Lecture 24

Hair Follicles

Internal epithelial root sheath

Is produced by matrix around the periphery of the bulb. It becomes thinner and the cells fuse just before it disintegrates below the opening of the sebaceous gland and gets mixed up with sebum.

External epithelial root sheath

Several layers of cells resembling those of the stratum spinosum of the epidermis with which it is continuous in the upper region of the hair follicle (continuous with the dermal papilla). It is separated from the dermal root sheath by a thick basement membrane (the glassy membrane) corresponding to the epidermal basement membrane.

Dermal papilla

The conical region of highly vascular connective tissue directly under the hair matrix.

Hair matrix

Cells that make up the hair bulb and cover the dermal papilla are hair matrix cells which correspond to stratum basale of epidermis. Proliferation of cells here results in hair growth. Hair matrix also contains melanocytes that contribute to hair colouration.

Dermal root (connective tissue) sheath

Dermal connective tissue cells and fibres arranged circularly and longitudinally around the hair follicle. In sinus hairs (large tactile hairs, e.g. whiskers), the dermal sheath is split in two concentric layers with an intervening blood-filled sinus. The sinus is traversed by trabeculae containing many nerve endings.

Hair Implantation

The keratinocytes in the cuticle of the hair follicle are in the opposite direction to The keratinised cells found in the internal epithelial root sheath. The result is a secure implantation of hair within the follicle = interlocking mechanism

Follicle Location

They typically sit in the in the papillary and reticular layers but not uncommonly found in the hypobiosis region

Musculus Arrector Pili

The arrector pili muscle is a fascicle of smooth muscle cells attached to the dermal sheath and angled upwards towards the epidermis. It is anchored in the papillary dermis by elastic fibres. The muscle is under autonomic nervous control and its contraction causes elevation of hair as in the bristling of hair on a dog's back (goose pimples in humans).

The red muscle pulling on the follicle

Elevation of the hair creates pockets of air which significantly improves insulation and helps maintain body temperature.

In several species, e.g. cats, this also adds to the apparent size of the animal which helps repel potential threats. These muscles may also act to assist in the emptying of sebaceous glands

Primary Hair Follicles

Larger, longer hair follicles that can produce both primary and secondary hairs and tactile hairs. They possess both sebaceous and sweat glands and an arrector pili muscle.

Secondary Hair Follicles

Smaller hair follicles with that may have a sebaceous gland but lack sweat gland and arrector pili muscle (but may have one in association with the primary follicle its shared with. Produce secondary or under hairs which lack a medulla. Secondary follicles may be found in isolation but are usually associated with primary follicles

Simple Follicles

One follicle with a single hair fibre emerging from the canal

Compound Follicles

Multiple follicles that have hairs emerging from the canal

Hair Clusters

A hair cluster is a variable number of hair follicles, whether primary or secondary, that are isolated by dermal connective tissue fibres from other similar follicles

Hair Types

Guard Hair/Primary Hair

The stiff, straight fibres that are regularly arranged in broad tracts creating the smooth appearing topcoat ****of animals; this regular arrangement functions in the runoff of rain. The shaft in primary hair fibres possess all three cuticular, cortical and medullary regions

Fine Hair/Secondary Hair

The fine wavy hair fibres that comprise the undercoat. These fibres often lack a medulla. Wool fibres are of this type.

Tactile Hair/Sinus Hair

A large (primary) hair in the head region which is highly adapted for tactile sensitivity and perception of touch and space, e.g. whiskers (vibrisae) of cats.

These arise from large simple follicles that have a blood-filled sinus between the two layers of dermal root sheaths. Tactile follicles are abundantly supplied by nerve endings and sometimes have skeletal muscle attachments which allow voluntary control.

1. epidermis

2. sebaceous gland

3. hair root

4. int. epith. root sheath

5. ext. epith. root sheath

6. blood sinus

7. blood sinus

8. dermal root sheath

9. nerve ending

10. trabecula

1. dermal root sheath

2. sebaceous gland

3. root of hair

4. nerve fibre

5. blood sinus

6. dermal papilla

Wool

Wool = textile fibre produced from sheep or other wool producing animals

Fleece = the woollen coat of a sheep or goat

3 types of hair fibres:

1. Kemp → the normal hair/primary hair. Contains cuticle, cortex and medulla but dyes poorly

2. True Wool → the undercoat. Lacks medulla. Very fine, often elastic and crimped

3. Heterotype → an intermediate type of hair fibre where the medulla region is not always continuous

The ratio of secondary to primary follicles (ns/np), therefore, is of industrial importance

Sheep have regular hair on their face, ears and distal limbs and everywhere else is pretty much dense wool growth

Has a primary trio follicular arrangement

= Has three primary follicles with many secondary follicles surrounding them

Hair Growth

Occurs in 3 phases

1. Anagen

   The growing phase = the matrix cells proliferate → new cells rise → pushes old cells towards the outside → pushes the hair out of the follicle.

