PREGNANCY AND EMBRYONIC DEVELOPMENT

 

Pregnancy is the process of carrying an offspring, called an embryo or fetus, in the uterus of a female mammal. The embryonic development is the process by which the embryo develops into a fetus.

The fertilization of an egg by a sperm marks the beginning of pregnancy. The fertilized egg, now called a zygote, undergoes several cell divisions as it travels down the fallopian tube towards the uterus. About five days after fertilization, the zygote forms a hollow ball of cells called a blastocyst, which implants itself into the uterine lining.

During the embryonic stage, which lasts from implantation until the end of the eighth week of pregnancy, the blastocyst develops into an embryo, and the three germ layers (ectoderm, mesoderm, and endoderm) form. These germ layers give rise to all the organs and tissues of the body.

 

 

By the end of the embryonic stage, the embryo has developed most of its major organs, including the heart, brain, and lungs. The fetus stage then begins, which is characterized by growth and maturation of the organs and tissues already formed. During this stage, the fetus continues to grow in size and weight, and its body systems become more complex and mature. At the end of the fetal stage, the baby is fully formed and ready for birth.

Throughout pregnancy, various factors can affect the development of the embryo or fetus, including genetics, maternal nutrition, exposure to toxins, and certain medications. It is essential for expectant mothers to receive proper prenatal care to ensure the health and well-being of both the mother and the developing baby.

 

• The placenta is an essential organ that connects the developing fetus to the mother’s uterus. It serves as a conduit for nutrition, respiration, and waste removal.
• The placenta is lined with finger-like projections called villi that are matched by depressions in the uterine wall, creating a large surface area for exchange of materials between mother and fetus.
• In humans, the placenta is hemochorial, meaning that maternal blood directly bathes the fetal chorionic villi. It is also non-deciduous and meta discoidal in shape.
• Unlike in simpler placental structures, only three barriers separate maternal blood from fetal blood in the human placenta.
• The umbilical cord is a flexible cord-like structure that connects the fetus at the navel to the placenta. It contains two umbilical arteries and one vein, which supply the fetus with nutrients and oxygen and remove waste materials.
• Both types of blood vessels in the umbilical cord are surrounded by Wharton’s jelly, a gelatinous substance that helps protect them from damage.
• In addition to its role in facilitating material exchange, the placenta also acts as an endocrine tissue. It produces hormones like human chorionic gonadotropin (hCG), human placental lactogen (hPL), estrogens, and progestogens, which help regulate maternal physiology and support fetal development. Later in pregnancy, the ovary also secretes relaxin.

 

PARTURITION AND LACTATION

After approximately nine months of pregnancy, the fully developed fetus is ready for delivery, which is known as parturition. The process of parturition is initiated by a complex neuroendocrine mechanism involving hormones such as cortisol, estrogens, and oxytocin. The powerful contractions of the uterus during labor are necessary for the delivery of the baby. The stages of labor during parturition include the dilation of the cervix in the first stage, the delivery of the baby in the second stage, and the delivery of the placenta and umbilical cord in the third stage.

The signals for the onset of parturition originate from the fully developed fetus and the placenta, which induce mild uterine contractions called the fetal ejection reflex. This reflex triggers the release of oxytocin from the posterior pituitary gland, which acts on the uterine muscles to cause stronger contractions.

The gestation period, which is the length of time from conception to birth, is typically around 280 days, calculated from the time of the mother’s last menstrual period (hence 266 days from conception).

During pregnancy, the mammary glands differentiate and begin to secrete milk after childbirth, a process known as lactation. The newborn baby is nourished by the mother’s milk during the initial few months of growth. The milk produced during the first few days of lactation is called colostrum, which contains numerous antibodies essential for developing the baby’s resistance to infections.

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