Exam 2 A&P I

Know the anatomical structures associated with a synovial joint besides the bone itself: articulating cartilage, the synovial fluid, ligaments, tendons, bursa, tendon sheath, fat pads, sensory nerves, and blood vessels.

Articulation cartilage - composed of hyaline cartilage. This covers all exposed articulating bone surfaces within the joint cavity. Avascular - does not have a direct access to a blood supply. Damage to this can contribute to arthritis. Synovial fluid - a thick, colorless, oily liquid that serves three main functions in the joint: Lubrication - lubricates the joint cavity and articulating surfaces to reduce friction. Protects the articulating ends of the bones Metabolic functions - supplies nutrients such as glucose to the cells in the joint cavity. Also removes metabolic waste products from these cells. Shock absorption - helps to evenly distribute the force and stress exerted on the articular surfaces of the bones during movement. Ligaments - a strand of dense regular collagenous connect tissue that connects one bone to another to strengthen and reinforce the joint. Intrinsic ligament - thickened regions of the articular capsule - found within the articular capsule Extrinsic ligament - not part of the articular capsule, they may be inside or outside the joint cavity. Tendons - a structural component of a skeletal muscle composed of dense regular collagenous CT. connects the muscle to a bone or another structure. Typically cross over or around a joint to that when a muscle contracts, the force generated is transmitted across the joint. The force results in motion as the tendon pulls on the bone to which it attaches. Often stabilize joints, particularly joints that have multiple tendons crossing them. Example - tendon of the long head of the biceps brachii muscles crosses the shoulder joint, stabilizing the head of the humerus in the joint cavity. Bursa - a synovial fluid-filled structure resembling a limp water balloon. A fibrous structure lined with a synovial membrane. May be attached to the articulate capsule or completely separate. Generally found in regions of high stress where bones, tendons, muscles and skin interact in a small space. Minimize friction between all the moving parts. Tendon sheath - long bursae that surround some tendons in high-stress regions of body. Protect long tendons as they course over and around the synovial joints. Fat pads - adipose tissue pads that protect the articular cartilage. Seen in the infrapatellar fat pad in knee. Sensory nerves - nerves that help muscles move in the joint. The knee joint is supplied by branches from the femoral, sciatic and obturator nerves. Blood vessels - joints are surrounded by a rich network of arteries and veins. Arteries give off subdivisions that join freely on its outer surface. Vessels formed branches leading to the fibrous capsule and ligaments to the synovial membrane. These are accompanied by nerves and they proliferate to form other networks which capillaries go to all parts of the membrane.

Explain and discuss the effects of aging on bone

As we age, individual bone growth hormone and sex hormones decline. A decrease protein synthesis in bone.

Name the components of the skeletal system:

bone, cartilage, tendons, ligaments, and joints

Each ligament and what it does to support the knee

1. Anterior Cruciate ligament(ACL) : controls rotation and forward movement of shin bone and is located in center of knee, ACL connects thigh bone and helps stabilize the knee joint. 2. Medial Collateral ligament (MCL): the inner side of knee connecting medial femoral condyle and the medial tibial condyle, it helps to stabilize the inner knee. 3. Posterior cruciate ligament (PCL): located on the back of knee, it controls backward movement of the tibia, it is larger and stronger than the ACL. 4. Lateral collateral ligament (LCL): located on the outer side of the knee and belongs to the extrinsic knee ligament and posterolateral corner of the knee, it gives stability to the outer knee.

be able to describe the pathology of the "unhappy triad"

Any activity that involves quick changes in direction can injure the knee. Athletes who participate in contact sports such as football or soccer are particularly at risk, especially if the knee is struck from the side or behind. A lateral blow - such as when a football player is illegally blocked below the knees- often ruptures the tibia collateral ligament. This often results in the tearing of the lateral meniscus. Such a force generally also ruptures the anterior cruciate ligament, forming this. The treatment almost invariably requires surgery to repair the damage to the anterior cruciate ligament and often the damaged meniscus. Followed by physical therapy. Page 276 in book. Reread.

Discuss bone growth: appositional and interstitial

Appositional - bones growing in width. Osteoblasts between the periosteum and bone surface lay down new bone. New circumferential lamellae are formed. Primarily thickens the compact bone of the diaphysis. Bone growth in width may continue after bone growth in length ceases, depending on factors such as hormones, diet. Interstitial - long bones lengthen during this process. Occurs not from the division of osteocytes or other bone cells, but from the division of chondrocytes in the epiphyseal plate.

