Animal Tissue

Tissue

A group of cells which are similar in structure, similar in origin, and specialized for a particular function, is known as a tissue.

The term "tissue" (from textre, to weave) was given by Nehemiah Grew with reference to plant anatomy. This term was introduced in zoology by the French anatomist and physiologist Marie Francois Xavier Bichat (1771-1802). He is considered the founder of animal histology.

The branch of biology concerned with the study of the microscopic anatomy of the cells and tissues of plants and animals is called histology (histos = web/tissue, logos = study), i.e. the study of micro-anatomy. A person who specializes in the science of histology is called a histologist. The Italian scientist Marcello Malpighi is considered the founding father of histology. The term "histology" was given by August Mayer.

Types of Tissue

There are four major types of tissue:

1. Epithelial tissue
2. Muscular tissue
3. Connective tissue
4. Nervous tissue

1. Epithelial Tissue

(Greek: epi = over/upper, thelium = covering)

The word "epithelium" was introduced by Ruysch. An epithelium is a tissue composed of one or more layers of cells covering the external surface of the body or the viscera (internal organs).

• Epithelia arise from all three primary germ layers of the embryo: ectoderm, mesoderm and endoderm. For example: the epidermis arises from the ectoderm, coelomic epithelium from mesoderm, and the epithelial lining of the alimentary canal from endoderm (giving rise to ectothelium, mesothelium and endothelium respectively).
• The tissue consists of cells of various shapes, closely arranged in one or more layers, forming a continuous sheet, and lacks intercellular space.
• The cells are held together by a cementing substance made up of a carbohydrate derivative, hyaluronic acid (a mucopolysaccharide).
• The epithelium rests on a thin, lowermost, non-cellular layer called the basement membrane.
• Epithelia generally lack blood vessels (avascular), but still have good power of repair after injury.
• They receive useful material and transfer waste material to the blood flowing in the blood vessels, across the basement membrane.

Functions of epithelial tissue

1. Protection: as it covers the surface, it protects the underlying tissue from mechanical injury, drying up, entry of germs (infection), and harmful chemicals.
2. Formation of exoskeleton: it produces many useful exoskeletal structures, such as scales, feathers, hairs, and claws/hooves in tetrapod vertebrates.
3. Secretion: epithelium lining the cavities gives rise to glands that provide valuable secretions, such as mucus and gastric juices.
4. Selective barrier: the epithelium checks the absorption of harmful material.
5. Respiration: the epithelium of lung alveoli brings about the exchange of CO2 and O2 between blood and air.
6. Reproduction: the germinal epithelium of the seminiferous tubules and ovaries produces spermatozoa and ova respectively.
7. Regeneration: the ability of epithelia to regenerate quickly helps in the healing of wounds.
8. Sensation: sensory epithelium in sense organs, e.g. the nasal chamber, picks up stimuli from the environment and sends nerve impulses, finally to the CNS.
9. Absorption: the epithelial lining of the intestine absorbs digested food.

Types of epithelial tissue

On the basis of layering, shape, structure and function, epithelial tissue is divided into three types:

1. Simple epithelium (one layer of cells): simple squamous, simple cuboidal, simple columnar (ciliated, brush-border), and pseudostratified.
2. Compound epithelium (many layers of cells): stratified squamous, stratified cuboidal, stratified columnar, and transitional.
3. Compound specialized epithelium (for particular functions): sensory, ciliated, glandular.

1) Simple epithelial tissue

• It consists of a single layer of cells.
• All the cells rest on the basement membrane.
• Being extremely thin, it is more effective as a sensory or absorptive surface.
• It is not very effective as a protective lining.
• It is of various types, on the basis of shape and arrangement.

a) Simple squamous epithelium

• Formed of flattened, scale-like (squama = scale), usually polygonal cells, each with a centrally placed large nucleus.
• Contains little cytoplasm.
• The tissue has an irregular margin called a tessellated margin.
• The cells are so closely fitted together that they look like tiles or a mosaic; also called pavement epithelium.
• Located in the alveolar lining (air sac) of lungs, Bowman's capsule of the kidney, the inner lining of blood vessels, and the lining of blood capillaries.
• This epithelium is suited for easy exchange of material by diffusion across it.

