ALBIO5: SKELETAL TISSUES

This unit dwells on different tissue types in animals and plants

Closely related to connective tissue, responsible for supporting the body and providing it with a rigid frame work. Like C.T. it consists of cells embedded in an organic matrix though it is comparatively hard. Two kinds of skeletal tissue occur in the vertebrates; i.e. cartilage [gristle] and bone. The skeleton of elasmobranches fishes, such as dog fish, sharks and rays is composed entirely of cartilage. The mammal, with its predominantly bony skeleton, has cartilage at the joints and in the discs between successive vertebrae.

Cartilage consists of an organic matrix, chondrin, in which are embedded groups of spherical cells called chondroblasts which secrete the matrix.

The simplest form, hyaline cartilage, consists only of chondrin and chondroblasts. Other more complex forms are strengthened by the presence in the matrix of connective tissue fibres. e.g., cartilage of intervertebral discs contains collagen fibres, whilst that in the nose and pinna of the ear contains elastic fibres.

Bone is much harder than cartilage. It consists of an organic matrix impregnated with calcium salts, mainly calcium phosphate. These salts confer upon bone its property of extreme hardness. They are secreted by stellate cells called osteoblasts which arrange themselves in concentric rings round nerves and blood vessels, thereby giving compact haversian bone. Another type of bone tissue is spongy bone. It is less compact and hard than haversian bone and forms a network of interconnected trabeculae. Compact bone is found in the shafts of the limb bones, such as femur, while spongy bone is found at the ends (epiphyses) of such bones.

The matrix in both bone and cartilage is a mucopolysaccharide.

Other animal tissues

Blood is a circulating tissue consisting of 3 types of cells suspended in fluid (plasma). Red blood cells (erythrocytes) are most numerous i.e. 5million/ml3 whose function is to transport oxygen. Less numerous are white blood cells (leucocytes). Collectively, they combat disease by destroying pathogenic micro-organisms which have got into the body. It also contains minute cell fragments i.e. platelets which play part in process of blood clotting. Hence main function of the complex tissue is transport and defense.

Nervous tissue transmits electrical massages from one part of the body to another. The individual nerve cells are elaborately interconnected. Nervous tissue is an intricate network of interconnected cells whose function is to transmit and sometimes store information.

wuyo

Muscular tissue consists of 3 types: visceral, cardiac and skeletal. Visceral (smooth or unstriated) muscle consists of sheets of densely-packed, elongated fibres running parallel to each other and bound together by connective tissue. Each muscle fibre is a singe cell containing a nucleus and numerous fine contractile fibrils. Controlled by the involuntary part of the nervous system, visceral muscle contracts and fatigues, comparatively slowly. It is found in places where slow, involuntary movements take place e.g. walls of the gut, bladder, blood vessels and ducts of glands as well as dermis of the skin.

muscle

Skeletal muscle is associated with the skeleton, cardiac muscle with the heart. Both appear striated when viewed under the microscope and both contain contractile fibrils. Skeletal muscle is controlled by the voluntary part of the nervous system and provides basis of all voluntary movement in the body such as locomotion and movement of the limbs.

Reproductive tissue is associated with ovaries and testis. This is concerned with production of gametes i.e. eggs and sperms respectively.

Reproductive tissue is composed of developing gametes in the process of division and differentiation, together with cells that provide support and nourishment.

rep

CONNECTIVE TISSUES

This is a major supporting tissue of the body and binds organs and tissues together.

Loose connective tissue (areolar tissue).

This is found allover the body, beneath the skin, connecting organs together and filling the spaces between adjacent tissues. It consists of a gelatinous glycoprotein matrix or ground substance having four types of cells and two types of protein fibre. The cells are the most cells, fat cells, macrophages, fibroblasts.

The fibres are collagen fibres and elastic fibres.

Uses

The most cells secrete the ground substance and anti-coagulant known as heparin.

The macrophages are large and have an irregular shape. They ingest a wide variety of foreign particles and are important in defending the body against disease.

The fat cell stores fat.

The fibroblasts produce the collagen and elastic fibres. The collagen fibres are for support and are flexible, strong but not stretchable. The elastic fibres are stretchable.

Diagram of an areolar tissue.

are

PLANT TISSUES

Meristematic tissue

MERI

A meristem is a group of cells which has the ability to divide by mitosis and produces daughter cells which grow and form the rest of the plant body. The daughter cells form the permanent tissue and this is a tissue which has cells which have lost the ability to divide.

