Changes

===Origin of the epidermis===

===Origin of the epidermis===

Following neurulation, the precursor of the epidermis is formed from the cells covering the embryo.  This then becomes 2 layers, the outer forming the '''periderm''' and the inner forming the '''basal layer''' or ''stratum basale''.  A further layer is formed from the basal layer called the '''spinous layer''' or ''stratum spinosum''.  The basal and spinous layers are termed the ''Malphigian layer'', the cells of which then divide to produce the '''granular layer''' or ''stratum granulosum''.  As the cells of the granular layer differentiate and keratinise, they form the outer '''cornified layer''' or ''stratum corneum''.

Following neurulation, the precursor of the epidermis is formed from the cells covering the embryo - the '''ectoderm'''.  This then becomes 2 layers, the outer forming the '''periderm''' and the inner forming the '''basal layer''' or ''stratum basale''.  A further layer is formed from the basal layer called the '''spinous layer''' or ''stratum spinosum''.  The basal and spinous layers are termed the ''Malphigian layer'', the cells of which then divide to produce the '''granular layer''' or ''stratum granulosum''.  As the cells of the granular layer differentiate and keratinise, they form the outer '''cornified layer''' or ''stratum corneum''.

==Structure and Function==

==Structure and Function==

*'''Stratum spinosum'''

*'''Stratum spinosum'''

Above the stratum basale is the ''stratum spinosum''.  Spinous cells are large polygonal cells with prominent '''desmosomal intercellular filaments'''.  When cells move into this layer, they shrink and move apart.  Keratinisation begins in the stratum spinosum and continues in the stratum granulosum.

Above the stratum basale is the ''stratum spinosum''.  Spinous cells are large polygonal cells with prominent '''desmosomal intercellular filaments''', which act like bridges between the cells.  When cells move into this layer, they shrink and move apart.  Keratinisation begins in the stratum spinosum and continues in the stratum granulosum.

*'''Stratum Granulosum'''

*'''Stratum Granulosum'''

===Dermis===

===Dermis===

The epidermis interfaces with the underlying dermis at the '''basement membrane zone (BMZ)'''.  The binding includes anchoring junctions called ''hemidesmosomes'' and ''focal adhesions''.  The former comprise ECM proteins such as '''laminin-5''' and '''collagen type IV''' molecules, which form the lamina densa part of the BMZ.  '''Collagen type VII''' is important in forming anchoring fibrils between the dermis and the basal lamina.

The epidermis interfaces with the underlying dermis at the '''basement membrane zone (BMZ)'''.  The binding includes anchoring junctions called ''hemidesmosomes'' and ''focal adhesions''.  The former comprise extra cellular matrix (ECM) proteins such as '''laminin-5''' and '''collagen type IV''' molecules, which form the lamina densa part of the BMZ, while the lamina lucida is composed of '''Collagen type V'''.  '''Collagen type VII''' is important in forming anchoring fibrils between the dermis and the basal lamina.

Rete ridges are epidermal projections into the dermis in areas of mechanical loading such as the nasal planum and footpads.

Rete ridges are epidermal projections into the dermis in areas of mechanical loading such as the nasal planum and footpads.

The '''dermis''' consists of dense fibrous tissue composed of '''collagen type I and III''', with some elastin fibres and provides a supporting mattress for the carriage of blood vessels, lymphatic vessels, nerves and sensory receptors, '''hair follicles''', sebaceous and sweat glands, with circulating white cells.

The '''dermis''' consists of dense fibrous tissue composed of '''collagen type I, III and V''', with some elastin fibres and provides a supporting mattress for the carriage of blood vessels, lymphatic vessels, nerves and sensory receptors, '''hair follicles''', sebaceous and sweat glands, with circulating white cells.

===Glands===

===Glands===

*'''Sweat glands'''

*'''Sweat glands'''

Sweat is considered to be the product of [[General Pathology - Necrosis#Apoptosis|apoptosis]] releasing cell contents into the gland lumen, together with secretion where exocytosis, microapocrine blebbing, transcellular and perhaps paracellular processes involved in varying degrees.  Sweat is composed of secretions and cellular debris and includes inorganic ions, water, immunoglobulins, amino acids and waste products including urea and lactic acid.  Equine sweat is rich in proteins including glycoproteins, albumin and gammaglobulins.

