Anatomy and Physiology of Children and Adolescents - M. R. Sapin 2007

Genitourinary System
Reproductive System
Placenta

After Fertilization (the fusion of the female and Male Germ Cells), a new Organism—the embryo—emerges. It undergoes intrauterine development within the uterine cavity for 9 months, right up until birth.

The connection between the developing embryo (fetus) and the maternal organism is maintained through an extraembryonic organ known as the placenta. The placenta supplies the embryo with essential nutrients and oxygen, while also shielding it from foreign substances. At the same time, certain medications, alcohol, nicotine, and drugs can cross the placental barrier and exert harmful, or even destructive, effects on the developing embryo.

The placenta is disk-shaped, measuring approximately 20 cm in diameter and about 5 cm in thickness, and is attached to the wall of the uterine cavity. It is formed by the proliferating trophoblast, which develops into the villous chorion—this constitutes the embryonic, or fetal, part of the placenta. The maternal, uterine portion is formed by the thickened mucous membrane of the Uterus, the endometrium. Within the placenta, maternal and fetal Blood flows through separate vessels and does not mix. Maternal blood bathes the chorionic villi externally, whereas the fetal Blood Vessels branch extensively within the core of these villi (Fig. 71, see color insert).

The total surface area of all chorionic villi reaches 7 m². Situated between the maternal and fetal bloodstreams is the so-called placental (or hemochorial) barrier, which consists of the endothelial lining of both maternal and fetal vessels, along with the intervening Tissues of the chorionic villi. This hemochorial barrier performs vital Functions. It mediates gas exchange between maternal and fetal blood, facilitates The transport of nutrients from the mother to the fetus, and clears Metabolic waste products in the opposite direction. Furthermore, the hemochorial barrier serves an important protective role.

The placenta connects to the embryo (fetus) via the umbilical cord, which encloses the fetal blood vessels. These include two umbilical Arteries, carrying deoxygenated blood from the fetus to the placenta, and one umbilical vein, which transports oxygen- and nutrient-rich arterial blood from the placenta back to the fetus. These blood vessels are embedded in an elastic mucous (gelatinous) tissue that protects them and prevents compression.

Despite this considerable degree of protection, the embryo and fetus remain highly vulnerable throughout their development. During intrauterine life, there are specific critical periods characterized by a heightened sensitivity of the developing organism to various environmental and Chemical factors (including alcohol, nicotine, X-ray radiation, and even certain medications). The most hazardous periods include gametogenesis (oogenesis and Spermatogenesis), fertilization, embryo implantation (on days 6–7 of Embryogenesis), the stage of accelerated Brain GROWTH AND DEVELOPMENT (weeks 15–20 of gestation), and the birth process itself.

Review and Self-Assessment Questions:

1. Describe The Structure of the spermatozoa and the ovum.

2. Indicate in which Organs and by what mechanisms male and Female Germ Cells develop.

3. At which stages of male and female germ Cell development do they acquire a single (haploid) set of Chromosomes?

4. What process in the female body is referred to as the Menstrual cycle? What does it involve, and what phases does it comprise?

5. Describe the structures that form the placenta and explain its primary functions.