Unit 2 : Integumentary System, Skeletal System, Histology

Integumentary System

Covering of the body, consists of skin, hair, nails and sweat glands (sudoriferous) and oil glands (sebaceous).

Epidermis

Superficial region. Sheet-like layers. Has epithelial stratified squamous cells and is dense connective. Nutrients from dermis diffuse into epidermis, giving them food.

Dermis

Underlines epidermis. Made up of loose & connective and dense connective tissue. Mostly fibrous connective tissue, vascular (rich in blood).

Hypodermis

Not technically apart of the skin because it is underneath the epidermis. It is the subcutaneous layer deep to the skin. Mostly made up of adipose tissue that absorbs shock and insulates. Anchors fat to underlying structures, mostly muscles.

What does fat do?

Fat is fuel preserve and also insulates the body.

Keratinocytes

Most abundant epidermal cells. They produce water proofing keratin protein that gives skin its protective properties.

What are keratinocytes connected by and why?

Keratinocytes are connected by desmosomes which provide tensile strength.

Melanocytes

Deepest part of the epidermis. They produce melanin, which is packaged melanosomes. The melanosomes get transferred to the keratinocytes and protect nucleus from UV damage.

Why do the nuclei of mitotically active cells need melanosome protection?

Nuclei house DNA = mitosis. Harmful UV rays can cause unchecked cell growth and can cause cancer.

Dendritic (Langerhans Cells)

Star-shaped macrophages that patrol deepest epidermis.

What do dendritic cells do?

- Macrophages that eat and ingest invaders. They prevent entry of bacteria into the vasculatured dermis.

Tactile (Merkel) cells

Sensory touch receptors that sense touch. They can feel light or fine touh micromovements.

Difference between thick and thin skin?

Thick skin has 5 layers (which include stratum lucidum) and thin skin only has 4 layers.

What are the five layers of skin? From most deep layer to most superficial.

Stratum basale, Stratum Spinosum, Stratum granulosum, Stratum lucidum, Stratum corneum.

Stratum basale (basal layer)

-Deepest epidermal layer firmly attached to the dermis
single row of stems. The layer right before the hypodermis.
-The single row of stem cells are highly mitotic and actively producing and dividing.
-10-25% of this layer is made up of melanocytes.

Why do the stem layers of stratum basale never deplete?

As the cells are mitotically active, when they divide, one daughter cell remains while the other migrates to the apical surface. It takes about 25-45 days.

Stratum spinosum (prickly/spiny layer)

• Several layers thick (where cells start to become dehydrated.

• Cell contain web-like system of intermediate prekeratin filaments attached to desmosomes.

• Desmosomes allow cells to resist tension and pulling.

• Keratinocytes in this layer appear prickly/spikey.

Stratum granulosum (granular layer)

Four to six cells thick, but the cells are flattened so layer = thin cell keratinization begins.

• Cell nuclei/organelles begin to disintegrate.

• Cells accumulate keratinohyaline that form keratin fibers.

• Accumulate lamellar granules.

• Any cells above this layer die because they are too far from dermal capillaries.

Stratum lucidum

Only found in thick skin. The clear because there are no nuclei. The cells are now dead.
- 2/3 flat rows.

Stratum corneum

Absent of nuclei, anucleated, keratinized dead layer of cells.

• 20/30 flat rows

• physical barrier to protect deeper cells from environment, prevent water loss, protects against abrasion and penetration.

• Acts as a barrier against biological, chemical and physical assaults.

What is apoptosis?

Programmed cell death. Dead cells in the skin slough off as dandruff and dead skin.

What is the Dermis?

• strong flexible connective tissue beneath the epidermis. It is made up of fibers in matrix that bind body together.

• Contains nerves, blood vessels and lymphatic vessels (vascularized).

What kind of cells are within the dermis?

Fibroblasts, macrophages and occasionally mast cells and white blood cells.

What do fibroblasts do?

Secrete cell matrix, collagen fibers and active stem cells.

What are macrophages and what do they do?

>Macrophages are large phagocytic cells that develop from some leucocytes
>Some wander through tissues looking for pathogens and destroying them
>Others have a fixed place and destroy pathogens that come to them
- Macrophages phagocytize and pinocytize.

What is the papillary layer?

The thinner layer of loose and airy areolar connective tissue of interlacing collagen, elastic fibers and blood vessels.

What are dermal papillae?

Superficial region of dermis that sense fingerlike projections up into epidermis.
- in thick skin, it lies on top of dermal ridges, which gives rise to friction ridges (finger prints!)

What is the friction ridge for?

Enhances gripping ability and contributes to the sense of touch. Sweat pores in ridges leave unique fingerprint-like pattern.

What do projections in the dermal papillae contain?

- Capillary loops, free nerve endings and touch receptors.

Does location of capillary loops, free nerve endings and touch receptors matter?

