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- Ovary
- produce female gametes & sex hormone
- are paired organs that can be divided into two portions: outer cortex + inner medulla
- surface = covered by layer of simple cuboidal surface epithelium (considered the source of most common form of ovarian cancer)
- tunica albuginea (a thick CT layer present beneath the epithelium)
- the rest of the cortex contains a compact, richly cellular stroma, which have follicles
- follicles are NOT present in the medulla, which contains CT, interstitial cells, nerves, lymphatics, & large blood vessels
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Follicle
- a structure made of follicular or granulosa cells that surrounds an oöcyte
- it is found in the stromal portion of the ovary CORTEX
- it undergoes a series of steps which cumulates in ovulation
- the oöcyte itself undergoes maturation steps which are completed if there is fertilization
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Primordial Follicle
- the earliest stage of follicle development
- each contains a primary oöcyte surrounded by one layer of squamous follicular (granulosa) cells resting inside a thin basal lamina
- the internal primary oöcyte has a large, vesicular nucleus w/ a large nucleolus (it's arrested in the diplotene stage of prophase of meiosis I)
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What are the predominant type of follicles present at birth in the ovary as well as before & after puberty?
PRIMORDIAL FOLLICLES arrested in prophase of meiosis I
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- primordial follicle histology
- central large oocyte surrounded by a FLATTENED layer of follicular cells
- there is a basal lamina surrounding the whole follicle structure until very late in follicular development
- may also see splotches in the oocyte which can be mitochondria; it has a lot of them
- outside are cells of the CT & the stroma (orient yourself)
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(Unilaminar) Primary Follicle
- starting at puberty & at the beginning of every future menstrual cycle chosen follicles (aka those with the most FSH receptors on their surface) begin to mature
- first thing to change is that the flattened follicular cells around the oocyte become cuboidal: this characterizes a primary follicle
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- unilaminar primary follicle histology
- follicular cells surrounding the oocyte become cuboidal
- basal lamina is still intact
- these oocytes/follicles becomes larger (esp. in comparison to primordial follicles
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Multilaminar Primary Follicle
- produced as a result of granulosa cell proliferation
- cellular stroma immediately around the follicle differentiates into theca folliculi (cells)
- follicle & oocyte are still enclosed by a complete basal lamina separating them from the theca cells
- zona pellucida starts to develop BTWN the oocyte & granulosa cells (contains glycoprotein)
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How do nutrients get to the oocyte?
- because there are no blood vessels to the oocyte, nutrients must diffuse through the outer basal lamina through granulosa cells to the oocyte
- even though it's a long distance, nutrient transfer is facilitated by gap junctions between both granulosa cells & the oocyte & granulosa cells (oocyte has long cytoplasmic processes that invaginate & touch granulosa cells through the zona pellucida)
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theca folliculi
- two layers of cells surrounding the granulosa cells (separated by the basal lamina) made up of theca cells
- theca interna: the has abundant SER + lipid droplets (the appearance of steroid producing/metabolizing cells)
- theca externa: CT-like
- BOTH layers are vascularized
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theca interna cells
- express receptors for LH and when signaled they synthesize androgens (androstenedione & testosterone) using cholesterol
- these male hormones diffuse INTO the follicle
- granulosa cells stimulated by FSH make aromatase, which converts the male hormones into estrogens (estradiol & estrone)
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Can the granulosa cells produce estrogens directly?
- no, they do not have the capacity to do so
- aromatase, a granulosa cell enzyme converts male hormones secreted by theca interna cells to estrogens for foster oocyte growth
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What is the purpose of a high concentration of estrogens in the microenvironment of the follicle?
- it is essential for the development of the oöcyte
- the level of the estrogens can be more than 100X higher in the follicle than in the blood facilitated by sex-hormone binding proteins IN the follicular fluid
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Which part of the follicle surrounding the oocyte is vascularized?
