homework on file
Topic I – physiological systems The Human Reproductive System and Gametogenesis 11:16 Learning Outcomes I1 Draw a diagram of the structure of the human reproductive systems I2 Outline how gametes are produced (oogenesis and spermatogenesis) with reference to meiosis, and understand the basic structure of egg and sperm I3 Understand how positive and negative feedback interact in the hormonal control of the menstrual cycle and the testes to illustrate positive and negative feedback mechanisms (endocrine control). I4 Outline fertilisation and the production of a zygote (how egg and sperm interact to form the zygote) 11:16 Introduction Humans, like other animals (and flowering plants), reproduce sexually. Sexual reproduction involves the fusion of a male gamete (sex cell) and a female gamete (fertilisation) to produce a zygote. In animals the male gametes are called spermatozoa or sperm, and the female gametes are called ova (singular ovum). 11:16 Sexual Reproduction Sexual reproduction combines genetic material from two parents to create a genetically unique offspring This increases variation within a population What is meant by variation and population in biological terms? 11:16 Variation and population Variation = Difference between cells, individual organisms, or groups of organisms of any species caused either by genetic differences or by the effect of environmental factors Population = A group of individuals of the same species inhabiting an specific area 11:16 The Reproductive Systems The male and female reproductive systems are groups of organs that work together to: Produce gametes Allow fertilisation to occur Provide a suitable environment for the growth and development of an embryo Provide suitable nutrition for a newborn baby 11:16 Male Reproductive System 11:16 https://www.youtube.com/watch?annotation_id=annotation_1675030759&feature=iv&src_vid=v22CjFYizi0&v=k60M1h-DKVY 11:16 11:16 Female Reproductive System 11:16 Female – front view 11:16 Female side view https://www.youtube.com/watch?v=9rs2gNchQig 11:16 Spermatogenesis Sperm (spermatozoa) are made in the testes in seminiferous tubules. Spermatogenesis: puberty old age. Occurs most efficiently at 35 °C, so the scrotum is outside the body. 11:16 Multiplication: germinal epithelial cells divide continuously from puberty → old age. After mitosis one daughter cell remains undifferentiated and divides again. The other cell is a spermatogonium. 11:16 Growth: The spermatogonia grow and accumulate the nutrients needed for later stages. When growth has finished the cells are called primary spermatocytes. 11:16 Meiosis: Primary spermatocytes become secondary spermatocytes (after the first meiotic division) and then spermatids (after the second meiotic division) 11:16 Differentiation: Spermatids differentiate into spermatozoa 11:16 Summary The developing cells are closely associated with Sertoli cells (nurse cells). Provide the developing sperm cells with nutrients & secrete substances involved in hormonal control of the testes. https://www.youtube.com/watch?v=krSMZDsjLuU 11:16 Oogenesis Oogenesis starts in the ovaries of the foetus. The primordial or germinal epithelial cells divide to form oogonia. These then grow to form primary oocytes. Primary oocytes enter prophase of meiosis I, then division stops. 11:16 Oogenesis A girl is born with about 2 million primary oocytes Have a single layer of granular cells around them forming primordial follicles. Around 200,000 left at puberty & about 450 mature fully & are released over a woman’s lifetime. The process of oogenesis starts again at puberty & occurs in cycles as part of the female menstrual cycle (unlike spermatogenesis). 11:16 Oogenesis A mature ovum is the product of a two week development process in the ovary. At the start of the process a number of primordial follicles accumulate small clusters of granular cells (as the granular cells divide) to become primary follicles. Usually just one of these follicles develops to maturity in each cycle. A mature follicle is called a Graafian Follicle. 11:16 The primary oocyte gains nutrients from secretions by the granular cells and grows to about 100 times its original size. The granular cells are separated from the primary oocyte by the zona pellucida (cytoplasmic extensions of the granular cells penetrate the zona pellucida to make contact with the primary oocyte). 11:16 Primary Oocyte 11:16 As the granular cells divide by mitosis they form an expanding cluster around the primary oocyte, and a new layer of cells called the theca (theca = ‘box’ in Greek) develops from ovarian connective tissue. These form around the outside of the follicle. Within the cluster of granular cells is a fluid-filled space called the antrum. 11:16 Antral Follicle 11:16 The granular cells secrete fluid into the antrum which causes it to expand so that the primary oocyte is just suspended by a strand of granular cells. The whole follicle swells as the antrum grows. It is now called a Graafian or tertiary follicle. 11:16 Graafian (tertiary) Follicle 11:16 Meiosis – continued… Just before ovulation, the swollen follicle can be observed as a bump on the surface of the ovary. About this time, the first meiotic division is completed (primary oocyte divides to form two haploid cells). The cytoplasm is not divided equally between the two daughter cells –the secondary oocyte retains almost all of the cytoplasm, whilst the other daughter cell, called a polar body, is little more than a nucleus and will break down. The second meiotic division proceeds as far as metaphase, but is not completed until fertilisation. 11:16 11:16 Ovulation At ovulation, the secondary oocyte is released when the outer lining of the ovary splits along a bulge created by the mature follicle. Antral fluid flows out of the ruptured follicle, carrying with it the secondary oocyte, zona pellucida and a few granular cells. After ovulation, the rest of the follicle forms the corpus luteum in the ovary. 