Sweat is considered to be the product of [[General Pathology - Necrosis#Apoptosis|apoptosis]] releasing cell contents into the gland lumen, together with secretion where exocytosis, microapocrine blebbing, transcellular and perhaps paracellular processes involved in varying degrees.  Sweat is composed of secretions and cellular debris and includes inorganic ions, water, immunoglobulins, amino acids and waste products including urea and lactic acid.  Equine sweat is rich in proteins including glycoproteins, albumin and gammaglobulins, which cause the sweat to froth and the horse is said to "lather up".

Sweat is degraded by bacteria leading to odour.  Dogs and cats produce very little sweat and use panting to lower body temperature.

Sweat is degraded by bacteria leading to odour.  Dogs and cats produce very little sweat and use panting to lower body temperature.

*'''Sebaceous glands'''

*'''Sebaceous glands'''

One or more sebaceous glands surround each '''hair follicle''' at a point about 1/3 of its length from the surface.  Each gland is composed of numerous ''acini'' attached to a common '''excretory duct''' through which they discharge their secretions onto the hair shaft and epidermis.  In the skin and eyelids such glands release the cell contents together with secretory products with holocrine secretion.  This secretion involves [[General Pathology - Necrosis#Apoptosis|apoptosis]] in which the secretory products and cell debris are released into the gland lumen.  The secretory products vary from species to species and include various fatty acids, diglycerides, cholesterol, diesters, wax esters and giant-ring lactones.

One or more sebaceous glands surround each '''hair follicle''' at a point about 1/3 of its length beneath the skin's surface.  Each gland is composed of numerous ''acini'' attached to a common '''excretory duct''' through which they discharge their secretions ('''sebum''') onto the hair shaft and epidermis.  In the skin and eyelids such glands release the cell contents together with secretory products with holocrine secretion.  This secretion involves [[General Pathology - Necrosis#Apoptosis|apoptosis]] in which the secretory products and cell debris are released into the gland lumen.  The secretory products vary from species to species and include various fatty acids, diglycerides, cholesterol, diesters, wax esters and giant-ring lactones.

The function of sebum is that of an emolient of the hair and skin, preventing excessive dryness.  It may combine with '''sweat''' to form an emulsion within the stratum corneum and a form of cement around the hair pores preventing bacterial invasion of the epidermis.

The function of sebum is that of an emolient of the hair and skin, preventing excessive dryness.  It may combine with '''sweat''' to form an emulsion within the stratum corneum and a form of cement around the hair pores preventing bacterial invasion of the epidermis.  Sebum is also released from the glands during piloerection, as the action of the ''arrector pili'' muscle squeezes the gland as it raises the hair.

 

In sheep, sebum is often referred to as "'''lanolin'''".  This is the cause of many skin allergies in humans.

*'''Circumanal and supracaudal tail glands'''

*'''Circumanal and supracaudal tail glands'''

*'''Submental and circumoral glands'''

*'''Submental and circumoral glands'''

Large glands in the [[Lips - Anatomy & Physiology|lip]] region of cats; presumed to have a territory marking role.  Circumoral glands are enlarged integumentary glands bordering on the mucocutaneous junctions of the mouth or oral margins associated with greeting and related behaviours, especially in scuiriomorph rodents.

Large glands in the [[Lips - Anatomy & Physiology|lip]] region of cats and some rodents; presumed to have a territory marking role.  Circumoral glands are enlarged integumentary glands bordering on the mucocutaneous junctions of the mouth or oral margins associated with greeting and related behaviours, especially in scuiriomorph rodents.

*'''Inguinal glands'''

*'''Inguinal glands'''

Sexually-related scent glands prominent in ungulates and lagomorphs.

Sexually-related scent glands prominent in ungulates and lagomorphs.  They are located at the base of the udders to allow offspring to locate the teats of their mother.

*'''Anal sacs'''

*'''Anal sacs'''