The organization of muscle spindles in the tenuissimus muscle of the cat during late development

Intrafusal muscle fibers in the tenuissimus muscle of the cat develop as two separate groups; one being a single nuclear bag fiber while the other comprises a second nuclear bag fiber along with all the nuclear chain fibers. The groupings are very distinctive in the late fetus (55 days gestation) and remain so until 18 days of age. In the adult, the grouping is less distinctive but can often be recognized and followed for considerable distances within the capsular region of the spindle. Each group develops under its own basement membrane and is separated from the other by fibrocytes. ATPase histochemistry indicates the isolated single nuclear bag fiber is slow twitch while the fibers of the other group, the second bag and all the nuclear chains, are fast twitch. The organization of intrafusal fibers in late development into two groups of different fiber types is discussed in relation to their selective innervation by γ fibers.