Early Development of the Zygote to a Blastocyst (1st Week) Ovulation releases ovum/secondary oocyte Fertilization in the oviduct → produces zygote Zygote undergoes cleavage as it moves along oviduct → produces morula As zygote divides, cells become smaller \ Morula stays same size Movement by cilia and peristalsis present in oviduct walls Morula develops into a blastocyst Trophoblast (outer layer of blastocyst) → nourishes future embryo Inner cell mass → will become fetus Fluid filled cavity → for protection (absorbs shocks, resists compression, ...) Blastocyst (≈100cells) implants itself in uterus lining Nourished by secretion from uterus Microvilli provide large surface area (→gas + nutrients exchange) Implantation Of The Blastocyst Into The Uterine Lining (2nd Week) Trophoblast secretes enzymes → digest tissues and blood vessel of endometrium Embryo uses released nutrients/products from digestion Blastocyst becomes buried within endometrium Microvilli are replaced by placenta Trophoblast secretes human chorionic gonadotrophin (hCG) hormone The Developing Fetus During gestation (→length of pregnancy) growth rate is in excess Placenta is the first organ to develop when blastocyst embeds itself in uterine lining Growth faster than embryo in early pregnancy Development increases in complexity Differentiating of inner cell mass of blastocyst First month → beginning of a gut, developed kidney, brain, beating heart Second month → all main organ systems present; embryo is called a fetus At the end of gestation placenta is discarded, but essential for the 1st 9 month of life Features of the Circulatory System of the Developing Fetus: Placenta is the Fetal Gas Exchange Organ → Fetal Lungs are Non-Functional Umbilical vein carries oxygenated blood from placenta to vena cava Blood in the heart bypasses through foramen ovale Oxygenated blood flows from right into left atria / flap valve prevents back flow of blood Some blood in right atria passes to right ventricle into pulmonary artery (to lungs) Blood bypasses lungs through ductus arteriosus Oxygenated blood flows from pulmonary artery into aorta Umbilical arteries carry deoxygenated blood from aorta to placenta Importance of Pulmonary Circulation Carry oxygenated blood to lungs To allow respiration in lungs / be ventilated before birth At Birth the Placenta is Replaced by Lungs as the Organ of Gas Exchange Umbilical vein constricts → prevents blood loss Ductus arteriosus constricts → blood leaving right ventricle is sent to lungs Blood pressure in left atrium exceeds that in right atrium Valve closes foramen ovale which fuses within atrial wall Prevents mixing of de- and oxygenated blood Deoxygenated blood in right ventricle is oxygenated in lungs [EXAM] Replacement of fetal to adult Hb takes ≈3months Each polypeptide/globin chain is coded by a separate gene Gene for fetal (gamma) globin is suppressed Gene for adult (beta) globin becomes active Structure of the Placenta Originates from fetal tissues and endometrium Fully developed ≈20cm across and ≈3cm thick Umbilical cord connects placenta with fetus 1 umbilical vein → oxygenated blood from placenta to fetal vena cava 2 umbilical arteries → deoxygenated blood from fetal aorta to placenta FICK'S LAW: (surface area x difference in conc)/thickness of surface µ rate of diffusion Microvilli grow into endometrium Each villi contains a network of fetal capillaries Surrounded by thin pool of maternal blood Supplied by uterine arteries and drained by uterine vein Max difference in concentration Fetal Hb has a greater affinity for O2 than adult Hb Flow of maternal and fetal blood in opposite direction Uterine artery to umbilical vein Maintains gradient/prevents concs reaching eqm Short diffusion path (≈3.5μm) Fetal and maternal blood supply is separated by 3 layers Capillary endothelium Thin layer of connective tissue Epithelium covering villi Exchange surface only one cell thick Maternal and fetal blood come close together but never mix Maternal blood may be genetically different from fetal blood Function of the Placenta Exchange of substances