Lecture exam #2 Flashcards
Components of the integumentary system
The integument consists of two distinct layers: a layer of stratified squamous epithelium called the epidermis, and a deeper layer of both areolar and dense irregular connective tissue called the dermis. Deep to the integument is a layer of areolar and adipose connective tissue called the subcutaneous layer, or hypodermis. The subcutaneous layer is not part of the integumentary system; however, it is described in this chapter because it is closely involved with both the structure and function of the skin.
Describe the structure, composition, arrangement, and functions of the five layers (strata) of the epidermis
The epidermis is organized into specific layers called strata. From deepest to most superficial, they are the stratum basale (deepest layer, with actively dividing keratinocytes), stratum spinosum, stratum granulosum, stratum lucidum, and stratum corneum (many layers of dead keratinocytes).
∙ Keratinization is the process by which keratinocytes fill up with the protein keratin, and as a result the cell dies. Keratinization begins in the stratum granulosum.
∙ Thick skin (palms of hands, soles of feet) has five epidermal strata, whereas thin skin (on the rest of the body) has four.
∙ Skin color is a result of hemoglobin in the blood vessels of the dermis, melanin pigment, and carotene pigment.
Stratum Basale
The deepest epidermal layer is the stratum basale, also known as the stratum germinativum, or basal layer. This single layer of cuboidal to low columnar cells is tightly attached by hemidesmosomes to an underlying basement membrane that separates the epidermis from the connective tissue of the dermis. Three types of cells occupy the stratum basale: keratinocytes, melanocytes, tactile cells
Keratinocytes
The most abundant cell type in the epidermis and are found throughout all epidermal strata. The stratum basale is dominated by large keratinocyte stem cells, which divide to generate new keratinocytes that replace dead keratinocytes shed from the surface. Keratin is one of a family of fibrous structural proteins that are both tough and insoluble. Fibrous keratin molecules can twist and intertwine around each other to form helical intermediate filaments of the cytoskeleton. The keratin proteins found in keratinocytes are called cytokeratins. Their structure in these keratinocytes gives skin its strength and makes the epidermis water resistant.
Melanocytes
long, branching processes and are scattered among the keratinocytes of the stratum basale. They produce and store the pigment melanin in response to ultraviolet light exposure. Their cytoplasmic processes transfer melanin pigment within granules called melanosomes to the keratinocytes within the basal layer and sometimes in more superficial layers. This pigment (which includes the colors black, brown, tan, and yellow-brown) accumulates around the nucleus of the keratinocyte and shields the nuclear DNA from ultraviolet radiation. The darker tones of the skin result from melanin produced by the melanocytes. Thus, “tanning” is the result of the melanocytes producing melanin to block UV light from causing mutations in the DNA of your keratinocytes (in the epidermis)
and fibroblasts (in the dermis).
Tactile cells
Tactile cells, also called Merkel cells, are few in number
and found scattered among the cells within the stratum basale. Tactile cells are sensitive to touch and, when compressed, they release chemicals that stimulate sensory nerve endings, providing information about objects touching the skin.
Stratum Spinosum
Several layers of polygonal keratinocytes form the stratum spinosum, or spiny layer. Each time a keratinocyte stem cell in the stratum basale divides, a daughter cell is pushed toward the external surface from the stratum basale, while the other cell remains as a stem cell in the stratum basale. Once this new cell enters the stratum spinosum, it begins to differentiate into a nondividing, highly specialized keratinocyte. The keratinocytes in the stratum spinosum attach to their neighbors by many membrane junctions called desmosomes.
