🥚 Fertilization and Development

Fertilization

Fertilization is the fusion of a sperm with an ovum to form a zygote (2n = 46 chromosomes). It occurs in the ampulla of the fallopian tube — the widest part of the oviduct, located near the ovary. Of the ~200–300 million sperm deposited during ejaculation, only about 100–200 reach the vicinity of the ovum, and ultimately only one sperm successfully fertilizes it.

Mechanism (11 Steps)

The process of fertilization is an intricate sequence of molecular events, each building on the previous:

1. Capacitation — After entering the female reproductive tract, sperm undergo biochemical changes over 5–7 hours. Cholesterol and glycoproteins are removed from the sperm membrane, exposing receptor molecules. This makes the sperm membrane more fluid and prepares the acrosome for the acrosomal reaction. Only capacitated sperm can fertilize.

2. Chemotaxis — The ovum and surrounding follicular cells release chemical signals (progesterone and other chemoattractants) that guide sperm toward the egg. The classic fertilizin-antifertilizin interaction also plays a role: fertilizin (glycoprotein on the egg surface) interacts with antifertilizin (on the sperm surface) ensuring species-specific recognition.

3. Acrosomal reaction — When the sperm contacts the outer egg coats, the acrosome (cap-like structure on the sperm head) fuses with the sperm membrane and releases its enzyme contents: hyaluronidase and acrosin.

4. Penetration of corona radiata — Hyaluronidase dissolves the hyaluronic acid cement holding together the cells of the corona radiata (the layer of follicular cells surrounding the ovum). Multiple sperm may help in dispersing the corona radiata, but only one will ultimately fertilize.

5. Penetration of zona pellucida — Acrosin (a serine protease) digests a path through the zona pellucida, the thick glycoprotein layer surrounding the oocyte. The sperm binds to ZP3 receptors on the zona pellucida in a species-specific manner.

6. Contact with oocyte membrane — The sperm membrane fuses with the secondary oocyte's plasma membrane at the equatorial region of the sperm head.

7. Cortical reaction — Immediately upon sperm entry, cortical granules (vesicles just beneath the oocyte surface) release their enzyme contents into the space between the oocyte and the zona pellucida. These enzymes modify ZP3 receptors, causing the zona pellucida to harden and become impermeable to other sperm. This is the block to polyspermy — ensuring that only one sperm fertilizes the egg (polyspermy would create a triploid or polyploid embryo, which is lethal).

8. Sperm entry — Only the head and middle piece of the sperm enter the oocyte cytoplasm. The tail degenerates outside. The sperm's mitochondria (in the middle piece) are also typically destroyed — which is why mitochondrial DNA is inherited exclusively from the mother.

9. Completion of meiosis II — The secondary oocyte, which had been arrested at metaphase II, is now stimulated to complete meiosis II. This produces the mature ovum (n) and the 2nd polar body (which degenerates).

10. Pronuclei formation — The sperm nucleus decondenses and swells to form the male pronucleus. Simultaneously, the egg nucleus forms the female pronucleus. Both pronuclei migrate toward each other.

11. Amphimixis (Syngamy) — The two pronuclei fuse, combining their haploid chromosome sets to form the zygote (2n = 46). This is the moment a genetically unique new individual is created.

Significance of Fertilization

Fertilization achieves several biologically critical outcomes:

• Restores the diploid chromosome number (2n) — combining n from the sperm and n from the ovum
• Determines the sex of the offspring: if the sperm carries an X chromosome → XX (female); if it carries a Y chromosome → XY (male). Since all ova carry X, the sperm determines sex.
• Introduces genetic variation through the combination of two different parental genomes — the basis of sexual reproduction's evolutionary advantage
• Activates the ovum for development — the metabolically dormant oocyte is stimulated to begin rapid cell division
• Provides the centriole (from the sperm's middle piece) needed for the first cleavage division of the zygote

---

Types of Eggs (Based on Yolk)

The amount and distribution of yolk (stored nutrients) in an egg strongly influences how the embryo develops. Yolk provides nutrition to the developing embryo before it can feed independently or receive nutrition from a placenta.

Type	Yolk Amount	Example
Alecithal	No yolk (or negligible)	Sea urchin, Amphioxus
Microlecithal	Very little yolk	Humans, placental mammals (yolk is minimal because the placenta provides nutrition)
Mesolecithal	Moderate yolk	Frog, Petromyzon (lamprey)
Macrolecithal / Megalecithal	Large amount of yolk	Birds, reptiles (the "yolk" of a chicken egg is the entire ovum)

Based on Yolk Distribution

Type	Distribution	Example
Isolecithal / Homolecithal	Yolk evenly distributed throughout the cytoplasm	Sea urchin, humans
Telolecithal	Yolk concentrated at the vegetal pole (lower half), with the nucleus and active cytoplasm at the animal pole (upper half)	Frog (moderately telolecithal), birds (strongly telolecithal)
Centrolecithal	Yolk concentrated in the centre of the egg, with the nucleus in a central island of cytoplasm	Insects (Drosophila)

Based on Shell / Egg Membranes

Every ovum is surrounded by protective membranes that play roles during fertilization and early development:

Membrane	Feature
Vitelline membrane	The innermost membrane, directly around the ovum's plasma membrane. Forms the first barrier.
Zona pellucida	A thick glycoprotein layer around the mammalian ovum. Contains ZP1, ZP2, and ZP3 proteins — ZP3 acts as the sperm receptor for species-specific binding. Undergoes hardening after fertilization to block polyspermy.
Corona radiata	The outermost layer — composed of follicular (granulosa) cells that remain attached to the oocyte after ovulation. These cells provide nutrients and are dispersed by sperm hyaluronidase during fertilization.

---

Cleavage

After fertilization, the zygote begins a series of rapid mitotic divisions called cleavage. These divisions are unique because they occur without any growth — the total cell mass stays the same, but the number of cells increases rapidly while individual cell size decreases progressively.

Types of Cleavage

Type	Description	Example
Holoblastic equal	The entire egg divides completely; resulting cells (blastomeres) are equal in size	Sea urchin, humans
Holoblastic unequal	Complete division but blastomeres are unequal — smaller micromeres (at animal pole) and larger macromeres (at vegetal pole, weighed down by yolk)	Frog
Meroblastic discoidal	Only a small disc of cytoplasm at the animal pole divides (the massive yolk at the vegetal pole prevents cleavage there)	Birds, reptiles
Meroblastic superficial	Division occurs only on the surface of the egg (the yolk in the center is not cleaved)	Insects

Why does yolk affect cleavage pattern?

Yolk is denser and more viscous than cytoplasm. During cell division, the mitotic spindle must pull the cell apart. In eggs with large amounts of yolk (macrolecithal), the yolk physically resists cleavage — the furrow cannot penetrate the dense yolk mass. This is why cleavage in bird eggs is restricted to a small disc of cytoplasm (the blastodisk) sitting atop the massive yolk. In eggs with little or no yolk (like human eggs), nothing impedes cleavage, so division is complete and equal.

Stages After Cleavage

1. Morula — A solid ball of 8–16 cells formed by about 3–4 days after fertilization. It resembles a mulberry (Latin: morula = mulberry). At this stage, the embryo is still in the fallopian tube, moving toward the uterus.

2. Blastula / Blastocyst (the blastula is called a blastocyst in mammals):

   • Trophoblast — the outer layer of cells. This will form the embryo's contribution to the placenta and other extra-embryonic membranes. Trophoblast cells do NOT become part of the baby.
   • Inner cell mass (ICM) / Embryoblast — a cluster of cells at one pole of the blastocyst. This mass develops into the entire embryo (all body tissues). Embryonic stem cells are derived from the ICM.
   • Blastocoel — a fluid-filled cavity within the blastocyst

---

Implantation

Implantation is the process by which the blastocyst embeds itself into the uterine wall, establishing the physical connection between mother and embryo.

