3.2 Gametogenesis, Fertilization & Development

Why This Section Matters

After the hormonal control of Section 3.1, the diploma turns to how gametes are made and how an embryo forms. The classic exam contrast is spermatogenesis versus oogenesis, and numerical-response items frequently ask for how many functional gametes a process yields. Knowing the germ layers and the placenta's functions secures the rest of Unit B's marks.

Spermatogenesis vs Oogenesis

Both begin with a diploid (2n) cell that undergoes meiosis to make haploid (n) gametes, but the products and timing differ sharply.

The unequal division in oogenesis concentrates nutrients into one large egg; the tiny polar bodies degenerate and are not functional gametes.

Fertilization

Fertilization normally occurs in the upper oviduct (Fallopian tube). When a sperm reaches the egg, the acrosome — a cap of enzymes on the sperm head — releases those enzymes to digest a path through the egg's protective layers.

Once one sperm penetrates, the egg's membrane changes to block additional sperm (preventing polyspermy) and the egg finally completes meiosis II. The sperm and egg nuclei fuse, restoring the diploid (2n) chromosome number (46 in humans) and forming a zygote. This fusion of two haploid (n = 23) gametes is the moment a new, genetically unique organism begins, and it is why each parent contributes half of the offspring's chromosomes.

Early Embryonic Development

After fertilization, development proceeds in well-defined stages:

1. Cleavage - rapid mitotic divisions split the zygote into many smaller cells without overall growth.
2. Morula - a solid ball of cells produced by cleavage.
3. Blastocyst - a hollow ball of cells with a fluid-filled cavity and an inner cell mass; this is the stage that implants in the endometrium (about day 6-7).
4. Gastrulation - cells migrate and reorganize to form the three primary germ layers.

Implantation marks the start of pregnancy proper, and the outer cells of the blastocyst (the trophoblast) begin forming the membranes and placenta while the inner cell mass becomes the embryo itself. Note that cleavage divisions are mitotic, so every cell keeps the full diploid chromosome set even as the cells become smaller and more numerous.

The Three Germ Layers

Gastrulation produces three layers, each giving rise to specific tissues:

• Ectoderm (outer) - skin (epidermis) and the entire nervous system, including brain and spinal cord.
• Mesoderm (middle) - muscles, bones, blood, heart, kidneys, and the circulatory and reproductive systems.
• Endoderm (inner) - the lining of the digestive and respiratory tracts, plus glands such as the liver and pancreas.

A reliable memory aid: ectoderm = "attract" (outside/touch and nerves), endoderm = "inside tubes" (gut and lungs), mesoderm = "middle/movement" (muscle and bone).

The Placenta and Its Functions

The placenta is a temporary organ formed from both fetal tissue (chorion) and the maternal uterine lining. It is the interface between mother and fetus, connected by the umbilical cord, which carries blood between the fetus and placenta.

Key functions:

• Exchange of nutrients and oxygen from maternal to fetal blood, and wastes (CO2, urea) from fetal to maternal blood — by diffusion, with the two blood supplies kept separate (they do not mix).
• Hormone secretion, including hCG (human chorionic gonadotropin), which maintains the corpus luteum early on, plus progesterone and estrogen later to sustain the pregnancy. hCG is what home pregnancy tests detect.

Extraembryonic Membranes

Four membranes form outside the embryo to support development:

The amnion holds the amniotic fluid in which the fetus is suspended.

Stages of Pregnancy

Human pregnancy lasts about 38-40 weeks, divided into three trimesters:

• First trimester (weeks 1-12): implantation, organogenesis (major organs and systems form), and the embryo becomes a fetus around week 8; the period of highest vulnerability to teratogens.
• Second trimester (weeks 13-27): rapid growth, movement is felt, and most organ systems become functional.
• Third trimester (weeks 28-birth): major weight gain, lung and brain maturation, and the fetus prepares for life outside the uterus.

Birth (parturition) is driven by hormones such as oxytocin, which stimulates uterine contractions through positive feedback: contractions push the fetus against the cervix, triggering more oxytocin and stronger contractions until delivery. After birth, prolactin promotes milk production and oxytocin triggers milk release, completing the reproductive cycle.