Pregnancy and Embryonic Development

Gastrulation

Definition : Gastrulation is a dynamic process involving critical changes in the embryo such as differentiation of cells, establishment of the three primary germ layers and transformation of the single walled blastula into a double walled gastrula.

Types of gastrular movement or morphogenetic movement : The movements of cells during gastrulation is called formative or morphogenetic movements. Following types of gastrular movements are found in different animals

(1) Epiboly : It involves the morphogenetic movement of prospective ectodermal (micromeres) blastomeres antero-posteriorly to envelop the presumptive endodermal and mesodermal blastomeres. It is found in telolecithal egg of frog.

(2) Emboly : It involves inward movement of prospective endodermal and chorda-mesodermal blastomeres from the surface of blastula. Emboly includes following methods :

(i) Invagination : It involves insinking of endodermal cells in the blastocoel to form archenteron. It is found in amphioxus.

(ii) Involution : It involves the rolling in of the chorda-mesodermal blastomeres inside the ectodermal cells over the lips of blastopore. It is also found in the gastrulation of frog.

(iii) Ingression or polyinvagination : In this, individual blastomeres migrate into the blastocoel either from only vegetal pole (called unipolar ingression e.g., Obelia) or from all sides (called multipolar ingression e.g., Hydra) to form a solid gastrula called stereogastrula.

(iv) Delamination : It involves splitting off the blastoderm into two layers by the appearance of grooves resulting the formation of hypoblast. It is found in birds.

Formation of layers by gastrulation : Gastrulation includes the formation of following structures

(1) Formation of endoderm : The blastodermic vesicle enlarges and cells present on the lower surface of the embryonal knob detach by delamination from the embryonal knob. The part of endoderm located under the embryonal knob is called embryonic endoderm which later forms embryonic gut, while the remaining part of endoderm along with trophoblast forms the yolk sac.

(b) Formation of embryonic disc and mesoderm : Meanwhile, the blastocyst continues to grow due to absorption of more and more uterine milk. The embryonal knob stretches and cells of Rauber start breaking off and dispersing. So the cells of embryonal knob from a regular layer called embryonic disc which becomes continuous with the trophoblast. Embryonic disc is differentiated into cephalic, embryonic and caudal regions. Formation of embryonic mesoderm starts at the caudal region of the embryonic disc where cells undergo rapid proliferation and form a localized thickening of the embryonic disc and form the mesodermal layer between ectoderm and endoderm.

(3) Formation of ectoderm : The remaining cells of blastodisc become columnar and form ectoderm.

Fate of germ layers : Each of the three germ layers gives rise to definite tissues, organs and systems of the body. Their fate in embryo and adult has been listed below.

Neurulation and organogenesis

Post gastrulation involves two main process. Neurulation is process of laying the neural plate to form the nervous system. The establishment of the germ layers initiates the final phase of embryonic development, i.e., organogenesis. The latter involves differentiation and specialization of groups of cells in the individual germ layers. The cells of such groups change their form and give rise to morphologically recognizable tissues and organs of the new individual. The groups of differentiated cells separate from their germ layers in an orderly manner and with unique precision. Separation of the differentiated cell groups may occur by folding off from the germ layer or by migration of cells individually and reaggregation at a new place. In this manner, the primordial cells of the germ layers gradually and accurately give rise to the tissues and organs of the offspring.

By four weeks after fertilization, the embryo has a simple heart, limb buds and eye rudiments. It also has a tail and pharyngeal pouches, the vestiges of its early vertebrate ancestors that disappear later in development. After the second month, the embryo is recognizable as a primate. From this stage on, the embryo is often called foetus.

Extra embryonic membrane

These membranes are formed outside the embryo from the trophoblast only in amniotes and perform specific function. Some of these membranes take part in the formation of placenta in mammals.

