Veterinary Histology UFF
Department of Morphology - Biomedic Institute
LaBEc - Laboratory of Cellular and Extracellular Biomorphology
Veterinary Histology Atlas
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Integumentary System

• The skin covers the surface of the body and is formed by the:

- Epithelial portion of ectodermal origin, the epidermis

- Connective portion of endodermal origin, the dermis

• Depending on the thickness of the epidermis, thick and thin skin can be distinguished:
- Thick Skin: paw pads
- Thin skin: rest of the body, in some species the body is covered by thick skin.



• Below this and continuous to the dermis, we find the hypodermis or subcutaneous cellular tissue that is not part of the skin, it only serves to bind it to the subjacent organs.
- Hypodermis: loose connective tissue that can contain many adipocytes that form the panniculus adiposus.

• The skin is one of the largest organs with up to 16% of the body weight and accomplishing uncountable functions
- It protects the organism against water loss and abrasion thanks to the stratum corneum of the epidermis
- It constantly receives information on the surrounding environment and sends this to the CNS through sensitive nervous ends.

- It cooperates with the thermal regulation of the body through blood vessels, glands and adipous tissue
- The melanin (pigment produced and accumulated in the epidermis) protects against ultraviolet rays

- In the skin, vitamin D3 is formed by the sun’s ultraviolet rays on the precursors synthesized in the organism.

>> The junction between the epidermis and dermis is irregular, the dermis presents projections, the dermal papillae, that fit in the incurvatures of the epidermis, increasing the cohesion between these two layers.



Thin skin Thick Skin  


• Formed by keratinized stratified squamous epithelium

Cell types

• The most abundant, each cell that forms the epithelium is a keratinocyte

• Produce the melanin pigment

Langerhans cells
• Antigen-presenting cells, located throughout the entire epidermis, between the keratinocytes.

Merkel’s cells
• Mechanoreceptors located in the deepest region of the epidermis, supported by the basal lamina and bound to the keratinocytes through the desmosomes


Layers of the Epidermis (Thick skin)



Stratum Basale
• Formed by prismatic or cuboidal basophilic cells that rest upon the basement membrane that separates the epidermis from the dermis
• It is rich in epidermal stem-cells and is also called the Germinal layer
• It presents intense mitotic activity and is responsible along with the stratum spinosum for the constant renewal of the epidermis.
It contains intermediate keratin filaments in its cytoplasm that become more numerous as the cell advances towards the surface.



Stratum Spinosum
• Formed by cuboidal or slightly squamous cells with a central nucleus, a cytoplasm with short expansions that contain bundles of keratin filaments(tonofilaments)
• The tonofilaments and desmosomes have an important role in maintaining the cohesion between the cells of the epidermis and in resisting abrasions.
• There are also keratinocyte stem-cells, and mitosis occurs in the stratum basale and at a lower rate in the stratum spinosum.



Stratum Granulosum
• With only 3-5 lines of polygonal flattened cells with a central nucleus and a cytoplasm rich in basophilic granules called keratohyalin granules.
• The keratohyalin granules are not surrounded by a membrane
• They present lamellar granules, visualized only under electronic microscopy, that contain lamellar discs formed by lipid bilayers and are surrounded by a membrane. These granules fuse themselves with the plasma membrane and release their contents into the intercellular space of the granular layer, where the lipid material is deposited and will contribute to the formation of a barrier against the penetration of substances and turning the skin impermeable to water and avoiding dehydration of the organism.
• The waterproofing agent appeared in reptiles first and is important under an evolutionary point of view so to allow life outside the water.

Stratum Lucidum
• The most evident layer of the thick skin, formed by a thin layer of flattened, eosinophilic and translucid cells whose nuclei and cytoplasmic organelles were digested by enzymes from the lysosomes and disappeared.
• Its cytoplasm presents numerous compacted keratin filaments and it is also possible to see desmosomes between the cells.





Stratum Corneum
• It has a variable thickness, formed by flattened, dead and nonnucleated cells
• The cytoplasm of these cells is filled with keratin
• The tonofilaments agglutinate along with the matrix formed by the keratohyalin granules. At this stage of differentiation, the keratinocytes are transformed into lifeless plates and continuously shed.



Layers of the Epidermis (Thin skin): the epidermis is simple, frequently lacking the stratum granulosum and lucidum, and present a much smaller stratum corneum.


• The color of the skin results from various factors of which the most important are: content of melanin and carotene, and the amount of capillaries in the dermis and the color of the blood in theses capillaries.
• They present a globular cytoplasm that originates extensions that penetrate the incurvatures of cells of the stratum basale and stratum spinosum and transfer melanin granules to the cells of these strata.
• The melanocytes do not form demosomes with the keratinocytes, but attach themselves to the basement membrane through hemidesmosomes.
• Once formed, the melanin granules migrate through the extensions of the melanocytes and are injected through yet unclear mechanisms into the cytoplasm of the keratinocytes that function as deposits of melanin and contain a greater amount of this pigment than the melanocytes.
• The melanin granules fuse with the lysosomes of the keratinocytes and that is why the more superficial cells of the epidermis do not contain melanin.
• In the epithelial cells the melanin granules are in a supranuclear position, offering maximum protection to the DNA against the hazardous effects of sun radiation.
• The genetic load influences the rate of activity of the melanocytes since almost everyone has the same amount.
• The darkening of the skin due to solar exposition occurs initially due to the darkening of the pre-existent melanin and the acceleration of the melanin transference to the keratinocytes. On a second stage, the synthesis of melanin is increased.