In addition to the keratinocytes, the stratum spinosum also contains the fourth epidermal cell type, called epidermal dendritic cells. Epidermal dendritic cells are immune cells that help fight infection in the epidermis. These immune cells are often present in the stratum spinosum and stratum granulosum, but they are not identifiable in standard histologic preparations. Their phagocytic activity initiates an immune response to protect the body against pathogens that have penetrated the superficial epidermal layers as well as epidermal cancer cells
Stratum Granulosum
The stratum granulosum, or granular layer, consists of three to five layers of keratinocytes superficial to the stratum spinosum. Within this stratum begins a process called keratinization, where the keratinocytes fill up with the protein keratin, and in so doing, cause both the cell’s nucleus and organelles to disintegrate and the keratinocyte dies. Keratinization is not complete until the keratinocytes reach the more superficial epidermal layers. A fully keratinized cell is dead, but it is structurally strong because of the keratin it contains.
Stratum Lucidum
The stratum lucidum, or clear layer, is a thin, translucent region of about two to three keratinocyte layers that is superficial to the stratum granulosum. This stratum is found only in the thick skin within the palms of the hands and the soles of the feet. Keratinocytes occupying this layer are flattened, pale cells with indistinct boundaries. They are filled with the translucent protein called eleidin, which is an intermediate product in the process of keratin maturation. This layer helps protect the skin from ultraviolet light.
Stratum Corneum
The stratum corneum, or hornlike layer, is the most superficial layer of the epidermis. It is the stratum you see when you look at your skin. The stratum corneum consists of about 20 to 30 layers of dead, scaly, interlocking, keratinized cells. The dead keratinocytes are anucleate (lacking a nucleus) and are tightly packed together.
Thick Skin Versus Thin Skin
Skin is classified as either thick or thin based on the number of epidermal strata and the relative thickness of the epidermis, rather than the thickness of the entire integument. Thick skin is found on the palms of the hands and the soles of the feet. All five epidermal strata occur in the thick skin. The epidermis of thick skin ranges between 0.4 mm and 0.6 mm thick. It houses sweat glands but has no hair follicles or sebaceous (oil) glands. Thin skin covers most of the body. It lacks a stratum lucidum, so it has only four specific layers in the epidermis. Thin skin contains the following structures: hair follicles, sebaceous glands, and sweat glands. The epidermis of thin skin ranges from 0.075 mm to 0.150 mm thick.
Skin color
Normal skin color results from a combination of the colors of hemoglobin, melanin, and carotene. Hemoglobin is an oxygen-binding protein present in red blood cells. It exhibits a bright red color upon binding oxygen, thus giving blood vessels in the dermis a reddish tint that is seen most easily in lightly pigmented individuals. If the blood vessels in the superficial layers vasodilate (i.e., the blood vessel diameter increases), such as during physical exertion, then the red tones are much more visible.
Melanin is a pigment produced and stored in melanocytes (described earlier in this section), and it occurs in a variety of black, brown, tan, and yellow-brown shades. Recall that melanin is transferred in melanosomes from melanocytes to keratinocytes in the stratum basale. Because keratinocytes are displaced toward the stratum corneum, melanocyte activity affects the color of the entire epidermis.
Skin markings
A nevus, commonly called a mole, is a harmless, localized overgrowth of melanocytes. On rare occasions, a nevus may become malignant, typically as a consequence of excessive UV light exposure. Thus, nevi should be monitored for changes that may suggest malignancy. Freckles are yellowish or brown spots that represent localized areas of increased melanocyte activity, not an increase in melanocyte numbers. A freckle’s degree of pigmentation varies and is dependent upon both sun exposure and heredity.
A hemangioma is an anomaly that results in skin discoloration due to blood vessels that proliferate to form a benign tumor. Capillary hemangiomas, or strawberry-colored birthmarks, appear in the skin as bright red to deep purple nodules that are usually present at birth and disappear in childhood. However, their development may occur in adults. Cavernous hemangiomas, also known as port-wine stains, involve larger dermal blood vessels and may last a lifetime.
