Changes

 

==Development==

==Development==

===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==

===Epidermis===

===Epidermis===

The epidermis is a stratified squamous epithelium and is composed of 4 cell layers anchored to a basal lamina of connective tissue.  Keratinocytes migrate through the epidermis from the basal layer.  This migration begins in the stratum basale, then moves up through the stratum spinosum, stratum granulosum and the stratum corneum.

The epidermis is a stratified squamous epithelium and is composed of 4 cell layers anchored to a basal lamina of connective tissue.  Keratinocytes migrate through the epidermis from the basal layer.  This migration begins in the stratum basale, then moves up through the stratum spinosum, stratum granulosum and the stratum corneum.

*'''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'''

*'''Langerhans' cells'''

*'''Langerhans' cells'''

These are dentritic cells derived from bone marrow.  They act as ''antigen presenting cells'' in the epidermis as part of the mononuclear phagocyte immune system.  Langerhans' cells express MHC I and II molecules and immunoglubulin Fc receptors and are a key feature of '''delayed hypersensitivity responses.'''

These are dentritic cells derived from [[Bone Marrow - Anatomy & Physiology|bone marrow]].  They act as [[Peyer's Patches - Anatomy & Physiology#M cells|antigen presenting cells]] in the epidermis as part of the mononuclear phagocyte immune system.  Langerhans' cells express MHC I and II molecules and immunoglubulin Fc receptors and are a key feature of '''delayed hypersensitivity responses.'''

*'''Melanocytes'''

*'''Melanocytes'''

*'''Merkel cells'''

*'''Merkel cells'''

These are part of the sensory apparatus of the skin and are located within the '''stratum basale''' of the epidermis where they act as slow-adapting maechanoreceptors.  They are usually closely associated with the afferent myelinated nerve fibres, arranged in a plate-like structure with the base of the Merkel cell, together forming a '''''Merkel's corpuscle'''''.

These are part of the sensory apparatus of the skin and are located within the '''stratum basale''' of the epidermis where they act as slow-adapting mechanoreceptors.  They are usually closely associated with the afferent myelinated nerve fibres, arranged in a plate-like structure with the base of the Merkel cell, together forming a '''''Merkel's corpuscle'''''.

===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 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 glands'''

Sweat is considered to be the product of [[Necrosis - Pathology#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".

 

 

 

 

 

 

 

Large glands in the [[Lips|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.

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

'''Rabbits''' have scent glands on the chin which are used to mark territory by excreting droplets onto structures in the environment. Male rabbits scent-mark more intensively than female rabbits, and dominant individuals more than subdominant animals. This is correlated with larger scent glands in dominant males.

*'''Mammary Glands'''

*'''Inguinal glands'''

These are greatly modified, enlarged sweat glands and are present in both male and female animals. Depending on species, they extend from the axillae to the groin, associated with teats and a complex array of sinuses and ducts.  Age, pregnancy and [[Endocrine System - Anatomy & Physiology - The Endocrine System|endocrine]] factors have an important role in determining glandular activity.

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

*'''Sebaceous Glands'''

'''Rabbits''' have pocket-like inguinal glands lateral to the genital openings. They are used to mark kits and cage surfaces and may contain dried yellow-brown sebaceous material. Does will attack young which have not been scent-marked, even if they are in the same breeding group.

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.

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.

Sebaceous (and modified apocrine in the dog) glands lie within the wall of the anal sacs, with a duct carrying material to the anal opening.  The foul smelling secretion may act as a territorial marker.  Secretions are expressed on defecation.  The secretions are composed of fatty and serous materials with cellular debris.

 

 

 

 

 

Pads are covered with a very thickened epidermis with no hair follicles.  They are hyperpigmented and have atrichial sweat glands opening to the surface under the influence of epinephrine.

 

There is little dermis and the hypodermis is composed of collagenous and elastic fibres mixed with adipose tissue to form a digital cushion that provides protection from mechanical impact.

 

The covering of the nose is very thickened with rete ridges providing anchoring.  The surface has a cobblestone appearance in dogs and cats.  Hair and sebaceous glands are absent, atrichial glands are present.

 

 

Cats can unsheath their claws by muscle action and retract them with an elastic ligament on the dorsal aspect of the middle and distal phalanges.

 

Depending on the pigmentation of the claw, the dermis may or may not be visible.  Care must be taken when clipping claws to avoid laceration of the 'quick' and subsequent haemorrhage.

 

 

The palpebral margin is thicker than the remainder of the eyelid and cilia grow from the margin.  There are various glands in the region, including '''Moll's glands''' that are modified sweat glands, associated with the cilia.  '''Meibomian glands, Zeis glands''' associated with the cilia and the '''lacrimal glands''' together produce the tear film.

==Immunology==

==Immunology==