Integumentary system

Functions

The skin has a significant capacity for renewal and crucial roles for the normal functioning of the human body. It is an effective barrier against potential pathogens and protects against mechanical, chemical, osmotic, thermal and ultraviolet radiation damage (through melanin). The skin also takes part in a variety of biochemical synthetic processes, such as vitamin D production under the influence of ultraviolet radiation, but also the production of cytokines and growth factors.

Skin also has a major role in controlling body temperature by increasing or decreasing the blood flow through the cutaneous circulation, which in turn affects the magnitude of heat loss. Sweating also assists this process. The skin is also a major sensory organ, containing a large number of nerve terminals for touch, temperature, pain and other stimuli. The skin greatly assists in locomotion and manipulation due to its good frictional properties given by its texture and elasticity.

Layers

The skin is anatomically organized as follows, from superficial to deeper layers:

• Epidermis
  
  • Stratum basale
    
  • Stratum spinosum
    
  • Stratum granulosum
    
  • Stratum lucidum
    
  • Stratum corneum
    

(Memorise these layers with the mnemonic: "British and Spanish Grannies Love Cornflakes", see video below)

• Dermis
  
  • Papillary dermis
    
  • Reticular dermis
    

If you want to find out more about the anatomy of the skin, have a look at this article.

Hairs

Hairs are filamentous cornified structures which grow out of the skin and cover most of the body surface. Several areas of the body like the palms, soles, flexor surface of the digits and specific parts of the reproductive organs are devoid of hairs. Hairs are important in sensing, thermoregulation and protection against injury and solar radiation.

There are two major types of hairs: vellus and terminal. Vellus hairs do not project beyond their follicles in some of the areas, however, they are short and narrow and cover most of the surface of the body. This hair type is most easily observed on children and adult women and is colloquially known as “peach fuzz”. Terminal hairs are longer, thicker and more heavily pigmented. They are mostly observed on males but also in the axillary and pubic regions of both sexes.

Hair follicle

The hair follicle is the sac containing the hair, out of which it grows. It is actually a downgrowth of the dermis and contiguous with the epithelium. The hair follicles go through a cyclic activity of hair growth and loss.

Hair bulb

The hair bulb is the lowest expanded extremity of the hair follicle that fits like a cap over the dermal hair papilla, enclosing it. The dermal hair papilla is a cluster of mesenchymal cells giving rise to several capillaries, which form a capillary loop. The hair bulb generates the hair and its inner root sheath.

The bulb consists of two parts: germinal matrix and the upper bulb. The germinal matrix consists of pluripotent keratinocytes, which gives rise to the upper bulb. As the cells from the matrix migrate apically and differentiate further, they form several structures and layers. From the interior to the exterior, these include:

• Hair shaft
  
  • Medulla
    
  • Cortex
    
  • Cuticle
    
• Inner root sheath
  
  • Cuticle
    
  • Huxley’s layer
    
  • Henle’s layer
    
• Outer root sheath
  
• Glassy membrane (basement membrane of hair follicle)
  

You can image these layers as tree rings in a cross-section of the hair follicle since they are concentric cylinders. Each layer completely encloses the previous one situated more internally.

Hair cycle and growth

The growth, rest and shedding of hair follicles occurs in cyclical stages of variable duration. During the growing (anagen) phase, the follicles produce an entire hair shaft from the dividing cells of the hair bulb.

During the shedding (catagen) phase, the epithelial cells in the hair bulb and the and outer root sheath die in a regulated fashion (apoptosis). Differentiation of the hair shaft also stops, and the bottom of it becomes sealed into a structure called the club.

During the resting (telogen) phase, the hair follicles lie dormant. No differentiation or apoptosis happens.  Shedding or loss of club hair happens when the cycle is re-initiated and the newly growing hair follicle pushes the old one out. The average rate of hair growth is between 0.2 and 0.44 mm in 24 hours.

Are you struggling with all the parts of the integumentary system? Check out our quizzes and learn all those parts in an easy and fun way. 

Nails

Nails are homologous to the stratum corneum of the epidermis and contain a variety of minerals, such as calcium. They consist of compacted and layered keratin-filled squames (scales). The arrangement and cohesion of the squames are responsible for the hardness of nails. A nail consists of: the nail plate, nail folds, nail matrix, nail bed and hyponychium.

Nail plate

The nail plate is a rectangular and convex structure embedded within the nail folds. It originates from the nail matrices, found at the base of the nails. The nail plate is completely free distally to the onychodermal band (distal margin of the nail bed).

Nail folds

The nail folds are the borders of the nail plate, located laterally and proximally, which are continuous around the nail plate. The cuticle (eponychium) is an extension of the proximal nail fold located on the dorsal aspect of the nail plate, overlying the root of the nail.

Nail matrix

The nail matrix is the structure out of which the nail plate grows. Cornified cells from the matrix are gradually extruded distally to form the nail plate.

Nail bed

The nail bed extends between the lunula (crescent shaped, white area of the nail bed) and the hyponychium (area under the free edge of the nail plate). The distal margin of the nail bed is called the onychodermal band. There is a perfect match between the nail bed and plate, forming a seal, which prevents microbial invasion and debris collection. The nail bed consists of two layers: the epidermis and dermis. The dermis is directly attached to the periosteum of the distal phalanx and it is richly vascularized. It also contains numerous sensory nerve endings, such as Merkel endings and Meissner’s corpuscles.

Sweat glands

Sweat glands are small, tubular structures located in the skin. They are exocrine glands, hence they secrete substances on the epithelial surface via ducts. The glands produce sweat, which is important for thermoregulation. There are two types of sweat glands, eccrine and apocrine and each one produces a different type of sweat.

Eccrine sweat glands

The majority of sweat glands are eccrine. They are long, unbranched, tubular structures with a highly coiled secretory portion situated deep in the dermis. A narrower duct emerges from the gland and it opens via a pore on the skin surface.

Apocrine sweat glands

These are large glands specifically located in the axillae, perianal region, nipples, periumbilical region, prepuce, scrotum, mons pubis, labia minora, nail bed, penis and clitoris.

Similar to eccrine glands, apocrine glands also consist of a secretory coil. However, the duct emerging from the gland opens inside the pilary canal above the duct of the sebaceous gland or directly on the surface of the skin.

Sebaceous glands

Sebaceous glands are small saccular structures located in the dermis, which cover most of the body. They consist of a cluster of secretory acini, which is continued by a duct which opens into the dermal pilary canal of the hair follicle.

The ducts can also open directly on the surface of the skin, as seen on the lips and buccal mucosa. Sebaceous glands secrete sebum, which is an oily and fatty secretion. Sebum is crucial in the epidermal barrier and the skin’s immune system.