Immunological aspects of pregnancy
Since embryos carry genes from both parents, their proteins naturally become foreign antigens for the maternal organism, creating a high risk of immune rejection. Throughout evolution, animals have developed various strategies to protect against this mechanism.
- Marsupial mammals. They have a very brief intrauterine development period, meaning the process of fetal immune rejection simply does not have time to fully develop before birth.
- Placental mammals. Characterized by a prolonged period of intrauterine development. Their primary evolutionary adaptation is the formation of the trophoblast. The function of the trophoblast is to create an insurmountable, active barrier between the maternal tissues and the developing embryo, completely preventing rejection.
Classification of placentas by origin
The placenta is formed by joining the chorion to the uterine wall. The mechanism involves the initial invasion of primary, followed by secondary, chorionic villi. Subsequently, blood vessels grow into the secondary villi, and the source of these vessels determines the type of organ.
- Yolk sac placenta. Blood vessels grow directly from the yolk sac. This type is typical of viviparous fish (sharks), amphibians, most marsupials, and reptiles.
- Allantoic placenta. Vessels penetrate the secondary chorionic villi from the allantois (found in reptiles).
Higher mammals exhibit a distinct developmental dynamic: the yolk sac placenta functions first, followed by its replacement by the allantoic placenta. However, there are exceptions; in the mole, rabbit, horse, and camel, both placentas may function simultaneously.
Structural types (by attachment to the uterine mucosa)
This classification is based entirely on the degree of connection between the chorion and the uterine mucosa, which directly influences the course of labor.
- Non-deciduate placenta. During birth, the chorionic villi detach from the uterine walls without any tissue damage or concomitant hemorrhage. This is characteristic of some marsupials, pigs, tapirs, cetaceans, camels, horses, hippopotamuses, lemurs, and many ruminants.
- Deciduate placenta. During birth, the organ separates along with a portion of the uterine mucosa. Due to tissue tearing, the process is invariably accompanied by hemorrhage. It is found in carnivores, rodents, some insectivores, bats, and primates.
> Important: Disruptions in the development of the reproductive system during embryogenesis inevitably lead to the formation of severe malformations.
Histological structure and villus distribution
Based on the pattern of villus distribution on the chorionic surface and the degree of destruction of the maternal-fetal blood barrier, four main types are distinguished:
- Diffuse (pl. diffusa). Villi are distributed across the entire surface. Histologically, this is an epitheliochorial placenta: villi contact the uterine epithelium without destroying it at all (pigs, horses, cetaceans).
- Cotyledonary (pl. cotyledonaria). Villi are gathered into localized groups known as cotyledons. Histologically, this is a desmochorial placenta: the uterine epithelium is destroyed, and the villi come into direct contact with connective tissue (ruminants).
- Zonary (pl. zonaria). Villi are arranged in a belt-like band around the fetal sac. Histologically, this is an endotheliochorial placenta: villi contact the endothelium of maternal blood vessels (carnivores).
- Discoidal (pl. discoidalis). Villi are localized in a restricted area, forming a flat disc. Histologically, this is a hemochorial placenta: vessel walls are completely destroyed, and the villi are bathed in maternal blood (rodents, primates, humans).
Briefly: answers to topical questions
Which tissue layers make up the placental barrier in the hemochorial type of placenta?
In the hemochorial type of placenta, the hematoplacental barrier is formed exclusively by fetal tissues, with maternal tissues completely excluded. The barrier consists of the following anatomical components (villus structures from outside to inside):
- Syncytium (syncytiotrophoblast) — the outer layer lacking distinct cell boundaries.
- Cytotrophoblast — the inner layer represented by a layer of cells or their remnants.
- Villus stroma — the connective tissue framework (layer of mesenchymal cells).
- Capillary endothelium — the inner layer lining the fetal blood vessels.
What successive developmental stages do chorionic villi undergo during placentation?
During placentation, chorionic villi sequentially pass through three developmental stages, progressing in complexity from epithelium to vessels:
- Primary villi — avascular clusters of cells consisting only of a two-layered epithelium (cytotrophoblast and symplastotrophoblast).
- Secondary villi — formed by the ingrowth of extraembryonic mesenchyme into the epithelial villi, resulting in a connective tissue stroma.
- Tertiary villi — formed during vascularization when blood capillaries develop from mesenchymal cells inside the villi.
What hormones are synthesized by the human trophoblast and placenta?
The human trophoblast and placenta synthesize a wide range of protein and steroid hormones, functioning as a temporary endocrine organ:
- Pituitary hormone analogs — human chorionic gonadotropin (synthesized by trophoblast cells), placental lactogen, chorionic somatomammotropin, as well as analogs of ACTH, GH, TSH, and MSH.
- Female sex hormones — estrogens and progestins (progesterone), synthesized by the symplastotrophoblast of the chorionic villi.
- Hypothalamic hormones — practically all releasing and inhibiting factors (CRH, TRH, somatoliberin, somatostatin, prolactoliberin).
- Relaxins — hormones that prepare the body for childbirth.
Which blood vessels run within the umbilical cord and what type of blood do they carry?
The umbilical cord contains three blood vessels that ensure exchange between the fetus and the placenta:
- Unpaired umbilical vein (v. umbilicalis) — carries arterial blood enriched with oxygen in the placental villus capillaries, directing it toward the fetal heart.
- Paired umbilical arteries (aa. umbilicales) — carry subvenous (mixed) blood from the fetal iliac arteries back to the placenta for gas exchange and waste elimination.
To remember the histological types according to the increasing degree of contact between the villi and the uterus, use the mnemonic EDEM: Epitheliochorial (epithelium intact), Desmochorial (desmos — connective tissue), Endotheliochorial (vessel endothelium), Hemochorial (hemo — blood).
Class and exam questions
What is the main difference between deciduate and non-deciduate placentas?
A deciduate placenta separates along with a fragment of the uterine mucosa during childbirth, causing bleeding. A non-deciduate placenta is delivered without damaging maternal tissues.
Which type does the human placenta belong to?
Humans form a discoidal hemochorial placenta. The villi are gathered in the shape of a disc and are directly bathed in maternal blood due to the destruction of the maternal vessel walls.
What else to study on this topic
- Stages of trophoblast formation
- Development of primary and secondary chorionic villi
- Anatomy and functions of the yolk sac
- The role of the allantois in embryogenesis
- Malformations of the reproductive system in embryogenesis
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