• The blastocyst reaches the uterus approximately 6–7 days after fertilization
• The trophoblast cells at the embryonic pole make contact with and attach to the endometrium → implantation begins
• Implantation usually occurs in the posterior wall of the uterus (upper body region)
• After attachment, the trophoblast differentiates into two layers:
  • Cytotrophoblast (inner layer) — a cellular layer of individual cells that serves as a stem cell population, continuously supplying new cells to the syncytiotrophoblast
  • Syncytiotrophoblast (outer layer) — a continuous, multinucleated mass (no cell boundaries) that is highly invasive. It erodes the endometrium, burrowing the embryo deeper into the uterine wall and eventually forming the fetal side of the placenta. It also begins secreting hCG (human chorionic gonadotropin) to maintain the corpus luteum.

---

Gastrulation

Gastrulation is one of the most important events in embryonic development — the conversion of the simple blastula (single-layered) into a gastrula with 3 primary germ layers. These three layers are the foundation from which all body organs and tissues will develop.

Germ Layer	Derivatives
Ectoderm (outer layer)	Skin (epidermis), hair, nails, sweat glands; entire nervous system (brain, spinal cord, nerves); lens of eye; enamel of teeth; adrenal medulla
Mesoderm (middle layer)	Muscles (skeletal, smooth, cardiac), bones, cartilage, connective tissue, blood and blood vessels, kidneys, gonads (testes/ovaries), heart, spleen, adrenal cortex
Endoderm (inner layer)	Lining of the alimentary canal (gut epithelium), lining of the respiratory tract, liver, pancreas, thyroid, parathyroid, thymus, urinary bladder, urethra lining

Methods of Gastrulation

The cells of the blastula rearrange themselves through several types of cell movements:

• Epiboly — spreading and thinning of the outer cell layer to cover the entire embryo
• Emboly — inward movement of cells to form the inner layers. This includes:
  • Invagination — infolding of a sheet of cells (like pushing in the side of a rubber ball)
  • Involution — inward rolling of cells over a lip or edge
  • Ingression — individual cells migrate inward from the surface
  • Delamination — splitting of a single layer into two parallel layers

---

Extra-Embryonic Membranes

These membranes develop from the embryo but are NOT part of the embryo itself — they provide protection, nutrition, and waste management during development and are discarded at birth.

Membrane	Origin	Function
Amnion	Ectoderm + somatic mesoderm	Encloses the embryo in a fluid-filled sac. The amniotic fluid acts as a shock absorber (protects against mechanical injury), prevents desiccation (drying out), maintains a constant temperature, and allows free fetal movement for proper musculoskeletal development.
Chorion	Trophoblast + somatic mesoderm	The outermost extra-embryonic membrane. Its surface develops finger-like chorionic villi that interdigitate with the uterine tissue to form the fetal component of the placenta.
Yolk sac	Endoderm + splanchnic mesoderm	In birds and reptiles, it nourishes the embryo from the yolk. In humans, it is vestigial (small, since humans have very little yolk) but serves an important early role: it is the first site of blood cell formation (hematopoiesis) in the embryo and gives rise to primordial germ cells.
Allantois	Endoderm + splanchnic mesoderm	In birds and reptiles, it stores metabolic waste (uric acid) and participates in gas exchange. In humans, it is largely vestigial but its blood vessels become the umbilical cord blood vessels (two umbilical arteries and one umbilical vein), which are the lifeline between fetus and placenta.

Why is amniocentesis performed?

**Amniocentesis** is a prenatal diagnostic procedure in which a small sample of **amniotic fluid** (containing fetal cells shed from the skin, urinary tract, and other surfaces) is withdrawn using a needle. These fetal cells can be cultured and their chromosomes analyzed (karyotyping) to detect genetic abnormalities like **Down syndrome (trisomy 21)**, **Turner syndrome (45,X)**, and **neural tube defects** (detected by elevated alpha-fetoprotein in the fluid). It is typically performed between weeks 15–18 of pregnancy.

