Bio Chapter 37- Animal Reproduction

Hormones control the ovarian and uterine cycles

*See slide 19 -There are both positive and negative feedback mechanisms -Positive feedback only occurs when there is a surge in LH in the middle of the cycle -Hypothalamus (GnRH) -Anterior pituitary (LH/FSH) -Ovary -Estrogen and progesterone -Uterus

Corpus luteum and the luteal phase

-After ovulation, LH stimulates remaining follicle cells to reorganize to form the corpus luteum. It secretes estrogens and progesterone for the remainder of the ovarian cycle, called the luteal phase. -If no fertilization of the released egg occurs, the corpus luteum will degenerate at the end of the luteal phase.

Human female reproductive tract

-All immature eggs are located in the ovaries -All females have 2 ovaries -Once a month, one immature egg is released from the ovary. It exits the ovary and enters the uterus, the region of the body where the baby will grow if the egg is fertilized -The ovary is connected to the uterus the ligament -The immature egg exits the ovary and enters the oviduct (fallopian tube) -The immature egg travels from the fallopian tube to the uterus *Oocytes mature in the ovaries, are released during ovulation, and travel through the oviduct into the uterus

Estrogen

-As the follicle grows, more and more estrogen is released into the bloodstream -Estrogen will feed back to the anterior pituitary to prevent further release of LH and FSH -Levels of LH and FSH decrease between day 14 and day 28 because they are inhibited by increasing levels of estrogen in the bloodstream *This is an example of negative feedback

Luteal phase

-During the luteal phase, the corpus luteum (structure of the cells that previously formed the mature follicle) continues to secrete hormones (they are endocrine cells) -One of those hormones is estrogen, and the other is progesterone -Elevated progesterone promotes thickening of endometrium during the luteal phase. -If no pregnancy occurs, corpus luteum degenerates, endometrium sloughs off during menstruation.

Gametes

-Eggs and sperm -Gametes are haploid (n) -In humans, formation of mature gametes begins in puberty and is controlled by hormones -The egg is much larger than the sperm

Peripheral endocrine gland hormones

-Estrogens -Progesterone -Testosterone

Estrogens

-Estrogens cause follicular growth and the LH surge

FSH

-FSH acts on cells in testes and ovaries to produce sex hormones

Oviduct

-Females have 2 oviducts (one on each side). Only one will take up the egg -If fertilization of the haploid egg occurs, it will happen in the oviduct before it gets to the uterus

Tropic hormones

-Follicle Stimulating Hormone (FSH) -Luteinizing hormone (LH)

LH surge

-Follicles contain cells that are responsive to FSH and LH. These cells secrete estrogen hormones until the largest follicle matures completely. The surviving preovulatory follicle continues to secrete estrogens. -During days 12-14, estrogen stimulation of the anterior pituitary causes a surge of LH (and FSH). The LH surge triggers ovulation, releasing the mature oocyte from ovary. *The change from negative feedback to positive feedback causes a 2 day surge in LH and FSH *Estrogen stimulation causes negative feedback

In Vitro Fertilization

-For cases in which fertilization cannot be achieved "in vivo", it can sometimes be carried out "in vitro." -A single sperm can be injected into a primary oocyte -In vitro fertilization occurs outside the body (in a test tube) -In vivo = inside the body -The embryo that is formed in the process of uniting the sperm and egg to form the zygote is implanted back into the mother, who carries out the rest of the pregnancy

Gamete formation in females

-Gamete formation in females is called oogenesis -The female body takes great care to make sure that the egg is ready to be fertilized -Because the egg is so important, only one egg matures every 28 days

Gamete formation in males

-Gamete formation in males is called spermatogenesis -In males, meiosis produces haploid sperm cells -Over time, they develop and become mature germ cells (2n)

Gamete formation

-Gamete formation is controlled almost entirely by hormones!

Where are gametes formed?

-Gametes are formed in the gonads from precursor cells called germ cells

Releasing hormone

-Gonadotropin Releasing Hormone (GRH)

Hormones

-Hormones regulate gamete formation -Hormones play critical roles in almost every aspect of mammalian sexual reproduction, both male and female

Oocytes

-Human oocytes remain arrested in Meiosis I from birth until ovulation occurs -Oocytes are immature egg cells -There are 100,000-200,000 oocytes in the female ovary -Oocytes remain immature until the female hits puberty

Human Chorionic Gonadotropin (hCG)

-If the released egg is fertilized, it will be implanted into the uterine wall. -Following implantation, a developing embryo begins to secrete human chorionic gonadotropin (hCG), which keeps the corpus luteum functional. -The corpus luteum will continue to secrete estrogens and progesterone during pregnancy. -Still want increased levels of estrogen and progesterone in order to maintain a thick lining for the embryo to implant -hCG tells the corpus luteum not to degenerate and maintain constant expression of progesterone and estrogen and maintain the thickness of the uterine lining -hCG is the only tropic hormone that is not secreted by the anterior pituitary. -The presence of circulating hCG is the basis for pregnancy testing. hCG should not be present unless an embryo is present

Follicle

-Inside the ovaries, primary oocytes are surrounded by endocrine support cells to form a primordial follicle -Each primordial follicle contains only one oocyte -Oocytes are part of a group of cells called a follicle -A follicle is a collection of cells that includes on immature egg (an oocyte), as well as supporting cells that support the immature egg (metabolic support, secrete hormones, etc.)