Understand the hormonal influence on bone deposition and resorption (PTH and thyroid hormone)

Bone deposition - carried out by osteoblasts in the periosteum and endosteum. (Osteoblasts) Make the components of the organic matrix as well as facilitate the formation of inorganic matric. Secrete certain proteoglycans and glycoproteins that bind to calcium ions, ATP, enzymes. These vesicles bind to collagen fibers, and their calcium ions eventually crystallize, which ruptures the vesicle and begins the overall process of calcification. Bone resorption - during this, osteoclasts secrete hydrogen ions from their ruffled borders onto the bone ECM. These make the PH more acidic which breaks down the PH sensitive hydroxyapatite crystals in the inorganic matrix. The released calcium ions and other minerals ma then be reused in the body. To degrade organic matric, osteoclasts secrete enzymes to catalyze reactions that break down proteoglycans, glycosaminoglycan and glycoproteins, the breakdown products are taken into the osteoclasts for possible reuse.

Identify different bone markings and attachment points for tendons and ligaments.

Depressions - may provide pathways for blood vessels and nerves, or sites for articulations or joins between bones. Openings - enclose delicate structures and allow them to travel through bones Projections - provide sites where bones articulate or where ligaments and tendons attach

Show and discuss the gross anatomy of long bones, naming its parts: diaphysis, metaphysis, epiphysis, medullary cavity, articular cartilage, and epiphyseal line.

Diaphysis - shaft of the long bone Metaphysis - narrow portion of a long bone between the epiphysis and the diaphysis Epiphysis - located at the ends of the bones which are covered with a thin layer of hyaline cartilage known as articular cartilage, which allows bones to rub together with reduced friction at joints. Medullary cavity - located inside the long bone, a hollow cavity. Where much of the marrow of a long bone is located at. Epiphyseal line - in adult bones. Also called growth plate. A hyaline cartilage plate in the metaphysis located at each end of a long bone.

Know the difference between periosteum and endosteum

Endosteum - all inner surfaces of bone, lined with this. A membrane. Contains different types of bone cells that help maintain bone homeostasis. This is thinner and lacks the fibrous outer layer that is in the periosteum. Periosteum - membrane that covers most of the surface of the long bone. Composed of dense irregular collagenous connective tissue that is richly supplied with blood vessels and nerves. Firmly attached to underlying bones by collagen fibers called perforating fibers.

Explain the extracapsular or intracapsular ligament.

Extracapsular or intracapsular ligament - the function is to hold the bones in place, provide stability and prevent dislocation injuries. Located inside the articular capsule. The knee has two of these - the MCL (located on the inside/medial surface of the knee) and the LCL which runs along the outside/lateral of the knee.

Explain the differences between male and female os coxae (hip bone).

Female - wider and shorter sacrum. Has a wide greater pelvis. Pelvic inlet is oval shaped. Acetabula is farther apart. Pelvic outlet is wide. Larger pubic arch - 90-100 degree angle. Males - narrow, longer sacrum and narrow greater pelvis. The pelvic inlet is heart shaped. Has a smaller pubic arc - 60-70 degree angle. Acetabula is closer together.

Explain that there are three (four) types of joints based on structural characteristics: fibrous joint, (bony joint), cartilaginous joint and synovial joint

Fibrous joint - sutures - dense fibrous CT. Syndesmoses - denser fibrous CT than a suture. Interosseous membrane - a broad sheet o dense fibrous CT. Can by synarthroses or amphiarthroses. Lacks a synovial cavity. Permits little to no movement. Cartilaginous joint - lack a synovial cavity. Held together with cartilage connective tissue. Permits little to no movement. Types: synchondroses and symphyses. Synovial Joint - articular cartilage on ends of long bones and a synovial cavity between articulating bones surrounded by accessory ligaments; freely moveable. Examples: hip, knee, elbow. Held together by ligaments. Have a nerve and blood supply and are surrounded by an articular capsule. Bony joint - between two or more bones without intervening cartilage or fibrocartilage. Infused by osseous tissue and in most cases are then regarded as a single bone. EX: frontal bones that fuse into one bone during infancy

Know in detail the five major types of movements at the various types of synovial joints: gliding motion, angular motion, rotational motion, and special movements and where you may find each one.

