Anatomy and Function
The testes have two main functions: spermatogenesis, the formation of male gametes, and testosterone synthesis, the principal male sex hormone. Ninety per cent of testicular mass consists of seminiferous tubules which are the site of spermatogenesis. Seminiferous tubules consist of:
- Sertoli cells: these are located in the walls of the seminiferous tubules and are the site of spermatozoa formation (spermatogenesis). They also synthesise inhibin under the influence of Follicle Stimulating Hormone (FSH).
- Leydig Cells: these are interspersed in the interstitial region of the testis between seminiferous tubules. They are responsible for testosterone synthesis in response to lutenising hormone (LH). LH binds to these cells and this stimulates the synthesis of testosterone from cholesterol. Testosterone is secreted within 30 minutes of an LH surge. LH is released in bursts and this is critical because it prevents the desensitisation of Leydig cells to LH which may occur if exposed for long periods of time. In addition, if testosterone where present in the blood constantly, negative feedback would prevent LH and FSH release.
The process of testosterone synthesis is summarised in the flow diagram below:
Cholesterol
↓
StAR - Rate Limiting Step
↓
Pregnenolone
↓
Progesterone
↓
17 OH-progesterone
↓
Androstenedione
↓
Testosterone
↓
Blood OR Sertoli Cells
↓
↓
Converted to Estradiol or DHT
In this process, StAR transfers the cholesterol to the inner mitochondrial membrane and is the rate-limiting step. Testoseterone is then produced which can either diffuse into the blood or, more commonly, into sertoli cells where it is converted to estradiol or dihydrotestosterone (DHT). DHT is the more active form of testosterone in the body.
Testosterone isn't very soluble in blood and so most of it travels bound to several substances. Forty-five per cent is bound to sex-hormone binding globulin (SHBG), 50% to albumin and roughly 0.5% is free and bioavailable. Testosterone and DHT bind to intracellular receptors, complex to DNA in the nucleus and increase gene transcription. In many tissues, such as the prostate and penis, DHT is more important than testosterone. However, in skeletal muscle and bones, testosterone is the most important.
Actions of Testosterone
Testosterone has several actions on the body, including:
- Anabolic Actions: enhances skeletal muscle and bone growth and stimulates protein synthesis while inhibiting its breakdown.
- Reproductive Actions: Testosterone is responsible for the differentiation of the male internal and external genitalia in the foetus. It is also involved in the growth, development and function of the male internal and external genitalia as well as the initiation and maintenance of spermatogenesis.
- Secondary Sex Characteristics: These are characteristics which distinguish between males and females but are not part of the reproductive system. Testosterone is responsible for:
- increasing sex drive
- territorial marking
- development of antlers, horns and tusk-like teeth
- pheromones: these are odorous molecules secreted in the saliva, urine or feces.
- development of combs and unique plumage.
The hormonal regulation of reproduction is primarily controlled by the interaction between the hypothalamus, anterior pituitary and testes. The hypothalamus secretes gonadotropin releasing hormone (GnRH) which stimulates the anterior pituitary to release FSH and LH. These hormones affect the testes by causing germ cell development, as well as the secretion of androgens, oestrogens and progesterone.
GnRH is released in a pulsatile manner and its secretion is influenced by a variety of internal and external signals including: stress, temperature, nutrition, light, social cues and steroid hormones.
Both males and females produce FSH and LH. FSH primarily stimulates gametogenesis as well as the secretion of inhibin which has a negative effect on FSH. LH stimulates secretion of androgens from the testes as well as oestrogens and progesterone in the ovaries. It is essential for final follicle maturation and ovulation. FSH and LH are known as gonadotropic hormones because they stimulate the growth and development of the gonads.
Regulation of the Male Reproductive System
Steroid hormones are the most important regulator of the male reproductive system. Steroid hormones provide negative feedback at the pituitary and hypothalamus as they slow the release of GnRH at the hypothalamus and reduce the sensitivity of the pituitary to GnRH, lowering the secretion of LH and FSH. Inhibin, synthesised by the sertoli cells, suppresses the release of FSH only.
Androstenone in Pigs
This substance is an androgen which is produced in the testes along with testosterone in pigs. It has a characteristic smell and acts as a sex pheromone in saliva by eliciting a copulatory stance in sows that are in oestrus. Androstenone is also responsible for 'boar taint' which makes boar meat difficult to market. For this reason, male pigs which are intended for market are often castrated.
Factors which Affect Male Fertility
Several factors affect male fertility in animals, this includes:
- Daylength: Daylength determines the breeding season in seasonal breeders. Daylength can be altered in order to adjust breeding seasons by using indoor lighting (this is often done in the poultry industry).
- Nutrition: Nutrition and feeding practise has a large effect on the fertility of an animal.
- Stress: this is usually associated with deleterious effects on reproduction. Stresses include social stressors, environmental stressors, transport, nutritional/metabolic stressors, disease/infection, and predation stressors.
- Social Cues: Pheromones provide information on the sexual or social status of the emitter and males use the scent of females to asses whether they are in oestrus or not.
Cryptorchidism is a condition in which one or both testes has failed to descend into the scrotum during development. It is common in stallions and boars and is the most common disorder of sexual development in dogs. It is important that cryptorchids are always desexed because the condition is inherited and tumours commonly develop within cryptorchid testes, these often turn cancerous.
That's all for this post, if you have any questions please feel free to ask : )
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