Friction ridges
Friction ridges are another type of skin marking. These ridge patterns follow the contours of the skin, varying from small, conical pegs (in thin skin) to the complex arches and whorls. Friction ridges are found on the fingers (fingerprints), palms, soles, and toes. These ridges are formed from large folds and valleys of both dermis and epidermis. When sweat glands and oil glands release their secretions, noticeable fingerprints may be left on touched surfaces. Examples are arch/loop/whorl/combination
Describe the structure, organization, and functions of the layers of the dermis
The dermisis deep to the epidermis. This layer of the integument is composed of connective tissue proper and contains primarily collagen fibers, although both elastic and reticular fibers also are found within the dermis. Additionally, researchers recently have dis- covered motile cells in the dermis called dendritic cells. These cells are similar to the epidermal dendritic cells in that they serve an immune function, except they are located in the dermis. Other structures within the dermis are blood vessels, sweat glands, sebaceous glands, hair follicles, nail roots, sensory nerve endings, and smooth muscle tissue associated with hair follicles. Two major regions of the dermis can be distinguished: a superficial papillary layer and a deeper reticular layer
Papillary Layer of the Dermis
The papillary layer is the superficial region of the dermis that is deep to the epidermis. It is composed of areolar connective tissue, and it derives its name from the projections of the dermis called dermal papillae. The dermal papillae interdigitate with deep projections of the epidermis called epidermal ridges. Together, the epidermal ridges and dermal papillae increase the area of contact between the two layers and interlock them. Each dermal papilla contains the capillaries that supply nutrients to the cells of the epidermis. Additionally, dermal papillae contain sensory nerve endings that serve as tactile receptors; these receptors continuously monitor touch on the surface of the epidermis.
Reticular Layer of the Dermis
The reticular layer forms the deeper, major portion of the dermis that extends from the papillary layer to the underlying subcutaneous layer. The reticular layer consists primarily of dense irregular connective tissue through which large bundles of collagen fibers extend in all directions. These fibers are interwoven into a meshwork that sur- rounds structures in the dermis, such as the hair follicles, sebaceous glands and sweat glands, nerves, and blood vessels.
Describe the structure and function of the subcutaneous layer
Deep to the integument is the subcutaneous layer, also called the hypodermis, or superficial fascia. It is not considered a part of the integument. This layer consists of both areolar connective tissue and adipose connective tissue.. In some locations of the body, adipose connective tissue predominates; thus, the subcutaneous layer is called subcutaneous fat. The connective tissue fibers of the reticular layer of the dermis are extensively interwoven with those of the subcutaneous layer to stabilize the position of the skin and bind it to the underlying structures. The subcutaneous layer pads and protects the body, acts as
an energy reservoir, and provides thermal insulation. Drugs often are injected into the subcutaneous layer because its extensive vascular network promotes rapid absorption of the drugs.
List and explain the varied functions of the integument
EPIDERMIS:
PROTECTION
PREVENTION OF WATER LOSS AND WATER GAIN
METABOLIC REGULATION
SECRETION AND ABSORPTION
IMMUNE FUNCTION
DERMIS:
TEMPERATURE REGULATION
SECRETION AND ABSORPTION
SENSORY RECEPTION
Explain how the skin helps retain warmth or cool the body
TEMPERATURE REGULATION IN DERMIS
Dilating blood vessels in the dermis release heat; constricting vessels conserve heat. Sweat glands release fluid onto the skin surface, and the body cools off by evaporation of sweat.
Describe the formation and function of nails
Nails are scalelike modifications of the stratum corneum layer of the epidermis that form on the dorsal edges of the fingers and toes. They protect the distal tips of the digits during jumping, kicking, or catch- ing. Fingernails also assist us in grasping objects.
Each nail has a distal whitish free edge, a pinkish nail body, and a nail root, which is the proximal part embedded in the skin. Together, these parts form the nail plate. The nail body covers a layer of epidermis called the nail bed, which contains only the deeper, living cell layers of the epidermis.
Most of the nail body appears pink because of the blood flowing in the underlying capillaries; the free edge of the nail appears white because there are no underlying capillaries. At the nail root and the proximal end of the nail body, the nail bed thickens to form the nail matrix, which is the actively growing part of the nail. The lunula is the whitish, semilunar area of the proximal end of the nail body. It has a whitish appearance because a thickened stratum basale obscures the underlying blood vessels.
