AP Biology Animal Form and Function

AP Biology Animal Form and Function Reproductive System

Reproductive System—Production of Eggs and Sperm • Characteristics that distinguish the sexes: • Primary sex characteristics—structures directly involved in reproduction (ovaries, uterus, testis) • Secondary sex characteristics—noticeable physical characteristics that differ between males and females (facial hair, deepness of voice, breasts, and muscle distribution)

Meiosis • In order for the newly formed organism to have the same number of chromosomes as its parents, the chromosome number must be halved when the egg and sperm are made. • This is accomplished by a process called meiosis. Then, when one sperm successfully fertilizes the egg, the diploid chromosome number is restored Each sperm cell surrounding this egg cell is trying to enter it and leave its packet of genetic information in the form of chromosomes containing DNA. Only one will be successful.

Male Reproductive Organs • Sperm cells are produced in the testis. Males have two testes, located in a sac called the scrotum. • A portion of the testis called the seminiferous tubules is where the sperm cells are actually made.

Spermatogenesis • Sperm cells begin as diploid body cells located in the testes called Primary Spermatocytes. • After dividing in Meiosis I, they are now haploid cells called Secondary Spermatocytes • After dividing again in Meiosis II, they are called spermatids. • They then mature into sperm cells in the epididymis

Interstitial Cells in Testes • Most of the cells in the testes are destined to become sperm cells by meiosis. • In between these cells are other cells, called Interstitial Cells. • These are the structures that produce the hormones involved in the male reproductive system.

Epididymis • After the sperm cells are formed by meiosis, they move into the epididymis—the coiled region extending from the testes. • Here, the spermatids mature into sperm cells.

The Journey of the Sperm • From the epididymis, the sperm moves through the vas deferens to the urethra • The urethra is the tube that releases both sperm and urine (not at the same time!) The seminal vesicles dump fluids into the vas deferens as the sperm cells pass through. This fluid adds fructose for energy, prostaglandins (which stimulate uterine contractions) and mucus, which helps the sperm swim more efficiently. • The prostate gland adds a basic • (pH >7) liquid to the mix to help combat the acidity of the vaginal region of the female.

Female Reproductive System • Ovary—the site of egg production • The egg leaves the ovary before it is fully mature and enters a structure called the oviduct (aka: Fallopian Tube) • The oviduct carries the egg from the ovary to the uterus.

Fertilization • When fertilized by a sperm cell in the oviduct (fallopian tube), after several days travelling in the tube, the egg will usually attach itself to the inner wall of the uterus (called the endometrium). The uterus connects to the vaginal opening via a narrowed portion called the cervix. The narrow opening through which the sperm travels up to the Fallopian tube is the vagina.

The Environment Through Which the Sperm Must Travel • As the sperm cells enter, they must survive the somewhat hostile environment that the female body presents • Its task is to find its way to the fallopian tube, where it must meet the egg and penetrate its outer surface to achieve successful fertilization. The sperm works its way through the vaginal region, up through the cervix, through the uterus, and into the fallopian tube. If the timing is right, an egg is in the tube and the sperm can fertilize the egg to produce a diploid zygote.

Oogenesis Oogenesis—the development of the egg, begins while the female is still an unborn embryo. Some of the fetal cells in the ovary of the embryo begin meiosis and stop at Prophase I. These are called Primary Oocytes and they stop further meiosis and development until the female enters puberty.

Oogenesis After puberty begins, each month, one of the Primary Ooctyes completes meiosis I. This produces a polar body and one Secondary Oocyte, a haploid cell. As the menstrual cycle continues, ovulation frees the Secondary Oocyte to travel into the Fallopian Tube.

Oogenesis The Secondary Oocyte travels down the Fallopian Tube and, if a sperm cell is present, may be fertilized. If a successful fertilization occurs, the secondary oocyte enters Meiosis II, again producing a Polar Body, as well as an egg which combines with the sperm to form an embryo.

