4 Main Types of Tissues Seen in Animals

The following points highlight the four main types of tissues seen in animals. The types are: 1. Epithelial Tissues 2. Connective Tissue 3. Muscular Tissue 4. Nervous Tissue.

Type # 1. Epithelial Tissues:

The cells taking part in this type of animal tissue have a regular, well-defined shape and scanty intercellular substance. They are usually in contact with one another on a definite base­ment membrane. The epithelial cells either form compact groups or form a continuous lining on all external or internal free surface.

These tissues are without vascularisation but contain nerve fibres and migratory cells. When epithelial tissue is made up of a single layer of cells, it is called simple epithelial tissue and when it is many-layered it is called com­pound or stratified epithelial tissue.

Accord­ing to the shape of the animal cells (Fig. 5.2) which are present near the surface, the epithelial tissue may be:

(A) Squamous—These are tile­-like flat cells. The simple squamous epithe­lium is seen in peritoneum of frog, alveoli of lungs and endothelium of blood vessels. The stratified squamous epithelium is found in epidermis, wall of oesophagus, cornea and a part of female urethra.

(B) Cuboidal—Cube-­like cells with polygonal outer surface, e.g., lining of thyroid vesicles and kidney tubules.

(C) Prismatic or Columnar—The simple columnar tissues include tall cells with dis­tinct elongated nucleus, e.g., lining of ali­mentary canal. The stratified columnar is seen in the pharynx, epiglottis as a pack of cells in which inner cells are fusiform and outer cells are tall and prismatic in appearance.

The columnar epithelium may be:

(1) Cili­ated—Possessing short vibratile cilia for re­moving fluid, mucous and other materials, e.g., lining of nasal tube, lining of bronchial tube and lining of Fallopian tube.

(2) Flagel­lated—Each animal cell possesses a single flagellum, e.g., flagellated cells in the endoderm of hy­dra.

(3) Stereo-ciliated—Here the cells are with one or two immobile cilia at their free end, e.g., lining of nostril, tongue and inter­nal ear. These are sensory cells for receiving different stimuli, i.e., smell, taste and hear­ing.

(4) Brush border—Free end of the animal cells has brush-like appearance. These are respon­sible for absorption, e.g., lining of the intes­tine. All the above mentioned epithelial tis­sues are included in this group.

These epithelia carry following functions:

(a) Absorption, e.g., Brush-border co­lumnar.

(b) Diffusion, e.g., squamous.

(c) Secretion, e.g., cubical.

(d) Protection, e.g., squamous, ciliated columnar.

(e) Sensory, e.g., stereo-ciliated.

According to its function, the epithelial tissues may be (A) Covering epithelium and (B) Glandular epithelium.

A. Covering Epithelium:

It includes the covering of external or internal surface.

B. Glandular Epithelium:

It includes spe­cialised epithelial tissues which have acquired either secretory or excretory properties. In general, the cells of the glandular epithelium possess large nuclei, prominent Golgi appa­ratus and the secretory products in the form of granules. The glandular epithelium may be present either scattered or in a group within other epithelial cells or may be variedly organised to form glands.

The glands are chiefly either tubular or saccular. The tubular glands may be simple (intestinal gland), branched (lacrimal gland) and coiled (sweat gland). The saccular glands are again simple (present in skin of toad), compound (salivary gland) and branched (sebaceous gland).

Some glandular epithelial cells give out only their product (merocrine), others throw their entire cell together with the con­tent (holocrine), e.g., sebaceous gland.

In the third type (apocrine) only the free regions of the animal cells burst and the products are extruded out, e.g., mammary glands. Some glandular epithelium secretes through definite duct or tube (exocrine glands), while there are cer­tain glandular epithelia which put their prod­uct directly into the blood stream (endocrine glands).

Type # 2. Connective Tissue:

Connective tissues are characterised by groups of animal cells which remain embedded in a extracellular material called matrix.

Accord­ing to the nature of the matrix the connective tissues be:

(A) Connective tissue proper,

(B) Skeletal tissue and

(C) Vascular tissue.

A. Connective Tissue Proper:

These tis­sues are concerned with binding and pack­ing of different parts.

Following tissues are included under this group:

(a) Mucous tissue

(b) Reticular tis­sue

(c) Areolar tissue

(d) Fibrous tissue and

(e) Adipose tissue (Fig. 5.3).

