Stages of Human Development Fertilization Embryo Cleavage Gastrulation Neurulation Fetus Childhood Puberty 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 43-04.jpg Figure 43.4 Figure 43.4

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.

Fertilized Egg Figure 43.5 Figure 43.5

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).

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

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

Cleavage: Cleavage is a period of rapid cell division without cell growth or gene expression. 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 Placenta develops Amnion Yolk sac 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 43-01.jpg Table 43.1 Table 43.1 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 1st trimester in humans Includes embryo & early fetal development Vulnerable to disruption Most miscarriages Susceptible to the environment (smoking, alcohol) 2nd and 3rd trimesters Further development of existing organs & growth

Childbirth Estrogen levels increase Oxytocin Cervix opens 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

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? Oogenesis Spermatogenesis The correct answer is B. Answer A is incorrect because meiosis in the female results in one egg and three polar bodies instead of four equally functional ova.

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? Embryonic stem cells Bone marrow stem cells Skin stem cells Answer A is correct because embryonic stem cells have undergone the least amount of differentiation. Therefore, they possess the greatest ability to become any tissue of the body. Bone marrow stem cells (hematopoietic stem cells) can become any of the blood cells, but they have already undergone a series of differentiation steps. Skin stem cells likewise have undergone a series of differentiation and, although they can develop into a number of different types of skin cells, their “plasticity” or degree of differentiation is limited.

Concept Quiz Fertilization of the human egg occurs in which location? Ovary Uterus Oviduct The correct answer is C. The ovary is where the egg is produced. During ovulation it is released and swept into the oviduct. Most successful fertilization events take place in the oviduct, after which the egg completes meiotic division and spends the next week rapidly dividing and moving downward into the uterus where it is implanted. Fertilization usually occurs long before the egg reaches the uterus. Several changes must take place in the uterus prior to its acceptance of and implantation of the fertilized egg.

How Identical Twins Form: