LO: Histology of the Female Reproductive Tract

  1. Differentiate between the histology of the ovary, fallopian tubes, uterus and vagina
  2. Describe the histological features of the ovary, tubes, uterus and vagina
  3. Identify the histological changes that occur in the ovary, endometrium and vagina during the pre-ovulatory and post-ovulatory phases

The ovaries:

  • In nulliparas women, the ovaries are paired, almond shaped, pinkish/white and measure about 3cm in length, 1.5cm in width and 1cm in thickness.
  • Each ovary is attached to the posterior surface of the broad ligament by a peritoneal fold, the mesovarium
  • The superior (or tubal) pole of the ovary is attached to the pelvic wall by the suspensory ligament of the ovary, which carries the ovarian vessels and nerves
  • The inferior (or uterine) pole is attached to the uterus by the ovarian ligament (be careful not to mix this up with the suspensory ligament of the ovary)
  • Before puberty, the surface of the ovary is smooth, but becomes progressively scarred and irregular owing to repeated ovulations. The ovaries of a postmenopausal woman is about one forth the size of a women n the reproductive period of her life

The ovary is composed of a cortex and a medulla

a)Medulla

Contains loose connective tissue, a mass of relatively large contorted blood vessels, lymphatic vessels and nerves

b)Cortex

Found in the peripheral portion of the ovary, surrounding the medulla. This contains the ovarian follicles, which are embedded in a rich, cellular connective tissue.

Scattered smooth muscle fibres are present in the stroma around the follicles.

The boundary between the medulla and cortex is indistinct.

“Germinal epithelium” instead of mesothelium covers the ovary

  • The surface of the ovary is covered by a single layer of cuboidal (and in some parts almost squamous) cells – known as the germinal epithelium
  • Continuous with the mesovarium
  • The tunica albuginea lies between the germinal epithelium and the underlying cortex

Ovarian follicles provide the microenvironment for the developing oocyte

  • Will be of various sizes (indicating developmental stage of the oocyte), distributed in the stroma of the cortex
  • At birth, oocytes are arrested in the first meiotic division
  • During the reproductive life span, a woman will produce around 400 mature ova (from a starting point of 600,000 – 800,000. Obviously, most of these go through atresia)
  • The oocytes that remain at menopause will degenerate within a few years

Follicle Development:

There are 3 different types of ovarian follicles, defined by their developmental state. These include; Primordial follicles, Growing follicles and the Graafian follicles. Growing follicles are further categorized into primary and secondary (or antral) follicles.

The Primordial follicle is the earliest stage of follicular development

  • First appear by the third month of foetal development
  • In the mature ovary, the primordial follicles are found in the stroma of the cortex, just beneath the tunica albuginea
  • A single layer of squamous cells surrounds the oocyte
  • Outer surface is bounded by a basal lamina
  • At this stage, the oocyte will have a large, eccentric nucleus that contains finely dispersed chromatin and one of more large nucleoli
  • The cytoplasm of the oocyte (ooplasm) contains a Balbiani Body, which is a localized accumulation of Golgi membranes and vesicles, ER, numerous mitochondria, and lysosomes
  • Also contain annulate lamellae, which resemble a stack of nuclear envelope profiles. Each layer of the stack includes pore structures morphologically identical to nuclear pores

The primary follicle is the first stage in the development of the growing follicle

  • Oocyte enlarges and the surrounding flattened follicle cells proliferate and become cuboidal (it is this latter change that identifies the follicle as primary as opposed to primordial)
  • The zona pellucida appears between the oocyte and the adjacent follicle cells. This is secreted by the oocyte and is rich in glycosaminoglycans and glycoproteins

Follicle cells undergo stratification to form the granulose layers of the primary follicle

