Histogenesis and organogenesis
Following the initial cleavage stages, embryonic development follows a strict sequence: first, cell layers form; then, the main body axes are established; after which organ formation begins, culminating in their maturation. The fourth and fifth stages of embryogenesis are dedicated directly to histogenesis and organogenesis.
Initial histo- and organogenesis (Stage IV) is characterized by the formation of the rudiments of virtually all future systems from mesenchyme and primary axial primordia. It is by the end of this period that the embryo is referred to as a fetus. This is followed by definitive histo- and organogenesis (Stage V), which occupies the greater part of embryonic development. During this period, the primary primordia transform into fully developed tissues and organs. The process concludes with the formation of a mature fetus ready for birth (in mammals) or hatching (in birds and reptiles).
Chronological stages of differentiation
The biological essence of development lies in the sequential specialization of cells. Based on the timing of trait manifestation, several early stages are distinguished:
- Ootypic stage. Localized at the level of the oocyte and zygote. Its key mechanism lies in the biochemical heterogeneity of the zygote cytoplasm. Certain zones (so-called presumptive areas) enter different cells during cleavage and predetermine the vector of their future development.
- Blastomere stage. At this stage, clear morphological and biochemical differences begin to appear between individual cells—blastomeres. At the 4- to 8-blastomere stage, these differences are still minor, and the cells retain totipotency. Clinically, this means that accidental division of blastomeres can result in monozygotic twins. However, as further cleavage proceeds, structural and functional changes increase and become completely irreversible.
Stages of differentiation and histogenesis
The final phases of cellular specialization are inextricably linked with the process of tissue formation. In this process, histologists distinguish three sequential steps:
- Gastrulation stage — spatial organization of cells and formation of primary germ layers.
- Early histogenetic stage — the appearance of the very first, still immature rudiments of future tissues.
- Late histogenetic stage — the final formation of definitive (mature) tissues and organs capable of performing their physiological functions.
The concept of the differon
To understand the principles of tissue organization, the concept of the differon is used. A differon represents the structural and functional unit of a tissue, which actively forms precisely during the stages of histogenesis. In essence, it is a continuous histogenetic lineage (differentiation line) that includes the entire sequence of development: from the initial stem cell to the fully matured (differentiated) form.
Depending on the capacity for self-renewal, two types of differons are distinguished:
- Complete differons — retain the stem cell population and the full composition of intermediate forms throughout the life of the organism.
- Incomplete (reducing) differons — over time, irreversibly lose their initial forms (stem cells) and intermediate stages of maturation.
Any adult organism, from a histological standpoint, can be considered as a complex aggregate of complete and incomplete differons, as well as the intercellular substance they produce during their vital activity.
Briefly: answers to topical questions
What specific cellular stages does a typical differon include?
A cellular differon (histogenetic lineage) is conventionally divided into three parts: the initial cambial, the middle differentiating, and the terminal highly differentiated parts. The initial form of the cell differentiation lineage is stem cells. These are followed by substem (committed) progenitor cells, which can differentiate in only one direction. The differon ends with the stage of mature functioning cells. Using hematopoiesis as an example, 6 classes of cells are distinguished: stem cells (HSCs), hemopoietic stem cells, unipotent (progenitor cells), blasts, maturing cells, and mature cells.
What methods of gastrulation exist in embryogenesis?
The following methods of gastrulation exist in embryogenesis:
- Invagination (folding inward).
- Epiboly (overgrowth).
- Delamination (splitting) and immigration (cell migration).
A mixed type is also found—a combination of various gastrulation methods. For example, a combination of immigration and delamination is characteristic of birds and mammals, while a combination of epiboly and invagination is characteristic of amphibians.
Which tissues and organs are formed from the embryonic ectoderm?
The embryonic ectoderm differentiates in three directions: the neural tube, ganglionic plates (neural crest), and surface ectoderm. From these are formed:
- From the neural tube — the spinal cord and brain.
- From the neural crest — peripheral nerve ganglia, skin melanocytes, and neuroendocrine system cells.
- From the surface ectoderm — the epidermis of the skin and its derivatives (sebaceous and sweat glands, hair, nails), the epithelium of the oral cavity, vagina, and anal rectum.
Which tissues develop from the embryonic mesenchyme?
The tissues of the internal environment and smooth muscle tissue develop from the embryonic mesenchyme. The general spectrum of derivatives includes:
- All types of connective tissue (including skeletal tissues — bone and cartilage, and the connective tissue of the skin dermis).
- Blood and hematopoietic tissue.
- Blood vessels.
- Smooth muscle tissue.
- Microglia (glial macrophages of the CNS).
To easily remember the global logic of embryonic development, use the acronym SLOM: Layers (Sloi) → Axes (Osi) → Organs (Organy) → Maturation (Sozrevanie).
Class and exam questions
At what stage does the embryo begin to be called a fetus?
The embryo acquires the name of a fetus by the end of Stage IV — initial histo- and organogenesis, when the rudiments of virtually all organs have already formed from the axial primordia and mesenchyme.
What is the underlying cause for the possible emergence of monozygotic twins?
At the early blastomere stage (at the 4–8 cell stage), biochemical differences between cells are minimal, and they are totipotent. Blastomere separation at this moment can lead to the development of independent identical organisms.
What are presumptive areas?
These are specific zones of the zygote cytoplasm possessing biochemical features. During cleavage, they are distributed among different blastomeres and predetermine the direction of their further differentiation.
What is the adult organism structurally composed of?
The adult organism is an aggregate of complete and incomplete (reducing) differons, as well as the intercellular substance produced by these cells.
What else to study on this topic
- Formation of germ layers during gastrulation
- Biochemical mechanisms of presumptive area localization
- Molecular causes of the loss of totipotency by blastomeres
- Axial primordia and mesenchyme: role in organogenesis
- Cellular composition of complete and incomplete differons using specific tissues as examples
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