(a) Mucous tissue:

These connective tis­sues are best seen in jelly fishes and in the eye of the vertebrates. It consists of a few branched cells and clear gelatinous ground substances called matrix.

(b) Reticular tissue:

It consists of a dense network of reticular fibres with free cells. The nature of the animal cell varies according to organ. This tissue forms the framework of various organs like lymph node, bone mar­row and liver.

(c) Areolar tissue:

It serves as a packing layer beneath the skin and fills up the gaps between different organs. It consists of a jelly­like matrix in which different cells and fibres are embedded.

Two types of cells are seen in the matrix, namely fibroblasts which are re­sponsible for the production of fibres and matrix cells for secreting the matrix. Two kinds of fibres are yellow elastic fibres as freely, branched network and white fibres containing bundles of collagen fibres.

(d) Fibrous tissue:

These tissues are entirely made up of fibres and they form sheath over the muscles, tendon and liga­ments. Tendons are responsible for connect­ing muscles with the bones or with other muscles.

In tendons the fibres contain colla­gen and are arranged in parallel fashion. Ligaments connect different bones and also bones with cartilages. Its fibres contain collagen and in addition, another protein called elastin, which permits the stretching and recoiling.

(e) Adipose tissue:

This tissue contains cells which are primarily responsible for car­rying reserve fats. This layer beneath the skin serves as insulating layer. As a covering around important organs, it works as a cush­ion to protect these organs from mechanical injury.

B. Skeletal Tissue:

Two types of skeletal tissues are known (a) Cartilage and (b) Bone (Fig. 5.4).

(a) Cartilage:

This tissue is rigid and at the same time elastic. It is capable of endur­ing mechanical stress. It consists of a ground substance which contains a protein and polysaccharide mixture called chondrin. Within this matrix are; suspended the carti­lage cells or chondrioblasts, which secrete the matrix.

The animal cells are enclosed in spaces within the matrix called lacunae where they multiply and remain in groups of two to four. A coat of areolar tissue called perichondrium, surrounds the cartilage.

Depending on the nature of the matrix the cartilages are divided into two main cate­gories:

(i) Hyaline and

(ii) Elastic cartilages.

The hyaline cartilage has a homogeneous and transparent matrix. It can be seen in larynx, trachea and suprascapula. The matrix of elastic cartilage bears yellow elastic fibres (seen in epiglottis) or with parallel collagen­ous fibres (seen in between the vertebrae).

(b) Bone:

It is a hard connective tissue which is responsible for forming the skeletal framework of the body. It is rigid and can endure great mechanical strain.

Each bone consists of a central cavity which contains bone marrow. The cavity is bounded by con­centric layers called bony lamellae. Through the lamellae pass numerous smaller chan­nels from the narrow cavity; these are called Haversian canals.

These canals are also lined by concentric rings of lamellae. Each lamella is made up of a matrix in which bone cells are arranged within smaller spaces called lacunae. The lacunae are interconnected by finer canaliculi. The matrix of bone is made up of protein fibres and mineral deposits like calcium phosphate, magnesium carbonate and fluorides.

The bone cells are known as osteoblasts, which are spider-shaped. These cells are interconnected by protoplasmic pro­cesses which pass through canaliculi. Exter­nally the bone is enclosed by a fibrous sheath called periosteum, which takes the blood ves­sels within the cavity of bone marrow.

C. Vascular Tissues:

It Includes blood, lymph and bone marrow (Fig. 5.5).

(a) Blood:

This tissue not only connects different parts but also performs certain other important functions. It has a liquid matrix called blood plasma in which are suspended various blood corpuscles.

(1) Blood plasma:

It is a pale yellow col­oured liquid in which various substances like food, waste products, hormones and gases remain in solution. It transports everything excepting oxygen in vertebrates and fat drop­lets. Within the body of invertebrate animals, the colouring pigments remain in the plasma, e.g., haemocyanin in prawn and haemo­globin in earthworm.

(2) Blood corpuscles:

Only one type of corpuscle is seen in the invertebrates which are amoeboid in nature. But in vertebrates several types of corpuscles are seen, e.g., Erythrocytes, Leucocytes and Thrombocytes. These corpuscles are formed within the marrow. They have a limited span of life after which they are destroyed.

