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