HUMAN EMBRYOLOGY

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