What is ontogenesis
The term originates from the Greek words onto (being) and genesis (birth, development). Essentially, this is the entire life of a specific individual, the fundamental basis of which is the genetic program inherited from the parents.
In biology, the general scheme of the life cycle is conventionally described by three key milestones:
- Fertilization.
- Birth (or hatching from egg membranes).
- Death.
For convenience of study, this entire prolonged process is divided into three major periods: pre-embryonic (progenesis), embryonic (embryogenesis), and post-embryonic (postembryonic).
Progenesis (pre-embryonic development)
This stage encompasses all processes that precede the actual formation of a new organism. Without progenesis, the formation of a zygote would be impossible.
Main stages of progenesis:
- Gametogenesis. The process of formation of germ cells (gametes) in parent individuals. Gametes include spermatozoa (male cells) and oocytes (female cells).
- Insemination. The mechanism that ensures the physical meeting of female and male gametes.
- Fertilization. The final stage of progenesis — the fusion of parental germ cells. The result of this fusion is the zygote (a diploid single-celled embryo).
Embryonic period
Embryonic development starts at the moment of zygote formation and ends when the organism is born or leaves the egg (embryonic) membranes.
Within this period, a strict sequence of stages is distinguished:
- Zygote stage. The embryo is represented by just a single diploid cell.
- Cleavage. The zygote actively divides, resulting in the formation of a multicellular, yet still single-layered embryo — the blastula.
- Gastrulation. The formation of germ layers takes place. The embryo becomes two- or three-layered and is called the gastrula.
- Histo- and organogenesis (neurulation). Tissues and organs begin to form from the germ layers. At this stage, an embryo with axial organs is formed, which is called the neurula.
Postembryonic period
This stage begins immediately after birth (or hatching) and lasts until the biological death of the organism. Depending on the reproductive status, it is divided into three age intervals:
- Prereproductive period. The time of active growth and development (associated with childhood and adolescence in humans). The organism is forming, but is not yet capable of sexual reproduction.
- Reproductive period. Puberty is reached. During this time, the individual actively participates in reproduction, leaving offspring.
- Postreproductive period. The final stage of life. Characterized by the loss of reproductive capacity and aging processes, which ultimately lead to death.
Briefly: answers to topical questions
Which specific germ layers are formed during gastrulation?
During gastrulation, germ layers are formed from blastula cells:
- Ectoderm — the outer germ layer.
- Mesoderm — the middle germ layer.
- Endoderm — the inner germ layer.
Gastrulation is considered fully complete when all three germ layers have formed.
Which axial organs are laid down at the neurula stage?
At the neurula stage, a complex of axial organs characteristic of all chordates is laid down. This complex includes:
- Neural tube — formed from the ectoderm on the dorsal side of the embryo.
- Notochord (Chorda) — a dense unpaired cord of cells along the embryo's axis.
- Intestinal tube (gut) — located directly beneath the notochord and formed from the endoderm.
The identical development and mutual arrangement of these organs in all chordates indicate their complete homology.
Which tissues and organs develop from the ectoderm during organogenesis?
The following develop from the ectoderm and its derivatives:
- Nervous system: from the neural tube — the brain and spinal cord; from the neural crest — sensory and autonomic ganglia.
- Body integuments: the epidermis of the skin and its derivatives — hair, nails, skin and mammary glands.
- Sensory structures: organs of vision, hearing, and smell; specifically, the retina of the eye, lens, anterior epithelium of the cornea, and the organ of Corti of the inner ear.
- Digestive system: the epithelium of the oral cavity, the anterior and posterior sections of the gut; the anal section of the rectum.
- Dental system: tooth enamel.
- Endocrine structures: the posterior pituitary gland, pineal gland, adrenal medulla.
- Skin cells: melanocytes / pigment cells of the skin.
Class and exam questions
At what moment does ontogenesis begin?
From the moment of fertilization and the formation of the zygote (a diploid single-celled embryo).
How does the embryonic period end?
It ends with the birth of the organism or its hatching from the egg membranes.
What are the blastula, gastrula, and neurula?
These are successive stages of embryogenesis: blastula — a single-layered multicellular embryo, gastrula — a two- or three-layered one, and neurula — an embryo with established axial organs.
What is the essence of the prereproductive period?
This is the time from birth to puberty, when the organism grows and develops, but is not yet ready for reproduction.
What else to study on this topic
- Mechanisms of gametogenesis: formation of spermatozoa and oocytes.
- Types of zygote cleavage in different species of organisms.
- The process of gastrulation and the formation of germ layers.
- Histogenesis and organogenesis: which layers give rise to specific tissues.
- Biological mechanisms of aging in the postreproductive period.
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