LH

-LH acts on cells in testes and ovaries to produce sex hormones

Which two pituitary gonadotropins coordinate male reproductive events? *See slide 22

-Luteinizing hormone and follicle-stimulating hormone *Testosterone is NOT a tropic hormone because it is not released by the anterior pituitary *LH and FSH are NOT female-specific hormones

Preovulatory follicle

-Only ONE of the growing follicles survives and fully matures -The surviving follicle is the preovulatory follicle, which contains the preovulatory oocyte -The preovulatory follicle will undergo ovulation

Tropic hormones

-Oocytes are still arrested in Meiosis I -There are no mature eggs in the ovary until one is stimulated to mature every month -Meiosis must be completed in order for the oocyte to become a true haploid egg -This is coordinated by tropic hormones (secreted by the anterior pituitary gland)

Ovulation is triggered by a surge in LH- see slide 12

-Ovulation is guided by changes in concentrations of hormones Day 0 -At day 0, there are primordial follicles. They have not matured yet -Hormones are released from the anterior pituitary Day 14 -Around day 14, there is a surge of LH. FSH is also highest during that phase -LSH and FSH act on cells in the primary follicle to stimulate growth of cells. This causes the follicle to grow -Day 14 is the midpoint of the ovulatory cycle -The surge in LH and FSH coincides with the release of the mature egg from the ovary -The anterior pituitary secretes FSH and LH. This stimulates the follicle cells to secrete their own hormones. Follicle cells are endocrine cells that respond to tropic hormones by secreting their own hormones. This hormone is ESTROGEN *Ovulation is triggered by a surge in LH and smaller surge in FSH Day 28 -Once this is over, the corpus luteum is formed and other hormones are secreted

Progesterone

-Progesterone prepares and maintains the uterus for the arrival of the blastocyst

Sperm

-Sperm is composed of DNA and a centriole

Testosterone

-Testosterone is responsible for male sex characteristics -Testosterone is secreted by cells in the testes -Male hormones also regulate the formation of gametes and their maturation -Testosterone is secreted by cells in the gonads -Testosterone is NOT a tropic hormone

Follicular phase vs. Luteal phase- see slide 16, 17

-The corpus luteum is the source of progesterone and estrogens during the luteal phase; follicle cells secrete estrogens during the follicular phase. -We have to have target cells that are responding to estrogen and progesterone -Ovulation is over now --We have a mature egg in the oviduct- fertilization can occur -Hormones prepare the uterus in case that egg is fertilized --If it's fertilized, the zygote will start to divide in the oviduct and form an embryo- the uterus must be ready to receive it -Corpus luteum is a source of both of these hormones, but you only see progesterone start at a later time point --Very low during the follicular phase, but at an elevated level throughout the luteal phase -The ovarian and uterine cycles are tightly coordinated. Hormones secreted by the ovary at different phases in the ovarian cycle trigger changes in the uterine lining -At the beginning of the cycles, the levels of estrogen and progesterone are too low to maintain the uterine lining, and menses begins -About a week into the ovarian cycle, the developing follicle increases in secretion of estrogen, and estrogen levels in the body to begin to rise --This triggers the uterine cells in the body to proliferate, and the lining becomes thicker -Just before ovulation, the level of estrogen in the body has reached its peak. Afterwards, the follicle cells within an ovary develop into the corpus luteum, a structure that releases estrogen and progesterone -The hormones maintain the uterine lining at a peak thickness and prepare it for embryo implantation -At the end of the cycle, if the egg is not fertilized or has not implanted, the corpus luteum will come down and stops releasing estrogen and progesterone --Without these hormones the uterine lining breaks down, initiating menses -Progesterone and estrogen are not triggering ovulation or acting on the mature egg. They are preparing the lining of the uterus to accept an embryo in case it is implanted or enters the uterus -Most often, it doesn't happen- egg is not fertilized, no embryo will be formed, and nothing is implanted into the uterine wall --Still, every single time, the same sequence of events happens, just in case -Takes a lot of ATP, despite the fact that there often is no pregnancy

Cumulus mass

-The cumulus mass is the mature egg cell and a few of the cells surrounding it -Openings take the cumulus mass to the uterus -Fimbria guide the cumulus mass into the oviduct