---

Placenta

The placenta is a remarkable temporary organ that develops during pregnancy, connecting the developing embryo/fetus to the uterine wall. It is the sole interface between maternal and fetal circulations — performing functions that the fetus's own organs cannot yet handle (lungs, kidneys, digestive system, endocrine glands).

Classification

Criterion	Type in Humans
By layers between maternal and fetal blood	Haemochorial — maternal blood directly bathes the fetal chorionic villi (only the fetal capillary endothelium separates fetal blood from maternal blood — the most intimate contact possible)
By shape	Discoid — disc-shaped, ~20 cm diameter and ~2.5 cm thick at term
By villi distribution	Deciduate — part of the uterine tissue (decidua) is shed along with the placenta at birth
By implantation	Interstitial — the blastocyst is fully embedded within the endometrium

Types of Placenta by Barrier Layers

The number of tissue layers separating maternal and fetal blood varies across mammals, affecting the efficiency of exchange:

Type	Layers	Example
Epitheliochorial	6 layers (all intact — all 3 maternal + all 3 fetal layers)	Pig, horse — least intimate contact
Syndesmochorial	5 layers (maternal epithelium eroded)	Cow, sheep, goat
Endotheliochorial	4 layers	Dog, cat
Haemochorial	3 layers (all maternal barriers removed; maternal blood directly contacts chorionic villi)	Humans, rabbit
Haemoendothelial	1 layer (only fetal capillary endothelium remains)	Rat, guinea pig — most intimate contact

Placental Hormones

The placenta functions as a powerful endocrine gland, producing several hormones essential for maintaining pregnancy:

Hormone	Function
hCG (Human Chorionic Gonadotropin)	Maintains the corpus luteum during early pregnancy (preventing its degeneration), ensuring continued progesterone production. It is the hormone detected by pregnancy tests (urine or blood). hCG levels peak at 8–10 weeks and then decline as the placenta itself takes over hormone production.
Relaxin	Produced in late pregnancy; softens the cervix and relaxes the pubic symphysis (the joint between the two pelvic bones) at the time of delivery, widening the birth canal
hCS / hPL (Human Chorionic Somatomammotropin / Human Placental Lactogen)	Stimulates mammary gland development in preparation for lactation; mobilizes maternal glucose and fatty acids to ensure adequate fetal nutrition (makes glucose available to the fetus by inducing maternal insulin resistance)
CRH (Corticotropin-Releasing Hormone)	Acts as a "placental clock" — rising CRH levels toward term trigger increased fetal cortisol production, which promotes fetal lung maturation (surfactant production) and initiates the hormonal cascade leading to labor

The placenta also produces estrogen and progesterone in increasing quantities. By the end of the first trimester (~12 weeks), the placenta takes over hormone production from the corpus luteum, which then degenerates. Progesterone from the placenta maintains the uterine lining and prevents contractions throughout pregnancy.

Functions of Placenta (9 Key Functions)