between maternal and fetal blood O2 and waste products (urea, CO2) cross placenta by diffusion Glucose enters fetal blood by facilitated diffusion Amino acids enter fetal blood by active transport Placenta contains many mitochondria Maternal antibodies are taken into villi by pinocytosis Infant has immunity to same diseases as its mother after birth Secretes hCG (oestrogen, progesterone) → maintains pregnancy Maternal Physiology Effect of Pregnancy on Aspects of Maternal Physiology GROWTH OF: uterus from ≈50g to 1kg / secreting tissue in breasts by progesterone ENLARGEMENT OF: smooth muscle fibres of uterus wall / ducts of breast tissue by oestrogen INCREASE OF: Body mass/thirst/metabolic rate/ventilation rate/cardiac output/blood volume/red blood cell number Ca2+ and glucose levels in bloodstream Dietary requirements of Carbohydrates(energy), Protein(growth), Fe(Hb), Ca(bones), Vitamins Minimises stresses imposed on female body → optimum environment for growing fetus (1) Changes in Thermal Balance Respiration + high growth rate increase heat Heat excess transferred to cooler maternal blood by heat gradient Mother loses this excess heat from her body (2) Changes in Cardiac Output and Blood Volume and their Significance High growth rate of fetus, placenta, maternal tissues (not just breast and uterus) This increases O2 consumption/respiration As maternal muscles have to work harder to move her increased size Increases CARDIAC OUTPUT (= STROKE VOLUME x HEART RATE) Heart beats faster Increase in stroke volume Increase in cardiac muscle / heart chambers enlarge / output increases by 40% Increase in maternal blood volume Changes in volume of plasma > increase in number of red blood cells Birth Fetus lies with its head down against stretched cervix Weak contraction of uterus every ≈30min / increase in strength and frequency Caused by hormone oxytoccin secreted by posterior pituitary gland When cervix is fully dilated Expulsion of baby by contractions of mother's abdominal muscles Umbilical cord shuts down, isolating baby from mother Rises CO2 content of the blood / stimulates baby's first breath Expulsion of placenta → ≈30min after birth Pregnancy lasts ≈38 weeks from implantation, 40 weeks from last period Hormonal Changes During And After Pregnancy Human Chorionic Gonadotrophin hCG Hormone Secreted by trophoblast and developing placenta Maintains corpus luteum past the time it normally disintegrates Endometrium is maintained and menstruation does not occur Female sex hormones still at high level Basis for pregnancy test → hCG can be detected in urine Peak in bloodstream after ≈2months followed by a slow decline Progesterone and Oestrogen Secreted by corpus luteum for first 3 months Maintain endometrium, development of uterus, prevent menstruation Inhibit FSH production from anterior pituitary gland Prevents development of further mature ovarian follicles in ovary Hormone secretion is taken over by placenta Corpus luteum degenerates Oxytocin causes Uterine Contraction (Birth) Oestrogen from placenta makes muscles of uterus sensitive to oxytocin End of pregnancy, level of oestrogen in blood rises, level of progesterone falls Oestrogen promotes uterine contraction, progesterone inhibits it Pressure of fetus against cervix stimulates stretch receptors Hypothalamus stimulates posterior pituitary glands to secrete oxytocin Oxytocin causes contraction of uterus Lactation and Prolactin High levels of progesterone inhibits prolactin At birth, progesterone levels fall → prolactin levels increase Suckling promotes production and ejection of milk Nerve impulses travel to hypothalamus Stimulates posterior pituitary gland to secrete oxytocin Stimulates muscles in walls of milk ducts to contract, squeezing milk out Stimulates anterior pituitary gland to secrete prolactin Stimulates production of more milk Prevents secretion of FSH and LH Thus, ovulation does not happen → mother is less likely to conceive Milk is bacteria free/ contains antibodies, essential nutrients, Ca2+ for bone growth, NO fibre/iron → baby will need solid food after ≈3-4months