Gametogenesis. Fertilization

Gametes are derived from

Primordial germ cells (PGCs)

primordial germ cells (PGCs)

Formed in epiblast and moves to wall of yolk sac at the end 3rd week

4th week: migrate to developing gonads

When do the primordial germ cells migrate from the yolk sac to the developing gonads?

4th week and arrive by the end of the 5th week

Oogenesis

the process in which PGCs differentiate into mature oocytes (ovum)

oogenesis location

cortex of ovary

When do PGCs differentiate into oogonia?

Before birth (embryo)

PGCs -> oogonium -> primary oocyte (arrested in prophase 1 of meiosis 1)

oogonia

When PGCs arrive at the developing gonads they differentiate into oogonia

Oogonia divide by mitosis to proliferate

primary oocyte (2n)

Oogonia divides by meiosis to form primary oocyte (arrested in prophase 1 of meiosis 1)

diplotene stage

prolonged resting phase in oogenesis - primary oocytes are arrested in prophase 1 of meiosis 1 until puberty

atresia

Oocyte degeneration

5th month: 7 million PGCs

At birth: 600000 primary oocytes

At puberty: 40000 primary oocytes

Effect of atresia with increasing age

Primary oocytes are vulnerable as age increases -> Risk of having child with chromosomal abnormality

secondary oocyte

At puberty: primary oocyte completes meiosis 1 just 24 hours before ovulation -> secondary oocyte (arrested in metaphase of meiosis 2)

Cytokinesis of meiosis 1 gives the 1st polar body

polar bodies

Receive little amount of cytoplasm and degenerate after

1 primary oocyte -> 4 daughter cells: 1 ovum + 3 polar bodies

Ovum (egg)

After fertilization: secondary oocyte finishes meiosis 2 -> ovum + 2nd polar body

No fertilization -> secondary oocyte degenerates

follicular development

primordial follicle

Primary oocyte + single layer of squamous follicle cells

End of 3rd month

Arrested in this phase by OMI

primary follicle

Puberty, primary oocyte, 15-20 primary follicles begin to grow each month

Follicular cells: squamous -> cuboidal (granulosa cells); single layer -> stratified

Granulosa cells: produce zona pellucida

Theca folliculi: CNT surrounding ovarian

zona pellucida

Glycoprotein membrane surrounding the oocyte

Produced by granulosa cells

Provides support and nutrients to oocyte

How are materials transported to the oocytes?

cytoplasmic projections from granulosa cells to oocyte (across zona pellucida)

secondary follicle

Primary oocyte + layers of granulosa cells

Antrum

Theca folliculi: interna (estrogen production) + externa

antral stage

Fluid accumulates between granulosa cells -> antrum

tertiary follicle (Graafian follicle)

Secondary oocyte

Cumulus oophorus

Corona radiata

How many follicles mature each month?

1 from puberty to menopause

Overall oogenesis

female reproductive cycle

Ovarian + uterine cycle (interconnected)

Average 28 days

summary of ovarian cycle

ovarian cycle

Produce eggs and hormones

Follicular phase

Ovulation phase

Luteal phase

follicular phase of ovarian cycle

LH promotes development of theca folliculi -> secretes estrogen -> estrogen spike -> surge in LH -> ovulation around day 14

ovulation phase of ovarian cycle

Day 14

LH peak + FSH slight rise -> stimulates ovulation

(secondary oocyte release from mature follicle)

luteal phase of ovarian cycle

Days 15-28

LH stimulates secretion of progesterone by the corpus luteum -> promotes uterine lining growth

uterine cycle

Prepare lining of uterus (endometrium) for blastocyst

Endometrium remains 9 days after ovulation

Menstruation

Shedding of the uterine lining without fertilization

GnRH, FSH, LH decrease

Progesterone

Produced by corpus luteum

Thickening of endometrium

Pregnancy: progesterone is produced until 3rd month of pregnancy (hCG)

LH during oogenesis

LH surge causes primary oocytes to finish meiosis 1 -> secondary oocytes

maturation of secondary sex characteristics

caused by FSH and LH (GnRH)

LH during follicular phase

APG releases LH -> theca folliculi release androstenedione + testosterone

Granulosa cells convert them to estrogen

hormonal control of menstrual cycle

Spermatogenesis

Seminiferous tubules

Spermatogonia -> spermatozoa (mature sperm)

After puberty

phases of spermatogenesis

maturation phase of spermatogenesis

mitosis + meiosis 1 + meiosis 2

Type A spermatogonia

Undergo mitosis to produce type B spermatogonia

Type A dark spermatogonia

Type A pale spermatogonia

type B spermatogonia

divided by mitosis to form primary spermatocyte

Why is spermatogonia connected to each other?

Cytokinesis is incomplete -> connected by cytoplasmic bridges through out maturation

primary spermatocyte

Arrested at prophase 1 for 22 days

Complete meiosis 1 to produce secondary spermatocyte

secondary spermatocyte

Undergo meiosis 2 to produce spermatid

Spermiogenesis

Transformation of spermatids into spermatozoa

steps of spermiogenesis

Formation of acrosome

Condensation of the nucleus

Formation of the neck, middle piece, and tail

Shedding of most cytoplasm

acrosome of sperm

contains hydrolytic enzymes to penetrate zona pellucida during fertilization

semen

Sperm cells and seminal fluid (secretion of prostate, seminal vesicle, bulbourethral glands)

Fructose

Alkaline pH (neutralize acidic pH of vagina)

Sertoli cells

Within seminiferous tubules

Provide metabolic support for the spermatids

Leydig cells

Between the seminiferous tubules

Produce testosterone

sperm cell

Male Genital System

hormonal regulation of male reproductive cycle

female genital system

Fertilization

sperm enters the cytoplasm of the egg cell -> fusion of 2 nuclei -> diploid zygote -> embryo

steps of fertilization

Recognition of sperm and egg

Activation of sperm: capacitation, acrosome reaction

Plasma membrane fusion

Further entry of sperm is blocked

Activation of oocyte

Fusion of oocyte and sperm nucleus

Intercourse ___ before ovulation and ___ can result in fertilization

1-2 days; next day

At ejaculation, the semen is transferred to the ___

posterior vaginal vault

How long does it take for sperm to reach egg?

60-70 minutes

Where do sperm and egg meet?

fallopian tube

chemotactic

Substance that attract sperm

Produced by oocyte

Capacitation

Thinning of plasma membrane -> acrosomal cap

acrosome reaction

digest zona pellucida

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