A-Level Biology Human Reproduction PDF

A-Level Biology MA Micallef HUMAN REPRODUCTION Topic Subject Content Knowledge expected 9.5 Human 9.5.1 Male human Structure and function of reproduc...

A-Level Biology MA Micallef HUMAN REPRODUCTION Topic Subject Content Knowledge expected 9.5 Human 9.5.1 Male human Structure and function of reproductive system. Histology of reproduction reproductive testis. Spermatogenesis. system 9.5.2 Female Structure and function of reproductive system. Histology of human ovary. Oogenesis. The menstrual cycle. reproductive system 9.5.3 Fertilisation Transfer of male gametes to female gametes, leading to fertilization. Capactiation; sperm penetration in oocyte to include acrosome reaction. 9.5.4 Human Cleavage, morula, blastula formation, implantation, Leading to development the formation of the placenta. 9.5.5 Human Structure and functions of human placenta. Placenta 9.5.6 Birth Three stages of parturition. 9.5.7 Lactation Colostrum, nutritional importance and passive immunity. 9.5.8 Hormones Roles of luteinising hormone (LH) also known as interstitial cell stimulating hormone (ICSH) in males, follicle stimulating hormone (FSH), testosterone, oestrogen, progesterone, human choronic gonadotropin (hCG), oxytocin, prolactin and prostaglandins (role of prostaglandins in menstruation only). 1) DIOECISM IN HUMANS  having male and female organisms  is more common in animals than in plants as male and female animals can move about and therefore there is less wastage of gametes.  In some animals such as reptiles, sex determination is environmentally controlled.  In mammals, this is genetically controlled. Embryonic gonads are indifferent during early development. The presence of a Y chromosome converts the gonads into testes. There is a Sex- determining region on the Y chromosome (A key gene termed SRY) that initiates the whole process.  In organisms such as humans the male and female organisms have complementary reproductive organs to enable internal fertilization of gametes. Human reproduction Page 1 of 24 A-Level Biology MA Micallef 2) THE MALE REPRODUCTIVE SYSTEM  Function i) Sperm production and the delivery of sperms in the vagina for fertilization during copulation. ii) Production of the hormone testosterone which gives rise to the secondary sexual characteristics.  Testosterone: - initiates and maintains secondary sexual characteristics. - controls the continuous production of sperms along with FSH.  Structure: - Consists of: a) testes (singular = testis) b) a system of genital ducts c) accessory glands d) the penis Human reproduction Page 2 of 24 A-Level Biology MA Micallef  Inside the scrotum:  Testes (pl: testis/testicles) - Are contained within the Scrotum (flap of skin) - Sperm development requires a temperature lower than that of the body (3o lower) which is achieved by suspension of the testes outside the body in the scrotum. - Testis are held inside the body during foetal development and eventually move downwards. - They are the site of sperm and testosterone production  Seminiferous tubules: - are tubules coiled inside the testis - about 1,000 present each being about 50cm long. These tubules would be more than 500 metres long if placed end to end. The large surface area allows each testis to produce thousands of sperm every day. - Lining the seminiferous tubule there are: i) Sertoli cells – have a supportive function. They nurse developing germ cells and help convert spermatids (immature sperm) into spermatozoa by engulfing their extra cytoplasm. These respond to the hormone FSH. ii) Germinal Epithelial Cells (or Germ Cells) - undergo differentiation to become mature spermatozo a (sperm) - Among the seminiferous tubules there are other interstitial cells called Leydig cells which synthesize and secrete Testosterone in response to Luthenizing Hormone (LH in males is also called ICSH) Human reproduction Page 3 of 24 A-Level Biology MA Micallef  Genital Ducts System:  Epididymis - 1mm in diameter and 6m long - consists of coiled tubes that store the sperms until fully mature and supplies them with nutrients - highly coiled and very compact tube - upon leaving it, the spermatozoa become motile  Vas Deferens (sperm duct) - 45cm long tube - the seminal vesicles empty their secretion into the vas deferens - has smooth muscle in its wall which helps to move sperm down its length by peristalsis over the ureter and passes behind the urinary bladder - as the vas deferens continues towards the base of the urinary bladder, it collects secretions from the prostate gland and finally empties into the urethra  Urethra - a single tube extending from the urinary bladder to the tip of the penis - conducts urine from the bladder during urination and provides path for sperms during copulation  Accessory Glands:  Seminal vesicles - contribute slightly more than half the seminal fluid volume - secrete a fluid which: a) contains fructose (a sugar used by sperm for energy) b) is alkaline (neutralises acidity of urine remaining in the urethra)  Prostate gland - lies below the bladder, where it surrounds the urethra and 2 ejaculatory ducts adds about 20 - 30% to the volume of semen - secretes a fluid which a) provides enzymes that aid in activating the movements of sperm thus helping them to reach and fertilize an ovum. b) maintains the proper pH for sperm, being alkaline  Cowper’s Glands (or Bulbourethral glands) - small paired glands located below the prostate gland - empty contents into the urethra through ducts located close to the base of the penis - secretions provide a) lubricants for sexual intercourse b) nutrients that sperm require for motility c) help to neutralise the acidity of any urine remaining in the urethra Human reproduction Page 4 of 24 A-Level Biology MA Micallef  Urethral glands (not in syllabus) - numerous bulges found in the walls of the urethra, some of which are branched chambers - connected to the urethra by short ducts - produce mucus that serves as a lubricant to semen  The Penis – Glans Penis: - Consists of three cylindrical bodies of spongy tissue surrounded by sheaths of connective tissue and skin. - The cylindrical bodies are called corpora cavernosa and corpus spongiosum. - The urethra passes through the corpus spongiosum. - The function of the corpus spongiosum in erection is to prevent the urethra from getting pinched and closing, thereby maintaining the urethra as a viable channel for ejaculation. To do this, the corpus spongiosum remains pliable during erection while the corpora cavernosum penis becomes engorged with blood. - Continued stimulation during sexual intercourse cause a series of intense nervous, glandular and muscular reactions to occur: i) The muscles in the walls of the epididymis, vas deferens and seminal vesicles contract in a series of peristaltic contractions. ii) The urethral sphincter closes, preventing the back-flow of sperm into the bladder and passage of urine into the urethra. iii) Semen is forced into the urethra, driven by contractions of the muscles surrounding the tubules. iv) During erection the tissues inside the penis fill with blood. Parasympathetic nerves release nitric oxide (NO), which stimulates dilation of arteries. v) Ejaculation is the ejection from the penis of about 2-5 ml of semen containing an average of 300 million sperm (maximum sperm recorded in ejaculation is 500 million sperm. Human reproduction Page 5 of 24 A-Level Biology MA Micallef 3) GAMETOGENESIS IN MALES: SPERMATOGENESIS - Sperm production  The seminiferous tubules contain a layer of germ cells (germinal epithelium) which first divides mitotically to form the spermatogonia. One cell produced is stored and remains as a spermatogonium. Other spermatogonium increase in size to produce primary spermatocytes.  The spermatogonium dives mitotically several times to finally produce a primary spermatocyte.  The spermatocyte divides mitotically. After meiosis I it forms haploid secondary spermatocytes.  Secondary spermatocytes undergo meiosis II to form spermatids.  Meiosis takes a long time, over a month.  During maturation the spermatids take nutrients from the Sertoli cells in the seminiferous tubules. Sertoli cells remove excess cytoplasm. Spermatids differentiate during maturation to form spermatozoa (sperms).  Spermatogenesis produces 4 spermatozoans from each primary spermatocyte.  As the spermatids mature they become motile and move from the seminiferous tubule to the epididymis. The nutritive fluid secreted by the Sertoli cells is reabsorbed. Sperms are stored between two to ten days in the epididymis. Sperms become fully motile in epididymis.  