REPRODUCTIVE HORMONES

androgen

the testes of male vertebrates secrete androgens, which are steroid hormones that stimulate and control the development and maintenance of the male reproductive system

testosterone

• principal androgen

• the male sex hormone

• stimulates sperm production and controls the growth and function of male reproductive hormones

testosterone stimulates puberty in males through (6)

• maturation of testes + penis

• increased secretion of body oils

• growth of facial and body hair

• development of muscles

• changes in the vocal cords (growth of larynx, lowering of the voice)

• development of sex drive

testosterone for spermatogenesis

• testosterone controls spermatogenesis, the process by which sperm cells are produced in the testes from precursor cells called spermatogonia

• the organs that produce and deliver sperm make up the male reproductive system

testes anatomy

• males have a pair of testes suspended in a baglike scrotum

• each testicle is packed with about 125 m of seminiferous tubules, in which sperm proceed through all the stages of spermatogenesis

• mature sperm flow from seminiferous tubules into the epididymis (a coiled storage tubule attached to the surface of each testes)

• rhythmic muscular contractions of epidydymis move sperm into a thick-walled, muscular tube, the vas deferens

spermatogenesis

• occurs in the seminiferous tubules of the testes

• the stem cells that give rise to sperm, spermatogonia, are located at the periphery of each seminiferous tubule

• developing sperm cells move toward the central opening (lumen) of the tubule as they undergo meiosis and differentiation

Sertoli cells

• supportive cells that completely surround developing spermatocytes in the seminiferous tubules

• they supply nutrients to the spermatocytes and seal off the spermatocytes from the body’s blood supply

Leydig cells

• located in the tissue surrounding the developing spermatocytes

• scattered in connective tissue between seminiferous tubules

• produce the male sex hormones (androgens), particularly testosterone

follicle stimulating hormone (FSH)

stimulates Sertoli cells to secrete a protein and other molecules required for spermatogenesis

luteinizing hormone (LH)

causes Leydig cells to produce and secrete testosterone and other androgens, which promotes spermatogenesis in the tubules

negative feedback: testosterone & hormones

• testosterone regulates blood levels of GnRH, FSH, and LH through inhibitory effects on the hypothalamus and anterior pituitary

• negative feedback circuits maintain androgen levels in the normal range

hypothalamus

• produces special hormones called neurohormones which travel through the blood into the pituitary gland

• secretes gonadotropin releasing hormone (GnRH)

types of neurohormones produced by the hypothalamus (2)

• releasing & inhibiting hormones

• hormones either stimulate or inhibit the release of specific hormones created in the anterior pituitary gland

anterior pituitary gland

• a two-lobed gland within the cranial cavity that produces hormones that control the other endocrine glands; the “master gland”

• synthesis and secretion of testosterone by cells in the testes are controlled by the release of LH and FSH from anterior pituitary gland

secretion of testosterone

• the hypothalamus secretes GnRH in brief pulses every 1-2 hours

• GnRH stimulates the anterior pituitary gland to secrete LH and FSH

testosterone secretion negative feedback mechanism

• if the concentration of testosterone falls in the bloodstream, the hypothalamus responds by increasing GnRH secretion

• if the concentration of testosterone becomes too high, the overabundance inhibits LH secretion

inhibin for testosterone production negative feedback

• overabundance of testosterone also stimulates Sertoli cells to secrete inhibin, which inhibits FSH secretion by the pituitary gland

• as a result, testosterone secretion by the Leydig cells drop off, returning the concentration to optimal levels in the bloodstream

female reproductive hormones

the ovaries of female vertebrates secrete estrogen and progesterone, steroid hormones that stimulate and control the development and maintenance of the female reproductive system

estrogen stimulates the maturation of the sex organs at puberty through (5)

• breast development

• increased secretion of body oils

• growth of body hair

• widening of the pelvis

• development of sex drive

progesterone

prepares and maintains the uterus for the implantation of a fertilized egg and the subsequent growth and development of an embryo

female reproductive anatomy

• pair of ovaries suspended in the abdominal cavity

• a fallopian tube leads from each ovary to the uterus

• uterus is a hollow structure with walls that contain smooth muscle; it is lined with endometrium which supports the fertilized egg

