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Chapter 26

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Biology 1225
Michael Butler

Chapter 26: Reproduction and Development (27) Overview Animals can reproduce either sexually or asexually. Human beings reproduce sexually. The human reproductive system consists of a pair of primary reproductive organs (testes in males, ovaries in females), accessory glands, and ducts. Testes produce sperm; ovaries produce eggs. Both reproductive organs release sex hormones in response to signals from the hypothalamus and pituitary gland. Human males continually produce sperm from puberty onward, whereas human females are fertile on a cyclic basis. Each month during their reproductive years, an egg is released from an ovary, and the lining of the uterus is prepared for pregnancy. Sexual intercourse is the natural mechanism that brings together sperm and eggs, permitting fertilization and the conception of a new individual (an embryo). Embryonic and fetal development is a continuum in which each stage of development builds on the tissues and structures formed during the stage that preceded it, thereby establishing a more complex body than was present at the end of the previous stage. A key event in early embryonic development is the formation of three distinctive layers: endoderm, mesoderm, and ectoderm. Every tissue of the body arises from one of these layers. Interactions between these layers stimulate the development of specific organs Responsibilities All of this chapter contains valuable material, but for section 26.6, only the part on cleavage and implantation is testable. Sections 26.7 and 26.8 will not be tested. In particular, be able to trace the pathway from the formation and release of sperm in the seminiferous tubules of the testis of a male on its way to the union and fertilization of an egg in a female's uterine tube. The textbook index refers to vasectomy and tubal ligation as being on page 517, but as far as I can tell they are not, but these important contraception measures are briefly discussed on page 523. In a vasectomy the vas deferens of a male is surgically cut so that sperm no longer appear in the semen, but contrary to a popular misconception (no pun intended) viable sperm are still produced, they just do not end up in the sperm. In a tubal ligation a woman's oviducts are tied to block exit of the egg. Human Sexual Reproduction and Development The male reproductive system Sperm production is located in the testes (the male gonads), in the seminiferous tubules. The seminiferous tubules are lined with spermatogonia - the diploid cells that differentiate in a lengthy process to form mature haploid sperm. The spermatogonia undergo mitosis to become primary spermatocytes, these become secondary spermatocytes after meiosis I, and become spermatids after meiosis II. The spermatids develop into sperm and are nourished by large Sertoli cells lining the tubules, which also exert control over the sperm developmental process. A mature sperm has a flagellum loaded with mitochondria to provide energy for propulsion, and a head that contains the genome. The tip of the head holds a special lysosome called an acrosome that contains hydrolytic enzymes that will assist in penetration of the egg at fertilization. Production of sperm is controlled by luteinizing hormone (LH), Follicle stimulating hormone (FSH) and testosterone (this last hormone is produced in Leydig or interstitial cells found in the testes). LH and FSH are released by the anterior pituitary under control of the hypothalamus, which releases gonadotropic releasing hormone (GnRH), which in turn stimulates the anterior pituitary to release LH and FSH. LSH and FSH stimulate Leydig cells to release testosterone, which stimulates sperm production. If the blood level of testosterone is too high this inhibits GnRH release, which in turn inhibits LH and FSH release and this in turn reduces the signal for the testes to produce testosterone - negative feedback. A hormone called inhibin is released by Sertoli cells when too many sperm are present, and this also inhibits GnRH release. Sperm complete their maturation and are stored in a coiled pair of ducts seated on each testicle called the epididymis. During sexual activity sperm travel down the vas deferens into the seminal vesicle which adds sugars and other material to form semen. Before sperm are ejaculated during intercourse, glands mix their secretions with the sperm to form semen, these glands are the seminal vesicles, bulbourethral glands, and prostate glands. The secretions include nutrient materials for the swimming sperm, chemicals that activate the sperm and that neutralize acids in the vaginal tract. The female reproductive system. All of a woman’s eggs are present at her birth but are immature (they developed from precursor cells called oogonia, and they are called primary oocytes). The primary oocytes are stored in the ovaries (the female gonads). All of these primary oocytes are “frozen” at Meiosis I. An oocyte matures (becoming a secondary oocyte) and is released from the ovary during the