Embryonic Development • Embryonic development begins as soon as the egg is fertilized to produce a diploid zygote. • This zygote then divides by mitosis many times without increasing the size of the embryo. • During these cleavage divisions, cytoplasm is distributed unevenly to the daughter cells, but genetic information is distributed equally. • Different cells will play different roles in the body of the future embryo.

Embryonic Development By Day 4 after fertilization, the embryo is a ball of cells called a morula. As it undergoes the next round of divisions, fluid will fill the middle and it will form a hollow ball of cells called a blastula. The blastula has two parts: the inner cell mass which becomes the embryo and a trophoblast, which becomes the placenta.

The Trophoblast • The trophoblast aids in attaching the embryo to the endometrium (the inner wall of the uterus). • It also produces human chorionic gonadotropin (HCG), which maintains the endometrium by ensuring the continued production of progesterone.

Gastrulation • The next major stage of embryonic development is gastrulation. • During gastrulation, cells separate into three primary layers called germ layers, which eventually give rise to the different tissues of an adult.

Endoderm, Mesoderm, Ectoderm • The three germ layers of the embryo are the endoderm, mesoderm and ectoderm. • Endoderm: inner germ layer: gives rise to the inner lining of the gut and the digestive system, liver, thyroid, lungs, and bladder • Mesoderm: intermediate germ layer: gives rise to muscle, the circulatory system, reproductive system, excretory organs, bones, and connective tissues • Ectoderm: outer germ layer: gives rise to the nervous system and skin, hair and nails.

How Do Cells Know What to Do? Induction: the influence of one group of cells on the development of another through physical contact or chemical signaling. Hans Spemann, a German embryologist showed, for example, that the cells of the notocordinfluence the development of the neural plate (which eventually becomes the nervous system). When the notocord was transplanted into a different part of an embryo, the neural plate grew in the new location.

Homeotic Genes • Homeotic genes regulate or “direct” the body plan of organisms. • For example, a fly’s homeotic genes help determine how its segments will develop and which appendages should grow from each segment. • Scientists interfering with the development of these animals have shown that mutations in these genes can lead to growth of organs where they should not be.

Homeotic Genes • These genes regulate the body plan of animals. • The DNA sequence of a homeotic gene that tells the cell where to put things is called the homeobox. • The homeobox is similar in organism to organism and has been found to exist in a variety of organisms: birds, humans, fish, and frogs.

Reproductive Hormones • The hormones involved in human reproduction are LH, FSH, estrogen, progesterone, and testosterone. • Estrogen and progesterone: continually circulate in the female bloodstream. The hypothalamus monitors these levels to determine when to release certain hormones. • When estrogen and progesterone are low, the hypothalamus secretes GnRH (gonadotropin-releasing hormone), which travels to the pituitary gland to induce the release of FSH and LH.

FSH and LH • FSH: Follicle-stimulating hormone (induces the development of the follicle that surrounds the primary oocyte during its development); it also causes the follicle to release estrogen • LH: luteinizing hormone (initiates ovulation—the release of a secondary oocyte from the ovary)

Effects of Birth Control Pills

Ovulation Caused by LH Surge • The LH surge causes further release of estrogen and progesterone from the follicle (which has now become a structure called the corpus luteum). The corpus luteum induces the thickening of the endometrium, the site of future egg attachment. At this point, the levels of estrogen and progesterone elevate enough so that the hypothalamus cuts off prodution of GnRH so that the LH and FSH levels drop back down.

If Fertilization Occurs…. • If fertilization has occurred in the fallopian tube, and if the embryo attaches successfully to the uterine wall, HCG will be secreted, which works to keep the corpus luteum alive. • As a result, estrogen and progesterone will remain high and will keep the endometrium intact. Left: Implanted Embryo at 6 days; Right: Implanted Embryo at 6 weeks

AP Biology Animal Form and Function