Before ejaculation, sperms are stored in the vas deferens/ sperm duct. Human reproduction Page 6 of 24 A-Level Biology MA Micallef Spermatozoon  The sperm cell consists of a head and a tail.  The head tip is the acrosome which contains hydrolytic enzymes needed to break down the zona pellucida of the ovum.  The head contains the haploid nucleus. It carries no other organelles. In humans it is only the haploid nucleus that is delivered into the ovum.  The neck or midpiece is rich in mitochondria to provide energy for tail movement.  The tail contains a flagellum which has a whip-like movement.  The change undergone by spermatozoa in the female genital tract that enables them to penetrate and fertilize an egg is called Capacitation Functions of Male Hormones  Stimulation by GnRH (gonadotropin-releasing hormone) from hypothalamus.  GnRH stimulates the anterior pituitary to produce two gonadotropins – these regulate testicular function  Luteinizing hormone (LH) / ICSH (Interstitial cell Stimulating Hormone) effects Leydig (interstitial) cells to secrete testosterone.  Follicle-stimulating hormone (FSH) stimulates Sertoli (sustentacular) cells to facilitate sperm development - results in spermatogenesis, and to secrete inhibin.  Puberty is triggered. Testosterone initiates development of secondary sexual characteristics.  Excess production of testosterone by the Leydig cells has an inhibitory effect on the anterior pituitary reducing the secretion of FSH and LH. This is a negative feedback mechanism which controls the level of testosterone.  Inhibin exerts feedback inhibition on FSH in a similar way. Human reproduction Page 7 of 24 A-Level Biology MA Micallef 4) SEXUAL MATURATION Sexual maturation occurs after puberty – when the organism is ready to produce offspring. Testosterone initiates these changes in males. Oestrogen initiates them in females. Human reproduction Page 8 of 24 A-Level Biology MA Micallef 5) THE FEMALE REPRODUCTIVE SYSTEM  Functions: i) To produce ova. ii) To deliver the ovum to the fallopian tube for fertilisation iii) For implantation and the development of the embryo iv) Production of the female hormones – oestrogen and progesterone  Structure: - Consists of six general regions: i) two Ovaries in which ova are produced ii) two tubular Oviducts through which eggs pass after release iii) a saclike Uterus in which the embryo develops iv) a Vagina, which leads from the uterus to the exterior v) Vulva a collective name for the external genitalia vi) Mammary Glands, used to provide milk for the offspring. (Also considered part of the female reproductive system)  Ovaries - almond-shaped, located on each side of the lower portion of the abdominal cavity - Ovaries develop more slowly than the testes - Ovaries contain microscopic structures called ovarian follicles - Each follicle contains a potential egg cell called a primary oocyte and smaller granulosa cells - The major functions of granulosa cells include the production of sex steroids, as well as a helping with the growth of oocyte during its development - including mature Graafian Follicles Human reproduction Page 9 of 24 A-Level Biology MA Micallef - There are also Atretic Follicles - follicles that cease to develop and subsequently degenerate - Ovaries produce Oestrogens - steroid hormones responsible for developing body form and other features characteristic of a female - They also produce the hormone: progesterone  Oviducts - fertilisation occurs in the oviducts - the end adjacent to the ovary consists of a mass of highly convoluted fingerlike projections called Fimbriae - internal surface of the oviduct consists of 2 cell types: i) Ciliated Cells - movements of cilia create a current to help carry an ovum away from ovary ii) Secretory Cells - provide nutrients for the ovum as it passes through the oviduct. Smooth muscle in the oviduct wall helps the ovum to move to the uterus by peristalsis  Uterus - a hollow, pear-shaped, thick-walled sac consisting of: i) an inner Endometrium: a complex layer of epithelial cells, glands and blood vessels ii) a middle layer, the Myometrium: contains smooth muscles, large blood vessels and connective tissue iii) an outermost Perimetrium: is continuous with the ligaments that support the uterus and uterine tubes  