The Reproductive System: Gonads and Development

• The reproductive system is the system in humans that contributes to the production of offspring to ensure survival of the human species.
• Reproductive organs produce gametes and contributes to the development of secondary sex characteristics

Gonads: Primary Sex Organs

• Testes are present in males.
• Ovaries are present in females.
• Gonads produce gametes or sex cells.
• Gonads secrete hormones.
• Sperm are the male gametes.
• Ova or eggs are female gametes.

Development Aspect

• Gender is determined at fertilization.
• Males have XY sex chromosomes.
• Females have XX sex chromosomes.
• Gonads start to form until the eighth week.
• Testes form in the abdominal cavity and descend to the scrotum one month before birth.
• A determining factor for gonad differentiation is testosterone.
• Reproductive system organs do not function until puberty.
• Puberty usually occurs between ages 10 and 15.
• The first menses usually occur about two years after the start of puberty.
• Most women reach peak reproductive ability in their late 20s.
• Menopause occurs when ovulation and menses cease entirely.
• Ovaries stop functioning as endocrine organs during menopause.
• There is no equivalent of menopause in males, but males experience a steady decline in testosterone.

Male Reproductive System

• Main organs consist of testes and a duct system which includes the epididymis, ductus deferens, and urethra.
• Accessory organs consist of a seminal vesicle, prostate gland, and bulbourethral gland.
• External genitalia consist of the penis and scrotum.

Testes

• There are two oval-shaped glands in the scrotum.
• Testes are the primary sex organs in the male reproductive system.
• Golf ball-sized testes are approximately 4 cm (1.5 inches) long and 2.5 cm (1 inch) wide.
• Testes produce sperm and secrete testosterone.

Coverings of the Testes

• Tunica albuginea surrounds each testis.
• Septa – extensions of the capsule that extend into the testis and divide into lobules.
• Each lobule contains one to four seminiferous tubules, which are tightly coiled structure & function as sperm-forming factories and empty sperm into the rete testis.
• Sperm travels through the rete testis to the epididymis.
• Interstitial cells produce androgens such as testosterone.

Epididymis

• The epididymis is comma-shaped and tightly coiled.
• It is located on the superior part of the testis and along the posterior lateral side.
• It functions to mature and store sperm cells (at least 20 days).
• It expels sperm with the contraction of muscles in the epididymis walls to the vas deferens.

Ductus Deferens (Vas Deferens)

• The ductus deferens carries sperm from the epididymis to the ejaculatory duct.
• It passes through the inguinal canal and over the bladder.
• It moves sperm via peristalsis and forms the spermatic cord with blood vessels and nerves in a connective tissue sheath.
• The ductus deferens ends in the ejaculatory duct which unites with the urethra.
• A vasectomy involves cutting the ductus deferens at the level of the testes which prevents transportation of sperm.

Urethra

• Extends from the base of the urinary bladder to the tip of the penis.
• Carries both urine and sperm.
• Sperm enters from the ejaculatory duct.

Regions of the Urethra

• Prostatic urethra -surrounded by prostate.
• Membranous urethra from prostatic urethra to penis.
• Spongy (penile) urethra runs the length of the penis.

Seminal Vesicles

• Seminal vesicles are located at the base of the bladder.
• Seminal Vesicles produce a thick, yellowish secretion (60% of semen) consisting of fructose (sugar), Vitamin C, prostaglandins, and other substances that nourish and activate sperm.

Prostate Gland

• The prostate gland encircles the upper part of the urethra and secretes a milky fluid to activate sperm and enters the urethra through several small ducts.

Bulbourethral Gland

• It is a pea-sized gland inferior to the prostate that produces a thick, clear mucus to cleanse the urethra of acidic urine and serves as a lubricant during sexual intercourse, it is secreted into the penile urethra.

Scrotum

• It is a divided sac of skin outside the abdomen which maintains testes at 3°C lower than normal body temperature to protect sperm viability.

Penis

• The penis delivers sperm into the female reproductive tract, consists of a shaft, glans penis(enlarged tip), and prepuce/foreskin (folded cuff of skin around proximal end).
• The prepuce or foreskin is often removed by circumcision.
• Ejaculation is the release of semen through the penis during orgasm.
• Emission phase: The sperm moves to your prostate from your testicles and mixes with fluid to create semen.
• Vas deferens contract to squeeze the semen toward the base of the penis.
• Expulsion phase: Muscles at the base of your penis contract every 0.8 seconds and force or shoot the semen out of your penis in several spurts.