Gliding motion - a sliding motion between the articulating surfaces of bones in a joint. Consider nonaxial b/c the bones slide past each other in a single plane but not around an axis. Find this movement between the intertarsal joints of the ankle and foot. Angular motion - increase or decrease the angle between the articulating bones. Several types include: Flexion - decreases the angle between the bones by bringing the two bones closer together. EX: action of the elbow joint in which the forearm is pulled toward the arm. Extension - opposite of flexion - increases the angle between articulating bones. EX: the elbow is flexed with the forearm close to arm. Hyperextension - extension beyond the anatomical position of the joint. EX: when you prepare to pitch a softball as your arm moves behind the trunk of your body. Abduction - motion of a body part away from the midline of body or another reference point. EX: the first movement of a jumping jack exercise - spreading the legs at the hips and raising the arms at the shoulders. Arms and legs are moving away from the midline of the body Adduction - opposite of abduction. Motion of a body part toward the midline of the body. EX: completed the second movement of the jumping jack adducts the arms and legs by bringing them back toward the body. Also bringing fingers and toes back toward the reference point. Circumduction - freely moveable distal bone moves around a stationary proximal bone in a cone-shaped motion. Best seen in the hip and shoulder joints. A complex movement that is the sum total of flexion-extension and abduction-adduction. EX: if you draw a circle on the wall by moving only your shoulder. Rotational motion - a nonangular, pivoting motion, which one bone rotates or twists on an imaginary line running down its middle, known as its longitudinal axis. EX: shaking head to indicate "no". Multiaxial joints such as hip and shoulder are also able to rotate. Internal (medial) rotation - of the hip is best appreciated by using the foot as a reference point when the hip is internally rotated the toes point medially toward the midline of the body where: External (lateral) rotation - toes point away from the midline of the body. Special movements - Opposition - occurs only at the thumb or first carpometacarpal joint which involves movement of the thumb across the palm of the hand. Using the thumb to touch the tip of your fingers Reposition - is the return of the thumb to its anatomical position Depression - the movement of a body part in an inferior direction. EX: opening the mouth depresses the mandible, which moves inferiorly relative to the maxilla. Elevation - is the opposite of depression. EX: mandible is pulled up toward the maxilla in a super direction. Protraction - moves a body part in the anterior direction. EX: moving the mandible forward so the inferior teeth stick out Retraction - opposite motion of protraction. EX: once the mandible is protracted, it must be retracted to return the mandible to its normal position Inversion - rotational motion of the foot in which the plantar surface (sole of the foot) rotates medially toward the midline of the body Eversion - opposite. Where the plantar surface of the foot rotates laterally away from the midline of the body. THESE ARE IMPORTANT FOR WALKING Dorsiflexion - involves the foot and ankle. The angle between the foot and the tibia decreases. The toes are pulled toward the head Plantarflexion - opposite. Angle between the foot and tibia increases. The toes point to the ground Supination & pronation - rotational movements that occur at the proximal radioulnar joint of the forearm and at the combination of the ankle and tarsal joints. Turning your and facing anteriorly (supination) and posteriorly (pronation).

Know the characteristics and give examples of the three types of fibrous joints: gomphosis joint, suture joint, and syndesmosis joint.

Gomphosis joints - a fibrous joint between a tooth and its corresponding alveolus in the mandible or maxilla. This is a synarthrosis, as its primarily function is to provide stability and hold the tooth in place. Suture joint - between bones that make up the skull. Held together by very short collagen fibers that are part of dense regular collagenous connective tissue. Makes the sutures very stable synarthrosis and well suited for reinforcing the bones that protect the brain. Syndesmosis joint - in which the articulating bones are joined by a long membrane - the interosseous membrane or ligament. Which is composed of dense regular collagenous connective tissue. An example of this is what is between (holding together) the ulna and the radius. Also between the fibula and tibia.

anatomical construct and purpose of each component of compact bone: Haversian canal, concentric lamellae, osteocytes, lacunae, canaliculi, Volkmann canals, external circumferential lamellae, internal circumferential lamellae, and interstitial lamellae.

Haversian canal - forms a network in bone and contains blood vessels. Also called the osteon. Functional unit of the compact bone. Roughly cylindrical. Blood vessels, nerve fibers and lymphatics pass through. Concentric lamellae - made up of 4 to 20 lamellae. This greatly enhances its strength. Osteocytes - bone cells. Formed when an osteoblast becomes embedded in the matrix Lacunae - small cavities that are filled with extracellular fluid and located between lamellae. Canaliculi - the lacunae are connected to one another by tiny canals called canaliculi. Volkmann canals - central canals of neighboring osteons are connected by a second type of canal called a perforating canal (which is also known as this). These run perpendicular to the osteons and they carry blood vessels from the periosteum that merge with vessels in the central canals. External circumferential lamellae - add strength and tension resistance to the bone as whole Internal circumferential lamellae - Interstitial lamellae - remnants of resorbed osteons.