Cervix - muscular, cylindrical neck of uterus separating it from the vagina - the cervix secretes mucus that fills the cervical canal. - helps clean vagina and lubricate it during intercourse  Vagina - provides a passage from the uterus to the outside of the body - sperm are deposited at the connection of the vagina to the cervix - initially covered by a membrane called the hymen  Vulva (External genitalia) - present at the body surface - outermost are a pair of fat- padded skin folds, the Labia majora - this are found around another pair of skin folds, Labia minora - the labia minora partly enclose the Clitoris (Organ for sexual stimulation) Human reproduction Page 10 of 24 A-Level Biology MA Micallef 6) GAMETOGENESIS IN FEMALES – Oogenesis  The production of the egg cells begins in the ovaries of the foetus but most of the stages of meiosis occur after puberty at least 12 years later. So in females, unlike in males, meiosis is paused and the production of the gamete occurs in two phases separated by years.  The outer surface of the ovaries is germinal epithelium. This undergoes meiosis to produce an ovum every menstrual cycle.  In the foetus the germinal epithelium divide mitotically to form oogonia, these migrate to the connective tissue of the ovary and grow to form primary oocytes.  A layer of follicle cells – granulosa cells (forming the primary follicle – which gradually matures into the Grafiaan Follicle) surrounds the primary oocytes. In the primary follicle the oocyte undergoes Prophase I of meiosis. Meiosis stops at this stage when the baby girl is born.  After puberty at least one primary follicle develops every menstrual cycle. In each cycle as the primary follicle matures, meiosis continues in the oocyte. The primary oocyte undergoes the first meiotic division and a secondary oocyte and a polar body are produced. The polar body degenerates. The secondary oocyte undergoes the second meiotic division but continues only till metaphase II.  At this stage ovulation occurs and the secondary oocyte (ovum) is released from the Graafian follicle. Meiosis continues only if fertilisation occurs. Otherwise the secondary oocyte comes out with the menstrual flow. Imp: Meiosis in females starts in the ovary but is completed in the fallopian tube upon fertilization. Human reproduction Page 11 of 24 A-Level Biology MA Micallef 7) THE MENSTRUAL CYCLE  At birth, ovaries contain about 1 million follicles. Each contains a primary oocyte that is stopped in prophase of meiosis I. At puberty, granulosa cells secrete oestrogen. This stimulates secondary sexual characteristics to develop – it triggers menarche, first menstrual cycle. The human menstrual cycle lasts around 1 month and is divided in into phases: follicular phase, ovulation and luteal phase. i) Follicular Phase - On the 5th day after menstruation has occurred hypothalamus releases GnRH. This hormone travels through a special vessel called a portal vein, which carries it directly to its target organ the anterior pituitary. The anterior pituitary then secretes FSH (Follicle stimulating hormone). - FSH stimulates the maturation of many primary follicles in the ovary. A primary follicle consists of a primary oocyte surrounded by a layer of protective/nourishing follicle cells. All of these primary follicles degenerate except the most developed one. LH (Luteinising hormone) also starts being released by anterior pituitary. - Only one follicle achieves full maturity as a Graafian follicle. The Graafian follicle produces the hormone oestrogen. As the level of oestrogen increases, it inhibits the pituitary from producing more FSH. In this manner no other follicles can develop. The Graafian follicle nourishes the developing oocyte. The primary oocyte completes meiosis I. It produces one large secondary oocyte and one tiny polar body. The secondary oocyte begins meiosis II but it stops in metaphase II. o Oestrogen helps to repair and increasing the blood vessels in the endometrium. (This is sometimes called the proliferative stage). o Oestrogen initially has a negative feedback on FSH to reduce the number of developing follicles since one has already reached the developmental stage required. o A peak in Oestrogen at around day 12 stimulates the pituitary to release more FSH and the Luteinising