intro to embryology

what is embryology?

the study of embryo development, starting from ovum fertilization to the fetal stage, helping particularly in understanding development of head and oral structures

what are the primary aims of studying embryology?

1. embryonic formation

2. understanding fetal development

what are the phases development of embryo is divided into?

1. prenatal development

2. postnatal development

when does prenatal development begin?

when the sperm and ovum fuse together to form a zygote, which is a process known as fertilization

what are the periods of prenatal development?

1. proliferation

2. embryonic

3. fetal

proliferation period

• also known as germinal period

• begins at fertilization (fusion of sperm and ovum), followed by cleavage (rapid cell division)

• extends to the end of the 2nd week of intrauterine life (I.U.L)

• ends with the beginning of the formation of the three germ layers

embryonic period

• extends from 3rd W.I.U.L

• ends at 8th W.I.U.L

• completes the formation of the 3 germ layers (ectoderm, mesoderm and endoderm)

• organogenesis, the formation of organs and body systems in their primitive forms, occurs

fetal period

• begins at the end of the 8th week and the start of the 9th week I.U.L

• continues from 9th week until birth (~ 38 W.I.U.L)

• no new major organs are formed

• main processes are growth and maturation (functional development) of organs already formed

which prenatal period is most susceptible to teratogens?

the embryonic period, as that is when major organ systems and structural body plans are forming, making them highly vulnerable to structural defects and irreversible damage

what are the periods of postnatal development?

• neonatal

• infancy

• childhood

• puberty

• adolescence

• adulthood

neonatal period

• extends from birth to two weeks of age

• characterized by a lack of teeth

infancy period

• extends from 2 weeks of age to 2 years of age

• deciduous teeth begin to erupt

childhood period

• extends from 2 years of age to 12 years of age

• deciduous teeth are replaced by permanent teeth (mixed dentition)

puberty period

• extends from 12 to 15 years of age for girls, and 13 to 16 years of age for boys

• onset of sexual maturity

adolescence period

• about 2-3 years after puberty

• extends from 15 to 18 years of age

• physical and emotional maturity occurs

adulthood period

• extends from 18-25 years

• growth completed

what is the proliferation period?

also known as the proliferative or germinal period, it is the beginning of embryonic development. during this period, the sperm fertilizes the ovum, which undergoes cleavage, beginning the formation of the placental membrane. it extends during the first 2 weeks I.U.L, kickstarting the formation of the 3 germ layers, which extends into the embryonic period

what are the events that occur during the proliferative period?

1. fertilization

2. cleavage

3. blastocyst formation

4. implantation

fertilization period

begins when the sperm fertilizes the ovum, forming a zygote, which is the first cell of the human body

cleavage period

also known as rapid cell division, the zygote divides into 2, then 4, then 8 and then 16 cells, forming a morula

what is a morula?

a solid multicellular mass

blastocyst formation

the morula enters the uterus and becomes filled with fluid, forming a blastocyst

implantation

the blastocyst attaches to the uterine wall, ending the germinal period

what does the structure of the blastocyst consist of?

1. trophoblast, the outer cell layer

2. embryoblast, the inner cell mass

3. blastocyst cavity, a fluid-filled space

what are trophoblasts?

outer cells that form the outer-lining of the blastocyst. they are responsible for implantation in the uterine wall, and they contribute to the formation of the placenta

what are embryoblasts?

the inner cell mass of a blastocyst, they give rise to the embryo. they differentiate into two layers, the epiblast and hypoblast, which form the bilaminar disc

epiblast

also known as the primitive ectoderm, it is located in the upper / outer layer of the bilaminar disc and is shaped like columnar cells

hypoblast

also known as the primitive endoderm, it is located in the lower / inner layer of the bilaminar disc and is shaped like cuboidal cells

what cavity is formed from the epiblast?

the amniotic cavity, a fluid-filled cavity that surrounds the embryo. it provides space for growth and protection (shock absorption)

what cavity is formed from the hypoblast?

the yolk sac cavity, which provides early nutrition to the embryo before placental circulation is fully established. it also plays a role in early blood cell formation and it contributes to the primitive gut

what occurs during the week 3 I.U.L, around days 15-16?

gastrulation, which is a critical and most important characteristic event that occurs during week 3

what occurs during gastrulation?

the bilaminar disc is reorganized and converted into the trilaminar disc, leading to the formation of the mesoderm, and all 3 germ layers are properly established

what are the germ layers?

1. ectoderm, the outer layer

2. mesoderm, the middle layer

3. endoderm, the inner layer

what does the ectoderm form?

• the skin

• hair

• enamel of the teeth

• the nervous system

what does the mesooderm form?

• muscles

• bones

• heart

• connective tissues

what does the endoderm form?

• lining of the digestive system

• lining of the respiratory tract

during the end of the 3rd week (around day 20), what process occurs?

neurulation, which is the formation of the nervous system

what are the steps of neurulation?

1. thickening of the ectoderm

2. formation of the neural plate

3. formation of the neural groove

4. formation of the neural plate

what occurs 4th week I.U.L?

neural crest cells (NCC) arise from the ectoderm, which then migrate beneath the ectoderm into the mesenchyme, particularly in the head and neck region. they are known as ectomesenchyme in this region

what are the derivatives of the neural crest cells?

1. melanocytes

2. most tooth structures, such as dentin, pulp and cementum, with the exception of the enamel, which arises from the ectoderm

3. supporting structures of the teeth: alveolar bone and periodontal ligament