Puberty

1. Secondary Sexual Characteristics
   
   • In the Female
   • In the Male
   • Growth Spurts
2. Hormone Changes in Puberty
   
   • Hypothalamus and Anterior Pituitary
   • Gonadotrophin Releasing Hormone  
   • Changes in Males
   • Changes in Females
   • Inhibin
   • Androgens
   • Leptin
3. Quiz

• Primary sexual characteristics (e.g. the ovaries, vagina, testes and penis) are determined before birth, while secondary sexual characteristics develop at puberty. The start point of puberty is determined according to weight rather than age, and this explains why the onset of menstruation has become progressively earlier over time. Puberty usually starts earlier in females than in males. Within each sex, secondary sexual characteristics usually develop in a set order, but the order is not the same in males and females.
• In females, puberty usually begins between about 8 and 13 years of age. The changes usually come in this order: thelarche (the start of breast development), pubic hair growth, the growth spurt in height (somatic growth), menstruation, adult distribution of pubic hair, and maturation of breast tissue.
• In males, puberty usually begins between about 9 and 14 years of age. The changes include: increased testicular volume, pubic hair growth, spermatogenesis (sperm production), the growth spurt in height (somatic growth), penile enlargement, and adult distribution of pubic hair.
• Males often grow taller than females. Oestrogen speeds up the maturation and fusion of the growth plates (epiphyseal plates, the growth zones at the ends of long bones) in both sexes. Differences in the timing and size of the pubertal growth spurt mean that males grow for longer.
• The hypothalamic-pituitary-gonadal axis (the hypothalamus, the anterior pituitary gland and the gonads, meaning the ovaries and testes) controls fertility through gonadotrophin releasing hormone (GnRH), luteinising hormone (LH), follicle stimulating hormone (FSH), and the sex hormones: oestrogens and androgens (male sex hormones such as testosterone). The axis is mainly governed by negative feedback.
• GnRH is released overnight and in a pulsatile manner. GnRH acts on the membrane of anterior pituitary cells and through the action of second messengers stimulates the release of LH and FSH, which causes the onset of secondary sexual characteristics.
• LH release in the male causes Leydig cells (the testosterone-making cells of the testis) to produce testosterone from cholesterol. Leydig cells sit between the seminiferous tubules (the coiled tubes in the testis where sperm are made). The tubules are made up of germ cells (that produce sperm) and Sertoli cells, which are sensitive to FSH and support developing sperm. Sertoli cells also produce inhibin, which acts to inhibit FSH.
• In females, FSH and LH act on granulosa and theca interna cells (the cells that surround the developing egg, or oocyte, in the ovarian follicle, the sac that holds it). Working together, these cells make oestrogens, and FSH and LH control maturation of the oocyte.
• Inhibin is produced by the Sertoli cells in testicles and granulosa cells in the ovary. It inhibits FSH production.
• Leptin is a hormone made by adipocytes (fat cells). It reduces hunger depending on the energy stores available. It can regulate GnRH levels separately from the LH feedback loop.

Puberty is a stage in the development of mammals in which sexual maturation and growth are completed, and which results in the ability to reproduce.

Primary sexual characteristics are established before birth and include only the reproductive organs themselves (e.g. the ovaries, testes, penis, vagina and uterus). The hormone system that controls these organs (described below) is active briefly in the fetus and in the first months after birth, but the organs only reach full adult function, including making mature eggs or sperm, after puberty.

Secondary Sexual Characteristics

Secondary sexual characteristics are the changes that occur to the body during puberty, including but not limited to, hair growth, somatic growth, breast development and voice changes. These changes are influenced more by nutritional status and body composition than by chronological age alone. Individuals who are smaller or undernourished in childhood may begin puberty later. In females, low energy availability or low body weight can delay the start of periods, or stop them once they have started. Significant weight loss or chronic energy deficiency, such as in anorexia nervosa (an eating disorder), can lead to hypothalamic amenorrhoea (periods stopping because the hypothalamus releases less GnRH, the hormone described below).

As already mentioned, the onset of puberty can vary. An interesting observation related to this fact is the historical average age of menarche (the beginning of menstrual periods). Soon after the industrial revolution, most studies report an average age of menarche of 15 to 16 years. In industrialised countries today it is 12 to 13 years, including in the UK. Poorer living conditions in the past are thought to explain the later menarche, and better conditions the earlier menarche today. An older idea, the "critical weight" hypothesis, suggested that menarche happens once a girl reaches a threshold body weight, but studies have not found a fixed weight at menarche.

Development of secondary sexual characteristics occurs in the same order within each sex, meaning that males typically develop in the same order as each other, and females typically develop in the same order as each other, even if these stages occur at different ages.
This linear development is demonstrated by the Tanner Scale (stages I-V), which is a series of developmental stages based on breast development in girls, genital development (including testicular volume) in boys, and pubic hair growth in both. Doctors use the Tanner scale to assess puberty, for example if puberty seems to start too early or too late, or to have begun but stopped.

Female Secondary Sexual Characteristics

The process usually begins between about 8 and 13 years and usually continues in this order.

• Breast buds develop (thelarche, the start of adult breast development)
• Pubic hair growth begins
• Somatic growth spurt (height)
• Onset of menstruation (menarche)
• Adult distribution of pubic hair
• Maturation of breast tissue

Male Secondary Sexual Characteristics

The process usually begins between about 9 and 14 years and usually continues in this order.

• Increase in volume of testicles
• Pubic hair growth begins
• Spermatogenesis (sperm production) begins
• Somatic growth spurt
• Enlargement of the penis
• Adult distribution of pubic hair

Growth Spurts

Somatic growth (increase in height) occurs in both males and females during puberty. However, men are on average taller than women, and this can be explained by the hormones that are present during puberty. The key hormones at play are oestrogens, androgens (testosterone), growth hormone and insulin-like growth factor 1. Insulin-like growth factor 1 is a hormone produced mainly by the liver with a similar structure to insulin and related signalling pathways, and is important for growth.

Adult height is reached and growth stopped when the epiphyseal growth plates (areas of growing cartilage near the ends of the long bones) fuse. This prevents the further lengthening of the long bones. The fusion of these epiphyseal plates is determined by the concentration of oestrogen in the body, as increased oestrogen causes earlier fusion.

The increased amount of oestrogen in females explains why the plates in women fuse faster. Although males do produce some oestrogen, it is a smaller amount than is produced in women. Males are also exposed to less oestrogen early in puberty, so their growth plates fuse later. Oestrogen still causes the fusion in the end, including oestrogen made from testosterone by an enzyme called aromatase (this conversion is called aromatisation).

There are many environmental and genetic factors at work when determining the height a child may achieve. Genetics are key, with taller parents often having taller children. However, there are also other considerations, such as nutrition; a child who is malnourished will not have adequate stores to grow tall and may see their growth stunted.

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