Pregnancy and Embryonic Development

Gastrulation: Gastrulation is defined as an early developmental process in which an embryo transforms from a one-dimensional layer of epithelial cells (blastula) and reorganizes into a 3-layered structure called the gastrula.

Gastrulation:

It involves the differentiation and movement of cells of a blastula from their original position to the sites where they finally settle. These movements are called morphogenetic movements. They include,   

• Epiboly: Overgrowth of cells to cover other cells. 
• Emboly: Migration of prospective endodermal and mesodermal cells from the surface into the interior of the embryo. 

Within the cytotrophoblast is the ball of ICM, and during the second week of human development, the ICM cells spread into a flattened tissue layer and differentiate into a two-layered tissue containing epiblast (columnar epithelial cells) and the hypoblast (cuboidal epithelial cells), which are together known as the bilaminar disc. Since the epiblast cell layer is located dorsally to the upper blastoderm, the formation of a bilaminar disc defines the dorsal/ventral axis. The anatomical location of the double-layer disc lies between the amniotic sac and the primitive yolk sac. Epiblast cells expand into the semi-sphere known as the amniotic cavity, while cells of the hypoblast expand to surround the yolk sac. Above the endoderm is a raised area of columnar cells known as the prechordal plate; this is the earliest boundary between the cranial and caudal. Development of the bilaminar disc directly precedes gastrulation, where the end goal during week 3 of development is to transform the human blastocyst into a multi-layered gastrula with endoderm, mesoderm, and ectoderm. 

Primitive Streak: 

The beginning of gastrulation is marked by the appearance of a groove in the caudal end of the epiblast layer known as the primitive streak. It clearly establishes the head and tail ends of the embryos, as well as its right and left sides. At the cranial end of the primitive streak, epiblast cells ingress at a greater rate forming a circular cavity known as the primitive pit. As the primitive streak and pit elongate, migrating epiblast cells join the streak at the cranial end, forming a mass of cells called the primitive node. Following the formation of the primitive streak, cells of the epiblast move inward below the primitive streak and detach from the epiblast. Some of these cells displace the hypoblast and then the hypoblast forms an endoderm. Other cells remain in between epiblast and hypoblast to form the intra embryonic mesoderm. By 22-24 days notochord is formed. 

Extra Embryonic Membranes: Extra embryonic region takes part in the formation of certain membranes called extra embryonic membranes.

Extra Embryonic Membranes:

The cellular layer formed by blastomeres remains as blastoderm. The central part of the blastoderm gives rise to the embryo proper, while the peripheral portion does not take part in the formation of the embryo. This peripheral part is known as the extra-embryonic region. The extra embryonic membranes provide facilities for nutrition, respiration, and excretion to the embryo. Extra embryonic membranes are of four types -

1. Amnion: With a gradual increase in size the amnion covers the embryo from all sides. After about eight weeks of fertilization, the amnion is completely incorporated into a connecting stalk, which finally forms the umbilical cord. Embryo, in this stage, is called as foetus remains hanging in amniotic fluid. 
2. Chorion: After implantation of the blastocyst, the trophoblast gives out several finger-like processes, the chorionic villi which get embedded into uterine endometrium Mesoderm also contributes in the formation of these villi. After a period of four months, these villi disappear from all parts except a disc-like area where they grow rapidly and participate in the formation of the placenta.
3. Yolk sac: Initially the size of the yolk sac is larger as compared to that of the embryo. About eight weeks after fertilization, the yolk is reduced in size and changes into a tubular structure.
4. Allantois: The mesoderm of allantois forms many small blood vessels in this region. These vessels connect the embryo with the placenta and ensure nutritional and respiratory supply to the embryo. In humans, allantois does not function to store the excretory wastes as it does in reptiles, birds, and protothians.

On the basis of the presence or absence of amnion, two groups of vertebrates are categorized.

1. Amniota: This group is characterized by the presence of amnion in the embryos of its members. For example, members of class Reptilia, Aves, and Mammalia.
2. Anamniota: Animals of this group are devoid of amnion in their embryos. For example, class Cyclostomata, Pisces, and amphibia.

Placenta: The placenta is an organic connection between the foetus and uterine wall for physiological exchange between the foetus and the mother's blood.

Placenta:  

The placenta develops at the point of implantation. At 1^st, the trophoblast cells absorb food and oxygen from the increasingly vascularised uterine lining. The allantois grows out from the embryo and fuses with the chorion to form the allanto-chorion which will develop into the placenta. The allantois gives rise to the umbilical cord which contains blood vessels connecting the foetus and placenta. 

Function of Placenta:  

• Nutrition: all the nutritive elements including glucose, fatty acid, amino acids, nucleotides, vitamins and minerals from the maternal blood pass into the foetus. 
• Respiration: Oxygen passes from the maternal blood to the foetal blood through the placenta and CO₂ pass in the reverse direction. 
• Excretion: Excretory products diffuse into maternal blood and are again excreted by the mother. 
• Storage: Stores glycogen, fat etc. 
• Barrier: Allows only essential materials to pass into the foetal blood. 
• Endocrine Function: Secretes hormones such as estrogen, progesterone, and human chorionic gonadotropin(hCG), hCS, and CRH. 

Changes in embryo during pregnancy: 

All these post-implantation steps lead to baby development. This takes 9 months for humans. Development is very slow and steady. First, the embryonic heart is formed, then the limbs, major organs, hair appearance, etc. At the end of 9 months, the foetus is fully developed. The limbs and external genital organs are well developed. The 1st movements of the foetus and the appearance of hair on the head are usually observed during the 5^th month. By the end of 24 weeks the body is covered with fine hair, eyelids separate and eyelashes are formed. By the end of the 9th month of pregnancy, the foetus is fully developed and is ready for delivery.