Semen

• Semen is a mixture of sperm and accessory gland secretions.
• Fructose provides energy for sperm cells.
• Alkalinity of semen helps neutralize the acidic environment of vagina.
• Semen inhibits bacterial multiplication, and elements of semen enhance sperm motility.
• Sperm are sluggish under acidic conditions (below pH 6) , relative alkalinity of semen as a whole (pH 7.2–7.6) it helps neutralize acidic environment (pH 3.5–4.0) of the female's vagina, protecting delicate sperm.

Spermatogenesis

• It is the production of sperm cells that begins at puberty and continues throughout life which occurs in the seminiferous tubules.

Processes of Spermatogenesis

• Spermatogonia (stem cells) undergo rapid mitosis to produce more stem cells before puberty.
• Follicle stimulating hormone (FSH) modifies spermatogonia division.
• One cell produced is a stem cell and the other cell produced becomes a primary spermatocyte.
• Primary spermatocytes undergo meiosis, and haploid spermatids are produced.
• Spermiogenesis allows late spermatids to be produced with distinct regions.
• The head contains DNA covered by the acrosome.
• Sperm cells result after maturing of spermatids, spermatogenesis takes 64 to 72 days.

Structure of Sperm Cells

• Sperm cells consist of a head, midpiece, and tail which function for genetic instructions, penetrating follicle cells, energy, and mobility.

Testosterone

• It is the hormonal product of testes which rises in blood level to stimulate the adolescent growth spurt, reproductive organs develop to their adult size, underlies sex drive, and causes male secondary sex characteristics to appear.
• It also causes the deepening of the voice as the larynx enlarges and increased hair growth all over the body, particularly in the axillary and pubic regions and on the face.
• It is also responsible for the enlargement of skeletal muscles to produce the heavier muscle mass typical of the male physique.

Female Reproductive System

• Main organs include ovaries and the duct system which includes the uterine (fallopian) tubes, uterus, and vagina.
• External Genitalia includes the labia and clitoris.

Ovary

• It is composed of ovarian follicles which are sac-like structures.

Structure of an Ovarian Follicle

• Composed of an oocyte or egg cell and follicular cells which are the cells surrounding the oocyte.

Ovarian Follicle Stages

• Primary follicle contains an immature oocyte.
• Graafian (vesicular) follicle is a growing follicle with a maturing oocyte.
• Ovulation occurs when the egg is mature and the follicle ruptures, and it occurs about every 28 days.
• The ruptured follicle is transformed into a corpus luteum.

Uterine (Fallopian) Tube

• Receives the ovulated oocyte to provide a site for fertilization which attaches to the uterus without physically attaching to the ovary, and it is supported by the broad ligament.
• Fimbriae are finger-like projections at the distal end that receive the oocyte.
• Cilia inside the uterine tube slowly move the oocyte towards the uterus (takes 3–4 days).
• Fertilization usually occurs in the uterine tube.

Uterus

• Located between the urinary bladder and rectum it is a hollow organ that functions to receive, retain, and nourish a fertilized egg.

Support for the Uterus

• The broad ligament is attached to the pelvis.
• The round ligament is anchored interiorly.
• The uterosacral ligaments are anchored posteriorly.

Regions of the Uterus

• The body is the main portion
• The fundus is the area where the uterine tube enters.
• The cervix is a narrow outlet that protrudes into the vagina.

Layers of the Uterus

• Endometrium is the inner layer that allows implantation of a fertilized egg and is sloughed off if no pregnancy occurs which is called menses.
• Myometrium is the middle layer of smooth muscle.
• Serous layer is the outer visceral peritoneum.

Vagina

• Extends from the cervix to the exterior of the body and is located behind the bladder and in front of the rectum.
• The vagina serves as the birth canal and receives the penis during sexual intercourse and the Hymen partially closes the vagina until it is ruptured.
• The Mons Pubis is a fatty area overlying the pubic symphysis which is covered with pubic hair after puberty.
• The Labia consist of skin folds known as the labia majora and labia minora.
• Vestibule is enclosed by labia majora, which contains the opening of the urethra and the greater vestibular glands that produce mucus.

Clitoris

• It contains erectile tissue and corresponds to the male penis.

Oogenesis

• The total supply of eggs are present at birth but the ability to release eggs begins at puberty and the reproductive ability ends at menopause.
• Oocytes are matured in developing ovarian follicles.
• Oogonia are female stem cells that are found in a developing fetus, they undergo mitosis to produce primary oocytes.
• Primary oocytes are surrounded by cells that form primary follicles in the ovary but Oogonia no longer exist by the time of birth., and primary oocytes are inactive until puberty.
• Follicle stimulating hormone (FSH) causes some primary follicles to mature.
• Meiosis starts inside the maturing follicle to produce a secondary oocyte and the first polar body.
• Meiosis is completed after ovulation only if sperm penetrates, and two additional polar bodies are produced.

