Provisional organs

General characteristics of temporary structures

During intrauterine development or development inside an egg, the embryo cannot independently provide itself with everything it needs. Its own organs form gradually, and for this transitional period, nature provides for the creation of provisional organs.

In higher vertebrates, these structures are traditionally called fetal membranes. They are laid down and develop from the material of the primary germ layers. Interestingly, the exact timing of their formation directly depends on two factors: the amount of nutrient reserves initially stored in the ovum, and the environmental conditions in which development takes place.

As soon as the fetus reaches the necessary degree of morphological and functional maturity, the need for these «helpers» disappears. Their ultimate fate is unambiguous: temporary structures either gradually resorb, integrating into other tissues, or are completely discarded at the moment of birth or hatching.

Amnion and chorion: aquatic environment and external barrier

Some of the most important fetal membranes are the amnion and the chorion. Both organs are of ectodermal origin (the somatopleure also participates in the formation of the chorion).

• The amnion is a closed ectodermal sac inside which the embryo itself is located. The cavity of the sac is filled with a specialized amniotic fluid. The main task of the amnion is to create an optimal and natural aquatic environment for the development of the fetus. In addition, it acts as a reliable shock absorber, protecting delicate tissues from potential mechanical damage and preventing critically dangerous desiccation.
• The chorion (or serosa) is the outermost fetal membrane. It is located directly beneath the shell in egg-laying species or closely adheres to maternal tissues in viviparous species. The main function of chorion is to ensure intensive metabolic exchange between the embryo and the external environment. In egg-laying species, the chorion is primarily responsible for respiratory gas exchange. In mammals, its functionality is significantly broader: it participates in respiration, nutrition, excretion, filters incoming substances, and is even capable of synthesizing certain compounds (e.g., hormones).

Yolk sac

This organ is of endodermal origin and is covered on the outside by a layer of visceral mesoderm. Anatomically, it maintains a direct connection with the gut tube of the developing embryo.

The functional role of the yolk sac varies drastically depending on the animal class:

1. In embryos with abundant yolk reserves (e.g., birds and reptiles), it performs its direct function—participating directly in the nutrition of the embryo by processing and delivering nutrients.
2. In mammals, whose ova are virtually devoid of yolk reserves, the organ changes its specialization.
3. The primary germ cells are laid down and formed in the endoderm of the sac.
4. The very first formed elements of the embryonic blood are formed in the mesodermal layer.
5. In addition, the sac cavity is filled with a specific fluid with a very high concentration of glucose and amino acids, which directly indicates its crucial role in regulating protein metabolism in the early stages of embryogenesis.

Allantois: from trash can to placenta

The allantois develops as a sac-like outgrowth of the ventral wall of the hindgut. In its structure, it consists of two layers: it is lined from the inside by endoderm and covered on the outside by the splanchnopleure. Its development and functions also strongly depend on the taxonomic affiliation of the animal.

In reptiles and birds, the allantois grows very rapidly and soon reaches the chorion. Within the confined space of the egg, it acts as a reservoir for isolating toxic end-products of nitrogen metabolism (urea and uric acid). At the same time, thanks to a powerfully developed vascular network, it ensures gas exchange together with the chorion. After hatching, the outer part of the allantois is discarded, while its inner part remains in the body and transforms into the urinary bladder.

In mammals (including primates), the allantois plays a key role in forming the connection with the maternal organism. By fusing with the chorion, it forms the placenta. In primates, the endodermal part of this organ remains rudimentary. However, mesodermal cells form a dense tissue cord extending from the cloacal region of the embryo to the chorion. It is along this mesodermal framework that the vital blood vessels of the allantois grow into the chorion. Through this vascular network, the placenta realizes its main functions: excretory, respiratory, and nutritional.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Origin of fetal membranes from ectoderm, endoderm, and mesoderm
• Synthetic function of the chorion in mammals: hormone production
• Features of gas exchange in egg-laying species via the allantoic vascular network
• The process of hematopoiesis in the mesoderm of the yolk sac

Other topics in the section «Periodization of Ontogenesis»

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