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Reilly et al. Plast Aesthet Res 2021;8:2 I http://dx.doi.org/10.20517/2347-9264.2020.153 Page 9 of 24

Figure 7. Collagen content in skin tends to increase until approximately the mid-20s. Thereafter, there is a progressive loss of collagen
through the decades. (By permission of MINERVA Research Labs Ltd - London)

Collagen fibres accumulate damage over time and this decreases their ability to function correctly.
Intrinsically aged skin is generally characterised by dermal atrophy with reduced density of collagen
[43]
fibres, elastin, and hyaluronic acid . In addition to reduced density, the collagen and elastin fibres can be
[44]
observed to be disorganized and abnormal in aged skin compared to young and healthy skin . As collagen
levels start to decline, the collagen structure becomes more fragile and brittle leading to a weakening of the
skin’s structural support. The skin loses volume and firmness and starts to thin and wrinkle. The reduction
in collagen production also coincides with a loss of hyaluronic acid further impacting on the hydration and
suppleness of the skin.

[45]
In a study published by Sibilla et al. , the researchers reported a peak in collagen content for subjects
between 25-34 years old, followed by a gradual decline equating to an approximate 25% decrease over 4
decades (Percent collagen score 73.28 ± 14.3 at age 25-34 vs. 55.3 ± 13.1 at age 65-74, n = 64, Figure 7). This
decline in collagen in aged skin has been measured using various methodological approaches, which are
generally in good agreement and support the hypothesis of collagen loss being a key determinant of age-
related deterioration of skin appearance [46,47] .

The endocrine system and hormonal effects on skin collagen
Changes in hormones levels associated with chrono-ageing affects different parts of the body in various
ways. With hormonal changes during teenage years and puberty many adolescents experience acne, caused
by an interaction of hormones, sebum-based oils, and resident bacteria and associated with inflammation,
redness, and spots. Acne can be severe in clinical presentation and can cause scarring of the skin. Scarring
of skin requires tissue remodelling, including remodelling of the collagen-based ECM, to repair the damage
associated with long term inflammation and tissue atrophy.

An increase in hormonal levels is accompanied by increased activity of sebaceous glands, with an increase
specially in androgens, resulting in an excess of sebum produced in skin. During early adult life, hormone
levels start decreasing, thus acne symptoms starts to lessen. However, facial lesions can affect people
throughout their entire adulthood [48,49] . Women may repeatedly suffer acne in adulthood as this may occur
with their menstrual period, especially for those who suffer from PCOS (Polycystic Ovarian Syndrome).
This hormonal disorder affects the menstrual cycle and can increase the severity of acne. Most women
[50]
suffer from acne disorders until menopause period when levels of oestrogen start decreasing rapidly .