Reproductive System

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings

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),

Estrogens and progesterone (females)


Male Reproductive System

The male gonads (testes) produce sperm and lie within the scrotum

Sperms are delivered to the exterior through a system of ducts: epididymis, ductus deferens, ejaculatory duct, and the urethra

Accessory sex glands:

Empty their secretions into the ducts during ejaculation

Include the seminal vesicles, prostate gland, and bulbourethral glands


The Scrotum and associated structures Sac of skin and superficial fascia that hangs outside

the abdominopelvic cavity at the root of the penis

Contains paired testicles separated by a midline septum

Its external positioning keeps the testes 3C lower than core body temperature (needed for sperm production)

Each testis is surrounded by two tunics:

The tunica vaginalis, derived from peritoneum

The tunica albuginea, the fibrous capsule of the testis


Testicular Thermoregulation Sperms are not produced at core body temperature

In the scrotum, the testes are kept 2-3°C cooler than in the pelvic cavity. This is essential for sperm production.

Cooling mechanisms

The cremaster muscle contains strips of the internal abdominal oblique muscle around the spermatic cord.

It can elevate or lower the testes.

The dartos muscle is a subcutaneous layer of smooth muscle that wrinkles skin reducing surface area of scrotum. Can lifts testis upwards

The pampiniform plexus is an extensive network of veins that surround the testicular artery in the spermatic cord, keeping the testes cooler countercurrent heat exchange that cools arterial blood entering testis


The Testes Septa divide the testis into

250-300 lobules, each containing 1-4 seminiferous tubules

Seminiferous tubules:

Produce the sperm

Converge to form the tubulus rectus

The straight tubulus rectus conveys sperm to the rete testis


Cells in the testes

Between the seminiferous tubules are clusters of interstitial (Leydig) cells, the source of testosterone.

A seminiferous tubule

lumen lined with germinal epithelium consisting of several layers of germ cells in the process of becoming sperm

Sustentacular (Sertoli) cells.


The functions of Sertoli cells and BTB Sustentacular cells protect the germ cells and promote their

development. Tight junctions between adjacent sustentacular cells form a blood-testis barrier (BTB)

The fluid inside the tubules contains high levels of androgens, potassium and amino acids

The BTB prevents the immune system from attacking the developing spermatozoa (contain specific Ag that are not found on any other cell)

Sertoli cell supply nutrients to the developing sperms

Phagocytize cytoplasm shed by spermatids

Secrete inhibin (we will talk about this hormone later)

Secrete androgen-binding-protein (ABP) – binds testosterone inside tubules to maintain high levels



Spermatic Ducts from rete testis to urethra

Epididymis (head, body and tail)

6 m long coiled duct adhering to posterior of testis

site of sperm maturation and storage (fertile for 60 days)

Ductus deferens/vas deferens (peristalsis during orgasm)

muscular tube 45 cm long passing up from scrotum through inguinal canal to posterior surface of bladder

Ejaculatory duct

2 cm duct formed from ductus deferens and seminal vesicle and passing through prostate to empty into urethra

Spermatic cord – encloses PNS and SNS nerve fibers, blood vessels, and lymphatics that supply the testes


Epididymis The epididymis functions:

The duct of the epididymis has stereocilia that:

Absorb testicular fluid

Pass nutrients to the sperm

Recycling center for damaged spermatozoa

Storage place for spermatozoa

Facilitate functional maturation preparing them for capacitaion.

The spermatozoa mature in the epididymis but are not motile yet


Ductus Deferens and Ejaculatory Duct

Runs from the epididymis through the inguinal canal into the pelvic cavity

Its terminus expands to form the ampulla and then joins the duct of the seminal vesicle to form the 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

Consists of three regions

Prostatic – portion surrounded by the prostate

Membranous – lies in the urogenital diaphragm

Spongy, or penile – runs through the penis and opens to the outside at the external urethral orifice


The Penis A copulatory organ designed to deliver sperm into the female

reproductive tract

Consists of an attached root and a free shaft that ends in the glans penis

Foreskin – cuff of skin covering the distal end of the penis

Erectile tissue – spongy network of connective tissue and smooth muscle riddled with vascular spaces

Corpus spongiosum – surrounds the urethra and expands to form the glans and bulb of the penis

Corpora cavernosa – paired dorsal erectile bodies bound by fibrous tunica albuginea

Erection – during sexual excitement, the erectile tissue fills with blood causing the penis to enlarge and become rigid


Accessory Glands: Seminal Vesicles Lie on the posterior wall of the bladder and secrete 60% of the volume of

semen

Join the ductus deferens to form the ejaculatory duct

The composition of the gland secretions include:

