells that possess suberized cell wall. The activity of cork cambium is more towards outer side than inner side. So, more phellem towards outer side than phelloderm towards inner side are formed. Cork cells prevent the loss of water by evaporation. They also protect the interior cells against the entry of harmful micro-organisms, mechanical injury, and unfavourable conditions.

At places of former stomata of the epidermis, phellogen produces aerating pores in the bark of plant instead of cork cells, called lenticels, for the exchange of gases and transpiration. They occur in woody trees but not in climbers. Lenticels are filled up by loosely arranged thin walled rounded, suberized or unsuberised cells, called complementary cells, which are formed by the division of sub-stomatal parenchyma cells. Complementary cells help in exchange of gases.

Figure 17 Secondary growth in dicot stem: a. & b. promeristem; c. young stem; d. formation of cambium ring; e. secondary growth in the first year; f. secondary growth in third year.

Figure 18 L. S. of Cambium ring showing fusiform and ray initials

Figure 19 T. S. of a part of two years old dicot stem showing secondary growth

Figure 20 T. S. of trunk showing sapwood and heartwood

Figure 21 Secondary growth due to activity of cork cambium

Internal structure of Isobilateral Leaf (Monocot Leaf)

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Isobilateral leaves of Paddy (Oryza sativa)
Image source: MB Chand

Here, I am posting a concise description of the anatomy of isobilateral leaf or monocot leaf.

Isobilateral leaves are found in monocotyledonous plants, such as maize. These leaves are normally erect and identical. Both the surfaces of such leaves are equally exposed to light. They are also called unifacial leaves. The internal structure of isobilateral leaf has epidermis, mesophyll tissue, and vascular bundles (see Figure 17).

1. Epidermis

Epidermis is the single layer of parenchyma found in both sides of leaf. This layer contains nearly equal number of stomata on both the sides, called amphistomatic. Each stoma opens into the substomatal cavity. In maize, cells in the upper stomata are modified into swollen bulliform or motor cells. Each stoma is guarded by a pair of dumb-bell shaped guard cells.

2. Mesophyll tissue

Mesophyll tissue is between the upper and lower epidermis. It contains large rounded parenchymatous cells with large cavities. It has reduced intercellular spaces. It is differentiated into palisade parenchyma and spongy parenchyma. Cells have abundant chloroplast.

3. Vascular bundles

Vascular bundles are parallelly arranged within the mesophyll tissues. They have different sizes. Each vascular bundle is conjoint, collateral, and closed type. Also, it is surrounded by a bundle sheath. Small vascular bundles are with less developed phloem and xylem whereas large bundles are with distinct phloem and xylem. The phloem is towards the lower epidermis and xylem is towards upper epidermis.

Phloem consists of sieve tubes and companion cells to transport food materials.

Xylem consists of two pitted oval metaxylem with tracheids in between them. The xylem parenchyma is less abundant. The protoxylem remains as a lysigenous cavity. Xylem conducts water and minerals.

Figure 17 T. S. of isobilateral leaf

Internal Structure of Dorsiventral Leaf (Dicot Leaf)

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Dorsiventral leaves of Syzygium cordifolia 

Image source: File:Dorsiventral leaves of Syzygium cordifolia IMG 1985.jpg - Wikimedia Commons

In this post, I am presenting a concise description of the anatomy of dorsiventral leaf .

The dorsiventral leaf is found in dicotyledonous plants. It has two surfaces – the upper or adaxial surface and the lower or abaxial surface. Its internal structure has epidermis, mesophyll tissues, and vascular bundles (See Figure 16).

1. Epidermis

Dorsiventral leaf shows distinct upper and lower epidermis.

Upper epidermis: The upper epidermis consists of a single layer of thin-walled and compactly arranged parenchymatous cells without intercellular spaces. Cells lack chloroplasts. Stomata are either absent or are fewer than in the lower epidermis. It is externally covered by cuticle.

Lower epidermis: Lower epidermis is made of single layer of parenchymatous cells. It is externally covered by cuticle. More stomata are present than on upper epidermis. Each stoma is surrounded by two kidney-shaped guard cells.

