Integument Microanatomy Pt 2

Langerhans Cells

• Langerhans cells are located within the dermis and are derived from bone marrow monocytes.
• They are not often seen with H&E staining but can be seen with immunochemistry or electron microscopy.

Epidermal Derivatives

General Info

• Epidermal derivatives include hair follicles, compound hair follicles, sinus (tactile) hairs, sebaceous glands, apocrine (sweat) glands, udders and mammary glands, hooves, claws, nails, footpads, anal sacs, and circumanal glands (hepatoid glands).

Compound Follicles

• Compound follicles contain clusters of several hair follicles that merge at the level of the sebaceous gland and emerge through one orifice.
• They are most common in carnivores.
• Compound follicles usually have one primary follicle with an apocrine sweat gland and several secondary hair follicles.

Arrector Pili Muscles

• Arrector pili muscles are smooth muscles attached to primary hair follicles that play an insulation role.
• Their contraction causes hair to stand up (goosebumps).

Hair

• Hair functions include insulation, camouflage, social display, sense/protection, and sex recognition.
• Hair is produced by hair follicles.
• Hair follicles are invaginations of the epidermis that contain an internal root sheath, external/outer root sheath, dermal papilla, and hair matrix cells.
• The inner root sheath is composed of a few layers of squamous cells and a cuticle.
• The external/outer root sheath is an external glassy membrane due to the thickened basal lamina, while the external root sheath is continuous with the epidermis.
• The dermal papilla is composed of connective tissue that carries blood supply to the hair.
• Hair matrix cells are comparable to the stratum basale epidermal layer, as it is the layer of cells that are constantly dividing and producing hair via the help of keratinocytes.
• Hair is composed of the medulla, cortex, and cuticle.
• The medulla is composed of loose cuboidal cells with areas of air.
• The cortex is composed of dense compact keratinized cells.
• The cuticle is composed of a single layer of flat keratinized cells.
• The hair shaft is located above the skin surface, while the hair root is located within the follicle and ends with a bulb.
• The hair cycle involves the following stages: anagen, catagen, and telogen.
• During the anagen period, hair bulb cells are mitotically active, meaning that the hair is growing.
• The catagen period is the "regressive stage" where metabolic activity slows down and the base of the follicle migrates towards the surface.
• The telogen period is the "resting" or "quiescent phase" where hair growth stops, and the base of the bulb is at the level of the sebaceous canal.
• New hair grows below the telogen follicle, and the old hair shaft gets shed.
• The hair cycle is controlled by daylight, ambient temperature, nutrition, and hormones (estrogen, testosterone, adrenal steroids, and thyroid hormones).

Hair Follicle Types

• Hair follicle types include primary and secondary hair follicles.
• Primary hair follicles are large in diameter, rooted deep in the dermis, and contain sebaceous glands, arrector pili muscles, and sweat glands.
• Examples of primary hair follicles include primary or guard hairs.
• Secondary hair follicles are small in diameter, rooted near the surface, may or may not contain sebaceous glands, and lack both apocrine sweat glands and arrector pili muscles.
• Examples of secondary hair follicles include secondary or under hairs.

Whiskers

• Whiskers are also referred to as "sinus hairs" which are tactile hairs composed of a very large single follicle.
• A blood-filled sinus is located between the inner and outer dermal root sheath.
• Whiskers contain nerve bundles penetrating the sheath.
• Whiskers are attached to skeletal muscle, which allows for voluntary movement.

Sebaceous Glands

• Sebaceous glands are located in the dermis, where they produce sebum.
• Sebum is a mixture of lipid and cell debris.
• Sebaceous glands partake in holocrine secretion, and aid in antibacterial and waterproofing.
• Their ducts empty into a follicle.
• They can be simple, branched, or compound glands.
• Specialized sebaceous glands include:
  • Supracaudal glands (in dogs)
  • Circumanal/hepatoid glands
  • Mental organs/glands (in cats)
  • Horn glands (in buck goats)
  • Preputial glands (in the smegma of horses)
  • Tarsal (meibomian) glands (in eyelids)
  • Uropygial glands (preen glands) (in birds)

Apocrine Sweat Glands

• Apocrine sweat glands are secreted by apical budding/pinches.
• Epithelial cells have apical secretory caps.
• Simple coiled tubular glands open into distal hair follicles.
• Contractile myoepithelial cells help express product.
• In domestic animals, the apocrine sweat gland is located throughout most of the skin, where they function to communicate via attraction or markers.
• Specialized apocrine sweat glands can be found in:
  • Mammary glands
  • Ciliary glands (of Moll) (in eyelid, making tears form)
  • Apocrine glands of anal sacs
  • Ceruminous glands (ear wax)
  • Mental organs and planum rostrale (of pigs)

Merocrine/Eccrine Sweat Glands

• Merocrine means that the method of secretion involves excretion via exocytosis.
• These glands open directly onto the skin surface rather than the hair follicle.
• Merocrine/eccrine sweat glands are minor in domestic animals except for the footpad of dogs, where they aid in thermoregulation and electrolyte balance.
• This function is completed via the secreted fluid onto the skin's surface whenever body temperature rises.

Mammary Gland

• Mammary glands contain tubuloalveolar glands that are connected by ducts and are separated into lobules by connective tissue septae and interstitium.
• Mammary glands are composed of clusters of alveoli forming lobules, and ducts that drain into sinuses.
• Mammary glands have smooth muscle in the teat (sphincter).
• The height of the epithelium relates to the activity of the gland.

Hooves and Claws

• Hooves and claws are skin modifications composed of a variation of stratum corneum, and are supported by a highly vascularized dermis.
• Hooves and claws lack stratum granulosum and stratum lucidum.
• The equine hoof is their distal phalanx encased in heavily keratinized epidermis (horn).
• In the equine hoof, the skin angles internally at the coronary band/groove, causing distal growth of the stratum corneum epidermal layer.
• The equine hoof has a white line that joins the sole with the wall.
• The corium (dermis) of the equine hoof is a highly vascular and innervated connective tissue (dermis).
• The laminar corium-primary dermal laminae (500-600) interdigitates with the primary epidermal laminae.
• There is papillae at the coronary corium, sole corium, and distal laminae (terminal papillae).
• The epidermis of the equine hoof (insensitive laminae) is avascular.
• The dermis in the equine hoof is the coronary corium, and laminar corium.
• The keratinized hoof wall has 3 layers: stratum externum (tectorium), stratum medium, and stratum Internum (lamellatum).
• Stratum externum (tectorium) is a continuation of perioplic epidermis, referred to as a "glaze".
• Stratum medium is composed of the majority of the wall and is produced from the coronary epidermis.
• Stratum Internum (lamellatum) is the primary epidermal laminae (~600), and is the insensitive laminae that interdigitates with the primary dermal laminae.
• Sensitive laminae is composed of laminar corium with primary dermal laminae.
• The horn is composed of keratin arranged into parallel microscopic tubules (like hair shafts) and intertubular horn.