Embryonic Development
Question of the Day
- How do you get from this (.) to...
Ovulation
- Female born with 400,000 oocytes
- One ovum released per month
- FSH stimulates follicles of ovary to ripen
- LH (lutenizing hormone) triggers ovulation (ripest follicle bursts, releasing ovum)
Ovum
- Ovum travels 4-5 days down muscular, cilia-lined Fallopian tube (oviduct)
- Ovum (0.1 mm) surrounded by
-- zona pellucida (clear zone, jelly-like)
-- "nurse" cells
- Unfertilized ovum dies after 48 hours
Testes
- Seminiferous tubules (2 x 400 ft) produce sperm when stimulated by FSH.
- Interstitial cells produce testosterone when stimulated by LH.
Vasectomy
- Vasectomy cuts vas deferens (sperm duct), not the blood vessels carrying testosterone.
- Contrast castration.
Seminal fluid (3 ml/ejaculate)
- Testes = 300-400 million sperm
- Seminal vesicles = fructose (sugar) for sperm motility
- Prostate = alkaline fluid to neutralize the normal acidity of female reproductive tract
- Bulbourethral glands = alkaline mucous neutralizes urine acidity before ejaculation
Mucous plug of cervix
- Microbes can’t penetrate the mucous.
- Mucous structure changes 2 days a month
- Only strongest sperm can penetrate.
- Mucous cleans bacteria off sperm.
- Female orgasm helps sperm thru plug.
Fertilization
- Only 100 sperm reach ovum.
- Enzymes from sperm’s acrosome digest their way through the zona pellucida.
- Change in zona pellucida prevents entry
of more than one sperm.
Fertilization & implantation
- Male and female nuclei fuse, resulting cell is called a zygote.
- Sperm penetration initiates "cleavage"
of zygote to form a hollow blastula.
- Blastula implants in uterine lining.
Three developmental processes
- Cleavage
- Gastrulation
- Induction
Cleavage
- Zygote divides repeatedly to produce a blastula, a hollow ball of cells.
- Increase in cell number, but no increase
in overall size.
- Pattern depends on amount of yolk in ovum; inert yolk divides more slowly.
Patterns of cleavage
- Even in sea urchin.
- Uneven in frog.
- Disc only in chicken.
- "Disc"-like in humans, too
Gastrulation
- Blastula becomes a 3-layered gastrula.
- Three primary germ layers:
-- ectoderm (outer)
-- endoderm (inner)
-- mesoderm (middle)
Induction
- The 3 primary germ layers interact to form organs and organ systems:
- Ectoderm = > skin and nervous system.
- Mesoderm => muscles, skeleton, kidneys, circulatory system.
- Endoderm = > digestive system, lungs.