By Lauren and Jocelyn
Dec 25, 2015
By Lauren and Jocelyn
Fertilization
The first step in embryonic development is fertilization (the joining of male and female gametes)
Fertilization has three parts:PenetrationActivationNuclei 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:EctodermMesodermEndoderm
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 locationFor 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 nourishment for the embryo. Gastrulation also takes place
3 weeks- neurulation occurs. By the end of the third week, over a dozen somites, which give rise to muscles, vertebrae, and connective tissues, are present and the blood vessels and gut have begun to develop. Embryo is about 2 millimeters long
4 weeks- organogenesis, which is the formation of body organs takes place. Eyes form, 30 somites are visible and the arm and leg buds have begun to form. Embryo is 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 and the
right atrium in the heart
Cleft lip deformity- incomplete closures in the area between the lip and the mouth
Anencephaly- part of the brain is absent
Why is it necessary? It is needed to form babies. Without this
system, there would be no way for fertilization 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 systemWithout the sperm and the egg, there would
be no babyThe female carries the fetus and provides
the eggThe male provides the sperm