Along the lateral and proximal borders of the nail, folds of skin called nail folds overlap the nail. The eponychium, also known as the cuticle, is a narrow band of epidermis extending from the margin of the nail wall onto the nail body. The hyponychium is the area of thickened epithelium underlying the free edge of the nail.
Describe the function of hair
Protection. The hair on the head protects the scalp from sunburn and injury. Hair within the nostrils entraps particles and prevents their entry deeper into the respiratory system, whereas hairs within the external ear canal protect the ear from insects and foreign particles. Eyelashes protect the eyes.
Heat retention. Hair on the head prevents the loss of conducted heat from the scalp to the surrounding air. Individuals who have lost their scalp hair release much more heat through the scalp than those who have a full head of hair.
Sensory reception. Hair follicles have associated tactile receptors (root hair plexuses) that detect light touch.
Visual identification. Hair characteristics are important in determining age and sex, and in identifying individuals
Compare and contrast the function and location of different types of exocrine glands of the skin.
sweat glands: merocrine sweat glands/apocrine sweat gland
sebaceous glands
other glands:
ceruminous glands (ear wax)
mammary glands
Distinguish between regeneration and fibrosis
Damaged tissues are normally repaired in one of two ways. The replacement of damaged or dead cells with the same cell type is called regeneration. This restores organ function.
When regeneration is not possible because part of the organ is too severely damaged or its cells lack the capacity to divide, the body fills in the gap with scar (fibrous) tissue. This process of scar tissue deposition in connective tissue during healing is referred to as fibrosis, and it binds the damaged parts together. The replacement scar tissue is produced by fibroblasts and is composed primarily of collagen fibers. Some structural restoration occurs; however, functional activities are not restored.
Describe the process of wound healing
- Cut blood vessels initiate bleeding into the wound. The blood brings clotting proteins, numerous leukocytes, and antibodies
- A blood clot forms, temporarily patching the edges of the wound together and acting as a barrier to prevent the entry of pathogens into the body. Internal to the clot, macrophages and neutrophils clean the wound of cellular debris.
- The cut blood vessels regenerate and grow in the wound. A soft mass deep in the wound becomes granulation tissue, which is a vascular connective tissue that initially forms in a healing wound. Macrophages within the wound begin to remove the clotted blood. Fibroblasts produce new collagen fibers in the region.
- Epithelial regeneration of the epidermis occurs due to division of epithelial cells at the edge of the wound. These new epithelial cells migrate over the wound, moving internally to the now superficial remains of the clot (the scab). The connective tissue is replaced by fibrosis.
BASAL CELL CARCINOMA
∙ Most common type of skin cancer
∙ Least dangerous type, as it seldom metastasizes (i.e., spreads to other locations within the body)
∙ Originates in stratum basale
∙ First appears as small, shiny elevation that enlarges and develops central depression with pearly edge
∙ Usually occurs on face
∙ Treated by surgical removal of lesion
SQUAMOUS CELL CARCINOMA
∙ Arises from keratinocytes of stratum spinosum
∙ Lesions usually appear on scalp, ears, lower lip, or dorsum of hand.
∙ Early lesions are raised, reddened, scaly; later lesions form concave ulcers with elevated edges.
∙ Treated by early detection and surgical removal of lesion
∙ May metastasize to other parts of the body
MALIGNANT MELANOMA
∙ Most deadly type of skin cancer due to aggressive growth and metastasis
∙ Arises from melanocytes, usually in a preexisting mole
∙ Individuals at increased risk include those who have had severe sunburns, especially as children.
∙ Characterized by change in mole diameter, color, shape of border, and symmetry
∙ Survival rate improved by early detection and surgical removal of lesion
∙ Advanced cases (metastasis of disease) are difficult to cure and are treated with chemotherapy, interferon therapy, and radiation therapy.