Yes! If found deeper in dermis, it detects deeper pressure, but if it is superficial, it will detect fine touch.

What is the reticular layer?

-makes up ~80% of dermal thickness.

• Consists of coarse, dense, fibrous connective tissue.

• Elastic fibers = stretch and recoil properties while collagen = tensile strength.

• Layer contains spillage of adipose tissue.

How do dermal modifications result in characteristic markings?

Surgeons are able to use specific lines on the body cause by collagen fibers in order to cut along and prevent scarring.

Cleavage (tension lines)

Caused by collagen fibers running parallel to skin surface.

Flexure line

Adheres skin/dermis to joints where it is adhered to deeper structures.

What causes skin color?

- Melanin, Carotene & Hemoglobin

Melanin

• Brownish color made in skin by melanocytes within the stratum basale.

• The more sun you are exposed to, the more melanin your skin secretes.

• Packages melanosomes to the keratinocytes to shield the nucleus from UV sunlight.

Carotene

• Yellow-orange pigment

• Most obvious in palms and soles. Accumulates to stratum corneum + hypodermis.

• Can be converted to Vitamin A for vision and epidermal health.

Hemoglobin

• Reddish pigment found in our RBCs.

• In lighter complexioned individuals, pinkish hue of fair skin is due to lower levels of melanin.

Why do caucasian individuals have transparent skin?

• The amount of melanocytes that people have in the skin are the same, however, the amount of melanin an individual secretes is entirely different.

• Caucasian individuals are more likely to be transparent and hemoglobin shows through more.

Eccrine (merocrine) sweat gland

• released to the exterior and is everywhere.

• abundant on palms, soles and forehead.

• function is thermoregulation

• Secretes sweat and is devoid of odor

Apocrine sweat gland

• Stinky sweat glands that are most confined to axillary (armpit)and urogenital areas (areas with pubic hair)

• Sweat secretion is viscous milky or yellowish sweat that contains fatty substances and proteins.

• Begins to secrete at puberty.

Why is apocrine sweat smelly?

Bacteria break down fatty substance/viscous protein sweat, leading to body odor because ammonia is release.
- Remember that protein is made up of amino acids that have the N group.

Sebaceous (oil) glands

Body relies on sebum to lubricate skin and hair follicles.

• Androgens activate sebaceous glands to be active @ PUBERTY.

• Sebaceous glands self destruct after a cell secretes.

Arrector pili muscle

An involuntary muscle fiber attached to the underside & base of the hair follicle.
- Causes the hair on your epidermis to stand up

What are some function of skin?

• chemical, physical and biological barrier.

• Skin protects against microorganisms, abrasions, temperature extremes and harmful chemicals.

Skin : Chemical barrier

-Secretes sweat (antimicrobial proteins), sebum & defensins that kill bacteria.
- Low pH of skin retards bacterial manipulation because bacteria cell wall is negative.

Skin : Physical barrier

• Glycolipids within the stratum corneum block most water and water-soluble lipids.

• Skin takes in lipid soluble substances like hydrocortisone because it is made of steroids = lipid soluble.

Skin : Biological barrier

• Epidermis contains phagocytic cells.

• Dendritic cells engulf antigens and present them to WBC to launch and activate immune response system.

• Dermis contains macrophages (phagocytic WBC)

Body Temperature Regulation : Insensible respiration

Resting body temperature, you sweat about 500 ml a day of unnoticeable sweat.

Body Temperature Regulation : Sensible respiration

Body temperature rises because of the dilation of dermal vessels and can increase sweat glands activity to produce 12 L of sweat.

Blood vessel dilation

- When blood vessels are brought closer to the dermis of the skin and the body gets hotter. Occurs when you're feeling hot so you can sweat (sweat cools the body).

Blood vessel constriction

- When blood vessels move further into the skin so heat cannot escape. When you're feeling cold.

Cutaneous sensory receptors

Part of nervous system. Exteroreceptors respond to stimuli outside body such as temperature and touch.
Free nerve endings sense painful stimuli.

Metabolic function of skin

• Synthesis of vitamin D needed for calcium absorption in intestine.

• Keratinocytes can activate some hormones (convert cortisone into hydrocortisone).

• Skin makes collagenase to aid in turnover of wrinkles

Skin / Reservoir of Blood

• Skin holds 5% of body's blood volume.

• Skin vessels can be restricted to shunt (redirect) blood to other organs.

Skin / Excretion

- Skin can secrete limited amounts of waste such as ammonia, urea and uric acid.
-Sweat = salt and water loss

Skeletal cartilage

made of highly resilient, molded cartilage tissue that consists primarily of water.
Contains no blood vessels or nerves

Perichondrium

Layer of dense connective tissue surrounding cartilage.

• Helps cartilage resist outward expansion.

• Contains blood vessels to diffuse nutrient delivery to the cartilage.