(i) Erythrocytes:

Presence of red blood cor­puscles (RBC) is peculiar to vertebrates ex­cepting some invertebrate forms e.g. Glycera, Phoronis, Area, Thyone. The shape and size of erythrocytes differ in various groups of ver­tebrates but always the erythrocytes carry an iron-containing compound called haemo­globin. Haemoglobin is a complex molecule composed of a compound of iron and globu­lin.

This compound can establish temporary affinity with oxygen and thus carries oxygen from respiratory organs to the different parts of the body in the form of oxyhaemoglobin. Erythrocytes are nucleated in all the verte­brates excepting mammals, where it is non- nucleated in mature stage.

They appear as oval discs in most vertebrates and appear to bugle out in the centre due to the presence of nucleus. But in mammals (except camels and llamas where the RBC are oval discs like non-mammalian forms) they are more or less circular in outline.

(ii) Leucocytes:

The leucocytes occur in much lesser number than the erythrocytes. These cells, which are also known as white blood corpuscles or WBC, may be of different type (Monocytes giant mononuclear amoeboid forms; Lympholcytes roughly cir­cular with one large nucleus; Granulocytes amoeboid with granular cytoplasm.

They are subdivided into neutrophils, eosinophils and basophils depending on an affinity for neutral, acid or basic dyes respectively), which not only differ in their structures but also in their functions. Their functions in­clude removal of dead tissue, killing of for­eign bodies and carrying of fat globules.

(iii) Thrombocytes:

These are small, spin­dle-shaped and nucleated cells. At the time of blood-shed, the thrombocytes (thrombus, clot: cyte, cell) break down under the influ­ence of tissue fluid and produce an enzyme which helps the conversion of fibrinogen into insoluble fibrin.

Fibrin forms an entangling mesh-like barrier through which the blood cells cannot easily pass and thus causes the blood to clot. The coagulation of blood pre­vents excess loss of blood from the injured region.

(b) Lymph:

It is regarded as a modified tissue fluid. Generally, it is pale yellow in colour but after meal it becomes milky due to the presence of emulsified fat droplets. Lymph contains 94% water and nearly 6% solid particles which include proteins, fats, carbohydrates and other substances.

Lymph also carries some wandering leucocytes. Lymph is responsible for (1) acting as a medium between blood and the cells, (2) conveying emulsified fat droplets and (3) protecting cells of the body from foreign invasion by its leucocytes.

(c) Bone marrow:

The spaces within the bones are filled up with a special type of vascular tissue called bone marrow.

The bone marrow may be of two types:

(1) Yellow bone marrow and

(2) Red bone marrow.

The yellow marrow contains fatty tissues and does not produce blood cells.

In the young indi­viduals most of the bones contain red mar­row, but with the advancement of age, in most bones (excepting upper ends of femur and humerus) yellow marrows replace the red ones.

The bone marrow serves following important functions:

(1) Produces erythro­cytes, leucocytes and thrombocytes, and

(2) through special kind of reticuloendothe­lial cells it destroys the old red blood cells.

Characteristics of connective tissue:

In strict sense the term connective tissue is a loose one because it includes completely diverse types of tissues.

The most important features about connective tissues are:

(1) they have their own blood supply,

(2) they have the ability to be converted from one type to another, e.g., conversion of cartilage to bone.

Considerable amount of research work is in progress on the connective tissues.

Whether cells of a particular connective tis­sue present in different organs have common affinity or not. Are cartilages of the knee and ear, alike? Do fibroblasts present in same fashion? These are a few of many unanswered questions.

Type # 3. Muscular Tissue:

A muscle means a pack of muscular tissue which is enclosed in a connective tissue called fasciculi, which in turn a covered by a sheath called perimysium. The muscular tissue is made up of muscle fibres which develop from a special type of cells called myoblasts. According to its structure the fibres may be striated, smooth and cardiac (Fig. 5.6).

The electron microscopy has revealed that each striated muscle fibril contains an alternate dark anisotropic or A band and a light iso­tropic or I band. Each I band exhibits central Z line and similarly A band exhibits a central hyaline transverse band called H band with a middle deep stripe called M line. The re­gion between two Z lines is known as sar­comere (Fig. 5.7).