A = Asymmetry: One-half of a mole or birthmark does not match the other.
B = Border: Edges are notched, irregular, blurred, or ragged.
C = Color: Color is not uniform; differing shades (usually brown or black and sometimes patches of white, blue, or red) may be seen.
D = Diameter: Affected area is larger than 6 mm (about 1/4 inch) or is growing larger.
E = Evolving: Change in the size, shape, or color of a mole or a change in symptoms, such as how a mole feels (how itchy or tender it feels) or what happens on the surface of a mole (especially bleeding)
List the components of the skeletal system
Our skeletal system includes the bones of the skeleton as well as cartilage, ligaments, and other connective tissues that stabilize or connect the bones.
Compare and contrast compact and spongy bone
Bones of the skeleton are the primary organs of the skeletal system. They form the rigid framework of the body and perform other functions, described shortly. Two types of bone connective tissue are present in most of the bones of the body: compact bone and spongy bone. Compact bone (also called dense or cortical bone) is a relatively rigid connective bone tissue that appears white, smooth, and solid. It makes up approximately 80% of the total bone mass. Spongy bone (also called cancellous or trabecular bone) is located internal to compact bone, appears porous, and makes up approximately 20% of the total bone mass.
Describe the types and locations of cartilage within the skeletal system
Cartilage is a semirigid connective tissue that is more flexible than bone. Mature cartilage is avascular (lacks a blood supply).
∙ Hyaline cartilage attaches ribs to the sternum (costal cartilage), covers the ends of some bones (articular cartilage), and is the cartilage within growth plates (epiphyseal plates). Hyaline cartilage also provides a model during development for the formation of the fetal skeleton.
∙ Fibrocartilage is a weight-bearing cartilage that withstands compression. It forms the intervertebral discs, the pubic symphysis (cartilage between bones of the pelvis), and the cartilage pads of the knee joints (menisci).
Explain the general functions of bone
Bones perform several basic functions: support and protection, levers for movement, hemopoiesis, and storage of mineral and energy reserves.
Describe the structural components of a long bone.
Long bones are greater in length than width. These bones have an elongated, cylindrical shaft (diaphysis). This is the most common bone shape. Long bones are found in the upper limbs (namely, the arm, forearm, palm, and fingers) and lower limbs (thigh, leg, sole of the foot, and toes). Long bones vary in size. The small bones in the fingers and toes are long bones, as are the larger tibia and fibula of the lower limb.
Compare the gross anatomy of other bones to that of a long bone
One of the principal gross features of a long bone is its shaft, which is called the diaphysis. The elongated, usually cylindrical diaphysis provides for the leverage and major weight support of a long bone. Extending internally from the compact bone along the length of the diaphysis are spicules (thin, needlelike structures) of spongy bone. The hollow, cylindrical space within the diaphysis is called the medullary (marrow) cavity. In children, this cavity contains red bone marrow, which later is replaced by yellow bone marrow in adults.
An expanded, knobby region called the epiphysis is at each end of a long bone. A proximal epiphysis is the end of the bone closest to the body trunk, and a distal epiphysis is the end farthest from the trunk. An epiphysis is composed of an outer, thin layer of compact bone and an inner, more extensive region of spongy bone. Spongy bone within the epiphysis resists stress that is applied from many directions. Covering the joint surface of an epiphysis is a thin layer of hyaline cartilage called the articular cartilage. This cartilage helps reduce friction and absorb shock in movable joints.
The metaphysis is the region in a mature bone sandwiched between the diaphysis and the epiphysis. This region contains the epiphyseal plate (or growth plate) in a growing bone. It is a thin layer of hyaline cartilage that provides for the continued lengthwise growth of the bone. The remnant of the epiphyseal plate in adults is a thin, defined area of compact bone called the epiphyseal line.