Meristematic cells are small, thin walled, have small sub-vacuoles and some of them have none, they also lack chloroplasts and they have prominent nuclei.

There are 3 types of meristem.

  • Apical
  • Cambium

Intercalary

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EPIDERMAL TISSUES

epidermis

 

The epidermis of the plant is the outermost layer of cells in the younger parts of the plant and can be replaced by the bark as the plant grows older. It is permanent in organs such as leaves. It is composed of cells which are flattened, lack chloroplasts except guard cells. These cells also have cellulose cell walls impregnated with fatty and waxy materials forming a cuticle.

Functions

  1. Protects the plants delicate tissue beneath it from parasites, abrasion and weather.
  2. It is transparent and elastic in the aerial parts hence allowing, flexibility, growth and entry of light for photosynthesis.
  3. In the aerial parts, it allows exchange of gases with the atmosphere through the stomata.

Has the cuticle which is impermeable to water and thus prevents excessive water loss

 

  1. In the roots, it avoids damage from abrasion by the soil particles and also allows absorption of water.
  2. Some epidermal cells have chloroplasts like in some plants which live in the deep shade.
  3. Anthocynin pigments can be found in the cell sap of epidermal cells of many petals and this gives them their distinct cebur to attract pollinators.
  4. The epidermis may posses on outgrowth known as a trichome e.g. root hairs.

 

The function of the trichome is to enhance its functions like protection and absorption. E.g. root hairs one the roots increase on the S.A for absorption of water.

(i) The stinging hairs of the Nottle are for protection.

Photosynthetic tissue

fotoz

Their main function is to photosynthesis. It is performed by the cells containing chloroplasts. These tissues are abundant in the leaves. This tissue is mainly made up of the mascephyll cells i.e. spongy and palisade mesophyll but there is a similar tissue in young and herbercious stems. The photosynthetic tissue has spaces for easy diffusion of gases to and from the cells with photosynthesis. The spaces form a continuous network of diffusion channels connecting with the atmosphere through the stomata

Parenchyma tissues

parench

Mechanical support in herbicious plants is achieved by means of fully turgid parenchyma cells packed tightly together in the stem.

 

This is also the packing tissue. It consists of living cells of uniform size and shape and is rounded.

Function

  • It fills up the spaces between other tissues.
  • As long as cells are turgid, it maintains the shape and firmness of the plant especially the leaves and herbercious stems.
  • Acts as a storage tissue of starch, oil, glucose, proteins in the plant as wells as vitamins and sucrose.

Occur in the vascular tissue in which they provide the living protoplast to control the functioning of the tissues and therefore play an important role in the movement of water and food products in the xylem and phloem respectively

Vascular tissue

vascular tissue

This is concerned with transport. These tissues transports water and food and are basically of 2 types i.e.

  • Xylem

Phloem

They also contain strengthening tissues i.e. scleronchyma and parenchyma. The primary vascular tissues arise from the primary meristem and the secondary vascular tissue from the secondary cambium

Xylem.

xylem

The xylom is for upward transporting of water and it is made up of channels made up of short sections and these are arranged from one end to the other end. Each section is the wall of a single cell thickened and strengthened by a secondary cell was inside the primary cell wall. The xylem cell is a blood cell. The secondary wall has cellulose and lignin. The lignin renders it impermeable to water and solutes. The xylem cell has no protoplast and doesn’t divide, respire or perform any processes associated with a living cell. It has tracheids and vessels.

Trecheids

tra

These are alongated empty cells with tapered ends and bordered pits. Water moves from cell to cell through the pits. The tracheids are also concerned with mechanical support. The tracheids occur in the gymnosperms and angiosperms.

Vessles

They occur only in angiosperms. They are composed of empty cells with complete or partial break down of the end walls joined to each other vertically. Through them, there is an uninterrupted flow of water. Cell which are joined to each other form a vessel and each vessel has components known as vessel segments/elements.

Scloronchyma tissue

scl

This consists of cells with uniformly thickened and lignified wall and sometimes with cellulose with open empty lumers. The cells are therefore dead and are of two types ie fibres and sc;ereids. Fibres are long and narrow while sclerids are short and branched cells. The sclerids are in addition to the cortex found in the covering of seeds and fruits ie testas and poricarps respectively while fibras are also found strengthening the ribs of ribbed stems.

