Embryology 2 - Oogenesis Study Guide PDF

STUDY GUIDE Faculty of Medicine General Embryology: Lecture 2 Oogenesis ILOs: student will be able to 1. Describe the steps of oogenesis. 2. Describe the structure of Graafian fo...

STUDY GUIDE Faculty of Medicine General Embryology: Lecture 2 Oogenesis ILOs: student will be able to 1. Describe the steps of oogenesis. 2. Describe the structure of Graafian follicle. 3. Describe ovulation & fate of corpus luteum. 4. Describe the steps of the ovarian cycle. 5. Describe the steps of the menstrual cycle. FEMALE GENITAL SYSTEM A. Primary sex organs: 2 Ovaries attached to the back of the broad ligaments on the sides of the uterus. The ovaries secrete female sex hormones (estrogen and progesterone) and after puberty they produce an ovum each cycle till the age of menopause. B. Secondary sex organs: 1. Fallopian (uterine) tubes: Their lateral fimbriated ends open into the peritoneal cavity close to the ovaries. Their medial ends open in the uterus. 2. Uterus: Formed of fundus, body and cervix. 3. Vagina. 4. External genitalia. 5. Mammary glands. OOGENESIS Aim: Formation of mature ovum (single, large and haploid). Site: In the ovary. Each ovary produces one ovum every 2 cycles. Duration: It starts before birth but arrested after birth till puberty. After puberty it is resumed till menopause. Steps: 1- During early fetal life, the oogonia (diploid = 44 + XX) divide by mitosis giving daughter oogonia. 2- The resulting daughter oogonia enlarge forming primary oocytes (diploid). Each primary oocyte becomes surrounded by flat follicular cells (derived from the ovarian tissue) and together they form a primordial follicle. When the surrounding flat cells become cuboid, the follicle is called the primary follicle. Further changes are arrested till puberty. The arrest of further intrauterine development is caused by the 1  oocyte maturation inhibitor which is secreted by the follicular cells. At birth, the female has about 40,000 primary follicles in her ovaries, about 500 only of them will be ovulated. 3- After puberty, each month, a number of primary follicles (15-20) begin to grow. All of them degenerate forming atretic follicles except one which continues the oogenesis process as follows: The follicular cells increase in the primary follicle changing it into growing follicle. The primary oocyte inside the growing follicle divides by meiosis I into secondary oocyte and 1st polar body: Secondary oocyte 1st polar body Haploid = 22+X Haploid = 22+X Receives most of the cytoplasm Receives a very little cytoplasm The follicular cells around it differentiate May or may not divide by meiosis II but and together they form the eventually all polar bodies degenerate Graafian follicle Ovulation: It means rupture of the mature Graafian follicle and liberation of the secondary oocyte and some of its coverings (the zona pellucida and corona radiate) into the peritoneal cavity to be picked by the fimbriated end of the uterine tube. If sperm is available and fertilization is going to happen, the secondary oocyte undergoes meiosis II giving mature ovum and second polar body. If not fertilized, the secondary oocyte degenerates after 24 hours without undergoing meiosis II. After ovulation, the remaining part of the Graafian follicle in the ovary is invaded by lutein (a yellow pigment) and within 3 days a corpus luteum is formed which secretes progesterone (mostly) + estrogen. Fate of the corpus luteum: - If fertilization occurs, it becomes the corpus luteum of pregnancy which keeps secreting hormones for 3-4 months until the placenta is formed and takes its role. - If no fertilization, it becomes the corpus luteum of menstruation which keeps secreting hormones for 9 days then regresses gradually until menstruation occurs. - In either case, it becomes fibrosed eventually giving the corpus albicans. 2  N.B:. Drugs which inhibit ovulation are used as contraceptives (they contain estrogen & progesterone).. Drugs which stimulate ovulation are used to treat sterility (e.g., parlodel) but may cause twins. Structure of a Graafian follicle : (½ cm in diameter) 1. Secondary oocyte (120 microns in diameter) surrounded by: a. Vitelline membrane: the cell membrane of the secondary oocyte. b. Zona pellucida: non-cellular glycoprotein coat separated from the vitelline membrane by the perivitelline space. c. Corona radiata: single layer of cells surrounding the zona pellucida. 2. Stratum granulosum: the outer wall of follicle, made of follicular cells. 3. Antrum: the cavity of the follicle, filled with liquor folliculi. 4. Cumulus oophorus: group of follicular cells attaching the secondary oocyte to one side of the follicle. The compressed ovarian stroma forms a capsule around the follicle called Theca Folliculi which is divided into: 1. Theca interna: formed of secretory cells. 2. Theca externa: mostly fibrous. Differences between spermatogenesis and oogenesis (but both produce haploid gametes) Spermatogenesis Oogenesis In testis In ovary Starts after puberty Starts during fetal life Continues till death Ends at menopause Primary spermatocyte gives 4-minute Primary oocyte gives one large ovum equal sperms Sperms are motile Ova are non-motile 3  ADULT FEMALE CYCLES Occur every 28 days from puberty till menopause. 1. Ovarian cycle: In the ovarian cortex and depends on pituitary hormones: 1. Follicular phase (14 days): Under the effect of pituitary follicle stimulating hormone (FSH), 15-20 primordial follicles grow → single Graafian follicle (secretes estrogen mainly) + several atretic follicles. 2. Ovulation: Under the effect of FSH and pituitary leuteinizing hormone (LH) with liberation of secondary oocyte and formation of corpus luteum. 3. Luteal phase (14 days): Under the effect of LH; the corpus luteum is formed (secretes progesterone mainly). Finally, the corpus luteum degenerates giving the corpus albicans. 2. Menstrual cycle: In the endometrium of the uterus and depends on ovarian hormones. 1. Menstrual stage (4days) → shedding of the superficial layers of endometrium → bleeding (50 – 60 cc of non - clotting blood). This shedding is due to lack of ovarian hormones. 2. Repair (3 days) → healing of the raw endometrial surface from the epithelial cells of the bases of the glands. 3. Proliferative stage (7days) → thickening of endometrium whose glands are straight. * The 2nd and 3rd stages are under control of estrogen of the follicular phase of ovarian cycle. 4. Secretory phase (14 days) → Further thickening of endometrium under control of progesterone of the luteal phase of ovarian cycle. The endometrium becomes differentiated into: a- Deep basal layer containing the straight basal parts of the glands. b- Superficial (functional) layer containing the spiral parts of the glands. It becomes shedded in the next menstrual phase. 4