Module 11

Gametogenesis

oogenesis and spermatogenesis

Oogenesis

development of oocyte

Spermatogenesis

development of spermatozoa

Oogonia

ovarian stem cell, where oogenesis starts; formed during fetal development

Primary oocyte

formed by division via mitosis of oogonia

Meiosis 1

primary oocytes are arrested and will resume years later during puberty until menopause

Menopause

cessation of reproductive function in females

Ovulation

release of an oocyte from an ovary; is a transition from puberty to reproductive maturity

Luteinizing hormone

surges prior to ovulation to trigger resumption of meiosis 1 in a primary oocyte

Secondary oocyte

formed by division in meiosis 1, along with a polar body

Sperm

penetrates barriers to reach egg and meiosis 2 resumes producing a haploid ovum

Haploid ovum

mature egg

Folliculogenesis

process of follicular development

Follicles

key to support the developing oocyte; when they stop developing, oocytes cannot be supported and released from the ovary

Primordial follicles

exist in newborns and stay in resting state until recruited after puberty

Puberty

few primordial follicles recruited everyday to join a pool of immature follicles

Primary follicle

immature follicle that transition to a different type of tissue and become secondary follicles, increasing in diameter

FSH

hormone that helps stimulate the growth of the tertiary follicle

LH

stimulates the production of estrogen by granulosa and theca cells

Mature follicle

raptures and releases the secondary oocyte

Corpus luteum

cells remaining in the follicle develop into this, it is important for progesterone secretion

Atresia

primordial follicles degenerate

Ovarian cycle

process of development of the secondary oocyte and tertiary follicles

Uterine cycle

driven by the hormones released from the developing follicles and corpus luteum

Follicular phase

first stage of the uterine cycle

Luteal phase

Entered once the secondary oocyte is released and ovulation took place and corpus luteum is developed

Development of the corpus luteum

triggered by the surge in LH secreted by the follicles and cells in the follicles

Estrogen

helps rebuild endometrium of the uterus

Progesterone

secreted once corpus luteum is formed, helps maintain endometrium of the uterus

Secondary oocyte not fertilized

does not complete meiosis II and will not form the zygote, does not need the support of the endometrium

Endometrium

helps support the developing zygote divisions

Implantation

progesterone levels remain high as secreted by corpus albicans

No implantation

progesterone levels drop and corpus albicans degrades 14 days post ovulation

Progesterone and estrogen low

signal that the cycle should start again, triggering menses and primordial follicle recruitment

Menses

endometrium and blood vessels are shed

Estrogen high

triggers switch from menses to proliferative phase

Proliferative phase

endometrium is rebuilt and becomes ready for the next ovulation

Follicular phase

hypothalamus increases secretion of gonadotropin-releasing hormones

Gonadotropin-releasing hormone

increase secretion of follicle stimulating and luteinizing hormone by the anterior pituitary

FSH and LH

increase follicle development and endometrial rebuilding, increase estrogen secretion

Estrogen

decreases secretion by hypothalamus and pituitary to prepare the cycles for ovulations

Ovulation

cycle is restarted

Progesterone

triggers a decrease in secretion by the hypothalamus and pituitary to prepare for the next cycle

Corpus luteum declines

progesterone declines and endometrium maintenance stops as there is no implantation of the zygote into the endometrium

Spermatogenesis

new spermatogonium divides by mitosis about every 16 days; occurs in the seminiferous tubule

Primary spermatocytes

undergo meiosis 1

Secondary spermatocytes

undergo meiosis 2 to form 4 equally sized spermatids

Spermatids

reach physical maturation once they become mobile through spermatogenesis

Physical maturation

starts in the epididymis and ductus deferens

To reach full mobility

sperm must pass the seminal gland all the way around the prostate

Sperm production process

lasts about 64 days and starts at puberty; no finite number of sperm as the new sperm is procured every 16 days

Hypothalamus and pituitary gland

regulate the production of testosterone and the cells that assist in spermatogenesis

GnRH

activates the anterior pituitary to produce LH and FSH

LH and FSH

stimulate Leydig cells and Sertoli cells

Negative feedback loop

end products of the pathway, testosterone and inhibin, interact with the activity of GnRH to inhibit their own production

Leydig cell

production of testosterone

GnRH

stimulates endocrine release of hormones from the pituitary gland

GnRH receptors on anterior pituitary gland

stimulates release of the two gonadotropins, luteinizing hormone (LH) and follicle-stimulating hormone (FSH)

FSH (men)

binds predominantly to the Sertoli cells within the seminiferous tubules to promote spermatogenesis and produce inhibin

Inhibin

inhibit FSH release from the pituitary, reducing testosterone secretion

Polypeptide hormones

correlate directly with Sertoli cell function and sperm number

Inhibin B

can be used as a marker of spermatogenic activity

LH (men)

binds to receptors on Leydig cells in the testes and upregulates the production of testosterone

Low testosterone

stimulate the hypothalamic release of GnRH

Testosterone at critical threshold

it will bind to androgen receptors on both the hypothalamus and the anterior pituitary, inhibiting the synthesis and secretion of GnRH and LH