Question

1. Note the primordial, primary, secondary and tertiary (Graafian) follicles. How do they differ from one another? What new structures are apparent as the follicles mature? What extant structures change during folliculogenesis?

2. The human uterus, unlike that of the mouse, has a unified structure (corpus). How does a bicornuate uterus serve the reproductive needs of the mouse (and why don’t humans have this type of uterine structure?

Answer 1

Ans 1: Follicular phase begins with FSH stimulation. It initiates the development of primordial follicles into primary follicles. As follicles enlarge, granulosa cells produce estrogen. When secondary follicle develops, it secretes inhibin and lowers the FSH levels by negative feedback.

Inhibin in females - follicle produces inhibin, reduces FSH production, which causes less follicles stimulated to develop from a primary follicle to a secondary follicle and how many secondary follicles become tertiary follicles and release that secondary oocyte.

Stages of Follicle Development:

1. Scattered throughout and embedded in connective tissue matrix of the cortex are 1000s of structures called ovarian follicles.
2. Under the influence of FSH at puberty a primary follicle enlarges into a secondary follicle and then into a mature (Graafian) follicle.
3.On the 14th day of a 28 day cycle, LH causes ovulation whereby the secondary oocyte and a polar body are released from the ovary and swept into the fallopian tube by fimbrae (finger like projections).
4. the remaining follicles reform into the Corpus luteum (yellow body) under the influence of LH.

Folliculogenesis:

Follicules in ovarian cortex consist of a female germ cell called as oocyte plus epithelial follicular cells within a surrounding basal lamina separating the follicle from the cortical stromal cells. Folliculogenesis begins prenatally and pauses throughout childhood and then becomes active during reproductive life. Follicles reach the preovulatory follicle stage from puberty until menopause. Oogonia arrest prenatally as primary oocytes in meiosis prophase I, 2n / 4d = 2 chromosomes / 4 chromatids, until ovulation when one daughter cell becomes the secondary oocyte in the pre-ovulatory (graafian, tertiary) follicle. Fertilization can starts the ovulated secondary oocyte to complete the second meiotic division in the oviduct one day after ovulation and generating a haploid ovum plus the non-viable second polar body within the zona pellucida. And upon fertilization the ovum becomes the diploid zygote having 2n of chromosomes. Continuing differentiation of multilaminar follicles to a mature follicles requires multiple cycles of transient hormone elevations FSH, estrogen and luteinizing hormone. The complete folliculogenic process for the ovulatory follicle occurs over many cycles requiring a year.And during each menstrual cycle in humans, only one or two preovulatory follicles become fully mature and all other maturing follicles undergo atresia at earlier stages of their differentiation.

Primordial follicle: immature oocyte surrounded by one layer of flattened follicle cells and the cells are simple squamous.
Primary follicle: 2 or more layers of cuboidal or columnar-shaped granulosa cells surround the oocyte and at this point the primary follicle has undergone mitosis and is diploid.

Secondary follicle: fluid-filled pockets present between the layers of the granulosa cells. These three follicle stages all contain a primary oocyte (arrested in prophase I held in prophase I)

The antral or the graafian follicle: the mature follicle and have the secondary oocyte. The fluid-filled cavity is very large and it bulges from the surface of the ovary and this is the antrum. Ovulation occurs when this mature follicle ruptures, ejecting the oocyte. After ovulation, the ruptured follicle will transform into the corpus luteum. The corpus luteum is a temporary endocrine structure because it secretes progesterone and it becomes a corpus albacans, progerstorone helps uterine or menstrual cycles. And if there is fertilization then there will be no corpus albacans, the corpus luteum will live longer and secrete HCG (a pregnancy hormone) and that HCG extends the life of the corpus luteum for three more months.

Tertiary (mature graafian) follicle: Usually 1-2 secondary follicles develops this each month. Primary oocyte continues from prophase 1, produces secondary oocyte that stops at metaphase II.

Ans 2: Bicornuate uterus: demonstrates two completely separate uterine cavities and one cervix. Partial nonfusion of the müllerian ducts. It represents 10% of müllerian duct anomalies. A definable fundal notch, concave surface or indentation, heart shaped uterus. The horns are not fully developed, smaller than those of didelphys uteri. Generally no surgery is indicated but some are candidates for metroplasty.

Human uterus: The uterus is pear shaped and has two main parts called as the cervix and the body (the fundus is the rounded portion of the body superior to the attachment of the uterine tubes). Wall of the uterus is composed of three layers. The inner endometrium (mucous membrane), the middle myometrium (smooth muscle) which is the largest layer and the perimetrium (outer incomplete layer of parietal peritoneum). Cavities of the uterus are small because of the thickness of theuterine walls. The cervix of the uterus dips slightly into the vagina producing spaces called the anterior and posterior fornix, these pool seminal fluid briefly after intercourse and increase the probability of reproductive success (fertilization).

Uterus permits sperm to ascend toward the uterine tubes (fallopian). If conception occurs, an offspring develops in the uterus. Myometrial contractions occur during labor and help push the offspring out of the mother body. And if the conception does not occur, outer layers of endometrium are shed during menstruation a cyclical event that allows the endometrium to renew itself.