The medulla usually possesses varying numbers of circular bodies called Hassall’s, or thymic, corpuscles (HC). The corpuscles are large concentric layers of flattened type VI epithelioreticular cells (Ep). They stain readily with eosin and can be distinguished easily with low magnification, as in top figure and lower left (arrows). The center of a corpuscle, particularly a large one, may show evidence of keratinization and appear somewhat amorphous.
The thymus gland remains as a large structure until the time of puberty. At that time, regressive changes occur that result in a significant reduction in the amount of thymic tissue. The young thymus is highly cellular and contains a minimum of adipose tissue. On the other hand, in the older thymus, much adipose tissue is present between the lobules. With continued involution, adipose cells are found even within the thymic tissue itself. Occasional plasma cells may be present in the periphery of the thymic cortex of the involuting thymus gland.
KEY BV, blood vessels C, cortex Cap, capsule Ep, epithelioreticular cells HC, Hassall’s corpuscles L, lobule M, medulla T, trabeculae arrowheads, nuclei of type VI epithelioreticular cells of Hassall’s corpuscles arrows, Hassall’s corpuscles
OVERVIEW OF THE INTEGUMENTARY SYSTEM / 488 LAYERS OF THE SKIN / 489 Epidermis / 489 Dermis / 491 CELLS OF THE EPIDERMIS / 493 Keratinocytes / 494 Melanocytes / 496 Langerhans’ Cells / 499 Merkel’s Cells / 500 STRUCTURES OF SKIN / 501 Nerve Supply / 501 Epidermal Skin Appendages / 503 Folder 15.1 Clinical Correlation: Cancers of Epidermal Origin / 492 Folder 15.2 Functional Considerations: Skin Color / 499 Folder 15.3 Functional Considerations: Hair Growth and Hair Characteristics / 504 Folder 15.4 Functional Considerations: The Role of Sebum / 505 Folder 15.5 Clinical Correlation: Sweating and Disease / 507 Folder 15.6 Clinical Correlation: Skin Repair / 512
The skin (cutis, integument) and its derivatives constitute the integumentary system. The skin forms the external covering of the body and is its largest organ, constituting 15% to 20% of its total mass. The skin consists of two main layers:  The epidermis is composed of a keratinized stratified squamous epithelium that grows continuously but maintains its normal thickness by the process of desquamation. Epidermis is derived from ectoderm.
 The dermis is composed of a dense connective tissue that imparts mechanical support, strength, and thickness to the skin. Dermis is derived from mesoderm.
The hypodermis contains variable amounts of adipose tissue arranged into lobules separated by connective tissue septa. It lies deep to the dermis and is equivalent to the subcutaneous fascia described in gross anatomy. In well-nourished individuals and in individuals living in cold climates, the adipose tissue can be quite thick.
The epidermal derivatives of the skin (epithelial skin appendages) include the following structures and integumentary products:
The integumentary system performs essential functions related to its external surface location.
Skin and its derivatives constitute a complex organ composed of many different cell types. The diversity of these cells and their ability to work together provide many functions that allow the individual to cope with the external environment. Major functions of the skin include the following:  It acts as a barrier that protects against physical, chemical, and biologic agents in the external environment (i.e., mechanical barrier, permeability barrier, ultraviolet barrier).