In mammals the female gamete released at ovulation is a secondary oocyte. It only completes meiosis II, and becomes a mature ovum, after a sperm enters it. It is often simply called the egg cell.
• Haploid nucleus: contains one set of chromosomes (23 in humans), paused at metaphase II.
• Large cytoplasm containing food reserves such as lipid droplets and proteins, plus mitochondria and ribosomes, to support the early embryo before it implants in the uterus.
• Cortical granules: vesicles just beneath the cell surface membrane containing enzymes. After one sperm enters, they release their contents by exocytosis in the cortical reaction, which hardens the zona pellucida so no further sperm can enter.
• Zona pellucida: a thick, clear layer of glycoproteins around the cell surface membrane. Sperm bind to receptors here, which triggers the acrosome reaction.
• Corona radiata: a layer of follicle cells from the ovary surrounding the zona pellucida, which sperm must pass through.Unlike a sperm, the egg cannot move by itself. After ovulation it is carried along the oviduct by the beating of cilia and by muscular contractions of the oviduct wall.The human egg is about 0.1 mm (100 µm) in diameter, one of the largest cells in the body and just visible to the naked eye.Worked example: the egg has a diameter of 100 µm and the sperm head a length of 5 µm, so the egg is 100 ÷ 5 = 20 times wider than the sperm head is long.
• Haploid nucleus: contains one set of chromosomes (23 in humans), paused at metaphase II.
• Large cytoplasm containing food reserves such as lipid droplets and proteins, plus mitochondria and ribosomes, to support the early embryo before it implants in the uterus.
• Cortical granules: vesicles just beneath the cell surface membrane containing enzymes. After one sperm enters, they release their contents by exocytosis in the cortical reaction, which hardens the zona pellucida so no further sperm can enter.
• Zona pellucida: a thick, clear layer of glycoproteins around the cell surface membrane. Sperm bind to receptors here, which triggers the acrosome reaction.
• Corona radiata: a layer of follicle cells from the ovary surrounding the zona pellucida, which sperm must pass through.Unlike a sperm, the egg cannot move by itself. After ovulation it is carried along the oviduct by the beating of cilia and by muscular contractions of the oviduct wall.The human egg is about 0.1 mm (100 µm) in diameter, one of the largest cells in the body and just visible to the naked eye.Worked example: the egg has a diameter of 100 µm and the sperm head a length of 5 µm, so the egg is 100 ÷ 5 = 20 times wider than the sperm head is long.