Functions

  • It is used for support because it is highly lignified.
  • For mechanical protection due to the presence of sclerids in testa, shells and endcarps.
  • Provides rigidity
  • Used for mechanical support due to presence of fibres.

Collenchyma tissue

 Consist of unevenly thickened cellulose walls. They remain living even in mature roots and stems. In the young stems, the cellonchyma is located in the outer layer of the cortex just below the epidermis.

Functions

(i) Support in young plants and leaves due to extra deposition of cellulose at the corners.

THE PHLOEM

phloem

It transports organic solutes i.e. substances that have been manufactured by the leaves. Translocation involves transport in xylem and phloem. The phloem is composed of several cells which include;

  • Sieve tube elements which are the conducting cells.
  • Companion cells which meet the metabolic requirements of the sieve cells.
  • Schlerenchyma and parenchyma cells for support.

In between the companion cells and the sieve tube elements are the plasmodasmala and they connect the two.

They sieve tub element has got sieve pores to allow passage of materials, it has mitochondria for energy production. It has the sieve plate which has forces, has fine cytoplasmic filaments for the passage of materials.

Cell division involves cells dividing to form daughter cells. There are 2 types of cell division i.e. mitosis and meiosis.

MITOSIS

This is the process by which a cell nucleus divides into two daughter nudei, each containing exactly the same number of chromosomes and therefore same genes as the parent nucleus. The nuclear division is followed by a cell division.

Mitosis is divided into four active stages i.e. telophase, prophase, anaphase and metaphase. These stages are not distinct but grade into each other. When it’s not under going mitosis, the cell rests and this is known as interphase.

During this state, it carries out activities such as energy production for cell division, replication for chromosome and protein synthesis

Prophase

At the beginning of prophase, the chromosomes appear as long threads which are tangled together and can be strained. They gradually become shorter and thicker by coiling into a spiral a process known as spiralisation. Each chromosome then appears as a double thread and each of the threads is called a chromotid. The chromotids are joined together at the centromere and it can be stained with a dye. Towards the end of prophase, the nucleolus disappears and nuclear membrane breaks down and chromosomes are dispersed in the cytoplasm with centrioles at the opposite end and are only present in animals and lower plants.

Metaphase

meta

 

In metaphase, the chromosomes came to lie on the equatorial part of the spindle to which they are attached by their centremores.

The centromores begin to divide.

Anaphase

ana

 

  • The centromores divide into two
  • The chromatids separate.
  • The spindle microtubes connecting the chromosomes to the poles shorten while those which are from pole to pole lengthen.
  • The poles move further apart and lengthen the cell. This results in each chromatid being pulled to its respective pole led by the centromore. Once the chomatids are separated, they are reffered to as daughter chromosomes.

Telophase

telophase

  • The spindle fibries disappear and the nuclear membranes form around the 2 sets of the chromosomes.
  • The nucleolus and centrioles also reform.
  • The chromosomes begin to unravel and become less dense and harder to see.
  • The final step is the division of the cytoplasm called cytokinesis.

In animal cells, a ring of centractile fibres tightens around the centre of the cell until the 3 cells have separated.

In plant cells, the division of the cell occurs differently with a carplate made of cell plate made of the cellulose cell wall building up from the inside of the cell outwards.

The plate is formed from fusion of vesicles produced by the Golgi body.

Cellulose is laid down on this plate to form the cell wall.

In both the animals are plant cells, the end result in the same. Two identical daughter cells are formed.

Differences between mitosis in plants and animals

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SIGNIFICANCE OF MITOSIS

Genetic stability

Mitosis produces 2 nuclei which have the same number of chromosomes as the parent cells. Since these chromosomes were derived from the parental chromosomes, by the exact replication of their DNA, they carry the same hereditary information in their ganes so the daughter cells are identical to the parent cell, resulting to genetic stability.

Growth

The number of cells within organism increases by mitosis.

Asexual reproduction

There are many organisms propergated increases by asexual methods involving mitotic division of cells

 Regeneration

Missing parts such as crustacean are regenerated by mitosis.

Cell replacement

e.g. when an organism has been damaged, cells can be replaced by mitosis.

Meiosis   

mei

A type of cell division in which the parent cell divides to form four daughter cells each having half the number of chromosomes of the parent cell and it occurs in 2 successive divisions. It is sometimes called reduction division because at the end, the daughter cells have half the number of chromosomes of the parent cell and it occurs in the reproductive cells of both plants and animals and the daughter cells formed are haploid. Each division is divided into 4 stages and these stages are not distinct but grades into each other. The 1st division is further divided into prophase 1, metaphase 1, anaphase 1 and telophase 1.

