Oogenesis and Follicular Development Flashcards
What is the fundamental reproductive unit ?
single ovarian follicle, composed of one germ cell (oocyte), surrounded by endocrine cells
Define menarche.
Menarche = beginning of menstrual cycles – av 11-13 y
What determines age at puberty ?
– genetics – nutrition – geographic location – exposure to light – body composition, fat deposition – exercise
What is the menstrual cycle ? What is it governed by ?
Female sexual cycle = Menstrual cycle: controlled by gonadotropins, gonadal hormones
This cycle can be described by the ovarian cycle (describes changes that occur in the follicles of the ovary), or by the endometrial cycle (describes changes in the endometrial lining of the uterus).
What are the main phases of the Ovarian cycle ?
– follicular phase: av 15 d (range 9-23 days)
– ovulatory phase: 1-3 d (and culminates with ovulation)
– luteal phase: 13 d (is less variable than follicular)
What are the main phases of the Endometrial cycle ?
- Menstruation
- Proliferative
- Secretory phases
Identify the main ovarian functions.
- Oogenesis ie production of gametes
- Maturation of oocyte
- Expulsion of the mature oocyte (ovulation)
- Secretion of female sex steroid hormones (oestrogen and progesterone) and peptide hormone inhibin.
Describe the initial form of the oocyte/ovarian follicle, and its final form.
In the development of oocyte and ovarian follicle, go from primordial follicle with v small oocyte, to a mature follicle which is one and a half cm of diameter, with oocyte centrally located within a fluid filled antrum and surrounded by the cells of the follicle.
Describe the main steps of oogenesis.
Steps for oocyte to develop into ovum:
PHASE 1 (IN FETAL LIFE)
1) Primordial germ cell (in the embryonic yolk sac 3 weeks post conception) in the developing fetus undergoes repeated cell divisions (mitosis) as we increase number of those.
2) These primordial germ cells then migrate to the ovarian cortex, at which stage we have the oogonia (AKA primordial ova) (max # ≈ 7 million).
At around the time of birth she has around 1 to 2 million oocytes present (millions of oocytes degenerate before birth)
3) At 8-10 wks of gestation, prophase of 1st meiosis starts, and oogonia become primary oocyte (surrounded by pre-granulosa cells called primordial follicle). There, primary oocyte is arrested in prophase 1, until menarche
PHASE 2
4) Every month, from menarche, to menopause, development starts again: stimulated by LH, primary oocyte resumes meiosis and completes meiosis 1 (haploid nuclei separate to form 2 cells), involving unequal sharing of the cytoplasm and resulting in the production of a large secondary oocyte and a first polar body (all polar bodies degenerate). This secondary oocyte is arrested in metaphase 2. One primary oocyte completes meiosis 1 every month.
5) Ovulation (secondary oocyte released from the follicle and from the ovary) occurs every month.
6) If fertilisation with sperm occurs, then meiosis 2 is completed and an ovum (zygote) + second polar body form. If fertilisation does NOT occur, no completion at that stage.
Does the oocyte development every complete ?
Yes, if fertilisation takes place (then meiosis 2 is completed)
Define mitosis, and meiosis.
MITOSIS
– process of cell division that results in two genetically identical daughter cells developing from a single parent cell.
MEIOSIS
– division of a germ cell involving two fissions of the nucleus and giving rise to four gametes, or sex cells, each possessing half the number of chromosomes of the original cell.
Identify factors which may contribute to cytogenetic abnormalities. What may these cause ?
Rate of cytogenetic abnormalities increases with increasing maternal age
The most common abnormality is aneuploidy and Down Syndrome is clinically the most frequently recognised
Identify the main differences between oogenesis and spermatogenesis.
- In females, mitotic proliferation of oogonia occurs prior to birth.
- In males, spermatogonia proliferate only after puberty.
- —— - In females, meiotic divisions of oocyte produces only one mature ovum.
- In males, meiotic divisions of primary spermatocyte produces 4 mature spermatozoa.
- ——- - In females, second meiotic division is completed only upon fertilisation.
- In males, the products of meiosis (spermatids) undergo substantial differentiation in the maturing process.
- ——- - Oogenesis is discontinuous (periods of arrest)
- Spermatogenesis is continuous
- ——- - Female: Normal body temperature
- Male: lower temperatures required
- ——- - Female: Meiosis begins before birth (initial stimulus is not steroidal)
- Male: Meiosis begins at puberty (indirectly dependent on progesterone)
- —— - Female: Results in production of finite numbers of oocytes (ovary has no stem cells)
- Male: Results in production of infinite numbers of sperm (testis has a stem cell population)
- ——- - Female: Results in production of immotile gametes
- Male: Result in production of motile gametes
What is the relationship between ova and follicles ?
Eggs exist in ovaries in structures known as follicles
Describe the process of follicular development.
FOLLICULAR GROWTH (pre-puberty) (follicular phase)
1. Primordial follicle = single layer of granulosa cells around oocyte (before birth)
2. (Primary follicle) Oocyte size increases, multiple (3) layers of granulosa cells. + separation of oocyte from granulosa cells by thick
layer of material (i.e. zona pellucida)
3. BUT cytoplamic processes cross the zona pelucida and form gap junctions with oocyte and nutrients and chemical messengers are passed to oocyte
4. Follicle grows by mitosis of granulosa cells and some differentiate to become theca
5. Antrum begins to form from amongst granulosa cells from fluid they secrete.
——– MENARCHE (still follicular phase, up until ovulation)
6. Pre-ovulatory, mature follicle, ready for its oocyte to be ovulated
7. Oocyte ovulated
CORPUS LUTEUM FORMATION (luteal phase)
8. Once oocyte is released at the time of ovulation, theca and granulosa cells go on to form corpus luteus which has a crucial function if fertilisation occurs (key hormonal role)
