Embryology: Stages and Branches

• Embryology studies the various stages of an embryo using different techniques, from fertilization to organogenesis.

Branches of Embryology

• Descriptive embryology describes the various stages of an embryo, from fertilization to organogenesis.
• Experimental embryology studies the fundamental developmental mechanisms of the embryo by removing, transplanting, or exchanging parts of the developing embryo.
• Comparative embryology studies and compares the embryological development of different animals.
• Chemical (biochemical) embryology studies malformation by studying developmental stages using biochemical, biophysical, and physiological techniques.

Stages of Embryonic Development

• Gametogenesis is the formation of male and female gametes.
• Fertilization is the fusion between the nucleus of sperm (1n) and the nucleus of ovum (1n), resulting in a zygote (2n).
• Cleavage is the successive mitotic division of a zygote, forming a ball of cells called a blastula.
• Blastulation involves a ball of cells lined by a blastoderm, enclosed with a blastocoele cavity.
• Gastrulation is the rapid cell movement of blastula cells, leading to the formation of germ layers: ectoderm, mesoderm, and endoderm.
• Neurulation occurs with the formation of an organ rudiment.
• Organogenesis is the formation of organs.

Male Reproductive System

• Consists of testes, glands, and ducts.
• Testes are located outside the body, enclosed in the scrotal sac.
• Each testis is surrounded by an outer tunica vaginalis and an inner tunica albuginea.
• Each testis is divided into approximately 250 lobules, each containing 1-4 seminiferous tubules.
• Seminiferous tubules are the structural units of the testis that produce sperm.
• Interstitial cells (Leydig cells) between the seminiferous tubules produce testosterone hormone.

Glands in the Male Reproductive System

• Seminal vesicles produce 60% of semen, which is rich in fructose.
• Prostate glands secrete 20% of milky semen.
• Cowper's (bulbourethral) glands produce 20% of mucoid semen.

Ducts in the Male Reproductive System

• Epididymis is a long, coiled tube responsible for the complete maturation of sperms.
• Ductus deferens transfers sperms to the ejaculatory duct.
• Urethra transports both sperms and urine to the outside.

Spermatogenesis

• Formation of sperms.

Stages of Spermatogenesis

• Multiplication phase involves primordial germ cells undergoing mitotic division to produce spermatogonia (2n), types A and B.
• Growth phase involves type B spermatogonia increasing in size without division to form primary spermatocytes (2n).
• Maturation phase: each primary spermatocyte undergoes the first meiotic division, producing two secondary spermatocytes (1n); each secondary spermatocyte then undergoes the second meiotic division to produce two spermatids (1n).
• Spermiogenesis involves the differentiation of spermatids into sperms.

Female Reproductive System

• Consists of the ovary, fallopian tube, and uterus.

Ovaries

• Small, lumpy, almond-shaped organs located near the lateral walls of the pelvic cavity.
• Produce immature female gametes (oocytes).
• Secrete female sex hormones, including estrogens and progestins.

Uterine Tubes (Fallopian Tubes/Oviducts)

• Hollow, muscular tubes measuring approximately 13 cm (5 in.) in length.
• Consist of three segments: infundibulum, ampulla, and isthmus.
• The infundibulum is the end closest to the ovary that forms an expanded funnel with fingerlike projections called fimbriae. The inner surfaces are lined with cilia.
• The ampulla’s smooth muscle layers in its wall gradually thicken toward the uterus.
• The isthmus is the short segment that connects to the uterine wall.

Uterus

• Provides mechanical protection, nutritional support, and waste removal for the developing embryo (weeks 1-8) and fetus (from week 9 to delivery).
• Contractions in the muscular wall aid in ejecting the fetus during birth.
• Consists of a thick, outer, muscular myometrium and a thin, inner, glandular endometrium (mucosa).
• The endometrium contributes about 10% of the uterus's mass and supports the growing fetus with glandular and vascular tissues.
• The myometrium forms about 90% of the uterus's mass and provides the force needed to move the fetus during birth.

Oogenesis

• Starts at fetal life.

Stages of Oogenesis

• Multiplication phase: Primordial germ cells undergo mitotic division, which gives rise to oogonia (2n).
• Growth phase: Oogonia increase to become primary oocytes (2n).
• Maturation phase: each primary oocyte produces one secondary oocyte (1n) and the first polar body (1n). Each secondary oocyte undergoes the second meiotic division (upon fertilization), giving rise to a mature ovum and the second polar body (1n). The first polar body may also undergo the second meiotic division to give two secondary polar bodies (1n).