Chapter 27: The Reproductive System
Gametes: male and female reproductive, haploid cells produced by meiosis, sperm in the male and ovum or oocyte in the female.
The combination of these gametes, fertilization, produces a zygote which is a single cell, new genetic combination that will divide and develop into an infant in approximately nine months.
The reproductive tract includes the following:
Reproductive organs or gonads, testes in the male and ovaries in the female, which produce gametes and hormones.
Reproductive tracts.
Accessory glands and organs that secrete fluids
Perineal structures know as the external genitalia.
Testes: Ovoid organs approximately 3-4 cm in length housed within the scrotum. The testicles develop inside the abdominal cavity near the kidneys and descend as fetal growth progresses. Undescended testes are not located in the scrotum and are at increased risk for developing testicular carcinoma.
Spermatic Cords: Connective tissue, fascia, and muscle enclosing blood vessels nerves and lymphatics supplying the testes. Spermatic cords include testicular artery, pampiniform plexus (draining veins) and nerves. The spermatic cord passes through the inguinal canal to reach the pelvis.
The Scrotum: divided internally into two separate chambers which house the two testicles. The tunica vaginalis is a serous membrane that lines each testicle and the inner surface of the scrotum. An abnormal collection of fluid in this space is called a hydrocele. The dermis of the scrotum contains the dartos muscle. Deep to the dermis is the cremaster muscle, contraction of which elevates the testes (cremaster reflex). Sperm production occurs in a very limited temperature range and the cremaster muscles play an important part in controlling the temperature the testicles are exposed to.
Varicoceles, varices of the pampiniform plexus can lead to low sperm count due to increased temperature in the scrotum.
Histology and spermatogenesis, incredibly briefly:
The epididymis: Tightly wound tubule (about 23 feet long) beginning at the upper margin of the testicle (head) and extending inferiorly (body) to the lower portion of the testicle (tail). It stores sperm as they mature.
The ductus deferens or vas deferens: begins at the tail of the epididymis and extend cephalad through the inguinal canal as part of the spermatic cord. Once in the pelvic cavity it extends to the posterior aspect of the prostate gland where it expands into the ampulla. The ampulla joins with the seminal vesicle to form an ejaculatory duct which empties into the prostatic urethra. The vas deferens carries, stores and aids in the maturation of sperm.
Accessory Glands:
Seminal vesicles: Paired structures lying posterior to the bladder. Join with the ampulla of the vas deferens to form the ejaculatory duct. Secretory glands which contribute about 60 percent of the volume of semen. These secretions activate previously inactive sperm starting “the big swim”.
The Prostate Gland: circular gland surrounding the prostatic urethra at the base of the bladder. Prostatic secretions account for about 20-30% of the volume of semen.
Bulbourethral glands: Small, paired glands which lie in the UG diaphragm and secrete a fluid which helps neutralize any urinary acids and provides some lubrication.
The Penis: Three portions; the root attaches the penis to the pelvis. The body or shaft includes the erectile tissue. The glans is the expanded distal end. The prepuce or foreskin covers the glans but is often removed in circumcision. The shaft of the penis consists of three cylindrical columns of erectile tissue. Two cylinders on the ventral surface of the flaccid penis are the corpora cavernosa. The single corpus spongiosum lies on the dorsal surface and contains the urethra. Please note, these terms refer to the flaccid penis. Apparently early anatomists assumed the penis was not normally flaccid (see dorsal artery, vein, nerve, of the penis).
The Female Reproductive System:
The Ovaries: Paired organs near the lateral pelvic walls and stabilized by ovarian and suspensory ligaments and mesovarium. Ovaries have an outer cortex where ovum are produced and an inner cortex.
The ovarian cycle: Oogenesis is the production of female gametes which occur in ovarian follicles. The stem cells which produce the primary oocytes complete their mitotic divisions before birth. So a female is born with all of the primary oocytes she will have. These primary oocytes are surrounded by simple squamous follicular cells and the combination comprises a primordial follicle. Rising FSH (follicle stimulating hormone) levels at puberty start ovarian cycles which run about 28 days and during which primordial follicles are stimulated to mature.
