Human Embryology: From Zygote to Fetus
Human Embryology: From Fertilization to Birth
Embryology, which deals with the single cell, starts with the fertilization of the sex cells. During fertilization, the single female cell (egg or ovum) is fertilized by the male cell (sperm). Gametes (sex cells) are haploid cells, and through fertilization, the diploid state is restored.
Key Stages of Embryonic Development
- Blastula Stage: The embryo is implanted into the uterine wall.
- Gastrulation: The formation of the three germ layers (ectoderm, mesoderm, and endoderm).
- Organogenesis: The formation of organs.
Developmental mechanisms are generally conserved through metazoans. Model systems, such as rats, plants, invertebrates, fish, and birds, help understand human development. These model organisms are chosen because they are easy to keep in lab conditions, develop quickly, and produce many offspring.
From Zygote to Blastocyst
A zygote is a fertilized egg. You can see the two nuclei in the zygote before they fuse. At some point, the cells will rearrange to form the blastocyst. The embryo will begin to develop from the top right portion of the blastocyst.
Germ Layer Differentiation
The blastula begins to differentiate into three layers:
- Mesoderm: Forms in the middle (muscles, red blood cells).
- Ectoderm: Forms on the outside (skin, teeth).
- Endoderm: Forms on the inside (lungs, pancreas).
Timeline of Early Human Development
- Weeks 1-3: Abnormal development will result in termination.
- Implantation to Week 8: Organs begin developing.
- Weeks 9 to Birth: Extremities and other structures form.
Preparing for Pregnancy: Gametogenesis
Gametogenesis is the formation and maturation of gametes. This process involves:
- Release of gametes from gonads.
- Transport to where fertilization occurs (fallopian tubes).
- Fertilization: Fusion of haploid sperm and haploid egg to produce a zygote.
- Implantation of the fertilized egg (embryo) in the uterus.
For further development, the embryo must travel from the upper area of the uterine tract to the lower area for nutrients.
Four Phases of Gametogenesis
- Extraembryonic origin of the germ cells and migration to the presumptive gonads.
- Increase in the number of germ cells by mitosis.
- Reduction in chromosome number to haploid by meiosis.
- Structural and functional maturation of the eggs and spermatozoa.
Germ Cell Origin and Migration
Germ cells originate from an extraembryonic origin. It begins with the ‘germ plasm’ located at the vegetal pole of the fertilized egg. The process is very intense at the animal pole and slow at the vegetal pole. Determinants in the germ plasm become packaged into a small number of cells during cleavage (cellularization). Cells containing germ plasm determinants are primordial germ cells (PGCs).
Origin and Migration of PGCs: PGCs migrate through the somatic tissue to the presumptive genital ridges. In the genital ridges, PGCs become oogonia (female) or spermatogonia (male). PGCs migrate, proliferate, and differentiate in the gonad. Primordial germ cells travel through the dorsal mesentery and into the gonad.
Teratomas: These are cancerous masses of differentiated cells in a disorganized state, due to PGCs not migrating to the gonads. They can differentiate into skin, hair, etc.
Mitotic Proliferation
- Female (Oogonia): Intense mitotic activity in the embryonic ovary (up to 7 million germ cells) occurs from 2-5 months. Over time, natural degeneration of germ cells (called atresia) occurs until menopause.
- Male (Spermatogonia): Mitosis begins in the early embryonic testis and continues throughout life. Seminiferous tubules of the testes are lined with a germinative population of spermatogonia.
Meiosis
The first step is prolonged because of mixing and genetic variation. In Metaphase I, chromosomes line up in the median line. In Anaphase I, whole chromosomes migrate to each pole. Telophase I separates them to become two separate cells. The two daughter cells will now go through mitosis. Metaphase II is when the chromatin migrates to the poles. Recombined and split into two identical daughter cells. The end of this phase produces four haploid gametes.
Maturation of Gametes
Gametes form from germ cells in the gonads. Mitotic divisions of germ cells produce spermatogonia in males and oogonia in females. These cells undergo meiosis to produce gametes.
Germ Cell Proliferation in the Ovary
From months 2-5, oogonia reach 7 million. Atresia follows by 7 months. Most oogonia have reached prophase of meiosis I (primary oocytes). Only about 2 million oogonia are present at birth, and thousands by puberty.
Oogenesis
Oogonia undergoes mitotic proliferation. In the womb, the oogonium has no follicle. At birth, it develops a primary follicle and is now called the primary oocyte. Meiosis is in progress (2n4c). After birth, the primary oocyte is arrested in the diplotene stage of the first meiotic division. After puberty, the secondary follicle forms. The first meiotic division is completed, and the second meiotic division occurs. The third follicle forms, and the secondary oocyte and polar body I form. The ovulated ovum is formed, now arrested in metaphase II. A fertilized ovum is created. A mature egg is created at the end of the ovarian cycle.
The internal follicle contains blood vessels and granule structures, surrounded by the theca (which forms the theca externa). Angiogenetic factors attract capillaries for blood vessel formation. Blood vessels develop in the second follicle stage for more nutrients. The light blue area is the zona pellucida. Theca cells can release testosterone, cross the membrane, and reach the granulosa cells. Inside, they produce aromatase, which converts testosterone into estrogens, affecting structures in the body.
Spermatogenesis
Spermatogonium (in seminiferous tubules) – primary spermatocyte (1st meiotic division completed) – 2 secondary spermatocytes (2nd meiosis division) – 4 spermatids – spermiogenesis forms 4 mature spermatozoa (haploid) nurtured by hormones from Sertoli cells (form blood barrier).
Spermiogenesis is the process of a spermatid becoming a mature spermatozoon. It develops an acrosome for fertilization. The flagellum begins, eventually forming a tail. Cytoplasm moves away from the nucleus and is eventually shed.
Spermatozoa: The head has a nucleus and acrosome, the midpiece has mitochondria, and the tail has flagella.
Hormonal Control in Males
Leydig cells are equivalent to the theca (releases testosterone, affects Sertoli cells).
Uterus Structure
The tube towards the embryo is the fimbria. The largest part farthest from the ovary is the ampulla. Epithelial granular cells inside the tube release mucus for sperm travel (cilia help). The isthmus is the middle tube area, and closest to the uterus is the intramural region. A mature female follicle has the stigma present (circular protrusion).
Hormonal Control
- Oxytocin: Responsible for birth and mammary gland function. Released by the neurohypophysis.
- Prolactin: Affects the breasts, released by the anterior pituitary.
Timeline
- 0-3 weeks: Early embryo.
- 3-8 weeks: Embryonic organogenesis.
- 8-38 weeks: Fetal period.
Menstrual dating is 40 weeks, while fertilization is 38 weeks.