Know and discuss the anatomical construct and manner of movement of the knee joint.

Largest diarthrosis in the body. A hinge joint that allows flexion and extension, which when flexed it allows some degree of rotation and lateral gliding. Has two separate articulations: Tibiofemoral joint - where the tibia and femur come together. Patellofemoral joint - where the patella and femur come together.

Know the classifications of bone determined by shape and where you find each one: long bones, short bones, flat bones, and irregular bones

Long bones -longer than they are wide. Include most bones in arms and legs. Short bones - roughly cube-shaped or about as long as they are wide. Wrist or carpals and ankle or tarsals. Flat bones - thin and broad bones. Ribs, pelvis, sternum and most bones in the skull Irregular bones - vertebrae and certain skull bones

Know the planes of motion at a synovial joint: uniaxial, biaxial, and multiaxial.

Nonaxial - allow motion to occur in one or more planes but do not move across an axis Uniaxial - allows motion around only one axis. EX: elbow joint Biaxial - allows motion around two axis. EX: metacarpophalangeal joint (in the hand) Multiaxial/triaxial - allows motion around three axes. EX: shoulder joint

Know the histology of bone ... i.e. types of cells, organic/inorganic material

Osteoblasts - active bone cells found in periosteum and endosteum Osteogenic cells - flattened cells that differentiate into osteoblasts when stimulated by specific chemical signals. Bone building cells that perform bone deposition (produces the bone) Inorganic matrix - about 65% of the total weight of bone. Contains calcium salts. Stores 85% of the total calcium ions in the body. Hydroxyapatite crystals which makes the bone one of the hardest substances in the body and gives bone its strength and ability to resist compression. Also included: bicarbonate, potassium, magnesium and sodium salts. Organic matrix - remaining 35% of the total weight of bone. Also known as an osteoid. Consists of protein fibers, proteoglycans, glycosaminoglycan, glycoproteins and bone specific proteins such as osteocalcin. These protein fibers are collagen fibers, which form cross-links with one another to help the bone resists twisting and tensile (pulling or stretching forces). Other components of organic matrixs perform support functions. Large compounds such as glycosaminoglycans and proteoglycans in the ECM draw water out of the blood vessels and cells by osmosis. Collagen fibers are the most important component of the ECM in terms of strength,

Understand that a synovial joint can be classed into six types: planar joint, hinge joint, pivot joint, condylar joint, saddle joint, and ball-and-socket joint.

Planar joint (plane) - simplest and least mobile. Features two bones whose flat surfaces sit next to each other. These are nonaxial - only side to side "gliding" movements. The intercarpal joints of the wrist are examples. Hinge joint - a convex surface of one bone fits into a concave depression of another bone. Motion is restricted to movement in only one place. EX: elbow, knee Pivot joint - a uniaxial joint in which the rounded surface of one bone fits into a groove on the surface of another bone. The stability at this joint is enhanced by a ring like ligament that surrounds the rounded bone and holds it in the groove of the second bone. Allows the bone to rotate or pivot on its longitudinal axis within this ring. EX: the joint between the atlas and axis (cervical vertebrae) is an example. Condylar joint/ellipsoid - A biaxial joint in which the oval, convex surface of one bone fits into the shallow concave surface of the other bone. Allows both flexion and extension, abduction and adduction. EX: metacarpophalangeal joints of the fingers. Saddle joint - resemble a horse-back riding saddle. The surface of each articulating bone has both convex and concave regions that complement each other. This allows for a good fit, much like putting one saddle on the found and then stacking a second one on top. Has a large amount of motion. Biaxial but allows greater motion than condylar joints. Found between your first metacarpal of the thumb and the trapezium. Allows the finger and thumb to work together for opposition and reposition. Ball and socket joint - a multiaxial joint in which the articulating surface of one bone is ball-shaped and fits into a cup or socket formed by the articulating surface of the other bone. Both the shoulder and hip joints are examples.

Know functions of the skeletal system: support, protection, movement, hematopoiesis, storage of minerals, and storage of energy reserves.

