Difficulty: Intermediate | Prerequisites: Anatomical terminology (Study Notes 01), cell structure basics (Study Notes 02).
Cells do not work in isolation. They organise into tissues, and tissues combine to form organs. This section covers the four fundamental tissue types and then focuses on the integumentary system (skin, glands, membranes), which is the body's largest organ and its first line of defence. Understanding tissue types is essential for every organ system that follows, and the integumentary system is a common exam topic because it ties together epithelial tissue, connective tissue, glands, temperature regulation, and wound healing in one package.
There are four tissue types: epithelial, connective, muscle, and nervous. The skin has three layers (epidermis, dermis, hypodermis), contains multiple gland types, and plays roles in protection, temperature regulation, sensation, and immune defence. Skin injuries are classified by depth (burn degrees) and assessed by area (Rule of Nines). Healing occurs via regeneration or fibrosis.
Epithelial tissue
Tissue that covers body surfaces, lines cavities and organs, and forms glands. Cells are tightly packed with minimal extracellular matrix. It is avascular (no blood supply of its own) and relies on diffusion from underlying connective tissue. In simple terms, this is the body's lining and covering material, found everywhere from your skin's surface to the inside of your stomach.
Connective tissue
Tissue that supports, connects, and separates other tissues and organs. Characterised by cells dispersed within an extracellular matrix (which can be liquid, gel-like, or solid). Includes bone, blood, cartilage, fat, and fibrous tissue. Think of it as the body's scaffolding and packing material.
Muscle tissue
Tissue specialised for contraction and movement. Three types: skeletal (voluntary, striated), cardiac (involuntary, striated), and smooth (involuntary, non-striated).
Nervous tissue
Tissue that generates and transmits electrical impulses. Composed of neurons (signal conductors) and neuroglia (support cells).
Epidermis
The outermost layer of the skin. Made of stratified squamous epithelium. Contains keratin (a tough, waterproofing protein) and melanin (a pigment that absorbs UV radiation). In simple terms, this is the layer you can see and touch.
Dermis
The layer beneath the epidermis. Made largely of connective tissue. Contains blood vessels, nerve endings, hair follicles, sweat glands, and sebaceous glands. Think of it as the skin's "working layer" where most of the action happens.
Hypodermis (subcutaneous layer)
The deepest layer, composed of fat (adipose tissue) and connective tissue. Not technically part of the skin itself, but closely associated. Provides insulation, energy storage, and cushioning.
Exocrine glands
Glands that secrete their products onto a surface (external or internal) via ducts. Examples: sweat glands, sebaceous glands, salivary glands.
Endocrine glands
Glands that secrete hormones directly into the bloodstream (no ducts). Examples: thyroid, adrenal glands, pituitary gland.
Eccrine glands
Sweat glands distributed across most of the body's surface. Produce a watery sweat for thermoregulation. Open directly onto the skin surface.
Apocrine glands
Sweat glands found in specific areas (axillae, groin). Produce a thicker secretion that, when broken down by skin bacteria, causes body odour. Open into hair follicles.
Sebaceous glands
Oil-producing glands associated with hair follicles. Secrete sebum, which lubricates and waterproofs the skin and hair.
Serous membrane
A membrane that lines closed body cavities (thoracic, abdominal) and covers the organs within them. Produces a thin, watery serous fluid that reduces friction. Examples: pleura (lungs), pericardium (heart), peritoneum (abdominal organs).
Mucous membrane
A membrane that lines body cavities open to the exterior (respiratory, digestive, urinary, reproductive tracts). Secretes mucus for protection and lubrication.
Rule of Nines
A clinical method for estimating the percentage of body surface area affected by a burn. Each major body region is assigned a percentage that is a multiple of nine (e.g., each arm = 9%, the anterior trunk = 18%).
ABCDE rule
A screening tool for suspicious moles: Asymmetry, Border irregularity, Colour variation, Diameter greater than 6 mm, Evolving (changing over time).
