Topic 11 Animal Physiology Notes
IB Biology
11.4 Sexual Reproduction
Nature of Science: Assessing the risks and benefits associated with scientific research: the risks to human male fertility were not adequately assessed before steroids related to progesterone and estrogen were released into the environment as a result of the use of the female contraceptive pill.
Understanding Statements:
 Spermatogenesis and oogenesis both involve mitosis, cell growth, two divisions of meiosis and differentiation. o Oogenesis – production of eggs in the ovaries.  Starts in the ovaries of a female fetus. o Germ cells (oogonia) in the ovary divide by mitosis and the cells formed distribute themselves through the cortex of the ovary. o Those cells start to divide by meiosis. By the time the baby is born, cells are still in the first division of meiosis and a single layer of cells called follicle cells has formed around them. No further development takes place until after puberty o Primary follicle = the cell that has started to divide by meiosis, together with the surrounding layer of follicle cells. o There are about 400,000 primary follicles in the ovary at birth. o At the start of each menstrual cycle, a small batch of primary follicles are stimulated to develop by FSH. Usually only one goes on to become a mature follicle, containing a secondary oocyte. o Spermatogenesis – production of sperm. o Occurs in the testes, which are composed of a mass of narrow tubes, called seminiferous tubules, with small groups of cells filling the gaps between the tubules
(interstitial cells or Leydig cells). o Seminiferous tubules are made of cells. The outer layer of cells is called the germinal epithelium. This is where sperm production begins. Cells in various stages of sperm production are found inside the germinal epithelium, with the most mature stages closest to the fluid filled center of the seminiferous tubule. o Cells that have developed tails are called spermatozoa. o Also in the wall of the tubule are large nurse cells called Sertoli cells
 Processes in spermatogenesis and oogenesis result in different numbers of gametes with different amounts of cytoplasm o Spermatogenesis – each mature sperm consists of a haploid nucleus, a system of movement, and a system of enzymes and other proteins that enable the sperm to enter the egg. The process of sperm differentiation eliminates most of the cytoplasm. From puberty onwards, the testes produce sperm continuously. At any time there are millions of sperm at all stages of development. o Oogenesis – the egg must increase its cytoplasm. All of the requirements for beginning the growth and development of the early embryo must be present in the egg. In females, the first division of meiosis produces one large cell and one very small cell. The small cell is the first polar body which eventually degenerates. The large cell goes on to the second division of meiosis, completing it after fertilization. Again one large cell and one very small cell are produced. The small cell is the first polar body and it also

degenerates and dies. Only the large cell, which is the gamete, survives. The result is that the egg is much larger than the sperm cell. The process of egg formation happens once per menstrual cycle in humans and usually only one egg per cycle is produced.
During the years from puberty to menopause only a few hundred female gametes are likely to be produced.
Spermatogenesis Oogenesis
SKILL: Annotation of diagrams of seminiferous tubule and ovary to show the stages of gametogenesis
(pg 501)
SKILL: Annotation of diagrams of mature sperm and egg to indicate functions. (pg 502)

 Fertilization involves mechanisms that prevent polyspermy. o Fertilization = union of sperm and egg to form a zygote. o The membranes of sperm have receptors that can detect chemicals released by the egg, allowing directional swimming towards the egg. o Once the egg is reached…
1.
Acrosome reaction – the zona pellucida is a coat of glycoproteins that surrounds the egg. The acrosome is a large membrane-bound sac of enzymes in the head of the sperm. In mammals, the sperm binds to the zona pellucida and the contents of the acrosome are released. The enzymes from it digest the zona pellucida.
2.
Penetration of the egg membrane – The acrosome reaction exposes an area of membrane on the tip of the sperm that has proteins that can bind to the egg membrane. The first sperm that gets through the zona pellucida binds and the membranes of the egg and sperm fuse together. The sperm nucleus enters the egg cell.
3.
The cortical reaction – sperm activates the egg – which releases the contents of the cortical granules from the egg by endocytosis. In mammals, the cortical vesicle enzymes result in the digestion of binding proteins so that no further sperm can bind.
 Fertilization in animals can be internal or external. o External fertilization – aquatic animals often release their gametes directly into water in a process that will lead to fertilization outside of the female’s body. Risks – o Internal Fertilization – terrestrial animals – ensures that sperm and ova are placed in prolonged close proximity to each other. Once eggs are fertilized, the developing embryo can be protected inside the female.
 Implantation of the blastocyst in the endometrium is essential for the continuation of
pregnancy. o After fertilization in humans, the fertilized ovum divides by mitosis to form two diploid nuclei and the cytoplasm of the fertilized egg cell divides equally to form a two cell embryo. These two cells replicate their DNA, carry out mitosis and divide again to form a four-cell embryo. The embryo is about 48 hours old at this point. Further cell divisions occur, but some of the divisions are unequal and there is also migration of cells, giving the embryo the shape of a hollow ball. It is called a blastocyst. At 7 days old, the blastocyst consists of about 125 cells and it has reached the uterus, having been moved down the oviduct by the cilia of cells in the oviduct wall. At this age the zona pellucida, which has surrounded and protected the embryo, breaks down. The blastocyst has used up the reserves of the egg cell and needs an external supply of food. It obtains this by sinking into the endometrium or uterus lining in a process called implantation. The outer layer of the blastocyst develops finger-like projections allowing the blastocyst to penetrate the uterus lining. The embryo grows and develops rapidly and by eight weeks has started to form bone tissue.
 hCG stimulates the ovary to secrete progesterone during early pregnancy. o Pregnancy depends on the maintenance of the endometrium, which depends on the continued production of progesterone and estrogen. These hormones prevent the degeneration of the uterus lining which is required to support the developing fetus.
Early in pregnancy, the embryo produces human chorionic gonadotropin – hCG – which stimulates the corpus luteum in the ovary to continue to secrete progesterone and estrogen.

 The placenta facilitates the exchange of materials between the mother and embryo. o Humans are placental mammals. o The placenta is needed because the body surface area to volume ratio becomes smaller as the fetus grows larger. o Placenta is made of fetal tissues, in intimate contact with maternal tissues in the uterus wall. Fetus also develops amniotic sac which contains amniotic fluid that supports and protects the developing fetus. o Placental villi increase in number during pregnancy to cope with the increasing demands of the fetus for the exchange of materials with the mother. o Maternal blood flows in the inter-villous spaces around the villi. Fetal blood circulates in blood capillaries, close to the surface of each villus. The cells that separate maternal and fetal blood from the placental barrier (selectively permeable).
 Estrogen and progesterone are secreted by the placenta once it has formed. o By the ninth week of pregnancy, the placenta has started to secrete estrogen and progesterone in large enough quantities to sustain the pregnancy, and the corpus luteum is no longer needed for this role.
 Birth is mediated by positive feedback involving estrogen and oxytocin. o During pregnancy, progesterone inhibits secretion of oxytocin by the pituitary gland and also inhibits contractions of the muscular outer wall of the uterus (myometrium). At the end of pregnancy, hormones produced by the fetus signal to the placenta to stop secreting progesterone, and oxytocin is therefore secreted. o Oxytocin stimulates contractions of the muscle fibres in the myometrium. These contractions are detected by stretch receptors, which signal the pituitary glands to secrete more oxytocin.
APPLICATIONS: