Epithelial tissue

Animal tissues take their first form when the blastula cells arising from the fertilized ovum, differentiate into three germ layers: the ectoderm, mesoderm, and endoderm. Through further cell differentiation or histogenesis, group of cells grow into more specialized units to form organs.
Animal cells are classified into four types based on fundamental tissues i.e epithelial, nervous, muscular and connective tissues.

EPITHELIAL TISSUES
They are divided into covering and glandular epithelia.

a. Covering Epithelia
i. Their structural features are as follows
· Epithelial cells generally have large nuclei, clear outlines, and a large amount of of granular protoplasm
· Their cells are attached to the underlying tissues by a basement membrane, made a network of white, wavy, non-elastic collagen fibres
· The junctions between epithelial cells tend to be strongly held together by desmosomes
· Epithelial tissue consisting of one or a few layers of cells, is found on the internal and external surfaces of organs
ii. Their functions are as follows
· protect the underlying tissues against mechanical injury
· protect against water loss and dehydration, infection of microorganisms and also radiant energy
· are involved in the physiological processes that take place across surfaces including respiratory gas exchange, excretion, elimination of waste and absorption
· Some of the modify to form special receptors for stimuli
iii. There are classified into two ways
Based on arrangement of cells,
· Simple epithelium, arranged in one/single layer
· Stratified/Compound epithelium, arranged in more than one layer
· Pseudo-stratified epithelium, Cells look like arranged in layers but every one is attached to the basement membrane
Based on their cell shapes,
· Squamous epithelium, Cells are flattened like scales or tiles
· Cuboidal epithelium, Cells more or less like cubes
· Columnar epithelium, Cells look like pillars, height is longer than their base
· Transitional epithelium, Cells can change shape when stretched
iv. There are eight types of covering epithelia
· Simple --à squamous ep. Is found in the outer layer of Bowman capsule,
endothelium of blood vessels and alveolar walls
--à cuboidal ep. Is found in the collecting ducts and tubules of
nephron
--à columnar ep. Is found in lining the innermost layer of intestine
and stomach
· Stratified --à squamous ep. Is found in the epidermis of skin and lining the
innermost layer of oesophagus
--à cuboidal ep. is found in the excretory ducts of sweat gland
--à columnar ep. Is found in the secretory ducts of the mammary
glands
· Pseudo stratified epithelium,is found in the innermost layer of trachea, bronchi and bronchioles
· Transitional epithelium, is found in the wall of urinary bladder

b. Glandular Epithelia
i. Glandular epithelia are gland cells derived from epithelia and can secrete liquid containing mucus, hormone or enzymes
ii. There are two types of glandular epithelia i.e exocrine glands and endocrine glands
iii. Exocrine glands
· they have ducts that secrete substances out
· there are little capillaries in them
· they produce liquid with proteins or enzymes
· their formation is by invagination of epithelia
iv. Endocrine glands
· they have no duct and the secretion is directly emptied into the blood capillaries around them
· a lot of capillaries are found within
· they produce hormones
· their formation is by detachment from epithelia as shown below
· examples are thyroid and adrenal gland

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Clegg, C. J. and D. G. Mackean, 1994, Advanced Biology, Principles and Application, John Murray Pub.
Encarta
Kooi, Foo Yuen, 2003, STPM Biology, Volume 1, Penerbitan Pelangi SDN. BHD. Malaysia
Kwan, Lam Peng and Eric Y. K. Lam, Federal Publications, An Imprint of Times Media, Singapore