HISTOLOGICAL STRUCTURE OF HIDE AND SKIN
The hides and skins forms the base for the leather. Let it be sheep,goat,cow and buffalo,every hides and skins have its own structure and different end uses.The basical histological structure surely have some influence on the end properties of the leather.
Hence it becomes necessary to understand the basical histological structure of the skin or the hide to cater the end needs.Although the hides and skins vary considerably from one another,their basical anatomical structure is more or less similar.The hide or skin is sharply divided in to three different layers.
The various layers are,
2.Derma or Corium---connective tissue
3.Flesh or adipose layer----subcutaneous tissue
During the process of tanning ,all the other layers except the corium are removed step by step.The corium is only made into tanned leather.The corium generally constitutes the collagen fibres to a maximum extent.
Epidermis is the comparatively thinner portion,covering 1-2% of the entire skin thickness which is removed during the course of liming.This layer basically gets the energy from corium.This layer are generally divided into,
The various other things (appendages) which are present in the epidermis are hairs,nails,sebaceous glands(fat glands),sudoriferous glands(sweat glands) erectorpili muscle and elastin .There is a layer called “hyaline layer” which is present between the junction of the epidermis and the grain layer which actually forms the grain layer after finishing.
It is the most important layer which gets converted into imputrifiable leather and it constitutes of two layers namely..
1.Corium minor(grain layer)
2.Corium major(reticular layer).
The layer consists of the collagen fibres which are interwoven among themselves at an angle to the grain surface of the leather. The angle is known as “angle of weave”.It has an profound impact on the final strength characteristics of the leather.
It constitutes for about 20-25% of the entire thickness of the leather.It contains all the epidermal appendages of the skin.It also contains the papillae which are organs sense or touch.The arrangement of the hair follicles and the papillae gives the animal hides or skins basic and distinct grain pattern.
This is below the corium minor and constitutes for 75-90% of total thickness of hide or skin.The collagen fibres present are surrounded by the recticulin sheath and hence also it got the name reticular layer. The fibres present in this layer are more thick than the fibres in minor and they have interfibrillary matter (albumin,globulein etc..)deposited in between them.In specific skins and hides fat cells are present.This is the most important layer from the tanner’s point of view.
ADIPOSE OR FLESH LAYER:
A thin layer which is attached to the corium major is called the flesh (or) Adipose layer. It is a loose connective tissue lying between the skin and the body of the animal. This layer consists of adipose, elastin, blood vessels, fat cells and voluntary muscles and the entire layer is removed during the liming operation.
And as per the histology we come to know about the basical microscopical structure of the skin or hide, the angle of weave of the fibre bundles, the amount of elastin, fat cells, the epidermal appendages such as fat glands , sweat glands, thickness of the fibre bundles, the compactness of the fibre bundles (etc) which determine the final qualities or strength of the leather and hence the knowledge about the basic histology becomes necessary to a leather technologist.
ANATOMICAL STRUCTURE OF A COW HIDE:
There are some 26 breeds basically found in India and the Indian hides are smaller in size and lesser in might when compared to the foreign hides and most of the reasons for that accounts for the tropical climate of our country and its folders.
EPIDERMIS OF COW:
· It covers approximately 0.6 to 1.8% of the total thickness of the hide or skin.
· It generally doesnot contain any papillation which accounts for roughness of grain, but rarely found in heavy hides.
· The average hairs per square inch account to 13,000 and pigment is found in epidermis. Although the number of hairs are fixed during birth, the surface area undergoes a 7 fold increase during the total growth .
· The hair follicles are rooted from 0.4 to 1.5 mm and the longer hairs are generally more pigmented.
· The sweat glands are comparatively small than that of a calf skin but enclosed by a common factor and they are more concentrated in the neck area.
· Also a row of erectorpili muscles can be seen and since the number of hairs are high all the epidermal appendages such as but and sweat glands are also high in number.
GRAIN LAYER OF COW:
CORIUM MAJOR OF COW:
· It covers approximately 62 to 85% of the total thickness of hide or skin.
· The collagen fibre bundles are quite big and are compactly woven
· The angle of weave varies between 45° and 80° and they are much less in the neck and belly which makes them less compact.
· More reticulin is present in cow hide than calf skin but when compared to buffalo hides, it is denser.
· The thinners of fibre bundles are more pronounced in fallen hides.
FLESH LAYER OF COW:
· It covers approximately 2 to 5% thickness of the entire layer of the skin.
· The connectively fibre tissues runs nearly parallel to the surface.
