Stages of Human Development

• Start of fertilization

a. Starts with zygote (two-cell stage)

b. Move to four-cell stage via mitosis

c. Morula stage (becomes 16 cells)

d. Blastocysts start to form in the Morula stage; it is the external border of the embryo.

• Implantation

a. The existence of the embryoblast is dependent on the implantation of the trophoblast to the uteri wall.

The first week of human development begins with fertilization of the egg by sperm forming the first cell, the zygote. Cell division leads to a ball of cells, the morula. Further cell division and the formation of a cavity in the ball of cells forms the blastocyst.

· Implantation

· Trophoblast

· Embryoblast

The blastocyst possesses an inner cell mass (ICM), or embryoblast, which subsequently forms the embryo, and an outer layer of cells, or trophoblast, which later forms the placenta. The trophoblast surrounds the inner cell mass and a fluid- filled, blastocyst cavity known as the blastocoele or the blastocyst cavity.

• Where gastrulation occurs

a. Process of establishing the 3 germ layers

• Notochord formation

a. Primitive spinal cord and nervous system

• Neurulation

• Development of Neural crest

a. Grooves or furrow coming from one side to another,

b. The future start of the head

• 3 germ layers:

1. Endoderm

a. Provides the epithelial layer of the gastrointestinal tract,

respiratory tract, and urinary bladder.

b. Forms the parenchyma of the thyroid, parathyroid, liver and pancreas.

c. Epithelial layer of the tympanic cavity and auditory tube.

2. Mesoderm

a. 3 types:

a. Paraxial mesoderm

b. Intermediate mesoderm

c. Lateral mesoderm

3. Ectoderm

a. Gives rise to organs and structures that maintain contact with the outer world.

b. Central nervous system, peripheral nervous system, sensory epithelium of nose, eyes, ears, skin, hair, nails, pituitary, mammary, sweat glands, enamel of teeth.

Derivatives of the Ectodermal Germ Layer. Neuroectoderm forms neural plate > neural folds and neural grooves > neural tube with anteriori and posterior neuropores. Neural crest cells - become sensory ganglia, sympathetic and enteric neurons, schwann cells and cells of adrenal medulla.

· Development of the face

· Rapid length increase

· Fetal motion (kicking)

· Wrinkled skin, fused CNS- Respiratory System

• Development of the face

• Rapid length increase

• Fetal motion (kicking)

• Wrinkled skin, fused CNS-Respiratory system

• Development of the face

• Placenta

a. Covers/encloses the embryo

b. Gas exchange because there is no oxygen within the womb because it is fluid filled and closed off with the external environment.

c. Exchange of nutrients and electrolytes (shared through placenta)

d. Transmission of maternal antibodies (passive immunity)

e. Production of hormones

f. Filtration and detoxification of the blood

g. Contains amnion (large sac that contains the amniotic fluid where the fetus is suspended within the fluid and connected to the placenta via umbilical cord).

h. Umbilical cord is the main connection of the fetus to the mother.

• Genetic factors which are caused by dominant and recessive traits from the parents.

• Environmental because of teratogens.

• Teratogens: agents or chemicals that will affect the normal development of the embryo.

a. Infectious agents

b. Physical agents

c. Chemical agents

d. Hormones

FETAL ALCOHOL SYNDROME (FAS)

· When the mother drank alcohol during pregnancy

· Features include indistinct philtrum, think upper lip, depressed nasal bridge, short nose, and flat midface

Functions

· Encloses the whole fetus

· For gas exchange; as there is no oxygen in the womb

· Exchange of nutrients and electrolytes

· Transmission of maternal antibodies aka passive immunity

· Production of hormones

· For filtration and detoxification

· Amniotic fluid = fluid inside