Fetal Development BIOL 314 PDF

Fetal Development BIOL 314 1  Fertilization and the Preembryonic Stage Embryo — term has varied meanings – Some authorities assert that the fertilized egg or the two-cell stage is an embryo – Other authorities assert that an individual becomes an embryo when...

Fetal Development BIOL 314 1  Fertilization and the Preembryonic Stage Embryo — term has varied meanings – Some authorities assert that the fertilized egg or the two-cell stage is an embryo – Other authorities assert that an individual becomes an embryo when it is 16 days old and consists of three primary germ layers. Ectoderm, mesoderm, and ectoderm Preembryonic stage is the first 16 days after fertilization 2  Sperm Migration 1 Egg must be fertilized within 12 to 24 hours of ovulation, if it is to survive Sperm must encounter the egg somewhere in the distal one-third of the uterine tube Vast majority of sperm ____________ make it to egg – Destroyed by vaginal acid or drain out of vagina – Fail to penetrate the mucus of the cervical canal – Destroyed by leukocytes in the uterus – Half go up wrong uterine tube – Of the 300 million that were ejaculated, about 200 spermatozoa reach the vicinity of the egg 3  Fertilization 1 Sperm are viable for up to 6 - 7 days after ejaculation – _______________________ optimal if sperm are deposited a few days before ovulation to 14 hours after When sperm encounters an egg, it releases enzymes needed for one sperm to penetrate the egg – Penetrates granulosa cells, then zona pellucida 4  Fertilization 2 5  Meiosis II The secondary oocyte begins meiosis II before ovulation but completes it only if fertilized – Through formation of the second polar body, the egg discards one chromatid from each of its chromosome Sperm and egg nuclei swell and become pronuclei Each pronucleus ruptures and the chromosomes of the two gametes mix into a single diploid set The fertilized egg, now called the ______________, is ready for its first mitotic division 6  Major Stages of Prenatal Development 1 The course of pregnancy is divided into 3-month intervals—trimesters – First trimester: from fertilization through 12 weeks More than half of all embryos die from natural causes in the first trimester Conceptus is _____________ to stress, drugs and nutritional deficiencies during this time 7  Major Stages of Prenatal Development 2 – Second trimester: weeks 13 through 24 Organs complete most of their development Fetus looks distinctly human Chance of survival (with intensive care) if born near end of this trimester Premature Birth at 24 weeks – Third trimester: week 25 to birth Fetus grows rapidly and organs achieve enough cellular differentiation to support life outside of womb At 35 weeks and 5.5 lb, fetus is considered mature Annabelle, 37 wks developed, 8 at 20 minutes old  The Preembryonic Stage 1 Preembryonic stage —First _____________ of development culminating in the existence of an embryo – Involves three major processes Cleavage Implantation Embryogenesis 9  The Preembryonic Stage 2 ___________ — mitotic divisions that occur in first 3 days while conceptus migrates down uterine tube – First cleavage occurs within 30 hours after fertilization Zygote splits into two daughter cells (blastomeres) – By the time the conceptus arrives in the uterus About 72 hours after ovulation Morula stage—solid ball of 16 cells that resembles a mulberry Sill no larger than the zygote Cleavage produces smaller and smaller blastomeres 10  The Preembryonic Stage 3 _______________ lies free in uterine cavity for 4 to 5 days – Divides into 100 cells or so – Zona pellucida disintegrates and releases conceptus, called blastocyst ________________: a hollow sphere – Trophoblast —outer layer of squamous cells Destined to form the placenta and play a role in nourishment of the embryo – Embryoblast —inner cell mass Destined to become the 11 embryo  The Preembryonic Stage 4 ___________________ attaches to uterine wall 6 days after ovulation – Usually on the fundus or posterior wall of the uterus Implantation: process of attachment to uterine wall – Begins when blastocyst adheres to endometrium Embryogenesis —arrangement of blastomeres into three primary germ layers in the embryoblast – Ectoderm, mesoderm, and endoderm 12  The Preembryonic Stage 5 Trophoblast also secretes human chorionic gonadotropin (HCG) – HCG stimulates the ____________ to secrete estrogen and progesterone Progesterone suppresses menstruation HCG levels rise in mother’s blood until end of second month Trophoblast develops into membrane called the _________________ Takes over role of corpus luteum making HCG unnecessary Ovaries become inactive for remainder of pregnancy