· Somethimes contains the lymphoid wandering cells which when not properly cured, due to bacterial action breaks up and indicates the inflammatory condition of the tissue.
HISTOLOGICAL STRUCTURE OF A BUFFALO HIDE:
Buffalo hides are thicker than most meat cattle hides of the equal weight, but the thikness varies from 3.35 to 6.89 mm (excluding the flesh layer).The hides of the female buffalos skins are usually thicker than those of male buffalos of the equal weight.
EPIDERMIS OF BUFFALO HIDE:
· It accounts for 1.2 to 2.4% of the entire thickness of the hide. It is made of up epithelial cells.
· Two types of hairs viz, long or coarse, and short or fine are found in buffalo hides. The coarse hairs are rooted deeper a little below the junction of the grain and corium.
· The coarse hairs are lesser in number.
· The hair follicles in buffalo are straight and dip down 0.3 to 2.3 mm below the surface.
· The cells of the stratum germinatum contains pigment granules constituting of melanin and the epidermis only in highly pigmented.
· The epidermis of the buffalo hides are highly papillated contributing to the roughness of the grain surface and the papillation is more in shank and belly.
· The average number of hairs are 900 to 1200 per square inch and are highly scattered altogether giving rustic grain pattern.
GRAIN LAYER OF BUFFALO HIDE:
CORIUM MAJOR OF BUFFALO HIDE:
FLESH LAYER OF BUFFALO HIDE:
From the quality and thickness and the histological characters the buffalo hides may be the best suited for the manufacture of harness leathers
HISTOLOGICAL STURCTURE OF A GOAT SKIN:
There are12 outstanding breeds of goat in India. Although goat skins are available in various parts of the world Indian goat skins are usually considered the best among them.
The normal size of the skin varies from 23*12 inch and 33*18 inch.Skins of sizes below the former and above the latter are called “kids” and “heavies” respectively. In many aspects the structure of a goat skins can be considered in between a cow calf and a sheep skin.
EPIDERMIS OF A GOAT SKIN:
· The epidermis of a goat skin covers approximately 1 to 2.6 % of the total thickness of the skin.
· The average number of hairs varies from 8000 to 18,000 hairs per square inch. Two types of hair are present(coarse and fine hairs).
· Like the calf the goat has straight hair follicles and hence straight hairs. The hair follicles are quite deeply rooted.
· The glands and fat cells are very much less in number in goat skins. The erectorpili muscle is well developed and long in goat skin.
GRAIN LAYER OF A GOAT SKIN:
· The grain layer of the goat skin usually occupies approximately 24 to 54 % of the total thickness of the skin.
· In the grain layer the collagen fibre bundles are compactly woven. Because of lesser number of cellular components and fat glands the goat skin is comparatively compact than sheep skin.
· In the goat skins there is more elastin and its covers approximately 2/3 rd of the entire layer of the grain. The presence of larger amount of elastin tissue in perhaps one of the reasons why greater efforts are needed to open up the grain structure during pre-tanning process.
· In good quality goat skins corium and grain layer merges uniformly.
CORIUM LAYER OF GOAT SKIN:
· The corium proper of the goat skin occupies approximately 45 to 75 % of the total thickness of the skin.
· The collagen fibres present in this layer are firmer and fuller than the corresponding ones in the sheep skins. It is nearly equal to that of an cow calf hide.
· In goat skins a very low angle of weave is present.
· Fat cells and fat droplets are rarely found in the corium proper of goat skin.
· A considerable amount of reticular tissue is present in goat skin.
FLESH LAYER OF GOAT SKIN:
It roughly covers 1 to 2 % of the total thickness of the skin.
· A considerable amount of elastin tissue is present in the layer.
· Natural fats and fat cells are present in this layer.
HISTOLOGICAL STRUCTURE OF SHEEP SKINS:
There are many varieties and breeds of sheeps in India. The sheep is bred mainly for wool and meat and the skin is usually a by-product. In general longer wool = thinner skins. The place of origin has a greater influence on the nature of the skin. In size sheep skins are almost equal to goat skins but in texture and strength they are less compact and weaker.
EPIDERMIS OF SHEEP SEPIDERKIN:
GRAIN LAYER OF SHEEP SKIN:
CORIUM LAYER OF SHEEP SKIN:
FLESH LAYER OF SHEEP SKIN:
HAIR SHEEP SKIN:
a. Firmness with a smooth strong grain.
b. Low angulation of fibre bundles with fairly compact weave.
c. Full fibres with good tensile strength.