hormone (LH). Here there is Positive Feedback from oestorgen on LH and FSH. Human reproduction Page 12 of 24 A-Level Biology MA Micallef ii) Ovulation - At around day 14 of the menstrual cycle, LH causes the mature Graafian follicle to burst. - LH causes ovulation - A mature ovum that is covered by a protein layer called the Zona Pellucida and a layer of protective and nutritive follicle cells leaves the ovary into the opening of the fallopian tube or oviduct. Cilia within the oviduct move the ovum towards the uterus. If sperm are present, fertilisation will occur in the oviduct itself. iii) Luteal Phase - LH also causes the remains of the follicle within the ovary to develop into a solid mass called the yellow body or Corpus Luteum. This secretes small amounts of oestrogen and large amounts of progesterone. o Progesterone thickens the lining of the uterus further and increases and maintains its blood supply in preparation for implantation of the embryo. o It also inhibits production of LH and FSH as there is no more need for follicles to develop. (If implantation occurs, progesterone inhibits contraction of the uterus that would expel the developing baby.)  Inhibin - Is a hormone that has a similar function as oestrogen – when it increases it inhibits/ stops the Production of FSH. (It is also present in men)  Prostaglandins - Prostaglandins are released by the endometrium during the menstrual cycle. Some women release more prostaglandins during menstruation than other women. - These higher levels of prostaglandins in women with severe dysmenorrhea (painful periods) results in increased uterine contractions and muscular spasm. - Prostaglandins cause spasmodic muscle tension. If fertilization and implantation do not occur, the spiral arteries of the lining close off, stopping blood flow to the surface of the lining. Surface layers fill with blood burst and, with the endometrial lining, form the menstrual flow. - Some prostaglandins cause relaxation, and some cause constriction. Prostaglandins also help during parturition (labour). Human reproduction Page 13 of 24 A-Level Biology MA Micallef 8) IF NO FERTILIZATION OCCURS  If the ovum is not fertilised within 24-48 hrs, the unfertilised secondary oocyte (it has not formed the proper ovum) dissolves and its contents are absorbed.  The corpus luteum regresses and degenerates. Since no implantation occurs, there is no hCG (hormone produced by embryo) received by brain, the corpus luteum forms the corpus albicans and is broken down by macrophages.  Progesterone production falls. The uterus cannot be maintained. This results in the gradual breakdown and release of blood rich endometrial wall.  This is known as menstruation or the menstrual period. The blood rich material that is released through the cervix and vagina is called the menses. 9) IF FERTILIZATION OCCURS  Copulation or mating is also called coitus or sexual intercourse. The act of copulation allows insemination - the process of depositing semen inside a woman’s vagina.  The sperm can live for 3-5days. The oocyte has a life span of 1 day (maximum of 2). If the sperm meet the oocyte in the fallopian tube while it is still viable, the oocyte will be fertilized.  The (secondary) oocyte completes meiosis II, forming a mature ovum and second polar body. If oocyte is fertilized, the corpus luteum is maintained by human chorionic gonadotropin (hCG) which is produced by embryo.  This keeps high levels of oestrogen and progesterone, and so prevents menstruation until placenta takes over hCG is tested for in all pregnancy tests.  In all sexually-reproducing animals, the first step is fertilization.  Fertilisation is the union of male and female haploid nuclei to restore the diploid number of chromosomes to form a zygote.  