Implantation:
The attachment of the blastocyst to the uterine wall is known as Implantation. This process starts 5- 6 days after fertilization.
The inner cell mass of blastocyst in placed against endometrium,the inner most layer of uterus.
The trophoblastic cell break the endometrium at the point of contact and the blastocyst sinks into a pit formed in endometrium.
Finger like projections (villi) are formed in the contact point in order to supply nutrients from mother to the developing embryo through placenta.
Chorion is also developed from tropoblast cells ,which secret human chorionic gonadotropin (HCG) hermone which maintains and stimulates the corpous leuteum to secrete progesterone hermone which prepares the endometrial wall for implantation and growth of the developing embryo.
Function of placenta-
Placenta is like a lifeline for the developing one which helps in various way to sustain life in the womb.
1. The oxygen and required nutrients are transmitted from mother to embryo through placenta via umbilical cord.
2. Wastes like ammonia, uria etc and carbondioxide are transfered from embryo to mothers blood stream through placenta .
The blood of mother and fetus does not mix up and there is a 6 layer barrier between placenta and the mother.
3. Placenta helps in immune response of fetus by transferring maternal Ig antibody to fetus and filters out harmful cytotoxic antibodies.
4. The hormones secreted from placenta are lactogen,estrogen and progesterone helps in growth of fetus.
Types of placenta-
According to placental shape and contact point.
Placenta are of different types, in different shapes and the area of contact between fetal and matternal tissue
1. Diffuse placenta-
In this type of placental formation almost entire surface of allantochorion forms placenta.
This type of placenta is found in horse and pigs.
2. Cotyledonary placenta-
In this type, there is multiple, discrete areas of attachment are formed by interaction of patches of allantochorion with endometrium. The area of attachments are called cotyledons and the matternal contact site are called carncles.
Ruminants bear this type of placentation.
3. Zonary placenta-
The placenta takes the form of a complete band of tissue surrounding the fetus.
This type is seen in dogs,cats,bears and elephants.
4. Discoid placenta-
In this type a single placenta is formed of discoid in shape in primates and rodentes.
Gastrulation:
After the formation of blastocyst, the blastomeres chage their positions and rearrangement of embryonic cell causes three layers( in triploblastic animals)in the embryo with a premitive digestive tube . This process is called gastrulation because of the formation of primitive gastric tube.
In some vertibrates after gastrulation process formation of two germ layer occurs (only ectoderm and endoderm) these animals are known as diploblastic animals.
Image source Wikipedia
What causes gastrulation?
Gastrulation process occurs due to
- change in the motility of cells
- cell shape change
- And change in the cellular adhesion to other cells.
After cleavage ,due to the morphogenetic movement ,blastocyst moves toward the uterus.
In the uterus implantation occurs. The gastrulation process starts 10 days after implantation of the blastocyst.
The inner cell mass of blastocyst forms endoderm layer and the primitive gut formed from the endodermal layer.
A part of this endoderm gives the alimentary canal and the other gives yolk sac.
After formation of alimentary canal the remaining mass of cell of endoderm gives embryonic disc.
The embryonic disc have three parts- cephalic margin, embryonic disc proper and caudal margin.
The caudal margin forms mesodermal layer by detachment from embryonic disc.
The cephalic margin and embryonic disc proper together makes ectodermal layer
of embryo.
Outcome of gastrulation
- Formationof gut.
-Formation of germ layer occurs.
- Archenteron and major body axis are formed.
Fate of germ layer or Organogenesis:
After formation of three germ layer in gastrulation process the cells of different layer gives rise different organs of the body by the process of differentiation ,which act as a cascade between three germ layer .
During this process the cells of embryo express specific sets of genes for formation of specific cell type.
Endoderm-
The cells of endoderm gives epithelial lining of various organs.
-It gives epithelial lining of whole digestive tract except part of mouth , pharynx and terminal part of rectum.
-It also forms the lining cells of glands open in to digestive tract.
-The epithellium of auditory tube and tympanic cavity are formed from endoderm and also trachea, bronci and alveoli of lungs.
-The bladder and part of urethra and follicular lining of thyroid gland and thymus is also formed from this layer.
Organs like the stomach, the colon,the liver ,the pancreas ,bladder, thyroid, parathyroid and intestine are formed from endodermal layer.
Mesoderm-
- This layer only present in triploblastic animals like frog,human etc
- Formation of mesodermal layer leads to the development of coelom, developes from tropoblast cell of blastocyst.
- The mesoderm develop in to four types like
1. Intermediate mesoderm , forms kidneys and gonads.
2. Paraxial mesoderm, which forms cartilage, skeletal muscles and dermis.
3. Lateralplate mesoderm, it forms circulatory system ( heart and spleen) and wall of gut.
4. Chorda mesoderm forms Notochord.
Mesoderm forms the muscles, both smooth and straight, bones cartilages,connective tissue, adipose tissue, circulatory system and notochord
Ectoderm-
The ectoderm developes in to surface ectoderm,neural crest and neural tube.
-The surface ectoderm forms epidermis ,hair, nails, lens of eye,sebaceous glands, cornea,tooth enamel.
-Neural crest forms peripheral nervous system, adrenal medulla, melanocytes,
dentine of teeth . Because of its importance neural crest is considered as 4th germ layer however it is originated from ectoderm.
-Neural tube developes in to brain, spinal chord,posterior pituitary, motor neurons and retina of eye .
-It also construct melanocytes.
Comments
Post a Comment