The Male Reproductive System

Sexual Reproduction

• Sexual reproduction involves the combination of genetic material to produce offspring.

• Gametes are specialized cells required for sexual reproduction.

  • Male gamete: sperm cell

  • Female gamete: ovum

• Fertilization is the union of a male and female gamete, forming a zygote.

Chromosomes

• Chromosomes are grouped in homologous pairs, carrying genes for the same inherited characteristics.

• Somatic cells (regular body cells) have homologous chromosome pairs, one from each parent.

• Homologous chromosomes are matched in length, centromere location, and gene loci.

• Humans have 22 pairs of homologous chromosomes called autosomes.

Sex Chromosomes

• X and Y chromosomes determine gender and are exceptions to the homologous chromosome rule.

• Females have a homologous pair of X chromosomes (XX).

• Males have an X and a Y chromosome (XY).

• X and Y chromosomes differ in size and genetic composition but act as homologous chromosomes.

Reproductive Systems

• Male and female reproductive systems function to produce gametes and sex hormones.

• Primary sex organs (gonads) produce gametes and sex hormones.

• Accessory reproductive organs include accessory sex glands and supporting structures.

• The perineum is the surface region containing much of the reproductive system, located between the pubic symphysis and the coccyx.

  
  

Male Reproductive System

• Primary sex organs in males are the testes.

• Accessory sex glands include seminal vesicles, prostate, and bulbourethral glands.

  • These glands secrete fluids for sperm function and delivery.

• Supporting structures include a system of ducts, the scrotum, and the penis.

• The urethra is part of both the urinary and reproductive systems in men.

The Scrotum

• The scrotum supports the testes.

• Externally, it is divided into two lateral portions by the perineal raphe running from the glans penis to the anus.

• Internally, the median septum separates the scrotum into two sacs, each containing a testis.

• The dartos muscle is found in the scrotal septum and subcutaneous layers, composed of smooth muscle.

The Scrotum (cont.)

• The cremaster muscle is a skeletal muscle associated with each testis and is an extension of the internal oblique muscle.

• The cremaster and dartos muscles regulate the temperature of the testes.

• Normal sperm production requires a temperature range between 34-35°C.

• The muscles contract and relax in response to environmental conditions to maintain proper temperatures for sperm production.

The Testes

• The testes (testicles) are covered by the tunica vaginalis, a serous membrane.

• The tunica albuginea is deep to the tunica vaginalis and consists of dense irregular connective tissue.

• It separates the testes into lobules through the formation of septa.

• Each lobule contains one to three coils of seminiferous tubules, responsible for sperm formation (spermatogenesis).

The Seminiferous Tubules

• Seminiferous tubules contain two cell types:

  • Spermatogonia: spermatogenic cells responsible for sperm formation.

  • Sertoli cells (sustentacular cells): support spermatogenesis.

• Leydig cells are located in the spaces between seminiferous tubules and secrete testosterone.

The Sertoli Cells

• Sertoli cells form the blood-testis barrier to prevent immune responses against sperm cells.

• Other functions include:

  • Nourishing immature and mature sperm cells

  • Phagocytizing excess cytoplasm released by sperm

  • Controlling movement of spermatogonia

  • Controlling sperm release into the tubule lumen

  • Producing fluid for sperm transport

  • Secreting inhibin, which regulate testosterone and FSH effects

Gamete Cells

• Most cells are diploid cells, with two homologous sets of chromosomes and two sex chromosomes.

  • Somatic cells are diploid.

  • Human diploid cells have 46 chromosomes.

• Gamete cells are haploid cells, with a single set of chromosomes and one sex chromosome.

  • Human haploid cells have 23 chromosomes.

• Meiosis is the process of nuclear division to form haploid cells used in sexual reproduction.

Meiosis

• Interphase occurs before meiosis, involving growth and chromosome duplication.

• Meiosis has two parts: meiosis I and meiosis II.

  • In meiosis I, homologous chromosomes come together in tetrads and then split into replicated chromosomes.

  • In meiosis II, the replicated chromosomes are split into individual chromosomes.

• Cell division occurs in both meiosis I and II.

Meiosis I

• Meiosis I consists of five phases.

• Prophase I is similar to prophase in mitosis.

  • The nuclear envelope breaks down, nucleoli disappear, and the spindle apparatus forms.

• The primary difference is how chromosomes interact in both processes.

Meiosis I (cont.)

• In prophase I, chromosomes condense and homologous chromosomes group into tetrads in synapsis.

• During synapsis, crossing over (homologous recombination) occurs, exchanging genetic material between chromosomes.

• After recombination, the chromosomes condense and are directed to the center of the cell by the spindle apparatus.

Meiosis I (cont.)

• In Metaphase I, the tetrads are arranged on the metaphase plate.

• In Anaphase I, the tetrads separate and are directed towards the poles by the spindle.

• The chromosomes do not separate into individual chromosomes; instead, each side has two copies of the same chromosome (double haploid).

Meiosis I (cont.)

• In mitosis, chromosomes $A1A1 A2A2 B1B1 B2B2$ are divided into $A1A2 B1B2$ and $A1A2 B1B2$.

• In meiosis I, chromosomes $A1A1 A2A2 B1B1 B2B2$ are divided into $A1A1 B1B1$ and $A2A2 B2B2$.