   Melanogenesis and pigmentation occurs at the same time giving rise to hair colouration.

2. Catagen

   Regressive phase = Mitosis and melanogenesis cease → follicle shrinks to about a third of its length. It also starts moving towards the epidermis becoming more superficial. Dermal papilla shrinks and separates from hair germ cells.

3. Telogen

   Resting phase = hair shaft has a clubbed base surrounded by a layer of germ cells and remains anchored to papilla by a thin filament. The keratinocytes at the base of the follicle have fully differentiated

   Hair remains in canal until pushed out by new growth. This gives rise to the physiologic shedding of hair

4. Renewed Anagen

   New cycle of hair growth

Hair growth is controlled by:

• Photoperiod

• Ambient temperature

• Nutrition

• Hormones → oestrogen, testosterone, adrenal steroids and thyroid hormones

Moulting

A large number of hair follicles are affected at once

Typically under influence of photoperiod and temperature

Mainly in spring and autumn is when moulting occurs

Horses, some breeds of dogs and cats and other animals can have different summer and winter coats

Specialised Skin

Anal Sacs and Glands

Anal sacs are invaginations of skin which occurs between the external and internal anal sphincters → forms diverticula

Typically seen at 4 and 8 o’clock

The inside of the sac is heavily keratinised epithelium

Each sac is associated with a modified sweat gland which produces a gross substance

The duct that connects the sac to the epidermis of the anus is narrow and can be obstructed easily → results in anal sac impaction, infection and/or abscessation, a relatively common clinical presentation

Circumanal Glands

These are modified sebaceous glands

Found in the anal region, groin and prepuce

Important because they can are sites of tumours of dogs

Eyelids

Also known as palpebrae

It is a fold of musculocutaneous tissue with a layer of skin on the outside and mucosa on the inside

Contains a mucutaneous junction at the palpebral margin

These marginal hairs are referred to as ciliae (pl) or eyelashes

Of note here, are the tarsal or Meibomian glands. These are multilobular sebaceous glands, best developed in the upper eyelid, with a central duct. The duct opens at the eyelid margin and could sometimes get blocked causing entrapment of secretions and a cystic swelling

External Ear

Has the pinna (auricle) of the ear and the ear canal

The pinna consists of two layers of skin with an elastic cartilage in between them. The skin here is haired and has both sebaceous and sweat glands

• Rich in blood vessels

The skin covering the external ear canal is haired and contains sebaceous glands as well as modified sweat glands known as ceruminous glands. Cerumen or ear wax is a mixture of glandular secretions (notably ceruminous glands) and desquamated keratinocytes of the ear canal which has a protective role

Nostrils

A thickened epidermis

While horses have regular skin surrounding their nostrils, dogs, cats and small ruminants have a planum nasale (nasal plate); pigs have a planum rostrale (rostral plate); and large ruminants possess a planum nasolabiale (nasolabial plate).

May or may not have skin glands/hair

May be smooth or have surface grooves or ridges

Foot Pads

The epidermis has a stratum lucidum = usually sits on top of the stratum granulosum (not seen in the image well)

Thick epidermis

The digital cushion has a mix of fat and connective tissue present to absorb shock

The dermis has merocrine sweat glands

Claws

They are a form of keratinised epithelium that covers the ungular process

This epidermis is supported by a thick dermis that merges with the underlying periosteum

The claw consists of a claw plate (dorsal ridge and walls) and sole - the dorsal ridge being the top angle, the walls, one on each side, and the sole as the bottom side form a triangle in cross-section. The walls of the claw plate meet at a high dorsal ridge where both stratum spinosum and stratum corneum are the thickest.

Horns

The skin that produces horn is highly specialised. Both dermis and epidermis differ from typical skin and the keratin of horn is a type of hard keratin which is also called horn (the term horn refers to both “the horns of an animal” as well as the material that makes up horns or hooves)

The epithelium produces 2 main types of horn depending on where the stratum basale lies; whether over the dermal papillae or in the valleys between them (epidermal pegs).

• Tubular horn is produced by the epithelium covering the dermal papillae. Horn tubules are similar to hair in structure however show more complexity with cells that are highly ordered.

• Intertubular (lamellar) horn is produced by the epithelium lying between the papillae. This horn cements the tubular horns into a solid coherent structure.

Chestnuts and Ergots

Seen in horses, mules, donkeys etc

Chestnuts always found medially

Ergots typically found on the palmar/plantar faces

The equine chestnut and ergot have a thick epidermis composed of tubular and intertubular horn interdigitating with long dermal papillae

Hair, arrector pili muscles and glands are absent in both of these structures

Avian Integuement

Keratinised stratified squamous epithelium → thinner than mammals

Extensive lipogenesis in epidermis

Completely aglandular (lacks glands)

(uropygial gland near tail is analogous to sebaceous glands)

Feathers are analogous to hair in mammals

Wetting the skin can lead to hypothermia of the bird