Embryonic Development: Gastrulation and Trophoblast Formation

Uterus

• Size: 7.5 cm long, 5 cm wide, and 2.5 cm thick
• Composed of three parts:

  Parts of Uterus

  • Fundus: upper dome-shaped portion superior to the uterine tubes
  • Body/Corpus: middle part and main part of the uterus
  • Cervix: inferior narrow portion that opens into the vagina

Vagina

• A tubular, 8-10 cm long canal lined with mucous membrane

• External opening is the vaginal orifice

• Guarded by a thin fold of vascularized mucous membrane called the hymen### Hymen and Virginity

• The hymen can be broken during the first sexual intercourse, but it can also be broken by other means such as a sudden fall, insertion of a vaginal tampon, or active participation in sports like horseback riding or cycling.

• The presence or absence of the hymen is not a reliable indicator of virginity or sexual experience.

Vulva and External Genitalia

• The vulva consists of the paraurethral glands, which produce bacteria-lactobacillus acidophilus.

ForeSkin and Prepuce

• The foreskin, also known as the prepuce, covers the glans penis and can be retracted back over the shaft of the penis.

Hormonal Regulation of Lactation

• Prolactin, produced by the anterior pituitary gland, stimulates milk production during lactation.
• Estrogen and progesterone prepare the body for pregnancy, and oxytocin stimulates milk ejection.
• The sucking action of the baby on the nipple stimulates prolactin secretion, which maintains lactation.

Composition of Human Milk

• Human milk contains lactose, casein (milk protein), and minerals like sodium, potassium, calcium, and phosphorus.
• Vitamins and passive immunity are also present in human milk.

Follicular Development

• Primary follicles develop from primordial follicles in the embryonic stage.
• The first meiotic division of oogenesis occurs in the fetal stage, and the second meiotic division occurs at the time of fertilization.
• Graafian follicles are the mature follicles that release an ovum during ovulation.

Menstrual Cycle

• The menstrual cycle is the phase between two menstruations, lasting around 28 days.
• The follicular phase starts at menstruation, and ovulation occurs around the 14th day.
• The luteal phase starts after ovulation and lasts until the next menstruation.

Ovulation and Fertilization

• Ovulation occurs when the ovum is released from the Graafian follicle.
• Fertilization occurs when a sperm fertilizes the ovum in the fallopian tube, resulting in a zygote with 46 chromosomes.
• The zygote then travels down the fallopian tube and into the uterus, where it implants in the endometrium.### Embryonic Development
• Gastrulation begins with the formation of the primitive streak on the surface of the epiblast, around 7-9 days after fertilization.
• During gastrulation, three germ layers are formed by morphogenetic movement of blastomeres: ectoderm, endoderm, and mesoderm.

Implantation

• Implantation is the attachment or embedding of the developing blastocyst with the wall of the uterus (endometrium) with the help of lytic enzymes.
• The blastocyst completely sinks into the wall of the uterus, and the endometrium becomes more vascularized.
• After implantation, the endometrium is known as the decidua.

Placenta

• The placenta is a structural and functional unit between the developing embryo and maternal body.
• It is responsible for the exchange of nutrients, hormones, gases, and antibodies between the mother and foetus.
• The placenta acts like an endocrine gland and releases hormones such as progesterone, human chorionic gonadotrophin, and human placental lactogen.

Gestation Period

• The gestation period is the duration between fertilization and parturition (childbirth).
• In humans, the gestation period is approximately 9 months, 38 weeks, or 266 days.
• The gestation period is divided into three trimesters: first trimester (1-3 months), second trimester (4-6 months), and third trimester (7-9 months).

Fetal Development

• During the first trimester, the major organs and systems are formed.
• During the second trimester, the fetus develops limbs and digits, and most organs are well-developed.
• During the third trimester, the fetus continues to grow and develop, and the body is covered with fine hair (lanugo).

Parturition

• Parturition is the process of childbirth, which is triggered by the release of hormones such as oxytocin and progesterone.
• The foetus is fully developed and ready for parturition by the end of the gestation period.
• The placenta releases hormones that stimulate uterine contractions and help in the expulsion of the fetus during childbirth.