A. Striated Muscle Fibres:

These elongated and cylindrical muscles are also known as somatic or voluntary muscles. Each fibre is enclosed within a sheath called sarcolemma. Each fibre contains a mass of protoplasm called sarcoplasm, which includes numer­ous delicate parallelly arranged fibrils called myofibrils.

In longitudinal section all myofibrils exhibit alternate dark and light segments in uniform position. This gives the striated appearance. These fibres are present in all voluntary parts and are innervated by the branch of central nervous system.

B. Smooth Muscle Fibres:

These are spin­dle shaped, elongated, uninucleated cells without investing sarcolemma. Only longi­tudinal fibrils are present. It forms the walls of involuntary organs and are innervated by nerves from autonomic nervous system.

C. Cardiac Muscle Fibres:

These are modi­fied striated muscles in which fibres are short and branched. The branches unite to form a network or syncitium. Each fibre contains only one nucleus. This type of fibre is seen only in the wall of heart.

Type # 4. Nervous Tissue:

This is a special kind of tissue which is responsible for receiving, transmitting and discharging various sorts of stimuli. It is made up of (A) nerve cells and (B) nerve fibres (Fig. 5.8).

A. Nerve Cells:

These are also known as neurones. Each nerve cell is a large spider- shaped cell having distinct nucleus and scat­tered Nissl’s granules in the cytoplasm. Each animal cell gives rise to several short, branched fi­bres called dendrites, and a large process called axon. The axon may be branched along its length and each branch terminates into several finer branches called end brush.

The axon either enters within a muscle fibre or unites with dendrites from other neurone. This union is called synapse. It was formerly thought that synapse involves a complete fusion between a fibre of axon and dendrite. But recent electron microscopic studies have shown that a gap exists between the two ends.

When message travels from axon of one cell to the dendrite of another a pack of chemical substance is released from the former to the latter. It is to be remembered that a nerve cell receives message through dendrites but always sends information through the axon.

Nerve cells are divided into three types depending on the existence of number of fibres.

They are:

(i) Unipolar,

(ii) Bipolar and

(iii) Multipolar nerve cells.

A bipolar nerve cell has one centripetal dendrite and one centrifugal axon or neurite. These types of nerve cells are abun­dantly found in fishes. A unipolar nerve is formed when the proximal ends of an axon and a dendrite emerge out from the cell body. A multipolar nerve cell has a single axon and several dendrites.

B. Nerve Fibres:

A nerve fibre consists of a centrally placed axon which is bounded by a sheath called neurilemma. The neuri­lemma also includes special type of cells called Schwann cells. Some nerve fibres contain a fatty layer called myelin or med­ullary sheath, between neurilemma and axon. These fibres are called medullated fibres.

Along its path the medullary sheath is con­stricted at different regions, these points are called nodes of Ranvier. Some fibres are without myelin sheath and are called non- medullated fibres, which are present in autonomic nervous system. The medullated fibres arise from various parts of central nervous system.

Nervous tissue constitutes important organs like brain, spinal cord, etc. and are responsible for co-ordinating the different activities of the body. This is done by electro­chemical mechanism within neurones.

According to their structure and function the three groups of neurones are known as:

(1) Sensory neurones—carry impulses from various parts of the body to the brain and spinal cord,

(2) Association neurones— present exclusively within the brain and spinal cord to form definite circuits for nerve conduction and

(3) Motor neurones—return instructions from central nervous system to the different parts of the body.

Tissue specification:

The four types of tissues constitute all the different organs hav­ing different functions. Apparently these tis­sues appear to be same but detailed exami­nations have established the presence of some amount of specificity in each case. In recent years, the coming up of cell dissociation tech­niques and studies of mixed aggregates have thrown much light on the question of cellu­lar affinity.

When embryonic kidney and heart cells from one species were mixed up they sorted out but when any one organ (heart or kidney) from two different species (mouse and chick embryos) was put together they do not sort out. These results demonstrate that whatever may be the nature of specificity it exists at tissue grade and not at species level.

All these abilities to sort out exist only at the embryonic level and completely di­minish in the adult state.

Regarding the prob­lem of tissue specificity two questions are yet to be answered:

(1) whether tissues from one organ can take part in the formation of other organ and

(2) what changes happen in the adult state, which prevent the cells from sorting out.