Estrogen and progesterone levels rise from chorion 13  The Embryonic and Fetal Stages Embryonic stage —begins when all three primary germ layers are present (usually day 16) Placenta forms over the next 6 weeks – Becomes embryo’s primary source of nutrition ___________________ —germ layers differentiate into organs and organ systems – Organs are present (but not fully functional) at 8 weeks: time when the embryo becomes a fetus 14  Embryonic Folding and Organogenesis 1 In embryonic stage, the flat embryonic disc is converted into a somewhat cylindrical form – Occurs during week 4 – Embryo grows rapidly and folds around a membrane called a yolk sac – Embryo becomes C-shaped, with head and tail almost touching – Lateral margins of the disc fold around the sides of the yolk sac to form the ventral surface of the embryo 15  Embryonic Folding and Organogenesis 2 As a result of the embryonic folding, the entire surface is covered with ectoderm – Will later produce the epidermis of the skin Mesoderm splits into two layers – One adheres to the ectoderm – The other to the endoderm – ____________ —body cavity between the two layers of mesoderm 16  Embryonic Folding and Organogenesis 3 Formation of organs from primary germ layers – At 8 weeks, all organs are present in fetus (3 cm long) Derivatives of ___________ – Epidermis, nervous system, lens and cornea, integumentary glands, internal ear Derivatives of ___________ – Skeleton, muscle, cartilage, blood, dermis, lymphoid tissue, gonads and ducts, kidneys and ureters Derivatives of ___________ – Gut and respiratory epithelium and glands, bladder, and urethra 17  Embryonic Membranes 1 Accessory organs develop with embryo: placenta, umbilical cord, and four embryonic membranes: amnion, yolk sac, allantois, and chorion Amnion —transparent sac that completely enclose the embryo – Penetrated only by the umbilical cord – Fills with amniotic fluid Protects embryo from trauma, infections, and temperature fluctuations Allows freedom of movement important to muscle development 18  Embryonic Membranes 2 Amniotic fluid (Continued) – Enables embryo to develop symmetrically – Prevents body parts from adhering to each other – Stimulates lung development as fetus “breathes” fluid – At first, amniotic fluid formed from filtration of mother’s blood plasma – Fetus contributes to fluid volume by ________________ into amniotic cavity (but fetus also swallows amniotic fluid) – At term, about 700 to 1,000 mL of fluid 19  Prenatal Nutrition 1 During gestation the conceptus is nourished in three different, overlapping ways 1. Uterine milk 2. Trophoblastic nutrition 3. Placental nutrition Uterine milk: glycogen-rich secretion of the uterine tubes and endometrial glands – Conceptus absorbs this fluid as it travels down the tube and lies free in the uterine cavity __________ implantation 20  Prenatal Nutrition 2 _________________________— conceptus consumes decidual cells of the endometrium – Progesterone from corpus luteum stimulates decidual cells to proliferate – They accumulate a store of glycogen, proteins, lipids – As conceptus burrows into the endometrium, the decidual cells are digested. – Only mode of nutrition for first week after implantation – Remains dominant source through the end of 8 weeks – Wanes as placental nutrition increases 21  Prenatal Nutrition 3 Placental nutrition—nutrients diffuse from the mother’s blood through the placenta into the fetal blood Placenta — disc-shaped organ attached to the uterine wall on one side; on the other, attached by way of umbilical cord to the fetus Placental phase —the period beginning week 9 – ____________ mode of nutrition from end of week 12 until birth 22  Fetal Development 1 By the end of 8 weeks – ______________________ are present – Individual is about 3 cm long – Now considered a fetus – Bones have begun to calcify – Skeletal muscles exhibit spontaneous contractions Too weak to be felt by the mother – Heart, beating since week 4, now circulates blood – Heart and liver are very large, forming the prominent ventral bulge – Head is nearly half the total body length 23 Fetal Development 2 24  Fetal Development 3 The fetus is the final stage of prenatal development – From the start of week 9 until birth – Organs mature to support life outside the mother Unique aspects of fetal circulation – __________________ – Presence of three circulatory shortcuts: shunts 25  Two Semester Summary Slide ________ is the study of structure. _________ is the study of function. We maintain internal balance (_________) with ___________ feedback loops. Too much, or too little of anything is _________. Moderation is important. ___________ is of lasting value. 26