Stages of Human Development 1 2 Fertilization Embryo
Stages of Human Development 1. 2. Fertilization Embryo 1. 2. 3. 4. 5. 6. Cleavage Gastrulation Neurulation Fetus Childhood Puberty Adulthood
Fertilization: n n Fertilization involves fusion of sperm and egg plasma membranes. Entry of sperm into egg triggers blocks to polyspermy, and in mammals, signals the egg to complete meiosis and begin development. See Figures 43. 4
figure 43 -04. jpg Figure 43. 4
Fertilization: Sperm and egg contribute differentially to the zygote. n The sperm contributes a haploid nucleus and, in some species, a centriole. n The egg contributes a haploid nucleus, cytoplasm including nutrients, ribosomes, mitochondria, and informational molecules that control early stages of development. n
Fertilization: The cytoplasmic contents of the egg are not distributed evenly, and are used to set up the major axes of the future embryo. n See Figures 43. 5 n
Fertilized Egg Figure 43. 5
Fertilization: The animal pole will become the future embryo. n The vegetal pole will become yolk or yolk cells (food for the developing embryo, like the yolk of a chicken egg). n
Stages of Animal Development 1. 2. 3. After fertilization comes: Embryogenesis The Embryo is from fertilization to early organ development. Three subphases: Cleavage 2. Gastrulation 3. Neurulation 1.
Cleavage: Repackaging the Cytoplasm Cleavage follows fertilization. n During cleavage, the cytoplasm of the zygote is repackaged into smaller and smaller cells. n
Cleavage: 1. Cleavage is a period of rapid cell division without cell growth or gene expression. 2. The zygote is partitioned into blastomeres (cells). 3. The result of cleavage is a mass of ~1000 cells called a blastula that is ~size as the egg.
Early Development: the Embryo n Cleavage ¨ Zygote to Blastula ¨ Increase in cell number, not size ¨ Blastula Hollow sphere of early dividing cells n Blastocyst—in mammals n
Embryonic Development n Blastocyst (the mammalian blastula) ¨ Organ in mammals for embryo development ¨ One week for egg to travel to uterus n About 100 cells in human ¨ Trophoblast n n Outer layer of blastocyst cells Becomes the placenta ¨ Inner n n (gray cells) cell mass (red cells) Clustered inside the sphere Develop into the embryo
Implantation n Into endometrium ¨ Inner n lining of uterus Placenta develops ¨ 2 Network of blood vessels ¨ Exchange nutrients and waste n Amnion ¨ Sac n surrounding embryo Yolk sac ¨ Produces blood cells ¨ Normally a source for but not in mammals food,
Embryo phase: Gastrulation n After cleavage, the blastula develops three layers and is called the gastrula ¨ Different layers give rise to various tissues Endoderm n Mesoderm n Ectoderm n ¨ Three layers move relative to one another
table 43 -01. jpg Table 43. 1 Do not memorize!
Embryo phase: Gastrulation n Gastrulation is when most embryonic genes are first expressed. ¨ Prior, most proteins came from the mother. ¨ Many Gastrulas die due to errors or mutations in the embryonic genome. ¨ Gastrula cells migrate. Many malignancies express genes allowing gastrula cells to move.
Embryo phase: Neurulation The Neurula follows the Gastrula. n Cells migrate over the blastopore and induce the overlying ectoderm to fold in on itself to form a neural tube. n The nervous system develops from the neural tube. n The next Figure shows neurulation views from the top and inside. n
Fetal Development n Follows neurulation ¨ Development of organs ¨ 1 st trimester in humans Includes embryo & early fetus n Vulnerable to disruption n Most miscarriages n Susceptible to the environment (smoking, alcohol) n ¨ 2 nd and 3 rd trimesters n Further development of existing organs & growth
Childbirth n n Estrogen levels increase Oxytocin ¨ Produced by fetus and mother’s pituitary gland ¨ Stimulates uterine muscle contraction ¨ Placenta produces prostaglandins ¨ Positive feedback n n Contractions cause release of more oxytocin Cervix opens ¨ Fetus is pushed out --birth ¨ Placenta follows --afterbirth
· Birth: · First stage: opening of the cervix. · Second stage: expulsion of the baby from uterine contractions. · Third stage: expulsion of the placenta.
Control of Development n n Genes are turned on or off during development Gene activation during development ¨ Transcription of genes is regulated ¨ Processing of transcribed RNA n Changes gene expression ¨ Morphogens n Switch genes on or off ¨ Hormones n Can control development ¨ Environment n Can influence gene regulation in many animals
Changes in Development n Apoptosis ¨ Programmed cell death ¨ Remove cells from further development Webbing between digits n Remove unstimulated neurons n
Childhood n Growth after birth ¨ Continuation n of fetal growth Humans—one quarter of life reaching full size ¨ Many tissues/organs developing Immune system n Brain n
Puberty Continuation of fetal growth n Many tissues/organs still developing n n n Brain till early 20 s Sexual development ¨ Secondary sex characteristics ¨ Reproductive organs develop
Adulthood Cessation of Growth n Sexual maturation n Cells still divide n Development only ends at what time? n
Post-Reproductive Years n Females ¨ Reproduce n less effectively after 40 years Eggs decline in quality ¨ Menopause n n End of reproductive life Males ¨ Decline n ¨ No in sperm Quality and quantity distinct end of reproductive life
Regeneration of Cells n Limited life span of cells ¨ Must regenerate and be replaced ¨ Example: skin cells ¨ Two methods for regeneration Simple cell division n Stem cells n
Stem Cells Unspecialized cells n Can develop into mature cells n Unspecialized to various degrees n ¨ Depending on tissue and stage of life ¨ Embryonic stem cells Cells haven’t undergone differentiation n Greatest potential for developing into any tissue n
Concept Quiz Meiosis is the process of producing gametes in both spermatogenesis and oogenesis. Which of the two developments results in four gamete cells during human reproductive cell production? A. Oogenesis B. Spermatogenesis
Concept Quiz Stem cells are considered to be a great potential source for regenerating damaged or nonfunctional tissue. Which of the following stem cells provide the greatest potential for regenerating different types of tissues? A. Embryonic stem cells B. Bone marrow stem cells C. Skin stem cells
Concept Quiz Fertilization of the human egg occurs in which location? A. Ovary B. Uterus C. Oviduct
How Identical Twins Form:
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