High concentration of fructose that is easily metabolized by spermatozoa

Prostaglandins that stimulate smooth muscle contractions in both male and female reproductive tract

Fibrinogen that forms temporary clot within the vagina after ejaculation

When the inactive sperms mix with the seminal fluid they undergo the first step of activation and the flagella start moving (in some book this is done by the help of the prostate)

The secretion of the seminal gland are discharged into the ejaculatory duct under the control of the sympathetic system


Accessory Glands: Prostate Gland

Doughnut-shaped gland that encircles part of the urethra inferior to the bladder

Produces 20-30% of the semen volume

Contains citrate (nutrient source) and enzymes

Fluid enters the prostatic urethra during ejaculation


Accessory Glands: Bulbourethral Glands (Cowper’s Glands)

Pea-sized glands inferior to the prostate

Produce thick, clear mucus prior to ejaculation that neutralizes traces of acidic urine in the urethra


semen

The combination of sperm and the fluids secreted by the accessory glands.

The ejaculate (fluid produced during ejaculation) contains 20-100 million spermatozoa/ml

The volume is about 2-5 ml


Ejaculation

The propulsion of semen from the male duct system

At ejaculation, sympathetic nerves serving the genital organs cause:

Reproductive ducts and accessory organs to contract and empty their contents

The bladder sphincter muscle to constrict, preventing the expulsion of urine

Propulsion of semen from the urethra (peristaltic contractions)


Meiosis

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)


Chromosomes in Mitosis and Meiosis


Spermatogenesis

Cells making up the walls of seminiferous tubules are in various stages of cell division

These spermatogenic cells give rise to sperm in a series of events

Mitosis of spermatogonia, forming spermatocytes

Meiosis forms spermatids from spermatocytes

Spermiogenesis – spermatids form sperm


Mitosis of Spermatogonia

Spermatogonia – outermost cells in contact with the epithelial basal lamina

Spermatogenesis begins at puberty as each mitotic division of spermatogonia results in type A or type B daughter cells

Type A cells remain at the basement membrane and maintain the germ line

Type B cells move toward the lumen and become primary spermatocytes


Spermatocytes to Spermatids

Primary spermatocytes undergo meiosis I, forming two haploid cells called secondary spermatocytes

Secondary spermatocytes undergo meiosis II and their daughter cells are called spermatids

Spermatids are small round cells seen close to the lumen of the tubule


Spermiogenesis: Spermatids to Sperm

Late in spermatogenesis, spermatids are haploid but nonmotile

Spermiogenesis – spermatids lose excess cytoplasm and form a tail, becoming sperm

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings http://distance.stcc.edu/AandP/AP/AP2pages/reprod/spermato.htm

spermatogenesis

spermiogenesis


Spermiogenesis: Spermatids to Sperm

Sperm have three major regions

Head – contains DNA and has a helmetlike acrosome containing hydrolytic enzymes that allow the sperm to penetrate and enter the egg

Midpiece – contains mitochondria spiraled around the tail filaments

Tail – a typical flagellum produced by a centriole


Brain-Testicular Axis

Hormonal regulation of spermatogenesis and testicular androgen production involving the hypothalamus, anterior pituitary gland, and the testes

Testicular regulation involves three sets of hormones:

GnRH, which indirectly stimulates the testes through:

Follicle stimulating hormone (FSH)

Luteinizing hormone (LH)

Gonadotropins, which directly stimulate the testes

Testicular hormones, which exert negative feedback controls


Hormonal Regulation of Testicular Function

The hypothalamus releases gonadotropin-releasing hormone (GnRH)

GnRH stimulates the anterior pituitary to secrete FSH and LH

FSH causes sustentacular cells to release androgen-binding protein (ABP)

LH stimulates interstitial cells to release testosterone

ABP binding of testosterone enhances spermatogenesis


Testosterone

The principal androgen (male sex hormone) is testosterone.

This steroid is manufactured by the interstitial (Leydig) cells of the testes.

Secretion of testosterone increases sharply at puberty and is responsible for the development of the secondary sexual characteristics of men.

Testosterone is also essential for the production of sperm.


Male Secondary Sex Characteristics Male hormones make their appearance at puberty and

induce changes in nonreproductive organs, including

Appearance of pubic, axillary, and facial hair

Enhanced growth of the chest and deepening of the voice

Skin thickens and becomes oily

Bones grow and increase in density

Skeletal muscles increase in size and mass

Testosterone is the basis of libido in both males and females

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Reproductive System

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