Hint: Eutherians are the placental mammals. Placenta is an organ that develops during pregnancy, it attaches to the wall of uterus and the umbilical cord arises from it. Though the common feature of placenta is that it is the structural and functional unit between developing embryo (fetus) and the maternal blood but the classification of placenta is done on several fundamental types.
Given below is the classification of placenta in humans.
1. Classification based on shape and contact points-
Diffuse: Most of the surface of the allantochorion is involved in the formation of placenta, for ex. horses, pigs, etc.
Cotyledonary: By multiple interaction in patches of allantochorion with the endometrial wall, discrete area of attachments in fetal portion called ‘Cotyledons’ are formed and the maternal contact sites are ‘Caruncles’. The cotyledon-caruncle complex is called ‘placentome’. This is generally seen in ruminants.
Zonary: Placenta forms either complete or incomplete band of tissues that surrounds the uterus. Such types are seen in dogs, cats, seals, bears, elephants, etc.
Discoid: A single discoid shaped placenta is formed and is present on the whole surface but later disappears except at the region of implantation. This is observed in humans and other primates.
2. Classification on the basis of layers between fetal and maternal blood-
Just prior to the formation of the placenta, there are a total of six layers of tissue that separates maternal and foetal blood. There are \[3\] foetal extra embryonic membranes-
i) The outermost, chorionic epithelium which is derived from trophoblast.
ii) Connective tissue in the form of chorioallantois mesoderm.
iii) Innermost, endothelium lining of allantois capillaries.
Similarly, \[3\] layers of maternal tissue-
i) Outermost endometrial epithelial cells.
ii) Connective tissue of the endometrium.
iii) Endothelium lining of endometrial blood vessels.
But the no. of maternal layers varies among different species-
Epitheliochorial: All \[3\] maternal layers are retained in placenta. Ex. horse, ruminants, etc.
Endotheliochorial: Only the uterine endometrial layer of the mother is retained, ex. dogs, cats, etc.
Hemochorial: None of the maternal layers are retained. Ex. Humans, rodents, etc.
3. Classification based on the pattern of implantation-
Superficial/Central: Blastocyst remains unembedded in uterine lumen, ex. pigs, dogs, etc.
Eccentric: The blastocyst lies for a time in a fold or a pocket which loses from the main cavity, for ex. rats, squirrel, other rodents, etc.
Interstitial: Blastocyst is completely embedded in the endometrium and the placenta penetrates deep in the uterus making a strong association between embryo and maternal part. Ex. humans, apes, etc.
4. Classification based on degree of association between fetal and maternal tissue-
Non-deciduous: The fetal epithelium comes are in contact with the maternal uterine tissue and at the time of birth, the villi are dawn out completely without tearing the uterine wall and so, no bleeding occurs such as in, horse, pig, cattle, etc.
Deciduate: The implantation is much more intimate and close association between chorionic and maternal villi is observed. During parturition, the mucosal covering of maternal tissue is damaged that leads to bleeding. Ex. human, rabbit, dogs, etc.
Contra-deciduate: The association here also, is intimate but bleeding does not occur at the time of birth because the fetal and maternal tissue are absorbed inset by maternal leukocytes, ex. parameles, moles, etc.
5. Classification on the basis of source of the vascular supply-
Chorio-vitelline/yolk sac placenta: Highly vascular yolk sac fuses with the chorion. Ex. metatherian- marsupials, didelphis, etc.
Chorio-allantoic: Allantois with its blood vessels fuses with chorion. Ex. some marsupials, eutherians.
Hence, Humans have a chorionic placenta.
To summarize, the human placenta is discoidal, hemochorial, interstitial, decidual and chorionic. Placenta in different species might be of different types but it often performs similar functions in all of them. The functions of the placenta are given below in the additional information section.
The functions of placenta are as follows:
Facilitates the supply of oxygen and nutrients to the embryo.
Removal of carbon-dioxide and excretory or waste materials produced by the embryo.
It acts as a temporary endocrine gland and secretes steroid hormones, estrogen and progesterone. Also secretes the proteinaceous hormones HCG (human chorionic gonadotropin) and HPL (human placental lactogen).
Both these hormones along with relaxing (secreted by ovary) are secreted only during pregnancy.