Fertilisation may occur without copulation via in-vitro fertilisation. In this process the egg and sperm are brought into vicinity outside the body system, dissolved in a fluid medium contained in a test tube. They are induced to fertilise each other. The fertilised ova are then implanted into the female’s uterus for normal development inside her body. This can give rise to multiple births. Human reproduction Page 14 of 24 A-Level Biology MA Micallef  Fertilization itself consists of three events: - Capacitation, Sperm penetration and membrane fusion - Egg activation - Fusion of nuclei  Capacitation, Sperm penetration and membrane fusion - Protective layers of egg include the jelly layer - corona radiata and the zona pellucida in mammals need to be penetrated. - After the ejaculation the sperm cells go through several essential physiological changes during their time in the female genital tract before they, at the end, are able to penetrate the oocyte membrane. The first change in this cascade is capacitation. The sperm cells accomplish this during the ascension through the female genital tract (in contact with its secretions). It has to do with a physiological maturation process of the sperm cell membranes, which is seen as the precondition for the next step to follow, namely the acrosome reaction. - Capacitation is a functional maturation of the spermatozoon. The changes take place via the sperm cell membrane in which it may be that receptors are made available through the removal of a glycoprotein layer. The area of the acrosomal cap is also so altered thereby that the acrosome reaction becomes possible. - Through the membrane alterations, the motile properties of the spermatozoon also change. Discharging whipping movements of the tail together with larger sideways swinging movements of the head take place. This type of motility is designated as hyperactivity. One can therefore say that the visible consequences of capacitation consist in hyperactivity of the spermatozoon. - The acrosome of sperm contains digestive enzymes that enable the sperm to tunnel its way through to the egg’s cell membrane. Membrane fusion permit sperm nucleus to enter directly into egg’s cytoplasm. - The head piece of the sperm has no other organelles in it. Only the nucleus of the sperm is supplied to the female ovum. All other organelles of the organism come from the mother. Human reproduction Page 15 of 24 A-Level Biology MA Micallef  Egg Activation - Capacitiation allows membrane fusion. - This triggers egg activation by the release of Ca2+ which initiates changes in the egg’s membrane potential and exterior coats. - The Zona Pellucida hardens. - These change blocks polyspermy (many sperm nuclei entering the egg’s cytoplasm) Enzymes from cortical granules remove sperm receptors and the ovum completes meiosis II. Human reproduction Page 16 of 24 A-Level Biology MA Micallef  Fusion of Nuclei and Zygote Formation - Cleavage: The fertilised egg cell is called a Zygote. Cleavage is the rapid division of the zygote into a larger and larger number of smaller and smaller cells called blastomeres. It is not accompanied by an increase in the overall size of the embryo. - A solid ball of cells called blastomeres form a ball called morula by the end of the third day. - After the cleavage has produced over 100 cells, the embryo is called a blastula. The blastula is usually a spherical layer of cells (the blastoderm) surrounding a fluid-filled or yolk-filled cavity (the blastocoel). - The outermost blastomeres in the ball of cells become joined by tight junctions. Innermost blastomeres pump Na+ into the intracellular spaces. This creates an osmotic gradient, which draws water. The result is the hollow ball of cells, the blastula, containing a fluid-filled cavity, the blastocoel. - The blastocyst comes after the blastula stage. The blastocyst contains an embryoblast, which is homologous to the blastula. However, it also includes the trophoblast, which goes on to form the placenta 1) Trophoblast = Outer layer of cells which forms placenta 2) Blastocoel = Central fluid-filled cavity 3) Inner cell mass = Located at one pole which forms the developing embryo Human reproduction Page 17 of 24 A-Level Biology MA Micallef 10) HUMAN DEVELOPMENT  Human development from fertilization to birth takes an average of 266 days, or about 9 months i.e. 40 weeks – this is called the gestation period.  