Protection - protects underlying organs: skull, sternum, ribs and pelvis. Support - supports weight of body and provides its structural framework Movement - serve as sites for attachment for most skeletal muscles

Understand the functions and location of the two types of bone marrow

Red Bone Marrow - consists of a network of reticular fibers supporting island of blood-forming, or hematopoietic cells. Remains mostly in the bones of the pelvis, the proximal femur and humerus (thigh and arm bones), vertebrae, ribs and sternum, the clavicles and scapulae. Also found in certain bones of the skull, but degenerates as we age. Yellow Bone Marrow - stores triglycerides, consists mostly of blood vessels and adipocytes. Infants and young adults most of the marrow is red bone marrow. By the time we are adults most of the marrow is yellow. Found in central cavities of long bones.

Discuss the three main spinal curvature deformities: kyphosis, lordosis, and scoliosis

Scoliosis - characterized by abnormal lateral curvatures in vertebral column. Give it a C or S shape when viewed from the posterior and anterior side. May be caused by deformities present at birth, caused by abnormalities of or trauma to the nerves and muscles around the vertebral column, or idiopathic which is of unknown cause. Server cases can put pressure of the heart and lungs and will require treatment. Lordosis (swayback) - characterized by exaggerated cervical and lumbar curvatures. Normal in young children but generally diminishes with age. Abnormal lordosis, somethings called hyperlordosis, is commonly seen in adults bearing extra abdominal weight. Puts extra stress on the lumbar vertebrae and can lead to lower back pain. Generally treated with weight loss. Kyphosis - exaggeration of thoracic curvature resulting in hunchback appearance. Caused by joint conditions such as arthritis, bone conditions such as osteoporosis and vertebral fractures and developmental abnormalities. Severe causes can lead to heart and lung dysfunction, nerve compression and significant pain, will require surgery.

Discuss bone fracture and repair.

Spiral - resulting from twisting forces applied to the bone Comminuted - in which the bone is shattered into multiple fragments; difficult to repair Greenstick - in which the bone breaks on one side but only bends on the other side. Compression - in which the bone is crushed under the weight it is meant to support. Common in the elderly and those with reduced bone mass Avulsion - in which a tendon or ligament pulls off a fragment of bone; often seen in ankle fractures Epiphyseal plate - involves at least part of the epiphyseal plate. Occurs only in children and young adults which may interfere with growth. Simple fractures - skin and tissue around fracture remain intact. Also called closed fractures. Compound fractures - skin and tissues around fracture are damaged. Also called open fractures. Know the phases which are listed on the notes you printed out. The last page. Also on page 206 in book.

Know and understand the differences between spongy bone and compact bone

Spongy bone - usually not the weight-bearing part of the bone. Does not have a dense structure like that of a compact bone. This resists forces from many directions and forms a protective framework for the bone marrow. Compact bone - as an outer shell. Subject to great amount of stress, or forces that tend to strain or deform an object. Built in a way that enables it to withstand these stresses quite well. Resembles a forest of small, tightly packed trees. Which is called an osteon.

Explain and understand the classification of joints based on their degree of motion; synarthrosis, amphiarthrosis, and diarthrosis.

Synarthrosis - doesn't have any movements. Examples: sutures of skull, provides the greatest stability. Amphiarthrosis - has slight movement. Intervertebral discs are an example. Provides a significant amount of stability. Diarthrosis - freely moveable. Found in appendages. Provides the least amount of stability

Compare, and contrast the two types of cartilaginous joints; synchondrosis joint and symphyses joint.

Synchondrosis joint - consists of bones united together by hyaline cartilage. Permits essentially no motion and so are functionally synarthroses. One example of this is the epiphyseal plate. Found in adulthood are found in the rib cage - the first sternocoastal join where the first rib connects to the manubrium of the sternum, stabilizes the rib cage. Additional are the costochondral joints - found where the ribs attach to their costal cartilages. Symphyses joints - in which the bones are united by a tough fibrocartilage pad. These joints permit small amount of motion and so are amphiarthroses. Best suited for regions of the skeleton that must resist compression and tension while still allowing a small amount of motion. An example: intervertebral joint - located between adjacent vertebral bodies of the spinal column. Another is the pubic symphysis - located between the two pubic bones of the pelvic girdle.

understand the purpose of the meniscus of the knee joint.

