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Revision Notes on Structural Organisation in Animals

Epithelial Tissues

  • An epithelium is a tissue composed of one or more layers of cells that cover the body surface and lines its various cavities.

  • It serves for protection, secretion and excretion.

  • The word ‘epithelium’ was introduced by Ruysch.

  • Epithelial tissue evolved first in animal kingdom.

  • It originates from all the three primary germ layers. e.g. Epidermis arises from ectoderm, Coelomic epithelium from the mesoderm and epithelial lining of alimentary canal from the endoderm.

  • Types of Epithelium

Glands

  • Multicellular exocrine glands are classified by structure, using the shape of their ducts and the complexity (branching) of their ducts system as distinguishing characteristics.

  • Shape include tubular and alveolar (Sac like).

  • Simple exocrine glands e.g. intestinal glands, mammalian sweat glands, cutaneous glands of frog etc. have only one duct leading to surface.

  • Compound exocrine glands have two or more ducts e.g. liver, salivary glands etc.

  • Structural classification of exocrine glands:

Type

Example

Simple tubular

Intestinal glands, crypts of Lieberkuhn in ileum.

Simple coiled tubular

Sweat glands in man

Simple branched tubular

Gastric (stomach) gland, and Uterine gland.

Simple alveolar

Mucous gland in skin of frog, Poison gland of toad and seminal vesicle.

Simple branched alveolar

Sebaceous glands

Compound tubular

Brunner’s gland, bulbourethral gland and liver.

Compound alveolar

Sublingual and submandibular parotid salivary gland

Compound tubulo alveolar

Parotid salivary glands, Mammary gland and Pancreas.

 

Important Tips

  • Study of tissue outside the body in a glass tube is known as in vitro, while study of living tissues in situ is known as in vivo.

  • Among epithelia, simple epithelia were first to evolve.

  • Transitional epithelium also called plastic epithelium or urothelium. It lacks basement membrane

  • False epithelium  derived from mesenchyma a diffuse network of tissue derived from embryonic mesoderm) and lining the synovial cavities.

  • Mammary glands without teats are present in prototheria.

  • A malignant tumour arising from an epithelium is called a carcinoma. If it arises from a squamous epithelium it is a squamous cell carcinoma and if it arises from glandular epithelium it is called an adenoma.

  • The epithelial lining of brain ventricles and central  canal of spinal cord is known as ependyma.

  • Stereocilia are elongated membrane outgrowths found in certain parts of male reproductive tract.

Muscle Tissues

  • Muscle cells are highly contractile (contracting to 1/3 or 1/2 the resting length).

  • Muscle cells lose capacity to divide, multiply and regenerate to a great extent. Study of muscle is called myology.

  • About 40% to 50% of our body mass is of muscles.

  • The muscle cells are always elongated, slender and spindle-shaped, fibre-like cells, These are, therefore called muscle fibres.

  • These possess large numbers of myofibrils formed of actin and myosin.

(f) Difference between three types muscle fibres

   S.No.

     Feature

Striated or Striped or Skeletal or Voluntary muscle fibres

 Non-striated or Unstriped or  Smooth or Visceral or Involuntary muscle fibres

      Cardiac muscle fibres

1.

Shape

Long cylindrical

Fusiform (thick in middle tapering at ends) (0.02 nm to 0.2 nm long)

Network of fibres

2.

Stripes

Dark A bands and light I bands present

Absent

Present

3.

Nucleus

Many (syncytial) at periphery

Single at the centre of each cell

Many nuclei between successive end plates central position

4.

Unit

Sarcomeres, cylindrical long myofibrils placed end to end forming cylindrical myofibrils

Fusiform cells with inconspicuous borders

Oblique cross-connecting fibres make this muscle an interconnected bundle of myofibrils

5.

Attachment

To bones

To soft organs or viscera

Not attached to other organs except major blood vessels which are isolated and covered by pericardium

6.

Sarcolemma

Distinct

Absent

Absent

7.

Sarcoplasmic Reticulum

Well developed

Less extensive

Poorly formed

8.

Blood supply

Rich

Poor

Rich

9.

Contraction

Quick, fatigue fast

Slow, sustained contraction

Rhythmic, contractions originate in heart (pace maker immune to fatigue)

10.

