Male Reproduction Pre-Reading

What are the two main functions of the male reproductive system?

• Gametogenic (production of mature spermatozoa)

• Endocrine (hormone production, including androgens and others)

Why are both gametogenic and endocrine functions essential?

They are necessary for fertility and male sexual development

What surrounds the testes?

Tunica vaginalis and tunica albuginea

What is the tunica vaginalis?

• Serous membrane on the outside

• Has a parietal and visceral layer

What is the tunica albuginea?

Dense fibrous connective tissue capsule

How are the testes internally divided?

Inward partitions (septa) divide each testis into around 250 lobules

What do the lobules contain?

• Seminiferous tubules (for sperm production)

• Sertoli cells

• Developing germ cells

• Leydig cells

• Blood vessels

• Connective tissue

What separates sperm production from androgen production?

• Sperm production occurs in the seminiferous tubules

• Androgen production occurs in the interstitial tissue between them

How do Sertoli and Leydig cells communicate?

Through paracrine signaling between the tubules and interstitial tissue

What is spermatogenesis?

The process where diploid spermatogonia develop into haploid spermatozoa

When does spermatogenesis begin and how long does it take?

• It begins at puberty and continues throughout life

• One full cycle takes about 64 days

What temperature is required for spermatogenesis?

2–3 °C lower than body core temperature

Why are the testes located in the scrotum?

To maintain the lower temperature necessary for sperm production

What happens if testicular temperature increases?

Sperm production may be impaired or halted

What are the main phases of spermatogenesis?

1. Mitotic proliferation

2. Meiotic divisions

3. Spermiogenesis

4. Spermiation

What happens during mitotic proliferation?

Spermatogonia divide to maintain the stem cell pool and produce differentiating cells

What happens during meiotic divisions?

Primary spermatocytes undergo meiosis I and II to form haploid spermatids

What is spermiogenesis?

The remodeling of spermatids into spermatozoa without further division

What is spermiation?

The release of mature spermatozoa from Sertoli cells into the lumen of seminiferous tubules

What are spermatogonial stem cells (SSCs)?

Diploid germline stem cells next to the basal lamina that self-renew and produce progenitor cells

What are Type A spermatogonia?

• Derived from SSCs

• Pale Type A maintain the stem cell line

• Dark Type A differentiate further as progenitors

What are Type B spermatogonia?

• Differentiated cells committed to spermatogenesis

• Still mitotically active

What are primary spermatocytes?

• Diploid (2n) cells formed after the last mitotic division

• Undergo meiosis I to form haploid secondary spermatocytes

What are secondary spermatocytes?

Haploid cells that rapidly undergo meiosis II to produce spermatids

What are spermatids?

Haploid cells that undergo spermiogenesis to become spermatozoa

What happens to spermatozoa after release?

They are transported to the epididymis for maturation and storage

What occurs during the Golgi phase?

• Golgi vesicles form the acrosomal vesicle

• Centrioles migrate to the opposite pole to initiate flagellum formation

What occurs during the Cap phase?

• The acrosomal vesicle forms an acrosomal cap over the nucleus

• The flagellar axoneme forms and nuclear flattening begins

What occurs during the Acrosome phase?

• The nucleus elongates and condenses (histones replaced by protamines)

• Mitochondria move to form the midpiece

• The manchette helps shape the cell

What occurs during the Maturation phase?

• Excess cytoplasm is removed

• Mitochondria form a helical sheath

• The flagellum elongates

• A mature spermatozoon is formed

What is spermiation?

Release of mature spermatozoa from Sertoli cells into the lumen of seminiferous tubules

What happens during spermiation?

• Sertoli–spermatid junctions break down

• Adhesion molecules are dismantled

• Cytoskeletal changes push sperm into the lumen

What happens if spermiation fails?

Mature spermatids are retained, reducing sperm output and fertility

What are the three main parts of a spermatozoa?

Head, midpiece, and tail (flagellum)

What is in the head of the spermatozoa?

A haploid nucleus and the acrosome containing enzymes (acrosin, hyaluronidase) for oocyte penetration

What is in the midpiece of the spermatozoa?

Contains mitochondria arranged helically to supply ATP for movement

What is in the tail of the spermatozoa?

A flagellum with a 9+2 microtubule axoneme and dynein for motility

What is the function of Sertoli cells?

Provide structural, nutritional, and metabolic support for developing germ cells

What key substances do Sertoli cells secrete?

• Androgen-binding protein (ABP)

• Inhibin

• Activin

• Anti-Müllerian hormone (AMH)

What are the roles of these secretions?

• ABP: Maintains high intratubular testosterone levels

• Inhibin: Suppresses FSH secretion

• Activin: Enhances FSH action

• AMH: Causes regression of Müllerian ducts during embryonic life

What enzyme do Sertoli cells contain for hormone conversion?

Aromatase (converts testosterone to estradiol)

How do Sertoli cells protect developing germ cells?

They form the blood–testis barrier via tight junctions and secrete FAS ligand to induce T-cell apoptosis

Where are Leydig cells located?

In the interstitial tissue between seminiferous tubules

What do Leydig cells secrete?

• Testosterone

• Dihydrotestosterone (DHT)

• Dehydroepiandrosterone (DHEA)

What are the functions of the androgens secreted by Leydig cells?

Support spermatogenesis locally and regulate development, metabolism, and secondary sexual traits systemically

What are the main physiological effects of androgens?

• Maintain spermatogenesis and libido

• Promote bone growth and closure of epiphyses

• Increase muscle mass and deepen voice

• Boost metabolism and erythropoiesis

• Affect skin (thickness, pigmentation, sebaceous secretion)

What is the fetal role of testosterone and DHT?

Testosterone directs Wolffian duct development and testicular descent; DHT develops external genitalia

How does testosterone interact with androgen receptors?

Moderately; it has balanced anabolic and androgenic effects and can convert to DHT or estradiol

How does DHT differ from testosterone?

DHT has 2–3x higher androgen receptor affinity, strong androgenic effects (skin, prostate, genitalia), and minimal anabolic action

What is the role of GnRH?

Pulsatile secretion from the hypothalamus stimulates FSH and LH release from the pituitary

What does LH do?

Acts on Leydig cells to promote testosterone production and release

What does FSH do?

Acts on Sertoli cells to promote ABP, inhibin, and other factors for spermatogenesis

How does testosterone regulate itself?

Through negative feedback on the hypothalamus and pituitary, suppressing GnRH and LH

What is the role of inhibin in feedback?

Inhibin from Sertoli cells selectively inhibits FSH release