Fertilization and Pregnancy

Reproduction and Development

Outcomes

• Trace the process of fertilization, implantation, and extra-embryonic membrane formation (placenta, amnion, chorion, allantois), followed by embryonic and fetal development, parturition and lactation

• Describe the events from fertilization to parturition in the context of the main physiological events that occur in the development of organ systems during each major stage (trimester)

• Identify major tissues and organs that arise from differentiation and morphological development of the ectoderm, mesoderm and endoderm in the embryo

Fertilization● Occurs in fallopian tube ● Union of sperm cell with secondary oocyte

- secondary oocyte divides into fertilized ovum (zygote) after fertilization

● Zygote: 46 chromosomes (23 from each parent)

● 3 – 5 days: fertilized ovum travels to uterus, undergoes many cell division in process - Cleavage: equal divisions of cells without increase in size (ie. the cells become more numerous but the ‘clump’ remains the same size as the original zygote)

● Once in uterus (~6th day), zygote developed into BLASTOCYST- composed of outer cells called TROPHOBLAST: extra-embryonic structures develop (becomes the chorion, which becomes placenta) - inner cell mass: embryo develops

● Blastocyst attaches to endometrial wall: IMPLANTATION

● After implantation, the trophoblast produces human chorionic gonadotropin (hCG) that maintains the corpus luteum and triggers a positive home pregnancy test

● Day 1-5: Zygote ● Day 5 - week end of week 8: Embryo ● After 8th week of pregnancy: Fetus ● Pregnancy lasts approx. 9 months (40 weeks) from last

menstrual period

● Endometrium must remain intact to support pregnancy: NO MENSTRUATION- results in high levels of progesterone and estrogen - PROBLEM!! This has negative feedback effect on gonadotropins; LH must remain high to sustain corpus luteum - if corpus luteum deteriorates, estrogen and progesterone levels drop, contractions begin!!

● Outside of Blastocyst is composed of 2 structures: -chorion (outside)-amnion (Eventually where baby will develop)

● chorion produces human chorionic gonadotropic hormone (hCG), which is similar to LH - maintains corpus luteum for first 3 months

Important Structures● Amniotic cavity: between embryo and amnion

- fluid-filled, insulates embryo ● Extraembryonic coelom: fluid-filled space between amnion

and chorion ● Yolk sac: site of early red blood cell formation, later

contributes to digestive tract (not used for nutrition)

● Placenta: site of materials exchange between mother and fetus - cells of endometrium combine with those of embryo (embryo and mother blood vessels are close enough for diffusion but blood does not mix)- fourth month: placenta produces estrogen and progesterone

● Chorionic villi: projections from placenta allowing fetal blood vessels access to maternal blood vessels

● Allantois: provides umbilical blood vessels in placenta- nutrients & oxygen diffuse into blood of fetus, wastes leave

● Umbilical Cord: connects embryo with placenta

Placenta Development

Multiple Embryos● Identical Twins: 1 egg, 1 sperm

- zygote divides in two in first couple days of division, identical DNA - share same placenta but can have separate amniotic sacs

● Fraternal Twins: 2 eggs, 2 sperm - develop independently, each to their own amniotic sacs and placentas

● Can develop natural triplets as well

● Multiples more common with use of reproductive technologies

First Trimester (week 1-12)● fertilization to end of 3rd month ● Morphogenesis: growth and differentiation ● 2nd week: inner cells of blastula flattened into 2

layers ● Gastrulation: 2-layered structure develops into

3 layers - GASTRULA, each layer gives rise to specific structures - ectoderm, mesoderm, endoderm (Table 1 pg 513)

● 4th week: 500 times larger than fertilized egg- organs and systems developing - heart formed, brain visible, limb buds

● 9th week: FETUS, arms & legs begin moving

Second Trimester● 8 cm long, all organs formed

but not developed ● Second trimester: organs

continue to develop, fetus increases in size

● Mother feels it move ● Hair covers entire body ● 6th month, eyelashes/lids

form ● Cartilage of skeleton

replaced by bone cells

4th month

Third Trimester● Baby grows rapidly, Body mass increases ● Organs enlarge, systems develop completely (digestive and

respiratory are last) ● Rapid brain cell formation ● If born at beginning of 3rd trimester, there is a chance baby

will survive ● At birth, infant is approx. 51 cm long, 3400 g

Sex Determination● Females: two X chromosomes (XX) ● Males: one X, one Y chromosome (XY) ● Gene responsible for sex determination on Y

- gene SRY (Sex-determining Region of the Y chromosome)- determines male phenotype

● Presence of SRY gene, developing gonad becomes a testis - production of hormones of the testis determines subsequent characteristics

● Females: determination not just lack of SRY - complex hormonal balance, both males and females can produce opposing sex hormones - expression of opposing sex hormones at wrong time during fetal development can transform an organism with male genes into an outwardly appearing female (and vice versa)

Environmental Factors on Development● Health of mother affects unborn child ● Proper nutrition: prevents developmental problems

Ex) spina bifida: lack of folic acid ● Lifestyle choices

- smokers: smaller babies, nicotine constricts blood vessels so less nutrients get to fetus

● Chemicals or microbes: teratogens - capable of causing developmental abnormalities in utero - alcohol: FASD - drugs: dependent infants, organs underdeveloped- medications: Ex) thalidomide: used to treat morning sickness, babies born with mishapen or missing limbs

Brain of FASD baby

Child who’s mother took thalidomide