b) Simple cuboidal epithelium

• These are cube-like cells, hence called cuboidal.
• The height and breadth of the epithelial cells are the same.
• Each cell has a central, spherical nucleus.
• The tissue lies on the basement membrane.
• Located in the tubular part of the nephron in the kidney, salivary gland, sweat gland, thyroid gland, and the ciliary muscle of the eye.
• Its main function is absorption, secretion, excretion, accommodation of the lens, and gamete formation.
• Cuboidal epithelium also covers the ovaries and lines the sperm-producing tubules of the testes (also called germinal epithelium), and produces new cells for the formation of gametes.

c) Simple columnar epithelium

• These are tall, column- or pillar-like cells, with the nucleus at the base, and their height exceeds their width.
• A single layer lining that rests upon the basement membrane.
• The free surface may have cilia or microvilli.
• Located in the stomach, intestinal villi, Fallopian tube, gall bladder, ureter and renal tubule.
• Main function: absorption, conduction, storage and excretion.

Simple ciliated columnar epithelium (a variant bearing cilia): found in the same regions where conduction/movement of surface fluid or particles is needed.

a) Pseudostratified epithelium

• One layer of columnar cells, but appears to be in two layers due to nuclei being at different levels; hence called pseudostratified epithelium.
• All the cells rest on the basement membrane.
• Such cells are ciliated.
• Found in the lining of the trachea and primary bronchi, and the respiratory tract and nasal epithelium.
• In the Fallopian tube, the cilia push the ovum toward the uterus; mucus traps bacteria and dust particles, and the ciliary beat expels them outward (filtration).

2) Compound epithelial tissue

• Consists of many layers of cells; only the lowermost layer rests on the basement membrane.
• These are thicker and stronger than simple epithelial tissue, and provide better protection to the underlying tissue.
• Found on surfaces exposed to wear, such as skin epidermis; it protects against chemical and microbial infection, friction and damage. Also found in the cornea of the eye and the inner surface of the mouth and oesophagus, etc.

It is of the following types:

1) Stratified squamous epithelium

The upper layer cells are flattened, hence called squamous epithelium. This epithelium consists of many layers of cells, and the deeper cells may be columnar or cuboidal. The main function of this epithelium is protection. It is of two types:

• i) Keratinized squamous epithelium: this tissue has a heavy deposit of the insoluble protein keratin in dead superficial cells, so it is impermeable to water and highly resistant to abrasion (protecting the underlying tissue). Located in the external surface of the epidermis of skin, nails, claws, horns, etc.
• ii) Non-keratinized squamous epithelium: occurs at wet surfaces where abrasion occurs but there is no risk of drying, such as the cornea of the eye, the inner surface of the eyelid, the lining of the buccal cavity, the lower part of the pharynx, and the vagina. It helps in protection and conduction. The squamous cells of the surface layer do not lose their nucleus, and keratin is absent.

2) Stratified cuboidal epithelium

Has two layers of cuboidal cells and rests on the basement membrane. Found in the lining of the salivary duct, pancreatic duct and sweat duct. It also forms the epidermis (scales) of fishes and the tails of urodeles (e.g. salamanders). Its function is to protect from mechanical or chemical stress, and also help in conduction.

3) Stratified columnar epithelium

Has a layer of columnar cells over many layers of cuboidal cells. Found in the lining of the duct of the mammary gland and part of the urethra; it also covers the epiglottis. This helps in secretion and protection from mechanical and chemical stress.

Transitional epithelium (urothelium)

• There is no true basement membrane in the usual sense.
• Consists of 2 to 6 layers of cells. The cells of the deepest layer are columnar, the middle layer is polygonal, and the upper layer consists of large, globular cells.
• The cells are stretchable.
• Found in the renal pelvis, urinary bladder and ureter.
• They have a thick membrane with thin regions that fold when the bladder contracts.
• Helps in urine conduction and storage.

