HUMAN EMBRYOLOGYMrs. DanielsAP Biology December 2005

Gametes join in fertilizationWeve already learned how gametes are produced (gametogenesis) through meiosisWhat process produces every other cell in the human body?MITOSIS

FertilizationDivided into 4 steps:1.Contact and recognition (Casual Introductions)Sperm undergo capacitation (further maturation) within the female reproductive tractRecall that sperm were produced in the testes and matured in the epididymus until ejaculationSperm reach the egg in the oviduct where fertilization will occur

2.Sperm EntryOnly ONE sperm is allowed to enterFast block - electrical charge in egg plasma membrane prevents polyspermySlow block - depolarization of egg plasma membrane due to Ca++ release

3.Egg ActivationThe release of calcium ions in egg plasma membrane also triggers protein synthesis

4.FusionThe sperm nucleus is propelled to the egg nucleus by microtubules

Let the Division Begin!

Cleavage follows fertilizationCleavage 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

GastrulationAt the end of the cleavage stage, cells making up the blastula move about and surface proteins help cells recognize each otherThe gastrula is formed, which consists of 3 germ layersEndodermMesodermEctoderm

Gastrulation

Extraembryonic MembranesThese 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 nourishmentFour 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 DevelopmentThe gestation period lasts 266 days from fertilization to birthOrganogenesis (development of the organs and organ systems) begins with the nervous system

Think on this:Do all animals have the same gestation period?