Skin

Skin (Integument) Overview

Objectives

• Identify skin as an organ

• Identify different parts of the epidermis, dermis, and hypodermis

• Identify sebaceous and sweat glands

• Identify different layers of the hair follicle

• Discuss structure and functions of different parts of the epidermis and dermis

• Discuss structure and functions of various cell types in the skin

• Discuss the role of skin in temperature regulation, ion-water balance, calcium homeostasis, sensory appreciation, and protection against external agents and ultraviolet rays

• Discuss the process of melanin synthesis and how this process differs among races

• Discuss possible clinical considerations associated with skin, e.g., Albinism, psoriasis, acne, skin cancer

Functions of the Skin

• Protection against:

  • External agents

  • Ultraviolet rays

  • Water loss (desiccation)

• Regulation of:

  • Temperature

  • Ions-water balance

  • Calcium homeostasis (via vitamin D synthesis in the sebaceous gland)

• Sensory appreciation

• Fat storage

• Clinical examination of the skin reveals ectoparasites and internal disease processes linked to endocrine and nutritional disorders.

General Organization of the Skin

• The skin consists of two main components:

  • Epidermis

  • Dermis

• Beneath the epidermis lies the hypodermis (subcutaneous tissue), which is a site for subcutaneous injections, consisting of loose connective tissue (CT) filled with fat and not considered part of the skin.

Epidermis

• Thickness varies among areas of the body; typically thinner in hairy skin.

• Layers of Epidermis:

  • Stratum (S) basale (germinativum)

  • Stratum spinosum (basale and spinosum combined termed the malpighian layer)

  • Stratum granulosum

  • Stratum lucidum

  • Stratum corneum

Characteristics of Epidermal Layers

• Stratum Basale:

  • Contains mitotic cells that continuously divide and replenish other epidermal layers.

• Stratum Spinosum:

  • Contains cells with thick keratin bundles called tonofilaments, terminating at desmosomes.

  • In thick skin, the spinosum layer is thicker with more tonofilaments and desmosomes, providing protection against friction and pressure.

• Stratum Granulosum:

  • Contains non-membrane bound, basophilic keratohyalin granules that integrate into keratin.

  • Contains membrane-bound lamellar granules that release contents in intercellular space, serving as a seal between skin cells.

• Stratum Lucidum:

  • Absent in thin skin.

• Stratum Corneum:

  • The outermost layer formed by the shedding of horny cells.

Epidermal Cell Types

• Keratinocytes:

  • Synthesize keratin and undergo continuous replacement through mitotic activity in the malpighian layer.

• Melanocytes:

  • Originating from neural crest cells, they reside in the malpighian layer and hair follicles.

  • Possess dendritic processes that interdigitate with keratinocytes.

  • Synthesize melanin, facilitated by the enzyme tyrosinase, converting tyrosine through several reactions (dopa, dopaquinone) to melanin.

  • Melanin granules are then transferred to keratinocytes. Notably, keratinocytes do not synthesize melanin, but they contain it.

  • The quantity of melanocytes is consistent among races; however, their melanin synthesis capability varies, influencing skin coloration and susceptibility to skin cancer (e.g., Albinos lack tyrosinase, hence cannot synthesize melanin).

• Langerhans Cells:

  • Function as antigen-presenting cells, originating from monocytes, presenting antigens to T lymphocytes for immune responses in the skin.

• Merkel’s Cells:

  • Function as sensory mechanoreceptors located in thick skin, richly innervated by free nerve endings.

Dermis

• Integral to many leather goods.

• Comprised of two layers:

  • Papillary Layer:

  • Contains loose connective tissue with all CT cell types and a robust blood supply.

  • Constitutes the majority of dermal papillae.

  • Reticular Layer:

  • Contains dense irregular connective tissue featuring a substantial number of type I collagen fibers, fewer cells compared to the papillary layer, smooth muscle, nerve fibers (vasomotor, pilomotor, sudomotor), and various receptors (touch, heat, cold, pressure).

Hypodermis (Subcutaneous Tissue)

• Composed of loose connective tissue filled with fat cells, large blood vessels, nerves, and skin glands (particularly prevalent in thick skin like foot pads).

• Not a part of the skin, this layer serves as a site for subcutaneous injections.

Blood Supply

• Deep or Subcutaneous Plexus:

  • Located in the hypodermis, formed by large blood vessels.

• Middle Cutaneous Plexus:

  • Associated with hairs and glands in the dermis.

• Superficial or Subpapillary Plexus:

  • Supplies blood to the papillary layer and epidermis.

• Superficial Arterio-Venous Anastomoses:

  • Thermoregulatory structures found in the papillary layer of certain species, absent in others (helps explain why pig skin is a model for studying human skin diseases).

Skin Glands

Sebaceous Glands

• Scattered across the body except in the foot pads.

• Typically associated with hair follicles, with exceptions in the lip, penis, and vulva.

• Comprise alveoli without a visible lumen; base cells divide while apex cells die and release contents (holocrine secretion).

• Secretion (sebum) lubricates hair shafts, protects skin from drying, reduces water loss, contains vitamin D precursors.

• Notable locations: infraorbital, inguinal, interdigital regions in sheep; base of goat horns; anal sacs of cats; prepuce and circumanal regions of dogs.

Sweat Glands

• Located in the lower dermis or hypodermis.

• Two types:

  • Eccrine (Merocrine):

  • Develops independently from hair follicles, opening via a long duct to the skin surface.

  • Comprises secretory cells that are cuboidal to columnar, encircling a narrow lumen.

  • Active throughout life, secreting a watery solution for heat and ion regulation.

  • Apocrine:

  • Associated with hair follicles, draining above the sebaceous gland opening.

  • Characterized by low cuboidal cells with or without apical protrusions surrounding a wider lumen.

  • Produces viscous secretions, becoming active at puberty.

• Myoepithelial Cells:

  • Surround secretory cells of sweat glands.

• Species Variations:

  • Absent in rats, rabbits, and birds; poorly developed in dogs and pigs; predominant in horses for thermoregulation.

  • Apocrine glands secrete significantly in horses, while secretion is minimal in other species, playing roles in temperature regulation and serving as sex attractants.

Hair

• Composed of thread-like epidermal structures.

• Distribution: Even in horses, cows; grouped into beds in pigs, dogs, cats.

• Components of a complete hair:

  • Hair Shaft: Free part above skin, consisting of three layers: medulla, cortex, and cuticle.

  • Hair Root: Portion beneath skin.

  • Hair Follicle: Sheath covering the root, including:

  • CT sheath

  • Glassy membrane

  • External root sheath (continuation of stratum germinativum)

  • Internal root sheath (Henle’s layer, Huxley’s layer with trichohyalin granules, cuticle)

• Hair Bulb: Expanded terminal part of the hair follicle where hair is rooted, indented by a dermal papilla (housing blood vessels).

• Arrector Pili:

  • Smooth muscle bundle attached to the CT sheath of hair follicles, responsible for hair elevation in response to cold or emotional stimuli.

Types of Hairs

• Cover Hair:

  • Most common.

• Wool Hair:

  • Found in fleece of sheep and goats.

• Tactile Hairs:

  • Long, thick, and contain blood sinus between CT sheath layers. Highly innervated (e.g., whiskers in carnivores).

Hair Growth and Replacement

• Hair formation arises from the germinal matrix covering the dermal papilla, characterized by cyclic growth phases:

  • Anagen: Active growth phase.

  • Catagen: Involution phase.

  • Telogen: Resting phase.

• Hair color is attributed to melanocytes within the germinal matrix.