GAMETOGENESIS

OVARIAN & UTERINE CYCLES

ANATOMY DEPARTMENT

OBJECTIVES• At the end of the lecture, the students should

be able to :• Describe the process of gametogenesis. • List the importance of meiosis.• List the stages of spermatogenesis.• List stages of oogenesis.• Compare between the 2 gametes.• Describe the ovarian cycle.• Describe the uterine cycle.

PROCESS OF GAMETOGENESIS• It is the process of

conversion of immature germ cells (spermatogonium or oogonium), into mature gametes (sperm or oocyte).

• This maturation is called spermatogenesis in males and oogenesis in females.

• During gametogenesis, a special type of cell division (meiosis) occurs, in which the number of chromosomes is reduced from diploid to haploid number.

• It prepares gametes for fertilization.

MEIOSIS• It is the cell division that

takes place in the germ cells only to produce male and female gametes.

• It consists of two cell divisions :

• meiosis I during which the chromosome number of the germ cells is reduced to half (23), the Haploid number (with double chromatid chromosoes).

• Meiosis II during which the haploid number (23) is retained (with single chromatid chromosome).

• (1) Reduces the chromosome number from the Diploid to Haploid.

• (2)Allows random movement of maternal and paternal chromosomes between the gametes (Segregation)

• (3)Allows Crossing Over of chromosome segments.

• (4)It enhances genetic variability through cross over and segregation (separation or disjunction of paired homologous chromosomes).

Segregation of chromosomes

Spermatogenesis

• It is a conversion of primitive germ cell spermatogonium into mature germ cell sperm.

• It begins at puberty and continues into old age.

• It occurs in the seminiferous tubules of testes.

• After several mitotic divisions, spermatogonia increase in number and grow into primary spermatocytes (46 chromosomes).

Spermatogenesis

• Each primary spermatocyte undergoes a reduction division- 1st meiotic division to form 2 haploid secondary spermatocytes.

• Secondary spermatocytes undergo 2nd meiotic division to form 4 haploid spermatids (half size).

• Spermatids are transformed into 4 mature sperms by a process called spermiogenesis.

Spermiogenesis• It is the last phase of

spermatogenesis.• The rounded spermatid is

transformed into elongated sperm.

• Note the loss of cytoplasm, development of the tail, and formation of acrosome, which is derived from Golgi region of spermatid.

• Acrosome contains enzymes that are released at the biginning of fertilization to help sperm in penetrating corona radiata & zona pellucida surrounding secondary oocyte.

• Spermatogenesis including spermiogenesis requires about 2 months for completion.

• It takes place in the Seminiferous Tubules.

• The sperms are stored and become functionally mature in the Epiddidymis.

The mature sperm

Mature sperm

Secondary oocyte (Mature female gamete)

• It is a free-swimming actively motile cell, consisting of a head, neck and a tail.

• The head, composed mostly of haploid nucleus.

• The nucleus is partly covered by a caplike acrosome, an organelle containing enzymes to help sperm in penetrating corona radiata & zona pellucida of secondary oocyte during fertilization.

• The tail of sperm consists of 3 segments : middle, principal & end pieces. it provides motility of sperm to the site of fertilization.

• The middle piece of the tail contains mitochondria, providing adenosine triphosphate (ATP) necessary for activity.

Oogenesis• It is a maturation of the germ

cells (oogonia) into mature secondary oocytes.

• It occurs in the ovary, which contains a large number of oogonia that differentiate into primary oocytes.

• It begins before birth (in early fetal life) and is completed after puberty and continues to menopause.

• No oogonia are found in ovary after birth because they differentiate into primary oocytes before birth.

Oogenesis : Prenatal maturation of Oocytes

• During early fetal life : Oogonia proliferate by mitosis to form daughter oogonia, they enlarge and grow to form primary oocytes before birth (each contains 46 ch.).

• A Single layer of flattened follicular epithelial cells from the cotex of ovary surrounds the primary oocyte, forming primitive or primordial ovarian follicles containing primary oocytes (A)

• At birth all primary oocyte (46 ch) remain arrested and do not finish their 1st meiotic division until puberty.

Oogenesis

During puberty, follicular epithelial cells become cuboidal then columnar and the primary oocyte becomes surrounded by zona pellucida (acellular glycoprotein material) forming growing follicle (B).

Then, Primary oocyte has several layers of follicular cells + Z.P.membrane, forming the primary follicle (C).

Oogenesis At puberty : cavities appear between

the follicular cells, then join together forming a single large cavity called (Antrum) filled with a watery fluid (liquor folliculi).

The follicular cells differentiated into : Stratum granulosum, forming the wall of follicle.

Cumulus oophorus : cells surrounding the ovum (primary oocyte).

Theca folliculi differentiated into : theca externa : outer fibrous & theca interna : inner vascular and cellular layer.

At this stage the growing primary follicle changed into mature secondary follicle or Graafian follicle.

Graafian Follicle

• Shortly before ovulation : Primary Oocyte completes first meiotic division :

• the Secondary Oocyte (23 ch) receives almost all the cytoplasm.

• The First Polar Body receives very little.

• It is small nonfunctional cell and soon degenerates.

• At ovulation : the nucleus of the secondary oocyte begins the second meiotic division, leading to :

• The secondary oocyte (23)• The 2nd polar body.

