Body Membranes: Types and Classification

• Body membranes cover surfaces, line body cavities, and create protective sheets around organs.

Classification of Body Membranes

• Membranes are divided into epithelial and connective tissue types based on tissue makeup.
• Epithelial membranes include cutaneous, mucous, and serous membranes.
• Connective tissue membranes are represented by synovial membranes.
• Skin (cutaneous membrane) provides outer protection.

Epithelial Membranes

• Epithelial membranes are covering and lining membranes, including cutaneous, mucous, and serous membranes.
• All epithelial membranes contain an epithelial layer overlying connective tissue.
• These membranes function as simple organs.

Cutaneous Membrane

• The cutaneous membrane is the skin and is composed of the epidermis and dermis.
• The epidermis is made of stratified squamous epithelium.
• The dermis consists of dense irregular fibrous connective tissue.
• The cutaneous membrane is dry and exposed to air, unlike other epithelial membranes.

Mucous Membranes

• Mucous membranes (mucosa) consist of epithelium resting on loose areolar connective tissue, known as the lamina propria.
• Mucosae line body cavities that open to the exterior, such as respiratory, digestive, urinary, and reproductive tracts.
• Mucosal epithelium varies by location; most contain stratified squamous epithelium (e.g., mouth, esophagus) or simple columnar epithelium (e.g., digestive tract).
• Mucosae are kept moist by secretions or urine.
• Mucosal epithelium is often adapted for absorption or secretion.
• Mucosae in the respiratory and digestive tracts secrete protective, lubricating mucus, while urinary tract mucosa does not.

Serous Membranes

• Serous membranes (serosa) are made of simple squamous epithelium on a thin layer of areolar connective tissue.
• Serous membranes line ventral body cavity compartments closed to the exterior.
• Serous membranes exist in pairs with parietal and visceral layers.
• The parietal layer lines a part of the ventral body cavity wall.
• The visceral layer covers the outside of organs in that cavity.
• The parietal serosa lines the cavity walls but is always fused to the wall.
• The visceral serosa clings to the organ's external surface.
• Serous layers are separated by a clear serous fluid secreted by both membranes.
• Serous fluid creates a potential space between the membranes.
• The lubricating serous fluid enables organs to slide easily across cavity walls, important for mobile organs like the heart and lungs.
• The peritoneum lines the abdominal cavity and its organs.
• The pleurae surround the lungs in the thorax.
• The pericardia surround the heart in the thorax.

Connective Tissue Membranes

• Synovial membranes are made of loose areolar connective tissue without epithelial cells.
• Synovial membranes line fibrous capsules around joints, providing a smooth surface and secreting lubricating fluid.
• They also line bursae and tendon sheaths, cushioning organs during muscle activity.

Integumentary System (Skin) Functions

• The skin and skin appendages are collectively known as the integumentary system.
• The integumentary system insulates, cushions, and protects against mechanical, chemical, thermal, and microbe damage.
• The skin prevents water loss from the body surface.
• Capillary networks and sweat glands in skin help regulate heat loss.
• Skin acts as an excretory system by losing urea, salts, and water through sweat.
• Skin produces proteins important to immunity and synthesizes vitamin D when modified cholesterol molecules are exposed to sunlight.
• Acidic skin secretions, the acid mantle, protect against bacterial invasion.

Structure of the Skin

• The skin comprises the epidermis and the dermis.
• The epidermis is the outer layer of stratified squamous epithelium.
• The dermis is the underlying layer of dense irregular connective tissue.
• Burns separate the epidermis and dermis, allowing fluid to accumulate and form blisters.
• Deep to the dermis is the subcutaneous tissue/hypodermis, made of adipose tissue.
• This layer anchors skin to organs, provides nutrient storage, acts as a shock absorber, and insulates deeper tissues.

Epidermis

• Most epidermal cells are keratinocytes that produce keratin.
• Keratinization is the process of making the epidermis a tough protective layer using keratin.
• Keratinocytes are connected by desmosomes.
• The epidermis is avascular, lacking its own blood supply.
• The epidermis includes the stratum basale, stratum spinosum, stratum granulosum, stratum lucidum, and stratum corneum.
• The deepest epidermal layer, the stratum basale, connects to the dermis and contains nourished epidermal cells.
• Stem cells in the stratum basale divide and produce new cells (stratum germinativum).
• Newly-produced cells either become epidermal cells or maintain the stem cell population.
• Daughter cells destined to become epidermal cells are pushed upward, away from nutrition sources, and become part of epidermal layers closer to the skin surface, becoming flatter and increasingly keratinized
• As cells leave the stratum granulosum, they die and form the stratum lucidum.
• The stratum lucidum is in hairless, thick-skinned areas like palms and soles and isn't present in all skin regions.
• Accumulation of keratin, glycolipid secretion, and distance from blood supply causes stratum lucidum and superficial epidermal cells to die.
• The stratum corneum is the outermost layer, 20-30 cell layers thick and accounts for three-quarters of epidermal thickness.
• The stratum corneum is composed of shingle-like dead cells filled with keratin, called cornified or horny cells.
• The stratum corneum provides a durable overcoat that protects, prevents water loss, and resists biological, chemical, and physical assaults.
• Cells from the stratum corneum shed slowly as dandruff and are replaced by division of stratum basale cells.
• The epidermis is replaced every 25-45 days.
• Melanin, produced by melanocytes in the stratum basale, ranges in color from yellow to brown to black.
• Freckles and moles are melanin concentrated in one spot.
• Epidermal dendritic cells are sentries that alert and activate immune system cells to bacterial or viral threats.
• Merkel cells, associated with sensory nerve endings, act as touch receptors called Merkel discs.

