14 Reproductive system

The propagation of species is not a small feat. The female and male reproductive systems have to produce gametes, and the respective ductal systems have to lead the gametes to meet and join. In addition to gonads, ducts, and accessory glands in both sexes, females have a uterus, an organ designed for nurturing and protecting a fetus during fetal development.

Male reproductive system

The male reproductive system consists of:

  • Gonads called testes (singular testis)
  • Ducts
    • Epididymis
    • Ductus deferens
    • Ejaculatory duct
    • Male urethra
  • Accessory glands
    • Seminal vesicles
    • Prostate
    • Bulbourethral glands
  • External genitalia

Testes

Testes are the place of sperm production. Each testis is divided into lobules, each containing one to four seminiferous tubules. The seminiferous tubules are tightly coiled, which adds to their length, and have a hollow center. Inside the seminiferous tubules are germ cells, called spermatocytes, in different stages of development and supporting cells called sustentacular cells or Sertoli cells. Germ cells are in close contact with Sertoli cells, an arrangement similar to marbles (germ cells) pushed into playdough (Sertoli cell). Sertoli cells secrete molecules that promote sperm production and control germ cell survival.

Figure 1: Transverse section through the seminiferous tubules of the testis.

The youngest spermatocytes, spermatogonia, are closest to the perimeter of the tubule, and the most mature spermatocytes, spermatozoa or sperm with tails, are closest to the middle. Spermatogonia are the largest of the germ cells, with rounded granular nuclei. During maturation, spermatocytes lose organelles and cytoplasm, and become smaller in size. The mature sperm—with the commonly known shape of a head with DNA, mitochondria along the neck and a long tail—lie closest to the lumen, have elliptical nuclei and no visible cytoplasm. Sometimes the tails are visible but not easy to find. Sperm and their tails are best observed in fresh preparations of ejaculate known as a sperm count.

Seminiferous tubules merge to form rete testis, a network of straight tubules that progress to the epididymis, and then to subsequent ducts, ending with the urethra.

Ducts of the male reproductive system and their characteristics

Ducts

image

Characteristics

Rete testis

Labyrinth of straight tubules lined by cuboidal or low columnar epithelium; each cell has one cilium

Epididymis

Long, coiled duct lined with pseudostratified epithelium with stereocilia
Thin muscularis layer
Site of sperm storage and maturation.

Ductus (vas) deferens

A long straight duct lined by pseudostratified epithelium with some stereocilia.
Thick muscular layer with peristalsis to move sperm toward ampulla

Ampulla of ductus deferens

A dilated portion of ductus deferens; same characteristics as ductus deferens

Ejaculatory duct

Narrower part of the ductus deferens that runs through the prostate gland; same characteristics as ductus deferens

urethra

A passage lined by stratified columnar or stratified cuboidal epithelium; moderate thickness muscularis.

Accessory glands of the male reproductive system

The accessory reproductive glands function to

    • Nourish and protect sperm in transport
    • Activate sperm before ejaculation
    • Clean the urethral tract before ejaculation
    • Produce secretions that assist in passage to the female reproductive tract

The three major accessory reproductive glands are the seminal vesicles, prostate, and the bulbourethral glands.

The prostate is the largest of the reproductive glands and straddles the urethra below the urinary bladder. It is composed of secretory acini, arranged into larger, irregular tubuloalveolar structures made of low columnar to cuboidal cells.

Figure 4: Prostate gland, low magnificaion

Female reproductive system

The female reproductive system consists of

  • Gonads called ovaries
  • Ducts
    • Ovarian (Fallopian) tubes
  • Accessory glands
    • Bulbourethral glands
  • Uterus
  • Vagina
  • External genitalia

Ovaries

Ovaries are the place of follicle maturation.

The ovary is divided anatomically into the cortex and medulla. The cortex of the ovary contains ovarian follicles in different stages of development. Within each follicle is a germ cell (oocyte) enveloped by hormone-producing follicular cells. In contrast to the male system, follicles become larger as they mature.

The ovarian medulla contains large arteries and veins, lymphatics and nerves in a loose collagenous matrix.

The ovary is covered by cuboidal epithelium that converts to squamous epithelium later in life.

