Integument

Skin or integument (L.,integumentum = covering) forms the boundary layer separating the organism from the outside world. It is a complex organ comprised of a tremendous variety of tissues and cells: lining epithelium, glandular epithelium, seve ral types of connective tissue, smooth muscle, bloodvessels, sensory receptors and more. It is typically among the largest organs in most craniates (e.g.,approximately 15% of body weight in humans; up to 40% or more in cetaceans) and serves an almost innumerable number of functions. The following is an incomplete list:

1) protection from external environm ent: abrasions, UV light, etc

.2) passive immunological barrier to pathogens

3) participates in water, ion and gas exchange

4. sensory reception (typically mechanoreceptors and thermoreceptors)

5) glandular secretions: protection (e.g.,mucus,) defense (e.g., toxins), signaling (e.g.,pheromones) , nutritive (e.g., mammary), cooling (sweat)

6). vitamin D synthesis - vitamin D functions in depositing salts into bone; deficiency (rickets) produces misshapen bones

7). coloration in defense and communication and protection

8) thermoregulation

an inner reticular layer. The former is loose connective tiss ue and the latter is irregular dense connective tissue (except in aquatic mammals where the collagen is regularly arranged into plies). The dermis of mammals is extensively vascularized for thermo regulation. Hair follicles and glands (sebaceous and sweat) project into the dermis. Associated with the hairs are smooth muscle arrector pilli. The dermis provides the membranous precursor for the membr ane (dermal) bones of the skull (dermatocranium) and pectoral girdle (clavicle). The hypodermis is relatively hypertrophied in mammals, presumably as an adaptation for endothermy (insulation and energy storage).Extending from the surface and embedded within the dermis are multicellular glands. There are two major types: sebaceous and sweat . Sebaceous glands (L.sebum, tallow) are branched alveolar [sac-like with obvious lumen] glands that produce an oily/waxy secretion (sebum ). The ducts typically open into hair follicles but are also found without hairs atspecial sites such as the eyelid (tarsal glands) and in the external acoustical meatus.

Sweat glands are typically of limited distribut ion (the ubiquitous sweat glands of humans are atypical) and absent