- the theca interna is HIGHLY vascularized
- granulosa cells remain avascular throughout the growth of the follicle & are separated from the theca by a thick basal lamina
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Secondary (Antral) Follicle
- during this stage, small areas filled with fluid (liquor folliculi) appear between the granulosa cells
- they coalesce to form one large cavity, the antrum
- at this stage in development the oöcyte has reached its full size (0.2mm) & the follicle is about 10 mm in diameter
- the 2ndary stage of follicular development is dependent on estrogens produced locally & the pituitary hormone FSH
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- Secondary (Antral) Follicle histology
- antrum: fluid formed between granulosa cells that coalesces
- granulosa cells are at the periphery
- outside of them are the 2 layers of theca cells
- corona radiata: granulosa cells which directly surround the ooctye and separate it from the antrum
- cumulus oophorus: granulosa cells between the corona radiata and the circular follicle that surrounds the whole antrum ('column on which the oocyte sits'); looks like a pedestal
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Mature (Graäfian) Follicle
- follicle has reached its largest size (2.5 cm) and can be observed by ultrasound (during IVF)
- immediately before ovulation the antrum increases greatly in size
- oöcyte is found on one side of the follicle, surrounded by a few layers of granulosa cells (corona radiata) on a pedestal of follicle cells (cumulus oöphorus)
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During which stage of development do granulosa cells acquire receptors for luteinizing hormone (LH)?
- as a Mature Graäfian Follicle
- this is critical for the development of the corpus luteum later on
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follicle order:
primordial -> unilaminar primary -> multilaminar primary -> secondary/antral -> mature/graäfian
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Ovulation
- a mid-menstrual cycle LH surge from the anterior pituitary occurs, instigating ovulation
- 16-24 hours after the LH surge, the secondary oocyte, it's surrounding zona pellucida & outer corona radiata detach from the cumulus oöphorus & float in the antrum
- collagenase activity + ischemia from increased antral pressure leads to the formation of a stigma in ovary cortex --> which ruptures, releasing the 2ndary oocyte with an intact corona radiata
- released oocyte is captured by the oviduct (fallopian tube)
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How long does the ovum in the oviduct have to get fertilized?
- unless it is fertilized within 24 hours, the ovum degenerates
- if the ovum is fertilized, the zygote undergoes cleavage, and makes the 3-5 day trip to the uterus for implantation
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To what age woman does each ovary belong?
- top: old; no follicles/oocytes to be found, only stroma
- bottom: young; filled with follicles
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Follicular Atresia
- About 7 million primary oöcytes are present in the fetal ovary by mid-gestation. At birth, about 400,000 oöcytes remain. Only about 450 of these will develop to maturity.
- a process undergone by follicles that don't mature and instead degenerate at various stages and undergo apoptosis.
- it can happen to follicles at any stage of development
- it begins in intrauterine life, becomes prominent at birth & shortly before puberty
- macrophages come in and remove the cellular debris, leaving only stroma cells and collagenous scar
- characterized by cell floating around in antrum, no longer specifically attached where they should be
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glassy membrane
- a thick structure made of the basal lamina between the granulosa cells and the theca cells during follicular atresia
- some of the thecal cells from atretic follicles may remain and become interstitial cells that (along with the adrenal glands) become sources of sex hormones after menopause
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Corpus Leuteum
- what remains of the follicle structure in the ovary cortex after the ovum is released
- it becomes a highly vascularized endocrine body who's granulosa cells produces progesterone in order to support a possible fertilization event and a subsequent pregnancy
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- Corpus Leuteum Cells
- granulosa cells -> granulosa lutein cells: puffier, pale-staining w/ lipid droplets + abundant SER in their cytoplasm [larger]
- theca interna cells -> theca lutein cells: smaller, darker-staining, still secrete androgens (in response to LH)
- aromatase in the granulosa lutein cells still converts androgens from the theca lutein cells into estrogen & now progesterone
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What happens to the Corpus Leuteum if fertilization doesn't take place?
