Introduction to Human Embryology

Introduction to Human Embryology

• Embryology is the science concerned with human origin and development from zygote to birth.
• Human development begins with fertilization of an ovum by a sperm forming a zygote, which then develops into a human.
• Puberty begins when secondary sex characteristics appear (e.g., pubic hair)
  • Females: 12-15 years old
  • Males: 13-16 years old
• Menarche is the first menstruation.
• Gametogenesis: the process of producing gametes.
  • Spermatogenesis in males at puberty (13-16 years)
  • Oogenesis in females during fetal period, stopping at meiosis I, then resuming at puberty.
• Gametes have a haploid number of chromosomes.
  • Sperm in males (from testes)
  • Oocytes in females (from ovaries)
• Oocyte: the female germ cell produced by the ovaries.
• Secondary oocyte: the mature oocyte.
• Sperm: the male germ cell produced by the testes (spermatozoa).
• Embryo: developing human during early stages of development.
• Zygote: the diploid cell (2n) formed by the union of an oocyte and a sperm, the beginning of a human being.
• Fertilized ovum: secondary oocyte impregnated by a sperm.
• Conceptus: the whole products of conception (embryo + membranes, such as the placenta) since fertilization.
• Primordium: the first indication of an organ or structure.
• Embryonic period: until the end of week 8.
• Fetal period: ninth week to birth.
• Differentiation and growth occur in the fetal period, with high organ function maturation in months 3 and 4.
• Weight increase is notable in the terminal months.
• Trimester: a period of three calendar months of pregnancy.
• Nine months are divided into three trimesters.
• Abortion: expulsion of an embryo or fetus before being viable.
• Viable: mature enough to survive outside the uterus.
• Embryology is important for understanding normal development, reasons for anomalies.
• Human embryonic stem cells are pluripotent.
• Anatomical position, terms include: Anterior/Posterior, Superior/Inferior, Lateral/Medial, Caudal/Cranial, Dorsal/Ventral.
• Coronal section, transverse section, and sagittal planes
  • Coronal section = frontal plane
  • Transverse section = horizontal section
  • Sagittal section = longitudinal section
• Somatic cells: 2n=46 (diploid number)
• Divide by mitosis.
• Germ line cells: 2n=46 (diploid number)
• Divide by meiosis.
• Gametes (sex cells): 1n=23 (haploid number).

Cell Division

• Two types of cell division: Mitosis and Meiosis.

Mitosis

• Process of cell division, creating two identical daughter cells to the parent cell.
• Before the process, DNA replicates (chromosomes are extremely long and spread diffusely throughout the nucleus).
• During prophase, chromosomes condense, shorten, and thicken.
• Chromosomes line up on the equatorial plane during metaphase.
• Chromosomes divide during anaphase.
• The processes of the cells migrate to opposite poles of the spindle.
• Cytokinesis: division of the cytoplasm.

Meiosis

• Cell division in germ cells, producing gametes (with half the number of chromosomes).
• Meiosis I & Meiosis II
• The number of chromosomes decreases to the haploid number of 23.

Crossover

• Exchange of chromatid segments between paired homologous chromosomes. . Critical event in meiosis I. . Segments of chromatids break, are exchanged, as homologous chromosomes separate, and temporary connection is observed as a structure called 'chiasma'.

Gametogenesis

• Formation and development of gametes.
  • Spermatogenesis in males starts at puberty.
  • Oogenesis in females starts during the fetal period; pauses at meiosis I, then resumes at puberty.

Spermatogenesis

• Transformation of spermatogonia into spermatozoa.
• Occurs within seminiferous tubules of testicles, in two to three months (64 days).
  • Spermatocytogenesis: division and growth of stem cells (spermatogonia) into spermatocytes.
  • Meiosis: spermatocytes divide to create haploid cells (spermatids).
  • Spermiogenesis: transformation of spermatids to sperm.

Spermatocytogenesis & Meiosis

• Spermatogonia (primitive germ cells)
• Present during fetal period
• After puberty, mitosis increases in size to establish primary spermatocytes.
• Primary spermatocytes, largest germ cells in seminiferous tubules (2n).
• Enter the first, then second meiotic divisions, producing spermatids (1n).

Spermiogenesis

• Transformation of spermatids into spermatozoa.
  • Development of head (nucleus & acrosome), neck, middle piece (mitochondria), principal piece, and tail.

