Rucete ✏ Campbell Biology In a Nutshell
Unit 7 ANIMAL FORM AND FUNCTION — Concept 46.5 In Placental Mammals, an Embryo Develops Fully Within the Mother’s Uterus
In placental mammals like humans, reproduction involves internal fertilization, development within the uterus, and live birth. A sequence of well-regulated stages—from fertilization to labor—is supported by hormonal control, placental function, and maternal physiological changes.
1. Fertilization and Early Development
- Fertilization occurs in the oviduct; zygote forms and begins cleavage
- A blastocyst forms by day 4 and implants into the endometrium by day 7
- Gestation lasts ~266 days from fertilization (40 weeks from LMP)
2. Placental Development and Function
- Trophoblast forms part of the placenta
- Placenta exchanges gases, nutrients, and wastes between mother and fetus
- Umbilical cord connects fetus to placenta
- Placenta provides immune protection and secretes hCG
3. Trimesters of Human Pregnancy
- First trimester:
- Organogenesis occurs; embryo becomes fetus at 8 weeks
- hCG maintains corpus luteum → progesterone rises → no menstruation
- Second trimester:
- Fetus grows and moves; placenta takes over hormone production
- hCG declines, corpus luteum degenerates
- Third trimester:
- Rapid fetal growth; limited space reduces movement
- Maternal organs compressed → discomfort
4. Labor and Birth
- Triggered by hormones:
- Estradiol increases uterine oxytocin receptors
- Oxytocin stimulates prostaglandins → contractions
- Positive feedback loop strengthens contractions
- Three stages:
- Dilation of cervix
- Expulsion of baby
- Delivery of placenta
5. Lactation
- Prolactin (anterior pituitary) stimulates milk production
- Oxytocin (posterior pituitary) causes milk ejection in response to suckling
6. Maternal Immune Tolerance
- Fetus is partially foreign, yet not rejected by the immune system
- Maternal immunity is modulated during pregnancy
7. Contraception and Abortion
- Barrier methods: condoms, diaphragms
- Hormonal methods: pills, patches, injections
- IUDs prevent implantation; RU486 blocks progesterone
- Sterilization: vasectomy, tubal ligation
- Natural methods are less reliable
- Spontaneous abortion (miscarriage) affects ~⅓ of pregnancies
8. Infertility and Reproductive Technology
- ~10% of couples are infertile; causes affect both sexes
- IVF and ICSI assist fertilization
- Ultrasound, amniocentesis, and CVS diagnose fetal conditions
- Noninvasive fetal DNA testing uses maternal blood
In a Nutshell
In placental mammals, internal fertilization leads to complex embryonic development within the uterus, supported by hormonal coordination and placental exchange. Human reproduction is regulated through pregnancy stages, birth, and lactation. Advances in medicine have enabled effective contraception, assisted reproduction, and prenatal diagnosis, expanding reproductive choices and outcomes.