The purpose of the meniscus of the knee joint is - provides shock absorption and cushioning between the articular cartilages of the femoral and tibia condyles. They also improve the fit between the two bones, which further stabilizes the joint.

Explain the pathology involved in various types of knee ligament and cartilage injuries

The tibia collateral ligament connects the femur with the tibia and attaches to the medial meniscus. Prevents the tibia from moving too far laterally on the femur. The fibular collateral ligament - links the femur to the fibular and prevents the tibia from moving too far medially on the femur. This does not attach to the lateral meniscus.

Explain and discuss the pathologic condition known as osteoporosis

This is a bone disease which is due to inadequate inorganic matrix in the ECM. Has a variety of causes, including dietary factors (such as calcium ion and vitamin D deficiency), being a female, advanced age, lack of exercise, hormonal factors (such as lack of protective estrogen in postmenopausal women), genetic factors, and diseases of the skin, digestive system, and urinary system. Many of these causes are preventable. And the methods to keep this from developing are: ensuring adequate dietary calcium salt intake, engaging in weight-bearing exercise and replacing estrogen in women, if appropriate. This makes the bone brittle and so dramatically raises the risk of fractures. This bone heals more slowly when it is fractured. Treated by preventing further bone loss and increasing more mass. Other treatments include drugs that inhibit osteoclasts or stimulate osteoblasts.

b. anatomical construct and purpose of each component of spongy bone: trabeculae and parallel lamellae.

Trabeculae - branching "ribs" of bone called this. Project into the marrow cavity. Covered with endosteum and usually do not contain osteons. Contains a canaliculi and lacunae housing osteocytes. No central or perforating canals are present within this. The cell obtains their oxygen and nutrients from blood vessels in the bone marrow. Parallel lamellae - in a Haversian system. Concentric circles enclosing a blood vessel.

Endochondral Bone Formation

all of the bones in the body below the head, except the clavicles are formed by this process. Begins during the fetal period for most bones, although some such as those in the wrist and ankle ossify much later. 1. The chondroblasts in the perichondrium differentiate into osteoblasts - the perichondrium becomes filled with blood vessel, and chemical signals trigger its chondroblasts to differentiate into osteogenic cells and then osteoblasts. The perichondrium is now called the periosteum. 2. The bone begins to ossify from the outside - a. Osteoblasts build the bone collar on the external surface of the bone - osteoblasts begin to secret organic bone ECM deep to the periosteum, forming a ring of early compact bone called the bone collar b. Simultaneously, the internal cartilage beings to calcify and the chondrocytes die- as the bone collar calcifies, ECM surrounding the internal chondrocytes calcifies. This cuts off the blood supply to the chondrocytes, causing them to eventually die. Their death leaves cavities surrounded by calcified cartilage. 3. The primary ossification center, osteoblasts replace the calcified cartilage with early spongy bone; the secondary ossification centers and medullary cavity develop. 4. As the medullary cavity enlarges, the remaining cartilage is replaced by bone; the epiphyses finish ossifying - the calcified cartilage is replaced with bone. (read more on page 198 in book)

Intramembranous Bone Formation

many flat bones, including the skull and clavicles, form during fetal development by this process. Bones formed in this way are primary bone, which is eventually resorbed and replaced with secondary bone. Occurs within a mesenchymal membrane - simply a flat sheet of one or more tissue. Embryonic connective tissue. 1. Osteoblasts develop in the primary ossification center from mesenchymal cells - these differentiate first into osteogenic cells and then into osteoblasts 2. Osteoblasts secrete organic matrix, which calcifies and trapped osteoblasts become osteocytes - the newly formed osteoblasts secrete the organic matric of bone I a few days the calcium salts and other components of the inorganic matric are deposited in the primary ossification center, a process called calcification, the early bone hardens. Trapped osteoblasts then become osteocytes. 3. Osteoblasts lay down trabeculae of early spongy bone, and some of the surrounding mesenchyme differentiates into periosteum - osteoblasts continue to lay down new bone, forming the trabeculae of the early spongy bone. Over time, the trabeculae enlarge and merge, forming larger trabeculae. Some of the mesenchyme surround the developing bone differentiates into the periosteum. Some of the vascular tissue in the forming spongy bone will become bone marrow. 4. Osteoblasts in the periosteum lay down early compact bone - osteoblasts with the periosteum continue to secrete organic bone matrix. This becomes more heavily calcified than the deeper spongy bone trabeculae, and its structure is remodeled to become the immature compact bone.