At puberty /Before ovulation

At ovulation

• If the secondary oocyte is fertilized by a sperm, the second meiotic division is completed otherwise it degenerates 24 hours after ovulation.

• Most of the cytoplasm is retained by the Mature Oocyte (Fertilized Oocyte).

• The rest is in the 2nd Polar Body which soon degenerats.

• Has a covering of a morphous material (Zona Pellucida) and a layer of follicular cells (Corona Radiata).

Comparison of Gametes

Sperm• It is highly motile.• It contains little cytoplasm.• It is not surrounded by Z.P & C.R.• It has 2 kinds of sex

chromosomes : 23,x and 23,y / so the difference in sex chromosome complement of sperms forms the basis of primary sex determination.

Secondary oocyte

It is immotile.

It has an abundance of cytoplasm.

It is surrounded by Z.P and a layer of follicular cells-the C.R.

It has only one kind of sex chromosome : 23,x

Oogenesis: postnatal maturation of Oocytes • No primary oocytes form after birth in

females, in contrast to continuous production of primary spermatocytes in males.

• Primary oocytes are formed in the ovaries in early fetal life and remain dormant in ovarian follicles until puberty.

• At puberty,as a follicle matures, each primary oocyte divides by 1st meiotic division into one large secondary oocyte (contains haploid chr.) and one small non-functional cell- first polar body (degenerates ,contains haploid chr.).

• At ovulation, secondary oocyte begins 2nd meiotic division, which is completed after fertilization, giving one ovum or fertilized oocyte + one small nonfuctional cell- second polar body , each one having haploid chr.

At puberty

At ovulation

Female reproductive cyclesFemale reproductive cycles

• Commencing at puberty and normally continuing throughout the reproductive years.

• Involving activity of :• Hypothalamus.• Hypophysis.• Ovaries.• Uterus.• Uterine tubes.• Vagina.• Mammary glands.

Female reproductive cyclesFemale reproductive cycles

• Gonadotropine releasing hormone by hypothalamus (GnRH) stimulates the release of :

• Follicle stimulating hormone (FSH); • Stimulates development of ovarian follicles.• Production of estrogen by follicular cells.• Lutinizing hormone (LH);• Trigger for ovulation (secondary oocyte)• Stimulate production of progesterone by follicular cells

and corpus luteum.• These hormones induce growth of endometrium.

Ovarian cycleOvarian cycle•FSH and LH produce cyclic changes in the ovaries-the ovarian cycle (development of follicles, ovulation, and corpus luteum formation).

Ovarian cycleOvarian cycle• (FSH) and (LH) produce cyclic

changes in the ovaries.• Follicular development, with the

resulting growth and proliferation of follicular cells, formation of zona pellucida, and development of theca folliculi.

• Subsequent estrogen production, necessary for reproductive organs development and function.

• Ovulation; occurs at mid menstrual cycle, stimulated by increase of (LH) production, and high estrogen level.

• Expulsion of the secondary oocyte, surrounded by zona pellucida, and corona radiata,

• Corpus luteum; the wall of ovarian follicle after expulsion of the secondary oocyte and develop under influence of (LH).

• It is of two types : corpus luteum of pregnancy and corpus luteum of menstruation (corpus albicans).

Ovarian cycleOvarian cycle

Ovarian cycleOvarian cycle

• Monthly series of events associated with the maturation of an egg.

• Follicular phase – period of follicle growth (days 1–14).

• Ovulation occurs midcycle.• Luteal phase – period of corpus luteum activity

(days 14–28).

Menstrual cycleMenstrual cycle• It is the cyclic changes in the endometrium

(every 28 days) under fluctuating effect of gonadotropic, ovarian follicle and corpus luteum hormones (estrogen and progesterone).

• It is the peroid during which the oocyte matures, ovulated, and enters the uterine tube.

Menstrual cycleMenstrual cycle

• Phases of the menstrual cycle :• Menstrual phase; the functional layer of

the uterine wall is sloughed, discarded with the menstrual flow, it lasts about 4-5 days.

• The endometrium is thin.

Menstrual cycleMenstrual cycle

• Proliferative phase :(follicular estrogenic), coincides with growth of the ovarian follicle and controlled by estrogen.

• It lasts about 9 days.• Increase thickness of endometrium.

Menstrual cycleMenstrual cycle

• Luteal phase; (secretory progesterone), coincides with formation, function, and growth of corpus luteum,

• It lasts about 13 days.• increase endometrial thickness under influence

of progesterone and estrogen of corpus luteum.

Menstrual cycleMenstrual cycle• If fertilization occurs; • corpus luteum under influence of hCG

continues to secrets estrogen and progesterone, the luteal phase persists and menstruation does not occur.

• The endometrium passes into the pregnancy phase.

• If fertilization dose not occur; corpus luteum degenerates, with fall of estrogen and progesterone level, menstruation occurs.

Menstrual cycleMenstrual cycle• Ischemic phase; due to hormonal

withdrawal as a result of non fertilization of the oocyte,

• Shrinkage of the endometrium, with patchy ischemic necrosis, resulting in bleeding in the uterine cavity.

• The cycle is repeated.

THANK YOUTHANK YOU