(1) Yolk sac : It is formed below the embryo. It contains fluid, not yolk. The yolk sac is a vestigeal organ inherited from the oviparous reptilian ancestors. Yolk sac encloses by outer mesoderm and inner endodermal layer.

Function : In human beings, it is vestigial. In human embryo it act as the site of blood cell formation until about the 6^th week, when the liver takes over this role.

(2) Amnion : It is formed above the embryo. It consist of outer mesoderm and inner ectoderm. The amnion and the fluid filled amniotic cavity it encloses, enlarge and nearly surround the embryo. Amniotic fluid secreted by both embryo and amnion.

Functions

(i) The amniotic fluid cushions the embryo.

(ii) It protects the embryo from jerk, injury and shocks.

(iii) It prevents desiccation of the embryo.

(3) Allantois : It is a fold of splanchnopleur developed from the hind gut of the embryo. It consist of outer mesoderm and inner endoderm.

Functions

(i) The cavity of the allantois serves as a urinary bladder. It stores the protein breakdown product in the form of water-insoluble crystals of uric acid and inside the egg upto the time of hatching.

(ii) The vascular 𠇌horioallantoic membrane” lies in a close proximity to the inner surface of the porous shell. It acts as an extraembryonic lung by supplying the embryo with oxygen.

(4) Chorion : It is outermost fold of somatopleur (outer ectoderm and somatic mesoderm) and surrounds the embryo. In reptiles, birds and prototherians, allantochorion act as extra embryonic lungs help in exchange of gases. But in primates including human beings, only chorion forms the placenta (chorionic placenta).

Function : It protects the embryo and forms placenta for metabolic exchange between the foetus and the mother.

Placenta

Definition : Placenta is defined as a temporary intimate mechanical and physiological connection between foetal and maternal tissues for the nutrition, respiration and excretion of the foetus.

Structure : Human placenta consist of chorion only. Hence, it is called a chorionic placenta. Allantois remains small. The allantoic blood vessels, however, extend to vascularize it. A large number of branching villi from the vascular chorion penetrate the corresponding pits, the crypts, formed in the uterine wall. The latter becomes very thick and highly vascular to receive the villi. The intimate connection established between the foetal membrane and the uterine wall is known as the placenta.

The placenta is fully formed by the end of the third month and it lasts throughout pregnancy. When complete, it is a reddish – brown disc. In the placenta, the foetal blood comes very close to the maternal blood, and this permits the exchange of materials between the two. Food (glucose, amino acids, simple proteins, lipids), water, mineral salts, vitamins, hormones, antibodies and oxygen pass from the maternal blood into the foetal blood, and foetal metabolic wastes, such as carbon dioxide and urea, also water and hormones, pass into the maternal blood. The placenta, thus, serves as the nutritive, respiratory and excretory organ of the foetus. The continuous uptake of oxygen by foetal blood is ensured by the difference in affinity for oxygen between foetal and maternal haemoglobin.

The maternal and foetal blood are not in direct contact in the placenta, because (i) the two may be incompatible (ii) the pressure of maternal blood is far too high for the foetal blood vessels and (iii) there must be a check on the passage of harmful materials (blood proteins, germs) into the foetal blood.

(iii) Functions

(1) Placenta helps in the nutrition of the embryo as the nutrients like amino acids, monosugars, vitamins, etc. pass from the maternal blood into foetal blood through placenta.

(2) It also helps in respiration of the embryo as O₂ of the maternal blood and CO₂ of the foetal blood diffuse through placenta into the foetal blood and maternal blood respectively.

(3) It also helps excretion of the embryo as nitrogenous wastes of foetal blood like urea pass into maternal blood through placenta.

(4) Though the placenta acts as an effective barrier for certain toxic chemicals like histamine but certain germs like AIDS virus, syphilis bacteria, viruses of German measles, etc, intoxicants like nicotine of cigarette smoke and addictive drugs like heroin and cocaine can pass through the placenta and cause the developmental defects.