Mature Sperm

• Head includes the nucleus and acrosome.
• Neck includes the mitochondria.
• Tail.

Spermatogenesis — Process Overview

• Spermatogenesis takes 2 months (64 days) to produce.

Oogenesis

• Production of female gametes (ovum)
• Begins during fetal development, halts at meiosis I.
• Resumes at puberty.
• One primary oocyte produces only one mature ovum (oocyte).

Oogenesis — Process Overview

• Developing follicles, primary follicle, secondary follicle.
• Maturing primary, secondary, and mature follicles contain oocytes.
• Ovulation and Fertilization

Transport

• Oocyte Transport into the Fallopian Tube, through the fimbria and via peristaltic movement to uterus.
• Sperms are moved in the male tract by peristalsis. Secretions are added to form semen. Millions of sperm are ejaculated to travel through the vagina.
• The process results in the formation of a vaginal plug that stops backflow.

Maturation of Sperms — Capacitation

• Fresh sperms cannot fertilize an ovum.
• 7-hour conditioning period.
• Glycoprotein coat and seminal proteins removed from acrosome's surface.
• Increased activity occurs in uterus or uterine tubes.

Viability of Gametes

• Oocytes should be fertilized within 12 hours; they degenerate after 24 hours.
• In vitro fertilization (IVF) can use these gametes even after being frozen.

Fertilization

• Fusion of male & female gametes to form a zygote.
• Contact between the oocyte and the sperm.
• Chromosomes intermingle at metaphase of 1st mitotic division of zygote
• Location: ampulla of the uterine tube.

Phases of Fertilization

• Passage of sperm through corona radiata.
• Penetration of zona pellucida; enzymes (acrosin) help in sperm passage.
• Fusion of plasma membranes of oocyte & sperm.
• Completion of the second meiotic division of oocyte.
• Formation of male and female pronuclei.
• Breakdown of the pronuclear membranes.

Results of Fertilization

• Stimulates the secondary oocyte's completion of the second meiotic division to produce the second polar body.
• Restores normal diploid number (46) in zygote.
• Results in variation in human species.
• Determines chromosomal sex of the embryo.
• Causes metabolic activation of the oocyte, which initiates zygote cleavage.

Cleavage of the Zygote

• Repeated mitotic divisions; blastomeres formation within the zygote.
• Compaction of the cells results in a compact ball after the 8-cell stage.
• Morula: a loose aggregation of 12-32 cells resembling a mulberry.
• 4 days after fertilization, it enters the uterine cavity.

Blastogenesis

• Early blastocyst formation (at day 4), with fluid-filled space.
• Late blastocyst (at day 5), zona pellucida sheds.
  • Trophoblast: outer cell layer (embryonic part of the placenta).
  • Inner cell mass (embryoblasts): cluster of cells at the periphery that will develop into the embryo.

Cleavage

• Occurs in the uterine tube.
• Zygote undergoes mitotic divisions increasing the number of cells.
• Blastomeres (smaller cells produced after cleavage).
• Cleavage begins approximately 30 hours after fertilization.
• Until the eight-cell stage, there is a loosely arranged clump of blastomeres.
• Blastomeres compact to form a compact ball of cells.

Morula

• 3 days after fertilization, the cells divide again
• forming a 16-cell morula (like a mulberry).
• The inner cell mass is composed of inner cells that will make up the embryo proper. . The outer cells form the trophoblast, later giving rise to the placenta.

Blastocyst

• Morula enters uterine cavity.
• Fluid penetrates the intercellular spaces, forming one cavity called the blastocele.
• The embryo (morula) becomes a blastocyst.
• Blastomeres are separated into the inner and outer cell masses (embryoblast and trophoblast).

Implantation

• Blastocyst enters the uterine wall.
• Blastocyst attaches to endometrial epithelium.
• Two layers of the trophoblast:
  • Cytotrophoblast: inner layer, proliferates and differentiates.
  • Syncytiotrophoblast: outer layer, mass of multinucleated protoplasm with phagocytic function.
• Implantation begins at approximately 6th-8th day and completes at approximately the 12th day

Explore the fascinating process of human development from the zygote to birth in this quiz. Learn about gametogenesis, puberty, and the stages of embryonic development. This is essential for understanding the biological foundations of human life.