Four Stages of Growth and Development
- Gametogenesis: formation of sperm and eggs
- Embryonic development: from fertilization until birth
- Childhood: birth to puberty (reproductive maturity)
- Aging process: from puberty to death
First year biology texts only cover stages one and two. We have previously covered gametogenesis (males, females), so now we will look at embryonic development.
Stages of Embryonic Development
- Fertilization: sperm penetrates the egg, forming a diploid cell called a zygote.
- Cleavage: cell division without cell growth
- Morula: a solid mass of cells about the same size as the original zygote
- Blastula: liquid fills the center of the morula, forming a hollow sphere of cells called the blastula (think “water balloon in swimming pool”).
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Gastrula: cells on the outside of the blastula push inward, forming a new central cavity that connects with the outside.
- The three primary tissue layers (ectoderm, mesoderm, endoderm) form.
- Extraembryonic membrane development: in the amniotes (reptiles, birds, and mammals), tissue develops outside the embryo to support the embryo's development.
- Organogenesis (“creation of organs”): the development of different tissues and organs
Fertilization
Research published last year in the journal Science describes “a significant advance in understanding how the sperm and egg find each other”:
Mammals and Sea Urchins
|
Mammals |
Sea Urchins |
| Outer coating |
Cumulus: follicle cells in a gelatinous matrix |
Jelly coat: noncellular gelatinous matrix |
| Extracellular matrix |
Zona pellucida: made of glycoproteins |
Vitelline layer (envelope): made of glycoproteins |
| Both are deuterostomes (“second mouths”). |
Fertilization in Sea Urchins
- Contact: sperm contacts egg's jelly coat.
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Acromosomal reaction:
- Hydrolytic enzymes are released from the sperm head, which cut a hole in the jelly.
- A protrusion called the acrosomal process begins to grow from the sperm head.
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Growth of acrosomal process:
- The acrosomal process grows and binds to receptors on the egg's vitelline layer.
- It continues to grow until it contacts the egg's plasma membrane.
-
Fusion of plasma membrane:
- Plasma membranes of sperm and egg fuse.
-
Known as the "Fast block to polyspermy"
- Occurs within 1–3 seconds of fusion.
- An influx of sodium ions depolarizes the membrane.
- This prevents more than one sperm cell from fusing with the egg's plasma membrane.
- Entry of sperm nucleus: sperm nucleus enters the egg's cytoplasm.
- Creation of fertilization membrane
- A result of the "cortical reaction"
- Known as the "Slow block to polyspermy"
- Occurs within one minute of fusion.
- Egg's endoplasmic reticulum releases calcium ions into cytoplasm.
- This causes cortical granules to release their contents into the space between the vitelline layer and the plasma membrane (perivitelline space).
- Enzymes from the cortical granules cause the vitelline layer and plasma membrane to separate from one another, and cause the vitelline layer to become impenetrable to sperm.
- It is now called the fertilization membrane.
Fertilization in Mammals