  • Rapid mitotic proliferation gives rise to a stratified epithelium, the membrane granulosa (stratum granulosa)
  • During follicular growth, extensive gap junctions develop between the granulose cells, however the basal layer of the granulose cells does not possess elaborate tight junctions (which differs from the sertoli cells of the testes) – this indicates that there is an absence of blood-follicle barrier (movement of nutrients and small informational macromolecules from the blood to the follicle is essential for development of the ovum and follicle)

Connective Tissue cells form the theca layers of the primary follicle

  • As the granulosa cells proliferate, stromal cells immediately surrounding the follicle form a sheath of connective tissue cells known as the theca folliculi (just external to the basal lamina)
  • The thecal folliculi further differentiates;

a)Theca interna: inner, highly vascularised layer of cuboidal, secretory cells. These possess ultrastructural features characteristic of steroid-producing cells and possess a large number of LH receptors. In response to LH, they synthesise and secrete androgens (precursors of oestrogen). IN addition, they also contain fibroblasts, collagen bundles and a rich network of small vessels (typical of endocrine organs)

b)Theca externa: outer layer of connective tissue cells, containing mainly smooth muscle and bundles of collagen fibres.

The boundary between these two layers is indistinct (which is different from the distinction between the granulose layer and the theca interna, which has a distinct basal lamina separating the 2 layers)

  • During follicular growth, the granulose layer is avascular

Maturation of the oocyte occurs in the primary follicle

  • Distribution of the organelles changes as the oocyte matures
  • Golgi elements (derived from Balbiani body) become scattered in the cytoplasm
  • Number of free ribosomes, mitochondria, small vesicles, multivesicular bodies and rER increases
  • Cortical granules are exhibited bust beneath the plasma membrane (known as the oolemma). These contain proteases that are released by exocytosis when the ovum is activated by sperm
  • Microvilli project from the oocyte into the perivitalline space (between the oocyte and the surrounding granulose cells as the zona pellucid is deposited)
  • Slender processes from the granulose cells develop and project towards the oocyte, intermingling with the oocyte microvilli and occasionally invaginating the oocyte plasma membrane, however although the contact the plasma membrane, they do not create cytoplasmic continuity between the cells

The secondary follicle is characterized by a fluid-containing antrum

  • Several factors are required for follicular growth;
  • FSH (follicle stimulating hormone)
  • Growth factors (epidermal growth factor, insulin-like growth factor I (IGF-I))
  • Calcium ions
  • When the stratum granulosum reaches a thickness of 6-12 cells, fluid-filled cavities appear (called liquor folliculi)among the granulosa cells and as the liquor folliculi continues to accumulate between the cells, the cavities begin to unite, eventually forming the crescent-shaped cavity known as the antrum
  • The above stage is what identifies the follicle as a secondary (or antral) follicle
  • At this stage, the oocyte will not undergo anymore growth, the inhibition of which is caused by the presence of a peptide called oocyte maturation inhibitor (OMI), which is secreted by the granulosa cells into the antral fluid

Cells of the cumulus oophorus form a corona radiata around the secondary follicle oocyte

  • As the secondary follicle enlarges, so does the antrum
  • The stratum granulosum has a relatively uniform thickness, except for the region associated with the oocyte, where the cells form a thickened mound, projecting into the antrum – the cumulus oophorus
  • The cells of the cumulus oophorus that immediately surround the oocyte and remain with it at ovulation are referred to as the corona radiate
  • The corona radiate is composed of cells that send penetrating microvilli throughout the zona pellucid to communicate via gap junctions with the microvilli of the oocyte
  • During follicular maturation, the no. of surface microvilli of granulosa cells increases and is correlated with an increase in the no. of LH receptors on the free antral surface