1. Nutrition — supplies all nutrients (glucose, amino acids, fatty acids, vitamins, minerals) from maternal blood to fetal blood via diffusion, active transport, and facilitated diffusion
2. Respiration — O₂ diffuses from maternal blood (PO₂ ~100 mmHg) → fetal blood (PO₂ ~30 mmHg); CO₂ moves in the reverse direction. Fetal hemoglobin (HbF) has a higher O₂ affinity than adult Hb, facilitating O₂ transfer.
3. Excretion — removes metabolic waste products (urea, CO₂, creatinine, bilirubin) from fetal blood into maternal blood for elimination by the mother's kidneys and lungs
4. Hormone production — hCG, hPL, estrogen, progesterone, relaxin, CRH (as detailed above)
5. Immunological protection — transfers maternal IgG antibodies across the placenta to the fetus, providing passive immunity that protects the newborn for the first few months of life
6. Storage — stores glycogen, fats, and iron in early pregnancy, releasing them as the fetus needs them
7. Barrier function — prevents the mixing of maternal and fetal blood (they remain separate). The placental barrier filters out most pathogens and harmful substances. However, it is NOT a perfect barrier — certain pathogens can cross: rubella virus, HIV, syphilis (Treponema pallidum), and toxoplasma. Many drugs, alcohol, and nicotine also cross freely.
8. Endocrine role — acts as a temporary endocrine gland producing multiple hormones essential for pregnancy maintenance
9. Maintains pregnancy — progesterone from the placenta suppresses uterine contractions (keeps the myometrium quiescent), preventing premature labor

---

Parturition (Childbirth)

Parturition is the process of delivering the fetus, occurring after a gestation period of approximately ~280 days (40 weeks / 9 months).

Neuroendocrine Mechanism (4 Steps)

The initiation of labor is a complex hormonal cascade:

1. Fetal cortisol rises — as the fetus matures, its adrenal glands produce increasing amounts of cortisol. This cortisol stimulates the placenta to increase CRH and estrogen production while decreasing progesterone.
2. Estrogen increases — rising estrogen levels have two critical effects: they stimulate the uterine myometrium to produce oxytocin receptors (making the uterus responsive to oxytocin), and they overcome the "progesterone block" that had been keeping the uterus relaxed.
3. Oxytocin released — the posterior pituitary gland releases oxytocin in response to cervical stretching. Oxytocin causes powerful rhythmic contractions of the uterine myometrium.
4. Positive feedback loop (Ferguson reflex) — uterine contractions push the baby's head against the cervix → cervical stretching sends nerve signals to the hypothalamus → more oxytocin is released → stronger and more frequent contractions → more stretching → even more oxytocin. This escalating cycle continues until delivery is complete.

Stages of Parturition

Stage	Events	Duration
Stage 1: Dilation	Regular uterine contractions progressively dilate the cervix to ~10 cm (full dilation). The amniotic membrane may rupture ("water breaking"). This is the longest stage.	~12 hours (first birth); shorter in subsequent births
Stage 2: Expulsion	The baby is delivered, usually head first (cephalic presentation). Strong uterine contractions combined with voluntary abdominal pushing expel the baby through the birth canal. The umbilical cord is clamped and cut.	20–60 minutes
Stage 3: Placental	The placenta detaches from the uterine wall and is expelled along with the fetal membranes — collectively called the afterbirth. Uterine contractions continue to minimize bleeding from the placental site.	15–30 minutes

---

Lactation

Lactation is the secretion of milk from the mammary glands, beginning after childbirth and continuing as long as the infant continues to suckle.

• Colostrum — the first secretion from the mammary glands, produced during the initial 2–3 days after delivery. It is a yellowish, thick fluid that differs significantly from mature milk:

  • Rich in IgA antibodies — provides passive mucosal immunity to the newborn's gut, protecting against gastrointestinal infections
  • High in proteins, vitamin A, and minerals
  • Lower in fat and lactose compared to mature milk
  • Also contains growth factors and immune cells (macrophages, lymphocytes)

• Prolactin (from the anterior pituitary) — the primary hormone responsible for milk production (mammogenesis and lactogenesis). Prolactin levels are high during pregnancy but milk production is suppressed by high progesterone and estrogen. After delivery, when placental hormones drop, prolactin can fully activate milk synthesis.

• Oxytocin (from the posterior pituitary) — stimulates milk ejection (the let-down reflex). When the baby suckles, nerve signals from the nipple travel to the hypothalamus, triggering oxytocin release. Oxytocin causes contraction of myoepithelial cells surrounding the mammary alveoli, squeezing milk into the ducts and toward the nipple.

