USMLE® Step 1 style questions USMLE
An 11-month-old infant boy is brought to the emergency department because of new bruises. His parents are distraught, as he has sustained multiple fractures in the past 3 months where they cannot recall any specific trauma. Vital signs are normal. Ophthalmology examination shows a blue-tinged sclera with no evidence of retinal hemorrhages. An X-ray of his left arm is shown below:
Credit: Wikimedia Commons
A hereditary defect of which of the following types of connective tissue is most likely present?
Cells live within an environment called the extracellular matrix, and it’s a bit like how homes have yards and streets that surround them.
Also, just like how lots of homes form a community, lots of cells form a tissue.
And there are different types of tissues - epithelial, connective, muscular, and nervous tissue.
Each tissue has an extracellular matrix that’s got a unique composition that’s adapted for each tissue’s unique needs.
Having said that, all of the different types of extracellular matrix are made up of three major molecules - adhesive proteins, structural proteins, and proteoglycans.
First, there’s adhesive proteins, which help to stick the individual cells together and organizes the tissue into a neat structure.
Now, adhesive proteins - like integrins and cadherins - are found on the cell’s surface and they’re like molecular velcro.
Cells use adhesive proteins to anchor themselves to other cells and to molecules in the extracellular matrix.
Adhesive proteins also help communicate messages from the extracellular matrix to the cell.
For example, signals relayed by integrins can help a cell decide when it’s time to grow, divide, differentiate, or even die - like in apoptosis.
Next, there are structural proteins which give our tissues their tensile and compressive strength.
Some examples are collagens, elastins, and keratins.
Collagen is the most common type of structural protein in the human body, mostly because it resists tension and it can also stretch.
When collagen is made and released into the extracellular space, it’s in the form of a precursor called procollagen.
Procollagen is a protein made up of three polypeptide strands, each coiled into a left-handed helix, and then all three are twisted together into a right-handed triple helix or "superhelix" with three loose strands at each end.
Once procollagen is in the extracellular space, it encounters a tiny band of enzymes called collagen peptidases that cleave the loose strands at both ends of the procollagen protein, turning procollagen into tropocollagen.
Once there are enough tropocollagen molecules, they bind with one another to form a stack of collagen fibrils.
Collagen fibrils are arranged in different ways depending on the tissue, and when there’s a bunch of collagen fibrils, they can join up to form a bigger tube called a collagen fiber.
The four most common types of collagen are I, II, III, and IV.