The 2nd division is divided into prophase 2, anaphase 2 and telophase 2 like mitosis, before the cell undergoes meiosis; it first goes through interphase when there is replication of chromosomes, storage of energy and protein synthesis.

Prophase 1

It is divided into 5 phases namely: leptotone, zygotone, pachytene, diplatone, diakinosis.

(i) Leptotone

This is the first phase and chromosomes appear as thin threads i.e. you can’t see the chromotids but the contromere is visible.

(ii) Zygotene

Chromatids can be seen joined by their centromeres. The homologous chromosomes are attracted to each other and they come together and pair up forming a bivalent. Starting from are end another, they zip up like a zip fastener. This process is known as pairing or synopsis.

(iii) Panchytene

Chromosomes become shorter and thicker by coiling around each other and cromatids.

(iv) Diplotene

Crossing over occurs, and breaks occur in the homologous chramatids due to strain of their coiling. The parts which broken off join up crosswise with the chromatid of the homologous chromosome.

(v) Diakinosis

Chromatids of homologous chromosomes are nolonger attrached to each other and they try to move a part but held together at the part where they crossed over. This point is known as chiasma (chiasmata if many). This point appears as a visible cross.

By the end of prophase, the nucleolus and nuclear membrane have broken down and in the animals and lower plants, a spindle in organized in the cytoplasm by the centrioles.

Crossing over

Diagrams below show how C.O between chromatids of homologous chromosomes leads to genetic variety. The new combinations possible in the gametes depend on the number of genes involved and their positions on the chromosomes relative to the chiasmata. Crossing over takes place during prophase of list meiotic division.

 

Metaphase 1

  • The bivalents oriented themselves on the spindle and there centromores come to lie on equal of equatorial plate and equidistant from it.
  • The homologous chromosomes strain to separate but are held together by their chaismata which may begin to slip towards the ends of the chromosomes a process known as terminalisation.

Anaphase 1

  • Microtubules connecting the chromosomes to the poles shorter while the spindle microtubules from pole to pole lengthen.
  • The pairs of homologous centromeres finally separate and more towards their respective poles centromere pulls the rest of the chromosome after it. In anaphase, homologous chromosomes separate.

Telephase 1

  • The spindle disappears and a nuclear membrane forms around

Each group of chromosomes. The length of telophase varies in different species, to complete nucleus may form and cell division may occur or cell division may not occur

Prophase 2                    

  • Similar o prophase of mitosts
  • Chromosomes appear as long threads, become shorter and thicker each chromosome appears as a double thread and each thread is called a chromatid. The chromatids are joined by a centromere.
  • Towards end prophase, nucleolus disappears, nuclear membrane breaks down and the chromosomes are dispersed the cytoplasm then the spindle is organized in the cytoplasm.

 

Metaphase 2

  • Similar to metaphase of mitosis
  • Chromosomes come to lie on the equatorial plate.

Anaphase 2

  • Similar to anaphase of mitosis.
  • Centromeres divide into two and chromatids separate.
  • Spindle microtubules connecting chromosomes to the poles shorter while spindle microtubules from pole to pole lengthen resulting in each chromatid being pulled to its respective pole led by the centromere.

Once chromatids are separated, they are known as daughter chromosomes.

Telophase 2

  • Spindle fibres disappear and nuclear membranes form around the four sets of chromosomes.
  • Centriole and nucleolus also reform hence there is division of cytoplasm resulting in four cells which are haploid.

Meiosis can only occur in diploid or polyploidy cells.

Meiosis occurs in the higher plants. It occurs in formation of spores in plants with alternation of generations. It also occurs in formation of sexual spores in fungi.

Significance of meiosis

  • The meiotic division before gamete fusion ensures that the chromosome number of the species is maintained in constancy through successive generations.
  • It brings about genetic variation due to;
  • Random distribution of the maternal and chromosome in the gametes. This occurs due to the orientation of chromosomes in metaphase 1 and 2.
  • Genetic variation due to the reshuffling of chromatin material due to crossing over.
  • The segregation or separation of allele so that each gamete carries only one allele of gone locus.

Differences between mitosis and meiosis.

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ASSIGNMENT : SKELETAL TISSUES Assignment MARKS : 30  DURATION : 24 hours

 

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