According to the servosystem theory of growth, which of the following hormones controls the growth of the primary cartilage?
**Core Concept**
The servosystem theory of growth proposes that growth and development are controlled by a negative feedback mechanism, where the growth of the primary cartilage is regulated by the rate of bone formation. This theory highlights the importance of hormones in modulating growth, particularly in the context of bone metabolism.
**Why the Correct Answer is Right**
Insulin-like growth factor 1 (IGF-1) plays a crucial role in the servosystem theory of growth by regulating the growth of the primary cartilage. IGF-1 acts as a growth factor, stimulating the proliferation and differentiation of chondrocytes in the growth plate. This, in turn, influences the rate of bone formation and growth. IGF-1 is secreted in response to growth hormone (GH), which stimulates the liver and other tissues to produce IGF-1.
**Why Each Wrong Option is Incorrect**
**Option A:** Growth hormone (GH) is involved in the servosystem theory of growth, but it does not directly control the growth of the primary cartilage. Instead, GH stimulates the production of IGF-1, which then regulates cartilage growth.
**Option B:** Thyroid-stimulating hormone (TSH) is involved in thyroid hormone regulation, which affects growth and development, but it is not directly involved in the servosystem theory of growth.
**Option C:** Parathyroid hormone (PTH) regulates calcium and phosphate metabolism, which is essential for bone health, but it is not directly involved in the servosystem theory of growth.
**Clinical Pearl / High-Yield Fact**
IGF-1 is often used as a marker of growth hormone excess or deficiency, and its levels can be affected by various conditions, including acromegaly, growth hormone deficiency, and Turner syndrome.
**Correct Answer:** C. Insulin-like growth factor 1 (IGF-1)