Who are you, subdistal appendages of centriole?

Abstract

This review summarizes data that assign morphological, biochemical and functional characteristics of two types of structures that are associated with centrioles: distal appendages and subdistal appendages. The description of centriole subdistal appendages is often a matter of confusion, both due to the numerous names used to describe these structures and because of their variability among species and cell types. Thus, we have summarized our current knowledge in this review. We conclude that distal appendages and subdistal appendages are fundamentally different in composition and function in the cell. While in centrioles there are always nine distal appendages, the number of subdistal appendages can vary depending on the type of cells and their functional state.

Keywords: centriole; distal appendages; subdistal appendages.

Figure 1.

Bessis & Breton-Gorius [8] discovered that the surface of leucocyte centrioles was often covered with growths, consisting of a stem with a round cap, which these authors called ‘massules’.

Figure 2.

Two consecutive ultrathin sections through the distal part of mother centriole in the cell of porcine kidney embryo cell line. (a) Distal appendages (DA) are always present in an amount of 9—one for each triplet (doublet) of MT of the mother centriole. (b) Heads of the distal appendages (hDA) are located more distally in comparison with points of attachment of the distal appendages to the surface of the centriolar cylinder and therefore are detected on the next ultrathin section. In the view from the proximal end of the centriole, the MT triplets are twisted counterclockwise, distal appendages are twisted in the opposite direction (clockwise). Scale bar, 100 nm. From [9].

Figure 3.

Ultrathin sections through the subdistal part of mother centriole in different types of cells. (a) Cell with three subdistal appendages from epithelial pig kidney (PK) embryo cell [9], conical form of subdistal appendages stem (sSDA) with spherical heads (hSDA) are visible. Each subdistal appendage based on 2 or 3 centriolar MT triplets. (b) Cell with four subdistal appendages from pig oviduct cell [4]; (c,d) cells with nine and six subdistal appendages from KE-37 human cell line [16]. Arrowheads show cylindrical subdistal appendages without visible heads and based on 1 MT triplet of centriolar cylinder. In (a,c,d) the triplets are twisted clockwise, hence the view from the distal of the centriole, for photo ‘b’ the triplets are twisted counterclockwise, hence the view from the proximal end of the centriole. Scale bar, 100 nm.

Figure 4.

Subdistal appendages can be on the different distance from distal end of centriole. (a,b) Two consecutives serial cross sections of distal part of mother centriole in the cell of BHK-21 (baby hamster kidney cell line), five subdistal appendages with well visible stem (sSDA) and head (hSDA) parts placed in ‘two flours’ (from [4]). (c) The view from the proximal end of the centriole (the MT triplets are twisted counterclockwise); near longitudinal section of active (mother) centriole (ac) from axolotl cell, subdistal appendages (SDA) placed in ‘three flours’ (from [18]). Scale bar, 100 nm.

Figure 5.

A scheme of centrosome in S-phase of cell cycle. Two subdistal appendages are shown here what is typical for epithelial pig kidney (PK) embryo cells. Serial sections cells of this cell line were used for model construction. Near the distal pole the MT ‘C’ terminates earlier than MT ‘A’ and MT ‘B’. In the region of triplet to doublet transition MT ‘C’ is uncomplete; this structure is called as ‘hook’. There are no distal appendages on the daughter centriole, but electronically dense plats (called ‘ribs’) are there on each triplet. Other terms are explained in the text. From [70,71] with small modifications.