Stages of Human Development 1. Fertilization 2. Embryo Cleavage Gastrulation Neurulation 3. Fetus 4. Childhood 5. Puberty 6. Adulthood (Johnson does not discuss development past the fetus, so I will assign some additional reading.)

Fertilization: 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 next figure.

Figure 43.4 figure jpg

Fertilization: Sperm and egg contribute differentially to the zygote. The sperm contributes a haploid nucleus and, in some species, a centriole. The egg contributes a haploid nucleus, cytoplasm including nutrients, ribosomes, mitochondria, and informational molecules that control early stages of development.

Fertilization: The cytoplasmic contents of the egg are not distributed evenly, and are used to set up the major axes of the future embryo. See next figure.

Figure 43.5 Fertilized Egg

Fertilization: The animal pole will become the future embryo. The vegetal pole will become yolk or yolk cells (food for the developing embryo, like the yolk of a chicken egg).

1. After fertilization comes: Embryogenesis 2. Three subphases: Cleavage Gastrulation Neurulation Stages of Animal Development

Cleavage: Repackaging the Cytoplasm Cleavage follows fertilization. During cleavage, the cytoplasm of the zygote is repackaged into smaller and smaller cells.

Cleavage: 1.Cleavage is a period of rapid cell division without cell growth or gene expression. 2.The result of cleavage is a mass of ~1000 cells called a blastula that is ~size as the egg.

Early Development: the Embryo Cleavage  Zygote to Blastula  Increase in cell number, not size  Blastula Hollow sphere of dividing cells Blastocyst—in mammals

Embryonic Development Blastocyst (the mammalian blastula)  One week for egg to travel to uterus About 100 cells in human  Trophoblast (gray cells) Is an outer layer of blastocyst cells Becomes the placenta  Inner cell mass (red cells) Clustered inside the trophoblast Becomes the embryo

Implantation Into endometrium  Inner lining of uterus Placenta develops  2 Network of blood vessels  Exchange nutrients and waste Amnion  Sac surrounding embryo Yolk sac  Produces blood cells  Normally a source for food, but not in mammals

Embryo phase: Gastrulation After cleavage, the blastula develops three layers and is called the gastrula  Different layers give rise to various tissues Endoderm Mesoderm Ectoderm  Three layers move relative to one another

Table 43.1 table jpg Do not memorize!

Embryo phase: Gastrulation 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 (move from one place to another).

Embryo phase: Neurulation The Neurula follows the Gastrula. Cells migrate over the blastopore and induce the overlying ectoderm to fold in on itself to form a neural tube. The nervous system develops from the neural tube. The next Figure shows neurulation views from the top and inside.

Fetal Development Follows neurulation  Development of organs  1 st trimester in humans Includes embryo & early fetal development Vulnerable to disruption Most miscarriages Susceptible to the environment (smoking, alcohol)  2 nd and 3 rd trimesters Further development of existing organs & growth

Childbirth Estrogen levels increase Oxytocin  Produced by fetus and mother’s pituitary gland  Stimulates uterine muscle contraction  Positive feedback 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.

Gene Expression: Development Embryo development depends on gene expression Timing of expression is complex, yet vital Cell and timing specific expression Controlled by cascades of gene expression that coordinate development of specific structures. (SRY is a cascade gene that changes the expression of many genes that cause the dev of a boy.)

Control of Development Genes are turned on or off during development Gene activation during development  Transcription of genes is regulated as is the processing of transcribed RNA Changes gene expression resulting in development  Chemicals and hormones Switch genes on or off, controlling or affecting development  Environment Can influence gene regulation in many animals Why an embryo/fetus must be protected fr/ env. insults (alcohol, drugs, pollution, etc.)

Changes in Development Apoptosis  Programmed cell death  Remove cells from further development Webbing between digits Remove unstimulated neurons

Childhood Dev does not end w/ birth Growth after birth  Continuation of fetal growth Humans—one quarter of life reaching full size  Many tissues/organs developing Immune system Brain

Puberty Continuation of fetal growth Many tissues/organs still developing Brain till early 20s Sexual development  Secondary sex characteristics  Reproductive organs develop

Adulthood Cessation of Growth Sexual maturation Cells still divide Development only ends at what time?

Post-Reproductive Years Females  Reproduce less effectively after ~40 years Eggs decline in quality  Menopause End of reproductive life Males  Decline in sperm Quality and quantity  No distinct end of reproductive life

Regeneration of Cells Limited life span of cells  Must regenerate and be replaced  Example: skin cells  Two methods for regeneration Simple cell division Stem cells

Stem Cells Unspecialized cells Can develop into mature cells Unspecialized to various degrees  Depending on tissue and stage of life  Embryonic stem cells Cells haven’t undergone differentiation Greatest potential for developing into any tissue

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: