Embryology: Development stages

• Embryology os a branch of biology.
• Embryology studies the stages of embryo development from fertilization to organogenesis, using various techniques.

Branches of Embryology

• Descriptive Embryology describes the various stages of embryo development from fertilization to organogenesis.
• Experimental Embryology studies fundamental developmental mechanisms by removing, transplanting, or exchanging embryo parts, or by altering environmental conditions.
• Comparative Embryology studies and compares the embryological development of different animals.
• Chemical Embryology studies malformation using biochemical, biophysical, and physiological techniques.

Stages of Embryonic Development

• Gametogenesis is the formation of male and female gametes.
• Fertilization is the fusion of the nucleus of sperm (1n) and the nucleus of ovum (1n) to give zygote (2n).
• Cleavage is the successive mitotic division of zygote to give a ball of cells called blastula.
• Blastulation is the formation of blastula.
• Blastula lined by blastoderm and enclosed with blastocoele cavity.
• Gastrulation is the rapid cell movement of blastula cells which form germ layers, including ectoderm, mesoderm, and endoderm.
• Neurulation is the formation of the organ rudiment.
• Organogenesis is the formation of organs.

Male Reproductive System

• Male reproductive system consists of testes, glands, and ducts.
• Testes: Two testes are located outside the body and enclosed inside the scrotal sac.
• Each testis is surrounded by two layers: outer tunica vaginalis and inner tunica albuginea.
• Each testis is divided into about 250 lobules, each containing 1-4 seminiferous tubules.
• Seminiferous tubule is the structural unit of the testis that produces sperms.
• Interstitial cells (Leydig cells) between seminiferous tubules produce testosterone hormone.

Glands in the Male Reproductive System

• Seminal vesicle produces 60% of semen which is rich with fructose.
• Prostate gland secretes 20% of milky semen.
• Cowper's (bulbourethral) gland produces 20% of mucoid semen.

Ducts in the Male Reproductive System

• Epididymis is a long coiled tube where sperms complete their maturation.
• Ductus deferens Transfers sperms to the ejaculatory duct.
• Urethra transports sperms and urine to the outside.

Spermatogenesis

• Spermatogenesis is the formation of sperms.

Stages of Spermatogenesis

• Multiplication phase: Primordial germ cells undergo mitotic division, producing spermatogonia (2n) of Types A and B
• Growth phase: Type B spermatogonia increase in size (without division), converting into primary spermatocytes (2n).
• Maturation phase: Primary spermatocytes undergo first meiotic division, yielding two secondary spermatocytes (1n); each secondary spermatocyte then undergoes a second meiotic division, producing two spermatids (1n).
• Spermiogenesis: Spermatids differentiate into sperms.

Female Reproductive System

• The female reproductive system is composed of the ovary, fallopian tube, and uterus.
• Ovaries are small, lumpy, almond-shaped organs near the lateral walls of the pelvic cavity.
• Ovaries form immature female gametes or oocytes.
• Ovaries secrete female sex hormones like estrogens and progestins.
• The Uterine Tubes (Fallopian tube or oviduct) are hollow, muscular tubes about 13 cm (5 in) long.

Segments of the Uterine Tubes (Fallopian tube or oviduct)

• The infundibulum is the segment closest to the ovary which forms an expanded funnel with finger-like projections into the pelvic cavity called fimbriae, lined with cilia beating toward the ampulla.
• The ampulla is a segment in the middle of the uterine tube, with smooth muscle layers increasing in thickness.
• The isthmus is a short segment connecting to the uterine wall.

The Uterus

• The 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 of the uterus are important in ejecting the fetus at the time of birth.
• The uterus consists of a thick, outer, muscular myometrium and a thin, inner, glandular endometrium, or mucosa.
• Endometrium contributes about 10 percent to the mass of the uterus.
• Glandular and vascular tissues of the endometrium support the physiological demands of the growing fetus, and is the site of implantation of the embryo.
• Myometrium is the thickest portion of the uterine wall, contributing about 90% of the uterus mass.
• Myometrium provides the force needed to move a large fetus out of the uterus and into the vagina.

Structure of Ovary

• Structures of the ovary includes the the primordial follicle, primary follicle, secondary follicle, tertiary follicle, mature follicle, oocyte, medulla, corona radiata, ovulated ovum, ruptured follicle, tunica albuginea and developing corpus luteum,

Oogenesis

• Oogenesis is the process that starts at fetal life.

Phases of Oogenesis

• Multiplication phase: primordial germ cells undergo mitotic division, giving oogonia (2n).
• Growth phase: oogonia increase in size (without division), converting into primary oocytes (2n).
• Maturation phase : primary oocyte undergoes division giving one secondary oocyte (1n) and 1st polar body (1n); then the secondary oocyte undergoes a 2nd meiotic division (when fertilization happens) and gives rise to one mature ovum and the second polar body (1n); also the 1st polar body may undergo 2nd meiotic division to give two secondary polar bodies (1n).