Coverings and Linings of Bone
A tough sheath called periosteum covers the outer surface of the bone except for the areas covered by articular cartilage. The periosteum consists of two layers. The outer, fibrous layer of dense irregular connective tissue protects the bone from surrounding structures, anchors blood vessels and nerves to the surface of the bone, and serves as an attachment site for ligaments and tendons. The inner, cellular layer includes osteoprogenitor cells, osteo- blasts, and osteoclasts. The periosteum is anchored to the bone by numerous collagen fibers called perforating fibers, or Sharpey’s fibers, which run perpendicular to the diaphysis.
The endosteum is an incomplete layer of cells that covers all internal surfaces of the bone within the medullary cavity. The endosteum, like the periosteum, contains osteoprogenitor cells, osteoblasts, and osteoclasts.
Compare and contrast the structure and location of the two types of bone marrow
Bone marrow is the soft connective tissue of bone that includes both red bone marrow and yellow bone marrow. Red bone marrow (also called myeloid tissue) is hemopoietic (i.e., blood cell–forming) and contains reticular connective tissue, developing blood cells, and adipocytes The locations of red bone marrow differ between children and adults. In children, red bone marrow is located in the spongy bone of most of the bones of the body as well as the medullary cavity of long bones. Much of the red bone marrow changes as children mature into adults. Primarily within the medullary cavities of long bones and inner core of most epiphyses there is a progressive decrease in developing blood cells and an increase in adipocytes. This fatty-appearing substance is called yellow bone marrow. As a result, adults have red bone marrow only in selected portions of the axial skeleton, such as the flat bones of the skull, the vertebrae, the ribs, the sternum, and the ossa coxae (hip bones). Adults also have red bone mar-
row in the proximal epiphyses of each humerus and femur.
Gross Anatomy of Other Bone Classes
Short, flat, and irregular bones differ in their gross anatomic structure from long bones. The external surface generally is composed of compact bone, the interior is composed entirely of spongy bone, and there is no medullary cavity. Observe the layer of spongy bone in between the roughly parallel segments of compact bone.
Name the four types of bone cells and their functions
- osteoprogenitor cells: stem cells derived from mesenchyme. divide by mitosis. another stem cell is produced along with a “committed cell” that matures into an osteoblast
- osteoblasts: formed from osteoprogenitor cells. main function is to synthesize and secrete initial semisolid organic form of bone matrix called OSTEOID. these osteoids later calcify as a result of salt crystal deposition. this traps osteoblasts in the matrix they created and become osteocytes.
- osteocytes: mature bone cells deriving from osteoblasts. function is to maintain bone matrix and detect mechanical stress on bone.
- osteoclasts: large, multinuclear, phagocytic cells. function is to break down bone during a process called bone resorption.
Composition of the Bone Matrix
The matrix of bone connective tissue has both organic and inorganic components. The organic component is osteoid, which is produced by osteoblasts. Osteoid is composed of both collagen and a semisolid ground substance of proteoglycans and glycoproteins that suspends and supports the collagen fibers. These organic components give bone tensile strength by resisting stretching and twisting, and contribute to its overall flexibility.
The inorganic portion of the bone matrix is made up of salt crystals that are primarily calcium phosphate, Ca3(PO4)2. Calcium phosphate and calcium hydroxide, Ca(OH)2, interact to form crystals of hydroxyapatite, which is Ca10(PO4)6(OH)2. These crystals deposit around the long axis of collagen fibers in the extracellular matrix. The crystals harden the matrix and account for the rigidity or relative inflexibility of bone that provides its compressional strength. A loss of protein, or the presence of abnormal protein, results in brittle bones; insufficient calcium results in soft bones.
Bone formation
Bone formation begins when osteoblasts secrete osteoid. Calcification, or mineralization, subsequently occurs to the osteoid when hydroxyapatite crystals deposit in the bone matrix. Calcification is initiated when the concentration of calcium ions and phosphate ions reaches critical levels and precipitate out of solution, thus forming the hydroxyapatite crystals that deposit in and around the collagen fibers. The entire process of bone formation requires a number of substances, including vitamin D and vitamin C (which is required for collagen formation), as well as calcium and phosphate for calcification.