- the corpus luteum of menstruation degenerates after about 14 days
- lipid droplets get bigger, sign of dying Corpus Leuteum/cells
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Corpus Albicans
- 'white body'
- all corpus luteum eventually will degenerate and is replaced by dense CT scar called corpus albicans
- the scar is larger for a corpus luteum of pregnancy than that of a corpus luteum of menstruation
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What happens to the Corpus Leuteum if fertilization does take place?
- the corpus luteum of pregnancy enlarges & is maintained for 6 months by human chorionic gonadotropin from the placenta
- after that it gradually declines but persists until the end of pregnancy
- in addition to estrogen and progesterone, it secretes relaxin, a polypeptide hormone which softens and dilated the cervix, inhibits uterine contraction, & loosens the fibrocartilage of the pubic symphysis, allowing the pelvic outlet to enlarge during parturition (birth)
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How can you tell the difference between a corpus leuteum and a corpus albicans?
- Leuteum: highly cellular (lutein cells)
- Albicans: just a scar, no cells, mostly collagen fibers
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Inhibin
a glycopeptide hormone secreted by the granulosa cells that reduces FSH secretion in the pituitary
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Activin
a glycopeptide hormone also secreted by the granulosa cells that stimulates the release of FSH from the pituitary
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oogonia that have already undergone mitosis then begin meiosis to become primary oocytes, which are arrested in _______________ from before birth until puberty.
- primary oocytes are arrested in prophase of meosis I from before birth until puberty
- - they don't proceed to meiosis II until ovulation
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What's one difference between male and female meiotic division?
- In females there are unequal meiotic divisions that produce a primary oocyte and a small polar body --> then another unequal meiotoc division that produces a secondary oocytes and a second small polar body
- In males, meiotic divisions are all equal and all meiotic derivatives produce spermatids
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When is the primary oocyte in a primordial follicle?
After the zona pellucida and the single layer of follicle cells form
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When is the primary follicle formed?
when the primary oocyte enlarges during puberty; follicular epithelial cells become cuboidal and then columnar, forming the primary follicle
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What happens after a primary oocyte finishes Meiosis I?
- It releases the first polar body and then enters Meiosis II where it is arrested at METAPHASE (II)
- arrested at metaphase II it's called a mature, secondary oocyte
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What triggers a secondary follicle to rupture, expelling the secondary oocyte by contraction of the smooth muscle-like _____ cells?
- prostaglandins - lipid paracrine hormone
- the smooth muscle-like THECA cells contract and expel the secondary follicle
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What surrounds the ovulated secondary oocyte?
the zona pellucida & one or more layers of follicle cells called the corona radiata
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How soon after the LH surge does ovulation occur?
within 12-24 hours
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What happens to the corpus luteum IF an oocyte is fertilized?!
- it becomes the corpus luteum of pregnancy, fortified by increased progesterone production
- this corpus luteum of pregnancy is functional for 20 weeks
- after this 20 weeks the placenta makes enough estrogen & progesterone to support the pregnancy
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What prevents the corpus luteum from degenerating and where is the source of this prevention?
- human chorionic gonadotropin (hCG) prevents the corpus luteum from degrading
- hCG comes from the implanted blastocyst (embryo)
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Oviducts (Fallopian Tube)
transport the egg; is the site for fertilization, transport the zygote
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Uterus
- with endometrium & myometrium; site of fetal development
- menstrual cycle involves proliferative, secretive, ischemic and menstrual phases
- Uterine cervix contains mucous glands
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Menstrual Cycle Phases (4)
- 1) Menstrual Phase: days 1-5
- 2) Proliferative Phase: days 5-14
- estrogen levels rise, stimulate re-growth of endometrium functional layer
- 3) Secretory Phase: days 14-27
- estrogen levels peak then fall
- progesterone levels are high
- if no fertilization occurs, corpus luteum degenerates, estrogen & progesterone levels fall, and the secretory endometrium enters the ischemic phase
- 4) Ischemic Phase: days 27-28
- progesterone & estrogen levels fall rapidly and the functional endometrium becomes ischemic
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