• lower end of the uterus, cervix, opens into a muscular canal, the vagina, which leads to the exterior

oogenesis

• the process by which the ovaries produce and release eggs (ova)

• ovaries do not release mature eggs; instead, they release oocytes, which have only undergone their first meiotic division

• before a female infant is born, each of her ovaries contains about 1 million oocytes in arrested development

ovulation

the monthly release of one (or a few) developing oocytes into the nearby fallopian tube

meiosis in eggs

• once ovulated, an ovum is pulled through the oviduct by a current produced by cilia, where fertilization may occur

• second meiotic division occurs only if the oocyte is penetrated by sperm; a mature ovum and polar body form, and the nucleus of the ovum fuses with the nucleus of the sperm to form a zygote

• zygote is propelled into the uterus by the cilia of the oviduct

• if the ovum is not fertilized, it degenerates

linking of ovarian and uterine cycles

hormone activity synchronizes ovulation with the establishment of a uterine lining that can support embryo implantation and development

hormone stimulation in ovarian cycle

• the beginning of the ovarian cycle is stimulated by the release of GnRH by the hypothalamus

• GnRH stimulates the anterior pituitary to release FSH and LH into the bloodstream

• FSH and LH stimulate follicle growth

follicle growth in ovarian cycle

• 6-20 oocytes finish meiosis I

• as oocytes develop, they become surrounded by cells that form a follicle (ovum and follicle cells)

• the follicle grows and develops (filled with fluid, 12-15 mm in diameter)

• only one follicle develops to maturity, with the release of oocyte by ovulation

• as the follicle enlarges, FSH and LH stimulate estrogen secretion by the follicles

how multiple births can occur

multiple births can result if two or more follicles develop and their eggs ovulate and are fertilized in one cycle

stages of ovarian cycle (3)

• follicular phase

• ovulation

• luteal phase

ovarian cycle: (1) follicular phase

• estrogen secretion increases steadily, and its level peaks about 12 days after the beginning of follicle development

• high estrogen level as a positive feedback effect on hypothalamus and pituitary gland, increasing secretion of GnRH and stimulating the pituitary to release FSH and LH

ovarian cycle: (2) ovulation

occurs after the burst in LH secretion, which stimulates the follicle cells to release enzymes that digest the wall of the follicle, causing it to rupture and release the oocyte into the fallopian tube

initiation of luteal phase

• LH causes follicle cells that remain at the surface of the ovary to grow into an enlarged structure, the corpus luteum

• initiates luteal phase, which prepares the uterus to receive a fertilized egg

ovarian cycle: (3) luteal phase

• corpus luteum acts as an endocrine gland that secretes estrogen, as well as progesterone and inhibin

• progesterone and inhibin have negative feedback effect on the hypothalamus and pituitary gland

• progesterone inhibits GnRH secretion (thus FSH and LH secretion by pituitary); inhibin inhibits FSH secretion

• decrease in FSH and LH diminishes signal for follicular growth, so no new follicles grow

negative feedback in luteal phase

• prevents maturation of another oocyte when a pregnancy may be underway

• if pregnancy does not occur, low GnRH levels at the end of the luteal phase causes corpus luteum to disintegrate, triggering decline in estrogen and progesterone concentrations

• this decline liberates hypothalamus and anterior pituitary from negative feedback; pituitary can secrete more FSH and LH to stimulate growth of new follicles, initiating the next ovarian cycle

human chorionic gonadotrophin (hCG)

an embryonic hormone that acts like pituitary LH in maintaining secretion of progesterone and estrogen by the corpus luteum through the first few months of pregnancy

uterine (menstrual) cycle phases (3)

• proliferative phase

• secretory phase

• menstrual flow phase

uterine cycle: (1) proliferative phase

estrogen secretion by growing follicles signals endometrium to thicken

uterine cycle: (2) secretory phase

after ovulation, estrogen and progesterone secreted by the corpus luteum stimulate maintenance and further development of the endometrium

uterine cycle: (3) menstrual flow phase

• if an embryo has not implanted in the endometrium by the end of the secretory phase, corpus luteum disintegrates

• drop in ovarian hormone levels causes arteries in the endometrium to constrict

• deprived of its circulation, the uterine lining largely disintegrates, releasing blood that is shed along with endometrial tissue and fluid

• during the phase, a new set of ovarian follicles begin to grow