3) Compound specialized epithelium

The cuboidal and columnar epithelia are modified to perform functions other than protection. Types:

a) Ciliated epithelium

The cells are made of columnar or cuboidal epithelium, but their margins are ciliated. Due to the presence of cilia, it conducts different products along the tissue. Found in the Fallopian tube, vas deferens, and nasal chamber. Its function is conduction, receiving sensitivity (e.g. detecting smell) and filtration.

b) Sensory epithelium

• A modified form of simple columnar epithelium.
• Each cell is elongated and slender, with its inner end connected to nerve fibres and its outer end bearing sensory hairs.
• These cells receive external stimuli and transmit nerve impulses to the brain.
• Found in sense organs, receiving sensitivity and making the body aware of stimuli.
• Found in the retina, taste buds, nasal chamber, and organ of Corti.

c) Germinal epithelium

Modified cuboidal epithelium that has the power to divide and form gametes (spermatozoa, ova) by reproduction. In males, found in the seminiferous tubules of the testes; in females, found in the outer lining of the ovary.

d) Glandular epithelium

Modified form of cuboidal cells that carry out the synthesis and secretion of important chemical substances in the body, such as gastric juice and intestinal hormones.

Gland

A cell, tissue or organ which secretes a useful material is known as a gland. Glands develop from epithelial tissue.

Secretion: a gland may secrete protein (e.g. pancreas), lipid (e.g. adrenal gland), a mixture of carbohydrate and protein (e.g. salivary gland), or a mixture of many kinds of materials (e.g. mammary gland).

Types of Glands

A. Based on site of secretion

According to the site where the secretion is released, glands are of three types: exocrine, endocrine and heterocrine.

1. Exocrine glands: send their secretion through a duct to the site of action. These include salivary glands, gastric glands, intestinal glands, tear glands, oil glands, sweat glands and mammary glands.
2. Endocrine glands: lack ducts. They discharge their secretion, called a hormone, into the blood, which carries it to the target organ. These include the thyroid, parathyroid and pituitary glands.
3. Heterocrine glands: have both exocrine and endocrine parts. The exocrine part sends its secretion by a duct, and the endocrine part releases its product directly into the blood. Example: pancreas.

B. Based on mode of secretion

Glands are of three types:

1. Merocrine gland: the secretion leaves the cell through the cell membrane; the cell remains intact on the cell surface. Example: goblet cell, sweat gland.
2. Apocrine gland: the secretion is stored in the apical part of the cell, which bursts and releases the secretion along with some part of the cytoplasm. Example: mammary gland.
3. Holocrine gland: the whole cell fills with secretion, then breaks down and is shed as the secretion. Example: sebaceous gland (oil gland).

C. Based on the involvement of a single or many cells

According to the involvement of one or many cells, exocrine glands may be categorized as unicellular or multicellular.

1. Unicellular exocrine gland: consists of a single cell which represents a single gland. This includes:

   • Mucous cells (goblet cells or mucocytes): secrete mucus; common in the columnar epithelium of the intestine.
   • Serous cells: secrete a clear, watery fluid containing enzymes; present in gastric glands.

2. Multicellular glands: formed by proliferation and sinking in of the glandular epithelium into the underlying connective tissue. They consist of a secretory part and a duct, both formed by epidermal cells. Types:

   • i) Tubular glands: have tube-like secretory parts. May be simple or compound. A simple tubular gland may be straight, coiled or branched, and opens into a single duct. Example: the glands in the male frog's nuptial pad and the crypts of Lieberkuhn in the human ileum are simple, straight tubular glands. The sweat gland in mammals is a simple coiled tubular gland.
   • ii) Saccular glands: the secretory part is wide and spherical (the acinus or alveolus) at the inner end of the tubule or duct. May be branched or unbranched. Example: the cutaneous (mucous) gland of the frog is a simple, unbranched saccular gland; the oil gland, and the exocrine part of the pancreas, are further examples.