11:16 https://www.youtube.com/watch?v=vyiTbTH9dMk 11:16 11:16 Similarities SpermatogenesisOogenesis Both start with germ cells / germinal epithelium of the gonad Both start with mitosis to produce many cells Both involve cell growth before mitosis Both involve meiosis / reduction division / forms haploid cells 11:16 Differences 11:16 Structure of Spermatozoan 11:16 Structure of Spermatozoan The sperm head is about 5 µm long, the total length head to tail is about 60 µm 11:16 Structure of Ovum 11:16 Structure of Ovum The egg cell is about 120 µm in diameter. Its mass is about x80,000 the mass of a sperm cell 11:16 Release of hydrolytic enzymes from the acrosome, which digest the corona radiata and the zona pellucida surrounding the secondary oocyte. Note: dozens of spermatozoa will release hyaluronidase enzyme. Only one sperm, with an intact acrosome, actually fuses with the egg nucleus, through release of acrosin, which allows it to break through the zona pellucida. Fertilisation in the Oviduct Capacitation - receptors on the spermatozoan membrane bind to specific female chemicals, increases cell motility, triggers the acrosome reaction. 11:16 Post sperm penetration: rapid sequence of events changes the nature of the zona pellucida - the cortical reaction. Cortical granules release enzymes by exocytosis. The zona pellucida gets harder and thicker and forms a structure - “fertilisation membrane”. The enzymes also destroy the sperm-binding receptors. The cortical reaction prevents polyspermy. 11:16 Post Fertilisation Completion of meiosis - haploid ovum nucleus (female pronucleus) and a second polar body. The nuclear envelope around the sperm breaks down and the male chromosomes de-condense. A new nuclear envelope develops (male pronucleus), the chromosomes re-condense. Microtubules form between the male and female pronuclei pulling them towards each other until… Nuclear envelopes fuse to form a diploid nucleus - the zygote is then complete. 11:16 Implantation The diploid zygote continues to develop through mitotic divisions, until it reaches a stage called the blastocyst. This structure becomes implanted into the endometrium (uterine lining), where it can connect to the mother’s bloodstream, eventually forming the placenta. 11:16 Overview of Fertilisation 11:16 1 The sperm binds to the surface of the egg 2 The acrosome releases its contents 3 Hydrolytic enzymes digest the zona pellucida 4 5 6 7 8 The sperm nucleus penetrates the egg cell membrane and enters cytoplasm The cortical reaction stops any other sperm from penetrating the egg The entry of the sperm nucleus stimulates the completion of meiosis in the egg, forming the haploid ovum nucleus Male and female pronuclei fuse to form a zygote The zygote divides by mitosis to form a blastocyst. The blastocyst takes 6-7 days to reach the uterus where it implants in the endometrium. Topic I – Physiological Systems Hormonal Control of the Testes & Menstrual Cycle 11:16 Learning Outcomes I3 Understand how positive and negative feedback interact in the hormonal control of the menstrual cycle and the testes to illustrate positive and negative feedback mechanisms (endocrine control). 11:16 Hormones A hormone is: A chemical substance produced in the body that controls and regulates the activity of certain cells, tissue or organs. Hormones are secreted by specific glands. 11:16 Regulation of Hormones The glands are controlled directly by the nervous system as well as other hormones produced by other glands. By regulating the functions of organs in the body, these glands help to maintain the body’s homeostasis. Homeostasis is: Maintaining a (roughly) constant internal environment inside the body 11:16 Principles of Endocrine Control The hypothalamus and the pituitary gland control the endocrine system as a whole The pituitary gland secretes a number of hormones including: Follicle Stimulating HormoneLuteinising Hormone Stimulates the follicle cells of the testes & ovaries to produce gametes (can be called Interstitial Cell Stimulating Hormone in males) stimulates the release of testosterone in the testes, and causes ovulation in females 11:16 Principles of Endocrine Control Hormonal control by the endocrine system often involves a control mechanism called negative feedback. https://www.youtube.com/watch?v=Iz0Q9nTZCw4 Negative feedback involves the regulation of a factor around a set-point. Any change away from the set point triggers events that reverse the effects of the change. Negative feedback promotes stability as it regulates factors around a set point, so is commonly used as a control mechanism in the body to maintain homeostasis. 11:16 Positive Feedback Control Positive feedback is a control mechanism whereby a change in conditions triggers events which reinforce the change. It does not promote stability, and is much less common than negative feedback. It only occurs for a set time as part of a more complex control system. 11:16 Hormonal Control of the Testes Germinal epithelial cells: divide to form the spermatogonia (starts in puberty, carries on throughout adult life. It declines but doesn’t stop even in old age). Sertoli Cells: regulate the development of spermatozoa. (Sometimes called nurse cells). Interstitial Leydig Cells: secrete testosterone Gonadotrophin releasing hormone 11:16 Male Reproductive Hormones 11:16 The hypothalamus secretes the hormone GnRH which stimulates the anterior pituitary gland to produce and secrete LH and FSH. FSH causes the Sertoli cells to stimulate spermatogenesis to occur, and LH stimulates the Leydig cells to secrete testosterone. Testosterone works with works with FSH to stimulate spermatogenesis. FSH also causes the Sertoli cells to secrete another hormone called inhibin. Inhibin and testosterone are involved in negative feedback mechanisms that control spermatogenesis. https://www.youtube.com/watch?v=GC58kOcsx9U 11:16 The Female Menstrual Cycle Starts in puberty (~12/13 years) , ends at menopause (~45/55 years) Cycle has variable length (24-44 days) – average is 28 days Purposes of the cycle: Ensure that a secondary oocyte is available to fertilise each month Post-fertilisation, that