• Breastfeeding acts as a natural contraceptive — this is called lactational amenorrhea. The mechanism: suckling stimulates prolactin secretion, and high prolactin levels suppress GnRH release from the hypothalamus → reduced FSH and LH → ovulation is inhibited → menstruation does not occur. This is effective for about 6 months if the mother is exclusively breastfeeding.

Why is breastfeeding recommended for at least 6 months?

The World Health Organization (WHO) recommends **exclusive breastfeeding for the first 6 months** of life because: (1) breast milk provides complete, perfectly balanced nutrition for the infant; (2) it contains antibodies (especially IgA) that protect against infections; (3) it is easily digestible; (4) it reduces the risk of allergies, asthma, and childhood obesity; (5) it promotes mother-infant bonding through skin-to-skin contact and oxytocin release; (6) it provides lactational amenorrhea (natural birth spacing); (7) it reduces the mother's risk of breast and ovarian cancer. After 6 months, complementary foods should be introduced alongside continued breastfeeding up to 2 years or beyond.

---

Gestation Periods of Animals

The gestation period (the time from fertilization to birth) varies enormously across mammals, generally correlating with body size and the degree of development of the offspring at birth:

Animal	Gestation Period
Rabbit	28–32 days
Cat	58–65 days
Dog	58–63 days
Pig	112–115 days (~3 months, 3 weeks, 3 days — a useful mnemonic)
Sheep/Goat	145–155 days (~5 months)
Human	270–290 days (~280 days / 40 weeks)
Cow	275–290 days (~9 months, similar to humans)
Horse	330–345 days (~11 months)
Elephant	620–680 days (~22 months) — the longest gestation of any land animal

