Gametes and Fertilization

Gametes

• Gametes are reproductive cells carrying half an organism's genetic material
• Sperm is the male gamete
• Eggs are the female gamete
• Gametes combine during fertilization to form a new organism

Gamete Sources

• Gametes are produced in specialized organs called gonads
• Sperm is produced in the testes in males
• Eggs are produced in the ovaries in females

Gametogenesis

• Gametogenesis produces gametes (sperm and eggs)
• Germ cells divide and mature into haploid gametes
• Spermatogenesis in males produces sperm
• Oogenesis in females produces eggs (ova)

Gametogenesis Location

• In males, gametogenesis occurs in the testes, specifically in the seminiferous tubules
• In females, gametogenesis occurs in the ovaries, where eggs are formed in follicles

Fertilization

• Fertilization is the process where a male gamete (sperm) and a female gamete (egg) combine to form a zygote
• The zygote divides and develops into a new organism
• Fertilization occurs in the fallopian tubes in females, where sperm meets and penetrates the egg
• Sperm and egg genetic material restores a full set of chromosomes (diploid number) and starts embryo development

Primordial Germ Cells (PGCs) Pathway

• Origin (Week 3): PGCs arise from the epiblast and migrate to the yolk sac wall near the allantois
• Entry into the Embryo (Week 4): PGCs leave the yolk sac and move into the hindgut
• Travel Through Dorsal Mesentery (Week 5): PGCs migrate along the dorsal mesentery of the hindgut toward the gonadal ridges
• Colonization of Gonadal Ridges (Week 6): PGCs reach developing gonads (ovaries or testes) and start differentiating into oogonia (in females) or spermatogonia (in males)

Oogenesis Facts

• Oogenesis commences while still a fetus
• At birth, about 2 million primary oocytes are in the ovaries
• The stage of cell cycle is suspended prophase I
• No primary oocytes are formed after birth
• At puberty, only 30 to 40 thousand remain
• Only 200-400 of these reach full maturity
• Usually, only one is expelled from the ovary at every cycle (28 days)

Oogenesis in the Fetus

• During fetal development, oogonia multiplies rapidly by mitosis inside the developing ovaries
• By the 5th month of fetal life, the number of oogonia reaches around 7 million
• Many oogonia undergo atresia (degeneration), and by birth, only about 2 million primary oocytes remain

Primary Oocytes and Meiotic Arrest

• Before birth, oogonia differentiate into primary oocytes by entering meiosis I
• Primary oocytes become arrested in prophase I of meiosis (specifically in the diplotene stage of prophase I) until puberty
• The arrest is caused by the oocyte maturation inhibitor (OMI), secreted by surrounding follicular cells
• No new primary oocytes are formed after birth

Puberty and the Menstrual Cycle

• At puberty, only about 30,000 to 40,000 primary oocytes remain due to continued atresia
• From puberty until menopause, a select group of primary oocytes mature each menstrual cycle under FSH
• Only one oocyte reaches full maturity per cycle and is ovulated (released from the ovary)

Secondary Oocyte Formation and Ovulation

• Just before ovulation, the primary oocyte resumes meiosis I under LH
• Meiosis I is completed, producing one large secondary oocyte and one small first polar body, which degenerates
• The secondary oocyte immediately begins meiosis II but becomes arrested in metaphase II
• The arrested secondary oocyte is released from the ovary and enters the fallopian tube during ovulation

Completion of Meiosis and Fertilization

• If fertilization occurs, the secondary oocyte completes meiosis II
• This produces a mature ovum (egg) capable of fusing with a sperm cell, and a second polar body, which degenerates
• If fertilization does not occur, the secondary oocyte degenerates without completing meiosis II

Oogenesis Key Facts

• Oogenesis produces a limited number of eggs, unlike spermatogenesis
• Oogenesis includes two meiotic arrests: Prophase I and Metaphase II
• Each oocyte is surrounded by granulosa and theca cells, forming a follicle
• Follicular cells secrete hormones that regulate oocyte development
• Only one oocyte is typically released per 28-day menstrual cycle

Ovarian Cycle

• The ovarian cycle begins at puberty
• The cycle is controlled by various hormones
• Primary follicles range from 15-20
• Secondary follicle
• Graafian follicle
• Ovulation
• Corpus luteum
• Corpus albicans

Ovarian Cycle Overview

• The ovarian cycle occurs in the ovaries each menstrual cycle
• It involves the development and release of an oocyte (egg cell) and hormonal regulation for reproduction
• There are three main phases: Follicular, Ovulation, and Luteal

Follicular phase

• The follicular phase lasts from days 1-14
• The ovarian cycle starts at puberty onset and continues until menopause
• Hormones, like follicle-stimulating hormone (FSH) and luteinizing hormone (LH), control the cycle
• At birth, the ovaries contain thousands of primordial follicles, each containing an immature oocyte
• In each cycle, 15–20 primary follicles develop under FSH influence
• Each follicle consists of an oocyte surrounded by granulosa cells
• Secondary follicles form as some follicles grow and has an enlarged fluid-filled cavity called the antrum
• One dominant follicle matures into a Graafian follicle while the rest undergo atresia (degeneration)