Location

Generally peripheral, tongue, proximal part of oesophagus

Central, in hollow visceral organs, iris of the eye, dermis of the skin

Only in heart

11.

Intercalated discs

Absent

Absent

Present

12.

T-tubule system

 Well developed

Lacking

Well developed

13.

Innervated nerves

Motor nerves from central nervous system (neurogenic)

Nerves from autonomic nervous system (neurogenic)

Nerves from central and autonomic nervous system (myogenic)

14.

Fibres

Unbranched

Unbranched

Fibres join by short oblique bridges

15.

Action

Voluntary

Involuntary

Involuntary

Connective Tissues

  • It connects and supports all the other tissues, the intercellular element predominating.

  • The cellular element is usually scanty. In function this tissue may be mechanical, nutritive and defensive.

  • It is a tissue made up of matrix (abundant intercellular substance or ground substance) and living cells that connects and support different tissues.

  • Connective tissue was called mesenchyme by Hertwig (1893).

  • Types of connective tissues

(1) On the basis of their texture:

The bones are divided into two categories spongy or cancellous or tubecular bones and compact or periosteal bones

Bone

Cartilage

1.     Matrix is composed of a tough, inflexible material, the ossein.

1.     Matrix is composed of a firm, but flexible material, the chondrin.

2.     Matrix is always impregnated with calcium salts.

2.     Matrix may be free or impregenated with calcium salts.

3.     Bone cells lie in lucunae singly.

3.     Cartilage cells lie in lacunae singly or in groups of two or four.

4.     Osteocytes are irregular and give off branching processes in the developing bone.

4.     Chondroblasts are oval and devoid of processes.

5.     Lacunae give off canaliculi.

5.     Lacunae lack canaliculi.

6.     There are outer and inner layers of special bone forming cells, the osteoblasts, that produce new osteocytes, which secrete new lamellae of matrix.

6.     There are no special cartilage-forming cells. Cartilage grows by division of all chondroblasts.

7.     Matrix occurs largely in concentric lamellae.

7.     Matrix occurs in a homogenous mass.

8.     Bone is highly vascular.

8.     Cartilage in nonvascular.

9.     Bone may have bone marrow at the centre.

9.     No such tissue is present.

(2) On the basis of origin of bone:

Ossification or osteogenesis is the process of bone formation. A bone is classified into four categories.

Characters

Spongy bone

Compact bone

Arrangement of lamellae

There is no regular Haversian system so have spongy texture.

Have regular Haversian system

Occurrence

In skull bones, ribs, centrum of vertebrae and epiphyses of long bones

In the shaft (diaphysis) of long bones

Marrow cavity

Broad

Narrow

Type of bone marrow

Red  marrow in the spaces between lamellae

Yellow marrow in marrow cavity

Function

Marrow forms RBCs and Granular WBCs

Marrow stores fats

(3) On the basis of treatment:

These are of two types :-

Characters

Dried bone

Decalcified bone

Type of treatment

Subjected to high temperature.

Subjected to dilute solution of HCl.

Nature of matter left

With only mineral matter.

With only organic matter.

Marrow cavity

Empty.

With bone-marrow.

Fate of cells

Periosteum, endosteum, osteoblasts and osteocytes are absent being killed by high temperature.

Periosteum, endosteum, osteoblasts and osteocytes all are present.

Lacunae

Lacunae present.

Lacunae absent.

(6) Number of RBC: The number of RBCs is counted by instrument haemocytometer. The total number of RBC per cubic mm of blood is called RBC count. RBC count is slightly lower in women than a man and number of RBC is more in people who live on mountains because there is less oxygen. RBC are absent in cockroach.

S.No.

Organism

Number of RBCs

1.

Male

5 – 5.4 million / cubic mm of blood

2.

Female

4.5 – 5 million / cubic mm of blood

3.

Infants

65 – 70 lacs/ cubic mm of blood

4.

Embryo

85 lacs/ cubic mm of blood

5.

Rabbit

70 lacs / cubic mm of blood

6.

Frog

4 lacs / cubic mm of blood

(7) Life span of RBC: The life span of red blood corpuscles circulating in the blood stream varies in different animals. RBCs have longest life span in blood. The mammalians RBC have short life span due to absence of nucleus, which is disappeared during development.

S.No.

Organism

Life span of RBCs

1.

Mammals and Human

120 days or 4 months

2.

Rabbit

80 days

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