The Reproductive System

The Two Sexes Male and female gametes (sex cells) combine their genes to form a zygote (fertilized egg) One gamete has motility: sperm (spermatozoon) Parent producing sperm considered male Parent with a Y chromosome is male Other gamete contains nutrients for developing embryo: egg (ovum) Parent producing eggs considered female Anyone lacking a Y chromosome is female In mammals, female is the parent that provides a sheltered internal environment and prenatal nutrition of the embryo

Overview of the Reproductive System Male reproductive system serves to produce sperm and introduce them into the female body Males have a copulatory organ (penis) for introducing their gametes into the female reproductive tract Female reproductive system produces eggs, receives sperm, provides for the union of the gametes, harbors the fetus, and nourishes the offspring Females have a copulatory organ (vagina) for receiving the sperm

Reproductive System Primary sex organs (gonads) – testes in males, ovaries in females Gonads produce sex cells called gametes and secrete sex hormones Accessory reproductive organs – ducts, glands, and external genitalia Sex hormones – androgens (males), and estrogens and progesterone (females)

Male reproductive system Gonads – testes Produces sperm and secretes hormones System of ducts – transport and stores sperm, assists in their maturation, and conveys them to the exterior Epididymis, ductus deferens, ejaculatory ducts, and urethra Accessory sex glands – adds secretions to semen Seminal vesicles, Prostate, and Bulbourethral glands Supporting structures Scrotum supports testes and penis delivers sperm into female reproductive tract

Male Reproductive System

Semen Milky white, sticky mixture of sperm and accessory gland secretions Provides a transport medium, nutrients and protection Slightly alkaline (pH 7.2 – 7.7) Prostaglandins Decrease the viscosity of mucus in the cervix Typical ejaculate = 2-5 ml fluid 60% seminal vesicle fluid, 30% prostatic fluid, and 10% sperm and spermatic duct secretions Contains between 20 – 100 million spermatozoa per ml

Accessory Glands: Seminal Vesicles Lie on the posterior wall of the bladder Produces seminal fluid secrete 60% of the volume of semen Slightly alkaline Seminal fluid contains fructose, prostaglandins and fibrinogen-like protein

Accessory Glands: Prostate Gland Doughnut-shaped gland that encircles part of the urethra About 25-30% of the semen Prostatic fluid is slightly acidic -contains seminalplasmin, an antibiotic -Plays a role in the activation of sperm

Accessory Glands: Bulbourethral Glands (Cowper’s Glands) Pea-sized glands inferior to the prostate Produce thick, clear mucus Alkaline to neutralize acidic urine in the urethra; Protects the sperm by neutralizing the acidity of residual urine in the urethra During sexual arousal, they produce a clear slippery fluid that lubricates the head of the penis in preparation for intercourse

The Scrotum Contains paired testicles separated by a midline septum Its external positioning keeps the testes 2-3C lower than core body temperature (needed for sperm production) Cremaster muscle Dartos muscle

The Testes Descent of the testes Movement of testes through inguinal canal into scrotum Occurs during fetal development Cryptorchidism

The Structure of the Testes

The Testes Each testis is surrounded by two tunics: The tunica vaginalis The tunica albuginea Septa divide the testis into 250-300 lobules, each containing 1-4 seminiferous tubules Seminiferous tubules: Produce the sperm Seminiferous tubules conveys sperm to the rete testis

Sperm production In seminiferous tubules Interstitial cells between seminiferous tubules Secrete sex hormones Sperm pass through rete testis

Histology of the Testis Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Blood vessel Seminiferous tubule Spermatids Sustentacular cell nuclei Tubule lumen Germ cells Connective tissue wall of tubule Interstitial cells (b) 50 µm © Ed Reschke

Seminiferous tubule

Interstitial Cells & Sertoli Cells Interstitial Cells (Leydig cells): responsible for the production of androgens- testosterone Steroli Cells (Sustentacular or Nurse cells): cells that extend from the basal lamina to surrounds developing cells Functions -Maintain Blood-Testis Barrier -Support Mitosis and Meiosis -Support spermiogenesis -Secrets inhibin -Secrets Androgen-binding Protein (ABP)

Epididymis Nonmotile sperm enter, pass through its tubes and become motile Upon ejaculation the epididymis contracts, expelling sperm into the ductus deferens

Ductus Deferens and Ejaculatory Duct Propels sperm from the epididymis to the urethra Vasectomy – cutting and ligating the ductus deferens, which is a nearly 100% effective form of birth control

Urethra Conveys both urine and semen (at different times) Three regions: Prostatic urethra Membranous urethra Penile or spongy urethra

The Penis A copulatory organ designed to deliver sperm into the female reproductive tract Corpus spongiosum Corpora cavernosa Prepuce, or foreskin – cuff of skin covering the distal end of the penis Circumcision – surgical removal of the foreskin after birth

Spermatogenesis Primordial germ cells form in yolk sac of embryo Colonize gonadal ridges and become spermatogonia Puberty brings on spermatogenesis Spermatogonia lie along periphery of seminiferous tubules and divide by mitosis One daughter cell of each division remains in tubule wall as stem cell: type A spermatogonium Other daughter cell migrates slightly away from wall and is on its way to producing sperm: type B spermatogonium

Spermatogenesis contd. Type B spermatogonium enlarges and becomes a primary spermatocyte Sustentacular cells protect it from the body’s immune system: blood–testis barrier (BTB) Primary spermatocyte undergoes meiosis I which gives rise to two equal-size, haploid, genetically unique secondary spermatocytes Each secondary spermatocyte undergoes meiosis II dividing into two spermatids—a total of four for each spermatogonium Spermiogenesis—four spermatids divide no further, but undergo a transformation in which it differentiates into a spermatozoon

Spermatogenesis

Spermatogenesis The sequence of events that produces sperm in the seminiferous tubules of the testes Each cell has two sets of chromosomes (one maternal, one paternal) and is said to be diploid (2n chromosomal number) Humans have 23 pairs of homologous chromosomes Gametes only have 23 chromosomes and are said to be haploid (n chromosomal number) Gamete formation is by meiosis, in which the number of chromosomes is halved (from 2n to n)

Spermatocytes to Spermatids

Spermatogenesis Once the primary spermatocyte undergoes meiosis, it becomes genetically different and needs to be protected from the immune system. The primary spermatocyte moves toward the lumen of the seminiferous tubule and a new tight junction between sustentacular cells forms behind it. Now protected by the blood–testis barrier closing behind it. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Cross section of seminiferous tubules Lumen of seminiferous tubule Sperm 5 Spermiogenesis Spermatid n n n n 4 Meiosis II Secondary spermatocyte n n 3 Meiosis I Blood–testis barrier Primary spermatocyte 2n Sustentacular cell 2 Type B spermatogonium 2n Tight junction Type A spermatogonium 1 2n Basement membrane of seminiferous tubule

Spermiogenesis: Spermatids to Sperm

Anatomy of spermatozoon Each spermatozoon has: Head Nucleus and densely packed chromosomes Middle piece Mitochondria that produce the ATP needed to move the tail Tail The only flagellum in the human body

The Mature Spermatozoon Acrosome Head Nucleus Basal body Mitochondrion Midpiece of tail Axoneme Principal piece of tail Endpiece of tail 2 µm (a) (b)

Hormonal Feedback and the Regulation of the Male Reproductive Function