Back to A&P syllabus Reproductive System
Human Development and Human Body Sections (updated 9/24/05)
A few sections of the chapter on Human Development relate to the chapters on the endocrine system, reproduction, and circulation. A haploid secondary oocyte, containing 23 double-stranded chromosomes, is ovulated, and the process of fertilization begins in the uterine tube. After the egg nucleus undergoes the second meiotic division and releases the second polar body, the male and female nuclei fuse to form the diploid zygote, containing the normal complement of 46 chromosomes.
Within about 10 days, the blastocyst stage of the embryo implants within the endometrium of the uterus, and begins to release human chorionic gonadotropin (HCG), which stimulates the corpus luteum to continue producing progesterone (a positive feedback mechanism).
Below are details of early stages in the embryological development of vertebrates.
Once the grastrula develops, the neural plate is formed above the notochord (a stiffening rod in all vertebrate embryos). Cell migration leads to the beginning of tissues and organs.
A side view of a fetal stage showing early organs, and an adult mouse.
The four extraembryonic membranes: the amnion (blue), chorion (green), yolk sac (large yellow), and allantois (small yellow), begin to form. The placenta is not an extraembryonic membrane.
Beginnings of the primary germ layers and their derivatives.
Human placenta shown below, in addition to chorion and amnion.
Human fetus shown in days 49-56, showing changes in development of head, ear, and limbs.
Note: only the inner lining of the digestive and respiratory systems are derived from endoderm; the remaining portions of these systems is composed of mesoderm. The epidermis and all of the nervous system are derived from ectoderm. The dermis and remaining systems are derived from mesoderm.
In the process of sexual development, as far as genetic influences are concerned, the male has the leading role in determining sex. Male mammals contain an X and a Y chromosome, while females contain two X chromosomes. Thus, 50% of male sperms contain an X chromosome and 22 autosomes, and the other 50% contain a Y chromosome and 22 autosomes. All ova (egg cells) contain an X chromosome. The combination of XY or XX usually determines the sex of the child; exceptions occur when the embryo fails to respond to the influences of the chromosomes or sex hormones. The illustration below shows examples of amniocentesis, chorionic villi sampling, and a normal karyotype of a human female.
When sexual development begins, the embryo is in a neutral stage, but under the influence of the chromosomes and proper level of hormones, differentiation occurs in most instances to produce a male or a female. The colors shown below illustrate homologous sexual structures, those that share the same embryological origins.
Below are fetal circulatory structures and the postnatal structures. Note the light shading around five structures below. Be able to discuss the changes in these structures at birth.
Be able to name the adult structures derived from each of the above fetal structures.
Sections of the body. Transverse section of head, showing frontal sinuses and lateral ventricles of brain.
Transverse section through eye, ethmoidal sinus, and third ventricle of brain.
Transverse section of head showing sphenoidal sinus and third ventricle of brain.
Transverse section of neck showing jugular vein and carotid artery.
Sagittal section of thoracic and upper abdominal cavity.
Transverse section of thorax showing heart and lungs.
Sagittal section of lower thoracic and upper abdominal cavity.
Three transverse sections of abdomen. Note labels and views of organs in each section.
Note that this portion of pancreas is retroperitoneal, while portion in section below is not.
Transverse section of abdomen showing retroperitoneal position of kidneys and aorta. Note that this portion of pancreas is not retroperitoneal, while portion in section above is.
Sagittal section of lower abdominal and pelvic cavity.
Transverse section of male pelvic section through prostate gland.
