WBR0763

Author	[[PageAuthor::Rim Halaby, M.D. [1]]]
Exam Type	ExamType::USMLE Step 1
Main Category
Sub Category
Prompt	[[Prompt::Osteogenesis is a complex process that results in the development of bone. Intramembranous and endochondral ossifications are 2 phenomena that lead to osteogenesis. Both modes are similar in their conversion of a preexisting tissue into bone. However, intamembranous and endochondral ossifications differ greatly in the means to form bone. Which of the following statements is true regarding osteogenesis?]]
Answer A	AnswerA::Flat bones of the skull are formed by endochondral ossification
Answer A Explanation	AnswerAExp::Flat bones of the skull are formed by intramembranous ossification.
Answer B	AnswerB::Osteoblasts are derived from mesenchymal cells that originate from the neural crest.
Answer B Explanation	AnswerBExp::Osteoblasts are indeed derived from mesenchymal cells that originate from the neural crest. In contrast, osteoclasts are derived from macrophage stem cells.
Answer C	AnswerC::Activation of transcription factor CBFA1 is involved in endochondral ossification
Answer C Explanation	AnswerCExp::CBFA1 is a transcription factor involved in the process of intramembranous ossification. In contrast, the two transcription factors Pax1 and Scleraxis, are involved in endochondral ossification.
Answer D	AnswerD::Endochondral ossification spreads inwards in long bones from the edges towards the center of the bone
Answer D Explanation	AnswerDExp::Endochondral ossification spreads outward in long bones from the center towards the edges.
Answer E	AnswerE::Osteoblast differentiation is regulated by osteoprotegerin and its ligand.
Answer E Explanation	AnswerEExp::Osteoprotegerin, also known as osteoclastogenesis inhibitory factor (OCIF) is a hormone that promotes the conversion of macrophage stem cell into an osteoclast.
Right Answer	RightAnswer::B
Explanation	[[Explanation::Both processes, the intramembranous and the endochondral ossifications, are required for osteogenesis. While endochondral ossification requires cartilage from mesenchyme cells to be ultimately replaced by bone, intramembranous ossification does not involve cartilage and bone is formed directly by the formation of osteoblasts from neural crest-derived mesenchymal cells that secrete collagen-proteoglycan matrix. The latter may bind to calcium salts to ultimately calcify the osteoid, which is the prebone. As osteoblasts aid in the calcification process, they are trapped within the matrix. Trapped cells are differentiated into osteocytes, or mature bone cells. Intamembranous ossification requires the activation of transcription factors, such as CBFA1. Endochondral ossification leads to formation of cartilage by chondrocytes that secrete the extracellular matrix. The cartilage forms a model is eventually replaced by bone starting at the center and then in the outward direction towards the edges. Osteoblasts start forming at the edge of the cartilage model and slowly replace cartilage by bone. Involvement of Pax1 and Scleraxis, 2 transcription factors, is required for formation of endochondral ossification. Osteoclasts, derived from macrophage stem cells, are linked to the matrix and pump hydrogen ions onto the matrix to acidify and dissolve it. Osteoprotegerin and its ligand are required for the conversion of macrophage stem cells into osteoclasts. The latter is under strict hormonal regulation at all times under physiologic conditions. Educational Objective: Osteoblasts are derived from mesenchymal cells that originate from the neural crest, while osteoclasts are derived from macrophage stem cells. Reference: Gilbert SF. Osteogenesis: The development of bone. in Developmental Biology. 6th ed. Sinauer Associates, Sunderland (MA): 2000 Educational Objective: References: ]]
Approved	Approved::No
Keyword	WBRKeyword::osteoblast, WBRKeyword::osteoclast, WBRKeyword::mesenchyme, WBRKeyword::mesenchymal, WBRKeyword::derived, WBRKeyword::derive, WBRKeyword::neural, WBRKeyword::crest, WBRKeyword::cell, WBRKeyword::cells, WBRKeyword::transcription, WBRKeyword::factor, WBRKeyword::factors, WBRKeyword::endochondral, WBRKeyword::intramembranous, WBRKeyword::ossification, WBRKeyword::osteogenesis, WBRKeyword::skull, WBRKeyword::bone, WBRKeyword::formation, WBRKeyword::chondrocyte, WBRKeyword::osteocyte, WBRKeyword::osteoprotegerin, WBRKeyword::differentiation, WBRKeyword::differentiate, WBRKeyword::CBF1, WBRKeyword::OCIF, WBRKeyword::Pax1, WBRKeyword::Scleraxis
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