Ovary with follicles in different stages. The arrow points to a secondary follicle.

Stages of follicle development

Follicles

Characteristics

primordial follicle

contain an oocyte surrounded by a single layer of flattened follicular cells
this is the resting stage

primary follice

follicular cells become more cuboidal and are now known as granulosa cells
granulosa cells are separated from the oocyte by a basement membrane called the zona pellucida
stromal cells around the follicles are now called theca cells.
this is the growing follicle

secondary follicle

spaces develop between granulosa cells that eventually form the follicular antrum
stromal cells surrounding the follicle form two layers, the theca interna, and the theca externa

Tertiary (Graafian) follicle

large preovulatory follicle with a very large central antrum
within antrum, the oocyte is surrounded by a layer of granulosa cells called the cumulus oophorus. The cells of the cumulus oophorus immediately adjacent to the oocyte are known as the corona radiata
clearly visible zona pelucida
close to the surface of the ovary
Primordial and primary follicles in the ovary

Ovarian ducts (Fallopian tubes)

The ovarian ducts, also called Fallopian tubes or oviducts, connect the ovary to the uterus. They transport the fertilized egg toward the uterus for implantation and the resulting pregnancy.

The ovarian duct walls have three layers: an inner mucosa, a middle muscular layer (smooth muscle), and the outer serosa. Fallopian tube mucosa is highly convoluted closer to the ovary. The folds decrease as the duct approaches the junction with uterus. The mucosa is covered in simple ciliated columnar epithelium with a large number of non-ciliated mucus-secreting cells. The cilia beat in the direction of the uterus, producing a current that propels the oocyte.

Oviduct
Oviduct mucosa

Uterus

The uterus is the muscular organ required for the maintenance of pregnancy. The upper part of the uterus is the body. The cervix is the narrow lower portion that projects to the vagina. The wall of the uterus has three layers. Going from the inside, they are the endometrium, the myometrium and the perimetrium.

The endometrium, or mucosa, is covered with simple columnar epithelium that rests on the lamina propria. Under the mucosa there are numerous tubular uterine glands that extend deep into the submucosa. Endometrium undergoes the cyclical changes called the uterine or menstrual cycle, that prepare the endometrium for pregnancy. If the pregnancy does not occur, the endometrium gets removed during menstruation and the cycle starts over.

The proliferative stage follows menstruation and is the stage of endometrium growth and repair. The uterine glands are tubular, relatively straight and narrow. Glands are distant from one another.

Endometrium: proliferative phase

The secretory stage follows the proliferative stage and is the preparation for implantation of the embryo. The endometrium is thicker, glands are closer to one another and are frequently distended with secretions. They have a characteristic convoluted and “saw-toothed” appearance.

Endometrium: secretory phase

The myometrium is a thick layer of smooth muscle arranged in two layers, an inner circular layer and an outer longitudinal layer. Myometrium is responsible for the uterine contractions during labor.

The perimetrium is the outermost layer and is composed of loose connective tissue, lymphatics and small arteries and veins with overlying mesothelium cells of peritoneum.

Uterine cervix

The cervix is the part of the uterus that extends into the vagina. The cervical canal runs in the middle from the vaginal opening (external os) to the body of the uterus. The outside (vaginal) surface of the cervix is lined with stratified squamous epithelium continuous with the epithelium of the vagina. The epithelium abruptly transitions to simple columnar epithelium in the cervical canal that leads to the inside of the uterus.

Transition zone of the epithelium from the stratified squamous epithelium lining the external surface to the simple columnar epithelium lining the cervical canal.

Vagina

The vagina has three layers, the mucosa, the muscularis, and the adventitia. The vaginal mucosa is made of stratified squamous epithelium. It undergoes cyclical changes following the ovarian and uterine cycles.

Stratified squamous epithelium of the vagina

Pap smear

By examining the cytology from the vagina or cervix, the approximate stage of the cycle can be identified, as well as the pathologies of the reproductive tract. The vaginal smear contains large numbers of round to oval squamous epithelial cells with large uniform nuclei.

Normal vaginal smear.
Pathological vaginal smear in HPV infection.

 

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Histology Copyright © by Malgosia Wilk-Blaszczak. All Rights Reserved.

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