Bone resorption
Bone resorption is a process whereby bone matrix is destroyed by substances released from osteoclasts into the extracellular space adjacent to the bone. Proteolytic enzymes released from lysosomes within the osteoclasts chemically digest the organic components (collagen fibers and proteoglycans) of the matrix, while hydrochloric acid (HCl) dissolves the mineral parts (calcium and phosphate) of the bone matrix. The liberated calcium and phosphate ions enter the blood. Bone resorption may occur when blood calcium levels are low.
Compare and contrast the microscopic structure of compact bone and spongy bone
Compact bone is composed of small, cylindrical structures called osteons, or Haversian systems. An osteon is the basic functional and structural unit of mature compact bone. Osteons are oriented parallel to the diaphysis of the long bone.
Unlike compact bone, spongy bone contains no osteons Instead, its structure is an open lattice of narrow rods and plates of bone, called trabeculae. Bone marrow fills in between the trabeculae. Between adjacent lamellae are osteocytes resting in lacunae, with numerous canaliculi radiating from the lacunae. Nutrients reach the osteocytes by diffusion through cytoplasmic processes of the osteocytes, which extend within the canaliculi that open onto the surfaces of the trabeculae.
Note that the trabeculae often form a meshwork of crisscrossing bars and plates of small bone pieces. This structure provides great resistance to stresses applied in many directions by distributing the stress throughout the entire framework.
List the bones that are produced by intramembranous ossification
Intramembranous ossification literally means “bone growth within a membrane.” It is so named because the thin layer of mesenchyme in these areas is sometimes referred to as a membrane. Intramembranous ossification also is called dermal ossification because the mesenchyme that is the source of these bones is in the area of the future dermis. Mesenchyme is an embryonic connective tissue that has mesenchymal cells and abundant ground substance.
Intramembranous ossification produces the flat bones of the skull (e.g., frontal bone), some of the facial bones (e.g., zygomatic bone, maxilla), the mandible (lower jaw), and the central part of the clavicle (collarbone).
List the four main steps in intramembranous ossification
- Ossification centers form within thickened regions of mesenchyme beginning at the eighth week of
development. Some cells in the thickened, condensed mesenchyme divide, and the committed cells that are formed then differentiate into osteoprogenitor cells. Some osteoprogenitor cells become osteoblasts and begin to secrete osteoid. Multiple ossification centers develop within the thickened mesenchyme as the number of osteoblasts increases. - Osteoid undergoes calcification. Osteoid formation is quickly followed by calcification, as calcium salts are deposited onto the osteoid and then they crystallize (solidify). When calcification entraps osteoblasts within lacunae in the matrix, the entrapped cells become osteocytes.
- Woven bone and its surrounding periosteum form.
- Lamellar bone replaces woven bone, as compact and spongy bone form.
List the bones produced by endochondral ossification
endochondral ossification begins with hyaline cartilage and produces most bones of skeleton including upper, lower limbs, pelvis, vertebrae, ends of clavicle.
List the steps in endochondral ossification of a long bone
- A hyaline cartilage model of bone forms.
- Bone first replaces hyaline cartilage in the diaphysis.
- Next, bone replaces hyaline cartilage in the epiphyses.
- Eventually, bone replaces hyaline cartilage everywhere, except the epiphyseal plates and articular cartilage.
- By a person’s late 20s, all epiphyseal plates typically have ossified, and lengthwise bone growth is complete.
Differentiate interstitial and appositional growth of bone
Bone growth occurs in length through interstitial growth within the epiphyseal plate and in width through appositional growth at the periosteum.
List the five zones of the epiphyseal plate & describe how growth in length occurs there
- Zone of resting cartilage. This zone is farthest from the medullary cavity of the diaphysis and nearest the epiphysis. It is composed of small chondrocytes distributed throughout the cartilage matrix. It resembles mature and healthy hyaline cartilage. This region secures the epiphysis to the epiphyseal plate.
