Structure of Cilia and Flagella

The cilium consists of a basal body from which arises a shaft. In some cells ciliary rootlets and hair like structures called mastigonemes are also present.

1. Basal Body. The basal body or kinetochore lies below the level of the cell membrane. It greatly resembles the centriole, except that it often shows a complicated internal structure. The wall of the basal body consists of 9 peripheral triplets. The triplets are so arranged that they project outward in a clockwise direction. Each triplet is composed of 3 subfibrils of subfibres named A, B and C from the inner to the outer side. Subfibril A appears circular in cross section while subfibrils B and C appear crecent shaped. Subfibril B shares a part of the wall of subfibril A, and subfibril C a part of the wall of subfibril B. The subfibrils are microtubules.



2. The Shaft. The shaft of the cilium varies in length from 5 to 10 um, The shaft of the filagellum may be up to 150 um in length.
The axis of the cilium consists of microtubules arranged in a nine fold radial symmetry and is called the axoneme. The diameter of the axomeme is embedded in a matrix, which is surrounded by as ciliary membrane.
The axoneme consists of 11 longitudinal fibrils which show a 9 + 2 arrangement, with 9 peripheral fibrils and 2 central fibrils.
The central fibrils originate from the basal plate, or more distally from a convex axial granule called the axosome.
The central fibrils lie in the so-called central sheath, and constitute the axial unit. Between the axial unit and the peripheral doublet microtubules is the matrix region.
The 9 peripheral fibrils are radially arranged around the axoneme axis at 40 intervals. Each peripheral fibrils consists of two subfibrils or microtubules, forming a doublet. Subfibre A usually consists of 12-13 protofilaments or subunits, and is a complete tubule. Subfibre B is an incomplete tubule consisting of 9-10 protofilaments.

Peripheral linkage. Peripheral doublet tubules are connected to each other (peripheral linkage) and to the central tubules (radial linkage).
Radial linkages have been demonstrated in the flagella of spermatids of Sarcophaga and Chlamydomonas. The radial link heads are connected to the central sheath through transitional junctions

3. Ciliary Rootlets. Attached to the proximal end of the basal body may be found striated structures called ciliary rootlets. These are commonly present in the ciliated epithelium of lower animals but are absent in the ciliated epithelium of mammals and in the ciliated Protozoa.

4. Mastigonemes. Mastigonemes are common in the algae and have also been reported in the fungi. There are two main types of mastigonemes, tabulur and nontubular.
Tubular mastigonemes are confined to the anterior flagellum and are comparatively larger.
Nontubular mastigonemes are usually smaller in diameter than the tubular mastigonemes, but may exceed 3um in length. They may be attached randomly on the flagellum or may be arranged in the spiral band. The mastigonemes function in locomotion.

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