• In Telophase I, chromosomes reach the poles, and two nuclei are formed.

• Cytokinesis usually occurs with telophase I, splitting the diploid cell into two daughter haploid cells.

Meiosis II

• Meiosis II is functionally identical to mitosis, except the starting cell is a haploid cell with duplicate chromosomes.

• The subphases are prophase II, metaphase II, anaphase II, telophase II, and cytokinesis.

• At the conclusion of meiosis II, one single diploid cell has generated four haploid gamete cells.

Spermatogenesis

• Spermatogonia are stem cells derived from primordial germ cells.

• After differentiation and migration through the blood-testis barrier, they become primary spermatocytes.

• A primary spermatocyte enters meiosis I, forming two haploid secondary spermatocytes.

• The two secondary spermatocytes enter meiosis II, forming four spermatids.

Spermatogenesis (cont.)

• Throughout meiosis, secondary spermatocytes and spermatids do not completely separate through cytokinesis; thin cytoplasmic bridges connect them.

• The maturation of spermatids into spermatozoa is called spermiogenesis.

• Spherical spermatids differentiate into elongated, flagellated sperm cells.

Sperm Cells

• A sperm cell consists of a head and a tail.

• The head is covered with an acrosome, containing digestive enzymes.

• The head contains the nucleus of the sperm cell and the flagellar basal body.

• The tail consists of three components:

  • Midpiece: contains mitochondria for energy production.

  • Principal piece: provides the majority of locomotive force.

  • Endpiece: the tapering end of the tail.

Hormonal Control of the Testes

• During puberty, the hypothalamus secretes GnRH, stimulating the release of LH and FSH.

• LH stimulates Leydig cells to produce testosterone.

  • Testosterone can inhibit LH secretion through negative feedback.

• FSH stimulates secretion of androgen-binding protein (ABP).

  • ABP and FSH assist testosterone in inducing spermatogenesis.

• Secretion of inhibin by Sertoli cells regulates spermatogenesis through negative feedback.

System Ducts

• The male reproductive system has several ducts for sperm excretion.

• Ducts of the testis convey sperm and fluid secreted by Sertoli cells from the seminiferous tubules into the epididymis.

  • These include the rete testis and efferent ductules.

• Epididymides is the site of sperm maturation.

  • Each epididymis is mostly composed of the tightly coiled ductus epididymis that reabsorb nonfunctional sperm.

  • The end of the ductus epididymis becomes wider and less coiled, turning into the ductus (vas) deferens.

System Ducts (cont.)

• The vas deferens conveys sperm during sexual arousal from the epididymis toward the urethra.

• It is part of the spermatic cord, containing the vas deferens, cremaster muscle, testicular artery, and pampiniform plexus.

• The terminal end of the vas deferens is the ampulla.

• Ejaculatory ducts are formed by the union of the ampulla and the secretory duct of the seminal vesicle.

• The ducts terminate in the urethra.

System Ducts (cont.)

• The urethra is composed of three parts based on its location:

  • Prostatic urethra: passes through the prostate, and the ejaculatory ducts empty into it.

  • Membranous urethra: passes through the deep muscles of the perineum (diamond shape between the anus and genitals).

  • Spongy urethra: enters the penis.

• Excretion of fluid occurs through the external urethral orifice.

Accessory Sex Glands

• The seminal vesicles secrete a thick fluid containing fructose and other carbohydrates, prostaglandins, and prosemenogelin (precursor to semenogelin).

  • Fructose provides fuel for sperm.

  • Prostaglandins assist in sperm motility and viability.

  • Prosemenogelin is the inactive form of semenogelin, a fibrin-like protein that clots semen into a gel-like consistency.

Accessory Sex Glands (cont.)

• The prostate secretes a slightly acidic fluid containing citric acid, serine protease, acid phosphatase, and seminalplasmin.

  • Citric acid provides fuel for sperm.

  • Serine protease and acid phosphatase contribute to the liquefaction of semen by breaking down semenogelin.

  • Seminalplasmin is a bacteriocide.

• The bulbourethral glands (Cowper’s glands) secrete alkaline fluid to neutralize acidic urine and lubricating mucus to minimize damage to sperm.

Semen

• Semen is a combination of mature spermatozoa and fluids secreted by the seminiferous tubules, seminal vesicles, prostate, and Cowper’s glands.

• The combined fluids are referred to as seminal fluid.

• Normal sperm content in semen is between 50-120 million sperm/ml.

• Seminal fluid has a pH range of 7.2-7.7.

• It provides spermatozoa with nutrients, a transport medium, and protection against acidic conditions.

The Penis

• The penis is responsible for semen ejaculation and urine excretion.

• It consists of the body, glans penis, and root.

• Blood vessels and nerves are primarily located dorsally.

• The body consists of three cylindrical structures:

  • Two dorsolateral masses: corpora cavernosa, surrounded by a tunica albuginea and separated by a median septum.

  • Midventral mass: corpus spongiosum, containing the spongy urethra.

The Penis (cont.)

• The glans penis is distal to the corpus spongiosum in the penis.

• The corona is the rounded projecting border of the glans penis.

• The external urethral orifice is found in the glans penis.

• The root of the penis is the proximal end of the penis.

  • It consists of the bulb of the penis (continuation of the corpus spongiosum) and two crura of the penis (continuations of the corpora cavernosa).