Male Reproductive Physiology

• This lecture covers male reproductive physiology.
• Primary function involves the production and storage of spermatozoa
• Accomplished through endocrine and exocrine activities

Endocrine Function

• Production and secretion of male sex hormones, like testosterone and androgen.
• Dependent on androgen and the anterior pituitary gland

Exocrine Function

• Secretion of protective fluids (semen) into the male reproductive duct system.
• Semen serves as a transport vehicle for spermatozoa

Structure of Male Reproductive System

• Primary sex organs, testes
• Accessory sexual organs and ducts consist of the epididymis, vas deferens, ejaculatory duct, and urethra (reproductive ducts)
• Seminal vesicles, prostate gland, ampulla, and Cowper's glands comprise reproductive glands.
• The penis is the copulatory organ

Testes

• Develop in the fetus near the kidney and then descend into the scrotum permanently, except in birds and elephants
• Birds and Elephants permanently retain testes in the abdominal cavity
• Some seasonal breeders withdraw into the abdominal cavity during periods of sexual inactivity

Testicular Structure

• Inside the scrotum, testes are surrounded by the tunica vaginalis (outer serous coat) and tunica albuginea (inner white vascular thick capsule).
• Parenchyma consists of seminiferous tubules
• Seminiferous Tubules are gathered in 300 lobuli testis, each containing 1-4 tubules
• Seminiferous Tubules have a 0.1-0.3 mm diameter, 50-100 mm length in the bull, and comprise 90% of the testis
• The unit is functional
• Seminiferous Tubules are arranged in lobules separated by connective tissue septa
• The wall consists of a basement membrane and a multi-layered sperm-producing germinal epithelium
• Composed of spermatogenic cells, which develop into spermatozoa, and Sertoli cells, which support
• The area between the seminiferous tubules contains blood vessels, lymphatic vessels, nerves, and Leydig or interstitial cells, which secrete androgen (testosterone).
• The rete testis connects with the epididymis through efferent ducts

Sertoli Cells

• Activated by FSH and have FSH-receptors
• Nourish developing sperm by secreting nutritional components like enzymes and amino acids
• The blood-testis barrier prevents some substances from crossing
• Due to this barrier it is called the "mother cell."
• Allow maturation of spermatids to spermatozoa
• Consume unneeded or excess spermatozoa
• Release spermatozoa from their apical cytoplasm during spermiation
• Transport androgens to the germinal epithelium
• Provides structural support via the blood-testis barrier, separating interstitial and luminal compartments
• Control the entry and exit of nutrients, hormones, and other chemicals into the tubules of the testis

Secretory Function of Sertoli Cells

• Anti-Müllerian Hormone (AMH) or Müllerian Inhibitory Factor (MIF) secreted during early fetal stages, inhibits the formation of fallopian tubes
• Inhibin and Activins secreted post-puberty, regulate FSH secretion through negative feedback
• Androgen Binding Protein (ABP) increases testosterone concentration in the seminiferous tubules
• Estradiol/Estrogen, converted from testosterone, controls spermatogenesis and regulates testosterone secretion
• Transferrin, a blood plasma protein, delivers iron ions
• Insulin-like Growth Factor 1 (IGF-I) stimulates differentiation and growth of germinal cells
• IGF-II function is unknown
• Gonadocrenine regulates LH secretion by decreasing the number/binding of LH receptors on Leydig cells

Spermatogenesis

• The process produces spermatozoa from spermatogonia stem cells within the lumen.
• Stem cells remain in contact with Sertoli cells and undergo mitosis and meiosis

Types of spermatogenesis

• A: Spermatocytogenesis, which involves spermatogonia division
• B: Spermiogenesis, where spermatids undergo metamorphosis and become spermatozoa

Spermatocytogenesis

• Type A spermatogonia divide into pale intermediate spermatogonia (Ap) and dark dormant type A cells (Ad).
• Intermediate cells divide into Type B spermatogonia, which then yield 16 primary spermatocytes.
• Four divisions yield 16 primary spermatocytes from one type A cell.
• Primary spermatocytes undergo meiotic prophase, resulting in secondary spermatocytes.
• Each secondary spermatocyte divides into 2 spermatids with half the number of chromosomes (haploid).
• 64 spermatids result from each stem cell.
• Dormant type A cells behave like their parents, ensuring continuity of spermatogenesis