The Graafian follicle contains the mature secondary oocyte

  • Because of the now large size of the mature follicle, it now extends through the full thickness of the ovarian cortex and causes a bulge on the surface of the ovary
  • As the follicle reaches its maximum size, the mitotic activity of the granulosa cells slows and the stratum granulosum appears to become thinner as the antrum increases in size
  • The oocyte and the cumulus cells are gradually loosened from the rest of the granulose cells in preparation for ovulation
  • During this stage, the thecal layers become more prominent and lipid droplets appear in the cytoplasm of the theca interna cells
  • Some androgens from the theca interna cells are being transported to the sER in the granulosa cells , which (in response to FSH) catalyse the conversion of androgens to oestrogens, which in turn, stimulate the granulose cells to proliferate and increase the size of the follicle
  • Increased oestrogen levels are correlated with increased sensitivity of gonadotropes to gonadotrophin-releasing hormone
  • A surge in the release of FSH and LH in induced in the adenohypophysis approx. 24 hrs prior to ovulation
  • In response to the LH surge, LH receptors on the granulose cells are down regulated and granulosa cells no longer produce oestrogens in response to the LH
  • Triggered by this LH surge, the first meiotic division of the primary oocyte resumes, which results in the formation of the secondary oocyte and the first polar body
  • The granulosa and thecal cells then undergo luteinization and produce progesterone

(Luteinization = the process by which a postovulatory ovarian follicle transforms into a corpus luteum through vascularisation, follicular cell hypertrophy, and lipid accumulation, the latter in some species giving the yellow colour indicated by the term)

Ovulation – hormone mediated process resulting in the release of the secondary oocyte from the Graafian follicle

  • A combination of hormonal changes and enzymatic effects are responsible for the actual release of the oocyte in the middle of the menstrual cycle. These include;
  • Increase in volume and pressure of the follicular fluid
  • Enzymatic proteolysis of the follicular wall by activated plasminogen
  • Hormonally directed deposition of glycosaminoglycans between the oocyte-cumulus complex and the stratum granulosum
  • Contraction of the smooth muscle fibres in the theca externa layer, triggered by prostaglandins
  • Just before ovulation, blood flow ceases in the small area of the ovarian follicle, over the bulging follicle
  • This area, known as the macula pellucida (or stigma) becomes elevated and then ruptures – leading to the oocyte, surrounded by the corona radiata and cells of the cumulus oophorus being forcefully expelled
  • At the time of ovulation, the fimbriae are closely opposed to the ovary and direct the oocyte
  • If the oocyte fails to be fertilised within 24hours, it will start to degenerate

The primary oocyte is arrested for 12 to 50 years in the diplotene stage of prophase of the first meiotic division

  • The first meiotic phase is not completed until just before ovulation
  • The long period of meiotic arrest exposes the oocyte to adverse environmental influences and may contribute to errors in meiotic division (eg. Nondisjunction and Down’s Syndrome)
  • As the first meiotic division is completed in the mature follicle, each daughter cell of the primary oocyte will receive an equal amount of chromatin, but one will receive the majority of the cytoplasm. The one with the majority of the cytoplasm becomes the secondary oocyte and the other, the polar body

The secondary oocyte is arrested at metaphase in the second meiotic division just before ovulation

  • As the secondary oocyte leaves the follicle at ovulation, the second meiotic division is in progress, but is arrested in metaphase and is completed only if fertilization occurs
  • In the case of fertilization, the secondary will become the mature ovum with the maternal pronucleaus containing 23 chromosomes. There is a second polar body created by the completion of the second meiotic division
  • The polar bodies will degenerate

Corpus Luteum – the collapsed follicle undergoes reorganization into the corpus luteum after ovulation