Primordial follicles develop into primary follicles in response to FSH. As maturation progresses thecal cells develop around the follicle and begin secreting estrogens, primarily estradiol.
Functions of estrogens:
Stimulate bone and muscle growth
Maintain female secondary sexual characteristics
Maintains libido
Initiates repair and growth of endometrium
Primary follicles continue to grow and become secondary follicles which become tertiary follicles at about day 10.
By mid cycle, about day 14, there is typically only one tertiary follicle measuring about 15 mm diameter.
Meiosis in the oocyte produces one ovum and three polar bodies which are essentially little packets of DNA.
Ovulation, initiated by a sudden release of LH, is the release of the oocyte which occurs at about day 14. The follicular phase of the menstrual cycle is from day one to ovulation, typically 14 days.
The involuting follicle develops into the corpus luteum under LH stimulation. The corpus luteum produces progestins which continue to prepare the endometrium for possible implantation.
If there is no implantation the corpus luteum involutes at about day 26 which causes a rapid drop in estrogen and progesterone levels. As the hormone levels fall, the endometrium begins to slough and the cycle starts again. The luteal phase of the cycle is from ovulation to the end of the cycle.
Uterine (fallopian) tubes: hollow muscular tube which transports the ovum to the endometrial cavity. Four parts:
Closest to the ovary is the infundibulum with its fimbria which sweeps the ovum toward the . . .
ampulla, the first portion of the tubular structure. The isthmus is the final portion near the uterus and the intramural portion actually lies in the wall of the uterus.
Fertilization usually occurs in the ampulla of the fallopian tube.
If scaring of the tube is present, typically as a result of previous infection (PID) the fertilized ovum may not be able to make it to the endometrial cavity which results in a tubal or ectopic pregnancy.
Midline pelvic structure which provides support for the embryo (up to 8 weeks) and fetus (9 weeks to delivery). Size varies; the uterus is larger in women that have been pregnant and is smaller in nulliparous and post-menopausal women.
Anatomy: body, fundus, cervix, external os, internal os, cervical canal, endometrial cavity
The endometrium lines the uterine cavity. The myometrium is the muscular body.
Uterine or menstrual cycle: Three phases; menses, proliferative phase and secretory phase.
Menses: Day 1. The onset of bleeding secondary to sloughing of the endometrial tissue as a result of decreasing estrogen and progesterone production by the corpus luteum.
Dysmenorrhea is the term for painful menses.
The proliferative phase begins at the end of menses as the endometrium begins to rebuild. This coincides with the enlargement of the primary and secondary follicles.
The secretory phase begins at ovulation and is marked by enlargement and increased secretions by the endometrial glands. The secretory phase lasts approximately 14 days and ends with the onset of menses.
Menarche is the onset of menses and occurs around 11 or 12 years of age.
Menopause is the end of menstrual periods and occurs about age 45 to 55.
The Vagina: Muscular, elastic tube extending from the vestibule of the external genitalia to the cervix. The recesses around the cervix are the fornices. The vagina and the vestibule are separated by the hymen which completely or partially blocks the entrance to the vagina.
The vulva or pudendum constitutes the external genitalia of the female. The vagina opens into the vestibule which is surrounded by the labia minora. The urethra exits just anterior to the vagina. The clitoris, which contains erectile tissue similar to the corpora cavernosa of the male projects just anterior to the urethra and is covered by the prepuce or hood.
The outer limits of the pudenda are formed by the mons pubis and labia majora, which are homologous to the scrotum of the male.
The mammary glands, anatomy: nipple, areola, lobes, lactiferous ducts, lactiferous sinus
.
Pregnancy (briefly): fertilization produces a zygote, which undergoes several divisions to produce a blastocyst. The blastocyst implants in the endometrium. Once a blastocyst implants in the endometrium it becomes and embryo. The outer cells of the blastocyst, the trophoblast will form the supporting tissues such as the placenta. A cluster of inner cells called the inner mass will develop into the fetus. It is the inner cell mass that contains pluripotent cells (cells that can differentiate into virtually any cell type in the human body) which are used in stem cell research. These cells generally come from blastocysts left over from in vitro fertilization.