Regeneration
Tissue repair by replacement with the same type of cell. Restores normal function.
Fibrosis
Tissue repair by replacement with scar tissue (collagen). Structural integrity is restored, but the original function of the tissue is not.
Epithelial: covers surfaces (skin), lines organs and cavities, forms glands. Key features: tightly packed cells, avascular, high regeneration rate. Classified by cell shape (squamous, cuboidal, columnar) and layering (simple, stratified).
Connective: the most diverse tissue type. Subtypes include loose connective tissue (areolar, adipose, reticular), dense connective tissue (tendons, ligaments), cartilage, bone, and blood. All share cells within an extracellular matrix.
Muscle: specialised for contraction. Skeletal is voluntary and striated; cardiac is involuntary and striated; smooth is involuntary and non-striated. Covered in detail in Study Notes 05.
Nervous: neurons conduct impulses; neuroglia support and protect neurons. Covered in detail in Study Notes 06.
Epidermis: the outer barrier. Contains keratinocytes (produce keratin for waterproofing), melanocytes (produce melanin for UV protection), Langerhans cells (immune function), and Merkel cells (touch sensation). Thickness varies by location (thickest on palms and soles).
Dermis: two sub-layers. The papillary layer (superficial, loose connective tissue, contains dermal papillae that create fingerprints) and the reticular layer (deeper, dense irregular connective tissue, provides strength and elasticity).
Hypodermis: primarily adipose tissue. Anchors skin to underlying structures, absorbs shock, insulates against heat loss.
Eccrine sweat glands: widespread, thermoregulatory. Most abundant on palms, soles, and forehead.
Apocrine sweat glands: limited distribution (axillae, groin, areolae). Begin functioning at puberty. Associated with body odour.
Sebaceous glands: secrete oily sebum into hair follicles. Keep skin and hair supple. Overactivity contributes to acne.
Serous membranes: line closed cavities. Each has a parietal layer (lining the cavity wall) and a visceral layer (covering the organ). Serous fluid between them reduces friction during movement. Key examples: pleura, pericardium, peritoneum.
Mucous membranes: line open cavities (digestive, respiratory, urinary, reproductive tracts). Mucus traps pathogens and particles, and keeps surfaces moist.
Burns are classified by depth:
First degree (superficial): epidermis only. Red, painful, no blisters. Example: mild sunburn.
Second degree (partial thickness): epidermis and part of dermis. Blisters, pain, redness.
Third degree (full thickness): destroys epidermis and dermis, may reach subcutaneous tissue. White or charred appearance, often painless at the burn site because nerve endings are destroyed.
Rule of Nines estimates total body surface area (TBSA) affected, which guides fluid resuscitation and treatment decisions.
ABCDE rule is used for early detection of melanoma.
Healing proceeds through inflammation, organisation, and regeneration or fibrosis. Tissues with high mitotic rates (epithelial, connective) regenerate well; cardiac and nervous tissue heal primarily by fibrosis.
The skin is central to thermoregulation.
Heat loss mechanisms: vasodilation (increased blood flow to skin surface), sweating (evaporative cooling).
Heat conservation mechanisms: vasoconstriction (reduced blood flow to skin), shivering (skeletal muscle activity generates heat), piloerection ("goosebumps," minimal effect in humans).
Thermoregulation is a classic example of negative feedback, with the hypothalamus as the control centre.
The Rule of Nines is used in emergency departments and burn units worldwide to quickly estimate burn severity and calculate IV fluid requirements (using formulas like the Parkland formula). The ABCDE rule is taught to the general public as a skin self-check tool for catching melanoma early, when survival rates are highest. Understanding serous membranes matters clinically because conditions like pleurisy (inflammation of the pleura) and pericarditis (inflammation of the pericardium) produce pain from friction between membrane layers that should normally glide smoothly.
