Integumentary System

Organs An organ is made of two or more tissues – Almost all have one or more connective tissue layers around the outside An organ serves a specific overall function to regulate homeostasis in some way An organ works with other organs in an organ system

Integumentary System The integumentary system consists of your skin (integument) and skin appendages – Can you name all four? We will use this as a model organ and model organ system – Note the different types of tissue found throughout the organ

Figure 5.1 Epidermis Hair shaft Dermis Reticular layer Papillary layer Hypodermis (superficial fascia) Dermal papillae Pore Subpapillary vascular plexus Appendages of skin Eccrine sweat gland Arrector pili muscle Sebaceous (oil) gland Hair follicle Hair root Nervous structures Sensory nerve fiber Pacinian corpuscle Hair follicle receptor (root hair plexus) Cutaneous vascular plexus Adipose tissue

Skin Layers The skin consists of two layers: – The epidermis (outer part) comprised mostly of epithelial tissue – The dermis (inner part) comprised mostly of connective tissue and blood vessels The skin has a layer below called the hypodermis which is most of the body’s fat reserves

Epidermis Structure Keratinized stratified squamous epithelium Cells of epidermis – Keratinocytes Produce fibrous protein keratin – Melanocytes Produce pigment melanin – White blood cells – Tactile (Merkel) cells—touch receptors

Figure 5.2b Melanocyte Melanin granule Tactile (Merkel) cell Sensory nerve ending Epidermal dendritic cell Dermis Keratinocytes Stratum corneum Most superficial layer; 20–30 layers of dead cells represented only by flat membranous sacs filled with keratin. Glycolipids in extracellular space. Stratum granulosum Three to five layers of flattened cells, organelles deteriorating; cytoplasm full of lamellated granules (release lipids) and keratohyaline granules. Stratum spinosum Several layers of keratinocytes unified by desmosomes. Cells contain thick bundles of intermediate filaments made of pre-keratin. Stratum basale Deepest epidermal layer; one row of actively mitotic stem cells; some newly formed cells become part of the more superficial layers. See occasional melanocytes and epidermal dendritic cells. Desmosomes (b)

Skin Stem Cells Keratinocyte stem cells (basal cells) live in stratum basale, close to the blood supply of the dermis – They are constantly undergoing mitosis Some of the cells become adult keratinocytes and slowly shrivel up and die and become tough – Keratin is also the protein in your hair and nails

Purpose of Melanin Melanin is a pigment that absorbs UV radiation – UV radiation can alter DNA when it hits the nucleus of a cell Melanin coats the nucleus of keratinocytes to prevent mutations – Mutations can potentially cause a cell to become cancerous, replicating out of control

Side Note: There is no scientific concept of race Melanin expression is largely genetic and a response to increased sunlight in the native regions – Some ethnic groups (families) express more than others as a result, but these do not translate into any notable difference in any other ability Race is a social construct and is therefore real, but only in our minds – Invented in the 1800s by nonscientists and supported by pseudoscience

Dermis Structure Strong, flexible connective tissue Cells include fibroblasts, macrophages, and occasionally mast cells and white blood cells Two layers: Papillary Reticular

Papillary and Reticular Papillary layer is loose fibrous connective tissue (also called areolar connective tissue) – Contains lots of blood vessels to nourish epidermis and sweat glands Reticular layer is composed of dense fibrous connective tissue – Irregular arrangement of fibers to give toughness in all directions

Appendages of Skin Hair – Densely packed keratinocytes Fingernails – Also densely packed keratinocytes Sebaceous glands – Secrete an oil called sebum starting at puberty Sweat glands – Two varieties: eccrine and apocrine sweat glands

Hair Structure Hair grows out of a hair follicle containing more keratinocyte stem cells The bulb (bottom area) of the follicle has a hair root plexus wrapped around it – Nerve that detects any vibrations in the hair – This is a very useful adaptation for detecting a mosquito landing on your skin Long, dark hairs are called terminal hair and contain more melanin, while thin, fine hairs are called vellus hair

Eccrine vs. Apocrine Eccrine sweat is mostly water and salt – Eccrine glands are all over the body, concentrated on the hands, feet and scalp Apocrine sweat also contains oil (sebum), proteins, and pheromones – Confined to axillary and anogenital regions – Activate at puberty, same as sebaceous glands

Thermoregulation The skin is the key to maintaining body temperature – Heating: arrector pili (smooth muscles) raise goosebumps – Cooling: sweat is produced to evaporate for cooling Blood is flushed to skin for cooling or pulled away to conserve heat Controlled by hypothalamus in brain

Negative Feedback Loop

Homeostasis Homeostasis is maintained by negative feedback loops – Positive feedback loops are self-amplifying and dangerous if not controlled Thermoregulation is just one example of these negative feedback loops

Thermoregulation

Skin Damage Skin damage poses health risks, primarily infection, dehydration and lack of temperature control – Burns that only go through part or all of the epidermis are called partial- thickness burns Includes sunburns (1 st degree) and burns that blister (2 nd degree) – Burns that go through the dermis are called full-thickness burns and are hideous to look at 3 rd degree burns typically turn unpleasant colors

That’s all folks! Bring your questions on Wednesday! Friday: Exam #1! – Talk to me ASAP if you need a different exam time!