menstrual cycle, which is of approximately 28 day length, and is under control of the hypothalamus via the anterior pituitary. Many reproductive biologists recognize three phases to the menstrual cycle: The follicular phase; menstruation occurs, the endometrium regenerates, an oocyte matures. Ovulation; an oocyte is released form the ovary. The luteal phase; hormones cause the endometrium to thicken and prepare for a possible pregnancy Follicles in the ovary contain the secondary oocyte. Under the influence of LH and FSH released by the anterior pituitary under control of GnRH from the hypothalamus, an oocyte begins to mature and grow in size. The levels of FSH and LH peak shortly before the mid point of the cycle (particularly LH), and the high level of LH is the trigger that induces ovulation - release of the mature oocyte - now called a secondary oocyte, from the swollen follicle. The cells of the burst follicle left behind when this secondary oocyte leaves, continue to develop and become a body called the corpus luteum This body secretes estrogen - the hormone that prepares the lining of the uterus (the endometrium) for a possible pregnancy. The corpus luteum also secretes progesterone - the hormone that maintains a pregnancy and also prepares the reproductive tract by causing a mucus plug to form at the cervix. Progesterone also causes the thickening of the endometrium, the layer of cells in the uterus that is heavily supplied with blood vessels and will form the maternal contribution to the placenta. While the corpus luteum persists, for about twelve days, the hypothalamus signals a decrease in FSH and this will prevent other follicles from maturing. If fertilization and implantation of the fertilized egg (the zygote) does not occur then the corpus luteum is destroyed, and the menstrual cycle soon ends as a result of a drop in estrogen and progesterone levels the corpus luteum was releasing. With the drop in these last two hormone levels the endometrium sloughs off, which is evidenced by the menstrual flow (menses) as the lining materials are expelled. When sperm meets egg, generally in the upper oviduct, fertilization occurs, the sperm enters. The sperm passes through a loose layer of follicle cells called the corona radiata that surrounds and then penetrates through the extensive inner layer that coats the egg cell membrane, the zona pellucida- with assistance from the enzymes released from the acrosome. The sperm then crosses the egg cell membrane and enters the egg cytoplasm. Once the head of the sperm containing the nucleus, is inside the egg, it triggers changes in the egg cell membrane that prevent other sperm from entering, and it also triggers the egg cell to complete meiosis, the two nuclei now fuse and a diploid nucleus forms Fetal Development The fertilized egg is now a diploid zygote. Mitotic cell divisions generate new cells (a process called cleavage) from the single diploid zygote cell that results after fertilization, the result is a solid ball of smaller cells called a morula. The repeated cell divisions that occur during cleavage do not involve synthesis of new cytoplasm, so the result is many cells to form a morula that is the same size as the original zygote. Next a blastocyst forms as a fluid filled space appears within the morula. The blastocyst is a layer of surface epithelium enclosing a clump of cells (the inner cell mass) which will begin to differentiate into the embryo, and the blastocyst will grow in size both as new cells are formed. Very shortly after its formation the blastocyst implants in the endometrial layer of the mothers uterus. Gastrulation then occurs, the cells rearrange and form the gastrula, the basis of the adult body plan – with the appearance of the three germ layer tissues called endoderm, mesoderm and ectoderm. The gastrula forms an indentation, called a blastopore, much the same way as if a finger is gently pushed into a balloon without bursting it, the small cavity so formed enlarges, and the cells that are thus turned inwards, into the interior, will become the lining of the future digestive tract. Endoderm in the adult becomes gut epithelia and lining of the respiratory tract etc, mesoderm becomes cartilage, bone, muscle, blood etc, ectoderm becomes nervous tissue, epithelia of sensory organs, epidermis etc. All of this busy very early process of embryonic development is taking place in the embryo at and around the time it implants into the endometrium. Soon after the blastocyst begins the formation of its contribution to the to the placenta, a stalk like connection to the area of the uterus where the blastocyst implanted. This placenta forms as an intimate association of maternal and embryo tissues such as small blood vessels and membranes, but while the blood vessels are intimately associated, maternal and fetal blood (normally) does not mix. The placenta allows carbon dioxide and metabolic wastes to be transferred from the growing fetus to the mother, and for the delivery of nutrients and oxygen from the mother to the fetus. There are a number of extra-
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