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