Ovulation Phase

• Ovulation occurs around day 14
• The LH surge triggers ovulation, causing the Graafian follicle to rupture and release the oocyte into the fallopian tube
• The released oocyte is now available for fertilization

Luteal Phase

• The luteal phase lasts from days 15-28
• After ovulation, the ruptured follicle transforms into the corpus luteum
• The corpus luteum is a temporary endocrine structure
• The corpus luteum secretes progesterone and some estrogen
• If fertilization does not occur, the corpus luteum degenerates into the corpus albicans, leading to menstruation
• If fertilization does occur, the corpus luteum continues to produce hormones to support early pregnancy until the placenta takes over

Hormonal Control

• FSH stimulates follicular growth in the early phase
• LH triggers ovulation and corpus luteum formation
• Estrogen is produced by the developing follicle, helping in endometrial proliferation
• Progesterone is produced by the corpus luteum and is crucial for maintaining the uterine lining

Oocyte

• The oocyte (egg cell) is located at the center, within the cumulus oophorus
• It is surrounded by the zona pellucida, a glycoprotein layer for fertilization
• The corona radiata is the innermost layer of granulosa cells attached to the oocyte after ovulation

Granulosa Cells

• Granulosa cells (membrana granulosa) form multiple layers around the antrum and the oocyte
• They produce estrogen by converting androgens produced by theca interna cells into estrogens via aromatase
• Granulosa cells are responsive to FSH, which stimulates follicular growth and estrogen secretion

Antrum

• The antrum is a large fluid-filled space that develops as the follicle matures
• It's filled with follicular fluid, containing hormones, growth factors, and nutrients
• The antrum increases in size as the follicle reaches ovulatory maturity

Cumulus Oophorus

• The cumulus oophorus is a specialized cluster of granulosa cells surrounding the oocyte and extending into the antrum
• It provides metabolic support and signaling for the oocyte

Theca Layers

• The theca layers are the outer layers of the follicle, divided into two distinct regions
• Theca Interna, which is highly vascularized and contains steroidogenic cells that produce androgens
• The Theca Interna responds to LH to stimulate androgen production
• Theca Externa is composed of connective tissue and smooth muscle cells, and provides structural support to the follicle

Basal Lamina

• The basal lamina is a thin layer separating granulosa cells from the theca interna
• It maintains the functions of granulosa and theca cells, preventing blood vessels from penetrating the granulosa layer

Capillaries

• Capillaries are found in the theca interna, providing blood supply and nutrients to the growing follicle
• The granulosa layer is avascular, relying on diffusion for nutrient exchange

Significance of the Graafian Follicle

• It represents the mature follicle ready for ovulation
• Under a surge in LH, the Graafian follicle ruptures, releasing the oocyte into the fallopian tube
• The remaining follicular structure transforms into the corpus luteum, which secretes progesterone to support the luteal phase
• It is crucial for female fertility, ensuring proper development and release of an egg for potential fertilization

Follicular Development in Ovary

• This begins with immature follicles developing under the influence of hormonal signals
• There is an early primary follicle, small follicles containing immature oocytes start to develop
• Another step is a growing follicle enlarging and begins accumulating fluid
• Followed by the secondary follicle continues growing, and the oocyte undergoes further maturation
• The follicle then approaches maturity when it reaches a stage where it is ready to release the oocyte
• The mature follicle (Graafian follicle) is the final stage of follicular development before ovulation

Ovulation Phase

• Ovulation (oocyte release) occurs around the middle of the menstrual cycle
• LH surge triggers ovulation
• The mature follicle ruptures, releasing a secondary oocyte into the fallopian tube
• This released oocyte is ready for fertilization

Post-Ovulation

• Post-ovulation results in the empty follicle turning into the corpus luteum
• The corpus luteum secretes progesterone to maintain the uterine lining
• If fertilization does not occur, the corpus luteum degenerates into the corpus albicans

Oocyte Journey and Fertilization

• Fimbriae at the end of the fallopian tube help sweep the oocyte into the tube
• Oocyte moves through the tube toward the uterus
• If sperm meets the oocyte in the fallopian tube, fertilization occurs, forming a zygote
• The zygote undergoes multiple cell divisions, forming a two-cell stage, four-cell stage, and eight-cell stage
• The developing embryo progresses to the morula stage and then becomes a blastocyst

Blastocyst and Implantation

• The blastocyst reaches the uterus and implants into the endometrium, leading to pregnancy
• If fertilization does not occur, the oocyte degenerates, and menstruation follows
• This process ensures the possibility of conception during each menstrual cycle, depending on whether fertilization occurs