• Strips of dense connective tissue.

Chondrocytes

Cells encased in small cavities (lacunae). They secrete the matrix of cartilage and become embedded into it.

Hyaline Cartilage

Provides support, flexibility & resilience.

• Most abundant, collagen only.

• Found in articular joints, costal ribs, respiratory tract and nasal.

Elastic cartilage

Contains elastic fibers, support, flexibility and stability.
- Found in external ear and epiglottis.

Fibrocartilage

Thick collagen fibers for great tensile strength.
- Found in areas with discs such as the knee and vertebral discs

Appositional growth

Perichondrium secretes matrix to widen the girth of bone.

Interstitial growth

Chondrocytes divide and secrete new matrix expanding the length of bone.

Calcification of cartilage

deposit of salt that hardens cartilage.
- Not yet bone, to become bone it must ossify.

Axial Skeleton

Long axis of body, skull, vertebral column and rib cage

Appendicular Skeleton

Bones of upper and lower limbs, girdles and skeletal girdle.
- Hip & clavicle + scapula.

Long bone

Longer than they are wide.
- Limb bones and phalanges.

Short bone

Cube shaped, width and length are equal.
- Found in wrist and ankles

Sesamoid

Small seed like shape, form within tendons (only example is patella)

Flat bone

thin, flat, slightly curved
- Found in sternum, scapulae, ribs and most skull bones

Irregular bone

complicated shapes
- Found in vertebrae + hip bones, sacrum

Compact bone

Dense outer layer on every bone that is smooth or solid

Spongy bone

made up of a honey comb. of small needle-like or flat pieces of bone called trabeculae

Periosteum

Covers the outside of the compact bone except joints.

Fibrous layer

Outer layer consists of dense irregular connective tissue

Osteogenic layer

Inner layer of abutting bone that contains primitive osteogenic stem cells that gives rise to most all bones

Endosteum

membranous lining of the hollow cavity of the inside of the compact bone

Where is red bone marrow found?

Scattered throughout spongy bone (diploe). No defined marrow cavity.

Where is yellow bone marrow found?

It is found within the yellow bone marrow cavity.

Diaphysis

Tubular shaft that forms long axis of bone. Contains medullary cavity of yellow marrow

Epiphyses

Ends of long bones that consist of compact bone externally and spongy bone internally.

Epiphyseal line

Ossified plate between diaphysis and epiphysis.
- Remnants where bone growth begins to occur.

If there is no epiphyseal line, what does that mean?

Growth plate means growth is still possible! Epiphyseal line may not be found in children who are still maturing.

Hematopoietic tissue in bones

As you age, your red bone marrow converts into yellow bone marrow.

What bone marrow do adults have?

Primarily yellow bone marrow.

What bone marrow do children have?

Primarily red bone marrow.

What happens if an adult / vperson if anemic?

Yellow bone marrow can convert to red.

Osteogenic cells

Mitotically active stem cells in periosteum and endosteum. When stimulated, they can differentiate into osteoblasts or bone-lining cells.

Osteoblasts

Bone-forming cells that secrete unmineralized bone matrix called osteoid. They are mitotically active.

Osteoid

Made up of collagen and calcium-binding proteins.

Osteocytes

Immature bone cells in lacunae that no longer divide. They maintain bone matrix and act as stress or strain sensors.

• Respond to mechanical stimuli such as increased force on bone or weightlessness. -Communicate to osteoblasts and osteoclasts so bone remodeling can occur.

• Connected through gap junctions.

• Deposit actual calcium into the matrix.

Bone-lining cells

On external bone called : periosteal cells.
On internal bone called : endosteal cells.

Osteoclasts

Hemopoietic stem cells merge to become osteoclasts, they have numerous amounts of nuclei.

• Osteoblasts send them signals in order to stay alive, if not they perform cell death / apoptosis.

• When active, cells are located in depressions called reabsorption bays.

• Function in bone resorption.

• Cells have ruffled borders that serve to increase surface area.

• Addition of acid to ionize calcium phosphate in bone matrix.

Osteon

Structural unit of compact bone.

• Consists of elongated cylinder that runs parallel to long bone axis.

• Lamellae contains collagen fibers that run in different directions in adjacent rings in order to resist twisting and withstand stress.

Canaliculi

Hairlike canals that connect lacunae to eachother and osteocytes via cell projections. Allows communication between all osteocytes of osteon and permits nutrients and wastes to be relayed.

Interstitial Lamellae

fill spaces between osteons

Circumferential lamellae

located deep to periosteum and superficial to endosteum and extend around entire circumference of the diaphysis and resist twisting of long bone

Haversian canal

one of a network of tubes running through compact bone that contains blood vessels and nerves

Organic components of bone

Osteogenic cells, osteoblasts, osteocytes, bone-lining cells, osteoclasts and osteoids.