By Lauren and Jocelyn

Fertilization  The first step in embryonic development is fertilization (the joining of male and female gametes)  Fertilization has three parts: Penetration Activation Nuclei fusion

Penetration  The sperm has swam up the female reproductive tract and meets the egg  In this stage, the sperm begins to tunnel into the eggs plasma membrane

Activation  Once a sperm penetrates the membrane, changes occur allowing no other sperm to enter  Stimulates second meiotic division making two egg nuclei  Establishes bilateral symmetry when the cytoplasm moves around the entry spot

Nuclei Fusion  The sperm nucleus fuses with the haploid egg nucleus and forms the diploid nucleus of the zygote.

Cell Cleavage  The second step in development is the division of the zygote into a large number of small cells  The overall volume of the zygote cytoplasm does not increase  The result is a mass of about 32 cells called a morula. Each cell in the morula is called a blastomere  Eventually, a hollow ball of 500 to 2,000 cells is formed called the blastula

Blastula  Each cell in the blastula is in contact with neighboring cells. These interactions determine what type of cell it will become.  Just before implant in the uterus, the blastula cells can develop into 210 different types of cells in the body (embryonic stem cells)

Gastrulation  Gastrulation is when certain groups of cells dent inward and roll inward from the surface of the blastula  Determine the basic developmental pattern of the embryo  By the end, the cells of the embryo have arranged into three primary germ layers: Ectoderm Mesoderm Endoderm

Gastrulation Cont.  The ectoderm forms the epidermis, central nervous system, sense organs, and neural crest  The mesoderm forms the skeleton, muscles, blood vessels, heart, and gonads  The endoderm forms the lining of the digestive and respiratory tract, liver, and pancreas

Neural Crest  It is a small strip of cells formed when the neural groove is pinched off. Incorporated into the roof of the neural tube  Differentiation of the neural crest cells depend on the location For example: At the anterior end of the embryo, they merge with the anterior portion of the brain. This helps in the development of skeletal and connective tissues of the head

Human Development  Human development, from fertilization to birth takes an average of 266 days  Divided into three periods called trimesters

First Month  30 hours- first cleavage occurs  60 hours- second cleavage occurs  6-7 days- embryo reaches the uterus  2 weeks- the placenta forms, this provides nourishmentfor the embryo. Gastrulation also takes place  3 weeks- neurulation occurs. By the end of the thirdweek, over a dozen somites, which give rise to muscles,vertebrae, and connective tissues, are present and theblood vessels and gut have begun to develop. Embryois about 2 millimeters long  4 weeks- organogenesis, which is the formation of bodyorgans takes place. Eyes form, 30 somites are visibleand the arm and leg buds have begun to form. Embryois about 5 millimeters long

Second Month  Morphogenesis which is the formation of shape takes place  The miniature limbs of the embryo begin to take their adult shape  The arms, legs, knees, elbows, fingers, and toes can all be seen. Also, a short bony tail!!!!  The embryo has grown to about 25 millimeters and weighs about one gram  The eighth week marks the transition from embryo to fetus, all major organs of the body have been established

Third Month  The nervous system develops  The arms and legs begin to move  The embryo begins to show facial expressions and can suck

Second Trimester  Fourth month- the bones begin to enlarge and the mother can feel kicking  Fifth month- the head and body grow a fine covering of hair called lanugo, lost later in development  By the end of the fifth month- rapid heartbeat of the fetus can be heard with a stethoscope. It has grown to be about 175 millimeters in length and is 225 grams  Sixth month- the baby weighs 600 grams and is over a foot long, still cannot survive outside the uterus without special medical attention

Third Trimester  The weight of the fetus doubles several times  Most of the nerve tract in the brain as well as many new nerve cells are formed  The brain is not complete though. That has to happen outside the uterus because the baby would get to be too big

BIRTH!!  The uterus releases prostaglandins which begin the uterine contractions  Oxytocin is also released and that works with prostaglandins to to further stimulate the contractions, forcing the fetus down  When they start, contractions occur a couple times per hour and then they become more frequent  The placenta is then expelled and the umbilical cord, still attached to the baby is cut

Postnatal Development  The baby continues growing after birth, at a very fast rate  Babies heads are disproportionate to the rest of the body because different parts grow at different speeds

Diseases  Atrial septal defect- a hole between the left andthe right atrium in the heart  Cleft lip deformity- incomplete closures in thearea between the lip and the mouth  Anencephaly- part of the brain is absent

Why is it necessary?  It is needed to form babies. Without thissystem, there would be no way forfertilization to form into a human

Evolutionary Development  The primitive chordates gave rise to vertebrates, they were initially slow- moving, filter-feeding-animals  The vertebrates became fast-swimming predators  Also, some fish release sperm into the water instead of straight into the female

Interdependency  Female and male reproductive system Without the sperm and the egg, there would be no baby The female carries the fetus and provides the egg The male provides the sperm