• Melanin is a dark-brown pigment produced by the melanocytes that are found in the junction between the dermis and epidermis or between the keratinocytes of the stratum basale of the epidermis.


• It is the connective tissue where the epidermis rests upon, and that binds the skin to the subcutaneous tissue or hypodermis.
The dermis presents a varying thickness and its outer surface is irregular, presenting saliencies, the dermal papillae, that accompany the corresponding invaginations of the epidermis.
The papillae increase the contact area of the dermis with the epidermis strengthening the link between these two layers, the papillae are most frequently found in the zones subject to aggressions and abrasions.
The dermis is composed by two layers with slightly distinct limits : The superficial Papillary Dermis and the deeper Reticular Dermis.
• The papillary layer is thin, composed of loose connective tissue that forms the dermal papillae. In this layer, special collagen fibrils were described, that insert themselves on one side into the basement membrane and on the other side penetrate deeply into the dermis, forming the dense connective tissue.
• The reticular layer is thick, composed of dense connective tissue.
• Both layers contain many fibers of the elastic system, partly responsible for the elasticity of the skin.
• Asides the blood and lymph vessels, and the nerves, the following structures derived from the epidermis are found in the dermis: hair follicles, sebaceous glands and sweat glands.


Sensory receptors of the Skin

Pacinian Corpuscles
• They capture special vibratory and tactile stimuli
• They are formed by a nervous fiber whose unmyelinated end portion is surrounded by many layers that correspond to various supporting cells.
• The end-layer is capable of sensing the application of pressure, which is transmitted to the other layers and sent to the corresponding nervous centers.

Merkel Discs
• They detect specially pressure and tactile sensibility.
• An afferent fiber is normally branched with many end-discs of this kind of nervous branching.
• These discs are comprised in a specialized cell whose distal surface is fixed on epidermal cells through an extension of its protoplasma.
• This way the movements of pressure and traction on the epidermis result in a stimulus.

Free Nerve Ends
• Sensitive to mechanical, thermal and specially pain stimuli
• Formed by a branched axon surrounded by Schwann cells, and these are both surrounded by a basement membrane.

>> Glabrous (hairless) skin exclusive receptors

Meissner’s Corpuscles
• Sensitive to touch stimuli
• They are in the hairless saliencies of the skin.
• Formed by a myelinic axon whose end-branches are entwined with accessory cells.

Krause End-Bulbs
• Cold-sensing thermal receptors
• They are formed by a nervous fiber whose ends have a club shape.
• They are placed in the boundary region of the skin and mucous membranes ( e.g.: around the lips and genitals)

Krause receptors
Ruffini receptors
Merkel discs
Touch and pressure
Vater-Pacini receptors
Meissner receptors
Free nervous ends
Mainly pain


• The hair follicle has a terminal dilation , the Hair Bulb, that contains the Dermal Papilla
• Covering the Dermal Papilla are the cells that form the root of the hair.
• The central cells of the hair root:
- Produce large, vacuolized and poorly keratinized cells that form the Hair Medulla
- Following this and in a lateral position, the cells that originate the Hair Cortex are seen.
- More peripheral epithelial cells originate the internal and external sheaths.

- External Sheath: continuous with the epithelium of the epidermis
- Internal Sheath: disappears once it reaches the region where the sebaceous glands release their products.
• Between the Hair follicle and the connective tissue around it is the Glassy Membrane.
• The connective tissue that surrounds the follicle is thicker and forms the Connective Sheath of the Hair Follicle
• Each one is an independent gland(compound alveolar tubule)
• Stretched out obliquely and inserted on one side in the Connective Sheath of the Hair Follicle and on the other in the Papillary Layer of the Dermis are the arrector pili muscles, whose contractions pulls the hair into a more vertical position, bristling it.
• The presence of melanocytes gives the hair its coloring and they are placed between the papilla and the epithelium of the hair root, and provide melanin to the cells of the hair root and cortex.



Hair Follicle Hair Trio  
Hair Trio
• Formed by the Hair Follicle, the arrector pili muscle and the sebaceous gland.





Sebaceous Gland

• Placed in the Dermis and its ducts generally open in the Hair Follicle
• However, in certain areas( lips, glans penis and labia minora of the vagina) the ducts open directly onto the surface of the skin. The glabrous skin of the palms and soles do not have sebaceous glands.
• The sebaceous glands are alveolar and generally a series of alveoli debouch into a short duct.
• The alveoli are formed by an outer layer of squamous epithelial cells that rest upon a basement membrane. These cells proliferate and differentiate into round cells that accumulate in their cytoplasm their secreting product of lipid nature.
• The nuclei gradually condense and disappear. The more central cells of the alveolus die and disintegrate, forming a sebaceous product.
• The sebaceous gland is an example of a holocrine gland, since its secretion results from the death of cells.




Sweat Gland

• The merocrine sweat glands are very numerous and throughout the skin, except in certain regions such as the glans penis.
• They are simple coiled tubular glands whose ducts open at the surface of the skin.
• The secreting portion is found in the dermis, the secreting cells are pyramidal and between them and the basement membrane are the myoepithelial cells that help release its secretion.
The duct of the gland opens at the surface of the skin and follows a helical course as it crosses the epidermis.
• It is formed by stratified cuboidal epithelium(two layers of cells) that rest on a basement membrane.



Accessory Gls.
Male Reprod.
Female Reprod.
Birds Histology