During this time the mother undergoes physiological and hormonal changes which return to normal after birth. This time is commonly divided into three periods called trimesters  Overview of First month - The zygote undergoes its first cleavage about 30 hr after fertilization. By the time the embryo reaches the uterus, 6-7 days after fertilization, it has differentiated into a blastocyst. - Trophoblast becomes an endocrine gland and begins to secrete hCG and hCG prevents autolysis of corpus luteum. Corpus luteum continues secreting more progesterone and oestrogens (for the first 16 weeks). High levels of progesterone suppress menstruation. - There is an increased growth of endometrium; no period. - Continued inhibition of anterior pituitary; no new 1o follicle stimulation - The blastocyst passes into the uterus, where it remains free in the uterine cavity for several days - During this time, the outer layer of cells of the blastocyst absorbs nutrients from the secretions present within the uterus - The zona pellucida disintegrates, and the blastocyst increases in size - When the blastocyst rests on the endometrium, trophoblast cells digest their way into the endometrium in the process known as implantation. - The developing chorion and mother’s endometrium engage to form the placenta. - Placenta starts producing hCG. This maintains mother’s corpus luteum. - Gastrulation occurs in the second week. - Neurulation occurs in the third week. - Organogenesis begins in the fourth week. Human reproduction Page 18 of 24 A-Level Biology MA Micallef Human reproduction Page 19 of 24 A-Level Biology MA Micallef  By the second month the embryo looks distinctively human. The ninth week marks the transition from embryo to foetus. During the third month secretion of hCG by the placenta declines, and so the corpus luteum degenerates. Placenta takes over hormone secretion – oestrogen and progesterone.  By the end of the second trimester bones enlarge and foetus is over 30 cm long, and weighs 600 gr. The third trimester is a period of growth and organ maturation.  By the end of this trimester, most of the major nerve tracts in the brain are formed but the brain continues to develop and produce neurons for months after birth. 11) FUNCTIONS OF HORMONES DURING PREGNANCY Oestrogen Progesterone Inhibits FSH Inhibits FSH Stimulates uterine and placental growth Maintains uterine lining Development of the mammary ducts Stimulates development of mammary glands Inhibits prolactin release Inhibits prolactin release Increases sensitivity to oxytocin Inhibits uterine contractions 12) EXTRAEMBRYONIC MEMBRANES  Chorion: Located near eggshell in birds. Contributes to the placenta in mammals (see below)  Yolk sac: Food source in bird embryos. Found in mammals, but it is not nutritive  Allantois: Unites with chorion in birds, forming a structure used for gas exchange In mammals, it contributes blood vessels to the developing umbilical cord accumulates the small amount of urine produced by fetal kidneys. Human reproduction Page 20 of 24 A-Level Biology MA Micallef  The amnion: - This is a lining that surrounds the embryo. It encloses the Amniotic fluid which  keeps the embryo buoyant  allows the embryo to grow and move freely  protects from sudden temperature fluctuations  acts as a shock absorber - In humans it bursts just prior to the birth of the child (called breaking of the waters).  The Placenta - After implantation the embryo forms another membrane called the chorion. - This surrounds the embryo. Projections of the chorion called villi are full of fine blood capillaries and penetrate the blood spaces in the mother’s endometrium to form the placenta. The placenta becomes fully functional in about 3 months. - The umbilical cord connects the embryo (at the navel) with the mother. - These are the placenta’s functions: i) Protection of the Embryo: o It hinders the entry of pathogens from the mother (though viruses, bacteria, drugs and alcohol may pass as they are very small.) o It allows the entry of antibodies from the mother (passive induced immunity). o It keeps the embryo separated from the mother’s higher blood pressure. o It prevents exchange of red blood cells avoiding the deadly possibility of agglutination. ii) Gas Exchange: o It supplies oxygen from the mother. o Allows excretion of carbon dioxide from the embryo to the mother’s blood. iii) Nutrient Supply: o Glucose, amino acids, lipids, vitamins and minerals pass to the embryo from the mother’s blood. Human reproduction Page 21 of 24 A-Level Biology MA Micallef iv) Endocrine: o The placenta takes over the role of the corpus luteum in maintaining the high progesterone levels required. These hormones maintain the thick uterine