Menstrual Cycle

• The menstrual cycle consists of cyclic changes of the endometrium regulated by cyclic production of estrogens and progesterone.
• Stages include:
  • (Days 1-5) Menstrual stage - functional layer of the endometrium is sloughed or shredded.
  • (Days 6-14) Proliferative stage - rebuilding/regeneration of functional layer of endometrium.
  • (Days 15-28) Secretory stage – endometrium increases in size and readies for implantation.

Hormonal Control of the Ovarian and Uterine Cycle

• This complicated process depends on hormones

Estrogen

• Produced by follicle cells, it causes secondary sex characteristics by causing enlargement of accessory organs, development of breasts, appearance of pubic hair, increase in fat beneath skin, widening and lightening of the pelvis, as well as the onset of menses.

Progesterone

• Produced by the corpus luteum, production continues until LH diminishes in the blood and helps to maintain pregnancy.

Mammary Glands

• These glands are present in both sexes, but only function in females and are modified sweat glands.
• Their function is to produce milk and are stimulated by sex hormones (mostly estrogens) to increase in size.

Stages of Pregnancy and Development

• Consists of fertilization, embryonic development, fetal development, and childbirth.

Fertilization

• The oocyte is viable for 12 to 24 hours after ovulation.Sperm are viable for 12 to 48 hours after ejaculation.
• Sperm cells must make their way to the uterine tube for fertilization to be possible.
• Membrane receptors on an oocyte pulls in the head of the first sperm cell to make contact.
• The membrane of the oocyte does not permit a second sperm head to enter.
• The oocyte then undergoes its second meiotic division which occurs when the genetic material of a sperm is combined with that of an oocyte to form a zygote.

The Zygote

• This is the first cell of a new individual which is the result of the fusion of DNA from sperm and egg.
• The zygote begins rapid mitotic cell divisions, and the zygote stage is in the uterine tube, moving toward the uterus.

The Embryo

• This is the developmental stage from the start of the cleavage, which is cell dividing without growth, until the ninth week.
• The embryo enters the uterus at the 16-cell state and floats free in the uterus temporarily where uterine secretions are used for nourishment.

The Blastocytes

• Consist of a Ball-like circle of cells and begins at about the 100 cell stage.
• It secretes human chorionic gonadotropin (hCG) to produce the corpus luteum to continue producing hormones.
• The blastocyst implants in the wall of the uterus around day 14.

Development after Implantation

• Chorionic villi (projections of the blastocyst) develop and cooperate with cells of the uterus to form the placenta.
• The embryo is surrounded by the amnion which is a fluid filled sac.
• An umbilical cord forms to attach the embryo to the placenta.

The Placenta

• Forms a barrier between mother and embryo (blood is not exchanged).
• Delivers nutrients and oxygen.
• Removes waste from embryonic blood.
• Becomes an endocrine organ (produces hormones) and takes over for the corpus luteum.
• Important in the creation of Estrogen, Progesterone, and other hormones that maintain pregnancy.

The Fetus

• At the beginning of the ninth week of pregnancy, all organ systems are formed by the end of the eighth week.
• Activities of the fetus are growth and organ specialization.
• It undergoes a stage of tremendous growth and change in appearance.
• Pregnancy is a period from conception until birth which results in both anatomical and physiological changes.

Anatomical Changes of Pregnancy

• Enlargements of the uterus.
• Accentuated lumbar curvature.
• Relaxation of the pelvic ligaments and pubic symphysis due to production of relaxin.

Physiological Changes of Pregnancy

• Effects on the Gastrointestinal system such as:
• Morning sickness is common due to elevated progesterone.
• Heartburn is common because of organ crowding by the fetus.
• Constipation is caused by declining motility of the digestive tract.
• Impact on the Urinary System:
• Kidneys have additional burden and produce more urine.
• The uterus compresses the bladder.
• Effects on the Cardiovascular system:
• Body water rises.
• Blood volume increases by 25 to 40 percent.
• Blood pressure and pulse increase, and Varicose veins are common.

The Childbirth

• Labor is the series of events that expel the infant from the uterus.
• Initiation of labor includes when Estrogen levels rise, Uterine contractions begin, The placenta releases prostaglandins, and Oxytocin is released by the pituitary, a combination of these hormones produces contractions.

Stages of Labor

• Dilation occurs when the cervix becomes dilated, uterine contractions begin and increase, and the amnion ruptures.
• Expulsion occurs when the infant passes through the cervix and vagina, and the normal delivery is head first
• Placental stage includes the delivery of the placenta.