embryology

Semen Composition: Semen is comprised of secretions from glands and sperm cells.
- Secretions consist of fluid that nourishes and supports sperm.
Ejaculation: Semen is ejaculated through the urethra during sexual intercourse.
- The male urethra is long, divided into three components.
- The ejaculation usually occurs during penetration, transporting sperm cells to the vaginal canal.

Ovaries: Paired structures producing ova (eggs), referred to as secondary oocytes released during ovulation, typically once a month.
Uterine Tube (Fallopian Tube): Carries the ovum toward the uterus for potential fertilization.
- Sperm enters the female reproductive system via the vagina through the cervix into the uterus.
- Cervix: The narrow portion of the uterus containing the cervical canal that sperm swims through.

Fusion of Sperm and Egg: Fertilization usually occurs in the uterine tube.
- The head of the sperm penetrates the ovum, while the tail remains outside.
- This process leads to the formation of the zygote, which is a fertilized ovum that undergoes mitotic division.

Zygote Division: The zygote divides through mitosis, progressing into cleavage and forming a morula (16-32 cells).
Blastulation: Formation of the blastocyst, which involves fluid moving into the interstitial area, creating a fluid-filled sac.
- The blastocyst has three components:
  - Embryoblast (Inner Cell Mass): Will develop into the embryo.
  - Blastocele: Fluid-filled cavity.
  - Trophoblast: Layer that attaches to the endometrium.
Implantation: Occurs about six to seven days after fertilization, completing in about five days.

Decidua Layers: Three distinct layers of the endometrium during implantation:
- Decidua Basalis: Underneath the blastocyst.
- Decidua Capsularis: Covers the blastocyst.
- Decidua Parietalis: The remaining tissue around the other regions.

The blastocyst continues to mature and change, forming structures including:
- Amniotic Cavity: Where the embryo/fetus develops, more fluid-filled.
- Primary Yolk Sac: Provides initial nourishment before the placenta forms.
- Embryonic Disc: Where the actual embryo develops.

Cytotrophoblast: Cellular layer that interacts with the endometrial tissue.
Syncytiotrophoblast: Multi-nucleated cells formed by merging trophoblast cells, aiding in implantation and forming the placenta.
Chorion Formation: Developed from the trophoblast, contributing to the formation of chorionic villi that anchor the embryo and establish nutrient exchange.

Hormones Produced: HCG (human chorionic gonadotropin) is produced and helps maintain the pregnancy by influencing the ovaries to secrete estrogen and progesterone.
Detection of HCG is the basis for early pregnancy tests.

Embryonic Period: Comprises four key stages:
1. Germ Cell Formation: Formation of the inner cell mass into two layers: epiblast (ectoderm) and hypoblast (endoderm).
2. Gastrulation: Development of a third layer (mesoderm) between the epiblast and hypoblast, crucial for forming various body structures.
3. Neurulation: Development of the central nervous system, deriving from ectoderm.
4. Somite Formation: Paired tissue blocks develop from mesoderm, differentiating into tissues such as bone (sclerotome), muscle (myotome), and skin (dermatome).