Term of the Day

Colles' Fracture: Fracture of the distal radius with dorsal displacement or angulation of the distal fragment. Typically results from forceful trauma, commonly from falling on outstretched hands. The ulna is unaffected, since it doesn't form part of the wrist joint.

Term of The Day

Middle Cranial Fossa: Basin-like depression inside the skull formed by the sphenoid bone and petrous portion of the temporal bone. The temporal lobe of the brain is supported by this fossa. Illustrated in pink and yellow in the diagram to the right.

Concept of the Week

The Structural and Functional Classification of Joints

Structurally, bones are classified according to the nature of the material that joins the bones together:

1. Fibrous Joints: The bone ends are united by collagenous fibers.

2. Cartilaginous Joints: The bone ends are united by cartilage (either fibrocartilage or hyaline cartilage).

3. Synovial Joints: The bone ends are covered with articular cartilage and enclosed within a joint capsule lined with synovial membrane.


Functionally, joints are classified according to the amount of movement permitted:

1. Synarthroses: Completely immovable joints (e.g. sutures)

2. Amphiarthroses: Slightly movable joints (e.g. interosseous membrane conncting radius to ulna)

2. Diarthroses: Joints that are freely movable in one or more planes of space (e.g. knee joint)

Questions, Comments, Concerns, Feedback

Use this thread to ask questions, make general comments on topics unrelated to this week's featured concept and key terms, or just leave feedback regarding how the blog might be improved. Check the bottom of the page for this week's crossword puzzle on the parts of the cell.

Term of the Day

Comminuted Fracture: A break in which the bone fragments into many pieces. Especially common in elderly people suffering from osteoporosis.

Term of the Day

Osteoid: The organic component of bone matrix, accounting for approximately 1/3 of the matrix. It is made up of collagen fibers, glycoproteins, and proteoglycans all secreted by the osteoblasts. These substances contribute to bone structure and give bone both flexibility and tensile strength.

Term of the Day

Papillary Dermis: Thin, superficial layer of the dermis composed of loose connective tissue and thrown into nipple-like projections called dermal papillae, which interdigitate with the overlying epidermis. The papillae contain capillaries, free nerve endings (which act as pain receptors) and Meissner's corpuscles (which act as touch receptors). On the palms and soles, the papillae lie on top of larger mounds called dermal ridges which, in turn, produce corresponding epidermal ridges. The latter give us our fingerprints.

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Term of the Day

Marfan's Syndrome: Genetic disorder of connective tissue. Abnormal connective tissue results from a defect in the protein fibrillin, which helps provide scaffolding and structural support between cells. Signs and symptoms include: excessive height, abnormally loose joints, long spiderlike fingers, poor vision, and a weakened heart and blood vessels due to inadequate connective tissue reinforcement. The trait is dominant and carried by the FBN1 gene. For you punk rockers out there, Joey Ramone was diagnosed with Marfan's.

Term of the Day

Proteoglycans: A component of the ground substance of connective tissue. They consist of a protein core, to which negatively charged polysaccharides called glycosaminoglycans (CAGs) are attached. The CAGs form an intertwining network that traps water and forms a substance that varies from a viscous fluid to a semisolid gel. The higher the CAG content, the stiffer the ground substance is. Chondroitin sulfate, keratan sulfate, and hyalouronic acid are three of the most prominent glycosaminoglycans.

Term of the Day

Mesenchyme (Mesenchymal Tissue): In embryology, the first true tissue formed from the mesoderm germ layer. It is a type of connective tissue composed of star-shaped mesenchymal cells surrounded by a matrix of fluid ground substance containing fine fibers. Mesenchyme eventually differentiates into all other connective tissues.

Concept of the Week

The Mitotic Spindle

During prophase of mitosis, cytoskeletal microtubules disassemble. As the centriole pairs migrate away from each other, they organize the assembly of a new system of microtubules called the mitotic spindle. Lengthening of these microtubules pushes the centrioles farther and farther apart, ultimately pushing them to opposite poles of the cell.

Then the centrioles cause asters (also composed of microtubules) to radiate outward from the ends of the spindle and anchor it to the plasma membrane. While all this is taking place, some of the spindle fibers attach to protein complexes called kinetochores on the centromere of each pair of sister chromatids. These fibers are referred to as kinetochore microtubules. Those spindle fibers which do not attach to chromosomes are called polar or nonkinetochore microtubules.

The kinetochore tubules pull on the chromosomes until they ultimately line up at the equator of the spindle to form the metaphase plate. During anaphase, the kinetochore fibers shorten, pulling each chromosome towart the end of the cell it faces. The polar microtubules slide past each other (due to the action of kinesin motor proteins), which forces the 2 poles of the cell further apart such that the entire cell elongates.

During telophase, the spindle finally breaks down and disappears.

Term of the Day

Ligands: Signaling molecules that bind specifically to plasma membrane receptors (e.g. neurotransmitters, hormones, paracrines, etc.). Binding of the ligand changes the shape of the receptor which, in turn, may act as an enzyme or an ion channel or be coupled to one of these by a G protein.