BIOL-200Follicles
The wool-producing ‘factory’ is the 50 million or so follicles embedded in the skin of sheep. This theme covers in detail, how these follicles form, what cellular and molecular processes produce the fibre, how genetics and nutrition affect these processes, and how genetic engineering might be used to produce a better fibre. With an understanding of the biology of the skin and the fleece, the characteristics of the wool follicle and fibre can be related to production, technology and the processing of wool.
The basic unit of the wool industry is the follicle. The follicle achieves the remarkable feat of producing a complex and unique fibre from undifferentiated cells in the follicle bulb. This theme examines the mechanisms responsible for this transformation.

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BIOL-200-100Follicle Function
The production of wool within the follicle is dependent on cell division, migration and keratinisation.
BIOL-200-100-100General follicle function
This module describes the basic processes that occur to produce a fibre from within the follicle. A diagram is used to illustrate the processes.
BIOL-200-150Structural Characteristics of the Wool Follicle
The characteristics of wool fibres are directly related to the structural characteristics of the follicle from which they derive. This topic examines the structural features of the wool follicle.
BIOL-200-150-050General follicle structure
This module describes the general structure of the follicle. The components of the follicle are listed and a diagram of the follicle is provided. The role and structure of the inner and outer root sheaths are described. The differences between primary and secondary follicles are illustrated in both vertical and horizontal sections.
BIOL-200-150-100Follicle accessory organs
This module describes and illustrates the accessory organs to the follicle, the suint and sebaceous glands. A brief description of the structure and function of these glands in the sheep and other species is provided with illustrations and photos.
BIOL-200-150-150Dermal papilla size
This module describes one of the characteristics related to volume of fibre produced – dermal papilla size.
BIOL-200-150-200Follicle size
A number of characteristics of the skin are associated with wool production. The wool follicle in its role as a production unit may be considered from two broad aspects. One concerns the processes involved in cell division, differentiation, growth and keratinisation. The other concerns the size/volume of the productive unit. This module describes how follicle size is associated with particular fleece characteristics and how it can be measured.
BIOL-200-150-250Blood vessels in the skin
This module describes one of the characteristics related to the supply of the nutrients to the productive unit – vascularisation of the follicle and contains diagrams and photomicrographs of blood vessels in the skin and around the follicles.
BIOL-200-150-300Glycogen in the follicle
Wool and hair follicles have high requirements for energy and amino acids to maintain their very high rate of cell division and protein synthesis and yet skin (and therefore the follicles) receives a highly variable supply of blood and oxygen. The skin and follicles have therefore developed some interesting features of energy metabolism to cope with this variation. The major energy substrates are glucose and glutamine. Glucose is metabolised by aerobic and anaerobic pathways, whereas glutamine is metabolised by glutaminolysis. There is also a store of glycogen in the outer root sheath. Glycogen is synthesised in the skin via two pathways; traditionally with glucose-6-phosphate as a precursor and de novo from lactate in a process called gluconeogenesis. Investigations of gluconeogenesis have been restricted to the demonstration of the enzymes and functional pathway. Glycogen is degraded in the skin to glucose-6-phosphate, a substrate available to the glycolytic and pentose phosphate pathways. The concentration of glycogen in the skin and hair follicle is influenced by the wound status, proliferative activity and nutrition of the tissue. This module describes the distribution and role of glycogen in the follicle.
BIOL-200-200Follicle Kinetics
Cell division is essential for follicle function. This topic examines cell migration and the role that stem cells play in follicle and fibre development.
BIOL-200-200-050Cell Cycle
Since cell division is essential for follicle function and fibre formation it is necessary to understand how cells replicate. This module provides a brief summary of the cell division cycle and mitosis.
BIOL-200-200-100Cell migration in the follicle
The cellular determinants of fibre production are the number of follicles, the number and size of cells in the bulb, the rate of cell division, the proportion of cells migrating to the fibre and the size of cells in the fibre. This module covers the migration of cells through the fibre canal, the inner root sheath and the outer root sheath. The module also describes some of the factors that may alter migration rate and how the proportion of bulb cells that migrate to the fibre can be estimated (production ratio).
