Paralysis and growth of the musculoskeletal system in the embryonic chick

TLDR

Avian embryos can be completely paralyzed by injection of neuromuscular‑blocking agents, enabling experimental manipulation of their development. The study aims to evaluate how paralysis influences embryonic skeletal growth by considering intrinsic and extrinsic factors. A single injection of decamethonium iodide at 7, 8, or 10 days of incubation was used to paralyze embryos, after which the growth of specific bones and associated muscles was measured. Paralysis differentially impaired bone growth—clavicle growth dropped to 27 % of normal, mandible to 77 %, and long bones to 52–63 %—and was linked to reduced muscle activity, especially in limb muscles, with clavicular retardation caused by sternal rudiment fusion, thoracic collapse, and loss of attaching musculature, illustrating that diminished movement and mechanical loading suppress skeletal development.

Abstract

Abstract Avian embryos can be completely paralyzed by injection of neuromuscular‐blocking agents. We used a single injection of decamethonium iodide to paralyze embryos at 7, 8, or 10 days of incubation and analyzed the growth of individual bones (clavicle, mandible, ulna, femur, tibia, humerus) and of individual muscles that act upon some of those bones (clavicular and sternal heads of m. pectoralis, and mm. biceps brachii, depressor mandibulae, pseudotemporalis, and adductor externus). Growth of the bones is not equally affected by paralysis. Only 27% of clavicular growth (by mass) but 77% of mandibular growth occurred in paralyzed embryos, whereas the four long bones exhibited 52–63% of their normal growth. Analysis of muscle weight, fiber length and physiological cross‐sectional area (weight/fiber length) indicate that there was greater reduction of the muscles acting on the limbs than of those acting on the mandible, i.e., diminished growth of the skeleton is correlated with reduced muscular activity. Specific retardation of clavicular growth is due to fusion of sternal rudiments and collapse of the thorax, as well as virtual absence of the musculature that normally attaches to the clavicle. We discuss these results in the light of intrinsic and extrinsic factors governing growth of tne embryonic skeleton. Paralysis reduces skeletal growth by reducing both the movements taking place in ovo, and the loads imposed on the bones by muscle contraction, changes that represent alterations in the mechanical environment of the skeleton.