Spermiogenesis

• Spermatids undergo slow metamorphosis (15 days) into spermatozoa without further division

Phases

• Golgi phase: the head forms, and the Golgi apparatus creates enzymes for the acrosome
• A thickened mid-piece develops, where the mitochondria gather and the distal centriole begins to form the tail
• Spermatid DNA undergoes packaging.
• Acrosome cap phase: the Golgi apparatus surrounds the condensed nucleus, forming the acrosomal cap
• Formation of Tail: distal centrioles elongate to form the tail
• Maturation phase: occurs under the influence of testosterone
• Unnecessary cytoplasm and organelles (residual bodies) are phagocytosed by surrounding Sertoli cells

Duration of Spermatogenesis

• The time between appearance of spermatogonial stem cells and the release of spermatozoa into the lumen for different species
• Ram: 49 days
• Bull: 60 days
• Rabbit: 52 days
• Man: 70 days

Spermatogenic Cycle

• The interval of spermatogonium replacement to give spermatozoa.
• Spermatogenesis duration is approximately 4 times the spermatogenic cycle duration
• The duration for different species is as follows
• Pig: 8 days
• Sheep: 10 days
• Cattle: 14 days
• Human: 16 days

Spermiation

• Release of spermatozoa from Sertoli cells occurs by gonadatropins through activating mucolytic enzymes
• Spermatozoa transported into the rate testis are non-motile and sterile
• These non-motile spermatozoa are transported to the epididymis via fluids secreted by Sertoli cells during peristaltic contractions
• The spermatozoa gain motility in the epididymis and become capable of fertilization

Hormones Controlling Testicular Function

• Hormonal control is interrelated and regulated by basic environmental factors affecting anterior pituitary function via the hypothalamus

Luteinizing Hormone (LH)

• Stimulates testosterone secretion from interstitial cells of Leydig.
• In males sometimes called Interstitial cell stimulating hormone (ICSH).

Follicle Stimulating Hormone (FSH)

• Stimulates divisions, growth, and maturation
• Stimulates Sertoli cells take up testosterone
• Stimulates synthesis of ABP and spermatogenesis
• Promotes the development of Sertoli cells
• In Males sometimes called Gametogenic hormone

Testosterone

• Stimulates the male androgen-dependent organs.
• Decreases GnRH via negative feedback
• It's needed for the development and maintenance of germinal epithelium.

Inhibin

• Decreases plasma level of FSH

Other Hormones

• Growth and thyroid hormones are necessary for spermatogenesis

Environmental Factors and Spermatogenesis

• Light: important and influences hypothalamus, pituitary, and testes
• Animals are classified based on light response:
• Decreasing daylight activates pituitary: sheep and goat
• Reproduce year-round, but reach maximum spermatozoa during autumn/winter
• Long light periods activate pituitary: horse and donkey, with stallions producing during April
• Sensitivity to light is difficult to characterize: cattle and pigs

Temperature

• Spermatogenesis requires temperature 4-7°C lower than body temperature
• The testes must descend into the scrotum.
• Scrotum regulates temperature via arterial blood precooling and cremaster muscle changes

Nutrition

• Balanced nutrition is necessary for spermatogenesis
• Low protein decreases volume and libido
• Vitamins E, A, and B deficiency causes atrophy and lowers sperm count

Noxious agents

• Cadmium and lead cause testicular necrosis
• Excessive radiation destroys epithelium
• Excessive estrogen depresses spermatogenesis
• Hypoxia, toxins, viruses and alcoholism depress spermatogenesis
• Triethylenemelamines "TEM" results in aspermia

Cryptorchidism

• A condition where one or both testis fail to descend into the scrotum.
• Most common birth defect in males
• May be bilateral or unilateral
• Causes degeneration of germinal epithelium