Fertilization and Early Development Fertilization Development and Birth

Fertilization and Early Development

Fertilization, Development and Birth l Fertilization (conception) l l l Union of a sperm with an oocyte (occurs in the fallopian tube) – ZYGOTE ** contains only mother’s mitochondrial DNA (mt. DNA) An egg will be fertilized by a SINGLE sperm l Preventions of POLYSPERMY: l l Change in the membrane of the egg What would happen if more than one sperm fertilized an egg? http: //www. youtube. com/watch? v=y. F 7 esx. WJj 1 Q

Acrosome Reaction (change in zona pellucida to prevent polyspermy)

Fertilization and Early Mitosis l Once the egg has been fertilized, it travels towards the uterus (“zygote”) l Zygote undergoes many CLEAVAGES (cell division without increase in size) l At ~16 cell stage – MORULA l At ~3 -5 days - BLASTOCYST l After ~4 days the “zygote” becomes an “embryo” l How to TWINS form? ? What is the difference between FRATERNAL twins and MATERNAL twins? l

Human Sex Determination l Determined by chromosomes § Differentiation into male or female occurs about 6 -7 weeks after conception Female = XX Male = XY l Y xsome l SRY-gene Developing gonads become testis Testis secrete male hormones (androgens) l l l Hormonal “balance” is critical during fetal development **Intersexed individuals**

Fertilization, Development and Birth l After ~ 6 days l Zygote becomes BLASTOCYST (blastula) l l l Outer cell layer = extraembryonic structures (chorion) Inner cell mass = embryo Once in uterus – becomes attached to uterine wall = IMPLANTATION!!

Maintenance of Pregnancy l Inhibition of menstruation = ESTROGEN AND PROGESTERONE levels high, LH high l WHY? ? l Upon implantation, the Blastocyst (more specifically the chorion) secretes the hormone h. CG l Human chorionic gonadotropic hormone l Maintains corpus luteum for first 3 months of pregnancy What hormones are secreted by the corpus luteum? ?

Prenatal Development Review l l Gametes form a zygote Zygote increases in size via mitosis Cleavage(1 -2), Morula(16 -64), and Blastocyst(64) • Zygote divides into two cells, then 4, then 8… – own nucleus – 46 chromosomes each • Placenta forms around zygote (not fully developed until about 3 months) • At about 64 cells, cells have begun to differentiate and accumulate on one side of the placenta and called a blastocyst • The blastocyst becomes attached to the wall of the endometrium (implantation) at about day 6 -7 http: //www. youtube. com/watch? v=l. XN_s. Dnd 1 ng

Gastrulation – from blastula to gastrula l l During gastrulation, 3 germ layers develop: Ectoderm l Outside layer (the skin) l Brain and nervous system Mesoderm l Middle Layer l Muscles, Bones and Blood Vessels Endoderm l Digestive tract and other internal organs

Extraembryonic Structures

• Placenta • Yolk sac – early red blood cell formation http: //www. nelson. com/ABbio 20 -30/student/protect/media/fetal_devt. html

Gestation (40 weeks) and The Placenta l l l l The Placenta isn’t fully formed until 3 months Takes over for CL to secrete PROG and EST Sits inside the uterus It consists of the entire area around the embryo The site of nutrients, oxygen, waste are exchanged between the mother and the embryo Blood is not exchanged Consists of 2 layers: l Chorion and chorionic villi l Amnion and amniotic cavity with fluid http: //www. nelson. com/ABbio 20 -30/student/protect/media/placenta. html

Placenta (fully formed at 3 months)