CHAPTER 12

​

EMBRYOLOGICAL AND FETAL DEVELOPMENT

INTRODUCTION

As a result of fertilization, the developing body in the female reproductive system till 2-3 weeks is known as zygotic development whereas embryological development is considered in the period of 2-8 weeks which is followed by the fetal development from 8th week of gestation.

SECOND WEEK OF DEVELOPMENT

The second week of development includes the development of trophoblast, bilaminar embryonic disc, amnion, yolk sac, sinusoids, extraembryonic coelom and the development of chorion.

• Development of Trophoblast

After 8 days of fertilization, the trophoblast divides into two layers, a syncytiotrophoblast (no distinct cells) and a cytotrophoblast (distinct cells).

• Syncytiotrophoblast
• Trophoblast

• Development of Bilaminar Embryonic Disc
• The embryoblast cells are divided into two layers after 8 days of fertilization, hypoblast and epiblast.
• There cells collectively form a flat disc-like structure known as a bilaminar embryonic disc in which a small cavity appears which enlarges to form the amniotic cavity.

• Development of the Amnion

Amnion forms the roof of the amniotic cavity which is domelike in shape whereas epiblast cells form the floor.

• Development of the Yolk Sac

It starts on 8th day after fertilization, the columnar cells migrates and become squamous cells to form a thin membrane referred to as the exocoelomic membrane.

Functions of the Yolk Sac

• It supplies the nutrition to the developing embryo during 2nd and 3rd week of development.
• It forms the part of gut.
• It acts as a shock absorber .

• Development of Sinusoids
• On the 9th day, the syncytiotrophoblast the cells expands and forms a small space within them known as lacunae.
• By the 12th day, the lacunae fuse by a network known as lacunar network and the maternal sinusoids are formed by the dilation of the capillaries surrounding the embryo.
• The maternal blood and secretions enter into the lacunar network which provides nutrition to the developing embryo.

• Development of the Extraembryonic Coelom

On the 12th day, the mesodermal layer of cells is developed from the yolk sac and forms the extraembryonic mesoderm.

• Development of the Chorion
• Two layers of trophoblast fuse with an extraembryonic mesodermal layer to form a chorion, that surrounds the embryo and the fetus and becomes part of the placenta .
• Chorion also secretes the hCG.
• Chorion also secretes the hCG. As the chorion develops, the extraembryonic coelom is known as the chorionic cavity.
• By the end of the 2nd week, a band of extraembryonic mesoderm connects the bilaminar embryonic disc to the trophoblast known as connecting (body) stalk. These stalks will be future umbilical cords.

THIRD WEEK OF DEVELOPMENT

From 3rd week, there is rapid development and differentiation begins for the next 6 upcoming weeks. This starts with the development of three primary germ layers.

• Gastrulation

After 15 days of fertilization, the bilaminar embryonic disc is transformed into trilaminar embryonic disc. This includes three primary germs layers:

• Endoderm
• Mesoderm
• Ectoderm

• Endoderm becomes the epithelial lining of the gastrointestinal tract, respiratory tract, and several other organs.
• Mesoderm forms the muscle, bones, peritoneum and other connective tissue.
• Ectoderm forms the epidermis of the skin and the nervous system.

​

Gastrulation

• Neurulation
• Notochord cells also stimulate the ectodermal cells to form the neural plate.
• The different neural folds approach each other and fuse to convert into neural tubes.
• After 4 weeks, the head end of the neural tube will divide to form the different parts of the brain.

• Intraembryonic Coelom

​

• Small spaces appear in the lateral plate of mesodermal layer which soon merges to form a larger cavity called the intraembryonic coelom.

​

• This cavity splits the lateral plate into two parts:
• Splanchnic mesoderm which forms the heart, blood vessels, connective tissues of the respiratory and digestive organs.
• Somatic mesoderm forms the ligaments, bones, blood vessels, the connective tissue of the limbs and the parietal layer of the serous membrane of the pericardium.

• Chorionic Villi and Placenta
• Chorionic villi are finger-like projections into the endometrial wall of the uterus. These are connected to the heart of the embryo by the umbilical arteries and the veins.
• Placenta is a pancake like structure, formed by the chorionic villi of the chorion of the fetus and the decidua basalis of the maternal endometrium. It is the site of exchange of the nutritions and the wastes between the mother and the fetus.

Schematic representation of the term placenta

• Cardiovascular System
• It is the first system to function. At the beginning of the 3rd week, angiogenesis occurs.
• Angiogenesis is the process of the formation of blood vessels. Mesodermal cells develop into hemangioblasts which will be converted into pluripotent cells to form the blood cells.
• A primitive structure of the heart is formed at the 4th week and starts beating from the 22nd day.

FOURTH WEEK OF DEVELOPMENT

In this week, organogenesis occurs. Organogenesis is the process of formation of the body’s organs and system, i.e., development of the gut, pharynx, cloacal membrane, upper limbs and the lower limbs etc.

• Respiratory System
• The lungs arise from the bud coming out of the pharynx which will divide to form the bronchial tree.
• The lower respiratory tract and lungs develop simultaneously.
• The lungs develop continuously till 8 years of age till the alveoli are fully developed.
• The type II alveolar cells start producing surfactant.
• Thorax movements are observed from 3rd month whereas the diaphragmatic movements are observed from 6th month

• Urogenital System
• The kidneys develop from the 4th week and start producing a small amount of urine from the 9th week.
• Kidneys become mature and fully functional at around the 15th week and constitute the amniotic fluid.
• The gonads start developing from the mesodermal layer at around 5th week.
• Differentiation occurs from the 7th week and the external genitalia in both sexes develops in the 9th week. Both sexes are distinguishable after the 12th week.

• Endocrine System
• The adrenal gland develops from the mesodermal layer and the neural crest cell at around 6th week.
• Adrenal glands produce the precursor of oestriols. They facilitate the maturation of the lungs, liver and epithelium of the digestive tract

• Digestive System
• It starts developing into a tube: foregut, midgut, hindgut.
• The liver, gall bladder and pancreas form 10% of the fetal weight by the 9th week.
• The liver starts producing bile from 12th week.
• Midgut contains the small intestine, caecum and vermiform appendix, ascending colon and transverse colon and is developed by 6th week.
• The hindgut is developed by the 7th week.
• The bile starts entering the duodenum at around the 13th week. Insulin is secreted from 10th week and the glucagon from 15th week.

• Integumentary, Skeletal and Muscular Systems
• By the 1st month, a thin layer of flattened cells covers the embryo. From 18 weeks, the fetus is covered by vernix caseosa.
• Hair start developing from the 9th week.
• By the 20th week, the fetus is covered with lanugo.
• Lanugo starts shedding from the 36th week.
• The skeletal muscles are developed from the mesodermal layer. The skeleton is first prepared as cartilage, later on replaced by the bones and the muscles continue to grow till skeletal maturity.