Ovulation and Implantation

• With each ovarian cycle, a number of primary follicles begin to grow, but usually only one reaches full maturity, and only one oocyte is discharged at ovulation.
• At ovulation, the oocyte is in metaphase of the second meiotic division and is surrounded by the zona pellucida and some granulosa cells.
• Sweeping action of tubal fimbriae carries the oocyte into the uterine tube.
• Before spermatozoa can fertilize the oocyte, they must undergo Capacitation, during which time a glycoprotein coat and seminal plasma proteins are removed from the spermatozoon head.
• The acrosome reaction, during which acrosin- and trypsin-like substances are released to penetrate the zona pellucida, occurs during fertilization.
• Three distinct layers can be recognized in the endometrium during the menstrual phase: a superficial compact layer, an intermediate spongy layer, and a thin basal layer.
• The human blastocyst implants in the endometrium along the anterior or posterior wall of the body of the uterus, where it becomes embedded between the openings of the glands.
• If the oocyte is not fertilized, venules and sinusoidal spaces gradually become packed with blood cells, and an extensive diapedesis of blood into the tissue is seen.
• When the menstrual phase begins, blood escapes from superficial arteries, and small pieces of stroma and glands break away.
• During the following 3 or 4 days, the compact and spongy layers are expelled from the uterus, and the basal layer is the only part of the endometrium that is retained.
• The basal layer, which is supplied by its own arteries, the basal arteries, functions as the regenerative layer in the rebuilding of glands and arteries in the proliferative phase.
• The inner cell mass, which is formed at the time of compaction and will develop into the embryo proper, is at one pole of the blastocyst.
• The outer cell mass, which surrounds the inner cells and the blastocyst cavity, will form the trophoblast.
• The uterus at the time of implantation is in the secretory phase, and the blastocyst implants in the endometrium along the anterior or posterior wall.
• If fertilization does not occur, then the menstrual phase begins, and the spongy and compact endometrial layers are shed.
• FGF and BMP4 ventralize mesoderm during gastrulation so that it forms intermediate and lateral plate mesoderm.
• Chordin, noggin, and follistatin antagonize BMP4 activity and dorsalize mesoderm to form the notochord and somitomeres in the head region.
• Formation of these structures in more caudal regions is regulated by the Brachyury (T) gene.
• Laterality (left–right asymmetry) is regulated by a cascade of signaling molecules and genes.
• FGF8, secreted62, is a part of this cascade.
• An ultrasound scan detects a large mass near the sacrum of a 28-week female fetus.
• On ultrasound examination, it was determined that a fetus had well-developed facial and thoracic regions, but caudal structures were abnormal.
• Kidneys were absent, lumbar and sacral vertebrae were missing, and the hindlimbs were fused.
• A child has polysplenia and abnormal positioning of the heart.
• Epiblast cells moving through the node and streak are predetermined by their position to become specific types of mesoderm and endoderm.
• By the end of the third week, three basic germ layers, consisting of ectoderm, mesoderm, and endoderm, are established in the head region, and the process continues to produce these germ layers for more caudal areas of the embryo until the end of the fourth week.
• Tissue and organ differentiation has begun, and it occurs in a cephalocaudal direction as gastrulation continues.
• The trophoblast progresses rapidly during the embryonic period.
• Primary villi obtain a mesenchymal core in which small capillaries arise.
• When these villous capillaries make contact with capillaries in the chorionic plate and connecting stalk, the villous system is ready to supply the embryo with its nutrients and oxygen.
• The embryonic period, which extends from the third to the eighth weeks of development, is the period during which each of the three germ layers, ectoderm, mesoderm, and endoderm, gives rise to its own tissues and organ systems.
• The ectodermal germ layer gives rise to the organs and structures that maintain contact with the outside world: Central nervous system, Peripheral nervous system, Sensory epithelium of ear, nose, and eye, Skin, including hair and nails, and Pituitary, mammary, and sweat glands and enamel of the teeth.
• The basal layer remains to regenerate the other layers during the next cycle.
• The corona radiata is the outermost layer of the zona pellucida.
• The zona pellucida becomes impenetrable to other spermatozoa when the oocyte finishes its second meiotic division and forms the female pronucleus.
• The head of the sperm separates from the tail, swells, and forms the male pronucleus during fertilization.
• Neural tube closure is initiated and proceeds in a specific week in gestation.
• Failure of neural tube closure can lead to cranial or caudal defects.
• Neural crest cells have an embryological origin that is ectodermal, mesodermal, or endodermal.
• Neural crest cells contribute to structures such as the eyes, ears, face, and heart.