BIOL-300Follicle Cycle
The wool-producing ‘factory’ is the 50 million or so follicles embedded in the skin of sheep. This unit 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.
Fibre growth in mammals is not uniform but cyclical with periods of rapid growth followed by resting stages. These cycles are under genetic, nutritional and hormonal control.

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BIOL-300-100Genotype and Follicle Cycle
Genotype influences the pattern of follicle cycling.
BIOL-300-100-100The effect of genotype on follicle shutdown
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. Rather the differences lie in the functioning of the skin and its population of follicles. Primitive sheep have a photoperiod driven follicle cycle that leads to an annual shedding of the fleece. However, Merino shedding appears to be driven by nutrition rather than photoperiod, but this should not be ignored. It is important to be aware of follicle shutdown as it results in a reduction in staple strength and the possible loss of fibres from the fleece. Better breeding and nutrition will help prevention of shedding This module describes the pattern of fibre shedding and follicle inactivity in sheep selected for increased staple strength and in finewool sheep with differences in CFW from the CSIRO Fine Wool flock.
BIOL-300-200Effect of Nutrition on the Follicle Cycle
While genotype appears to be the main controlling factor in cycling, nutrition also plays a role.
BIOL-300-200-050Effect of season and nutrition on follicle shutdown
Primitive sheep have a photoperiod driven follicle cycle that leads to an annual shedding of the fleece. However, Merino shedding appears to be driven by nutrition rather than photoperiod, but photoperiod should not be ignored. It is important to be aware of follicle shutdown as it results in a reduction in staple strength and the possible loss of fibres from the fleece (See module “”Staple Strength – Length Variability””). Better breeding and nutrition will help prevention of shedding This module contains photomicrographs showing the effects of season on wool follicle morphology and describes the effect of a Mediterranean environment and nutrition on fibre shedding in Merinos.
BIOL-300-200-100Glycogen and hair cycle
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 predominantly occurs in the outer root sheath of hair follicles in adult mammals. 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 of glycogen during the follicle cycle.
BIOL-300-300Hormones and Follicle Cycling
Control of follicle cycling seems to reside at the follicle level rather than systemic control. Growth factors are likely to be the regulating agents.
BIOL-300-300-050Follicle Cycle
Fibre growth is intermittent with periods of activity when the follicle is producing the fibre, alternating with periods of rest. The period of activity and rest varies with species. This module describes the follicle cycle in mammals.
BIOL-300-300-100Epithelial-mesenchymal reactions in the adult follicle
The dynamics of the fibre growth cycle vary:

  • between species,
  • between different body sites in the same species, and
  • between different follicle types in the same body site.

The local (non-systemic) control of the follicle cycle is poorly understood. However there are similarities between follicle initiation and the follicle cycle, so it is possible that similar mechanisms control both processes. This module demonstrates the similarity between follicle initiation and the follicle cycle and the role the dermal papilla plays in both situations.

BIOL-300-300-150Local vs systemic control of follicle cycle
This module demonstrates the interaction between local and systemic control agents on the follicle cycle.
BIOL-300-300-200Intrinsic control of the follicle cycle
In another module the molecular events underpinning the developmental processes of follicle morphogenesis were described. This module describes the local molecular interactions that function in the mature follicle. The systemic control of hair cycle by hormones such as prolactin, melatonin, oestradiol, testosterone, adrenal steroids and thyroid hormones, which are known to play an active role in regulating the hair cycle, have been covered in other modules.
BIOL-300-300-250EGF, FGF5 and the follicle cycle
This module describes the role of EGF and FGF5 on the follicle cycle and includes information that lead to the development of Biological Wool Harvesting. Useful reference: Stenn et al., 1999, Exp. Dermatol.,8, 229-236.
BIOL-300-400Follicle Shutdown
Follicle cycling has important practical effects on wool production and fibre quality.
BIOL-300-400-100Fleece shedding in sheep
Primitive sheep have a photoperiod driven follicle cycle that leads to an annual shedding of the fleece. However, Merino shedding appears to be driven by nutrition rather than photoperiod, but this should not be ignored. It is important to be aware of follicle shutdown as it results in a reduction in staple strength and the possible loss of fibres from the fleece. Better breeding and nutrition will help prevention of shedding This module describes the shedding patterns in primitive sheep.
BIOL-300-400-200Follicle shutdown in sheep
This module describes the potential long term effects of follicle shedding and outlines some methods to prevent fibre shedding.
BIOL-300-400-300Follicle shutdown and staple strength
This module describes the relationship between follicle shutdown and staple strength.