lining during pregnancy ensuring exchange of materials between mother and developing baby. o Progesterone also prevents contraction of the uterus by inhibiting oxytocin. o Prepares the mother’s body for birth and lactation (oestrogen and progesterone promote growth of mammary glands). v) Excretion: o Metabolic wastes, CO2 and urea, pass from embryo into the mother’s blood. vi) The Umbilical Cord o two Umbilical Arteries - carry foetal blood (deoxygenated blood with CO2 and wastes) to the placenta o one Umbilical Vein - carries oxygenated blood (with oxygen and nutrients) from mother/placenta to the foetus 13) MOVEMENT OF MATERIALS ACROSS THE PLACENTA  Umbilical vein - oxygenated blood - O2, nutrients, hormones, vitamins, minerals and amino acids - are taken from mother to foetus  Umbilical artery - deoxygenated blood - less nutrient than umbilical vein - CO2 & urea taken away from foetus to mother  Substances pass from mother to foetus via: B = Both Directions M = leave mother, F = leave Foetus Diffusion e.g. gases (B), fatty acids, Na+, Cl- (M) and urea (F) Facilitated diffusion e.g. glucose (M) Osmosis e.g. water (Bulk flow) (B) Active transport e.g. amino acids, water soluble vitamins (M) Pinocytosis e.g. Immunoglobulins (enclosed in small vesicles) (M) Endocytosis and Exocytosis – large proteins (M)  Adaptations of the Placenta i) Has numerous villi to increase surface area so that exchange can occur faster. ii) Maternal blood and foetal blood flows in opposite direction to increase diffusion gradient iii) Maternal blood capillaries and foetal blood capillaries are separated by a short distance for diffusion of materials. Human reproduction Page 22 of 24 A-Level Biology MA Micallef 14) BIRTH  Horrmonal Control during birth. - Foetal pituitary gland will secrete Adrenoconcotrophic hormone (ACTH). This will stimulate the foetal adrenal glands to secrete Corticosteroids. - Corticosteroids diffuse into the mother through the placenta and decrease the production of progesterone. The decrease in progesterone will cause an increase in the hormone oxytocin. - Oestrogen secretion increases and stimulates mother’s uterus to release prostaglandins, and produce more oxytocin receptors. - Oxytocin will induce waves of contractions in the uterine wall to expel the foetus out. - Oxytocin is further induced by the pressure in the womb. The pressure of the head on the cervix and feedback from the uterus will increase the production of oxytocin. - Oxytocin and prostaglandins further stimulate uterine contractions. This is an example of positive feedback where nervous impulses (detecting pressure on womb) stimulate further production of a hormone (oxytocin).  Steps involved in labour a) During the first stages of labour the contractions widen the cervix which has been softened by the hormone Relexin. b) The pressure raptures the amniotic sac. Water escapes through the vagina and this is referred to as “breaking of the waters”. c) During the second stage there is the emergence of the head. Once the head has emerged the birth of the rest of the body is quicker. Baby is called a neonate. d) The third stage of labour involves continuing uterine contractions to expel the placenta and associated membranes - Collectively called the afterbirth. e) Powerful contractions of the uterus constrict blood vessels and prevent bleeding from the uterus. Human reproduction Page 23 of 24 A-Level Biology MA Micallef 15) NURSING  Milk production (lactation) occurs in alveoli of mammary glands when stimulated by the anterior pituitary hormone prolactin. Milk is secreted into alveolar ducts  During pregnancy, progesterone stimulates development of mammary alveoli and oestrogen stimulates development of alveolar ducts.  After birth, anterior pituitary secretes prolactin. The sensory impulses associated with baby’s suckling trigger the posterior pituitary to release oxytocin.  Oxytocin stimulates contraction of smooth muscles surrounding alveolar ducts and milk flows out (milk let-down reflex).  The first milk produced after birth, colostrum, is rich in nutrients & maternal antibodies (therefore obtaining passive immunity). IMP: Both processes in which oxytocin is involved i.e. birth and the “milk-let down reflex” are examples of positive feedback Human reproduction Page 24 of 24