- Zone of proliferating cartilage. Chondrocytes in this zone undergo rapid mitotic cell division, enlarge slightly, and become aligned like a stack of coins into longitudinal columns of flattened lacunae. These columns are parallel to the diaphysis.
- Zone of hypertrophic cartilage. Chondrocytes cease dividing and begin to hypertrophy (enlarge in size) in this zone. The walls of the lacunae become thin because the chondrocytes resorb matrix as they hypertrophy.
- Zone of calcified cartilage. This zone usually is composed of two or three layers of chondrocytes. Minerals are deposited in the matrix between the columns of lacunae; this calcification destroys the chondrocytes and makes the matrix appear opaque.
- Zone of ossification. The walls break down between lacunae in the columns, forming longitudinal channels. These spaces are invaded by capillaries and osteoprogenitor cells from the medullary cavity. New matrix of bone is deposited on the remaining calcified cartilage matrix.
Describe the steps of appositional growth
In this process, osteoblasts in the inner cellular layer of the periosteum produce and deposit bone matrix within layers parallel to the surface, called external circumferential lamellae. These lamellae are analogous to tree rings: As they increase in number, the structure increases in diameter. Thus, the bone becomes wider as new bone is laid down at its periphery. As this new bone is being laid down, osteoclasts along the medullary cavity resorb bone matrix, creating an expanding medullary cavity. The combined effects of bone growth at the periphery and bone resorption within the medullary cavity transform an infant bone into a larger version called an adult bone. Appositional growth continues throughout an individual’s lifetime.
Describe bone remodeling & explain how this remodeling is affected by mechanical stress on bone
The continual deposition of new bone tissue by osteoblasts and resorption of bone by osteoclasts are called bone remodeling. Mechanical stress occurs in the form of weight-bearing movement and exercise, and it is required for normal bone remodeling. Stress is detected by osteocytes and communicated to osteoblasts. Osteoblasts increase the synthesis of osteoid, and this is followed by deposition of mineral salts. Bone strength increases over a period of time in response to mechanical stress. Mechanical stresses that significantly affect bone result from skeletal muscle contraction and gravitational forces.
List the hormones that influence bone growth and bone remodeling & describe their effects
Growth hormone: Stimulates liver to produce the hormone IGF, which causes cartilage proliferation at epiphyseal plate and resulting bone elongation
Thyroid hormone: Stimulates bone growth by stimulating metabolic rate of osteoblasts
Calcitonin: Promotes calcium deposition in bone and inhibits osteoclast activity
Calcitriol: Stimulates absorption of calcium ions from the small intestine into the blood
Parathyroid hormone: Increases blood calcium levels by encouraging bone resorption by osteoclasts
Sex hormones (estrogen and testosterone): Stimulate osteoblasts; promote epiphyseal plate growth and closure
Glucocorticoids: Increase bone loss and, in children, impair bone growth when there are chronically high levels of glucocorticoids
Serotonin: Inhibits osteoprogenitor cells from differentiating into osteoblasts when there are chronically high levels of serotonin
Explain the activation of vitamin D to calcitriol
- Ultraviolet light converts the precursor molecule in keratinocytes of the skin (7-dehydrocholesterol, a modified cholesterol molecule) to vitamin D3 (cholecalciferol), which is released into the blood. (Vitamin D3 also is absorbed from the small intestine into the blood from the diet.)
- Vitamin D3 circulates throughout the blood. As it passes through the blood vessels of the liver, it is converted by liver enzymes to calcidiol by the addition of a hydroxyl group (—OH). Both steps 1 and 2 occur continuously with limited regulation.
- Calcidiol circulates in the blood: As it passes through blood vessels of the kidney, it is converted to calcitriol by kidney enzymes (when another —OH group is added). Calcitriol is the active form of vitamin D3. The presence of parathyroid hormone increases the rate of this final enzymatic step in the kidney. Thus, greater amounts of calcitriol are formed when parathyroid hormone is present.
