BIOS3070_Ellis_2_MaleRepro

Learning Objectives

• Understand the basic anatomy of male fertility.

• Understand the developmental origins and similarities of cell types within the testis.

• Compare endocrinological regulation of fertility in men and women.

• Why do men stay fertile throughout life while women have a limited span?

Male Basic Structures and CellsKey Structures:

• Urinary Bladder: Stores urine before it exits the body.

• Seminal Vesicle: Produces seminal fluid that nourishes and helps transport sperm.

• Prostate Gland: Secretes prostate fluid, a component of semen that aids in sperm motility and viability.

• Testis: The primary reproductive organ that produces sperm and hormones.

• Epididymis: A long, coiled tube where sperm mature and are stored after production.

• Penis (corpus cavernosum, corpus spongiosum): Penis structures that facilitate erection and penetrative intercourse.

Cell Types:

• Spermatogonia: Stem cells in the testis that undergo mitosis; they can either replicate themselves or develop into primary spermatocytes, initiating spermatogenesis.

• Primary Spermatocyte: Diploid cells that undergo meiosis I, producing two secondary spermatocytes that are haploid.

• Secondary Spermatocyte: Formed from primary spermatocytes, these haploid cells undergo meiosis II, resulting in spermatids.

• Spermatids: These haploid cells undergo morphological changes through spermiogenesis, transforming them into mature spermatozoa capable of fertilization.

• Sertoli Cells: Support sperm development by providing nutrients and a conducive environment for germ cell development; perform phagocytosis to eliminate residual cytoplasm during spermiogenesis.

• Leydig Cells: Produce testosterone in response to luteinizing hormone (LH); interspersed between seminiferous tubules, they play a critical role in regulating male reproductive functions.

Functions of the TestisProduces:

• Spermatozoa: The male gametes essential for reproduction, which undergo a complex maturation process.

• Hormones: Most notably testosterone, a key hormone for the development of male secondary sexual characteristics (such as facial hair, deeper voice, increased muscle mass) and libido.Compartmentalization:

• The testis is divided into interstitial spaces containing Leydig cells and seminiferous tubule compartments where spermatogenesis occurs; this separation is crucial for the proper hormonal regulation and development of male gametes.Temperature Sensitivity:

• Optimal temperature for spermatogenesis is approximately 2°C below body temperature (around 34°C), facilitating the proper development of sperm without causing thermal damage.

Leydig CellsLocation:

• Found in the interstitial compartment of the testis, strategically positioned near blood vessels to efficiently release testosterone into the circulation.Function:

• Synthesize roughly 4-10 mg of testosterone daily, influencing male characteristics and reproductive functions.

• They maintain the immunological environment of the testis to protect developing sperm from immune system attacks.Structural Features:

• Abundant smooth endoplasmic reticulum (SER) and mitochondria for steroid biosynthesis; these organelles are crucial for quick and efficient production of steroid hormones.

Sertoli CellsRole:

• Essential for the formation of the testis and promoting the progression of germ cells to sperm through direct cellular interactions, physical support, and forming blood-testis barriers that protect them.

• Produce androgen-binding protein (ABP) to concentrate testosterone within the seminiferous tubules, enhancing its action on spermatogenesis.Secretion:

• Produce inhibin and activin, which play pivotal roles in regulating spermatogenesis through negative feedback mechanisms to the pituitary gland for modulating FSH release.

Spermatogenesis ProcessOccurs in seminiferous tubules:

1. Mitosis: Spermatogonia undergo mitosis to generate primary spermatocytes, increasing the sperm pool.

2. Meiosis: Primary spermatocytes undergo meiosis, splitting into two secondary spermatocytes (1st meiotic division), which further divide into four spermatids (2nd meiotic division).

3. Maturation: Spermatids develop into spermatozoa through spermiogenesis. This involves forming the flagellum, shedding excess cytoplasm, and acquiring the acrosome which contains enzymes critical for fertilization. The entire process of spermatogenesis takes approximately 64 days from spermatogonia to mature spermatozoa.

Key Differences in Male and Female Reproductive Systems

• Ovaries: In women, ovarian function follows a monthly cycle and has a finite reproductive life, culminating in menopause characterized by the cessation of menstruation and loss of ovarian hormone production.

• Testes: Males have continuous production of sperm and androgens, maintaining fertility throughout life typically until extreme old age, allowing for potential fatherhood at older ages.Meiosis Initiation:

• In ovaries: Initiated during fetal development, resulting in a fixed number of oocytes present at birth, which do not regenerate.

• In testes: Initiated at puberty, leading to the ongoing production of sperm throughout life, allowing a more flexible reproductive capability.

Endocrine Regulation of FertilityHypothalamic-Pituitary-Gonadal (HPG) Axis:

• Gonadotropin-Releasing Hormone (GnRH) from the hypothalamus stimulates the anterior pituitary gland to release LH and FSH into the bloodstream.

• FSH: Targets Sertoli cells, promoting spermatogenesis, stimulation of androgen-binding protein production, and maturation of sperm.

• LH: Targets Leydig cells, stimulating production of testosterone, creating a feedback loop that inhibits further LH release when testosterone levels are adequate, ensuring homeostasis.

Effects of Androgen Deficiency on InfertilityThis deficiency can result in:

• Reduced sperm production and maturation, leading to potential infertility.

• Impaired function of Leydig cells, leading to significantly decreased testosterone levels that affect secondary sexual characteristics and libido.

• Decreased mitotic proliferation and meiotic progression in germ cells, which further compromises fertility rates.

Testosterone Supplementation and EffectsDependence:

• Intra-testicular testosterone levels are approximately 100 times higher than in systemic circulation, which is crucial for local stimulation of spermatogenesis.Effect on Fertility:

• Exogenous testosterone supplementation can paradoxically lower intra-testicular testosterone levels, potentially acting as a contraceptive by inhibiting sperm production through negative feedback on the HPG axis.

Fetal Development and Retinoic AcidRetinoic Acid's Role:

• Essential for germ cell commitment to meiosis in females during fetal development; in males, meiotic entry is regulated differently, highlighting distinct pathways in male reproductive development.Meiotic Entry:

• Retinoic acid influences the timing and initiation of meiosis; this emphasizes the developmental differences between male and female reproductive biology.

ConclusionThe male reproductive system is characterized by continuous sperm production and androgen release, crucial for maintaining fertility throughout life. This contrasts sharply with the cyclic and finite nature of female reproduction. The intricate balance of hormonal regulation and testicular function ensures sustained male fertility, supporting reproductive health across the lifespan.