“Mkhitar Gosh” University, Armenia Medical biology courses : Lecture N 17 BASES OF ONTOGENESIS

ONTOGENESIS — is individual development of the organism from a zygote formation to its death

Division of ontogenesis into periods Periods of ontogenesis Prezygotic Embryonic Postembryonic (pre-embryonic (prenatal) (postnatal) or progenetic)

Periods of ontogenesis The pre-zygote period — is a period of formation and maturation of those parental sex cells that will form a zygote in future. The embryonic or prenatal period starts with the moment of a zygote formation and ends with birth of a new organism or its leaving egg membranes. The post-embryonic or post-natal period — lasts from birth of an organism or its leaving egg membranes and to death.

The pre-embryonic period — is a period of formation and maturation of those parental sex cells that will form a zygote in future

The embryonic or prenatal period starts with the moment of a zygote formation and ends with birth of a new organism or its leaving egg membranes.

Embryogenesis of the human includes: 1. Germinative or initial period — the 1st week after fertilization, a zygote is being split. 2. Embryonic period — the 2nd-3rd weeks after fertilization, a blastule and a gastrule are formed, germinal layers and axial organs are being germinated. 3. Pre-fetal period — the 4–8th weeks, formation of germs of all organ systems and the placenta. 4. Fetal period — from the 9th week an embryo is called a fetus; growth of the fetus is going on, its organs and organ systems are being formed.

Characteristic of embryogenesis stages. Zygote is a unicellular development stage of a multicellular organism; it was formed on fusion of a male and female gamete. The type of splitting a zygote is determined by an ovum type that depends on the amount of nutrients (yolk) and their distribution. Cells that are formed in splitting are blastomeres. The process of splitting a germ in some animals reminds a raspberry (morula). Blastomeres of the morula are located on the periphery in one layer and form a blastula — a one-layer germ with a cavity inside. This layer of cells is called blastoderma. The cavity of the blastula is a blastocele. The blastula stage is followed by gastrullation — formation of a gastrula, a two-layer germ.

Division of the postnatal human ontogenesis into periods Neonatal period (1–10 days): Breast-feeding period (11 days – 12 months Early childhood period (1–3 years): The 1st childhood period (4–6 years): The 2nd childhood period (7–11 years in girls, 7–12 years in boys) Adolescent period (12–15 years in girls, 13–16 years in boys) Juvenile period (16–20 years in girls, 17–21 years in young men Middle age, I period (21–35 years in women, 22–35 years in men Middle age, II period (36–55 years in women, 36–60 years in men) Advanced age (56–75 years in women, 61–75 years in men) Senile age (76–90 years) Age of long-livers (over 90 years)

There are critical periods in the postnatal human ontogenesis: Neonatal period (the first days after birth) Puberty period (12–16 years) Period of sexual wasting away (about 50 years in women, 60–70 years in men) Growth. Growth types of tissues and organs in the human. Acceleration.

Basic growth types for tissues and organs - a general type: the whole body, muscles, skeleton, respiratory organs, liver have a maximum growth in the 1st year of life and in puberty; - a lymphoid type: the thymus, lymphatic nodes and the lymphoid tissue of the intestine, spleen, tonsils; a maximum increase of their mass occurs till the age of 11–12 years and then involution; - a cerebral type: the brain and the spinal cord, eyes, the head develop earlier than other parts of the body — after birth and to 10–12 years; - a reproductive type: various parts of the reproductive system — a fast growth in puberty.