Gametogenesis: Spermatogenesis & Oogenesis

• Gametogenesis involves the formation and development of gametes (sperm and oocytes), reducing the chromosome number by half through meiosis.

Types of Gametogenesis

• Spermatogenesis forms mature sperm cells, starting at puberty and continuing until death.
• Oogenesis forms mature oocytes, beginning before birth, arresting at prophase I, continuing at puberty, and ceasing at menopause.

Meiosis in Gametogenesis

• Utilizes meiosis rather than mitosis to produce germ/sex cells.
• Diploid precursor cells undergo division and differentiation to produce haploid germ cells.
• Meiosis involves two divisions: Meiosis I (reduction division) reduces the chromosome number from 2n=46 to n=23.
• Chromosomes and cytoplasm are altered to prepare sex cells for fertilization, with significant shape and size changes, especially in males.

Meiosis

• Special type of cell division occurs only during gametogenesis in germ cells and involves two meiotic cell divisions.
• Transforms diploid germ cells into haploid gametes (sperms and oocytes).
• The process is similar in males and females but differs in timing.

First Meiotic Division (Reduction Division)

• Before DNA replication in the S phase, chromosomes exist as single chromatids (n=23).
• After DNA replication, chromosomes become double chromosomes (2n=46).
• Chromosome number is halved per cell and homologous chromosomes pair during prophase I.
• Homologous chromosomes segregate/crossover during anaphase I.
• The products of the first meiotic division are secondary oocytes or spermatocytes containing a haploid chromosome number with double chromatid chromosomes.

Second Meiotic Division

• Occurs without interphase (no DNA replication).
• Each double chromatid divides into two sister chromatids, resulting in one chromatid per chromosome.
• Each daughter cell has a haploid number of chromosomes with single-chromatid chromosomes.

Oogenesis

• Sequence of events by which oogonia are transformed into mature oocytes.
• Begins before birth, arrests at prophase I, and continues during puberty (12-15 years) before ceasing at menopause.

Prenatal Maturation of Oocytes

• Early fetal life: oogonia proliferate through mitotic division.
• Oogonia enlarge to form primary oocytes before birth.
• Primary oocytes become primary follicles when enclosed by a single layer of flattened follicular epithelial cells.
• During puberty, primary oocytes are enclosed by columnar follicular cells, forming the primary follicle.
• Primary oocytes enlarge, and follicular epithelial cells become cuboidal, forming the primary follicle, which is surrounded by the zona pellucida.
• Primary follicles become secondary follicles when surrounded by more than one layer of follicular cells.

Oocyte Development

• Oocytes migrate to the cortex of the embryonic gonad/primordial ovary by the 6th week of fetal development.
• Mitosis of primordial germ cells (oogonia) occurs up to the 5th fetal month.
• By the 3rd fetal month, meiosis starts before birth, and follicular cells and glycoproteins surround the oocyte.
• Primary oocytes enter the dictyotene phase, a resting phase in prophase I, maintained by the release of oocyte maturation inhibitor from follicular cells. This phase ends with luteinizing hormone surges during each menstrual cycle.

Ovulation: First Continuation of Meiosis

• Meiosis continues from prophase I, triggered by physiological events of ovulation.
• One secondary oocyte arrested in metaphase II and one first polar body are produced per cycle.
• Only the follicle with the lowest threshold for LH and FSH is stimulated to release the secondary oocyte.
• The first polar body is immediately extruded.

Fertilization: End of Meiosis

• Meiosis continues to anaphase II and finishes telophase II, triggered by penetration of one sperm through the zona pellucida.
• Results in one one-cell diploid ovum and one second polar body.
• Further divisions of the ovum are mitotic.
• Without fertilization, the secondary oocyte will degenerate and be excreted through the reproductive tract.

Postnatal Maturation of Oocytes

• Begins during puberty when one follicle matures each month, and ovulation occurs.
• Long durations in the first meiotic division increase the risk of errors and congenital anomalies like nondisjunction.
• No new primary oocytes are formed after birth; all are formed before birth and decrease throughout life.
• Follicle maturation increases primary oocyte size, and the first meiotic division completes shortly before ovulation, resulting in unequal cytoplasmic division.
• Produces a secondary oocyte (receives almost all cytoplasm) and a first polar body (small, non-functional cell).

Oogenesis Numbers

• Newborn females have 2 million primary oocytes, but regression occurs during childhood, leaving about 40,000 by adolescence.
• Only about 400 become secondary oocytes and are expelled during reproductive age, with few maturing or being fertilized.

Oogenesis Review points

• The only follicles present before birth are primordial follicles containing primary oocytes arrested at prophase I.
• A group of follicles will be called upon to be primary follicles.
• Primary follicles develop into secondary/antral follicles.
• Only one follicle will mature into the Graffian follicle.
• The oocyte will arrest in metaphase II if fertilization doesn't occur
• The Graafian follicle turns into the corpus luteum which turns into the corpus albicans id not fertilized or the corpus luteum of pregnancy if it gets fertilized

Spermatogenesis

• Spermatogonia divide to produce primary spermatocytes, a continuous process starting at puberty with no arrest.
• Cycle length is constant and specific.

Spermatogenesis Subprocesses

• Spermatocytogenesis, where spermatogonia undergo mitosis to form primary spermatocytes.
• Meiosis, where spermatocytes undergo division to produce spermatids.
• Spermiogenesis, where spermatids differentiate into spermatozoa.

Spermiogenesis

• Spermatids differentiate into spermatozoa with changes including loss of cytoplasm, tail development, and acrosome formation.
• Prominent Golgi apparatus appears adjacent to the nucleus.
• Pro-acrosomal granules appear and coalesce to form a larger acrosomal granule.
• Acrosomal granules are enclosed by an acrosomal vesicle and accumulate over time.
• The acrosomal vesicle adheres to nuclear envelope.

Cap Phase

• The acrosomal vesicle enlarges to cover the entire anterior hemisphere of the nucleus.

Acrosomal Phase

• The nucleoplasm condenses further.
• The centriole migrates to initiate the axoneme formation of the sperm tail.
• Cytoplasm is displaced posteriorly.
• The plasma membrane, acrosome, and nucleus remain in the anterior position.
• Mitochondria migrate posteriorly and gather around the axoneme's initial segment.

Maturation Phase

• Excess cytoplasm is shed off during the maturation phase.
• Sertoli cells phagocytose residual bodies.
• The residual bodies help in the interconnection.
• Spermatozoa become fully independent and mature.

Spermiation

• Spermatozoa are released into the lumen of the seminiferous tubule.
• Spermatids orient with the nuclear head towards the base and the flagellum towards the lumen.

Spermatozoa Contents

• Contains plasma membrane, acrosomal cap, and nucleus.
• Contains the mitochondria with centrioles to produce ATP for sperm activity and motility.
• Sperm analysis examines the ejaculate, especially for couples with difficulty conceiving.

Ejaculate Makeup

• Volume: 2-6 ml with an average of 3.5 ml.
• Speed: 1.5mm/min
• Sluggish inside the vagina because of acidic pH and moves faster in the uterus because of alkaline pH.
• Sperms: only 10%
• Secretions from accessory glands: 90% Normal sperm count is 100 million sperms/ml.