Classification of placenta

(1) According to the foetal membrane involved in the formation of placenta.

(i) Yolk sac placenta : In metatheria or marsupials, such as kangaroo (macropus) and opossum (Didelphys), placenta is derived from yolk sac and chorion.

(ii) Allantoic placenta : In the majority of Eutherian, the chief organ of embryonic nutrition is the allantoic placenta consist of allantois and chorion and also called allantochorionic placenta. Outside Eutheria, a primitive allantoic placenta occurs only in perameles (bandicoot) which is a metatherian.

(iii) Chorionic placenta : It occurs in primates (man and apes) and is formed only by chorion. Allantois remains small, burrows into body stalk (umbilical cord) and does not reach chorion. However, its mesoderm and blood vessels grow upto chorion whose villi enter the uterine crypts forming chorionic placenta.

(2) On the presence or absence of above barriers histologically placenta is divided into following types

(i) Epithelio-chorial : Most primitive and simplest type with all six placental barriers.

Examples : Odd hoofed mammals such as horse, ass, pig and lemurs.

(ii) Syndesmo-chorial : Uterine epithelium absent, with five placental barriers.

Examples : Even hoofed mammals such as cow, Buffalow, sheep, goat, camel, Girraffe etc.

(iii) Endothelio-chorial : Uterine epithelium and uterine connective tissues are absent, with four placental barriers.

Examples : Carnivores (dog, cat, lion, tiger etc.), Tree shrew and mole.

(iv) Haemo-chorial : Uterine epithelium, uterine connective tissue and endothelium of maternal blood vessel absent, with 3 foetal layers.

Examples : Primates (man, apes and monkey).

(v) Haemo-endothelial : Foetal capillaries indirect contact with maternal blood, only one placental barrier.

Examples : Rat, guinea pig and rabbit.

According to shape and distribution of villi : Depending on the shape of placenta, manner of distribution of villi, degree of connection between foetal and maternal tissues and behaviour of placenta at the time of birth, the following types and subtypes of allantoic placenta can be recognized.

(i) Non deciduous placenta : In most mammals villi are simple, unbranched and merely opposed without intimate contact between foetus and uterine wall. At the time of birth or parturition, villi are easily withdrawn from maternal crypts without causing any tissue damage. Thus no part of uterine tissue comes out and no bleeding occurs. Non deciduous or non-deciduate placenta has following subtypes according to the manner of distribution of villi.

(a) Diffuse : Villi remain scattered all over the surface of allantochorion. e.g., pig, horse, lemur.

(b) Cotyledonary : Villi are arranged in separate tufts or patches called cotyledons. e.g., goat, sheep, cow, deer.

(iii) Intermediate : Villi are arranged in cotyledons as well as scattered. e.g., camel, giraffe.

(ii) Deciduous placenta : Villi are complicated, branched and intimately connected. At birth, a variable amount of maternal tissue is pulled out with the shedding of blood. Deciduous or deciduate placenta is also differentiated in the following subtypes

(a) Zonary : Villi form an incomplete (e.g., racoon) or complete girdle encircling the blastocyst. e.g., cat, dog, seal, Lion, Tiger, Elephant etc.

(b) Discoidal : Villi are restricted to a circular disc or plate on the dorsal surface of blastocyst. e.g., insectivores, bats, rodents (rat, mouse), rabbit, bear.

(c) Metadiscoidal : Villi are at first scattered but later become restricted to one or two discs. It is monodiscoidal in man and bidiscoidal in monkeys and apes.

(iii) Contra-deciduous : Foetal villi and uterine crypts are so intimately connected that even most of foetal placenta is left behind at birth to be broken and absorbed by maternal leucocytes e.g., bandicoot (perameles), mole (Talpa).

Gestation period and parturition

Gestation period : Gestation period is the duration between fertilization and parturition.

Parturition : It is the expelling of the fully formed young from the mother’s uterus after the gestation period (about 280 days in human female).