Fertilization is the process by which eggs and sperm fuse into a zygote. It is a basic feature of sexual reproduction, and is commonly found in the animal and plant kingdoms, but people usually refer to animals most often. Animal fertilization is at the cellular level. The fertilization process includes three main stages: egg activation, adjustment, and bisexual prokaryotic fusion. Activation can be regarded as the starting point of individual development, mainly manifested by changes in the permeability of the omental membrane, efflux of cortical granules, formation of fertilization membranes, etc .; adjustment occurs after activation and is the first in the egg necessary to ensure the normal division of the fertilized egg Amphoteric prokaryotic fusion plays a role in ensuring parental inheritance and restores diploidy. Fertilization not only initiates DNA replication, but also activates genetic information such as mRNA and rRNA in the egg to synthesize proteins required for embryonic development.
Figure 1. Fertilization.
After fertilization, the blastocysts in the late stage of the blastocysts are gradually embedded and covered by the endometrium after 6 to 7 days. Need to go through three processes: 1. positioning; 2. adhesion; 3. penetration. Metamorphosis occurred quickly after implantation.
Animal sperm does not have obvious chemotaxis like lower plants, such as bryophytes, but instead reaches the egg by its own active movement or by the cilia of the reproductive tract epithelial cells. Human fertilization time: within 24 hours after ovulation. Fertilization site: tubal ampulla.
It is known that when many mammalian sperm pass through the female reproductive tract or cross the cumulus, the foreign proteins surrounding the sperm are removed, and the physicochemical and biological characteristics of the sperm plasma membrane change, enabling the sperm to participate in the fertilization process.
When the capacitated sperm of a mammal contacts the egg membrane or zona pellucida around the egg, it specifically binds to a certain glycoprotein on the egg membrane and stimulates the sperm to produce an acrosome reaction: part of the plasma membrane around the acrosome disappears, and the acrosome is inside the outer membrane. Pits, vesicles, and acrosome contents include some hydrolytic enzymes to escape. The acrosome reaction helps sperm to penetrate the egg membrane further. On the sea urchin eggs, the acrosome response of the sperm is a kind of polysaccharide in the peridrum membrane. Most of the eggs have egg membranes on the periphery. The thickness of various egg membranes varies. The main component is mucin or mucopolysaccharide; only a few are naked eggs, such as those of coelenterate.
Spermatogenesis is regulated by luteinizing hormone (LH), follicle stimulating hormone (FSH), and testosterone secreted by testicular mesenchymal cells. Leydig cells, also known as Leydig cells, are located in the interstitial tissue between the seminiferous tubules. They synthesize and secrete testosterone into the seminiferous tubules to promote spermatogenesis. Testosterone production is controlled by LH released by the pituitary. FSH secreted by the pituitary gland stimulates the support cells to synthesize and secrete androgen-binding protein. It has strong affinity with testosterone to maintain the concentration of testosterone in seminiferous tubules and maintain its effect on spermatogenesis. In addition, FSH can directly start spermatogonia division and stimulate the development of early germ cells.
Luteinizing hormone (LH)
Luteinizing hormone (LH) is a glycoprotein gonadotropin secreted by pituitary cells that promotes the conversion of cholesterol into sex hormones in gonadal cells. For women, it works with follicle stimulating hormone (FSH) to promote follicle maturation, secretes estrogen, ovulation, and the generation and maintenance of the corpus luteum, and secretes progestin and estrogen. For men, luteinizing hormone stimulates testicular mesenchymal cells to synthesize and release testosterone.
Figure 2. Effects of LH on the body.
Follicle-stimulating hormone is a hormone secreted by basophils in the anterior pituitary gland. The component is glycoprotein, which mainly promotes follicle maturation. Follicle-stimulating hormone can promote the proliferation and differentiation of follicular granulosa cells, and promote the growth of the entire ovary. And its action on the seminiferous tubules of the testis can promote sperm formation. Follicle-stimulating hormone is secreted in the human body in pulses, and women change with the menstrual cycle. The determination of follicle stimulating hormone in serum is of great significance for understanding the pituitary endocrine function, indirectly understanding the functional status of hypothalamus and ovary, predicting ovulation time, diagnosis and treatment of infertility and endocrine diseases.
Figure 3. Protein structure of Follicle-stimulating hormone.
1. Jiang X.; et al. Structure of follicle-stimulating hormone in complex with the entire ectodomain of its receptor. Proceedings of the National Academy of Sciences of the United States of America. 2012, 109 (31): 12491-6.