  • Post ovulation, the remaining granulosa and thecal cells left in the ovary, are thrown into deep folds as the follicle collapses and is transformed into the corpus luteum (or the luteal gland)
  • At first, bleeding from the capillaries of the theca interna into the follicular lumen leads to the formation of the corpus hemorrhagicum
  • Connective tissue from the stroma then invades the former follicle and the granulosa and thecal cells increase in size and become filled with lipid droplets (this gives the corpus luteum it’s yellow colour)
  • The cells also demonstrate abundant sER and mitochondria with tubular cristae, which are features associated with steroid-secreting cells
  • The cells are then called granulosa lutein cells (formally the granulosa cells and located centrally) and the theca lutein cells (derived from the theca cells and peripherally located)
  • As the corpus luteum begins to form, a rich vascular network and lymphatics (from the theca interna) begin to invade the granulosa layer – this highly vascularised area now secretes progesterone and oestrogens, which stimulate the growth and secretory activity of the lining of the uterus
  • If fertilisation does not occur, the corpus luteum will remain active for 14 days
  • In the absence of hCG and other luteotropins, the rate of secretion of progesterone and oestrogens declines and the corpus luteum begins to degenerate about 10-12 days after ovulation
  • After pregnancy or menstruation, the corpus luteum will undergo involution and the cells become loaded with lipids, decrease in size and undergo autolysis – this white scar, called the corpus albicans, will sink deeper into the ovarian cortex as it disappears over a period of several months

Atresia – most ovarian follicles are lost by atresia mediated by apoptosis of granulosa cells

  • Atresia is thought to be a mechanism whereby a few follicles are stimulated to maintain their development through the programmed death of other follicles, thus, at any stage of maturation, the follicle may undergo atresia
  • Atretic follicles shrink and eventually disappear from the stroma of the ovary as a result of the repeated apoptosis and phagocytosis by granulosa cells
  • The cells are reabsorbed and disappear and the surrounding stromal cells migrate into the space previously occupied by the follicle (therefore, no trace is left)
  • In atresia of large, growing follicles, the degeneration of the oocyte is secondary to the follicular degeneration, therefore indicating that once the oocyte has achieved a certain level of maturity, it is no longer sensitive to the same stimuli that initiate the atresia in the granulosa cells
  • The follicular changes include the following;
  • Initiation of apoptosis within the granulosa cells, indicated by cessation of mitosis and expression of endonucleases and other hydrolytic enzymes within the granulosa cells
  • Invasion into the granulosa cell layers by neutrophils and macrophages
  • Invasion into the granulosa cell layers by strands of vascularised connective tissue
  • Sloughing of the granulosa cells into the antrum
  • Thecal cell hypertrophy
  • Collapse of the follicle as the degeneration occurs
  • Invasion of connective tissue into the cavity of the follicle
  • The oocyte undergoes changes which are typical with autolysis and degeneration and the remnants are phagocytosed by macrophages
  • The zona pellucida, which is resistant to the autolytic changes occurring around it, becomes folded and collapses as it is slowly broken down in the cavity of the follicle – macrophages in the CT are involved in phagocytosis of the zona pellucida
  • The BM between the follicular cells and the theca interna may separate and increase in thickness, forming a way hyaline layer called the glassy membrane. This characteristic is used to identify follicles in a late stage of atresia
  • This has been shown to be controlled by NAIP (neural apoptosis inhibitory protein), which is present when the follicle is growing, but absent in follicles undergoing atresia
  • A high level of gonadotrophs inhibits apoptosis by increasing the expression of NAIP in the ovaries

The interstitual gland arises from the theca interna of the atretic follicle

  • As the atretic follicles continue to degenerate, a scar with hyline streaks develops in the centre of the cell mass, giving it the appearance of a small corpus albicans
  • This structure will eventuallydisappear as the ovarian stroma invades the degenerating follicle
  • The stands of the luteal follicle then can become scattered in the stroma in cords and can contribute to the interstitual gland and produce steroid hormones – the largest numbers of these strands generally occurs in the time of most atresia, namely in the first year of life and at puberty
  • Interstitual cells are an important source of oestrogens that influence the growth and development of the secondary sex organs
  • Ovarian hilar cells are also found in the hilum of the ovary in association with vascular spaces and nonmyelinated nerve cells. These contain Reinke crystalloids which appear to respond to hormonal changes during pregnancy and menopause. Hyperplasia or tumours associated with these cells usually leads to excess androgens, leading to masculinisation

Uterine Tubes