BIOL-200-200-150Stem cells
Stem cells are the original cells needed for the replacement of cells from the bulb and the epidermis. This module describes what stem cells are and how they can be identified.
BIOL-200-200-200Stem cells in the follicle
Stem cells are the original cells needed for the replacement of cells from the bulb (related module – “Stem cells in the wool follicle”). This module describes the location of stem cells in mice follicles.
BIOL-200-200-250Stem cells in the wool follicle
Stem cells are the original cells needed for the replacement of cells from the bulb. This module describes an experiment undertaken to locate the stem cells in the Merino wool follicle and provides photomicrographs of the location.
BIOL-200-300Effect of Hormones on Follicle Function
Wool growth is under endocrine control, some hormones having a direct effect and others an indirect effect on follicle function.
BIOL-200-300-100Direct Effect of Hormones on Wool Growth
Variation in wool growth between sheep within genotypes reflects the effects of the environment and physiology of the sheep. Environmental effects include both the external environment of the sheep, such as climate and day length, and the internal environment of the sheep, such as pregnancy and disease. Many physiological and environmental effects are mediated by an endocrine response. This module summarises the direct effects of hormones on follicle activity and wool growth.
BIOL-200-300-150Cell communication
A vital part of wool biology (and multicellularity of all organisms) is the ability of individual cells to work together as a whole organism. Communication between cells is required to regulate their development and organisation into tissues, to control growth and division and to coordinate diverse activities. Cells in higher animals communicate by signalling molecules, including proteins, small peptides (growth factors), amino acids, nucleotides, steroids, fatty acid derivatives and even dissolved gases such as nitric oxide and carbon monoxide. The extracellular matrix in the dermal matrix plays an important role in cell communication (see “Communication Between Extracellular Matrix and Cells” module). This module describes the general processes of how cells communicate using signalling molecules, and the endocrine, paracrine and autocrine systems.
BIOL-200-300-200Chemical Signalling – Hormones and Growth factors
This module focuses on the similarities and differences between hormones and growth factors as examples of chemical signals.
BIOL-200-300-250Chemical Signalling – receptors
This module describes how chemical signalling occurs through the use of specific receptors, including cell surface receptors, intracellular receptors, and hydrophyllic and hydrophobic receptors.
BIOL-200-300-300Hormonal interactions and synergism between hormones
This module describes how chemical signals interact with each other and the resultant synergistic effect.
BIOL-200-400Nutrient Supply to the Follicle
Wool growth rate is directly related to nutrient supply. Follicle cells have a high requirement for energy and amino acids which are extracted from capillaries surrounding the follicle.
BIOL-200-400-050Nutrition and wool follicles
Nutrition is the major environmental factor influencing the realisation of genetic potential for wool production. Wool growth rate is constrained by feed intake and the supply of protein and energy provided by the diet. This module examines the effects of nutrition on follicle bulb and papilla dimensions, cortical cell type and specific changes that different diets can cause.
BIOL-200-400-100Metabolic activity of the follicle
Wool and hair follicles have a high requirement for energy and amino acids to maintain their very high rate of metabolic activity for cell division and protein synthesis. This module illustrates the high metabolic activity of the follicle by estimating the number of bulb cells and the total length of wool that are produced per year, rate of protein synthesis in the skin and the total amount of energy required for wool production.
BIOL-200-400-150Is protein or energy more important for follicle activity and wool growth
This module describes the contribution of energy and protein to follicle activity and wool growth and illustrates the energy and amino acid requirements of follicles in vitro
BIOL-200-400-200Factors that influence the supply of energy to the follicle
There are four factors which influence energy supply to the follicle – the supply, concentration, transport and use of energy substrates in the follicle. This module describes these factors.
BIOL-200-400-250Factors that influence the supply of nutrients to the follicle
Wool and hair follicles have high requirement for energy and amino acids to maintain their very high rate of cell division and protein synthesis. There are four factors which influence nutrient supply to the follicle:

  • blood flow to the follicle;
  • how much nutrient is present in the blood;
  • the movement of the nutrient from the blood to the cells in the follicle; and
  • how well the cells use the nutrient.