---

Beginner's Box — Practice Questions

Set 1: Fertilization and Embryology

1. The cortical reaction prevents: Answer: Polyspermy

2. The outer layer of the blastocyst is called: Answer: Trophoblast

3. Implantation occurs approximately ______ days after fertilization. Answer: 6–7 days

4. Which germ layer gives rise to the nervous system? Answer: Ectoderm

5. What type of placenta do humans have? Answer: Haemochorial

Set 2: Placenta and Parturition

1. Which hormone is the basis of pregnancy tests? Answer: hCG (Human Chorionic Gonadotropin)

2. Colostrum is rich in which type of antibody? Answer: IgA

3. The hormone responsible for milk ejection reflex is: Answer: Oxytocin

4. The positive feedback mechanism of parturition is called: Answer: Ferguson reflex

5. Which class of antibody can cross the placental barrier? Answer: IgG

Set 3: Miscellaneous

1. Spermatogenesis takes approximately how many days? Answer: 65–74 days

2. The primary oocyte is arrested in which stage of meiosis? Answer: Prophase I (diplotene stage)

3. Centrolecithal eggs are found in: Answer: Insects

4. Which extra-embryonic membrane is responsible for shock absorption? Answer: Amnion (amniotic fluid)

5. Gestation period of elephant is approximately: Answer: 22 months (620–680 days)

---

Summary Cheat Sheet

Concept / Topic	Key Details / Explanation
Fertilization	Site: ampulla of fallopian tube Mechanism: sperm undergoes capacitation (in female tract) → acrosome reaction (releases hyaluronidase + acrosin to penetrate zona pellucida) → sperm fuses with oocyte membrane Cortical reaction: prevents polyspermy (cortical granules harden zona pellucida) Triggers completion of Meiosis II in secondary oocyte Significance: restores diploid chromosome number (2n = 46), determines sex (XX or XY)
Egg Types — By Yolk Amount	Alecithal: no/negligible yolk (e.g., humans, placental mammals) Microlecithal: small amount (e.g., sea urchin) Mesolecithal: moderate yolk (e.g., frog) Macrolecithal/Megalecithal: large amount (e.g., birds, reptiles)
Egg Types — By Yolk Distribution	Isolecithal: yolk evenly distributed (e.g., sea urchin) Telolecithal: yolk concentrated at vegetal pole (e.g., frog, bird) Centrolecithal: yolk in centre (e.g., insects)
Egg Membranes	Primary: vitelline membrane (secreted by oocyte itself) Secondary: zona pellucida (secreted by follicle cells) — in mammals Tertiary: albumen, shell membrane, shell (secreted by oviduct/uterus) — in birds
Cleavage Types	Holoblastic (complete): entire egg divides — equal (isolecithal eggs) or unequal (mesolecithal) Meroblastic (incomplete): only cytoplasmic part divides — discoidal (telolecithal, e.g., birds) or superficial (centrolecithal, e.g., insects) Human cleavage: holoblastic, equal, relatively slow
Morula and Blastocyst	Morula: solid ball of 16–32 cells (day 3–4) Blastocyst: hollow ball with fluid-filled blastocoel (day 5–6) Trophoblast: outer cell layer → forms placenta Inner cell mass (ICM/embryoblast): becomes the embryo
Implantation	Blastocyst embeds in endometrium at ~6–7 days post-fertilization Trophoblast invades endometrium → forms chorionic villi Site: usually posterior wall of uterus (upper body) Ectopic pregnancy: implantation outside uterus (usually fallopian tube) — medical emergency
Gastrulation	Process of forming 3 germ layers from ICM: Ectoderm (outer): skin, hair, nails, nervous system, sense organs, tooth enamel Mesoderm (middle): muscles, bones, blood, heart, kidneys, gonads, connective tissue Endoderm (inner): lining of GI tract, liver, pancreas, lungs, thyroid, urinary bladder
Extra-Embryonic Membranes	Amnion: encloses embryo in amniotic fluid (shock absorption, prevents desiccation) Chorion: outermost membrane, forms chorionic villi → contributes to placenta Yolk sac: nutrition in egg-laying animals; in humans produces early blood cells Allantois: waste storage in egg-laying animals; in humans contributes to umbilical cord blood vessels
Placenta	Type in humans: haemochorial (maternal blood directly bathes foetal chorionic villi) Connected to foetus via umbilical cord (2 umbilical arteries carry deoxygenated blood to placenta; 1 umbilical vein carries oxygenated blood to foetus)
Placental Hormones	hCG (Human Chorionic Gonadotropin): maintains corpus luteum in early pregnancy; basis of pregnancy tests hPL (Human Placental Lactogen): prepares breasts for lactation Estrogen: uterine growth Progesterone: maintains endometrium, prevents uterine contractions Relaxin: softens cervix and pelvic ligaments near delivery
Placental Functions (9)	Nutrition, respiration (O₂/CO₂ exchange), excretion (waste removal), hormone production, immune protection (IgG antibodies cross placenta), storage, barrier function (blocks most pathogens but NOT viruses like rubella, HIV), endocrine organ, haematopoiesis (early)
Parturition (Childbirth)	Ferguson reflex: positive feedback — foetal head pushes against cervix → nerve signals → hypothalamus → oxytocin release → uterine contractions → more pressure → more oxytocin 3 stages: (1) Dilation (cervix dilates to 10 cm, longest stage), (2) Expulsion (baby delivered), (3) Placental stage (afterbirth — placenta + membranes expelled)
Lactation	Colostrum (first milk, 2–3 days): rich in IgA antibodies, proteins, less fat Prolactin (anterior pituitary): stimulates milk production Oxytocin (posterior pituitary): stimulates milk ejection (let-down reflex) Breastfeeding acts as natural contraceptive (lactational amenorrhoea)
Gestation Periods	Rabbit: 28–32 days Cat/Dog: 58–65 days Pig: 112–115 days (~3 months, 3 weeks, 3 days) Sheep/Goat: 145–155 days (~5 months) Human: 270–290 days (~280 days / 40 weeks) Cow: 275–290 days (~9 months) Horse: 330–345 days (~11 months) Elephant: 620–680 days (~22 months) — longest of any land animal