Dermis

• It's a strong, stretchy envelope that helps to bind the body together.
• The dermis consists of the papillary and reticular areas.
• Both regions consist of areolar and dense irregular connective tissue, respectively.
• The dermis' thickness varies by body location; thicker on palms/soles and thinner on eyelids.
• The Papillary Layer is the superficial dermal region with peglike projections called dermal papillae, which indent the epidermis.
• The Dermal papillae contain capillary loops that furnish nutrients to the epidermis and other pain/touch receptors.
• Papillae on palms and soles create looped, whorled ridges on the epidermal surface for increased friction and gripping.
• The genetically determined ridges of fingertips are well-provided with sweat pores that leave unique fingerprints.
• The Reticular Layer is the deepest layer of dense irregular connective tissue, blood vessels, sweat/oil glands, and lamellar corpuscles (deep pressure receptors).
• The skin contains cutaneous sensory receptors (touch, pressure, temperature, and pain) that provide info about our external environment.
• Epidermis contains phagocytes to prevent microbes from penetrating deeper into the body.
• Collagen fibers are responsible for the dermis' toughness and attract/bind water to keep the skin hydrated.
• Elastic fibers give skin elasticity in youth; collagen and elastic fibers decrease with age.
• The dermis has an abundant blood supply to maintain body temperature homeostasis.
• Dermal capillaries become engorged with heated blood when body temperature is high, allowing body heat to radiate from the skin surface.
• Blood bypasses dermis capillaries in cool temps to conserve body heat and maintain high internal body temperatures.

Skin Color

• Three pigments contribute to skin color: melanin, carotene, and hemoglobin.
• Melanin: Amount and kind (yellow, reddish-brown, black) in the epidermis; exposure to sunlight stimulates melanocytes to produce melanin, leading to tanning.
• Carotene: Amount deposited in the stratum corneum and subcutaneous tissues.
• Hemoglobin: The amount of oxygen-rich hemoglobin (pigment in red blood cells) in the dermal blood vessels gives light-skinned people a rosy glow.

Appendages of the Skin

• Skin appendages are epidermal-derived structures located in the dermis.
• Skin appendages include cutaneous glands, hair, hair follicles, and nails, maintaining body homeostasis.

All Cutaneous exocrine Glands

• Cutaneous exocrine glands release secretions to the skin surface via ducts.
• Cutaneous endocrine glands are formed by stratum basale cells.
• Cutaneous endocrine glands push into the deeper skin and reside almost entirely in the dermis.
• Sebaceous (oil) glands-all over skin except palms/soles-empty into hair follicle (some onto skin surface directly)
• Sebum: A mixture of oily substances and fragmented cells that lubricate skin/hairs and kill bacteria.
• Sebaceous glands activate when androgens(male sex hormones) increase during adolescence, resulting in oilier skin.
• Sweat glands (sudoriferous glands)-Over 2.5 million sweat glands per person widely distributed in the skin.
• Eccrine: all over body+produces sweat ( clear secretion of water, salts NaCl, vitamin C, metabolic wastes, ammonia, urea, uric acid, lactic acid,)
• Sweat (pH from 4-6)-inhibits bacterial growth and eliminates facial "pores" when we talk of our complexion because these are external outlets of hair follicles NOT sweat pores
• Eccrine: part of body's heat regulating function due to them secreting sweat with nerve endings when the external/body temp increases
• It can get up to 7Ls on a hot day but more than a few degrees, 37C (98.6F) can be life threatening

Hairs and Hair Follicles

• Hairs are composed of a flexible epithelial structure
• Produced by hair follicles, millions are found on the body(except on palms, soles, nipples, lips).
• Humans are born with as many hair follicles as they will ever have
• Hairs are among the fastest tissues growing protect head from bumps, shield the eyes and keep particles out of respiratory tract
• Hormones account for the development of hairy regions
• Hairs are a flexible epithelial structure enclosed in the hair follicle is the root and the part projecting from the surface of scalp/skin is the shaft

Nails

• Nails are a modified form of the epidermis( corresponds w/ hoof and claws from other animals)
• Each nail ha a free edge, body and root
• -The border of nail are overlapped by the skin called nail folds