Each of these factors is described in relation to wool growth.

BIOL-200-400-300Glutamine as an energy source for the follicle
Wool and hair follicles have high requirements for energy and amino acids to maintain their very high rate of cell division and protein synthesis and yet skin (and therefore the follicles) receives a highly variable supply of blood and oxygen. The skin and follicles have therefore developed some interesting features of energy metabolism to cope with this variation. The major energy substrates are glucose and glutamine. Glucose is metabolised by aerobic and anaerobic pathways, whereas glutamine is metabolised by glutaminolysis. There is also a store of glycogen in the outer root sheath. The concept that glutamine has a major role as an energy substrate for the skin and follicle is relatively recent. In contrast to the extensively studied metabolism of glucose in the skin and follicle, the relevant literature on the metabolism of glutamine is scant. This module describes the role of glutamine as an energy source.
BIOL-200-400-350Transport of glucose in the follicle
Wool and hair follicles have high requirements for energy and amino acids to maintain their very high rate of cell division and protein synthesis and yet skin (and therefore the follicles) receives a highly variable supply of blood and oxygen. The skin and follicles have therefore developed some interesting features of energy metabolism to cope with this variation. The major energy substrates are glucose and glutamine. Glucose is metabolised by aerobic and anaerobic pathways, whereas glutamine is metabolised by glutaminolysis. There is also a store of glycogen in the outer root sheath. This module describes how glucose, the primary fuel of the follicle is transported into the follicle.
BIOL-200-400-400The effect of nutrition on glycogen in the skin
Wool and hair follicles have high requirements for energy and amino acids to maintain their very high rate of cell division and protein synthesis and yet skin (and therefore the follicles) receives a highly variable supply of blood and oxygen. The skin and follicles have therefore developed some interesting features of energy metabolism to cope with this variation. The major energy substrates are glucose and glutamine. Glucose is metabolised by aerobic and anaerobic pathways, whereas glutamine is metabolised by glutaminolysis. There is also a store of glycogen in the outer root sheath. The concentration of glycogen in the skin and hair follicle is influenced by the wound status, proliferative activity and nutrition of the tissue. This module describes how the glycogen concentration changes with nutrition levels.
BIOL-200-400-450Transport of specific amino acids into cells
Protein metabolism in the follicle is essential for wool growth. Without protein metabolism, fibre formation would be impossible (see the module “Basic fibre composition”). Protein metabolism is much more than simple fibre formation. It also includes transport of amino acids from the plasma into the cells, amino acid metabolism and polyamine synthesis. This module describes how alanine, proline, glycine, leucine, isoleucine, phenylalanine, valine, cysteine, histidine and arginine are transported across cell membranes. The site of expression of mRNA for a cysteine transporter in wool follicles is also described.
BIOL-200-400-500Polyamine metabolism
Polyamines are compounds that are present in all tissues that undergo cell division and protein synthesis. This module describes polyamines and their function, the metabolism of polyamines and the role of polyamines in follicle function using follicle culture as a model.
BIOL-200-500Follicles and Wool Growth
Skin and follicle characteristics determine the rate of wool growth, and the quality of fibre produced. This topic examines the genetic differences in skin and follicle traits which relate to fibre growth differences.
BIOL-200-500-100Associations between follicles and wool growth
Variations in wool production and quality are evident between breeds, strains and individuals in the same flock due to differences in the rate of wool growth and the characteristics of the fibre. Studies of sheep with different genetic potential for wool production and fibre diameter have found that differences in whole body functioning only play a minor role in genetic differences (see module “Whole Body Differences that Affect Wool Growth”). Rather the differences lie in the functioning of the skin and its population of follicles. This module describes differences in follicle kinetics, and follicle populations that contribute to different wool producing ability between sheep. Three experiments that measured the differences in skin characteristics between sheep of different strains, different selection criteria and the CSIRO Finewool flock are used to illustrate how skin differences are associated with wool production and quality. Heritabilities and genetic correlations between skin, follicle and fleece characteristics can be found in the subject “Wool Production”.