Female reproductive cycle

-The female reproductive cycle consists of two linked cycles: 1. Ovarian cycle 2. Uterine (menstrual) cycle

Early embryo

-The fertilized egg undergoes rapid mitotic division to form a ball of cells inside of the oviduct called the early embryo. -That structure ends up in the uterus and is implanted

Primordial follicle

-The follicle starts out as a primordial follicle -As gonandotropins (tropic hormones that are secreted by the anterior pituitary) start to increase in the bloodstream, the cells in the primary follicle have receptors that respond to those tropic hormones -Every month, tropic hormones stimulate some follicles to grow. Only one follicle makes it to full maturity. In a fully mature follicle, the support cells have increase in number via mitosis -Inside of the follicle, there is a fluid-filled cavity -The oocyte is arrested in Meiosis I, but supporting cells induce the primary oocyte to undergo Meiosis II. It becomes a haploid gamete and can undergo fertilization and create a zygote

Follicular phase

-The follicular phase is when the gonadotropins FSH and LH promote the growth of a small number of primordial follicles at the start of each ovarian cycle -Growth is due to mitosis of support cells *In the follicular phase of the ovarian cycle, tropic hormones are acting on the follicle

Germ cells

-The gametes are formed in the gonads from precursor cells called germ cells -Germ cells originate very early in the developing embryo -Germ cells are diploid (2n) *All germ cells undergo meiosis, even before the baby is formed

Gonadotropins

-The gonadotropins (FSH and LH) promote the growth of a small number of primordial follicles at the start of each ovarian cycle. This is called the follicular phase

Corpus luteum

-The majority of the cells that were previously surrounding the immature oocyte remain in the ovary and come together to form a new structure called the corpus luteum

Ovulation- see slide 11

-The mature oocyte undergoes ovulation -Ovulation is the release of the mature oocyte from the follicle and its entry into the oviduct -Ovulation is a physical rupture of the membrane surrounding the ovary. This releases the mature egg cell and a few of the cells surrounding it (called the cumulus mass) -The mature follicle gets large enough that it will rupture the membrane of the ovary. This is called ovulation -Ovulation is the physical release of the mature egg cell outside of the ovary -Ovulation is the release of mature eggs from the ovaries *Be able to recognize what's going on in diagram!

The ovarian and uterine cycles

-The ovarian cycle is controlled by interplay of hormones from the pituitary gland and the ovary itself -A few days before the beginning of the cycle, the anterior pituitary increases its secretion of FSH and LH -FSH and LH stimulate ovarian follicles to grow -As the follicles grow, they begin to secrete estrogen --Increasing levels of estrogen feed back on the pituitary to inhibit the release of additional FSH and LH -During the next wee, the levels of FSH and LH drop -Around day 12, the increasing levels of estrogen suddenly have the opposite effect on the pituitary gland --Instead of exerting a negative feedback on the pituitary, these hormones produce a positive feedback, stimulating the pituitary to release FSH and large amounts of LH -LH reaches a peak at day 14 of the ovarian cycle --This LH surge triggers the mature follicle to release the egg- ovulation -LH triggers the remaining follicle cells to differentiate into the corpus luteum, which secretes estrogen and progesterone -Corpus luteum remains in the ovary, secreting estrogen and progesterone, for the last two weeks of the cycle *Correspond how levels of estrogen change when corresponding to levels of FSH and LH

Ovarian cycle

-The ovarian cycle produces on mature haploid egg per cycle -The ovarian cycle causes the release of the mature haploid egg from the ovaries -The ovarian cycle repeats about every 28 days in humans -Every 28 days, a new oocyte matures to become a haploid egg cell -A woman's fertile years total about 450 ovarian cycles (30+ years)!

Uterine (menstrual) cycle

-The uterine cycle prepares the lining of the uterus (the endometrium) for the possible arrival of a developing embryo -During the uterine cycle, an environment is formed that can allow an egg to enter if it becomes fertilized

hCG

-hCG stimulates the corpus luteum to continue releasing estrogen and progesterone

Ovulation is triggered by...

...a surge in LH

Sexual reproduction depends on...

...gamete formation and fertilization

In animals, gametes are formed by...

...meiosis

The developing ovum is called an...

...oocyte

An egg is also called an...

...ovum -Every female baby has begun to form an egg cell -The egg cell is arrested in Meiosis I after the baby is born

Day 0 to Day 14 is...

...the follicular phase of the ovulatory cycle

Two phases of the ovulation cycle

1. Follicular phase -Leads to ovulation 2. Luteal phase

Summary of the hormones involved in oogenesis

Anterior pituitary -FSH (Follicle Stimulating Hormone) -LH (Luteinizing Hormone) Ovaries -Estrogens -Progesterone Cells of the growing embryo -hCG (Human Chorionic Gonadotropin)

Where do eggs mature?

Eggs mature inside follicles