Epidermis helps in protection of the inner tissues from microbes, exchange of gases, and transpiration.

2. Mesophyll tissue

Mesophyll tissue lies between upper and lower epidermis. It is made of two types chlorophyll bearing parenchyma – palisade parenchyma and spongy parenchyma.

Palisade parenchyma: Palisade parenchyma lies just below the upper epidermis. The cells are radially elongated with numerous chloroplasts. These cells remain in 2 to 3 layers.

Spongy parenchyma: Spongy parenchyma is located between palisade parenchyma and lower epidermis. The cells are irregular and loosely arranged. They have fewer chloroplasts than in palisade parenchyma. They have large intercellular spaces, called air chambers. Some air chambers open into substomatal cavities. They perform photosynthesis.

3. Vascular bundles 

Vascular bundles are present in the mid rib and major veins. Each vascular bundle is made of xylem and phloem. The xylem is towards the upper epidermis (adaxial side) and the phloem towards lower epidermis (abaxial side). Each vein has a single vascular bundle. The vascular bundle is conjoint, collateral, and closed type. The bundle sheath surrounds each vascular bundle.

Xylem: Xylem consists of tracheary elements. Protoxylem lies towards upper epidermis and the metaxylem towards the lower epidermis.

Phloem: Phloem lies below xylem. It consists of sieve tubes and the companion cells.

Figure 16 T. S. of Dorsiventral leaf
Image source: MBC Biology

 

Anatomy of Monocot Stem

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 A maize stem bearing a newly sprout fruit

In this post, I am presenting a concise description of the internal structure of monocot stem.

A typical monocot stem consists of epidermis, hypodermis, ground tissues, and vascular bundles (see Figure 15).

Epidermis

Epidermis is the outermost layer of monocot stem. It is composed of compactly arranged, elongated, barrel-shaped parenchymatous cells without intercellular spaces. It contains stomata. Each stoma has a pair of specialized dumb-bell shaped cells, called guard cells. It helps in the protection of internal tissues, exchange of gases through stomata, minimize the surface transpiration by cuticle, and prevent the entry of harmful organisms.

Hypodermis

Hypodermis lies below epidermis. It is made up of 2- to 3-layered thick-walled dead lignified sclerenchyma. It helps in mechanical support.

Ground tissues

Ground tissue lies below the hypodermis. It is homogenous mass of tissue that is not differentiated into cortex, endodermis, pericycle, pith, and medullary rays. It is made up of oval or rounded parenchymatous cells with intercellular spaces. It helps in storage of food.

Vascular bundles

Vascular bundles in monocot stem are atactostele i.e. vascular bundles are scattered throughout the ground tissue. They are small and numerous towards the periphery but they are large and few towards the centre. They are oval or rounded.

Each vascular bundle contains both phloem and xylem. Xylem lies towards the centre and phloem lies towards the periphery. Cambium is absent. This type of vascular bundle is called conjoint, collateral, and closed. Each vascular bundle is surrounded by sheath of parenchyma, called bundle sheath.

Phloem

Phloem consists of sieve tubes, companion cells, and a few phloem fibres. Phloem parenchyma is absent. Sieve tubes conduct organic matters.

Xylem consists of vessels, tracheids, xylem parenchyma, and a few xylem fibres. Protoxylem is smaller than metaxylem. Protoxylem is first formed xylem whereas metaxylem is later formed xylem. Protoxylem lies at the centre while metaxylem lies towards periphery. This type of xylem is called endarch xylem. Vessels are arranged to form ‘V’ or ‘Y’. Some of the protoxylem vessels and xylem parenchyma cells are separated during rapid growth of stem and form a cavity, called protoxylem cavity (schizo-lysigenous cavity). Protoxylem cavity stores water. Xylem helps in conduction of sap and provides mechanical support.

Figure 15a T. S. of monocot stem (diagrammatic)

Figure 15b. T. S. of monocot stem (detailed view)