Chapter 12: Fertilisation and Baby Development Flashcards
Fertilisation
Sperm must come contact with ovum (egg) by sexual intercourse.
1.
Requires male penis erection. Sexual stimulation causes blood to fill erectile tissue of penis (enlarging it).
2.
Male orgasm involves contraction of epididymis, vas deferens and 3 glands = ejaculation.
3.
Pathways: Seminiferous tubules → epididymis → vas deferens → urethra → penis → vagina → cervix → uterus → uterine tubes (only several 1000 of 100-300 million sperm reach uterine tubes where fertilisation occurs. Many die, reasons many sperm needed for fertilisation).
Erectile tissue in vagina
Fills with blood, tightens increasing penis stimulation during sexual intercourse in aroused female. Also produces copious mucus by glands around cervix and vagina lubricating vagina for easy penis entry.
Fertilisation 2 (in uterine tube)
Secondary oocyte (cell in ovary) is released during ovulation at metaphase 2 surrounded by 2 cell layers:
Corona Radiata
Outer made of follicle cells. Joined by acidic material surrounding oocyte membrane.
Zona Pellucida
Inner, a glycoprotein matrix surrounding oocyte membrane.
Fertilisation 3:
Sperm acrosome has enzyme which breaks acid material in corona radiata, but amount of enzyme in 1 sperm is ineffective. So, reason why many sperm is needed to break it, allowing 1 sperm through corona radiata.
2.
Sperm goes through corona radiata and reaches zona pellucida triggering acrosome reaction secreting digestive enzymes breaking glycoprotein matrix in zona pellucida. Gives sperm access to oocyte membrane for fusion of sperm and oocyte membrane.
3.
When sperm enters, it activates meiosis completion for oocyte. Also activates secondary oocyte to form membrane around oocyte preventing access of other sperm, ensures only 1 haploid chromosomes set of sperm joins oocyte chromosomes.
4.
Sperm enters oocyte it’s tail absorbed (with mitochondria) and head moves through ovum’s cytoplasm turning into male pronucleus (haploid sperm nucleus).
5.
Sperm entrance also triggers secondary oocyte to form female pronucleus (haploid egg nucleus) that fuses with male pronucleus forming 1 nucleus with diploid chromosomes. Fertilisation completes and fertilised egg is zygote.
Gestation
Time between fertilisation and birth (parturition). 280 days or 40 weeks.
Birth stages
1st 2 weeks after fertilisation is germinal stage.
Week 3-8 is embryonic period.
Week 9-birth is foetal period.
Birth trimesters
1st trimester: 1-12 weeks.
2nd trimester: 13-26 weeks.
3rd trimester: 27 week-birth.
Hormonal changes required before birth
- Ligaments soften (makes pelvis easier to move apart for birth).
- Increase strength+sensitivity of uterus for birth.
Embryo
Early baby development stage.
Foetus
Unborn baby. Foetal (relates to foetus/baby).
Maternal
Relates to mom
Zygote —> blastocyst (baby development 1):
Zygote, fertilised egg (diploid single cell) becomes new life (totipotent stem cell).
3-5 days
Zygote travels down uterine tube and divides by mitosis, producing identical totipotent cells (2, 4, 8). Twins occur by 2 totipotent cells separating, becoming 2 genetically identical embryos.
6 days
Zygote reaches uterus and totipotent cells specialise, turning zygote to blastocyst.
Blastocyst
Hollow cells ball filled with fluid. Made from inner cell mass and outer layer.
Inner cell mass
Embryoblast. Has 30 embryonic pluripotent stem cells that differentiate to different human body cells to form embryo for developing foetus.
Outer layer
Trophoblast. Forms placenta and other tissues for foetus development.
Implantation
7-8 days after fertilisation, blastocyst sinks into uterus wall (endometrium). For food ensuring growth, development by absorbing nutrients from glands and blood vessels of endometrium. So, blastocyst developing to embryo relies on endometrium.
What maintains endometrium?
High oestrogen and progesterone levels maintains endometrium, preventing it to breakdown. During embryonic period (early pregnancy), corpus luteum keeps hormone levels high till developing placenta takes over after 8-12 weeks.
