Macrofibril Formation

Abstract

Macrofibrils are the main structural component of the hair cortex, and are a composite material in which trichokeratin intermediate filaments (IFs) are arranged as organised arrays embedded in a matrix composed of keratin-associated proteins (KAPs) and keratin head groups. Various architecture of macrofibrils is possible, with many having a central core around which IFs are helically arranged, an organisation most accurately described as a double-twist arrangement. In this chapter we describe the architecture of macrofibrils and then cover their formation, with most of the material focusing on the theory that the initial stages of macrofibril formation are as liquid crystals.

De novo filament formation by human hair keratins K85 and K35 follows a filament development pattern distinct from cytokeratin filament networks

“…In this model, the abundant hair keratin pair, K85 and K31, builds up a high concentration of ULFs in the lower cortex cells, and any further lengthening of the ULFs into short IFs will quickly result in the formation of a concentrated oriented phase, the mesophase, appearing initially as spindle‐shaped tactoids, i.e., early‐stage proto‐MFs. It is hypothesized that incorporation of K35 into the ULFs by subunit interchange with K31 is a trigger for the initiation of ULF lengthening because the large N‐terminal head domain (96 amino acids) of K35 compared with the small head domain (55 amino acids) of K31 causes thermodynamic conversion of the ULFs from a stable (blocky) form to a shaggy form that can concatenate with each other [26,35]. Although K31 was not tested in this study, the results obtained here and our previous in vitro data [48] are consistent with the earlier model in that the K85–K35 pair readily forms the short IF bundles on their own and they can gradually elongate in cells.…”

“…In the cortex of a hair or wool fiber, hair keratin IFs are arrayed into bundles, in which each IF is surrounded by a matrix material composed of keratin‐associated proteins (KAPs) [3,4,24–26]. The supramolecular structures organized by hair keratin IF bundles and KAPs, termed macrofibrils (MFs), have a diameter of 200–500 nm and a maximum length of ~10 µm and fill the cortex by lateral and longitudinal assembly along the fiber [4,17,25–37].…”

“…Some TEM images of early proto‐MFs showed that distinctly separated IFs are arranged on a hexagonal basal lattice like a columnar hexagonal liquid crystal, with no matrix proteins interpolated between the filaments [4,34,35,37], indicating that the early proto‐MFs composed of just a few type I and type II hair keratins are substantially complete before the numerous KAPs are expressed. In the middle to upper cortex, small proto‐MFs coalesce with each other into mature MFs [4,26,28,30,31,34,35]. The initiation of proto‐MFs appeared to be limited to the lower cortex, and the increases in keratin content in the middle to upper cortex result from increases in the size of existing proto‐MFs, rather than initiation of new ones [31].…”

“…The supramolecular structures organized by hair keratin IF bundles and KAPs, termed macrofibrils (MFs), have a diameter of 200–500 nm and a maximum length of ~10 µm and fill the cortex by lateral and longitudinal assembly along the fiber [4,17,25–37]. Transmission electron microscopy (TEM) studies of MF formation have revealed the appearance of hair keratin IF bundles in the lower cortex, i.e., proto‐macrofibril (proto‐MFs) with a width of 50–100 nm and a length of several hundred nanometers, some of which are attached to a desmosome, while others are isolated in the cytoplasm [4,26,28,30,31,34,35,37]. Some TEM images of early proto‐MFs showed that distinctly separated IFs are arranged on a hexagonal basal lattice like a columnar hexagonal liquid crystal, with no matrix proteins interpolated between the filaments [4,34,35,37], indicating that the early proto‐MFs composed of just a few type I and type II hair keratins are substantially complete before the numerous KAPs are expressed.…”

Processing hair follicles for transmission electron microscopy

“…The morphogenesis of a hair from the follicle bulb to mature shaft is a complicated process and the anagen phase follicle that actively grows hair is a challenging structure for TEM [1][2][3] ; more challenging than the TEM of above-skin hair fibres, which can be cleaned, dehydrated and embedded into a resin. [4][5][6][7] For convenience, we focus on a discussion of TEM sample preparation techniques for assessing the anagen VI stage follicle, but most points apply to any stage in the follicle cycle.…”