Histology of the female reproductive system

Mammary gland

The mammary glands of the breasts are found at the anterior thoracic wall, anterior to the deep fascia and pectoral muscles; separated from them by the retromammary space. Each breast consists of mammary glands and surrounding connective tissue.

The mammary glands are modified apocrine sweat glands. They are structurally dynamic, meaning that the anatomy changes depending on a woman’s age, menstrual cycle phase and reproductive status. The glands are active in adult women after childbirth (postpartum period).

The gland is comprised of 15-20 secretory lobes which are separated by fibrous bands called the suspensory ligaments of the breast (of Cooper). The secretory lobes contain numerous lobules comprised of the tubuloalveolar glands. The lactiferous ducts converge and open into the nipple. Near their openings, the lactiferous ducts are lined with stratified squamous keratinized epithelium. 

Histologically, there are two main types of cells associated with mammary ducts and lobules, glandular epithelial and myoepithelial cells. Glandular epithelial cells are found lining the duct system, whereas myoepithelial cells reside deep within the epithelium between the epithelial cells and the basal lamina. The myoepithelial cells have a further function: their contraction assists in milk ejection during lactation.

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Ovary

The ovaries are small almond shaped structures in which oocytes develop and female sex hormones are produced.

The ovaries are externally ensheathed by a layer of simple squamous to cuboidal epithelium known as ovarian mesothelium (an extension of the peritoneum), except at the attachment site of the mesovarium which suspends the ovary. The ovary itself is enclosed by a thick capsule of dense fibrous connective tissue, the tunica albuginea, lined by the surface epithelium.

Most of the ovary consists of a cortex, in which ovarian follicles can be found. The most internal part of the ovary is the highly vascular medulla.

During embryonic/fetal development, germ cells start their differentiation into oogonia, undergo cell division (mitosis) before finally entering the prophase of meiosis I, without completing it. At this stage, the cells become known as primary oocytes and are typically formed by the fifth month of fetal development and remain dormant in this meiotic division until just before ovulation (thereby remaining dormant up to fifty years). 

The cortex of the ovary is populated by ovarian follicles which may be seen in various stages of development. These include primordial follicles, primary follicles, secondary follicles, tertiary follicles and mature follicles.

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Uterine tube

The uterine tubes, also known as the oviducts or Fallopian tubes, are paired tubes joined to the uterus and extending towards the ovaries. From distal to proximal, each uterine tube can be divided into four parts: the infundibulum, the ampulla, the isthmus and the uterine part.

Histologically, the wall of the uterine tube consists of several layers:

• Serosa: Primarily composed of mesothelium and connective tissue.
• Muscular layer: This layer is further divided into an inner, relatively thick circular layer and an outer, thinner longitudinal layer.
• Mucosa: This layer demonstrates thin longitudinal folds that project into the lumen of the uterine tube. The mucosa is lined with simple columnar epithelium with two types of cells: ciliated epithelial and nonciliated epithelial cells.

The uterine tube and its epithelial lining undergo cyclical changes along with those of the uterus, in response to changes in hormonal levels. During the follicular phase, the epithelial cells undergo cyclic hypertrophy, while during the luteal phase they tend to atrophy.

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Uterine wall

The menstrual cycle causes a series of changes each month (under normal circumstances) that are produced by the action of hormones in the female reproductive organs. It involves two cycles that interact and overlap; the ovarian cycle and the uterine cycle.

• The ovarian cycle goes through three phases: follicular, ovulation and luteal. All of these phases allow the maturation and release of the ovum.
• The uterine cycle equally has three phases: menstruation, proliferative and secretory. The uterine phases act to prepare the uterus for possible fertilization and pregnancy.

Thus, cyclic changes of the uterine wall (endometrium) during the menstrual cycle are represented by different phases.

• In the proliferative phase (days 1-14) of the uterine cycle, the endometrium prepares for implantation. 
• In the secretory phase (days 14-28), the endometrium becomes a nutritionally rich environment for implantation of the fertilized egg. 
• If no egg is fertilized, the endometrium is shed, signifying the start of menstrual phase. Hormone production by the ovary declines, as the corpus luteum begins to degenerate.

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Cervix of uterus

The cervix is the terminal, cylindrical region of the uterus that opens into the vagina. It consists of two parts (supravaginal, vaginal), two openings (internal os, external os) and a cervical canal.

Four types of cell layers can be found in the ectocervix (superficial to deep):

• Superficial cells 
• Intermediate cells 
• Parabasal cells 
• Basal cells.

The cervical canal, also known as endocervix, is lined with glandular epithelium, specifically mucus-secreting columnar epithelium.

The transformation zone marks the site of abrupt transition between the vaginal stratified squamous epithelium and the cervical simple columnar epithelium. In women of reproductive age, the transition zone is located just outside the external os, compared to before puberty and after menopause, where the transformation zone is found in the cervical canal.

The cervical mucosa is found to house large, branched cervical glands. The amount and viscosity of the mucus secreted by the glands varies considerably during the menstrual cycle and depends on the ovarian hormones. 

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Placenta

The placenta is an organ which develops from fetal and maternal tissues and provides gas exchange, nourishment and protection to the developing fetus. It consists of a fetal portion, formed by the chorion frondosum, and a maternal portion, formed by the decidua basalis. 

The placental disc is composed of two different surfaces, the fetal surface, also known as the chorionic plate, and the maternal surface, also known as the basal plate. The chorionic plate is covered by the amnion, or amniotic membrane, which secretes amniotic fluid to provide protection and cushion for the fetus, while also facilitating exchanges between the mother and fetus. Deep to the amnion lies the chorion, a thicker membrane continuous with the lining of the uterine wall. Originally, early in the development of the placenta, the entire chorionic plate is covered with chorionic villi. These villi increase in size and produce the chorion frondosum or fetal portion of the placenta. The chorionic villi of the fully developed placenta contain a network of fetal capillaries, allowing a maximal contact area with the maternal blood. The exchanges between the fetal and maternal circulation occur in the intervillous space. 

The maternal surface of the placenta, or basal plate, is an artificial surface, which emerges from the separation of the placenta from the uterine wall during delivery. This surface is composed of the decidua, which is composed of three parts: the decidua basalis, the decidua capsularis and the parietal decidua. 

Additionally, several specialized cells can be found here such as decidual cells, trophoblastic 'X-cells' and cytotrophoblasts.

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Related articles

Articles within this topic:

• Corpus albicans
• Endocervix
• Granulosa cells
• Myometrium
• Perimetrium
• Syncytiotrophoblast