3rd week
Inner cell mass (embryoblast) of blastocyst differentiate from pluri to multipotent stem cells forming 3 germ layers that produce all the tissues of foetus.
Ectoderm
Outer germ layer, forms outer layers of body. Skin, hair, cornea, epithelium of openings, nervous system, anterior pituitary.
Mesoderm
Middle germ layer. Muscle, connective tissues, skeletal & smooth & cardiac muscles, lymphoid tissue, epithelium of kidneys, stomach.
Endoderm
Inner germ layer. Epithelium of alimentary canal, bladder, urethra.
Embryonic membranes
Blastocyst also forms this in embryonic period outside embryo for protection, nourishment
Amnion
1st embryonic membrane to develop. Encloses embryo and amniotic cavity (formed within embryoblast).
Amnion function
Secretes amniotic fluid, a shock absorber protecting embryo from physical injury, controls temp (keeping baby warm), free movement for baby.
Chorion
Forms from blastocysts outer layer cells and becomes main foetal part of placenta by containing amnion (fuse chorion+amnion as amnion enlarges not amnion role). Attaches to mother via chorionic villi.
Yolk sac and allantois
2 embryonic membranes helps form outer structure of umbilical cord.
Placenta
Organ made from half foetal and half maternal tissues during 1st 3 months of pregnancy. Completely formed after 3 months to maintain endometrium (corpus luteum disintegrates). Placenta attached to foetus by umbilical cord
Placenta function
Supplies nutrients and antibodies (immunity), remove waste (excretory), produce oestrogen and progesterone, provides nutrients & gas exchange for lungs by blood transport.
Chorionic villi
After implantation, trophoblast of blastocyst develop finger-like projections from chorion that penetrate into endometrium called chorionic villi (has many blood vessels).
Chorionic villi purpose
As chorionic villi penetrate into endometrium it’s surrounded by mother’s blood in endometrium allowing material exchange by diffusion and active transport, but foetal and maternal blood don’t mix because a cell layer separates them.
Chroionic villi diffusion
Oxygen, nutrients from mother diffuse into foetal blood and waste leaves foetus by diffusing into maternal blood. The large number of chorionic villi provides surface area so substances pass.
2 Umbilical arteries
Transports deoxygenated blood away from foetus to placenta, mother.
Umbilical vein
Transports oxygenated blood from mom, placenta back to foetus.
(By placenta) uterine arteries
Mom’s blood enters placenta by uterine arteries to exchange.
(By placenta) uterine veins
Blood goes back to mom by uterine veins from placenta.
Week 4 (1 month)
Blocks of mesodermal tissue becomes muscles and vertebrae of vertebral column. Heart, liver, brain begins developing. 3 clefts of throat becomes face, throat, associated glands.
Week 5
Arm and leg buds appear.
Week 8 (embryonic period)
Embryo is now human-form and now called foetus. All organs present but not functional. Sexual organs present allows to determine which sex.
Week 16 (4th month)
18cm long, 100g. Foetus rapidly growing. Mom senses movement & mom’s abdomen will bulge. Heart rate 120-160.
Week 20 (5th month)
25cm long, 300g. Foetal movements felt clearly.
Week 24
27-35cm long, 500g. Obvious pregnancy.
Week 28
38cm long, 1 kg. Vigorous movement, male testes descend into scrotum. Brain enlarges significantly.
Week 32
41-45cm long, 1.8-2.2kg.
Week 36
46-48cm long, 2.7kg. Foetus circulatory system developed but digestive system still needs to develop.
Week 40
50cm long, 3.4 kg. Very slow growth as placenta begins to fail. Full term, no room to move. Male heavier than female.
Changes to mom
Growing abdomen and foetus, pressing on bladder feeling like it’s filled with urine (more urination).
Breasts enlargement because development of milk secreting tissues by hormones: progesterone from corpus letuem, prolactin from pituitary and oestrogen.
Changes to mom pt.2
Increased heart size and urine production for extra blood flowing through placenta.
Emotional changes by hormones and pregnancy fears.