Students often think the hypodermis is a layer of the skin. Strictly speaking, it sits beneath the skin (the dermis is the deepest true skin layer), though it is closely associated and often studied alongside it.
The epidermis is frequently assumed to contain blood vessels. It is avascular; nutrients reach it by diffusion from the dermis.
Students sometimes confuse serous and mucous membranes. The key distinction: serous membranes line closed cavities (no opening to the exterior); mucous membranes line cavities that open to the outside.
Third-degree burns are sometimes assumed to be the most painful. They often are not, at the burn site itself, because nerve endings in the dermis have been destroyed. Pain is felt at the margins where surrounding tissue has second-degree damage.
⚠️ Know the four tissue types, their defining characteristics, and where each is found.
⚠️ Be able to list the layers of the skin in order and describe what each contains.
⚠️ The distinction between eccrine and apocrine sweat glands is a commonly tested detail.
⚠️ Serous vs mucous membranes: know the difference and be able to give examples of each.
⚠️ Burn classifications and the Rule of Nines are high-yield exam material.
⚠️ Understand regeneration vs fibrosis and which tissues regenerate well.
True or false: Epithelial tissue has its own blood supply.
The skin layer that contains blood vessels, glands, and nerve endings is the __________.
True or false: Apocrine sweat glands are found all over the body.
A membrane that lines a closed body cavity is called a __________ membrane.
Fill in the blank: Healing by replacement with scar tissue is called __________; healing by replacement with the same cell type is called __________.
(Answers: 1. False, epithelial tissue is avascular. 2. Dermis. 3. False, they are found in specific areas such as the axillae and groin. 4. Serous. 5. Fibrosis; regeneration.)
Q: Compare and contrast serous and mucous membranes. Include location and function.
A: Serous membranes line closed body cavities (pleural, pericardial, peritoneal) and secrete thin serous fluid to reduce friction between organs and cavity walls. Mucous membranes line cavities open to the exterior (digestive, respiratory, urinary, reproductive tracts) and secrete mucus to trap pathogens and keep surfaces moist.
Q: A patient has a burn that covers the entire anterior trunk and the entire left arm. Using the Rule of Nines, estimate the total body surface area (TBSA) affected.
A: The anterior trunk is 18% and one entire arm is 9%, giving a total of 27% TBSA.
Q: Explain why third-degree burns may be painless at the burn site itself.
A: Third-degree burns destroy the full thickness of the epidermis and dermis, including the nerve endings in the dermis. Without intact nerve endings, the burned area cannot transmit pain signals. Pain is typically felt at the surrounding margins where partial-thickness (second-degree) damage has occurred.
Q: What is the role of melanocytes, and in which skin layer are they found?
A: Melanocytes produce melanin, a pigment that absorbs ultraviolet radiation and protects deeper tissue from UV damage. They are located in the epidermis.
Tissue types connect to every subsequent system: epithelial tissue lines the digestive and respiratory tracts, connective tissue forms the basis of the skeletal system, muscle tissue is the focus of the muscular system, and nervous tissue is central to the nervous system. Temperature regulation (integumentary) ties back to feedback mechanisms (Study Notes 02) and will appear again in the cardiovascular and endocrine systems. Wound healing connects to the immune response covered in the lymphatic system.
tissue types, epithelial tissue, connective tissue, muscle tissue, nervous tissue, squamous, cuboidal, columnar, simple, stratified, skin layers, epidermis, dermis, hypodermis, subcutaneous, keratin, melanin, melanocytes, keratinocytes, Langerhans cells, Merkel cells, exocrine glands, endocrine glands, eccrine glands, apocrine glands, sebaceous glands, sebum, serous membrane, mucous membrane, pleura, pericardium, peritoneum, parietal, visceral, Rule of Nines, burn classification, first degree burn, second degree burn, third degree burn, ABCDE rule, melanoma screening, regeneration, fibrosis, scar tissue, thermoregulation, vasodilation, vasoconstriction, integumentary system, anatomy and physiology, A&P