Reproductive Systems and Gametogenesis

A. Reproductive Systems

• The female reproductive system comprises two ovaries, two uterine tubes, a uterus, and a vagina.
• The uterine wall has three layers: endometrium, myometrium, and perimetrium.
• Fertilization occurs within the uterine tubes.
• The male reproductive system includes two testes, two epididymis, two seminal vesicles, two ejaculatory ducts, a prostate, and a penis.
• The primary sex organs (gonads) produce gametes: testes in males and ovaries in females.

B. Gametogenesis

• Gametes develop from germ cells that originate in the umbilical vesicle and migrate to the gonads.
• Gametogenesis transforms germ cells (46 chromosomes, diploid) into male and female gametes (23 chromosomes, haploid).
• The female gamete is a secondary oocyte (X).
• The male gamete is a sperm (X or Y).
• Gametes contain 22 autosomes and one sex chromosome.
• Errors in meiosis can lead to abnormal gametes, such as in Down syndrome.

C. Female Reproductive Cycles

• The ovarian cycle involves cyclical changes in the ovary monthly.
• The menstrual cycle involves cyclical changes in the endometrium monthly.
• The ovarian cycle phases are follicular, ovulation, and luteal.
• The menstrual cycle phases are ischemic, menstrual, proliferative, and luteal.
• Breastfeeding can temporarily cease menstruation.

D. Fertilization

• Fertilization is the fusion of male and female gametes to form a zygote.
• Fertilization typically occurs in the ampulla of the uterine tube.
• Fertilization restores the diploid chromosome number (46 chromosomes).
• Fertilization determines the embryo's sex (XX or XY).
• Embryonic age begins two weeks after the last normal menstrual period (LNMP).

E. Cleavage (Segmentation)

• Cleavage involves repeated mitotic divisions of the zygote, producing blastomeres.
• Cleavage occurs in the uterine tube during the first week of pregnancy.
• The morula is a solid ball of cells formed during cleavage.
• The morula develops into a blastocyst with an inner cell mass (embryoblast) and outer layer (trophoblast).

F. Implantation

• Implantation is the embedding of the blastocyst into the endometrium of the uterus.
• The usual implantation site is the upper posterior wall of the uterus's fundus.
• By day 10, the blastocyst is fully embedded, creating a closing plug within the endometrium.
• By the end of the second week, the endometrium is filled with edematous cells rich in glycogen and lipids (decidua).

G. Gastrulation

• Gastrulation is the beginning of morphogenesis (body form development), occurring during week 3.
• The first sign of gastrulation is the primitive streak formation, a thickened linear band of epiblast cells.
• The primitive node is at the cranial end of the primitive streak.
• The primitive streak's growth leads to the development of the notochord.

H. The 3rd Week

• The notochordal process forms a plate, which folds to form the notochord, the primordial axis of the embryo.
• The notochord is crucial for central nervous system and axial musculoskeletal development.
• The end of the 3rd week finds the embryonic disc changing shape to a flat, elongated form with a broad head and narrow tail.
• Tertiary chorionic villi form during this period.

I. Folding of the Embryo

• During the fourth week, the flat embryonic disc folds to form a cylindrical embryo.
• Folding involves the formation of a head, tail, and lateral folds, resulting in the umbilical ring.
• The chorionic villi at the embryonic pole grow and branch, forming the chorion frondosum part of the placenta.

J. Derivatives of the Germ Layers

• The epiblast forms the three germ layers (ectoderm, mesoderm, endoderm).
• The ectoderm differentiates into neuroectoderm and dermoectoderm.
• The neuroectoderm gives rise to the neural tube and crest.
• The mesoderm is segmented into paraxial, intermediate, and lateral plate mesoderms.
• The paraxial mesoderm forms somites, crucial for skeletal, muscular and connective tissue development.
• The number of somites can be used to determine the embryo's age.
• The intermediate mesoderm contributes to the urogenital system.
• The lateral plate mesoderm forms the intraembryonic coelom and subsequently the pericardial, pleural and peritoneal cavities.

K. Placenta

• The placenta is the fetal-maternal organ responsible for gas and nutrient exchange.
• The full-term placenta is discoid, weighing approximately 500g and expelled within 30 minutes of birth.
• The fetal side of the placenta, derived from the chorion frondosum, is smooth and covered by the amnion.
• The maternal side, derived from the decidua basalis, is irregular and divided into cotyledons.

L. Umbilical Cord

• The umbilical cord is about 50 cm long at full term.
• The cord contains two arteries and one vein, enveloped in Wharton's jelly.

M. Twins

• Dizygotic twins arise from two separate zygotes.
• Monozygotic twins arise from one zygote.
• Conjoined twins result from incomplete separation.

THE Amnion

• The amnion forms a fluid-filled sac that surrounds the embryo, maintaining homeostasis and supporting growth.
• Amniotic fluid volume varies but typically 800-1000ml.
• The umbilical vesicle, an older name for the yolk sac, is a pear-shaped structure connected to the midgut by a duct. It plays a role in germ cell development.