HUMAN EMBRYOLOGY Mrs. Daniels AP Biology December 2005 Gametes join in fertilization • We’ve already learned how gametes are produced (gametogenesis) through meiosis • What process produces every other cell in the human body? • MITOSIS Fertilization • Divided into 4 steps: • 1. Contact and recognition (“Casual Introductions”) • Sperm undergo capacitation (further maturation) within the female reproductive tract • Recall that sperm were produced in the testes and matured in the epididymus until ejaculation • Sperm reach the egg in the oviduct where fertilization will occur • 2. Sperm Entry • Only ONE sperm is allowed to enter • Fast block - electrical charge in egg plasma membrane prevents polyspermy • Slow block - depolarization of egg plasma membrane due to Ca++ release • 3. Egg Activation • The release of calcium ions in egg plasma membrane also triggers protein synthesis • 4. Fusion • The sperm nucleus is propelled to the egg nucleus by microtubules Let the Division Begin! Cleavage follows fertilization • Cleavage is a series of rapid mitotic divisions (without cell growth) • The two-celled zygote divides repeatedly until a ball of 32 cells is formed • This is the morula - 32 cells • Continued divisions make the hollow blastula • These few cells are pluripotent (have the potential to become ANY of the 220 types of cells in the human body). • These are embryonic stem cells Gastrulation • At the end of the cleavage stage, cells making up the blastula move about and surface proteins help cells recognize each other • The gastrula is formed, which consists of 3 “germ layers” – Endoderm – Mesoderm – Ectoderm Gastrulation Extraembryonic Membranes • These membranes develop from the germ layers, but are NOT part of the embryo (they are lost at birth) • They lie outside of the embryo & provide protection and nourishment • Four exist in terrestrial vertebrates: • Chorion, amnion, allantois (stores nitrogenous waste in reptiles), & yolk sac (not found in humans, but yolk aids in formation of RBC Human Development • The gestation period lasts 266 days from fertilization to birth • Organogenesis (development of the organs and organ systems) begins with the nervous system • Think on this: • Do all animals have the same gestation period?